The morphology and relationships of the hybodont shark Tribodus limae with a phylogenetic analysis of hybodont sharks (Chondrichthyes, Hybodontiformes).

摘要

'Hybodonts' (Hybodontiformes Maisey, 1989) have been proposed to represent the monophyletic sister group to Neoselachii (extant sharks and rays). Hybodontiformes have been considered to include the suborder Hybodontoidei, as well as two problematic taxa, Tristychius and Onychoselache. However, classifications of Hybodontiformes have been traditionally based on isolated dermal elements, and skeletal material is known from only 17 hybodontiform species and 11 genera (out of a total of over 44 species previously described).;New, exceptionally well preserved fossil material of the atypical hybodontiform Tribodus limae, from the Lower Cretaceous of Brazil, provides the first detailed description of skeletal morphology in this genus, shedding new light on hybodontiform diversity. Morphology of the cranial and postcranial skeleton and dermal elements are described. A three-dimensional braincase of Tribodus is examined and described using CT scanning and 3-D digital imaging, and cranial anatomy of another hybodont (Egertonodus basanus) is also examined for comparison. Reconstructions of cranial arterial circulation and the embryonic cranial cartilages in Tribodus are presented. The visceral skeleton of Tribodus is also described, and the first reconstruction of jaw musculature in a hybodont is presented. CT scanning reveals the presence of supportive endochondral "struts" in force-bearing regions of the jaws, similar to those found in extant batoids. A revised reconstruction of the pectoral fin skeleton of Tribodus is presented based on the new material.;Skeletal morphological data from Tribodus are then combined with that from 19 other hybodontiform taxa and 13 outgroup taxa to produce a matrix of 33 total taxa and 129 characters. A phylogenetic analysis is performed using the software programs PAUP* and T.N.T., testing the monophyly of Hybodontiformes, their proposed sister-group relationship with neoselachians, their interrelationships, and the phylogenetic position of Tribodus. Most-parsimonious trees are summarized using strict consensus, and Bremer support and bootstrap are used to test node strength.;Results of phylogenetic analysis support the monophyly of Hybodontiformes above Tristychius and their sister-group relationship with neoselachians, but suggest that Hybodontiformes including Tristychius is paraphyletic unless Neoselachii is included. Results of this analysis support monophyly of Hybodontoidei, Hybodontidae, and Hybodontinae. The position of Tribodus as sister to Asteracanthus is supported.