Trionyx sp.

Rollot, Yann, AbdelGawad, Mohamed K., Hamdan, Mohamed A., El-Barkooky, Ahmed N., Hassan, Safiya M. & Joyce, Walter G., 2025, Trionychian turtles from the Early Miocene (Burdigalian) Moghra Formation, Egypt, including a new species of Carettochelyidae, Swiss Journal of Palaeontology (30) 144 (1), pp. 1-37 : 28-30

Trionyx sp.

Description

General comments. DPC 7789 consists of six fragments that together represent a nearly complete carapace ( Fig. 18 ). The carapace is round and consists of a single unpaired nuchal, seven neurals, and eight pairs of costals ( Fig. 18 ). As in all trionychids, peripherals, suprapygals, and pygals are absent ( Meylan, 1987). Although many trionychid plastral elements are available in the collections of the DPC, most of these are fragmentary. We, therefore, only formally figure and describe three elements that bear sufficient information with regards to the taxonomic identity of the remains (see Fig. 19 ). Those elements are a nearly complete right hyoplastron (DPC 4466), a partial left hypoplastron (DPC 4122), and a nearly complete left hypoplastron (DPC 6436). All metaplastic portions of the shell bones are covered by a distinct sculpturing that consists of pits separated by vermiculated ridges, which tend to line up towards the margins. Scutes and their sulci are universally absent.

Nuchal. The nuchal forms the most anterior aspect of the shell and the anterior shell margin ( Fig. 18 ). The nuchal is anteroposteriorly deep, approximately three times wider than long, and contacts costal I posterolaterally and neural I posteriorly ( Fig. 18 ). The lateral half of the contact between the nuchal and costal I is strongly curved towards the anterior ( Fig. 18 ). The costiform process forms wing-like processes that are not fully covered dorsally by the metaplastic ossification of the bone and slightly expand over the margin of the carapace anteriorly and anterolaterally ( Fig. 18A, B ). In the middle of its posterior half, the nuchal ventrally forms paired depressions that provided space for the articulation of the eighth cervical vertebrae with the thoracic vertebral column ( Fig. 18C, D ). Suprascapular fontanelles are absent ( Fig. 18A, B ).

Costals. Costal I is curved anteriorly, relatively reduced in mediolateral width, and follows the shape of the posterior suture of the nuchal while tapering slightly laterally ( Fig. 18 ). Costal II is slightly curved anteriorly, albeit less than costal I, and laterally expanded mediolaterally ( Fig. 18 ). Costals III and IV are almost straight, whereas costals V, VI, and VII are curved posteriorly with the degree of curvature progressively increasing posteriorly ( Fig. 18 ). Costal VIII is a roughly triangular element that forms the posterior margin of the shell ( Fig. 18 ). Costal I only contacts the first neural, whereas costals II to IV contact neurals I–II, II–III, and III–IV, respectively ( Fig. 18A, B ). Left costal V contacts neurals IV, V and VI, but its right counterpart meets only neurals IV and V ( Fig. 18A, B ). Conversely, while left costal VI contacts neurals IV–VII, right costal VI contacts neurals V, VI, and VII. Costal VII only contacts neural VII ( Fig. 18A, B ). Costals VII also have a medial contact with one another along their posterior half, posterior to neural VII, while costals VIII medially contact one another for their entire length ( Fig. 18A, B ). The right costal VII also contacts the left costal VIII posteromedially ( Fig. 18A, B ). Although some of the lateral ends of the ribs are damaged, all of them extended beyond the metaplastically-ossified portion of the costals as wide, convex projections with the exception of the lateral end of costal rib VIII ( Fig. 18 ).

Neurals. DPC 7789 includes seven neurals that form an uninterrupted series ( Fig. 18A, B ). Neural I is hexagonal with straight lateral margins ( Fig. 18A, B ). Neurals II to IV are hexagonal with lateral margins that are slightly oriented posterolaterally and short posterolateral sides ( Fig. 18A, B ). Neurals V and VI are pentagonal and asymmetrical, thus jointly forming the neural reversal ( Fig. 18A, B ). Neural VII is the shortest of all neurals, heptagonal in shape, has five contacts, and prevents costals VII from contacting one another for their anterior half ( Fig. 18A, B ). Neurals I to IV each contact two pairs of costals: costals I–II, II–III, III–IV, and IV–V respectively ( Fig. 18A, B ). Neural V contacts costal V on the left side but costals V and VI on the right side, whereas neural VI contacts costals V and VI on the left side but only costal VI on the right side ( Fig. 18A, B ). Neural VII contacts costal VI and VII ( Fig. 18A, B ).

Hyoplastron. DPC 4466 is a nearly complete right hyoplastron that only lacks its anterolateral tip, which prevents us from determining the number of anterolateral processes ( Fig. 19A, B ). The hyoplastron is approximately four times wider than long ( Fig. 19A, B ). The hyoplastral callosity is well developed and covers most of the element with the exception of the anteromedial part, where at least four finger-like processes are preserved ( Fig. 19A ). The anterior margin of the hyoplastral callosity protrudes slightly into the axillary notch formed by the processes and is nearly straight ( Fig. 19A ). The hyoplastron forms a suture along its posterior margin for articulation with the hypoplastron ( Fig. 19A, B ). Medially, the callosity forms a smooth margin, which shows that the bilateral hyoplastra were not medially sutured to one another ( Fig. 19A ).

Hypoplastron. DPC 4122 and DPC 6436 together preserve nearly the entire morphology of a left hypoplastron, which is broad and elongate ( Fig. 19C–F ). The hypoplastral callosity is well developed and medially covers most of the medial processes ( Fig. 19E, F ). Posterolaterally, the hypoplastron forms two large lateral processes that protrude beyond the margin of the callosity ( Fig. 19C, D ). The anterior margin of the hypoplastron is nearly straight and forms a sutural surface for contact with the hyoplastron ( Fig. 19E, F ). Medially, the margin of the hypoplastral callosity is smooth, suggesting that a sutural contact with its counterpart is absent ( Fig. 19F ). Posteriorly, the hypoplastron forms a deep notch for the articulation with one of the anterolateral processes of the xiphiplastron ( Fig. 19E, F ). Although posteromedial processes are not apparent beyond the hypoplastral callosity, the ventral surface of the hypoplastron is crossed by at least three processes that suggest the former presence of an uneven medial comb ( Fig. 19E ).

Figures

Fig. 18 Carapace of Trionyx sp. (DPC 7789) from the Early Miocene (Burdigalian) Moghra Formation, Egypt. A picture in dorsal view.B drawing of carapace in dorsal view. C picture in ventral view. D drawing of carapace in ventral view. co I costal I, co VIII costal VIII,ne I neural I, nu nuchal

Fig. 19 Plastral elements of Trionyx sp. from the Early Miocene (Burdigalian) Moghra Formation, Egypt.A right hyoplastron (DPC 4466) in ventral view. B right hyoplastron (DPC 4466) in dorsal view. C posterolateral part of a left hypoplastron (DPC 4122) in dorsal view. D posterolateral part of left hypoplastron (DPC 4122) in ventral view. E left hypoplastron (DPC 6436B) in dorsal view. F left hypoplastron (DPC 6436B) in ventral view. Black arrowheads indicate the medial and posteromedial processes of the hypoplastron