Chelus undetermined

Cadena, EA & Jaramillo, CA, 2015, The first fossil skull of Chelus (Pleurodira: Chelidae, Matamata turtle) from the early Miocene of Colombia, Palaeontologia Electronica 24 (1), pp. 1-10 : 4

CHELUS sp.

Figure 2.7 -14

Referred material. MUN-STRI-dbid 38473. Partially preserved skull.

Locality, horizon, and age. Chelus sp. MUNSTRI-dbid 38473 comes from the lower segment of the Castilletes Formation, locality number 290666- 3 (Braincase), Kaitamana section, dated as ~ 16.2 m.y., radiometric age for the lower segment ( Moreno et al., 2015).

Description and remarks. Chelus sp. MUN-STRIdbid 38473 is a partially preserved skull with a maximum length of 67 mm, maximum width of 50 mm, and maximum height of 27 mm (all measurements as preserved). On the dorsal surface of the skull, most of the parietal bone is preserved exhibiting the characteristic dorsal surface corrugation as in the extant Chelus fimbriata ; this sculpturing pattern is related to the abundant number of skull scutes with very deep sulci, and irregular in number and shapes.

At the most anterior edge of both parietals the contact with the frontals is at least visible at the most right lateral portion of the skull; however, the frontals are only preserved as a very thin layer of the bone (the most ventral portion of bones). Portions of the pterygoids are also preserved as very thin layers of bone, highly eroded, with their medial contacts with the parietals hidden in dorsal view occurring under the most lateral portions of parietals, as in C. fimbriata . On the left side of the skull, a small portion of the squamosal bone is preserved, and a portion of the supraoccipital bone at the most posterior medial region. On the ventral surface, the basisphenoid has the characteristic elongated dagger shape, as in C. fimbriata , separating most of the pterygoids, which only have a short medial contact anteriorly. The basisphenoid is the only skull bone in Chelus sp. MUN-STRIdbid 38473 with clearly delimited sutural contacts and shape. Direct examination of skulls of C. fimbriata (Appendix 1) and MUN-STRIdbid 38473 described here, allow us to conclude that the basisphenoid bone for the genus Chelus has a very distinct osteological pattern, where the bone tissue grows extremely acicular and tapering anteriorly; this pattern can also occur in the most medial portions of the pterygoid bone. The basioccipital, preserved only at its most dorsal extension, is similar to that in C. fimbriata , with a rectangular shape and with lateral acute tip processes facing posteriorly. Very small medial portions of opisthotics, prootics and exoccipitals are preserved; however, the sutural contacts with the basisphenoid and basioccipital are not clearly defined.

Braincase morphology. The CT images ( Figure 3.1 -7) show that most of the internal structure of the braincase, as well as the cavum tympani, are not well preserved and have mostly been eroded and filled by the rock matrix, which includes small bivalves and other shell (mollusc) fragments ( Figure 3.3-4 ). However, some features are still well recognized, including the ascending dorsal portions of the pterygoids ( Figure 3.2-3 ), the contact between the frontal and parietals, and the contact between the parietals and the supraoccipital ( Figure 3.4-5 ). The ascending lateral processes of the basisphenoid are also well delimited ( Figure 3.6 ), as are portions of the descending ventral process of the parietal ( Figure 3.7 ). In all morphological features and sutural contacts, the fossil skull of Chelus sp. described here, resembles the skull of the extant C. fimbriata , except for the size of the endocranium, see below.

Figures

FIGURE 2. Skulls of Chelus fimbriata SM 37178 and Chelus sp. UNT-STRI-dbid 38473. Photographs and interpretative drawings. Chelus fimbriata SM 37178. 1-2, dorsal view; 3-4, ventral view; 5-6, posterior view. Chelus sp. MUN- STRI-dbid 38473 fossil from Castilletes Formation, Colombia. 7-8, dorsal view; 9-10, ventral view; 11-12, right lateral view; 13-14, posterior view. Abbreviations: bo, basioccipital; bs, basisphenoid; ex, exoccipital; fjp, foramen jugulare posterius; fm, foramen magnum; fn, foramen nervi hypoglossi, fp, fenestra postotica; fpcci, foramen posterior canalis caroticus cerebralis; fpp, foramen palatinum posterior; fr, frontal; fst, foramen stapedio temporalis; fts, fossa temporalis superior; ju, jugal; mx, maxilla; op, opisthotic; pa, parietal; pf, prefrontal; pl, palatine; pm, premaxilla; po, postorbital; pr, prootic; pt, pterygoid; qu, quadrate; so, supraoccipital; sq, squamosal; vo, vomer. Dotted line in 2 indicates sulci of skull scutes. Light grey areas in 8 indicate preservation of the dorsalmost surface of the bones. Scale bar applies for all figures.

FIGURE 3. CT images of Chelus sp. MUN-STRI-dbid 38473. 1, skull in dorsal view, arrows indicate the position of the cuts shown in 4-7. 2, horizontal cut on the lower portion of the skull, anterior to the foramen magnum. 3, horizontal cut at the level of the widest portion of the skull. 4, sagittal cut on the left portion of the skull. 5, sagittal cut at the midline of the skull. 6, coronal cut very close to the level of the basisphenoid-basioccipital contact. 7, coronal cut on the anterior portion of the skull. Abbreviations are as in Figure 2, plus apb (anterior process of basisphenoid).