Skull mechanics and functional morphology of Brasilodontidae, the sister clade to mammals

SALCIDO, CHARLES; RAYFIELD, EMILY; GILL, PAMELA; MARINA BENTO SOARES; AGUSTIN MARTINELLI

Congreso; 78th Annual Meeting, Society of Vertebrate Paleontology,; 2018

Triassic vertebrate assemblages of the Santa Maria Supersequence in Brazil have yieldedmany specimens of cynodont therapsids including members of the clade Brasilodontidae.Recent phylogenetic analyses place this clade as the sister clade to Mammaliaformes.While these specimens have been described in detail, little work has been done to quantifythe skull mechanics. The skulls of the brasilodontids Brasilodon, Brasilitherium, andMinicynodon are analyzed to compare their functional morphology and biomechanics tothe Early Jurassic mammaliaforms Morganucodon and Kuehneotherium, and anysimilarities could be interpreted in light of the ontogenetic status of the comparablebrasilodontids. We here compare the results of biomechanical and finite element analysis(FEA) of the mandibles of Minicynodon and Brasilodon, both to each other and to those ofMorganucodon and Kuehneotherium from a previous study.Specimens were μCT scanned, segmented in Avizo 9.4.0, meshed in Hypermesh to preparefor FEA, and then imported into Abaqus for testing. Biomechanical analysis includes beamanalyses such as second moment of area, polar moment of inertia, and section modulusresults that were obtained from Moment Macro in ImageJ. Although the overall trends arethe similar in Minicynodon and Brasilodon, the latter has higher resistance to torsion andbending strength than Minicynodon. Also, for Brasilodon, the posterior end has a nearconstant resistance to torsion, with a sudden drop between pc4 and pc3. These differencesmay support an ontogenetic relationship and niche partitioning between the different agegroups. The polar moment of inertia results also show that the mandibles of thebrasilodontids are able to resist torsion in a similar manner to Morganucodon and mayindicate a similar mode of feeding. During segmentation, the postdentary bones of Brasilodon and Minicynodon were found to be preserved and are more similar to those of Morganucodon (thin and rod-like) than other nonmammalian cynodonts (e.g., Probainognathus jenseni and Riograndia guiabensis). This further supports the close connection between brasilodontids and mammaliaforms, displaying a transitional stage in the evolution of the mammalian jaw joint and middle ear right before the mammaliaform condition.