Chelus colombiana 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 : 698-702

Chelus colombiana Wood, 1976

( Figs 5-8)

Chelus colombiana Wood, 1976: 3 .

TYPE SPECIMEN. — UCMP-78762 , an articulated shell ( Wood 1976) ( Fig. 5A, B)

REFERRED MATERIAL. — Specimens referred by Wood (1976): IGMp-002045 , incomplete shell lacking part of the right side of the carapace and anterior plastral lobe, possibly a male ( Sánchez-Villagra et al. 1995a: fig. 1A; Fig. 6); IGMp-002049 , a partially disarticulated shell; IGMp-002085 , a left epiplastron. This study: VPPLT-1748 , nearly complete carapace ( Fig. 5 C-G), VPPLT-380 , complete plastron and MT-22 , complete shell ( Fig. 5 H-J) .

OCCURRENCES AND AGE. — Villavieja and La Victoria formations, Huila Department, Colombia. Middle to Late Miocene 16 to 10.52 Ma ( Mora-Rojas et al. 2023).

REVISED DIAGNOSIS. — Chelus colombiana differs from all other extant and fossil Chelus by having narrow knobs that form continuous dorsal ridges on the carapace that do not expand laterally, a feature that is present independently of size. It shares with C. fimbriata an almost rectangular carapace, with parallel lateral margins. Chelus colombiana and Chelus lewisi share and differ from the extant taxa by the presence of an axillary buttress that extends onto costal 2, instead of being restricted to costal 1 (extant taxa); inguinal buttress 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). It shares with the two extant taxa a wider anterior plastral lobe than C. lewisi . It differs from C. fimbriata by cervicals 7 and 8 having longer and more dorsoventrally projected neural spines, and broader internal scapular angle of 85°.

DESCRIPTIONS

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

VPPLT-1748 is a nearly complete carapace, missing most of its left and posterior margins ( Fig. 5C, D). The carapace is almost rectangular in shape, exhibits three dorsal ridges formed by knobs, one on the left costals, one along midline of the neurals, and one on the right costals. The knobs are narrow and do not extend radially or laterally. The surface of the bone is well preserved, allowing clear identification of sutural contacts between the bones and the sulci left by the scutes.The nuchal bone is hexagonal in shape.The neural series is composed of six bones, most of which are wider than long (except neural 4) and have a hexagonal shape. VPPLT-1748 has eight pairs of costal bones. The peripherals have a marked lateral indentation at the contact between the marginals. The sulci indicate that the cervical scute was almost rectangular in shape. Five vertebral scutes were present, of which vertebrals 1 to 4 were wider than long and rectangular to slightly hexagonal in shape, while vertebral 5 was trapezoidal. Pleural scute 1 was large, covering most of costals 1 and 2, and peripherals 1 to 3. The other pleurals were slightly smaller than pleural 1 and rectangular to trapezoidal in shape, with pleural 3 being the smallest of the four pairs. Some of the sulci between the marginals are preserved. Marginals 1 to 3 were restricted to the anterior peripherals and marginals 8 to 10 were potentially restricted to the posterior peripherals, without reaching the costals. On the ventral surface of the carapace ( Fig. 5E, F), most of both axillary buttresses are preserved reaching costals 2. The inguinal buttress scar left is restricted to costal 5, while the iliac scar covered most of costal 8 and potentially the suprapygal. In right lateral view ( Fig. 5G), the carapace exhibits a low dome shape showing the left and right costal series of knobs forming the ridges, which are narrow without extending radially or laterally.

MT-22 is an almost complete shell ( Fig. 5 H-J). It has a rectangular shape, with the left and right lateral margins almost parallel to each other. The three ridges of knobs are narrow without expanding radially or laterally on the dorsal surface of the carapace. Some of the sutural contacts between contacts are visible. The nuchal bone is hexagonal, slightly wider than long, and its anterior margin a bit eroded. Bioerosion marks are well preserved on one of the left peripherals and some costals ( Fig. 5I). In left lateral view ( Fig. 5J), the carapace shows a low dome shape and the narrow and low knobs without developing peaks or extending laterally.

IGMp-002045 is an articulated shell ( Fig. 6), preserving most of the left half of the carapace and a portion of the medial region of the right half. Most of the anterior plastral lobe and right bridge with the carapace region are missing. The missing parts of the carapace and right bridge region were reconstructed with plaster originally during the preparation of the specimen in the 1970s.

The carapace of IGMp-002045 ( Fig. 6A, B) exhibits on its dorsal surface three ridges of narrow knobs, without developing lateral or radial expansion. The sutural contacts between bones are poorly identifiable. However, the sulci left by the scutes are well-preserved. There were five vertebral scutes. Vertebral 1 was almost rectangular in shape and vertebrals 2 and 3 hexagonal and wider than long. Vertebral scute 4 was almost equally wide than long and vertebral scute 5 trapezoidal in shape. There were eleven marginal scutes and four pleural scutes, of which pleural 1 was the largest. On its ventral surface, the carapace of IGMp-002045 exhibits an axillary buttress that reaches costal 2, and a very large costovertebral tunnel ( Fig. 6C). The plastron is complete and articulated with the carapace ( Fig. 7E). The sutures between bones, as well as the sulci left by the scutes are clearly identifiable. The preserved sulci indicate that the femoral was longer than the abdominal and pectoral, and the anal was the shortest one of these scutes ( Fig. 7F). The xiphiplastra exhibit a large and deep U-shaped anal notch and long and narrow posterior tips, as well as they are characterized by a deep medial concavity. The anterior plastral lobe is wide and has evidence of two pairs of extragulars and an elongated gular scute.

The plastron of IGMp-002045 is well-preserved, without any evidence of crushing, only some minor cracks, and missing most of the anterior plastral lobe ( Fig. 6D, E). The sutures between the bones, as well as the sulci left by the scutes are clearly identifiable. The preserved sulci indicate that the femoral was longer than the abdominal, and the anal was the shortest one of these three scutes. The xiphiplastra exhibit a large and deep U-shaped anal notch and long and narrow posterior tips. The plastron is characterized by a deep. medial concavity ( Fig. 6 E-H). These features suggest that the specimen potentially represents a male. Even though most of the anterior plastral lobe is missing, its left margin is preserved enough to indicate that this lobe was wider than the posterior.

The pelvic girdle of IGMp-002045 is well preserved, showing a strong sutural contact between the ilia and the carapace and the ischia and pubes with the plastron ( Fig. 6 F-H). In left lateral view ( Fig. 6F, G), the acetabulum is well defined, only slightly dislocated from its original anatomical orientation due to some fractures present in the left ischium. The ilia are robust at their contact with costals 8, suprapygal and possibly reaching a portion of the pygal ( Fig. 6H).

VPPLT-380 consists of a nearly complete plastron, only missing the most lateral portions of the left hyoplastron and hypoplastron ( Fig. 7). In visceral view ( Fig. 7A, B), the pubic scars are oval in shape and the ischia scars are nearly trapezoidal in shape indicating that the ischia almost touch each other medially. In ventral view ( Fig. 7C, D), the sutures between the bones and the sulci left by scutes are well-preserved. The anterior plastral lobe is wider than the posterior, having a large entoplastron and a relatively long medial contact between both epiplastra. The hyoplastra are slightly larger than the hypoplastra. There were two pairs of extragular scutes (I and II), enclosing together with the humerals the gular scute, which had a hexagonal shape and barely reached the anteromedial margin of the plastral lobe. The plastral formula was Abd> Fem> Pec = Hum = Gul> Ana> ExtII. The xiphiplastra exhibit a large and deep U-opened anal notch, long and narrow posterior tips and a medial concavity at the medial sutural contact between each other, indicating that the specimen was potentially a male.

VPPLT-1756. It is an articulated shell, preserving most of the plastron (missing the anterior regions of epiplastra and entoplastron), most of the peripherals and the pygal (missing portions of peripherals 1 and the nuchal), and most of costals 1 and lateral portions of costals 5 ( Fig. 8). It also preserves the right femur, most of the right scapula, the right coracoid, cervical vertebrae 7 and 8, and remains of some thoracic vertebrae.

Carapace

The shape of the carapace is rectangular, slightly curved at its anterior and posterior margins. Both costals 1 are preserved;

including ventrally the long axillary buttress that likely reached costals 2. Other preserved regions of the carapace are the lateral portions of costals 5, most of the peripherals and the pygal. However, peripherals 1 and the nuchal are missing ( Fig. 8A).

Plastron

The plastron is almost completely preserved, missing only the most anterior portions of both epiplastra and the most anterior corner of the entoplastron ( Fig. 8B, C). The sutures between bones, as well as the sulci left by the scutes are clearly identifiable. The femoral scute was the longest of the scutes, resulting in a plastral formula Fem> Abd> Pec> Ana = Hum. The xiphiplastra exhibit a large and deep U-shaped anal notch and long and narrow posterior tips. Portions of both pelvic girdles are still attached to the plastron via the ischia and pubis. The plastron lacks of deep concavity at the medial region of xiphiplastral, suggesting that it was possibly a female individual.

The right femur is preserved, missing most of the major and minor trochanters ( Fig. 8 D-F). The femoral shaft is arched and exhibits an oval-shaped outline. The femoral head is oval in shape, eroded at its more proximal region and slightly projected laterally. In all aspects, the femur of VPPLT-1756 resembles the femur of the extant specimens of C. fimbriata and C. lewisi .

VPPLT-1756 preserved the right scapula and coracoid ( Fig. 8G, H, J). The anterodorsal process of the scapula has an oval-shaped when seen in cross section, and it is broken, missing most of its distal end. There is a moderate notch in proximity to the glenoid fossa. The ventromedial prong (acromial process) is flatter than the anterodorsal and exhibits a dorsomedial ridge.The internal angle between the two processes is 85° ( Fig. 8H). In C. fimbriata this angle is 75° ( Fig. 8I). The coracoid has a flat distal blade which, although broken, is preserved enough to indicate that this region formed a broad surface, similarly to the coracoid of C. fimbriata ( Fig. 8K).

Cervicals

Cervicals 7 and 8 are preserved slightly crushed ( Fig. 8 L-U). Cervical 7 has a biconcave centrum, exhibiting a long and robust dorsoventrally projected neural spine, with postzygapophyses facing ventrally and meeting medially, and a long ventral keel ( Fig. 8 L-P). Cervical 8 has a biconvex centrum, exhibiting also a long dorsoventrally projected neural spine that ends at the postzygapophyses as cervical 7, which face ventrolaterally and meet medially. It is shorter than cervical 7, and it has a very long ventral keel that ends in a dropshape anterior tip ( Fig. 8 Q-U). VPPLT-1756 cervicals 7 and 8 resemble the centra of the same vertebrae in C. fimbriata (ICN-1781; Fig. 8V). In the extant taxon, however, the neural spines of these vertebrae are shorter, broader and more dorsally projected.