Oromobula dakhlaensis, 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 : 145

OROMOBULA DAKHLAENSIS SP. NOV. ( FIG. 6 )

Type locality: DAK3. Unit 2, level B2; from two sites south of Ad Dakhla (35.5 km and 41.5 km), southwestern Morocco ( Adnet et al., 2010).

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

Synonymy: Mobula sp. ( Adnet et al., 2010) in text only.

Etymology: Species named after the peninsula of Dakhla, south-western Morocco.

Holotype: DAK 2B-5 ( Fig. 6E–H ).

Diagnosis: Same as the genus.

Description: Crown is three or more times higher than root. Its lateral extremities are flared as in holotype ( Fig. 6E ) to convex ( Fig. 6A ) in labial or lingual view. Strongly labiolingually flattened, its lingual face is concave between a basal transversal roll of enameloid, overlapping the root/crown boundary, toward the transversal crest. Labial face is however, principally convex with a curve that follows those of lingual face in profile ( Fig. 6B ). Labial visor is only visible on the smallest tooth ( Fig. 6N ) but disappears in larger ones ( Fig. 6B, F ) and root is totally overhung in labial view ( Fig. 6C, G ). Nearly all the labial surface (visor included) is ragged by numerous deep vertical folds that form partially anastomosing lamellae around the mid part of labial face. The basal part is devoid of grooves, especially on the small teeth ( Fig. 6M–P ) considered as the youngest specimen. The apical part of the labial face that edges the transversal crest (making an occlusal surface) is flat, nearly horizontal, and smooth (except on the smallest elements, Fig. 6M–P ) because of scouring by functional wear. Occlusal contour is chaotic ( Fig. 6D, H, L ), with no geometric structure indicating possible interlocking columns of teeth, even weakly. The transversal crest is irregular and largely overlaps the lingual face. The lingual face is relatively smooth except under the transversal crest where a few pronounced vertical folds extend below the transversal crest ( Fig. 6E, M ). Root is extremely reduced in size, both in height and depth. Except on some of the youngest specimens of the type series ( Fig. 6M, N ), root is principally at the polyaulacorhize state. Root lobes are lamella-like and are well-spaced, except on the youngest specimens ( Fig. 6M, N ), in which the root shows a more compact shape. Small scattered foramina are sometimes visible on the labial and lingual faces of root, as in holotype ( Fig. 6E, G ).

Discussion: Teeth of † O. dakhlaensis gen. et sp. nov. come only from the level B2 (see Adnet et al., 2010 for geological details) and have never been found together with those of A. barbei gen. et sp. nov. from the underlying level B1. Distinct in time, these two taxa are also distinct in morphology with the youngest († O. dakhlaensis gen. et sp. nov.) having more strongly labiolingually compressed teeth with a higher crown without well-marked bulge on the labial face, a non-ornamented lingual face and a higher root strongly compressed labiolingually compared to the crown with well-spaced short lamellae.

Neither sexual nor monognathic heterodonty are seen in our limited sample. However, ontogenetic heterodonty is substantial within the smallest specimens ( Fig. 6I–P ), which display teeth showing more morphological affinities with those of Mobula (e.g. holaulacorhize to polyaulacorhize root, labial face partially ornamented, presence of an individualized labial visor). The labiolingual flattening as well as the enameloid ornamentation is moreover only slightly marked on the smallest teeth. Affinities with the Oligocene † E. kowaldi and † P. kruibekensis are marked as they share a number of similar characters such as a reduced labial visor combined with a concave lingual face in large teeth and a flat occlusal surface with a transversal crest often ragged (contrary to † Plinthicus ). However, the enameloid layer on the lingual face is globally smooth (except some scattered folds) as in † Eomanta and contrary to † Plinthicus . The teeth are strongly constricted labiolingually with a high crown/root ratio and the root possesses thin regular lamellae as in † Plinthicus and contrary to † Eomanta . Indeed, this genus represents an intermediate form with a mixed suite of characters existing in both enigmatic genera. Comparisons may be extended with care to † P. stenodon , as some small specimens of this species with large teeth recovered from the mid Miocene of North Carolina, USA (H. Cappetta, unpubl. data), resemble those of † O. dakhlaensis gen. et sp. nov. However, we remain unsure whether these unstudied and unfigured teeth really represent juvenile specimens of † P. stenodon or a different lineage.

Figures

Figure 6. Oromobula dakhlaensis gen. nov., sp. nov. A-D, DAK2B-4, tooth position unknown. A, lingual view; B, profile; C, labial view; D, occlusal view. E-H, holotype DAK2B-5, tooth position unknown. E, lingual view; F, profile; G, labial view; H, occlusal view; I-L, DAK2B-6, tooth position unknown; I, lingual view; J, profile; K, labial view; L, occlusal view. M-N, young element DAK2B-7, tooth position unknown. M, lingual view; N, profile. O-P, young element DAK2B-8, tooth position unknown. O, lingual view; P, occlusal view. Vertical scale bar for A-H, horizontal scale bar for I-P.