Abstract
Until recently, it was considered axiomatic that the skull of lizards and snakes arose from that of a diapsid ancestor by loss of the lower temporal bar. The presence of the bar in the living New Zealand Tuatara, Sphenodon, was thus considered primitive, corroborating its status as a ‘living fossil’. A combination of new fossils and rigorous phylogeny has demonstrated unequivocally that the absence of the bar is the primitive lepidosaurian condition, prompting questions as to its function. Here we describe new material of Tianyusaurus, a remarkable lizard from the Late Cretaceous of China that is paradoxical in having a complete lower temporal bar and a fixed quadrate. New material from Jiangxi Province is more complete and less distorted than the original holotype. Tianyusaurus is shown to be a member of the Boreoteiioidea, a successful clade of large herbivorous lizards that were dispersed through eastern Asia, Europe and North America in the Late Cretaceous, but disappeared in the end-Cretaceous extinction. A unique combination of characters suggests that Tianyusaurus took food items requiring a large gape.
Highlights
The reptilian group Lepidosauria encompasses Squamata and Rhynchocephalia (Sphenodon and its fossil relatives). Living members of these two subgroups differ in several respects, but the one most widely cited is the presence in the Sphenodon skull of a complete lower temporal bar
For more than a century, the classic view held that the diapsid skull of the ancestral lepidosaur experienced gradual reduction in the lower temporal bar, thereby ‘freeing’ the quadrate in squamates (e.g. Romer 1956; Robinson 1967)
Research over the last 30 years has demonstrated unequivocally that lepidosaurs inherited a skull without a lower temporal bar, the quadrate was firmly fixed to the skull by the pterygoid and squamosal (e.g. Whiteside 1986; Evans 2003, 2008; Muller 2003)
Summary
The reptilian group Lepidosauria encompasses Squamata (lizards, snakes and amphisbaenians) and Rhynchocephalia (Sphenodon and its fossil relatives). Living members of these two subgroups differ in several respects, but the one most widely cited is the presence in the Sphenodon skull of a complete lower temporal bar. We extend the description with three skulls from contemporaneous deposits in Jiangxi Province ( Nanxiong Formation, ca 66 Myr; see the electronic supplementary material). These skulls are more complete and less distorted than the holotype, and permit this remarkable lizard to be placed more fully into phylogenetic and functional context. Two skulls are significantly larger than the holotype and one is smaller
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More From: Proceedings of the Royal Society B: Biological Sciences
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