Phylogenetic relationships between Monotremata and Monotremaformes: parallelism with appendages and habits of aquatic moles

For a long time, Monotremata included Mesozoic taxa such as Steropodon and Teinolophos in addition to Cenozoic species. However, fossil discoveries from the Cretaceous revealed a major difference between Cenozoic monotremes and their ancestors, with Cenozoic monotremes being more closely related to each other than to earlier forms, thus forming the crown group of Monotremata described by Chimento et al. (2023). Among living mammals, moles have several morphofunctional parallels with monotremes. These parallels may reflect similar evolutionary trajectories. Our objectives were: to find additional morphological characters to differentiate Monotremata from their ancestors, and to trace possible evolutionary parallels between them and Talpidae. To this end, specimens of monotremes and talpids stored in the collection of the Museu Nacional-UFRJ were evaluated, as well as bibliographical research on the evolution, morphology and fossil record of these groups. Among the non-dental morphological characteristics identified here are the presence of a lateral projection in the anterior portion of the mandible that extends beyond the mandibular symphysis, forming the bony support of the “beak” in living monotremes and the presence of a reinforced shoulder girdle, with several hypertrophied bones and joints not seen in other tetrapods. The swimming method of platypuses, which involves a series of alternating beats of the front limbs, is very similar to that of moles when digging or swimming. Thus, it is possible that, like semi-aquatic moles, monotremes descended from an ancestor with a specialized fossorial lifestyle.