Title: Evaluating Machaeroprosopus Spp. Squamosal Morphology Based on New Material from the Martha’s Butte Beds: Chinle Formation, Arizona

Abbreviated Abstract: Occurrences of fragmentary phytosaurian (Machaeroprosopus spp.) squamosal bones are common in the Martha’s Butte Beds (Chinle Formation: Sonsela Member) of northeastern Arizona. Within the paleontology collection at Southern Connecticut State University (SCSU-P), multiple fragmentary squamosals display higher morphological variability than has been previously documented within Machaeroprosopus. Past studies have suggested that characters associated with the postorbital-squamosal bar are taxonomically informative among phytosaurs. The occurrence of squamosals outside the range of documented morphological variability raises questions regarding the extent of inter- and intraspecific variation among Machaeroprosopus spp. and the utility of fragmentary squamosal material in determining species-level taxonomic identification. Here, I will describe in detail the morphology of 13 Machaeroprosopus squamosals in SCSU’s vertebrate paleontology collection. Published accounts of Machaeroprosopus squamosal anatomy will be referenced to determine the degree of morphological variation found in the SCSU-P specimens. Further, I will describe the morphology of four complete squamosals with referrals to Machaeroprosopus spp. and incorporate these into a specimen-level phylogenetic analysis alongside the SCSU-P fragmentary squamosals to investigate whether species-level affinities can be established for the SCSU-P specimens. Although species-level assignment was not possible for most specimens, I identified three morphotypes among the SCSU-P squamosal specimens, suggesting that multiple phytosaur morphs were present in this paleoecosystem. Such variation could suggest subtle differences in jaw muscle organization and biomechanical function depending on species, skull size, sexual dimorphism, and/or ontogenetic stage. My results highlight the need for large-scale documentation of inter- and intraspecific variation and investigation into the relationships between morphology, function, and ecology among Machaeroprosopus.