DYROSAURIDAE de Stefano 1903

Sarr, Raphaël, Hill, Robert V., Jenkins, Xavier A., Tapanila, Leif & O'Leary, Maureen A., 2024, A Composite Section of Fossiliferous Late Cretaceous- Early Paleogene Localities in Senegal and Preliminary Description of a New Late Maastrichtian Vertebrate Fossil Assemblage, American Museum Novitates 2024 (4013), pp. 1-32 : 24-26

DYROSAURIDAE de Stefano 1903

Gen. et sp. indet.

Figure 13

REFERRED MATERIAL: SEN 057 (M902033), base of tooth crown; SEN 059, proximal end of right metatarsal II; SEN 062, caudal vertebra with partial neural arch and right transverse process; SEN 062 and SEN 064, proximal caudal vertebrae; SEN 065, midcaudal vertebral centrum; SEN 067, tooth crown; SEN 068 (M902036), fragmentary tooth crown; SEN 069, proximal half of right metatarsal II; SEN 073, distal caudal vertebra; SEN 074 (M902040), fragmentary distal caudal vertebral centrum; SEN 079 (M902026), two fragmentary tooth crowns. Material that is figured only in the database MorphoBank have MorphoBank numbers in parentheses.

DESCRIPTION: The fragmentary crocodyliform material is tentatively referred to Dyrosauridae on the basis of vertebral morphology. The caudal vertebrae in the sample are ventrally concave in lateral view and nearly amphiplatyan, with only shallow median depressions on their cranial and caudal surfaces. This morphology closely resembles that described for hyposaurine dyrosaurids by Schwarz et al. (2006).

Five caudal vertebrae are preserved. SEN 062 is a large caudal centrum with an articulated neural arch and a nearly complete right transverse process (caudal rib). The cranial and caudal articular surfaces are very shallowly concave. The ventral surface of the vertebral body is gently concave in lateral view. The downcurved caudal border bears worn facets for the articulation of the hemal arch. Caudodorsally the neural arch is observed to articulate with the body along an undulating suture, indicating the closure if not complete fusion of the neural arch. In dorsal view the right transverse process curves caudally. The details described above are all consistent with an anterior (first through fourth) caudal vertebra of a hyposaurine dyrosaurid ( Schwarz et al., 2006).

SEN 064 is another caudal vertebra, also likely from the anterior part of the series. It has a complete neural arch, articulating with the centrum along a very faint suture, and the stump of the broken right transverse process. Both the cranial and caudal faces of the centrum bear articular facets for the hemal arch. The ventral margin is strongly concave in lateral view. SEN 065 is a small, heavily weathered and preserves only the centrum. There is a strongly concave ventral margin in lateral view.

SEN 073 is a caudal centrum with a badly damaged neural arch. Unlike the caudal vertebrae described above, this one is more than twice as long as it is wide, indicating a position as the 14th caudal vertebra or very likely well beyond as it compares closely with the 31st caudal vertebra described by Schwarz et al. (2006). This vertebra has prominent articular surfaces for hemal arches both cranially and caudally. SEN 074 is an extremely weathered centrum that appears to bear a short hypapophysis.

Appendicular remains attributable to crocodyliformes include two proximal metatarsal fragments. Both represent the right metatarsal II, indicating the presence of more than one individual. The proximal articular surfaces possess the offset morphology that allows for imbrication of the metatarsals ( Turner and Gatesy, 2021). The two bones are morphologically indistinguishable; however, SEN 059 is about twice the size of 069. Crocodyliform metatarsals are morphologically conservative ( Turner and Gatesy, 2021); thus the presence of two similar bones differing only in size suggests either different species or different developmental stages, both plausible alternatives.

Broken tooth crowns (SEN 057 and 068) preserve only the base of the crown, and SEN 079 preserves only the apex. The most complete tooth is SEN 067, a nearly complete crown that compares closely with known dyrosaurid dentition. This tooth is long and slightly recurved, with a lentiform cross section. The subtle mesial and distal carinae are smooth, lacking any denticles.

DISCUSSION: A relatively complete dyrosaurid specimen was collected by French geologists in the 1930s during geological exploration of western Senegal but only recently described in detail ( Malavoy, 1935; Jacquet, 1936; Martin et al., 2019). The specimen was found in Paleocene rocks of the Ndayane Formation in the vicinity of the Ndayane Cliff at Poponguine locality in our composite section (fig. 3) and assigned only to Dyrosauridae for lack of diagnostic skull material ( Martin et al., 2019). Our new records from the North Quarry at Poponguine are Late Cretaceous and thus the oldest dyrosaurids known from Senegal. Dyrosaurids, which are common fossils in nearshore marine outcrops of West Africa, are hypothesized to be of African origin and to have dispersed to South America in the Late Cretaceous ( Hastings et al., 2011). This new and relatively ancient specimen from Senegal was present at one of the most westerly parts of the African continent and thus well situated to have been part of a larger trans-Atlantic dispersal.

Testudines Linnaeus, 1758

Pleurodira Cope, 1864

Pelomedusoides Cope, 1868

Figure 14

REFERRED MATERIAL: SEN 054, partial shell preserving left epiplastron, hyoplastron, and axillary buttress (bridge), and peripheral carapace fragment; SEN 055 (M902031) and SEN 066 (M900945, M900946), fragmentary plastral elements; SEN 080, partial neural bone. Material that is figured only in the database MorphoBank have MorphoBank numbers in parentheses.

DESCRIPTION: SEN 054 preserves most of the left anterior portion of the plastron. The epiplastron is broken medially, but its suture with the hyoplastron is clearly discerned. A sliver of the entoplastron is preserved, demonstrating a curved caudolateral border. The epiplastron-hypoplastron suture meets the entoplastron far caudally, in contrast to most fossil Pelomedusoides described by Gaffney et al. (2006). The bone surface is too weathered to discern any scale sulci. The carapace fragments include a peripheral element likely from near the cranial end. The margin of the shell is thickened and upturned. The suture with the adjacent costal bone is preserved. A partial neural is preserved, recognized by the incorporated vertebral arch in internal (ventral) view. It is unclear whether this bone was pentagonal or hexagonal as well as from where in the shell it originated.

DISCUSSION: Most West African pelomedusoids from the Late Maastrichtian–Early Paleogene have been assigned to Taphrosphyini, however, this new Maastrichtian specimen is too fragmentary for such refined assignment. The specimen is nonetheless significant because Pelomedusoides from the Maastrichtian are relatively rare compared with those from the Paleogene and it remains an open question whether the clade underwent a post-Cretaceous-Paleogene boundary radiation or if this pattern is a preservational artifact.

Figures

FIGURE 13. Late Maastrichtian dyrosaurid crocodyliforms of the Cap de Naze Formation discovered at the North Quarry of Poponguine.A, B, Caudal vertebrae (SEN 062 and 064) from proximal region of tail in right lateral view; C, caudal vertebra (SEN 065) from middle of tail in right lateral view; D, distal caudal vertebra (SEN 073) in right lateral view; E, proximal right metatarsal II (SEN 069) in dorsal view; F, proximal right metatarsal II (SEN 059) from a larger individual; and G, isolated tooth crown (SEN 067) in labial view. Scalebars: A–F = 2 cm; G =1 cm.

FIGURE 14. Late Maastrichtian Testudines from the Upper Cap de Naze Formation of the North Quarry of Poponguine.A, Partial plastron in ventral view (SEN 054). Dotted lines mirrored to show extent of preservation. B, Peripheral carapace fragment in dorsal view (SEN 054). C, Partial neural bone in dorsal view (SEN 080). Abbreviations: ent, entoplastron; epi, epiplastron; hyo, hyoplastron. Scale = 2 cm.