Chelus lewisi Wood, 1976

Cadena, Edwin-Alberto, Link, Andrés, Vanegas, Andrés, Avellaneda-Otero, Carlos A., Perdomo, César, Urueña-Carrillo, Diego A., Sánchez, Rodolfo & Vanegas, Rubén, 2023, New insights into the fossil record of the turtle genus Chelus Duméril, 1806 including new specimens with information on cervicals and limb bones, Comptes Rendus Palevol 22 (34), pp. 689-709 : 692-698

Chelus lewisi Wood, 1976

( Figs 2-4)

Chelus lewisi Wood, 1976: 7 .

TYPE SPECIMEN. — MCNC-239 , an articulated shell ( Wood 1976) ( Fig. 2A, B).

REFERRED MATERIAL. — Specimens referred by Wood (1976): MCZ-VPRA-4338 , a complete shell ( Fig. 2C, D); MCZ-VPRA-4337 , a complete shell; MCNC-240 , a costal bone ; MCNC-241 , posterior half of a carapace and plastron ; MCNC-242 , a crushed vertebra (probably a cervical) associated with a right xiphiplastron. Specimens referred by Bocquentin (1998) : UNEFM-1323 , a plastron and an isolated bone from the anterior half of the carapace ; UNEFM-1371 , a complete, articulated shell ( Sánchez-Villagra et al. 1995a: fig. 1B, Fig. 2E, F) ; UNEFM-1415 , anterior half of a plastron. Specimen described bySánchez-Villagra et al. 1995a): MCNUSB-150-85-PB , a complete shell. This study: AMU-CURS-600 , an articulated shell ( Fig. 2G, H) , AMU-CURS-119 , an articulated shell ( Fig. 2I, J), AMU-CURS-207 , an articulated shell ( Fig. 2K, L) , and AMU-CURS-1244 , an articulated shell preserving most of the left forelimb bones, some bones of the left hindlimb and the right femur ( Figs 3; 4).

OCCURRENCES AND AGE. — Socorro and Urumaco formations, Falcón State, Venezuela, Middle to Late Miocene ( Quiroz & Jaramillo 2010).

REVISED DIAGNOSIS. — Chelus lewisi differs from all other extant and extinct Chelus by a marked posterior widening of the carapace that creates a tapering anterior margin and by a neural ridge that is smaller than the two costal ridges (left and right). Chelus lewisi differs from C. colombiana by having a very narrow anterior region of the anterior plastral lobe, with a marked notch in the epiplastron at the lateral contact between the extragular I and the humeral scute. Chelus lewisi and Chelus colombiana share and differ from the extant taxa by the presence an axillary buttress that extends onto costal 2, instead of being restricted to costal 1 (extant taxa); inguinal buttress that extend onto costal 5, instead of costal 4 (extant taxa); two or three pairs of extragular scutes that encapsulate the gular, instead of gular reaching the anterior margin of the plastron and a single pair of extragular scutes (extant taxa); vertebral scute 1 almost rectangular, instead of pentagonal (extant); and vertebral scute 1 almost same width of vertebral scute 2, instead vertebral 1 wider than 2 (extant).

DESCRIPTIONS

Measurements for the new and all other previously described specimens are provided in Table 1 .

AMU-CURS-600 is a complete, articulated shell. The carapace is oval in shape, having three dorsal ridges of knobs, one on the left costals, one along midline of the neurals, and one on the right costals. It is much wider posteriorly, with anterior and posterior peripherals having indentations at the contact between marginal scutes ( Fig. 2G). The bone surface of the carapace is highly eroded, making bone sutural contacts and sulci poorly recognizable. Only some of the sutural contacts between the posterior peripherals are visible. The plastron is slightly shorter than the carapace, with its posterior lobe longer than the anterior. The margins of the anterior lobe taper anteriorly, ending in a narrow straight anteriormost edge and exhibiting marked lateral indentations on the epiplastra where the contact between extragular I and humeral scutes potentially were located ( Fig. 2H, O). The bone surface of the carapace is badly preserved, making any sulci impression unrecognizable, but the sutural contacts between the bones are visible.

AMU-CURS-119 is a complete, articulated shell. The left side of the carapace and the plastron are affected by crushing. The bone surface of both carapace and plastron is covered with a layer of gypsum that makes it difficult to discern sutural contacts and sulci. The more intact right margin suggests that the carapace was originally oval in shape, being wider posteriorly than anteriorly, and exhibited three knobby dorsal ridges ( Fig. 2I). The anterior plastral lobe ( Fig. 2J, P) exhibits the same shape and pattern described for AMU-CURS-600.

AMU-CURS-207 is a complete, though highly crushed articulated shell. The bone surface of both carapace and plastron is covered with a hard, black layer of rock matrix mixed with gypsum that rendered any sutural contacts and sulci unrecognizable ( Fig. 2K, L).

AMU-CURS-1244 is a nearly complete, articulated shell that is considerably affected by crushing and missing the posteromedial region of the carapace ( Fig. 3 A-E). The bone surface of the carapace is badly preserved allowing only the identification of sutural contacts between some of the bones. The plastron, by contrast, is complete and its bone surface is better preserved.

The carapace of AMU-CURS-1244 exhibits three knobby ridges on its dorsal side ( Fig. 3A, B). The nuchal has a pentagonal shape, being wider than long ( Fig. 3). The neural series is composed of at least six bones. Neural 1 is almost rectangular and positioned between costals 1. Neurals 2 to 5 are almost hexagonal in shape. Neural 6 is slightly deformed by crushing into a trapezoidal shape. Six pairs of costals are well defined by their sutures. Costals 2 and 4 exhibit almost the same width, while costals 3 and 5 are slightly anteroposteriorly narrower than the others. The shape of the carapacial scutes is not possible to establish due to the eroded bone surface that erased the sulci. However, it is possible to outline the sulci between the cervical and marginals 1.

The plastron of AMU-CURS-1244 is complete, exhibiting a bit of crushing at its central region and at the beginning of the xiphiplastral tips ( Fig. 3C, D). The anterior plastral lobe exhibits the same shape and pattern described above for AMU-CURS-600 and other specimens of C. lewisi ( Figs 2 M-P; 3C, D). It is slightly shorter than the posterior lobe. The epiplastra have a long median contact between each other and a straight anterior edge. The entoplastron has a diamond shape. The hyoplastra are longer and larger than the hypoplastra and the xiphiplastra exhibit narrow and long posterior process (tips). The sulci indicate that the gular scute had a diamond shape and was located between the two pairs of extragulars (I and II), and the humerals ( Fig. 3D). The extragulars I were almost rectangular in shape and the extragulars II were triangular, barely touching each other medially. The humerals and femorals almost had almost the same length medially and are both longer than the pectorals, the abdominals, and the anals, for a plastral formula of Fem = Hum> Ana> Abd> Pec> Gul> ExtII.

Most of the bones of the left forelimb of AMU-CURS-1244 are preserved, including the humerus, the ulna, radius, and the manus, including the ulnare, two centralia, the distal carpals, intermedium, metacarpals, some phalanges, and some distal phalanges ( Fig. 4A, B). These resemble the forelimb of Chelus fimbriata (NMW-1859, Fig. 4C; Sánchez-Villagra et al. 2007: figs 1H, 2E) both in number of elements and their shape, except for the missing (not preserved) phalanges and distal phalanges. From the hindlimbs, both femora are preserved ( Fig. 4 D-J). The right femur exhibits a robust, oval-shaped head and a relatively narrow trochanter minor that is located close to the femoral head, as well as a dorsally arched shaft ( Fig. 4 G-J). The left fibula, three metatarsals, and three distal tarsals are also preserved ( Fig. 4E, K).

Most of the left ilium of AMU-CURS-1244 is preserved showing the concave surface that contributed to the acetabulum ( Fig. 4D, E).