Eorhinobatos, Marramà & Carnevale & Naylor & Varese & Giusberti & Kriwet, 2021

Marramà, Giuseppe, Carnevale, Giorgio, Naylor, Gavin J. P., Varese, Massimo, Giusberti, Luca & Kriwet, Jürgen, 2021, Anatomy, taxonomy and phylogeny of the Eocene guitarfishes from the Bolca Lagerstätten, Italy, provide new insights into the relationships of the Rhinopristiformes (Elasmobranchii: Batomorphii), Zoological Journal of the Linnean Society 192 : -

GENUS † EORHINOBATOS

Zoobank registration: urn:lsid:zoobank.org:act:54BA1FD9-9C01-4E61-8327-9680F1AF50E8

Type species: Rhinobatus primaevus De Zigno, 1874 . Etymology: The name is derived from the word Eocene, second epoch of the Palaeogene period, and Rhinobatos ; hence a Rhinobatos -like guitarfish from the Eocene; gender masculine.

Diagnosis: A rhinobatid guitarfish with wedge-shaped pectoral disc slightly longer than wide; rounded, blunt anterior apex forming an angle of about 57 degrees; horn-like processes on nasal capsules absent; antorbital cartilages robust, triangular in shape with regular outline, articulating distally to the anteriormost portion of the propterygia; arcualia dorsalis absent; ten to 12 pairs of ribs; propterygia distally segmented, with the first segment lying near the level of the mouth, without reaching the level of the nasal capsules; propterygial radials extend as far as the nasal capsule level; 62–64 pectoral-fin radials of which about 30 are propterygial, 11–13 mesopterygial, one to two neopterygial and 20 metapterygial; dermal covering formed by small, numerous and closely packed dermal denticles showing smooth pentagonal or hexagonal crown; large thorns absent; few scattered roundish thornlets on the scapular region; teeth extremely small (up to 650 µm); crown high but mesiodistally short; lingual visor convex; long central uvula with rounded and not enlarged extremity; two incipient and divergent lateral uvulae; transverse keel separating the labial and lingual faces of the crown with a wide obtuse angle; enameloid surface completely smooth; root bilobed, massive, and almost as broad as the crown; root with undulated margins; very marked and deep furrow with a single central nutritive foramen; two additional foramina on lingual root face; weak collar on the upper part of the root stem.

Included species: Type species only.

† EORHINOBATOS PRIMAEVUS ( DE ZIGNO, 1874) COMB. NOV.

( FIGS 5B, 8–10)

Rhinobatus primaevus Zigno , mss.; De Zigno, 1874: 176 (original occurrence of name and description)

Rhinobatus primaevus Zigno ; De Zigno, 1878: 450, pl. 16; Woodward, 1889: 82; Eastman, 1905: 351.

Rhinobatus primaevus de Zigno ; Jaekel, 1894: 99; Blot, 1980: 344

Rhinobatis primaevus Zigno ; Eastman, 1904: 27.

Rhinobatus primaevus De Zigno ; D’Erasmo, 1922: 12.

Rhinobatus primaevus de Zigno, 1874 ; Carnevale et al., 2014: 41.

‘ Rhinobatus ’ primaevus ; Marramà et al., 2018a: 298, fig. 11b.

Holotype: MGP-PD 26278 , nearly complete articulated skeleton in a single slab, lacking the posterior part of the tail and caudal fin; 333.9 mm DW ( Fig. 8).

Type locality and horizon: Pesciara site, Bolca Konservat-Lagerstatte, Italy; Lower Eocene, Upper Ypresian, Middle Cuisian, SBZ 11, † Alveolina dainelli Zone and NP 14a (CN 6) calcareous nannofossil Zone (see: Papazzoni et al., 2014, 2017).

Description: Counts and measurements of † E. primaevus are listed in Supporting Information, File S1, Table S1. Although most of the skeletal elements of the cranial and pectoral regions are recognizable, the pelvic girdle and portion of the tail posterior to the first dorsal fin are missing ( Fig. 8). The pectoral disc has a maximum width of about 29 cm, being slightly longer than wide. The pectoral disc is wedge-shaped, with a rounded, blunt anterior apex forming an angle of about 57 degrees. The anterior lateral margins of the disc are nearly straight, contrary to those of † P. dezignii that are slightly concave. It is difficult to detect whether the posterior corner of the pectoral fins overlap with the anterior edge of the pelvic fins or not, since the latter are not preserved. The anteriormost dorsal fin is clearly recognizable, while the second one and the caudal fins are not preserved. The specimen mostly exposes its dorsal side, as suggested by the exposure of the anterior fontanel, the scapular processes of the scapulocoracoid, and the synarcual showing the dorsal ridge.

Neurocranium: The anterior portion of the rostral cartilage is incompletely preserved ( Fig. 9). Therefore, it is not possible to describe the rostral node and appendices. The anterior fontanel is wider than that of † P. dezignii ; the morphology of the posterior fontanel is difficult to determine. The preserved nasal capsule appears to be large and ovoid in shape, with the main axis anterolaterally directed, with no hornlike processes on its anterior aspect. The antorbital cartilage is robust, triangular in shape, and regular in outline. The antorbital cartilage articulates with the posterolateral margin of the nasal capsule, whereas its distal tip does not reach the anteriormost portion of the propterygium ( Fig. 5B).

Jaws, hyoid and gill arches: The palatoquadrate and Meckel’s cartilages appear straight and narrow. The hyomandibulae are short and nearly straight, slightly tapering in their medial section. The hyomandibulae directly articulate with the Meckel’s cartilage. The first four ceratobranchials are stout and robust, whereas the fifth pair appear narrow, posterolaterally directed and possibly articulated to the coracoid bar. The pseudohyals are small and rod-like.

Synarcual and vertebral column: The synarcual is strongly mineralized, tubular and anteriorly articulated to the chondrocranium ( Fig. 9). A narrow median crest can be recognized in the dorsal midline of the synarcual. The lateral stays are well developed and originate approximately at the midlength of the synarcual. Synarcual centra and spinal nerve foramina are difficult to observe. There are no arcualia dorsalis. Since the posterior part of the specimen is missing, it is not possible to estimate the total number of vertebral centra. There are 10–12 pairs of ribs, although the original number was probably higher.

Appendicular skeleton and fins: Since the specimen is mostly exposed in dorsal view, it is not possible to describe the morphology of the coracoid bar. The scapular processes of the scapulocoracoid are narrow and short, with their distal apices fitting into the indentation of the suprascapular cartilage, which is barely distinguishable. Only a single propterygium is visible. It is long, arched and extends nearly to the margins of the pectoral disc ( Fig. 9). The propterygium is distally segmented, with the first segment lying near the level of the mouth, without reaching the level of the nasal capsules. Conversely, the propterygial radials extend up to the level of the nasal capsule. The proximal section of the propterygium lies level with the procondyle, whereas the meta- to mesocondyle distance appears greater than the pro- to mesocondyle distance. The mesopterygia and metapterygia are incompletely preserved. There are possibly 62–64 pectoral-fin radials of which about 30 are propterygial, 11–13 mesopterygial, one to two neopterygial and 20 metapterygial. Each pectoral radial is segmented at least three or four times and bifurcates distally once. Pectoral fins are of plesodic type, with radials extending to the fin border.

The puboischiadic bar, basipterygia and pelvic radials are not preserved.

Dermal denticles: The dermal covering is formed by numerous and closely packed dermal denticles ( Fig. 10A). The denticle size is quite uniform across the body varying from 200 to 300 µm. The crown is completely smooth, pentagonal or hexagonal in shape. It is likely that this type of denticle covered both the dorsal and the ventral sides of the specimen, since no other type of denticle or thorn seems to be present, with the exception of a very few scattered roundish thornlets on the scapular region.

Dentition: The teeth of † E. primaevus have the typical rhinobatoid appearance and are more similar to those of † P. dezignii than those of other Eocene ‘ Rhinobatos ’ species ( Fig. 10B–F). Teeth are extremely small, up to 650 µm of mesiodistal crown width. The crown is higher and mesiodistally shorter than in † P. dezignii , thus resulting in a more globular appearance in occlusal view. The lingual visor is convex. The lingual face of the crown is characterized by a long, central uvula, which is more developed than that of † P. dezignii . The apex of the uvula is rounded and not enlarged. There are two incipient and divergent lateral uvulae. A transverse keel separates the crown into labial and lingual faces. Like in † P. dezignii , the transverse keel does not reach the lateral margins of the crown, and forms a wide obtuse angle in lingual view. The enameloid surface is completely smooth and there are no folds or other ornamental structures. The root is bilobed, massive and almost as broad as the crown. In basal view, the two root lobes are triangular in shape, with undulated margins. The root lobes are separated by a very marked and deep furrow bearing a single central nutritive foramen. Two additional foramina are visible on the lingual root face. There is a weak collar covering the upper part of the root stem.