Its the 11th installment of Fossil Sirenia of the West Atlantic and Caribbean Region!

Today sees the publication of the most recent addition to the series on fossil sirenians from the Western Atlantic and Caribbean (WAC) region (Velez-Juarbe and Domning, 2015). In our new paper we describe another new taxon from Puerto Rico (in July 2014 we described Priscosiren atlantica Velez-Juarbe and Domning, 2014). The material we describe in this new paper includes cranial and postcranial material from several individuals (Figures 1-5) and collected from several adjacent localities of the late Oligocene Lares Limestone in northwestern Puerto Rico*. We dubbed this new taxon Callistosiren boriquensis which translates into "Boriquen's most beautiful sirenian". Boriquén is the aboriginal name for the island of Puerto Rico, and "most beautiful" is in reference to the superb preservation of the type material (Figure 4).
*I've actually written and shown images of this fossil previously on this blog (here, here, here and here).

Figure 1. Callistosiren boriquensis is known from multiple elements of about five individuals (each color represents elements known from one or more specimens; white = unknown). Outline of skeleton modified from Cope (1890).
(Click on the image to see larger version.)

The material we described in our paper includes two skulls, which actually account for the first and second sirenian skulls I found and collected, one in 2003, the other in 2005! The rest of the material we described which consists of ribs and vertebrae, was collected during an NSF-funded trip in 2009 with co-author Daryl Domning and with the help of my undergraduate advisor Hernán Santos and my colleagues Alvin Bonilla and Diana Ortega.

Figure 2. Left, me at the type locality during the first day (April 9, 2005) digging around the holotype skull (USNM 540765) (photo by MPT); top right, the skull (USNM 540765) during the second day (April 10, 2005) and ready to be jacketed; bottom right, early stages of preparation of the holotype skull (September 7, 2005).

Figure 3. Top, some of the postcranial material referred to Callistosiren boriquensis prior to being collected, including the third lumbar (L3), sacral (S1) caudal (Ca1-4) vertebrae, and two chevrons (Ch). Below, my co-author Daryl Domning collecting other postcranial elements at type locality. Collected in June 2009.

What is Callistosiren?
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.


References

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