Plesioheros chauliodus, Perez & Malabarba & Papa, 2010

Perez, Patricia Alano, Malabarba, Maria Claudia & Papa, Cecilia del, 2010, A new genus and species of Heroini (Perciformes: Cichlidae) from the early Eocene of southern South America, Neotropical Ichthyology 8 (3), pp. 631-642 : 633-639

publication ID	https://doi.org/10.1590/S1679-62252010000300008
DOI	https://doi.org/10.5281/zenodo.15987237
persistent identifier	https://treatment.plazi.org/id/03E9D352-6A43-0179-FF2B-FAEDFDCBFCE5
treatment provided by	Carolina
scientific name	Plesioheros chauliodus
status	sp. nov.

Treatment

† Plesioheros chauliodus , new species

Figs. 2-6 View Fig View Fig View Fig View Fig View Fig

Holotype. CNS-V10026 , an articulated individual preserved as an impression in lateral view, with the posteroventral region of the body missing ( Fig. 2 View Fig ).

Locality. Alemanía, northwestern Argentina, level Faja Verde II , Lumbrera Formation , Lower Eoceno (~ 48 Ma).

Diagnosis. As for the genus.

Etymology. From the Greek word chauliodous (χαυλιοδουσ), meaning having large teeth, referring to the enlarged dentary teeth of the specimen. Treated as noun in apposition.

Description

Shape. Morphometric data for † Plesioheros chauliodus is presented in Table 1 View Table 1 . † Plesioheros is a deep-bodied cichlid with the greatest body depth is at about the seventh spine of the dorsal fin. The dorsal contour of head is steep, slightly arched from the snout to the supraoccipital crest. The dorsal body contour is convex from the dorsal-fin origin to the caudal peduncle. As judged from the prepelvic region the ventral body contour is curved.

Skull. The skull is roughly round in lateral view, a little deeper (21.53 mm) than long (17.08 mm), with a rounded snout and a terminal, slightly inferior mouth ( Fig. 3 View Fig ). The anteroventral portion of the vomer is preserved and presents a rounded outline. The frontal follows the contours of the orbit dorsally and contacts the lateral ethmoid anteroventrally and the supraoccipital posteriorly, but most of its surface is concealed by the scales. The supraoccipital crest is moderately high with the posterior margin slightly curved. Part of the sensory canal system and three foramina can be observed in the frontal. The median frontal lateralis openings are coalesced forming a single coronalis foramen (NLF0 of Stiassny, 1991) not covered by the supraoocipital. The parietal crest is low, starting in the posterior region of the frontal, near frontal lateralis foramen 3 and extending through the parietal and epiotic. The mesethmoid and lateral ethmoid are represented by impressions inside the orbit.

Over the anterior region of the skull roof, dorsal to frontal, there is a weak impression of a roundish protuberance, apparently covered by small scales. Although the bad preservation does not allow a more secure identification, it could be a “nuchal hump”, very common in males of some cichlid species. Circumorbital series. The orbital region show signs of distortion and disarticulation. The orbit is nearly oval, with the vertical diameter (6.01 mm) longer than the horizontal (5.06 mm). Only the posterior half of the lachrymal is visible, it appears nearly square with the posteriordorsal corner projected and extending over infraorbital 2. Only two pores of the sensory canal are observed on the lachrymal: a posteroventral and a posterodorsaly which is directed to infraorbital 1. Posteriorly to the lachrymal, there are four infraorbitals. They are short, nearly tubular and slightly dislocated from the original positions.

Jaws. The mouth is terminal and short, with the upper jaw slightly longer than the lower ( Fig. 4 View Fig ). The premaxilla is robust with a round symphysial region. The ascending arm is barely longer (5.72 mm) than the dentigerous arm (5.69 mm); however, as its spine is not completely preserved dorsally, we are assuming the ascending arm is a little longer. The confluence of the ascending and dentigerous arms forms a slightly obtuse angle (97°). The ascending arm exceeds half of the orbit diameter; it is wide (width 27% of height) with an articular process moderately distinct. The dentigerous arm is slightly curved ventrally with no symphysial extension or processes.

The dentary is short and deep with the posterodorsaly portion (coronoid process) concealed by the maxilla. There are five openings in the dentary for the lateral canal. The anguloarticular is deeper (4.73 mm) than long (4.16 mm), with the primordial process wide, low and slightly forward projected. The coulter area is nearly squared, almost so deep (2.16 mm) as wide (2.18 mm). The section of the mandibular canal in the anguloarticular is short and slightly oblique. The posterior border of the coulter area is slightly curve, almost straight with the retroarticular forming the posteroventral region.

The hyomandibular has a clear vertical crest and is dorsally expanded; it is ventrally sutured to mesopterygoid. The quadrate is typically triangular with the ventral border following the horizontal arm of the preopercle. The robust condyle articulates with the mandible at the level of the middle of the orbit.

Teeth. In the premaxilla, impressions of alveoli and teeth are preserved in the most anterior and posterior regions of the dentigerous arm, indicating that the teeth were implanted along its entire length and arranged in three rows symphysially ( Figs. 3 View Fig , 4 View Fig ). Four conical teeth are preserved: two in the anterior region and two at the caudal end ( Fig. 4 View Fig ). The most anterior teeth measure 0.45 mm; the posterior ones are smaller, measuring 0.2 mm.

There are seven tooth impressions in the oral border of the dentary. The teeth increase size toward the symphysis. The two most anterior teeth are conical with their apex slightly recurved and a very small lingual cusp; they measure 0.5 and 1.13 mm and probably belong to the outer row. The next posterior tooth is much smaller (0.41 mm), conical and anteriorly directed; it appears to be broken and to belong to an inner row; it is not possible determine the presence of a lingual cusp. The fourth tooth in the sequence is large (0.7 mm), conical, with the apex slightly recurved rearward. The next tooth is small (0.43 mm) and badly preserved; it appears to be pedunculate, with a triangular tip (spatulate tooth?). Posteriorly, it seems to have four more teeth in the sequence, but it is not possible to define their shapes. Based on this observation, the dentition of the lower jaw of this species appears to be made up of at least two rows: the outer row includes enlarged anterior teeth, conical, slightly curved and bearing a smaller lingual cusp; and an inner row would include smaller conical (with or without lingual cusp) and doubtfully spatulate teeth. The presence of the enlarged canines bearing an accessory lingual cusp in the outer jaw rows is exclusive of heroins among Neotropical cichlids ( Casciotta & Arratia, 1993b; Kullander, 1996, 1998; Chakrabarty, 2007).

Opercular apparatus. The preopercle is narrow with the horizontal arm much shorter than the vertical arm, which form an approximately rectangle angle. It is crossed longitudinally by the preopercular sensory canal presenting two terminal and four medial pores. The opercle is trapezium-shaped with a round posteroventral angle. The subopercle is narrow with a long process projecting between the opercle and preopercle. The interopercle is ovoidal tapering rostrally.All opercular elements have smooth surfaces and borders and they have impressions of cycloid scales all over their surface (except for the preopercle).

Gill rakers. Just below the most anteroventral part of the preopercle there are impressions of three gill rakers ( Fig. 3 View Fig ). They are short, fingerlike and aligned, close together, on its caudal border. Based on the position they fossilized, it is assumed they should belong to first gill arch.

Vertebrae and supraneurals. The vertebral column is almost completely preserved, only the most posterior region is missing including the caudal peduncle and the fin. There are 10 abdominal and 10 caudal vertebrae and a short hypapophysis is preserved in the fourth abdominal vertebra.

There are two narrow and parallel supraneurals with no neural spines associated with them ( Fig. 6 View Fig ). Ten abdominal vertebrae is the basal percoid number and is too low for South American cichlids, whichhave 12-14abdominalvertebrae ( Kullander, 1998). In † Plesioheros the first preserved neural spine is projected between the second and third dorsal pterygiophores, whereas American cichlids with two supraneurals have the first neural spine inserted between the supraneurals. Therefore, it is assumed here that there was a vertebra with the neural spine inserted between the supraneurals, resulting in 11 abdominal vertebrae for † Plesioheros .

The last preserved caudal vertebra (10 th vertebra) lies just before the caudal peduncle ( Fig. 5 View Fig ). Posteriorly, there is part of the dorsal margin of the caudal peduncle where it is possible to observe 4 procurrent rays ( Fig. 5 View Fig ). Considering the distance between the last preserved vertebra and the most anterior procurrent ray (5 mm) and the distance between the neural spines (1.2 mm), we estimate that 2-3 caudal vertebrae are missing from our vertebral count. Then, including these 2-3 missing vertebrae, † Plesioheros has 12-13 caudal vertebrae, totalizing 23-24 vertebrae.

Girdle and pectoral fin. Most of the pectoral girdle and fin are preserved. Dorsally in the girdle, there are parts of the posttemporal and the proximal (medial) extrascapular adjacent to the neurocranium. The supracleithrum is not visible. The cleithrum contours the opercle and subopercle ending in a point anteroventrally. Just above the pectoral insertion, the cleithrum has a notch with a lamina projecting for the proximal postcleithrum articulation. The fin rays conceal most of the postcleithra leaving only part of the proximal and the ventral pointed end of the distal postcleithrum observable. The scapula and coracoid are partially preserved articulating with each other and with the four rectangular radials. The pectoral fin has 12 soft rays and appears rounded.

Girdle and pelvic fin. Both pelvic basipterygia are preserved. They are robust and triangular bones, measuring 5.9 mm. Their most proximal end is covered by the cleithrum and it is not visible. Distally the pelvic bone is expanded to articulate with the rays; the processus medialis is short and stout. The articulation between basipterygium and the pelvic fin spine is strong and evident. There are I + 5 rays in the pelvic fin.

Dorsal fin. The dorsal fin is completely preserved with XI + 12 rays. The base of the spiny portion is longer (14.9 mm) than the soft portion (12.1 mm), and they are continuous with no gap between them. The first four spines increase gradually in length; the length does not vary from the 5 th to 10 th spine; the last one (eleventh) conspicuously longer. The soft dorsal fin is represented mainly by the proximal (unsegmented) portion of the soft rays; the last four soft rays are more completely preserved including their distal segmentation. There are 22 pterygiophores; the first three are about the same length; they decrease in length from the fourth to the last.

Anal and caudal fins. As stated above, the anal and caudal fins were not preserved. However, it is possible to observe the dorsal profile of the caudal peduncle, where 4 procurrent caudal-fin rays are present ( Fig. 5 View Fig ). This allows us to obtain a probable length for the caudal peduncle, which was measured from the end of the dorsal-fin base to the last point preserved (7.7 mm). We estimated that 2-3 caudal vertebrae referring to the caudal peduncle were lost.

Onlythepresumedfirstanal-finpterygiophore (itisnotpossible determine how many pterygiophores), whi ch is contacting the first caudal vertebra (11 th vertebra), is preserved.

Scales. The squamation was preserved as impressions all over the body ( Fig. 2 View Fig ). In the head, differently sized scales are present in the cheek and postorbital region. Cheek scales are ctenoid and irregularly distributed, with a few covering the rostral border of the preopercle. Differently sized cycloid scales fully cover the opercle, subopercle and interopercle, but not the preopercle. The scales covering the body are mainly cycloid, large sized (1.9-2.8 mm) with the chest scales slightly smaller than those on the flanks. In the occipital region, the scales are ctenoid. There is a predorsal series with about six scales ( Fig. 6 View Fig ), decreasing in size caudad, from the supraoccipital crest to dorsal-fin origin; however, it is not possible to determine accurately whether they constitute a uniserial or triserial arrangment. There are scale impressions on the bases of spines and rays and over the soft portion of the dorsal fin.

Lateral line. As typical in cichlids, the lateral line in † Plesioheros is divided into two segments: upper (anterior) and lower (posterior). The upper segment begins next to supracleithrum region and it extends posteriorly, ascending gradually and finishing at the vertical through the 4 th caudal vertebra (5 th soft ray). The trajectory of the lower section of the lateral line initiates at the vertical through the 6 th caudal vertebra (7 th soft dorsal-fin ray), following close to the vertebral column and entering the caudal peduncle. It is counted 17 scales (impressions) in the upper and 6 in the lower lateral line; there is no overlap of the upper and lower sections. We estimate that 3 lateral line scales referring to the caudal peduncle (see Vertebrae and supraneurals) were lost, totalizing 26 scales in the lateral line. There are 3 scales between the upper lateral line and dorsal fin anteriorly, and 2 posteriorly; 2 scales between the lateral lines.