Eoplinthicus underwoodi, Adnet & Cappetta & Guinot & Sciara, 2012

Adnet, Sylvain, Cappetta, Henri, Guinot, Guillaume & Sciara, Giuseppe Notarbartolo Di, 2012, Evolutionary history of the devilrays (Chondrichthyes: Myliobatiformes) from fossil and morphological inference, Zoological Journal of the Linnean Society 166 (1), pp. 132-159 : 147-148

EOPLINTHICUS UNDERWOODI SP. NOV. ( FIG. 7 )

Material: Four complete teeth and many fragments.

Type locality: Locality DAK2. Unit 2, levels B1 and B2, south of Ad Dakhla, south-western Morocco ( Adnet et al., 2010).

Type horizon: Samlat Formation, Gerran member ( Ratschiller, 1967) – late Middle Eocene/Late Eocene.

Etymology: After C. J. Underwood, for his work on the Fayum faunas in Egypt.

Holotype: DAK2A-2 ( Fig. 7A–E ).

Diagnosis: The teeth have a small size, less than 5 mm broad, with a roughly hexagonal outline in occlusal view. Marginolabial and marginolingual angles very obtuse and rounded. Occlusal face as developed or less developed than the complete width of the crown, and very hollowed transversally. Oblique and well-developed labial face covered by corrugated enameloid. Sharp and salient labial visor and well-marked lingual bulge at the base of the crown. Root as high as the crown, with about six lobes separated by rather deep and broad W- shaped furrows. Sharp basal face of the lobes. The two marginal lobes may be shorter and diverging more than the others.

Description: The holotype ( Fig. 7A–E ) is about 2.4 times broader (4.8 mm) than long (2 mm) and 2.3 mm high. In occlusal view ( Fig. 7C ), the outline is roughly hexagonal, with marginal angles less than 90°. The occlusal face is narrower than the crown at the level of marginal angles and its shape is elliptical on the whole. This face has a deep transverse hollow with an angular bottom. This hollow is asymmetrical, with a labial wall more abrupt than the lingual one. Its surface is sometimes irregular, with a shiny enameloid. Its lingual margin is more irregular than its labial margin, which is rather convex. The labial face ( Fig. 7B ) is oblique in profile ( Fig. 7E ) and slightly convex. The labial visor is sharp and distinctly overhangs the root. The lower part of the visor is rather flat and well developed labiolingually. In occlusal view, the visor outline consists of a labial transversal segment slightly convex and of two shorter and straighter marginal segments, with strongly marked and rounded angles. The labial face is strongly corrugated. The lingual face is less developed than the labial one and more vertical in profile ( Fig. 7E ). It is also very ornamented, with smooth vertical folds mainly in the median part of the face. In lingual view ( Fig. 7A ), oblique segments unite the marginal angles with the extremities of the occlusal face of the crown. The transversal bulge of the base of the lingual face is well developed, rounded, and salient. The root is practically as high as the crown. It consists of six lobes separated by broad and deep W- shaped furrows. The marginal lobes are shorter than the median ones and even diverging. Their basal face is sharp and not flattened ( Fig. 7D ).

The other specimens ( Fig. 7F–O ) have a similar morphology. Their occlusal face is sometimes more transversally developed, with vertical segments joining their extremities and the marginal angles in lingual view ( Fig. 7F ). The largest specimen ( Fig. 7H–J ) has a polyaulacorhize root that consists of nine lobes, irregularly spaced, whereas the smallest ( Fig. 7K–O ) has a less concave occlusal face.

Discussion: Compared with the type species † Eoplinthicus yazooensis Cappetta & Stringer, 2002 , from the Priabonian of Louisiana, USA, several differences can be noted. The teeth of † E. underwoodi sp. nov. are smaller and much less transversally elongated. Their occlusal face is more lingually developed, with a deeper transverse hollow that does not show the ‘lip-like’ design of the type species. The ornamentation of the labial face is less corrugated. The differences are more marked at the level of the root. In the type species, the lobes are more numerous (six versus 11/12), with a flat basal face, and the base of the root is parallel to the base of the crown in labial or lingual view. Such differences seem to indicate a less derived morphology in † E. underwoodi sp. nov. compared to † E. yazooensis and species of † Plinthicus , resembling the morphology of some species of † Burnhamia . Cappetta & Stringer (2002) suggested that the crown was little marked by functional wear as in mobulids and contrary to the rhinopterids and myliobatids. The same functional assumption could be seriously made with the concave occlusal surface observed in material from Dakhla. At present, the new species † E. underwoodi sp. nov. is known only from the Bartonian–Priabonian deposits of south-western Morocco although its presence may be spatially extended to the contemporaneous deposits from Fayum, Egypt ( Underwood et al., 2011 as † Eoplinthicus sp. ). The occurrence of this species elsewhere is suspected but currently remains debatable. Case & West (1991) reported a single broken tooth as Rhinoptera sp. from the Late Eocene of Pakistan that shows a deflected extremity of the crown in labial view ( Case & West, 1991: pl. 4, fig. 1), indicating that the occlusal face is straighter than the base of the crown, as in † Eoplinthicus and contrary to Rhinoptera .

Figures

Figure 7. Eoplinthicus underwoodi sp. nov. A-E, DAK2A-2, holotype: tooth position unknown. A, lingual view; B, labial view; C, occlusal view; D, basal view; E, profile. F-G, DAK2A-3, paratype, tooth position unknown. F, lingual view; G, occlusal view. H-J, DAK2B-2, tooth position unknown. H, lingual view; I, labial view; J, occlusal view. K-O, DAK2B-3: young element, tooth position unknown. K, lingual view; L, labial view; M, occlusal view; N, basal view; O, profile.