Abstract
Abstract: Skull shape variation in thalattosuchians is examined using geometric morphometric techniques in order to delineate species, especially with respect to the classification of Callovian species, and to explore patterns of disparity during their evolutionary history. The pattern of morphological diversity in thalattosuchian skulls was found to be very similar to modern crocodilians: the main sources of variation are the length and the width of the snout, but these broad changes are correlated with size of supratemporal fenestra and frontal bone, length of the nasal bone, size of the orbit and premaxilla and position of the narial opening. Patterns of shape variation, in combination with discrete‐state morphology and stratigraphic and geographic range data were used to distinguish nine species of teleosaurid and 14 species of metriorhynchid, with the four currently recognized Callovian species being split into eight. Metriorhynchids were found to be more disparate from the average shape of morphospace than teleosaurids. However, short‐snouted metriorhynchids and long‐snouted teleosaurids showed the greatest amount of disparity with respect to snout morphotypes, indicating that each group tended to explore opposite areas of morphospace. Phylogeny was found to have a moderate influence on the pattern of morphospace occupation in metriorhynchids, but little effect in teleosaurids suggesting that other factors or constraints control the pattern of skull shape variation in thalattosuchians. A comparison of thalattosuchians with dyrosaur/pholidosaurids shows that thalattosuchians have a unique skull morphology, implying that there are multiple ways to construct a ‘long snout’. Moreover, the skull geometry of the problematic speciesPelagosaurus typuswas found to converge on the teleosaurid area of morphospace. Finally, the temporal distribution of thalattosuchian species and morphotypes demonstrate a clear and highly correlated relationship with sea level curves and mass extinction events through the Jurassic and the Early Cretaceous.
Highlights
As with crown-group crocodilians (Pierce et al 2006, 2008; Pierce 2007), there is a long tradition of separating thalattosuchians, a clade of marine-adapted Jurassic crocodylomorphs, into two broad groups based on the relative proportions of rostral length and width (e.g. Andrews 1913; Wenz 1968; Young 2006)
As thalattosuchian species have been spuriously diagnosed by proportional differences of the skull, applying the method of landmark-based geometric morphometrics will provide a unique look at thalattosuchian morphology
The specific objectives of this study are to (1) characterize the main pattern of shape variation in the skull roof of the Teleosauridae and the Metriorhynchidae the two main clades of Thalattosuchia; (2) determine the number of valid teleosaurid and metriorhynchid species and examine their distribution within morphospace; (3) explore the relationship between skull shape and phylogeny; (4) compare morphospace occupation between the Teleosauridae, the Metriorhynchidae, and other longirostrine ‘mesosuchian’ fossils groups; (5) determine how each family and skull shape contribute to total morphological disparity; and (6) evaluate changes in the occupation of morphospace through thalattosuchian history
Summary
As with crown-group crocodilians (Pierce et al 2006, 2008; Pierce 2007), there is a long tradition of separating thalattosuchians, a clade of marine-adapted Jurassic crocodylomorphs, into two broad groups based on the relative proportions of rostral length and width (e.g. Andrews 1913; Wenz 1968; Young 2006). An examination of Adams-Tresman’s (1987a, b) cranial characters shows that measurements do overlap and, her results could be underestimating or overestimating the relationship between length and width of the skull and its utility for delineating thalattosuchian species. As one of the most important shape changes in thalattosuchians is related to a shift from brevirostrine to longirostrine skull morphologies, failure to differentiate allometric shape differences from size-independent differences may potentially underestimate species numbers. The present study re-examines thalattosuchian taxonomy using landmark-based geometric morphometrics, which allows one to capture and analyse morphological information in terms of size-independent shape variation (Bookstein 1991). In addition to dealing with the problem of size, geometric morphometrics provides a much more precise picture of shape differences amongst specimens than can be achieved using linear measurements. Species from all geological ages and geographical locations in which thalattosuchians occur will be analysed in order to quantify and explore the full extent of shape variation present within the Thalattosuchia and to evaluate AdamsTresman’s (1987a, b) delineation of Callovian species
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
