SPATIAL AND TEMPORAL DISTRIBUTION OF DESMOSTYLIA (MAMMALIA) AND DESMOSTYLIAN EVOLUTION RELATED TO OTHER MARINE MAMMALS

Desmostylia is one clade of marine mammals that inhabited the North Pacific Rim from Oligocene to Miocene. It has many unique morphologies not seen in other mammals and has no extant relatives, and the ancestral animals have not been clarified to date. Therefore, much of their evolution/extinction process has not been clarified thus far. One aspect of desmostylian paleoecological study that has been inferred based on stratigraphic and geographic distributions of their fossil record is their habitat and temporal changes. Previous studies suggested that each desmostylian taxon occupied its own specific habitat because multiple desmostylian taxa did not co-occur in a single locality. Beyond that, however, little was known about their paleoecology based on their spatial and temporal distribution．Recently, by analyzing a very rich set of databases, including PalroDB, we have been able to become available to use more data than ever, making analyses of spatial and temporal distributions of various desmostylians a plausible project.

For this study, I compiled all the reported information on desmostylian fossil localities and investigated the relationship between distribution changes and paleoenvironmental changes. Based on these data, I determined the appearance and extinction times of Desmostylidae and Paleoparadoxiidae and their contributory factors.

There was a competitive relationship in the Desmostylidae, which is evident after the extinction of other marine mammals. Desmostylidae expanded their habitat after the extinction of the toothed Mysticeti, and Odobeninae diversified after Desmostylidae’s extinction. Therefore, the marine succession feeder changed from toothed baleen whales to Desmostylidae and Odobeninae. However, there was no rapid diversification of the cardinals after the extinction of Paleoparadoxiiidae, which is considered herbivorous. Therefore, it is likely that niche exchange did not take place in herbivores.