Remarkable multicuspid teeth in a new elusive skate (Chondrichthyes, Rajiformes) from the Mediterranean Pliocene

Alberto Collareta,Frederik H Mollen,Simone Casati,Marco Merella,Andrea Di Cencio

doi:10.1007/s12542-020-00542-7

Abstract

Here we report on four highly peculiar skate teeth from Arcille and Certaldo, two Pliocene localities of Tuscany (central Italy). While being attributable to Rajiformes and somewhat reminiscent of Dipturus and Rostroraja, these specimens display an unusual multicuspid tooth design that does not match any extinct or extant skate taxon known to date. The studied teeth are thus referred to a new genus and species of Rajiformes, Nebriimimus wardi gen. et sp. nov., which is here tentatively assigned to the family Rajidae. Based on pronounced morphological similarities between the rather large-sized teeth of the latter and those of extant nurse sharks, we hypothesise that N. wardi might have been capable of actively foraging upon relatively large food items compared to other rays. This extinct skate species was likely not a common component of the Pliocene Tuscan marine vertebrate assemblages. The palaeoenvironmental scenarios that N. wardi inhabited were marginal-marine and open shelf settings characterised by tropical climate conditions. As N. wardi is currently known only from lower to mid-Pliocene deposits of the Mediterranean Basin, it is tempting to speculate that its speciation dates back to an earliest Pliocene phase of diversification that also contributed to the emergence of the Mediterranean endemic stock of extant skate species.

Highlights

Among extant rays (Chondrichthyes: Elasmobranchii: Batomorphii), those assigned to the order Rajiformes number some 290 species arranged in 38 genera, accounting for almost half of the global alphadiversity of rays (Naylor et al 2012a; Last et al 2016; Weigmann 2016)
We report on four skate teeth from two Pliocene localities of Tuscany
Nebriimimus wardi is assigned to the skate order Rajiformes by virtue of the following combination of characters: tooth size subcentimetric; crown cuspidate, robust, mesiodistally broad, provided with a distinct transverse cutting edge as well as with features for tooth interlocking; root stout, holaulacorhize, bilobate, weakly flaring basally

Summary

Introduction

Among extant rays (Chondrichthyes: Elasmobranchii: Batomorphii), those assigned to the order Rajiformes (sensu Naylor et al 2012a) number some 290 species arranged in 38 genera, accounting for almost half of the global alphadiversity of rays (Naylor et al 2012a; Last et al 2016; Weigmann 2016). Into four families, namely, Anacanthobatidae, Arhynchobatidae, Gurgesiellidae, and Rajidae, the latter being the most diverse one by accounting for more than half of the living species of Rajiformes. The morphological analyses do not unambiguously support the monophyly of all the aforementioned groups, either recognising a single rajiform family (i.e. Rajidae) articulated in several subfamilies and tribes (McEachran and Dunn 1998) or recovering rajids and gurgesiellids as para-/polyphyletic groups (Marramà et al 2019). The fossil record of Rajiformes is scanty to date, and the low number of diagnosable extinct rajiform species contrasts markedly with the high alpha-diversity of the recent stock (Herman et al 1996). While attributable to Rajiformes, these specimens display an unusual multicuspid tooth design that does not match any extinct or extant skate taxon known to date. An analysis of the relationships of this new taxon and a discussion of its palaeoecological and evolutionary significance are undertaken on the basis of a detailed characterisation of the idiosyncrasies and affinities of its remarkable tooth morphology

Materials and methods

Findings

Discussion