*I've actually written and shown images of this fossil previously on this blog (here, here, here and here).
Callistosiren is a dugongine, which is the name given to the group of seacows that are more closely related to the dugong (Dugong dugon) of the Indopacific region than to Steller's seacow (Hydrodamalis gigas) and manatees. In fact, dugongines seemed to have originated and diversified in the Western Atlantic and Caribbean region and the group was present there until the mid to late Pliocene (Domning, 2001). Although one of the oldest dugongines, Callistosiren has morphological features that groups it amongst more derived members of the group. One of these features is that the enlarged tusks (I1 in Figure 4) of Callistosiren had enamel is confined to the medial (inner) surface, while the outside consist only of dentine. The result of this is that the lower edge of the tusks would wear off unevenly, forming a self-sharpening edge. This is something we also see in other dugongines such as some Dioplotherium and would presumably have been advantageous when cutting and uprooting seagrasses.
|Figure 4. Dorsal, ventral and right lateral views of skull of Callistosiren boriquensis (modified from Velez-Juarbe and Domning, 2015:figs.1-3).|
|Figure 5. Comparison between vertebrae (top) and ribs (bottom) of Callistosiren boriquensis (left) and Priscosiren atlantica (right). (Click on image to see larger version.)|
A lightweight among sirenians
When I first encountered the ribs and vertebrae of Callistosiren I was surprised by how skinny they were relative to those of other similarly sized sirenians such as Priscosiren (Figure 5). You see, sirenian bones are usually pachyosteosclerotic which means that they are dense and thickened (Domning and Buffénil, 1991). This is an adaptation that evolved very early in the evolutionary history of the group, with pachyostosis (thickened) and lightly osteosclerotic (dense) bones already present in one of the oldest sirenians, the middle Eocene Pezosiren portelli, and apparently becoming fully pachyosteosclerotic by the late Eocene (Buffrénil et al., 2010). This adaptation helps sirenians achieve neutral buoyancy and move in the water column with minimal effort, functioning in a similar fashion as a divers weight belt. There are few exceptions where sirenian bones deviate from this condition. One of these are the protosirenids, an extinct group of early sirenians, Callistosiren and extant dugong. One possible explanation for the lack of pachyostosis in dugong is that this species occasionally dives to depths greater than 10 meters, which is around when lungs begin to collapse, thus reducing buoyancy (Domning and Buffrénil, 1991). This may have been the case in Callistosiren, whose vertebrae and ribs are osteosclerotic, but not pachyostotic. Interestingly, Domning (2001) predicted the discovery of Caribbean sirenians with reduced ballast as another strategy for niche partitioning in sirenian multispecies communities and has now become true with the discovery of Callistosiren.
Buffrénil, V. de, A. Canoville, R. D'Anastasio, and D. P. Domning. 2010. Evolution of sirenian pachyosteosclerosis, a model-case for the study of bone structure in aquatic tetrapods. Journal of Mammalian Evolution 17:101-120.
Cope, E. D. 1890. The extinct Sirenia. American Naturalist 24:697-702.
Domning, D. P. 2001. Sirenians, seagrasses, and Cenozoic ecological change in the Caribbean. Palaeogeography, Palaeoclimatology, Palaeoecology 166:27-50.
Domning, D. P. and V. de Buffrénil. 1991. Hydrostasis in the Sirenia: quantitative data and functional interpretations. Marine Mammal Science 7:331-368.
Velez-Juarbe, J., and D. P. Domning. 2014. Fossil Sirenia of the West Atlantic and Caribbean region. XI. Priscosiren atlantica, gen. et sp. nov. Journal of Vertebrate Paleontology 34:951-964.
Velez-Juarbe, J., and D. P. Domning. 2015. Fossil Sirenia of the West Atlantic and Caribbean region. XI. Callistosiren boriquensis, gen. et sp. nov. Journal of Vertebrate Paleontology e885034