Megachasma alisonae, Shimada & Ward, 2016

Shimada, Kenshu & Ward, David J., 2016, The oldest fossil record of the megamouth shark from the late Eocene of Denmark, and comments on the enigmatic megachasmid origin, Acta Palaeontologica Polonica 61 (4), pp. 839-845 : 840-841

Megachasma alisonae sp. nov.

Fig. 2A .

Etymology: In honor of Alison Ward ( Fig. 1D), who assisted DJW with the field work and sediment sorting that led to the discovery of the specimen described here.

Holotype: NHMUK PV P73711 (thus far only known specimen of this species). A nearly complete tooth (either upper right or lower left lateral tooth based on distal cusp inclination and asymmetrical root; Fig. 2A, B ).

Type locality: A sea cliff along Moesgård Strand in Denmark; coordinates: 56º04’53” N, 010º15’07” E ( Fig. 1A, B) .

Type horizon: Pyt Member of the late Eocene Søvind Marl Formation.

Diagnosis.—Dental morphology similar to Megachasma applegatei by possessing apicobasally shorter crown with strong lingual cusp inclination, one pair of lateral cusplets, and massive bilobate root ( Shimada et al. 2014: 281) but distinctly differing from it by exhibiting much smaller lingual protuberance, more labially situated and relatively larger lateral cusplets, and more widely spaced root lobes.

Description.—Small tooth with TH of 4.0 mm, TW of 4.5 mm, and TT of 1.8 mm. Crown apicobasally short with CH of 2.1 mm, CW of 4.1 mm, and CT of 1.2 mm; crown base mesiodistally broad and narrows rapidly just above base, developing apically into sharp, narrow cusp; lateral extensions of crown base with rounded shoulders extending shortly onto each root lobe on labial face; pair of prominent lateral cusplets present and situated slightly labially compared to position of main cusp; height of lateral cusplets about half of main cusp, but mesial cusplet slightly larger and taller than distal one; minute heellike rise along mesial margin of mesial lateral cusplet present; each cusplet triangular and lingually recurved, well separated from main cusp; smooth mesial and distal cutting edges present along main cusp and lateral cusplets, but absent between main cusp and each lateral cusplet; main cusp flexed lingually; labial crown face overall weakly convex; crown foot gently concave, lacking basal ledge, groove, or ornamentation (e.g., striations); lingual crown face strongly convex and smooth, lacking ornamentation; tooth neck encircles crown foot and particularly well developed immediately basal to both mesial and distal sides of main cusp on lingual face; crown slightly asymmetrical with weak distal inclination of main cusp. Root proportionally massive in relation to crown, dwarfing crown, with RL of 3.2 mm and RW (= TW) of 4.5 mm; root strongly bilobate; mesial and distal root lobes widely spaced; lingual root face gently rounded with robust protuberance basal to main cusp; mesial and distal root lobes rounded and connected with gently arched basal concavity; shallow, weak nutritive groove present on lingual root face that continues to basal root concavity.

Remarks.—The genus Megachasma is now known from three species: M. pelagios , M. applegatei , and M. alisonae sp. nov. Figure 2C shows Shimada et al.’s (2014: fig. 6B) graph plotting CH-to-CW ratios against RL-to-RW ratios. Whereas the difference between M. pelagios and M. applegatei in RH/RW ratio is relatively small, the graph reveals that M. pelagios tends to have more slender crowns compared to M. applegatei that has crowns with similar height and width. If NHMUK PV P73711 with a RL/RW ratio of 0.71 and a CH /CW ratio of 0.66 is plotted ( Fig. 2C ), it clearly falls in the range of M. applegatei . As its diagnosis suggests (see above), M. alisonae sp. nov. indeed bears characteristics of M. applegatei , likely suggesting their close phylogenetic affinity. However, the observed morphological differences in the robustness of the lingual protuberance, the position and size of lateral cusplets, and the spacing between the two root lobes are large between the two species and justify NHMUK PV P73711 to be considered a separate species from M. applegatei (and M. pelagios ), hence the decision to erect a new species, M. alisonae sp. nov.

Stratigraphic and geographic range.— Type locality and horizon only.

Figures

Fig. 2. Tooth of megamouth shark Megachasma alisonae sp. nov. (NHMUK PV P73711) from the Pyt Member (mid-Priabonian) of the upper Eocene Søvind Marl Formation of Moesgård Strand, Denmark. A. Photograph in labial (A 1), lingual (A 2), basal (A 3), distal (A 4), mesial (A5), and apical (A6) views. B. Line drawing showing crown (light gray) and root (dark gray) as well as missing portions (white). C. Scatter plots between CH/CW ratios and RL/RW ratios comparing NHMUK PV P73711 with extant M. pelagios (n = 23) and type series of M. applegatei from Aquitanian (early Miocene) of California, USA (n = 67) (after Shimada et al. 2014: fig. 6B). Abbreviations: CH, crown height; CW, crown width; RL, root length; RW, root width.