Chelonioidea (Baur, 1893)

Poust, Ashley W., Holroyd, Patricia A. & Deméré, Thomas A., 2023, An Eocene sea turtle from the eastern North Pacific fills a Paleogene gap, Acta Palaeontologica Polonica 68 (1), pp. 47-51 : 48-50

Chelonioidea Baur, 1893

Gen. et sp. indet.

Fig. 2 .

Material. —SDSNH 103374, a partial nuchal and right and left first peripherals (collected by Gino Calvano, March, 2004) from SDSNH locality 5570 (“fish bed”), middle Eocene (Lutetian, 43–45 Ma), San Diego County, California, USA. This locality occurred within informal member B of the Santiago Formation. Additional locality information is available on request from SDNHM.

Measurements (in mm).—Width 120.6; ant/post height, lateral 21.2; ant/post height, midline 33.7; maximum thickness 6.7; width of “keel” 14.6.

Description. —SDSNH 103374 is a thin, “boomerang”-shaped fossil 6.7 mm at its thickest point, tapering to 2 mm at the acute anterior edge. The nuchal and partial right and left first peripherals are preserved, together forming what would have been a slightly dorsoventrally arched embayment extending concavely into the anterior portion of the carapace. The sutures are poorly expressed, but a complex suture is visible 22.1 mm from the midline on the visceral surface (equivalent to ventral view) of the left side of the nuchal embayment. The concave anterior opening measures 91.9 mm in width. The dorsal surface is subtly sculptured, and no sulci indicating the position of the scales are visible. The external cortical bone is thin and spongy; cancellous internal bone (diploe) with large pore spaces was exposed by erosion. This bone structure is typical in cryptodiran marine turtles, in contrast to marine or freshwater pleurodires and more pelagic cryptodires which generally have a thicker external cortical wall and thinner internal diploe ( Scheyer 2009; Scheyer et al. 2014). On the visceral surface a thickened and slightly raised midline ridge is present for articulation with the 8th cervical vertebra. Due to erosion, it is not possible to determine whether the articular surface was formed as a blunt facet or raised pedestal (see e.g., character 120 of Cadena and Parham 2015; or characters 62 and 126 of Anquetin 2012). The posterior edge of the preserved nuchal is worn, so it is not possible to determine whether it was sutured to adjacent bones or separated by fontanelles. Two minute foramina are located bilaterally close to the midline of the nuchal. These structures are rimmed with intact cortical bone and are not taphonomic artefacts. The transverse torus is well expressed, and there is no costiform process. The first peripherals are facultatively fused to the nuchal, although a small indentation in the surface of the nuchal opening and faint impressions in the dorsal surface indicate where the bones meet. The first peripherals are incomplete, but the exterior outline would have been angled or “elbow-shaped” as seen in some other chelonioids ( Lapparent de Broin et al. 2018).

The wider-than-long nuchal is dissimilar in shape and dimensions to that of dermochelyids, but consistent with that seen in other chelonioids with marked nuchal embayments. The curvature of the nuchal embayment is less than in many Cretaceous chelonioids (e.g., ctenochelyids; Zangerl 1953; Gentry 2018) and greater than in Eocene stem-cheloniids (e.g., European Puppigerus Cope, 1970 [ Moody 1974], Ashleychelys Weems and Sander, 2014 , Procolpochelys Hay, 1908 , and Carolinochelys Hay, 1923 [ Weems and Sanders 2014]). In thinness and degree of the nuchal embayment, SDSNH 103374 is most similar to Eochelone brabantica Dollo, 1903 , and to a lesser extent Eochelone monstigris Grant-Mackie, Hill, and Gill, 2011 , and/or Argillochelys sp. ( Grant-Mackie et al. 2011; Lapparent de Broin et al. 2018). It differs from E. monstigris in lacking an acute corner at the edge of the nuchal embayment. SDSNH 103374 is more strongly curved and emarginate than similarly thin-shelled forms such as Erquelinessia gosseleti (IRSNB 1628; Lapparent de Broin et al. 2018).

The phylogenetic placement of incomplete carapacial specimens is challenging, due to both a paucity of characters and a lack of Paleogene taxa in most recent phylogenetic analyses of chelonioids. When included, Paleogene taxa are basal to extant taxa (e.g., Evers and Benson 2019). Weems and Brown (2017) provide the most comprehensive recent phylogenetic analysis including multiple Paleogene taxa) based largely on cranial characters, and consistently recovered several weakly-supported clades among Paleogene and Neogene chelonioids. Notably, they recognized a clade comprising Eocene Erquelinessia gosseleti and the Miocene Californian and Peruvian taxon Pacifichelys . This phylogenetic hypothesis suggests a long ghost lineage for the origin of Pacifichelys .

Lapparent de Broin et al. (2018) informally grouped several thin-shelled Eocene–Oligocene species, including European Eochelone spp. , Puppigerus camperi , and Glarichelys spp. , united in part by the shape of the nuchal region, i.e., having a slight protrusion of the anterior carapacial border delimited by an “elbowed” peripheral 1 and an “anterior transversal” or slightly notched nuchal border. These authors did not consider Eochelone monstigris from New Zealand, but both it and SDSNH 103374 would appear to fall within this informal group. However, it is not clear if this is a monophyletic group, or whether it corresponds to one or more of the clades recognized by Weems and Brown (2017) as the closest Paleogene sister group to crown Cheloniidae .

Figures

Fig. 2. Photographs of sea turtle nuchal and peripherals of Chelonioidea gen. et sp. indet. (SDSNH 103374) from the Eocene Santiago Formation of California, USA (SDSNH loc. 5570). Ventral (A 1, A 4), anterior (A 2), and dorsal (A 3, A 5) views.