Protostegidae, Cope, 1872

Protostegidae

Incertae sedis

Figs 2–4 View Figure 2 View Figure 3 View Figure 4

Referred material: UR*CP*0126 nearly complete left humerus . UR*CP*0127 , a nearly complete right xiphiplastron, left hypoplastron? fragment, a bridge region peripheral, and a costal bone fragment (single individual). UR*CP*0128 , proximal to the shaft regions of a right humerus . UR*CP*0145 , at least 10 per* ipheral bones from the anterior and bridge regions of the shell (single individual). UR*CP*0146 , distal portion of a humerus. UR*CP*0147 , nearly complete right epiplastron .

Although UC*CP*0128 and UR*CP*0146 were found in the same stratigraphic layer, a couple meters away from each other, we avoided considering them as belonging to a single individual, due to the impossibility of assembling them as a part of a single bone. However, this could be an artefact of the taphonomic events that both bones have suffered.

Remarks: The fossil material described herein can be attributed as belonging to Cryptodira by: the humerus having a smaller lat* eral process ( Gaffney 1990), as indicated in UR*CP*0126 ( Fig. 2 View Figure 2 ); a humerus exhibiting an anterior shoulder at the proximal articular surface (caput humeri) ( Gaffney 1990, Cadena and Parham 2015), seen in UR*CP*0128 ( Fig. 2 View Figure 2 ); and lack of sutural articulation of the pubis and ischium to the xiphiplastron ( Joyce 2007, Cadena 2015, references therein), as in UR*CP*0127 and UR*CP*0147 ( Fig. 4 View Figure 4 ). The humeri described here also shared with chelonioids the relatively low position of the lateral process along the humerus shaft ( Zangerl 1953, Gaffney and Meylan 1988, Hirayama 1994, Evers et al. 2019). The material is attrib* uted to Protostegidae by: (i) having a lateral process of the hu* merus enlarged within the anterior portion of shaft, not easily visible from ventral view ( Hirayama 1992, 1998), as exhibited by UR*CP*0126 ( Fig. 2E View Figure 2 ) and UR*CP*0128 ( Fig. 2K, L View Figure 2 ); (ii) a sigmoidal curvature of the shaft in anterior view and strong waist of the humerus, considered primitive for Cretaceous marine turtles ( Lehman and Tomlinson 2004); (iii) distal humerus trochlea absent, instead having a rounded epiphyseal surface without clearly defined articulation facets (Evers et al. 2019), UR*CP*0126 shows a very incipient trochlea, indicating its re* duction; (iv) also the flattening of the humeri indicates a fore* limb of flipper* type, characteristic of chelonioid marine turtles, including protostegids (Evers et al. 2019, Joyce et al. 2021b); and (v) ectepicondylar foramen in a groove (seen in UR*CP*0126), similar as in most chelonioids, including protostegids ( Hirayama 1994).

Descriptions

Measurements of the all fossils described herein are presented in Table 1 View Table 1 and Supporting Information, Fig. S1 View Figure 1 .

Humeri: UR*CP*0126 is a left humerus, missing most of its lat* eral and medial processes, as well as most of its proximal region ( Fig. 2A–E View Figure 2 ). In dorsal view ( Fig. 2A, B View Figure 2 ), a small portion of the caput humeri is preserved. The bone surface is smooth to slightly striate, and from the shaft region the bone starts increasing its width towards the most distal portion of it, exhibiting a slightly eroded bone surface. In ventral and distal views ( Fig. 2C–F View Figure 2 ), the distal region exhibits well*defined entepicondylar and ectepicondylar surfaces separated by a moderately deep depres* sion, and the ectepicondylar groove is particularly visible in both views. The trochlea, although existent, is very incipient. In an* terior view ( Fig. 2G View Figure 2 ), the humerus exhibits a sigmoidal curve from the proximal to the distal ends, and a general flat volume along its entire length. The preserved portion of the caput humeri is projected dorsally approximately 140º from the mid* line axis of the bone.

UR*CP*0128 is a right humerus, preserved from its shaft to the proximal region ( Fig. 2H–L View Figure 2 ). In dorsal view ( Fig. 2H, I View Figure 2 ), the bone surface is moderately eroded and the most remarkable fea* ture preserved is the slightly rounded caput humeri with a narrow lateral shoulder. In ventral view ( Fig. 2J View Figure 2 ), the proximoventral portion of the bone is completely eroded. In posterior view ( Fig. 2K, L View Figure 2 ), the bone exhibits an incipient sigmoidal shape, showing the prominent caput humeri and the base of the humerus medial process completely eroded.

UR*CP*0146 is a distal portion of a right humerus ( Fig. 2M–P View Figure 2 ), with most of its entire bone surface eroded and the ectepicondylar groove filled with a hard layer of haematite. In cross*section ( Fig. 2P View Figure 2 ) the bone exhibits a nearly oval shape, with thin cortical bone, and is densely filled to the centre with cancellous bone.

Carapace: Bridge to posterior region peripherals is preserved for two different individuals: UR*CP*0127 ( Fig. 3A–C View Figure 3 ) and UR*CP*0145 ( Fig. 3D–M View Figure 3 ). The peripherals are large in size (see Table 1 View Table 1 ), all of them exhibiting a smooth to slightly vermiculated bone surface in dorsal and ventral views ( Fig. 3A, D, E, H, J, M View Figure 3 ). On their medial surface, all of them exhibit a circular and deep pit for the insertion of the most lateral ending tip of the costal rib ( Fig. 3B, F, L View Figure 3 ). In anterior or posterior views, they are tri* angular in shape with a V*shaped medial embayment ( Fig. 3C, G View Figure 3 ). Some of them preserve evidence of the sulcus, indicating the contact between marginal scutes ( Fig. 3A, D, E View Figure 3 ). Two of the peri* pherals of UR*CP*0145 show evidence of attacks and possible bioerosional activity on the bone. The first corresponds to bite marks characterized by deep and almost parallel cuts ( Fig. 3H, I View Figure 3 ), and the second is a circular pit exhibiting bone remodelling ( Fig. 3R, S View Figure 3 ).

Some peripherals from the anterior margin of the carapace of UR*CP*0145 are longer than wide, and lack of pit for the costal rib end. They exhibit smooth bone surface in ventral view ( Fig. 3J, M View Figure 3 ). In ventral view, the suture between peripherals is well de* fined ( Fig. 3K, N View Figure 3 ), and in medial view the peripherals are oval in shape with the posterior edge being much narrower than the anterior ( Fig. 3L, O View Figure 3 ). A fragment of a large costal bone is pre* served for UR*CP*0127 ( Fig. 3D, E View Figure 3 ), possibly a right costal 3 or 5. In ventral view ( Fig. 3E View Figure 3 ), the costal rib is well defined. The pre* served carapacial bones are indicated in an outline of the cara* pace of Desmatochelys lowi ( Zangerl and Sloan 1960) redrawn from that study ( Fig. 3V View Figure 3 ).

Plastron: Plastral bones from two different individuals UR*CP*0127 and UR*CP*0147 represent two large turtles. The first of these bones corresponds to a nearly complete right xiphiplastron, UR*CP*0127 ( Fig. 4A–G View Figure 4 ), rectangular in shape. In ventral view ( Fig. 4A, B View Figure 4 ), the sutural edge with the hypoplastron is preserved at its most medial region. The bone surface exhibits a vermiculate decoration and some striations, particularly at the sutural region that contacted with the left xiphiplastron ( Fig. 3C View Figure 3 ). Also, in this view, the sulcus between the anal and femoral scutes is well defined. In dorsal view ( Fig. 4D, E View Figure 4 ), the xiphiplastron lacks any scars for attachment with the pelvic girdle bones, and the indented sutural edge that contacted with the left xiphiplastron is visible. At its most posterolateral margin, the contact between the visceral and the dorsal surfaces is indicated by parallel striations of the bone surface. Other plas* tral bones of UR*CP*0127 are very fragmentary ( Fig. 4H, I View Figure 4 ) and seems to be part of the left hypoplastron.

UR*CP*0147 corresponds to a large left epiplastron ( Fig. 4J–L View Figure 4 ), having a blade*shape. In dorsal view ( Fig. 4K, L View Figure 4 ), the bone exhibits a striate surface and the contact with the entoplastron is indicated by a concavity suggesting plastral kinesis between these two bones ( Fig. 4M, N View Figure 4 ). The preserved plastral bones are indicated in an outline of the plastron of Desmatochelys lowi redrawn from Zangerl and Sloan (1960) ( Fig. 4O View Figure 4 ).