Cranial anatomy and phylogenetic affinities of Bolosaurus major, with new information on the unique bolosaurid feeding apparatus and evolution of the impedance-matching ear

Resolving the phylogenetic relationships of early amniotes, in particular stem reptiles, remains a difficult problem. Three-dimensional morphological analysis of well-preserved stem-reptile specimens can reveal important anatomical data and clarify regions of phylogeny. Here, we present the first thorough description of the unusual early Permian stem reptile Bolosaurus major, including the first comprehensive description of a bolosaurid braincase. We describe previously obscured details of the palate, allowing for insight into bolosaurid feeding mechanics. Aspects of the rostrum, palate, mandible, and neurocranium suggest that B. major had a particularly strong bite. We furthermore found B. major has a surprisingly slender stapes, similar to that of the middle Permian stem reptile Macroleter poezicus, which may suggest enhanced hearing abilities compared to other Paleozoic amniotes (e.g., captorhinids). We incorporated our new anatomical information into a large phylogenetic matrix (150 OTUs, 590 characters) to explore the relationship of Bolosauridae among stem reptiles. Our analyses generally recovered a polyphyletic ‘Parareptilia,’ and found Bolosauridae to diverge from the reptile stem after Captorhinidae + Araeoscelidia. We also included Bolosaurus major within a smaller matrix (10 OTUs, 27 characters) designed to explore the interrelationships of Bolosauridae and found all species of Bolosaurus to be monophyletic. While reptile relationships still require further investigation, our phylogeny suggests repeated evolution of impedance-matching ears in Paleozoic stem reptiles.

Methods
Two datasets were analyzed using phylogenetic methods (i.e., parsimony and Bayesian). See publication for more specific methods, citation below:

K.M. Jenkins, W. Foster, J.G. Napoli, D.L. Meyer, G.S. Bever, B-A.S. Bhullar. In Review. Cranial anatomy and phylogenetic affinities of Bolosaurus major, with new information on the unique bolosaurid feeding apparatus and evolution of the impedance-matching ear. The Anatomical Record. DOI: https://doi.org/10.1002/ar.25546

1) The first dataset analyzed is an original taxon-character matrix (select "Download full dataset" to view). Supplemental to this are two additional files (1) an illustrated character list ("StemReptile_Megamatrix_IllustratedCharacterList_2024"), and (2) the list of taxa scored ("Taxa_Examined_2024"). The trees produced from this dataset include parsimony ("All_Parsimony_tre_files") and Bayesian ("Bayes_150_FBD") results. The input file for the Bayesian trees is "Full_Matrix_Bayes_150.nex".

2) The second dataset is a smaller, previously published and edited dataset focused on bolosaurid taxa. The taxon character matrix for this dataset is "BoloMatrix_Falconnect_edited" and trees produced are shown within the text (Figure 16).