Iterative Evolution of Sympatric Seacow (Dugongidae, Sirenia) Assemblages during the Past ∼26 Million Years

Extant sirenians show allopatric distributions throughout most of their range. However, their fossil record shows evidence of multispecies communities throughout most of the past ∼26 million years, in different oceanic basins. Morphological differences among co-occurring sirenian taxa suggest that resource partitioning played a role in structuring these communities. We examined body size and ecomorphological differences (e.g., rostral deflection and tusk morphology) among sirenian assemblages from the late Oligocene of Florida, early Miocene of India and early Pliocene of Mexico; each with three species of the family Dugongidae. Although overlapping in several ecomorphological traits, each assemblage showed at least one dominant trait in which coexisting species differed. Fossil sirenian occurrences occasionally are monotypic, but the assemblages analyzed herein show iterative evolution of multispecies communities, a phenomenon unparalleled in extant sirenian ecology. As primary consumers of seagrasses, these communities likely had a strong impact on past seagrass ecology and diversity, although the sparse fossil record of seagrasses limits direct comparisons. Nonetheless, our results provide robust support for previous suggestions that some sirenians in these extinct assemblages served as keystone species, controlling the dominance of climax seagrass species, permitting more taxonomically diverse seagrass beds (and sirenian communities) than many of those observed today

Evolution and biogeography of seagrasses

© Springer International Publishing AG, part of Springer Nature 2018. Seagrasses are an organismal biological group united by their ability to grow in marine environments. As marine flowering plants they have evolved a combined suite of adaptations multiple times enabling the four known lineages containing species of seagrass to survive, and thrive, in the sea. Unlike many other biological groups of plants however, seagrasses are all derived from a single order of flowering plants, the Alismatales. This order, being derived early in the evolution of the monocotyledons, is comprised predominantly of aquatic plants, of all forms- emergent, submerged, freshwater, estuarine and marine. A review of seagrass fossils suggests that new discoveries of seagrass fossils along with confirmation of some earlier finds lead to a clear signal that some seagrass species had a wider distribution in the past compared with today. The discovery of new fossil sites should be encouraged as this will likely produce important valuable information on the evolution of this group. In general the biogeography of seagrasses suggests that these organisms evolved successfully in the Tethys Sea of the Late Cretaceous. However, the modern division into two groups, temperate and tropical tends to suggest that at some point an ecological separation occurred in both the Northern and Southern Hemispheres. There are a disproportionately large number of temperate seagrass species in southern Australia and there is significant endemism shown in Posidonia, Amphibolis and a unique species of Halophila (H. australis). The use of genetic and genomic techniques has begun to explain these distributions but we can expect a much bigger picture to emerge in the near future