Ontogeny of the temporomandibular joint and middle ear of the Virginia opossum (Didelphis virginiana) and their implications for mammalian cranial evolution
Abstract
[EMBARGOED UNTIL 05/01/2026] The evolution of the modern mammalian middle ear from the primitive jaw joint is a remarkable example of vertebrate evolutionary history. We have a deep understanding of how the bones of the ear and jaw have evolved from the fossil record, however the knowledge surrounding the evolutionary origins of soft tissues of the temporomandibular joint (TMJ), such as the pterygoideus muscle, articular disc, and discomalleolar ligament (DML), is limited. Previous hypotheses have suggested that the disc and DML are vestiges of the lateral pterygoideus muscle becuase the malleus is its primitive attachment, but these remain untested. Ontogeny of the Virginia opossum (Didelphis virginiana) TMJ resembles the evolutionary trajectory of mammalian jaw and ear bones, making it a useful model to study soft tissue development. We explored the soft tissue connections between the TMJ, middle ear, and jaw musculature in a neonate, weanling, and adult opossum using high-resolution DiceCT. In the adult, we identified that the TMJ articular disc is continuous with the lateral pterygoideus rostrally and the DML caudally. The DML continues through the petrotympanic fissure to attach to the malleus. Similar connections are identifiable in the neonatal and weanling specimens, suggesting evolutionary and developmental connectivity of the pterygoideus, disc, DML, and malleus. These results further the understanding of opossum ontogeny and contribute to the overall understanding of mammalian cranial evolution.
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M.S.