Evolutionary transformations of the ossified Meckelian cartilage in mammaliaforms

Developmental and fossil evidence shows that the mammalian Meckelian cartilage plays a transient but crucial role in forming both the mandible and the middle ear. However, despite its broad occurrence as the Meckelian groove or ossified Meckelian cartilage in Mesozoic mammaliaforms, the evolutionary trajectory of the ossified Meckelian cartilage remains poorly understood. Here, we report the middle ear of the Early Cretaceous eutherian Cokotherium and integrate paleontological and developmental data to reconstruct the evolution of the Meckelian cartilage. Fossil evidence from the Early Cretaceous eutherian Cokotherium reveals a gracile, posteriorly confined ossified Meckelian cartilage with no substantial connection to the middle ear. We propose two evolutionary trajectories of the reduction of the Meckelian cartilage in early mammaliaforms, featured by different morphological configurations of the Meckelian cartilage and the middle ear bones. Comparisons with developmental data in extant mammals suggest that this evolutionary sequence mirrors the ontogenetic fate of the Meckelian cartilage. The ossified Meckelian cartilage likely originated in response to mandibular miniaturization during the origination of mammaliaforms, functioning as a temporary structural stabilizer linking the middle ear and mandible. The major morphological transformations of the ossified Meckelian cartilage thus reflect a stepwise pattern toward ear detachment over phylogeny.