The first three-dimensional fossils of Cretaceous sclerorhynchid sawfish: Asflapristis cristadentis gen. et sp. nov., and implications for the phylogenetic relations of the Sclerorhynchoidei (Chondrichthyes)

摘要

A new fossil batoid (ray), Asflapristis cristadentis gen. et sp. nov., is described from six exceptionally well-preserved, three-dimensional skeletal remains from the Turonian (Late Cretaceous) of Morocco. Mechanical and acid preparation and computed tomographic scanning of these specimens reveal details of much of the proximal skeleton, especially the skull, synarcual and pectoral skeleton, with only the more distal parts of the skeleton missing. These fossils represent a relatively large animal (62 cm preserved length, estimated total length approximately 2 metres) possessing a robust rostrum that lacks enlarged rostral denticles. It has a narrow and small chondrocranium with jaws that are relatively large compared to the rest of the skull and robust with highly ornamented teeth that lack cusps. The branchial skeleton shows a large second hypobranchial without an anterior process, which was probably fused to the basibranchial as in other sclerorhynchoids. The synarcual is large and lacks centra through its entire length, with no direct connection to the pectoral girdle observed. Pectoral fins probably possessed enlarged proximal elements (propterygium, mesopterygium and metapterygium); the articulation facet between the coracoid and the pectoral elements was reduced. A phylogenetic analysis using both parsimony and Bayesian methods was performed incorporating this new taxon. Both analyses recovered a phylogenetic topology that places the sclerorhynchoids in a close relation to rajoids and clearly separated from the morphologically similar Pristidae within the Rhinopristiformes. With respect to the extant taxa, the phylogenies generated are similar to that obtained from molecular analysis of modern batoids. The palaeoecological implication of this discovery suggests that the Asfla assemblage was not from a 'normal' open carbonate shelf but rather a restricted environment favouring a low-diversity chondrichthyan fauna.