Morphometric analysis of phytosaur premaxillae and maxillae

When it comes to studying organisms, having size independent measures for maturity are important for many aspects of organismal biology, and may be crucial for determining taxonomic affinity, and morphological signals associated with ontogenetic age (i.e., juvenile vs. adult) and sexual dimorphism. This is because the size of an organism can be the result of many factors that are not necessarily indicators of maturity (Chabreck and Joanen, 1979; Ferguson, 1984; Mazzotti et al., 1986; Deeming and Ferguson, 1989; Brandt, 1991). This problem is particularly pronounced when researchers are studying extinct species. The purpose of my research project was to investigate and understand patterns of morphological variation in the phytosaur premaxilla and maxilla and to determine the degree to which morphological variation is a result of ontogeny. For example, such patterns might include the number, size and location of alveoli or the presence of prenarial crests. I conducted this research by gathering information on the premaxilla and maxilla of all phytosaur elements present in the University of Texas at Austin Vertebrate Paleontology Lab collection. I then performed statistical analysis on the data, and compared my results to those of previous authors to see if I could identify any ontogenetic signal. I did not identify size-independent ontogenetic influence on morphology with certainty but I did find some possible features that merit additional investigation in future studies. Those include the presence of one to three diastemas located primarily at the anterior end of the premaxilla, a wide interpremaxillary fossa but small alveolar ridge, and alveoli whose size mirror the width of the premaxilla (for example wide areas in the premaxilla are associated with larger alveoli whereas narrow areas in the premaxilla are associated with smaller alveoli). My study also confirmed the previous findings of Hungerbühler (2002) that the alveoli of phytosaurs are heterodont and exist in three distinct location-specific patterns, and the work of other researchers that prenarial crests are present only in larger specimens (Camp, 1930; Ballew, 1986; Hungerbühler, 2002; Stocker, 2010).