Icaphoca choristodon, Dewaele & Muizon, 2025

Dewaele, Leonard & Muizon, Christian de, 2025, Icaphoca choristodon n. gen., n. sp., a new monachine seal (Carnivora, Mammalia) from the Neogene of Peru, Geodiversitas 47 (11), pp. 465-499 : 468-492

Icaphoca choristodon n. gen., n. sp.

( Figs 2-6 ; Tables 1-5 )

urn:lsid:zoobank.org:act:D62656C2-2C7D-4C7C-87F9-2F892F941620

TYPE MATERIAL (HOLOTYPE BY MONOTYPY). — Peru • 1 specimen (original specimen; subcomplete skull with associated left and right mandibles, atlas, axis, third, fourth, and fifth cervical vertebrae); Ica Department, Cerro La Bruja ; Pisco Formation , P1 sequence, unit P1- 2; Cenozoic, Neogene, Tortonian; c. 14°32’08.13”S, 75°39’39.45”W; 1981; Muizon leg.; MNHN.F.PPI270 .

DIAGNOSIS. — Small to medium-sized lobodontin monachine, having an elongated rostrum, similar to, but not to the same extent as Acrophoca longirostris . Icaphoca choristodon n. gen., n. sp. is a lobodontin, based on the following unambiguous synapomorphies, as detailed by the phylogenetic analysis, below: a parasagittal orientation of the medial margins of the tympanic bullae; the presence of a lip in the anteromedial region of the mastoid, abutting the posteromedial edge of the tympanic bulla; and the transverse foramen of the atlas is at least partially visible in dorsal view. Icaphoca choristodon n. gen., n. sp. differs from all other monachine except Acrophoca longirostris in the presence of well-developed diastemata between all post-canine teeth, the diastema between P4 and M1 being distinctly larger. Icaphoca also shares with Acrophoca the anterior end of the maxillary process of the jugal reaching above or dorsomedially to the infraorbital foramen. It differs from Acrophoca in having six upper incisors as in Magophoca brevirostris, and Noriphoca gaudini (a condition, absent in all other monachines), in having a moderately inflated ectotympanic, and an atlas in which the transverse foramen is visible in posterior view only.

ETYMOLOGY. — from the Greek choristos (separated) and odontos (tooth) referring to the wide separation between the upper and lower postcanine teeth, to enhance this major difference with the stratigraphically close Magophoca brevirostris.

TYPE LOCALITY. — The holotype of Icaphoca choristodon n. gen., n. sp. was discovered in 1981 by one of us (CM) at the base of the slopes of the Cerro La Bruja in the Ica Desert on the west side of the road from Ocucaje to Lomitas , west to the area named “Pampa Los Pescadores” ( Carta Nacional 1/100 000, Hoja 30-1, Lomitas ; Instituto Geografico militar Lima-Peru 1977; Ica Department ) ( Fig. 1 ). Since no GPS coordinates were available in 1981, an approximate collecting area of the specimen described here is outlined as a red oval on the west side of the road on Figure 1 .

TYPE HORIZON AND AGE. — The holotype of Icaphoca choristodon n. gen., n. sp., has been extracted from beds slightly older than those of the CLB level. The latter correspond to the base (unit P2-1) of the P2 sequence of the Pisco Formation, and the beds which have yielded the holotype of I. choristodon n. gen., n. sp. therefore likely proceed from the younger beds (unit P1- 2) of the P1 sequence of the Pisco Formation. The age of the specimen should therefore be slightly older than that of the CLB level (in which Magophoca brevirostris remains have been discovered). According to Di Celma et al. (2017) the strata of the P1 sequence of the Pisco formation were deposited between 9.5 and 8.9 Ma. Therefore, I. choristodon n. gen., n. sp. could be as old as 9 Ma, which corresponds to the middle part of the Tortonian.

DESCRIPTION AND COMPARISON

CRANIUM ( Figs 2-6 ; Tables 1-5 )

General

The holotype (MNHN.F.PPI270) specimen of Icaphoca choristodon n. gen., n. sp. is also the only known specimen of the species. The skull is fractured and notably deformed. However, the overall morphology is well preserved, allowing a detailed description of most visible bones.

The dental formula of Icaphoca is I3/2 C1/1 4/4P 1/1M (char. 39[0], 42[0], 51[1]). The presence of six upper incisors (three per upper quadrant) and four lower incisors (two per lower quadrant) has been considered a plesiomorphy, present in most terrestrial and aquatic Carnivora and Phocinae, whereas Monachinae are traditionally characterized by two upper incisors and two lower incisors (e.g., Muizon 1981; Berta & Wyss 1994; Bininda-Emonds & Russell 1996). Notable exceptions are the monachine genus Mirounga , and the phocine species Cystophora cristata , having two upper incisors and one lower incisor. Although all extant and most extinct Monachinae have two upper and two lower incisors, two extinct Monachinae conform the plesiomorphic condition of having three upper incisors, besides Icaphoca . These are Noriphoca gaudini from the early (?) Miocene of the Abruzzo region in Italy ( Guiscardi 1871; Dewaele et al. 2018), and Magophoca brevirostris from the late Miocene of the Cerro La Bruja locality in Peru ( Dewaele & Muizon 2024).

Skull dimensions of Icaphoca are comparable to most other extinct Monachinae , but smaller than extant Monachinae ( Tables 1 ; 2 ). A notable exception is Australophoca , which does not have any published skull specimens, but is the smallest monachine based on postcranial remains. Churchill et al. (2014) presented two equations to estimate overall body length of pinnipeds based on cranial measurements. The equation used for Phocidae is:

0.37 × Log (width of skull across canines) + 0.80 × Log (width across occipital condyles) + 1.39 With a width of 43.4 mm and a width of 59.2 mm across the canines and across the occipital condyles respectively, the individual to which the holotype skull belongs, had an estimated body length of c. 224 cm. This is roughly similar to the size of Magophoca as calculated by Dewaele & Muizon (2024).

ROSTRUM

The rostrum of Icaphoca choristodon n. gen., n. sp. is elongate ( Figs 2-4 ). As measured from the anterior edge of the orbit to the apex of the premaxilla it is 93.5 mm long. In the paratype of Magophoca (MNHN.F.PPI269)the length of the rostrum is 79.5 mm. therefore the rostrum of Icaphoca choristodon n. gen., n. sp. is 5% longer than that of Magophoca brevirostris. This difference is regarded as significant since the two specimens have the same estimated body length 224 cm vs 223 cm respectively.

The premaxilla lines the medial wall of the nasal cavity. The ascending ramus of the premaxilla largely remains within the nasal cavity, with only the posterodorsal triangular shaped portion of the ascending ramus of the premaxilla extending outward and being visible in lateral view (char. 2[1]). Consequently, the maxilla-premaxilla suture follows the dorsal edge of the nasal cavity in its median portion and is not visible in lateral view in the anteroventral and median portions of the rostrum.

When compared to other Phocidae , this condition conforms with Monachinae . In Phocinae, on the other hand, the ascending ramus of the premaxilla is visible in lateral view along the entire length of the nasal cavity. Notable exceptions are the phocine Histriophocini, in which this ascending ramus is marginally visible in lateral view at the middle part of the nasal cavity. In the phocine Cystophora cristata and the monachine Mirounga , the premaxilla is only visible in lateral view in the anterior portion of the nasal cavity. This condition can be considered an osteological adaptation to accommodate the “hood” of Cystophora and the proboscis of Mirounga .

The premaxilla of Icaphoca bears small dorsally pointing tuberosities at the anteromedial end of the nasal cavity. The premaxillary tuberosities are generally absent or at least very weak in extant Monachinae , except for Hydrurga . Within fossil monachines, premaxillary tuberosities are weak in Eomonachus and Sarcodectes , whereas they are conspicuous in Acrophoca , Hadrokirus , Homiphoca, Magophoca , and Piscophoca . Furthermore, Homiphoca , and to a lesser extent Piscophoca , are characterized by a strong anterior projection of the tuberosities.

The lateral margin of the nasal cavity in Icaphoca appears to be strongly concave dosrally (char. 4[1]), with the opening of the nasal cavity oriented more dorsally than anteriorly (char. 3[1]). Both conditions vary among Monachinae . Whereas most Monachinae have a nasal cavity with a rather strongly curving lateral margin and an opening that faces dorsally rather than anteriorly, this curvature is gentle in Lobodon , and the nasal cavity faces rather anteriorly in Ommatophoca .

The posterodorsal extremity of the ascending ramus of the premaxilla contacts the nasal(char.1[0]). This contact is extensive, a plesiomorphic character shared with Phocinae,except Cystophora and Mirounga , in which the posterior part of the ascending process is lost. Among Monachinae , this character varies and appears to largely split between extant and extinct taxa, with Acrophoca , Hadrokirus , Homiphoca, Magophoca , and Sarcodectes also exhibiting an extensive contact between the premaxilla and the nasal.This contact is reduced in Leptophoca , Piscophoca , and the tribe Monachini (not preserved in Pliophoca ). In Hydrurga , Lobodon , Mirounga and Ommatophoca the premaxilla and the nasal do not contact.

The left and right palatal processes of the premaxilla form an acute triangle, with the incisors arranged in a curved arch. The incisive foramen forms a deep groove,located within the premaxillary portion of the palate, but aligning the premaxilla-maxilla suture. Both left and right incisive foramina are well separated by a thick septum. Among other Monachinae , this aligns with the extinct Eomonachus , Hadrokirus, Magophoca , Piscophoca , and Sarcodectes and the extant Hydrurga , and Ommatophoca . Among other Monachinae , the incisive foramina are shallow or absent.

The nasals form a long V-shaped wedge between the frontals.

The anterior margin of the right nasal over the external naris is U-shaped to V-shaped with the lateral branch being distinctly longer than the medial one. Therefore, the anterior edge of the nasals is distinctly W-shaped. This condition is similar to that observed in Magophoca and Neomonachus schauinslandi , Hydrurga , Leptonychotes , and Lobodon , but differs from that in Homiphoca , Ommatophoca , and Neomonachus tropicalis ,in which it is U-shaped to concave anteriorly or from that of Monachus , in which the nasals are protruding anteriorly. The condition in Acrophoca and Piscophoca is unknown because this region of the skull is not preserved on the specimens available during this study.

The posterior end of the nasal of Icaphoca reaches at least midway between the level of the anterior margin of the orbit and the superior squamosal process of the jugal, which correspond to the posterior edge of the orbit (char. 5[1], 6[1]). Except for Erignathus and Neomonachus schauinslandi , this character agrees with other Phocidae : in Erignathus and N. schauinslandi , the nasals still form a V-shaped wedge between the frontals but have a more rounded distal termination. The posterior extension of the nasals between the frontals varies strongly among Monachinae ; even intraspecifically (LD, pers. obs). Generally, the nasals terminate posterior to the anterior margin of the orbit, but do not reach the level of the superior squamosal process of the jugal posteriorly. In Acrophoca it extends more posteriorly than in Icaphoca and Magophoca, whereas in Hadrokirus , Homiphoca , Sarcodectes , Mirounga, Monachini , and Ommatophoca , it is clearly more anterior.

The ventral margin,or palatal process,of the maxilla of Icaphoca is horizontal, resulting in a horizontal maxillary tooth row, or alveolar process.This condition conforms with Phocidae , except for the extant monachine Ommatophoca , and the extinct monachines Hadrokirus and, to a lesser extent, Piscophoca , in which the ventral margin of the maxilla is upturned, facing anteroventrally (char. 18[1]) ( Muizon & Hendey 1980; Muizon 1981; Amson & Muizon 2014; Govender 2015). The alveolar process is reduced, with the base of the tooth row raised only little over the palate. In cross-section, the palate is flat, as in other Monachine, but unlike Phocinae, which have a more arching palate.

The maxillary tooth row is constricted at the level of P2 (char. 19[1]), with the palate widening posteriorly being almost twice as wide at the level of M1, as at the level of P2 ( Table 2 ). The condition of Icaphoca differs from that in Magophoca, in which the palate constriction is at the level of P1. Among other Monachinae , the tooth row is mostly constricted at the level of P1 or P2, and the palate also significantly widens posteriorly. Notable exceptions are Hydrurga , and Mirounga , and to a lesser extent Lobodon ), which have maxillary tooth rows that do not diverge posteriorly, or only slightly, respectively.

The palate is incompletely preserved but appears largely flat. The alveolar process is strongly reduced, with the base of the toothrow only very little elevated over the palate.

The palatine sulcus is shallow and overall weakly outlined, being more pronounced posteriorly, where it terminates into the palatine foramen. This small, rounded palatine foramen is located at the level of the diastema between P4 and M1, and within the palatomaxillary suture. The position of this foramen, with respect to the postcanine teeth, as well as the palatomaxillary suture, differs notably among Phocidae . In Phocinae, this foramen is located well posterior to the level of M1, and on the palatomaxillary suture or on the palatine bone. An exception is the genus Phoca , in which it is anterior to the suture. Among Monachinae , this condition varies widely. In most fossil species, the foramen is located in the palatomaxillary suture in Piscophoca , Acrophoca , and Homiphoca as in Icaphoca . In Hadrokirus the foramen is anterior to the suture and in Magophoca its position is variable (anterior to the suture in the holotype and on the suture in the paratype). In extant monachine the foramen is anterior to the suture in Monachus , Neomonachus , Mirounga , Leptonychotes , Lobodon , and Hydrurga , but it is on the suture in Ommatophoca . In most extant monachines the foramen is at the level or slightly anterior to M1 except in Hydrurga in which it is positioned more anteriorly, at the level of P3.

ORBIT AND ZYGOMA

The antorbital process forms a distinct prominence, which is rather uncommon among Phocidae , only shared with the extant monachine Hydrurga , and the extinct monachines Acrophoca , Hadrokirus , Homiphoca , and Piscophoca . Icaphoca choristodon n. gen., n. sp. differs in this respect from Magophoca, in which the antorbital process is weak and low, being barely present in the holotype.

On the lateral surface of the maxilla, is the anterior opening of the infraorbital foramen, which pierces the antorbital wall. This foramen is located at the base of the zygomatic process of the maxilla, dorsal to M1. It is subcircular and large in Icaphoca (char. 11[1]). It is however distinctly smaller than in the paratype of Magophoca brevirostris, an individual which is similar in size to the holotype of Icaphoca choristodon n. gen., n. sp. (diameter of the left foramen is 9.8 mm in the former and 7 mm in the latter). Among other Monachinae , the infraorbital foramen is subcircular in Acrophoca , Hadrokirus , Leptonychotes, Magophoca, and Monachini ; whereas it is oval in cross-section in other Monachinae . Also, among other Monachinae , the infraorbital foramen is located dorsal to M 1 in most taxa, apart from extant Lobodontini , in which it is located slightly anterodorsal to M1, and Mirounga , in which it is located posterodorsal to M1.

The zygomatic process of the maxilla is perforated by the infraorbital canal. It extends posteriorly in a thin blade appressed against the ventromedial aspect of the maxillary process of the jugal almost reaching the base of the of the superior squamosal process of the jugal. On its dorsolateral side, the jugular process of the maxilla has a long, oblique, and sigmoid suture with the ventromedial side of the maxillary process of the jugal. This agrees with the condition in most other Monachinae (char. 12[0]). In Phocinae, as well as in the monachines Hadrokirus and Mirounga , the jugo-maxillary suture is rather subvertical and both the jugular process of the maxilla and the maxillary process of the jugal are stout.

The maxillary process of the jugal tapers anteromedially and forms the anterior wall of the orbit. As observed in anterior view, the anterior limit of this process terminates dorsal to the medial wall of the infraorbital foramen, a condition shared with Acrophoca and Magophoca. This condition differs from that of Homiphoca , Piscophoca , Hadrokirus and extant Monachini , in which it is slightly lateral to or dorsal to the lateral wall of the foramen) and (char. 13[1]). In other Phocidae , the anterior tip of the maxillary process terminates lateral to the level of the infraorbital foramen.

Posteriorly, the jugal splits into a superior and an inferior squamosal process, forming a mortized jugosquamosal contact (char. 15 [1]). In addition, the superior squamosal process is more slender than the inferior squamosal process.All this agrees with other Phocidae .In Icaphoca , the angle between the inferior and superior squamosal branches of the jugal is roughly 90°.

Although the cranium of the holotype of Icaphoca choristodon n. gen., n. sp. is severely distorted, in lateral view, the lowest point of the ventral margin of the zygomatic arch is dorsal to the level tooth row (char. 14[0]). This condition agrees with other extinct Monachinae (except Acrophoca , Eomonachus , and Homiphoca ) and the extant Hydrurga and Monachini .

The zygomatic process of the squamosal projects anterodorsally, like the condition in Acrophoca , Homiphoca , and Magophoca. In other extant and extinct Monachinae , this process is oriented more dorsally (i.e. with a greater dorsal component), as is the case in Eomonachus , Hadrokirus , Mirounga , Monachus , Neomonachus tropicalis , Ommatophoca , and Piscophoca , or anteriorly (i.e. with a greater horizontal component), as in Hydrurga , Leptonychotes , Lobodon , Neomonachus schauinslandi , and Sarcodectes . The zygomatic process of Icaphoca is very slender as in Magophoca, being strongly expanded at the apex differing, in this respect, from the condition observed in Magophoca. In other extinct monachines the process is more massive. In Icaphoca , the ventral edge is straight, as is observed in Hadrokirus , Homiphoca , and Magophoca, and differs from the conspicuously sigmoid ventral edge observed in Acrophoca and Piscophoca .

The glenoid fossa is deep and strongly semi-circular in cross-section, with a moderately well-developed preglenoid process and a strongly developed postglenoid process. Among other Monachinae , this condition corresponds most with Hadrokirus , Hydrurga , Piscophoca , and Sarcodectes . In this respect, Icaphoca differs from Magophoca, in which the glenoid fossa is shorter transversely and distinctly shallower.

The orientation of the long axes of the glenoid fossae remains difficult to ascertain, due to the fractured nature of the skull. However, as it appears, the long axes are parallel to one another, a condition shared with other Monachinae , contrasting with Phocinae, in which these axes converge posteriorly (char. 20[0]).

The interorbital bridge, formed by the frontals, is long and narrow ( Table 2 ), and at its narrowest in the posterior portion of the bridge slightly anterior to the level of the apex of the superior squamosal process of the jugal (char. 16[0], and char. 17[1]). In other Phocidae , the location of the narrowest portion of the interorbital bridge varies between Monachinae (except Ommatophoca ) and Phocinae.The condition in Icaphoca follows that in other Monachinae (except Ommatophoca ) in which it is close to the posterior region of the orbit, whereas in Phocinae and Ommatophoca , it is narrowest anteriorly (in the anterior region of the orbit), gradually broadening posteriorly towards the braincase. Among other Phocidae , the width of the interorbital bridge is similar, proportionally to the bizygomatic width, in most taxa, with the exception of the phocines Histrophoca, Pagophilus , and Pusa , in which it is extremely narrow, and Cystophora , Erignathus , Homiphoca , Lobodon , and Ommatophoca , in which this it is proportionally broad.

BASICRANIUM AND BRAINCASE ( Figs 5 , 6 )

The braincase of Icaphoca is sub-rounded in dorsal view. Posterior to the interorbital bridge, the sagittal crest is only weakly developed. The nuchal crest on the posterodorsal margin of the braincase is semi-circular in outline (in dorsal view) and moderately well developed, projecting slightly over the posterior portion of the braincase.

The tympanic bulla of Icaphoca is isosceles triangular in ventral outline. It is slightly more inflated than in the other Pisco monachines, although it approaches in this respect the condition observed in Magophoca brevirostris (especially on the holotype). This triangular outline, which corresponds with the condition in other Monachinae , is in part attributed to the moderate inflation of the entotympanic and ectotympanic as compared to Phocinae. In the latter, the tympanic bulla is strongly inflated, resulting in a rather sub-rounded outline in ventral view.

The straight medial margins of the left and right tympanic bullae appear parasagittal and parallel (char. 21[1]), which it shares with other Lobodontini . In other Phocidae , the medial margins of the tympanic bullae diverge posteriorly. However, it should be noted that this part of the holotype cranium of Icaphoca is severely fractured, and this condition cannot be ascertained unambiguously.

The bulla is constituted by the ectotympanic and entotympanic. The two bones are fused on the holotype of Icaphoca choristodon n. gen., n. sp. but a discrete uneven line can be observed from the anteromedial angle of the bulla, extending posterolaterally towards an anterolateral swelling of the bulla and from there turning anterolaterally toward the groove for the facial nerve (CN VII) leading to the stylomastoid foramen ( Figs 5 , 6 ).

The moderately-inflated ectotympanic forms the anterolateral portion of the tympanic bulla. It bears a gentle swelling, as mentioned above at the posterolateral portion of its suture with the entotympanic. Posteromedial to the ectotympanic the bulla is formed by the entotympanic. The inflation of the entotympanic is strongest in the central portion of the tympanic bulla, in the anterolateral portion of the entotympanic. The ectotympanic is triangular to club-shaped in ventral view, with the long “handle” along the anterolateral region of the entotympanic and the “head” projecting anterolaterally as a prominent ectotympanic lip, covering the external auditory meatus (char. 22[1]). The lateral tip of the ectotympanic reaches lateral to the center of the glenoid fossa and bears a moderately developed ectotympanic tubercle.

The entotympanic is subtriangular in outline as well, with the anterolateral and posterolateral margins forming a right angle. At the anteromedial tip of the bulla, the anterior lacerate foramen and the opening for the musculotubal canal coalesce and open anteriorly. The posterior tip of the entotympanic –and of the tympanic bulla– is blunt. It almost totally covers the posterior region of the pars petrosa of the petrosal and almost borders the anterior edge of the posterior lacerate foramen.

The posterior carotid foramen forms a large, circular, and posteriorly oriented opening on the posteromedial region of the entotympanic. This foramen opens posteroventrally and is visible in ventral view (char. 29[0]). This condition in Icaphoca follows other Monachinae . It differs from that in Phocinae, in which the posterior carotid foramen opens totally medially (thus facing its counterpart) and is not visible in ventral view, due to the inflation of the tympanic bulla. The posterior carotid foramen of Phocinae opens directly below and is coalescent with the posterior lacerate foramen.

In Icaphoca and other monachines the posterior carotid foramen is located entirely in the tympanic bulla and does not open directly on the posterior lacerate foramen from which it is separated by a thin lamina of the tympanic bulla (char. 30[0]).

The posterior lacerate foramen is inadequately preserved to allow a detailed description and comparison with other Phocidae .

On the anterior edge of the posterior lacerate foramen, the entotympanic extends posteriorly; it almost totally covers the pars petrosa of the petrosal but a narrow strip of petrosal is visible ventrally. A similar condition with the entotympanic almost totally covering the petrosal is also observed in Acrophoca , Hadrokirus, Magophoca , and Piscophoca . This condition differs from that in Monachini and Phocinae in which the pars petrosa is widely exposed ventrally at the anterior edge of the posterior lacerate foramen.

On the lateral edge of the posterior region of the tympanic bulla, the entotympanic interlocks with the anteromedial edge of the canal for the auricular branch of the CN X, which is excavated in the mastoid. In this region, the mastoid features a thin lip which overlaps the posterolateral edge of the entotympanic. This mastoid lip is approximately 9 mm long on the holotype of Icaphoca choristodon n. gen., n. sp. Such a structure has been observed in the four genera of extant lobodontines, and among fossil monachines, in Acrophoca , Hadrokirus , Homiphoca, Magophoca , Piscophoca , and Sarcodectes , as well as the tympanic bulla attributed to the nomen dubium Monotherium ? wymani ( Muizon & Hendey 1980; Muizon 1981, 1982; Amson & Muizon 2014; Rule et al. 2021; Dewaele & Muizon 2024). It is noteworthy however that in Icaphoca and Magophoca this lip is less developed than in the other fossil and extant lobodontins. This structure was observed for the first time by Repenning & Ray (1977) and has been regarded as a synapomorphy of the Lobodontini . ( Muizon 1981, 1982; Amson & Muizon 2014; Dewaele & Muizon 2024).

In the Phocinae and Monachini , a large portion of the pars petrosa of the petrosal is visible at the anterior edge of the posterior lacerate foramen and the posterior edge of the entotympanic is more anterior than in the extant and fossil Lobodontini , Sarcodectes , Monotherium ? wymani, and Miroungini. In contrast in the latter taxa, the enotympanic totally (or almost totally) covers the petrosal posteriorly (char. 33[0]) and the petrosal is not inflated, as is the case in most Phocinae and Monachini .

On the medial edge of the posterior apex of the entotympanic, facing posteromedially is a small foramen overhanging (in ventral view) the posterior lacerate foramen. We interpret this foramen as the passage of the tympanic nerve, which connects the CN IX to the minor petrosal nerve within the tympanic cavity. Such a foramen is present in Acrophoca , Hadrokirus , Homiphoca , and Piscophoca (pers. obs.) as well as in the four extant Lobodontini . Although not mentioned by Dewaele & Muizon (2024), it is also present in Magophoca. On the posterolateral edge of the entotympanic, just anteromedial to the mastoid lip is a small foramen, which is the opening of a small anterolaterally oriented canal. We have no interpretation for this foramen, which is clearly distinct from the tympanic nerve foramen ( Figs 5 , 6 ), although it could simply be a nutrient foramen.

On the posterolateral margin of the tympanic bulla, the contact between the bulla and the swollen mastoid is invaginated. At the posterolateral surface of the ectotympanic, the pit for the tympanohyal descends into this contact. The pit for the tympanohyal is a small and shallow groove. Among other Monachinae , the pit for the tympanohyal is bordered by elevated lips, which partially cover it as is observed in Acrophoca , Hadrokirus , Homiphoca , Leptonychotes , Lobodon , and Monachus . In contrast, in Magophoca, the edges of the stylomastoid groove are smoothly rounded, a condition that is probably also present in Icaphoca but not possible to ascertain due to the preservation of the specimen.

The stylomastoid foramen is located at the contact between the ectotympanic and the mastoid. It is a large and circular foramen, which opens anterolaterally into a semi-cylindrical groove.The latter extends anterolaterally at approximately 45° to the sagittal plane and conveys the facial nerve (CN VII), which exits the skull through the stylomastoid foramen and extends on the lateral and dorsal aspect of the skull to innervate the anterior region of the head. Posterior to the stylomastoid foramen, the mastoid is notably swollen along the posterior half of the posterolateral margin of the tympanic bulla. This swelling largely separates the stylomastoid foramen anteriorly from the auricular foramen posteriorly.However, this swelling remains separated from the tympanic bulla, thus separating both foramina only incompletely. Such a swelling separating the stylomastoid foramen from the auricular foramen is present in all Phocidae ( Deméré et al. 2003) . Bordering the swelling medially is a deep groove oriented posterolaterally for the origin of the digastricus muscle ( Piérard 1971). Anteromedial to the digastric groove, and at approximately 45° to it, is a narrow groove leading to the auricular foramen. The auricular foramen conveys the auricular branch of the vagus nerve (CN X), which runs in a groove along posterolateral edge of the entotympanic, from the posterior lacerate foramen (where the vagus nerve exits) to the facial nerve (CN VII) laterally.

The robust mastoid aligns and abuts the posterolateral margin of the tympanic bulla. As in other Monachinae (except Ommatophoca ), the mastoid is not visible in dorsal view (char. 25 [0]). This condition contrasts with that observed in Phocinae and Ommatophoca , in which the mastoid is visible in dorsal view of the skull due to the medial inflexion of the lambdoid crest.

The paroccipital process on the exoccipital forms a slender hook-like process (char. 32[1]), comparable to other Monachinae . However, it is noteworthy that it is considerably more gracile in Icaphoca than in any other Monachinae .

A large hypoglossal foramen is observed distal to the posterior lacerate foramen. Although this region of the skull is strongly fractured, it seems clear that it was closer to the posterior edge of the posterior lacerate foramen that to the occipital condyle.

The occipital condyles are slender and strongly convex. Lateroventral to the occipital condyle, a distinctly concave ventral condyloid fossa separates the condyle from the paroccipital process. The fossa in Icaphoca is deeper and wider than in Magophoca. Dorsally, the occipital condyles are strongly diverging (char. 35[1]). The articular surfaces of the occipital condyles are ventrally fused, and the ventral margin of the foramen magnum is roughly parabolic.Among other Monachinae , a similar condition is observed in Hadrokirus , Homiphoca , Hydrurga, Magophoca, Monachini , and Piscophoca . In lateral view, the occipital condyles strongly protrude distally more so than in Magophoca probably because of the larger condyloid fossa. This condition in Icaphoca is also probably enhanced because of the more anterior position of the nuchal crest (in lateral view) as compared to that of Magophoca.

The nuchal crest of Icaphoca protrudes distal to the paroccipital process.Both left and right halves form a parabolic crest in dorsal view, with both halves at slightly less than 90° at one another. The crest is moderately raised. The parabolic shape of the nuchal crest resembles the condition in Magophoca, and Piscophoca , but differs from the V-shaped nuchal crest of Acrophoca , Eomonachus , Homiphoca and Hadrokirus . Among extant monachines a parabolic nuchal crest is present in Hydrurga , Neomonachus schauinslandi , and Ommatophoca , whereas a V-shaped crest is observed in Leptonychotes , Lobodon , Monachus , and Neomonachus tropicalis (with some variation in the former two genera).

The sagittal crest is only lowly-raised where the interorbital bridge transitions into the braincase, but dissipates onto the braincase. This condition resembles that of Magophoca but differs from that in Acrophoca , Hadrokirus and Piscophoca , in which the sagittal crest extends up to the nuchal crest posteriorly. A sagittal crest is absent in Homiphoca . Among extant monachines, a well-developed sagittal crest contacting the nuchal crest posteriorly is present in Hydrurga and Monachus ; a weak sagittal crest is observed in Neomonachus and it is absent in Leptonychotes , Lobodon , and Ommatophoca .

MANDIBLE ( Fig. 7 ; Table 3 )

Both left and right mandibles are well preserved for the holotype specimen,MNHN.F.PPI270. The anterolateral portion of the chin is damaged in both mandibles, as well as the vertical ramus of the left mandible.

The mandible is gracile and mediolaterally thin, comparable to the condition in most other Lobodontini , with the notable exceptions of the extinct Acrophoca and Hadrokirus , in which the mandible is even more gracile and slender, or very robust, respectively. The right mandible of the holotype of Icaphoca choristodon n. gen., n. sp. is almost complete and misses probably no more than a few mm. As preserved it is 191.2 mm long being distinctly longer than that of the paratype of Magophoca brevirostris (MNHN.F.PPI269) which is 168.3 mm long, when both specimens have been estimated to be of similar body length. This comparison confirms one of the major differences between both taxa which is the relative length of the snout, much longer in Icaphoca than in Magophoca.

The length of the mandibular tooth row on the right mandible is 81.8 mm, which represents 42.7% of the dentary’s total length. Similar proportions are observed in Magophoca in which the length of the tooth row is 71.3 mm which represent 42.3% of the dentary’s overall length. This condition is comparable with most other Monachinae , except for Hydrurga and Lobodon , in which the tooth row is either approximately half the total length of the mandible, or more than half the total length of the mandible, respectively.

In dorsal view the dentary of Icaphoca is almost straight (very smooth medial bending), whereas the bending is distinctly more pronounced in Magophoca, possibly because of the shorter rostrum of this genus.

In lateral view, the horizontal ramus (corpus mandibularis) is roughly equally high throughout, with a smoothly convex chin running from the anterior extremity to p2. The horizontal ramus is slightly upturned dorsally, anteriorly, resulting in the incisor(s), the canine, p1, and p2 being located higher than p3-m 1 in lateral view. In other Monachinae , the horizontal ramus of the mandible is equally high throughout, except for Hydrurga in which the horizontal ramus of the mandible is proportionally significantly higher anteriorly. Transversely, the horizontal ramus of the mandible of Icaphoca becomes gradually more slender, posteriorly.

The symphysis has an ovale-shaped outline. It is short and small, terminating just ventral to the anterior edge of p2. In most other Monachinae , the symphysis terminates at the level of p2 (or just posterior to it) except in Lobodon , Mirounga , and Ommatophoca , in which it extends further posteriorly.

Two mental foramina have been identified on the lateral margin of the horizontal ramus: one sub-circular mental foramen ventral to the anterior root of p2, and one deep and horizontally elongate foramen ventral to the posterior root of p3.

The retromolar space is long, much longer than in Magophoca. Just distal to m1, the horizontal ramus initially widens very gently, in lateral view; widening gradually more strongly toward the vertical ramus (ramus mandibularis). Among other Monachinae , the retromolar space is short in Hadrokirus , Hydrurga , Lobodon, Magophoca , and tribe Monachini . The retromolar space is moderately long in the extinct monachines Homiphoca , Piscophoca , and Pliophoca , and long in all other Monachinae : Acrophoca , Leptonychotes , Mirounga , and Ommatophoca .

Overall, the ventral margin of the dentary is straight, only deflecting slightly ventrally at the posterior end of the mandible, vertically at the level of the coronoid process, before rising to the angular process.

The vertical ramus is approximately three times as high as the horizontal ramus, at the level of the maximum elevation of the coronoid process.

Compared with other Monachinae , a similarly proportionally high vertical ramus is also present in extinct Monachinae (e.g., Acrophoca , Hadrokirus , Homiphoca, Magophoca , and Piscophoca ) and Monachini . In other Monachinae , such as some Lobodontini (e.g., Hydrurga and Lobodon ) and Mirounga , the vertical ramus is proportionally not as high since these taxa have a very robust and high horizontal ramus.

The coronoid crest is rather straight, terminating posterodorsally in the coronoid process. Transversely, the coronoid process is thin. The coronoid process is a blunt, rounded apex, a condition that varies among other monachines, ranging from a concave “hook” in Hydrurga , Leptonychotes , and Neomonachus ; a straight posterior margin in Lobodon, Magophoca , Mirounga , and Monachus , to an anterodorsal-posteroventrally slanted ridge in Ommatophoca . Among fossil monachines, it is wide and stout in Acrophoca , Hadrokirus , and Magophoca; it is relatively narrow and slender in Homiphoca and very thin, narrow, and strongly oriented posteriorly in Piscophoca . The coronoid process of Icaphoca more resembles that of Acrophoca than any other extinct monachines.

The lateral aspect of the of the vertical ramus of the mandible receives the masseter muscle ventrally and part of the temporalis muscle dorsally. The insertion of the masseter is weakly outlined but clearly separated into two elongate insertion areas by the condyloid crest. Ventral to the condyloid crest is an elongated fossa for the masseter superficialis (on the ventral edge of the fossa) and for the larger masseter intermedius. Dorsal to the condyloid crest and ventral to the coronoid process is a roughly oval-shaped fossa for the masseter profundus. Dorsal to this latter fossa is the insertion area for the temporalis lateralis muscle, on the lateral aspect of the coronoid process.

On the medial surface of the mandible, a shallow, roughly reniform fossa along the coronoid notch was receiving the insertion of the temporalis medialis muscle.

The mandibular foramen on the medial surface is located horizontally at the level of the tooth row, but vertically at the level of the posterior margin of the coronoid process.

The condyloid process (condyle) is tilted anterolaterallyposteromedially, as in Hadrokirus and Magophoca. The condyloid process roughly forms a horizontal, cylindrical bar. The articular facet on the condyloid process is contorted: at the lateral tip, it projects more anterior than at the medial tip, but at the medial tip, it projects more ventral than at the lateral tip. It remains uncertain whether the latter is a diagnostic characteristic for Icaphoca or may be attributed to intraspecific variation and only present in the holotype specimen. The condyloid process is seated on a slender and long root.

The angular process is strongly reduced, forming a blunt projection, just posterior to the body of the mandible (char. 36[1]). This condition follows the general condition among Monachinae , with few exceptions: there is no –or only a weakly– discernible angular process in Hydrurga , Leptonychotes , and Magophoca. It is large (for a phocid) and strongly projecting posteriorly in Homiphoca . The angular process is small in most other Monachinae .

DENTITION

Upper teeth and alveoli ( Figs 3 , 4 , 8 ; Tables 2 , 4)

In the holotype specimen of Icaphoca choristodon n. gen., n. sp., most of the upper teeth are preserved except the left canine and P4. However, the state of preservation of the incisors and canines is poor, as well as the left M1, which is missing a part of the labial side of its main cusp. The incisors and right canine are worn (especially the canine) and the left I2 is broken. The left canine is broken at the base of the crown and only the roots are preserved for the left P4.

The upper dental formula for Icaphoca is 3I 1C 4P 1M. Except for the number of incisors, the upper dental formula is consistent among all Phocidae . The number of incisors is often quoted as a characteristic separating both subfamilies, Monachinae and Phocinae: monachines have two upper incisors, and phocines have three upper incisors. However, the extant phocine Cystophora has two instead of three upper incisors and the extinct monachine Magophoca has three instead of two upper incisors ( Dewaele & Muizon 2024).

The upper incisors are notably compressed transversely (especially I1 and I2), with peg-like crowns, like the condition in Phocinae, and in contrast to other Monachinae , in which this transverse compression is less pronounced (char. 40[0]). The incisors increase in size from I1 to I3. The third incisor is incisiform, yet, distinctly larger than the other inner incisors, being 5.3 mm in labio-lingual length, whereas this length is 3.7 mm on I2 and 2.8 mm on I1 (char. 41[0]). A flat facet, oblique on the lingual side of the right second incisor represents an occlusal tooth wear facet.

The canines are too incompletely preserved to allow description, other than that they appear to have a strong anteroventral orientation.

The postcanine teeth are little specialized, compared with the highly specialized postcanine teeth in extant Monachinae (except Monachus and Neomonachus ). Among other extinct Monachinae , the degree of specialization varies, The postcanine teeth of Icaphoca trend to homodonty, as in other Monachinae . Similar to most other Phocidae , the postcanine teeth of Icaphoca are double-rooted, with the exception of P1, which is single-rooted (char. 47[1], 50[0]). Exceptions are Hydrurga and Lobodon which, although they do not have a double-rooted P1, feature a bilobed single root for P1. Furthermore, Mirounga , and the phocine species Cystophora , and Halichoerus , have single-rooted P1-M1.

All postcanine teeth are widely separated by diastemas.These range from three millimeters to six millimeters between the premolars ( Table 2 ). In contrast, the diastema separating M1 from P4 is approaches one centimeter, or approximately one postcanine tooth length in length. This condition is notable, because among other Monachinae , only Acrophoca has large diastemata between all the postcanine teeth. The condition of Icaphoca strongly differs from that of Magophoca, in which postcanines are not separated by diastemata. Sarcodectes also presents an enlarged diastema between P4 and M1, but no diastema is observed between the premolars. All postcanine teeth of Icaphoca are oriented parallel to the tooth row axis and no tooth imbrication or tooth crowding has been observed (char. 46[0]).

The notable spacing of the postcanine teeth of Icaphoca is a unique condition among monachines that foreshadows the spectacular lengthening of the rostrum of Acrophoca (see below, phylogenetic analysis section).

In occlusal view, the first upper premolar has a semi-circular lingual margin and a weakly convex labial margin. The well-developed lingual margin sports a prominent, broad, lingual cingulum. There is no labial cingulum on P1. The apex of the main cusp bears a flat wear facet, oblique on the lingual side of the cusp. Tooth wear on the enamel is also present down from this wear facet, towards the lingual cingulum. Anteriorly and posteriorly, the main cusp is flanked by accessory cusps. Whereas the anterior accessory cusp is miniscule and barely visible, the posterior accessory cusp has a little prominence.

The double-rooted premolars P2-4 are anteroposterioly elongate. The crowns are approximately twice as long as they are high. The lingual cingulum is well-developed overall, forming a prominent posterolingual shelf. The labial cingulum is narrow, but disappears just below the main cusp. The crown is dominated by a large and anteroposteriorly elongate main cusp, anteriorly and posteriorly bound by sharp crests. Anterior and posterior to the main cusp, there are the small anterior and posterior accessory cusps. Tooth wear is moderate, but wear facets are present on the apices of the main and accessory cusps some of them being significantly shortened. Moreover, on the right P4 the wear is prominent, since it almost totally obliterates the posterior accessory cusp and extends onto the posterolabial margin of the tooth. It remains questionable whether tooth wear as profound as the tooth wear observed in the right P4 of MNHN.F.PPI 270 is common for the species of Icaphoca choristodon n. gen., n. sp. or intraspecifically variable.

M1 is noticeably smaller than P4. Among other Monachinae , M1 is generally smaller than P4, but the degree of the size discrepancy varies: for example, M1 is comparable or only marginally smaller in the extant Hydrurga and in the extinct Acrophoca , Hadrokirus , Homiphoca , and Magophoca; it is considerably smaller in the extant Leptonychotes and in the extinct Piscophoca .

The labial and lingual cingula are strongly reduced and the main cusp is comparatively large, in relation to the entire tooth crown, when compared to premolars. Towards its apex, the main cusp tends to curve posteriorly and has a small and flat wear facet. The accessory cusps of M1 are vestigial.

Lower teeth and alveoli ( Figs 7 , 9 ; Tables 2 , 5 )

For the holotype mandibles, only the postcanine teeth are preserved adequately complete, allowing a detailed description. One right incisor and the right canine are preserved as well, but too poorly to allow a detailed description and comparison. A well-preserved and isolated lower incisor has been associated with the holotype too.

Each mandible sports two incisors, one canine, four premolars, and one molar. Although none of the mandible preserves all its incisors it is likely that the holotype specimen bears two incisors in each lower quadrant, since two alveoli are observed on the left mandible. They are anteroposteriorly disposed and the smaller anterior alveolus is anteroventral to the larger one. The larger posterior alveolus is medial to the anterior edge of the canine. Another cavity, below the incisors’ alveoli, most likely represents the anterior mental foramen. Phocidae generally have two lower incisors per quadrant, except for the extant monachine genus Mirounga and the extant phocine Cystophora . A dental formula with 3I/2i is shared with all Phocinae (except Cystophora ) and the sympatric monachine Magophoca brevirostris.

For the (isolated) incisor, it appears that the root is slightly flattened transversely. The crown forms a stubby peg in anterior view. The tooth crown is slightly recurved lingually, and it has a small lingual cingulum. Assuming that there is not enough space in the right mandible to accommodate the isolated incisor with the other incisor already in place, this isolated incisor can be assumed to be a left incisor. This incisor has a large wear facet mesiolabially, again, considering this incisor to be a left incisor. This tooth wear facet is long and deep, cutting through the entire thickness of the enamel.

The lower canine appears to be oriented almost straight vertical, compared to the horizontal ramus.

As for other Monachinae , p1 is single-rooted and p2-m1 are all double-rooted (char. 47[1]). Apart from the smaller p1, the lower postcanine teeth are all of comparable size, with low crowns, approximately half as high as they are long. The lower postcanine dentition of Icaphoca is marked by wide diastemas between p2, p3, p4, and m1 ( Table 5 ), otherwise only known in Acrophoca . The diastema between p1 and p2 is smaller. The lower postcanine teeth of Icaphoca are implanted parallel to the long axis of the tooth row.

The first premolar, p1 is roughly oval in occlusal view, with a slightly straighter labial surface than the lingual surface. The main cusp dominates the crown. However, it is not high, less than half as high as the tooth is long ( Table 5 ). It does not point straight vertical but is instead slanted anteriorly. Posteriorly, the main cusp is flanked by a small accessory cusp. Both are aligned labial to the long axis of p1, thus creating a prominent lingual cingulum forming a relatively wide plateau. The labial cingulum is strongly reduced.

The other lower premolars, p2, p3, and p4 are morphologically comparable to one another. The crown forms an elongate oval in occlusal view, and is approximately twice as long as it is high. A prominent main cusp is located centrally on the crown and has sharp, angular anterior and posterior ridges descending to an anterior and posterior accessory cusp. These accessory cusps are roughly similar in size, but much smaller than the main cusp. Whereas the labial cingulum is strongly reduced, the lingual cingulum is well-developed. The distolingual portion of the lingual cingulum is most prominent in p2, and gradually smaller in p3 and p4. The tooth crowns of p2, p3 and p4 are affected by slight tooth wear. The apex of the main cusp is worn in all p3 and p4. In p3 and, especially, p4, the anterior half of the tooth crown is significantly worn down.

The lower first molar, m1, differs notably from the premolars. The main cusp is slightly inflated, thus reducing the labial and lingual cingula. This cusp is slightly turned posteriorly, giving the anterior portion of the main cusp a markedly convex appearance in lateral view. The posterior accessory cusp is similar in size to the posterior accessory cusps in the premolars. The anterior portion of the tooth is worn to the extent that it is impossible to determine whether there was an anterior accessory cusp present or not before being worn down.

AXIAL SKELETON

The known axial skeleton of Icaphoca choristodon n. gen., n. sp. is limited to the cervical vertebrae, which were recovered articulated to the skull of the holotype specimen, MNHN.F.PPI 270, comprising the atlas, axis, and Ce3-5.

Atlas ( Fig. 10 A-C; Table 6)

The atlas is incompletely preserved, missing its left wing; however, the right wing is almost complete and allows a complete description of the atlas.

The atlas of Icaphoca is not particularly distinctive from the average monachine in its overall dimensions. Exceptions among Monachinae are Acrophoca and Hadrokirus , with the former having an anteroposteriorly elongate atlas with posteriorly projecting wings, and the latter having a large and very stout atlas.

The transverse process of the atlas is roughly quadrate and projects only moderately, laterally. The lateral margin of the transverse process flares posteriorly, with a slightly convex anteromedially-posterolaterally oriented lateral margin and a rather transverse posterior margin, which bears a small, pointed process on its lateral half. On the posterior edge of the transverse process is a deep triangular fossa, which extends from the posterior articular facet for the axis to the small pointed process. The fossa forms an isosceles triangle whose base is along the articular facet.

Although the anterolateral angle of the transverse process is damaged it overhangs the alar notch. Because of this slight breakage of the transverse process, it is not possible to evaluate the depth of the alar notch.

This morphology of the margin of the transverse process in the atlas of Icaphoca differs from the posteriorly projecting processes of Acrophoca and from the rather triangular ones of Piscophoca . In contrast, it resembles the rather convex and rounded to quadrate processes of other Monachinae .

The degree of the lateral eversion of the transverse process in Icaphoca is moderate as observed in Magophoca and conforms with the genera condition in Monachinae . However, in a few taxa, i.e. Hadrokirus , Monachus , and Piscophoca , the transverse process projects more strongly laterally.

In lateral view, the transverse process of the atlas of Icaphoca is straight and oriented obliquely (char. 53[0]). Compared to other Phocidae , this corresponds with the condition in Phocinae and the monachine taxa Acrophoca , Hadrokirus , Leptonychotes, Magophoca , and Neomonachus ; whereas this process is oriented subvertically in other Monachinae .

The transverse foramen at the base of the transverse process is small, comparable in dimensions to Acrophoca and Hadrokirus , and slightly smaller than in Magophoca and Piscophoca ; it is much smaller than in extant Monachinae . On the anteroventral surface of the transverse process, the transverse foramen opens in a large and wide atlantal fossa, which is larger than in Acrophoca , Hadrokirus , and Magophoca, resembling in this respect to the condition of Piscophoca . It is transversely elongate rather than roughly circular as in Acrophoca , Hadrokirus ,and Magophoca. The posterior opening of the transverse foramen is visible in dorsal view, as in other Monachinae , with the notable exception of Mirounga , and Lobodon (char. 52[1]).

The anterior articular fovea for articulation with the cranial condyles is reniform and strongly concave. The posterior articular fovea for articulation with the axis is weakly concave and has a roughly rectangular outline, and both left and right posterior articular foveae are at a slightly obtuse angle to each other.

The neural canal of the atlas of Icaphoca is vertically oval-shaped, being conspicuously higher than wide. The internal processes are smaller than in Magophoca but distinctly present.

The dorsal tubercle is low and is not bordered laterally by deep fossae for the m. rectus capitis dorsalis minor as observed in Hadrokirus , and Magophoca. In contrast, this condition resembles that of Acrophoca , and Piscophoca .

The ventral bar is thin and slender compares favorably with that of Magophoca and Piscophoca , differing from the stronger morphology observed in Acrophoca , and Hadrokirus .

Axis ( Fig. 10 D-F; Table 7 )

The axis of the holotype of Icaphoca is missing the left transverse process and the apex of the right one. Nonetheless, the right transverse process is preserved, allowing a nearly full description of the axis.

The dens forms a thick and stubby peg, round in cross-section, with a horizontally flat dorsal margin and an anteriorly ascending ventral margin. The robustness of the dens is comparable to that of Magophoca, and thicker than the dens of the axis of Hadrokirus , which already has a dens more robust than other Monachinae ( Amson & Muizon 2014).

The cranial articular surfaces are subcircular in outline and at an obtuse angle to each other of 107°. This condition resembles that observed on the holotype of Piscophoca pacifica MNHN.F.SAS 564 (107°). It is conspicuously wider than in the paratype of Magophoca brevirostris MNHN.F.PPI269 (87°), the holotype of Acrophoca longirostris MNHN.F.SAS 563 (93°), and Homiphoca (100°, mean of three specimens; MNHN.F.AFS 38, 40, and 41), but narrower than in the holotype of Hadrokirus martini MNHN.F.SAS1627 (115°).

The dorsal margin of the spinous process is slightly abraded but its anterior and posterior extremities are preserved. The dorsal margin of the spinous process is roughly straight resembling the condition in Acrophoca , but differing from Magophoca and Piscophoca , in which it is convex. The posterior angle of the spinous process forms a narrow and elongated process, which projects dorsally and posteriorly to a greater extent than in Magophoca, and Piscophoca , a condition, which clearly resembles that of Acrophoca . The posterior projection extends well posterior to the postzyghapophyses, whereas in Magophoca brevirostris, it is anterior in the paratype (MNHN.F.PPI269) and it is only slightly posterior in the holotype (MNHN.F.PPI276). In the holotype of Piscophoca pacifica it is also only slightly posterior to the postzygapophyses. The anterior angle of the spinous process is sharp and extends anteriorly almost above the apex of the dens as is observed in Acrophoca , Hadrokirus, Magophoca , and Piscophoca . The spinous process remains equally thin along its entire length. The spinous process of the axis of Icaphoca is morphologically comparable to that of the extinct Acrophoca , but notably shorter anteroposteriorly, It is proportionally longer than that in the other extinct monachines Hadrokirus, Magophoca , and Piscophoca , and slightly larger than in extant Monachinae , in which the posterior projection of the spinous process is never as pronounced as in Icaphoca .

The centrum is comparable in length to that of Magophoca; it is apparently slightly longer, but comparative measurements are difficulted by the absence of the posterior epiphysis of the axis of the paratype of Magophoca brevirostris (MNHN.F.PPI269). The centrum is much shorter than in Acrophoca , and conspicuously longer than in Homiphoca , and Piscophoca . It has a semi-cylindrical dorsal surface; whereas the ventral surface is strongly excavated into two deep fossae on both sides of the ventral keel. This is similar to the condition in Acrophoca , and Magophoca, but unlike Hadrokirus , and Piscophoca , in which the fossae are shallower.

The neural canal is higher than it is wide, subtriangular in outline in anterior view. The dorsal surface of the centrum bears an elevated square (in section) carina, which divides it into two deep fossae. In posterior view the neural canal is U-shaped.

The neural arch, overarching the neural canal, sports postzygapophyses on both lateral sides of the spinous process. The postzygapophysis is robust, bearing a flat and rounded postzygapophyseal articular facet that faces ventrolaterally. The postzygapophyses reach well posterior to the posterior articular surface of the centrum, yet not as far posterior as the spinous process.

The transverse process is simple, long, and slender, projecting posterolateroventrally, posteriorly almost reaching the level of posterior articular surface of the centrum. It bears a large, triangular transverse foramen.

Other cervical vertebrae ( Fig. 11 ; Table 8 )

In addition to the atlas and axis, three other cervical vertebrae have been recovered articulated to the skull of the holotype of Icaphoca choristodon n. gen., n. sp.: Ce3, Ce4, and Ce5. Just as the atlas and axis, these cervical vertebrae are overall well preserved.

The centrum of Ce3 is slightly longer (40.3 mm) than that of Ce3 of the paratype of Magophoca brevirostris (38.2 mm). as for the axis, the centrum is proportionally much shorter than in Acrophoca . The centrum is smaller, compared to the neural arch and the transverse processes than in the paratype of Magophoca brevirostris. On the three cervical vertebrae described, the anterior articular surface of the centrum is roughly rectangular, wider horizontally than it is high vertically and the posterior articular facet is slightly oval-shaped to sub-circular. On the ventral aspect of the centrum, a narrow and sharp ventral keel is present. It is low and thin anteriorly but gradually becomes prominent and blunt at the posterior margin, where it forms a thick posterior tubercle. Overall, the vertebral body appears roughly parallelepiped, with the posterior articular surface offset ventrally compared to the anterior articular surface, in lateral view. As a consequence, in lateral view with the length of the centrum horizontal, the articular surfaces appear strongly oblique the anterior one being anteroventrally and the posterior one posterodorsally oriented. The length of the centra increases posteriorly from Ce3 to Ce5 (39.2 mm; 41.2 mm; 44.7 mm).

In anterior view, the neural canal is roughly U-shaped in the anterior cervical vertebra (Ce3), but becomes more triangular in the posterior cervical vertebrae (Ce4-Ce5): the ventral margin of the neural canal remains slightly convex, but the dorsal margin becomes rather angular.

The prezygapophyses are simple. The prezygapophyseal articular facet is elongated, oval-shaped on Ce3 and becomes more semi-circular and flat on Ce3-4 They face dorsomedially approximately at a 90° angle in Ce3, and slightly more open in the more posterior cervical vertebrae. Also, in Ce3, the medioventral portion of the prezygapophyseal articular facet is slightly curving around the long axis of the vertebra being distinctly concave. On the lateroventral surface, the prezygapophysis bears a faint mammillary tuberosity. The postzygapophysis is dorsoventrally thicker than the prezygapophysis. On the lateroventral surface, the postzygapophyseal articular facet is flat and elongate semi-circular. Whereas the dorsomedial surface of the postzygapophysis is marked by a prominent but blunt mammillary process in Ce3, the dorsomedial surfaces on the postzygapophyses of the more posterior cervical vertebrae bear small anteroposteriorly oriented ridges. The spinous process is insignificant in Ce3, but is more prominent in more posterior cervical vertebrae and increases in height posteriorly. In the more posterior cervical vertebrae (Ce4-5), the spinous process is slanted anteriorly: the blunt apex of the spinous process is located anterior to the level of the anterior opening of the neural canal. The posterior margin of the spinous process is more strongly slanted anteriorly than the anterior margin of the spinous process.

The transverse process is completely preserved in only Ce3 and Ce4.All the preserved cervical vertebrae, with the exception of the atlas and axis, have large, oval, and anteroposteriorly ori - ented transverse foramina.The transverse processes are oriented lateroventrally with the ventral component being much more pronounced than the lateral one. In lateral view the transverse processes are oriented almost fully ventrally differing from the condition in Acrophoca , and Magophoca, in which they are strongly slanted posteriorly. In anterior view, the transverse processes slightly diverge ventrally. In anterolateral view, they, widen distally and bifurcate, being divided into a small and more slender superior branch and a larger and wider inferior branch. The medial aspect of the transverse processes is almost oriented ventrally. The bifurcation is less prominent in Ce3, than it is in Ce4-5. In Ce3, the ventrally-oriented inferior and lateroposteriorly-oriented superior branches end as blunt processes, with the former being larger. In Ce3 the inferior branch is a thick process, whereas in the more posterior cervical vertebra, the inferior branch is transversely flattened and anteroposteriorly elongate.In Ce3-5 the superior branch forms a tubular process. The inferior branch of Ce3 resemble that of Magophoca but differs from the blade-like branch observed in Acrophoca . Although well-developed, the blade-like inferior branch of Ce4-5 are much smaller than in Acrophoca being approximately 50% shorter than it is in this genus.

PHYLOGENETIC ANALYSIS

The phylogenetic analysis using PAUP*4.0a169 yields fifty most parsimonious trees, with a tree length of 257 after 415 965 tried rearrangements. The strict consensus tree ( Fig. 12 ) highlights that the topology of Monachinae is unresolved. Consequently, we performed a second analysis, downweighing homoplasy via implied weighing (Goloboff criterion, with k = 3). The heuristic analysis resulted eight most parsimonious trees with a best score of 258 and Goloboff fit –71.33016 after 44527 tried rearrangements ( Fig. 13 ). The consistency index is 0.4380, and the retention index is 0.6902.

The topology of the outgroup taxa, i.e. the stem pinnipedimorph Pteronarctos goedertae , the otariids Arctocephalus pusillus , and Otaria byronia , and the odobenid Odobenus rosmarus is as expected, with the former branching off first, followed by the otariids, followed by Odobenus . Phocidae form a monophyletic clade, with 23 synapomoprhies, including 17 unambiguous ones. The unambiguous synapomorphies of Phocidae are: the strongly concave anterior border of the nasal cavity in anterior view (char. 4[0> 1]); the position of the posterior end of the nasals posterior to the level of the maxilla-frontal suture (char. 5[0> 1]; the absence of the alisphenoid canal (char. 10[0> 1]); a mortised jugal-squamosal contact (char. 15[0> 1]); the lateral extremity of the tympanic bulla reaching lateral to mid-width of the glenoid fossa (char. 22[0> 1]), the pit for the stylohyal oriented ventrally and anteriorly (char. 24[0> 1]); a heavily pachyosteosclerotic mastoid (char.26[0> 1]); a paroccipital that is well separated from the mastoid (char. 28[0> 1]); the posterior opening of the carotid canal and posterior lacerate foramen are separated (char. 30[1> 0]); the pterygoids are laterally tilted and flaring posteriorly (char. 34[0> 1]); the occipital condyles diverging dorsally (char. 35[0> 1]); an intermediately large, posteriorly and medially protruding angular process on the mandible (char. 36[0> 1]); all premolars are double-rooted (char. 47[2> 1]); a reduced acromion process on the scapula (char. 58[0> 1]); an unreduced intermediate phalanx on digit V (char. 74[1> 0]); a moderately well developed iliopectineal eminence on the innominate (char. 80[0> 1]); and no lesser trochanter on the femur (char. 83[0> 1]). Ambiguous synapomorphies are characters 23, 62, 66, 81, 90, and 91 (further details, see Appendices 3, 4).

The topology of the four species representing Phocinae correspond well with that in previous studies (e.g., Bininda-Emonds & Russell 1996; Higdon et al. 2007; Fulton & Strobeck 2010; Rule et al. 2020a, b). In the current phylogenetic analysis, which focuses on Monachinae , Monachinae are supported as a monophyletic group by 17 synapomorphies, with no contact between the premaxilla and nasal (char. 1[0> 2]); the nasal cavity opens more dorsally than anteriorly (char. 3[0> 1]); there are two upper incisors per quadrant (char. 39 [0> 1]); an anterior orientation of the posterior border of the scapula in lateral view (char. 57[0> 1]); a reduced scapular spine (char. 59[0> 1]); a humerus with a greater tubercle below the level of the head (char. 63[2> 0]); the presence of a transverse bar in the bicipital groove of the humerus (char. 64[0> 1]); a flattened distal end of the styloid process of the ulna (char. 71[0> 1]); a femur with a long epicondylar crest (char. 85[0> 1]); an anterolaterally-oriented fossa for m. peroneus longus on the femur (char. 86[0> 1]); a strongly developed anterior tibial fossa (char. 89[0> 1]); and MtIII approximately 50% (or more) shorter than MtI (char. 93[0> 1]) as unambiguous synapomorphies. Apart from these twelve unambiguous synapomorphies, five others are ambiguous: characters 2, 14, 60, 69, and 91 (further details in Appendices 3, 4).

The resulting topology of the clades in this phylogenetic analysis clearly return the Miroungini and Monachini tribes. The Lobodontini tribe forms a polytomy at its base, with a clade of extant Lobodontini , most extinct Lobodontini , Sarcodectes , and Acrophoca + Icaphoca .

Unlike the previous study by Dewaele & Muizon (2024), Miroungini are the first to branch off, with Lobodontini and Monachini being sister groups to each other. Following previous studies, Eomonachus and Pliophoca are returned as Monachini , together with Monachus and Neomonachus ( Tavani 1941; Berta et al. 2015; Berta et al. 2018; Rule et al. 2020a, b; Dewaele & Muizon 2024).

Within the polytomy, the four extant Lobodontini , Hydruga, Leptonychotes , Lobodon , and Ommatophoca , group together as crown lobodontins. No extinct lobodontin included in this analysis falls within this crown group.

The polytomy of Lobodontini in the strict consensus tree underlines that a detailed phylogenetic analysis of extant and extinct Monachinae , with a special focus on Lobodontini is much needed. Such an analysis should include Magophoca and Icaphoca (which analyses of Rule et al. 2020a, b do not) as well as some specimens of the collection of the MUSM referrable to new taxa and remarkable material (partial skulls and a sub-complete skeleton) of Australophoca changorum , whose holotype is a severely weathered incomplete humerus, partial radius and ulna. Such an analysis is currently beyond the scope of this study (which focusses on the description of a new monachine genus and species from the Pisco Formation) and will be undertaken in a work in progress by the authors.

The tribe of Lobodontini is supported by six synapomorphies, including five unambiguous synapomorphies: a parasagittal orientation of the medial margins of the tympanic bullae (char. 21[0> 1]); the presence of a lip in the anteromedial region of the mastoid, abutting the posteromedial edge of the tympanic bulla (char. 27[0> 1]); the transverse foramen of the atlas is at least partially visible in dorsal view (char. 52 [0> 1]); a large size difference exists between the distal condyles of the femur (char. 84[0> 1]); and a well-developed articular surface for the fibula on the calcaneum (char. 92[0> 1]). A sixth character, 60, is an ambiguous synapomorphy (Appendices 3, 4). The extant, i.e. crown lobodontins are supported by eight synapomorphies. Four are unambiguous: the anterior opening of the infraorbital foramen is located anterodorsal to M1 (char. 11 [1> 0]); the posterior border of the scapula is oriented proximodistally in lateral view (char. 57 [1> 0]); the radial tuberosity is located on the posteromedial side of the radius (char. 69[0> 1]); and a manus with a strongly reduced intermediate phalanx on digit V (char. 74[0> 1]). Four characters 17, 36, 62, and 66, are ambiguous. The extinct lobodontins are united by seven synapomorphies; all of which are ambiguous (characters 14, 53, 64, 70, 73, 75, and 80) (Appendices 3, 4).

Whereas Sarcodectes from the Northern hemisphere branches off before other extinct Lobodontini from the Southern hemisphere in the analysis by Dewaele & Muizon (2024), they form a polytomy with extant Lobodontini in the present study. Of the seven extinct lobodontins of the Southern hemisphere included in the present analysis, Hadrokirus , Homiphoca , Kawas, Magophoca , and Piscophoca , form a clade that is retained in the strict consensus trees of both phylogenetic analyses with and without implied weighting. The analysis agrees with the previous analyses by Dewaele & Muizon (2024) that Kawas is a monachine and not a phocine, as stated by Cozzuol (2001) in the original description of the species.

Acrophoca and Icaphoca are sister taxa, united by five synapomorphies, of which two are unambiguous: the anterior end of the maxillary branch of the jugal reaches dorsal to dorsomedial to the infraorbital foramen (char. 13[0> 1]), and the diastema between P4 and M1 is enlarged (char. 48[1> 0]. Other synapomorphies are ambiguous. These are characters 40, 56, and 87 in the character list (Appendix 3).

Low bootstrap values (<50 %) for Monachinae , support the above statement that a detailed phylogenetic reassessment of extant and extinct Monachinae is much needed to create a more robust phylogenetic framework of the subfamily.

Figures

FIG. 2. — Cranium of Icaphoca choristodon n. gen., n. sp. (holotype, MNHN.F.PPI270). A, Cranium in dorsal view; B, corresponding labelled line drawing. Areas in gray in the line drawings are incompletely preserved,either severely abraded or fractured,or obscured from view by sediment matrix.Premaxillae in green, nasals in purple, maxillae in yellow, jugal in blue, basioccipital in brown. Other bones not colored due to the fusion or incomplete preservation of the sutures separating the bones.Dashed lines represent the presume+d sutures between the nasals, maxillae and frontals. Abbreviations of the teeth follow the convention used in the main text. Scale bar: 10 cm. Photo by L. Cazes & P. Loubry (CR2P); drawing by L. Dewaele.

FIG. 3. — Cranium ofIcaphoca choristodon n.gen.,n.sp.(holotype,MNHN.F.PPI270).A, cranium in left lateral view;B, corresponding labelled line drawing;C, same cranium in right lateral view;D, corresponding labelled line drawing.Areas in gray in the line drawings are incompletely preserved,either severely abraded or fractured,or obscured from view by sediment matrix. Premaxillae in green,nasal in purple, maxilla in yellow,jugal in blue, tympanic bulla in red, basioccipital in brown. Other bones not colored due to the fusion or incomplete preservation of the sutures;but the glenoid area and mastoid,together with the tympanic bulla, comprises the temporal. Abbreviations of the teeth follow the convention used in the main text. Scale bar:10 cm. Photos by L. Cazes & P. Loubry (CR2P);drawings by L. Dewaele.

FIG. 4. — Cranium of Icaphoca choristodon n. gen., n. sp. (holotype, MNHN.F.PPI270). A, cranium in ventral view; B, corresponding labelled line drawing. Areas in grey in the line drawings are incompletely preserved, either severely abraded or fractured, or obscured from view by sediment matrix. Premaxillae in green, maxillae in yellow, jugals in blue, palatine in pink, pterygoids in orange, tympanic bullae in red, petrosals in teal, basioccipital in brown. Other bones are not colored due to the fusion or incomplete preservation of the sutures; but the glenoid area and mastoid, together with the tympanic bulla, comprises the temporal. Dashed lines represent the presumed sutures between the premaxillae, nasal, maxillae and frontals. Abbreviations of the teeth follow the convention used in the main text. Scale bar: 10 cm. Photo by L. Cazes & P. Loubry (CR2P);drawing by L.Dewaele.

FIG. 5. — Basicranium of Icaphoca choristodon n. gen., n. sp. (holotype, MNHN.F.PPI270a); right auditory region in ventral view. Scale bar: 3 cm. Photo by L. Cazes & P. Loubry (CR2P).

FIG. 6. — Basicranium of Icaphoca choristodon n. gen., n. sp. (holotype, MNHN.F.PPI270a); Left auditory region in ventral view. Scale bar: 3 cm. Photo by L. Cazes & P. Loubry (CR2P).

FIG. 1. –– Geography and geology of the Cerro La Bruja Locality. A, Geographic location of the study area of Cerro La Bruja;B, C, photos of the locality (red oval indicates the place where the holotype of Icaphoca choristodon n. gen.,n. sp.was extracted);D, geological map indicating the CLB level in the Cerro La Bruja locality; white star indicates a radioisotopic age obtained from a volcanic ash layer (Di Celma et al.2017). Credits:A,D, modified from Di Celma et al.(2017); B,C,Google Earth.

FIG. 7. — Right mandible of Icaphoca choristodon n. gen.,n. sp. (holotype,MNHN.F.PPI270). A, mandible in lingual view;B, the same mandible in labial view;C, the same mandible in occlusal view; D-F, corresponding labelled line drawings. Areas in gray are abraded or obscured from view by sediment matrix. Abbreviations of the teeth follow the convention used in the main text. Scale bar: 10 cm. Photos by L. Cazes & P. Loubry (CR2P); drawings by L. Dewaele.

FIG. 8. —Upper dentition of Icaphoca choristodon n. gen., n. sp. (holotype,MNHN.F.PPI270). A, stereoscopic view of the upper right cheek teeth in occlusal view; B, upper right cheek teeth in lateral view in occlusal view. Scale bar: 3 cm. Photos by L. Cazes & P. Loubry (CR2P).

FIG. 9. —Lower dentition of Icaphoca choristodon n. gen., n. sp. (holotype, MNHN.F.PPI270). A, Stereoscopic view of the left lower cheek teeth in occlusal view; B, right lower cheek teeth in lateral view; C, right lower cheek teeth in medial view. Scale bar: 3 cm. Photos by L. Cazes & P. Loubry (CR2P).

FIG. 10. — Atlas and axis of Icaphoca choristodon n. gen., n. sp. (A-C, holotype atlas, MNHN.F.PPI270; D-F, holotype axis, MNHN.F.PPI 270): A, atlas in anterior view; B, atlas in dorsal view; C, atlas in left lateral view; D, axis in anterior view; E, axis in right lateral view; F, axis in dorsal view. Abbreviation: m. rect. cap. dors. min., musculus rectus capitis dorsalis minor. Scale bar: 10 cm. Photos by L. Cazes & P. Loubry (CR2P).

FIG. 11. — Cervical vertebrae of Icaphoca choristodon n. gen., n. sp. (holotype, MNHN.F.PPI270). A, Ce3 in anterior view; B, Ce3 in right lateral view; C, Ce3 in dorsal view; D, Ce4 in anterior view; E, Ce4 in right lateral view; F, Ce4 in dorsal view; G, Ce5 in anterior view; H, Ce5 in right lateral view; I, Ce5 in dorsal view. Scale bar: 5 cm. Photos by L. Cazes & P. Loubry (CR2P).

FIG. 12. — Phylogenetic relationships of Icaphoca choristodon n. gen., n. sp. among Phocidae. Strict consensus tree of the 50 most parsimonious trees of the phylogenetic analysis. Note the extremely poor resolution of the topology of Phocidae. Extinct species are preceded by a dagger.

FIG. 13. — Phylogenetic relationships of Icaphoca choristodon n. gen.,n. sp. among Phocidae,with Goloboff k-value set at three to downweigh homoplasy.Strict consensus tree of the eight most parsimonious trees of the phylogenetic analysis. I. choristodon groups with Acrophoca longirostris. Bootstrap values equal to or higher than fifty are shown. Extinct species are preceded by a dagger.Black bars represent the known age ranges of the different extinct taxa, clearly showing that I. choristodon is the oldest of the Peruvian seals. Abbreviations: Ma, million years; Plioc., Pliocene; Quat., Quaternary.