Mosaic postcranial evolution and aquatic adaptation in the Miocene otter Tyrrhenolutra: Insights from morpho-functional analysis.

Otters are characterized by great adaptability, displaying a wide geographic distribution and being a common presence in all continents except for areas such as Australia and some remote islands. Otters inhabit a wide variety of environments, ranging from terrestrial and freshwater ecosystems to the exclusively aquatic habitat of the sea otter (Enhydra lutris). For this reason, otters provide interesting insights on the evolution of adaptations to aquatic lifestyle in living mammals. The taxonomic identification of the material housed at the Naturhistorisches Museum of Basel allowed us to identify abundant and undescribed postcranial material attributable to Tyrrhenolutra, including well-preserved ulna, radius, and calcaneus. Tyrrhenolutra helbingi is a poorly known Tortonian otter, described on some dental remains from the Baccinello mines and a few other localities. This study employs three-dimensional (3D) landmark-based geometric morphometrics to quantify the relative influences of phylogeny, allometry, and locomotor performance on the fore- and hindlimb morphology of Tyrrhenolutra in comparison to a dataset of 32 different species of extant Musteloidea. Our results reveal morphological affinities with semiaquatic mustelids and, to a lesser extent, semifossorial taxa. The forelimb morphology suggests that Tyrrhenolutra possessed advanced swimming capabilities, being capable of rapid aquatic maneuvering, aligning with modern piscivorous species. Conversely, the hind limb of Tyrrhenolutra retains a more basal morphology, suggesting a lack of the specialized aquatic propulsion seen in extant lutrines. This different specialization between forelimb and hindlimb observed in Tyrrhenolutra but also in other Neogene otters suggests that the transition to an aquatic lifestyle in early otters occurred through a mosaic pattern, with distinct traits originating independently at different times.