Asoriculus gibberodon (Petényi, 1864)

Bert, Hugo, Costeur, Loic, Lazarev, Sergei, Schulz, Georg, Vasilyan, Davit & Maridet, Olivier, 2025, An almost complete cranium of ASoriCUlUS gibberodon (Petényi, 1864) (Mammalia, Soricidae) from the early Pliocene of the Jradzor site, Armenia, Swiss Journal of Palaeontology (19) 144 (1), pp. 1-19 : 6-9

Asoriculus gibberodon (Petényi, 1864) .

Figs 3-4

Emended diagnosis (upper teeth only) of Episoriculus gibberodon from Reumer (1984): “A rather small member of the Soriculini with only weakly pigmented teeth; four upper antemolars, of which the A4 may be variable in its development; upper molars variable in their morphology, with a moderate posterior emargination.”

Distribution: First appearance in Hungary during the MN12 (Tardosbánya: Mészáros, 1998). Presence in Spain, France, Austria, Germany, Poland, Romania, Slovakia, Hungary, Italy, Bulgaria, Greece, Turkey and Morocco until the mid-Pleistocene ( Botka & Mészáros, 2017; Geraads, 1995; Reumer, 1998b; Rofes & Cuenca-Bescós, 2006; Rzebik-Kowalska, 1998).

Measurements (mm)

Left tooth row: I: L = 1.49, LT = 0.57, A1: L = 0.72, A2: L = 0.59, A3: L = 0.43, A4: L = 0.37, P4: bL = 1.65, lL = 1.05, peL = 0.97, W = 1.35, M1: bL = 1.54, lL = 1.38, peL = 1.20, aW = 1.46, pW = 1.63, M2: bL = 1.28, lL = 1.23, peL = 1.07, aW = 1.58, pW = 1.49, M3: L = 0.70, W = 1.22.

Right tooth row: A3: L = 0.38, A4: L = 0.36, P4: bL = 1.66, lL = 1.05, peL = 0.97, W = 1.38; M1: bL = 1.55, lL = 1.35, peL = 1.21, aW = 1.52, pW = 1.72, M2: bL = 1.27, lL = 1.22, peL = 1.12, aW = 1.59, pW = 1.52, M3: L = 0.67, W = 1.21.

Description

Cranium: Te dorsal part the skull ( Fig. 3A, B) is poorly preserved. Posteriorly, most of the squamosal, parietal and occipital bones are preserved but crushed into pieces so the contact between them is not visible. For the same reason the lambdoid crest, the occipital condyles and the sagittal crest are not preserved. Anteriorly, the rostrum is partially preserved but the premaxillary, maxillary and nasal bones are broken so their contacts are lost.

Te buccal view of the specimen ( Fig. 3D) confirms that most of the dorsal part of the skull is indeed crushed. Te parietal foramen is consequently not observable. In contrast, the infraorbital and lachrymal foramen are well preserved. Te infraorbital foramen is located above the P4 and the anterior part of the M1. Te relatively wide lachrymal foramen is about half the diameter of the infraorbital foramen. It is located at the level of the infraorbital foramen and above the middle anterior part of the M1. Te bony bridge separating both foramina is narrow. More anteriorly, above the A3, the supra infraorbitalis foramen is also preserved.

Te basicramium is better preserved in occlusal view

( Fig. 3C), so several anatomical features can be described. Te left tooth row is fully preserved. It is composed of one incisor, four antemolars, one premolar and three molars. Te size of the antemolars decreases from A1 to A4, whereas the P4 is much larger. Te M1 is the largest molar, about as large as the P4, whereas the M3 is the smallest. Between the tooth rows, the palate is also well preserved. Both incisive foramen are located at the contact between the maxillar and premaxillar but mostly open in the premaxillary part of the palate. Te greater palatine foramen is well preserved on the right side of the palate next to the M1 (visible but partly crushed on the left side). Te posterior part of the maxillary shows a small zygomatic process ending before the level of M3 mesostyle.

Te upper and lower articular facets of the mandible are preserved on both sides of the skull. Te hamulus of the pterygoid is curved buccally and forms a loop enclosing the foramen ovale. Te two basisphenoidalis lateralis foramina are visible on both sides, at the posterior end of the pterygoid fossa, where a fragment of the vomer is also preserved. Te ectotympanic rings are not preserved, but both petrosal bones, located laterally on both sides of the basioccipital suture, are relatively close to the medial axis of the skull. Finally, the foramen magnum is visible at the posterior end of the skull, but the occipital area is heavily damaged, preventing further description of its anatomy.

I: I is strongly fissident and its main apex is hook-like. In lateral view, the talon is well developed, triangular and pointed. An undulated cingulum is present, ending as another small cusp in lingual view. In buccal view, an internal groove is opening posteriorly along the root.

A1-A4: Tese teeth overlap each other, and A3 but most notably A4 are compressed anteroposteriorly. In buccal view, all these upper antemolars have rather triangular shape, whereas their outline is rather quadrate in occlusal view. Tere are slightly recurved lingually. A1 and A2 are similar in height, A3 is slightly smaller, and A4 is less than half the size of A3. A weak cingulum surrounds the teeth but disappears anteriorly. In lingual side, the cingulum makes a small cusp aligned with the main cusp and linked to it by a faint crest. A faint cingulum conule is also present at the posteriolingual corner. In occlusal view, all are more stretched buccally than lingually. Te A 4 is fully visible in buccal view .

P4: Te anterior border is oblique with the paracone being more anteriorly located than the protocone. Te hypoconal and metaconal flanges are both strongly developed, delimiting a deep posterior emargination.

In buccal view, the P4 paracone is large and conical; its postparacrista is moderately long and has a constant height. In lingual view, protocone, hypocone, and the cingulum of the hypoconal flange have a stepped descending shape ( Fig. 4. B 2).

M1-M2: In occlusal view, the posterior emargination is less pronounced than that of the P4 (the hypoconal flange is short with no contact with the posterior tooth), and the anterior margin is not oblique. Te protocone is U-shaped with a well-developed metaloph and a small anterior conule. Te division of the paraloph forms a small pit on the anterior border between the paracone and the protocone. Te metacone and the protocone are connected either by a prolongation of the postprotocrista (metaloph) or by an additional low crest that connects them directly. Tere is no hypoloph so the hypocone is isolated from the protocone by a deep sinus. Te hypoconal flange is broad and rounded, and extends posteriorly further than the metastyle. Te posthypocrista is short and curved toward the posterior emargination.

M3: It is half the length of M2. It shares the features of M1-M2 but lacks hypocone and metastyle. Te posterior cingulum bears two faint conules, the lingual one being probably a vestigial metacone.

Comparisons

Dental pigmentation, mandible shape, and lower teeth morphology are often used as key-characters for shrew identifications (e.g., Repenning, 1967; Reumer, 1998a; Zaitsev & Rzebik-Kowalska, 2004) that we cannot use for the Jradzor cranium identification. However, additional characters such as the fissident upper incisor ( Repenning, 1967; Reumer, 1998a), the number and the relative size of antemolars ( Hutterer, 2005b), the buccal shape of P4 ( Repenning, 1967), the presence of metalophs and hypocones in upper molars ( Repenning, 1967) allow to refer our specimen to the Soricinae subfamily and Neomyini tribe ( Reumer, 1998a). We observed a well-pronounced buccal curvature of the hamulus at the base of the skull in the Jradzor specimen, a feature that is also strongly expressed in the Neomyini tribe but absent in other Soricinae , such as the genus Sorex (as noted by the authors).

Among the Neomyini tribe, five genera have four upper antemolars: Nesiotites Bate, 1945 (although the number of antemolars is variable between 3 and 4, see Reumer, 1980 and Pons-Monjo et al., 2012), Neomys Kaup, 1829 , Soriculus Blyth, 1854 , Episoriculus Ellermann & Morrison-Scott, 1966 , and Asoriculus Bate, 1945 ( Dubey et al., 2007; Repenning, 1967).

Te cranium of Jradzor differs from Neomys by its generally smaller size and the shape of its antemolars: almost rectangular in occlusal view, main short cusp at the anterior part of the teeth and posteriorly recurved, lingual cusp poorly developed and posteriorly positioned. It also slightly differs from the Soriculus and Episoriculus by the position of its lachrymal foramen, anteriorly and dorsally displaced, compared to other Neomyini shrews ( Francisk et al., 2019; Motokawa & Lin, 2005).

Te taxonomy of Asoriculus is in need of a revision. Six species of Asoriculus have been described and are still considered valid so far: A. burgioi Masini & Sarà, 1998 , A. corsicanus (Bate, 1945) , A. gibberodon (Petényi, 1864) , A. similis (Hensel, 1855) , A. maghrebiensis Rzebik-Kowalska, 1988 , A. thenii Malez and Rabeder, 1984 (e.g., Geraads, 1995; Janossy, 1973; Malez and Rabeder, 1984). Tese six species, with others, have been discussed to be either morphotypes, subspecies, or even juvenile ontogenetic stage of the same species Asoriculus gibberodon (e.g., Reumer, 1984; Rzebik-Kowalska, 1994). However, these points of views are not widely accepted and there is no consensus so far on the systematics of this genus. For example, Koufos et al. (2001) described seven morphotypes of Asoriculus in Osztramos 7; most recent studies also keep maintaining the use of several morphotypes or sub-species (e.g., Vasileiadou & Doukas, 2022). However, solving this systematic issue is beyond the scope of the present study.

Our specimen is close to the Reumer’s molars morphotype A of A. gibberodon also described in Popov (2003) and Vasileiadou et al. (2012). Nevertheless, it differs from it by its metalophs and paralophs division, a feature that has been rarely described for Asoriculus . For instance, A. gibberodon from the early Pleistocene of Greece presents divided metalophs but not divided paralophs ( Koufos et al., 2001).

Terefore, not knowing the morphological variability of the population from Jradzor, we simply refer this cranium to Asoriculus gibberodon . Tis identification is coherent with mandibles of Asoriculus gibberodon found from the slightly younger stratigraphic layers (JZ-3, JZ-3s, JZ-13) of Jradzor section. Tese layers are included in the FA2 sedimentary unit composed of pyroclastic tails deposited between 3.98 and 4.1 Ma. Te FA2 lays above the FA1 sedimentary unit—the diatomite where from the skull has been recovered ( Lazarev et al., 2023).

Inner ears of Asoriculus gibberodon from Jradzor Fig. 5

Description

Cochlea: Both left and right cochleae are well preserved. Te number of turns is about 1.5. A deep secondary bony lamina stretches over most of the basal turn. Te blunt apex is detached from the basal turn. Te latter is thick. Te aspect ratio is low with a value of 0.48, and the cochlea is thus rather flat. Te cochlear aqueduct could not be reconstructed. Its thick starting point on the right bony labyrinth seems to be medio-laterally oriented.

Vestibule: It is crushed on both bony labyrinths so that the saccule and utricule are not clearly visible. Te stapedial fossa is well preserved on the left bony labyrinth. It is rather elliptical with a ratio of 1.55.

Vestibular aqueduct: It is poorly preserved on both bony labyrinths. Its starting point is visible on the left one and its course is apparently preserved on the right one. Te aqueduct is thin and starts on the vestibule medially (more that the common crus). It is parallel to the common crus while being apparently slightly undulating. Te aqueduct extends at least up to the splitting point of the common crus.

Semi-circular canals: Te semi-circular canals are crushed or broken on both bony labyrinths and especially strongly flattened on the right one. However, it is possible to observe that the lateral semi-circular canal enters the vestibule posteriorly slightly in front of the posterior ampulla, preventing the formation of a secondary common crus. Te posterior semi-circular canal has a slight undulation. Te posterior and anterior semi-circular canals seem to extend beyond the dorsalmost extension of the common crus. Te visible ampullae at the base of the semi-circular canals are rather ellipsoid in shape but still quite bulky.