Shape analysis of the preorbital bar in caviomorph rodents.

BECERRA, FEDERICO; VASSALLO, ALDO

Journal of Morphology

Wiley

Año: 2023

The highly specialized masticatory apparatus of rodents raises interesting 10 questions about how their skull withstands the great and sustained efforts produced 11 by biting on hard items. In these mammals, major systematics were explored for a long 12 time based on the adductor muscles’ architecture and the related bony structures. The 13 infraorbital foramen stands out, where a hypertrophied head of the 14 zygomaticomandibular muscle passes through –in hystricomorphous rodents– as a 15 direct consequence of the lateral and posterior shift of the preorbital bar. Interestingly, 16 this bar moved laterally and backwards –enlarging the foramen– but it never 17 disappeared throughout evolution, even showing morphological convergence among 18 rodents. Previous research proposed this bar as behaving mechanically similar to the 19 postorbital bar in ungulates, i.e. a safety structure against torsion stress while chewing. 20 We analysed its morphology by mathematically modelling it under bending and torsion 21 scenarios (linearly and elliptically shaped, respectively), and as for biting load 22 propagation (catenary curve). Although the preorbital bar primarily seems to be 23 shaped for withstanding torsion stress (as the postorbital bar in ungulates) and as an 24 escaping point for force propagation, these forces are not consequence of chewing 25 and grinding foods, but preventing the zygomatic arch for failing when the powerful 26 laterally-displaced jaw adductor muscles are pulling downwards.