Sirenian diversity in the past

Its been quiet here for a while as I’ve been busy working on the preparation of two sirenians skull, as well as getting ready for the upcoming field season.

It’s also been a while since I wrote something about sirenians so, here it goes.

Living sirenians can be divided into two families, Trichechidae (manatees) and Dugongidae (dugongs). Most people are probably more familiar with the manatees, after all, there are three species, West Indian, Amazonian and African, whereas there is only one species of dugong. The geographic distribution of extant sirenians is such that there is mostly no overlap between the different species. As the only living herbivorous marine mammals, it might be that by living in separate regions it reduced the chances of competing for the same resources (i.e. seagrasses). But what about in the past, what does the fossil record of sirenian tells us about their paleoecology.

When we look at the fossil record, sirenians were much more speciose, including multispecies communities in some regions (Domning, 2001). Now lets look at one good example.

The Late Oligocene of Florida

The Late Oligocene sirenian fauna of Florida includes at least three species of dugongids*. The dugongines, Crenatosiren olseni and Dioplotherium manigaulti, and the halitheriine Metaxytherium sp. (Domning, 1989, 1997, 2001). (See illustration below).

*The family Dugongidae includes three subfamilies: Dugonginae, Halitheriinae & Hydrodamalinae.

Illustration of known Late Oligocene sirenians from Florida (all at the same scale). Top, Crenatosiren olseni (modified from Domning, 1997); middle, Dioplotherium manigaulti (from Domning, 1989); bottom, Metaxytherium sp. (this last drawing based on a very similar skull from Puerto Rico, tusks not preserved, but presumed to be small as in the Fl specimen). The numbers in the circles are the degrees of rostral deflection. Mandibles absent in the middle and bottom specimens.

These three species, as you can see, differ in size, and to a lesser degree in rostral deflection. Also different from each other is the size of their tusks, increasing in size from Metaxytherium - C. olseni - Dioplotherium manigaulti. Taken as a whole, these differences (specially tusks size) could be indicators of different feeding habits, with small-tusked sirenians feeding of small rhizomes* and large-tusked sirenians feeding on larger ones (Domning, 2001; Domning & Beatty, 2007). Dugongids most likely used their tusks as a tool to dig out the rhizomes, with the most extreme specialization observed in the dugongines, including very large blade-like tusks as well as cranial adaptations that seemed to have help withstand the forces exerted when digging (Domning & Beatty, 2007).

*Rhizomes = the nutrient-rich, underground stems of seagrasses.

Other examples of sirenian multispecies communities are found in the Early Oligocene of Puerto Rico and the Early Miocene of India, among others (more on this sometime in the future). In addition, in the Pacific, sirenians were not the only herbivorous marine mammals. In the northern Pacific region, sirenians seem to have shared their resources with the desmostylians (see picture below), an interesting (and bizarre) group of mammals that lived from the Oligocene to the Miocene and were presumably feeding and spending time in the marine realm (Domning et al., 1986; Inuzuka et al., 1994). Whereas, in the southeastern Pacific, fossils of aquatic sloths (Thalassocnus spp.) have been found in the same formations as sirenians (Muizon & McDonald, 1995; Canto et al., 2008; Muizon & Domning, 1985; Bianucci et al., 2006; Domning & Aguilera, 2008).

Mounted cast of Palaeoparadoxia tabatai taken at the AMNH.

So, why is it so different in modern times, why do we see such a reduced diversity of sirenians and/or lack of any other herbivorous marine mammals? There has been, apparently, little change in the marine seagrass communities since the Eocene, so what happened? The answers for these and other questions could be answered with more fossils and more research. For now, we can certainly say that, like their close relatives, the proboscideans (elephants), sirenians are the last remnants of a once much more diverse group of animals.

References

Bianucci, G., S. Sorbi, M. E. Suárez & W. Landini. 2006. The southernmost sirenian record in the eastern Pacific Ocean, from the Late Miocene of Chile. Comptes Rendus Palevol 5:945-952.

Canto, J., R. Salas-Gismondi, M. Cozzuol & J. Yáñez. 2008. The aquatic sloth Thalassocnus (Mammalia, Xenarthra) from the Late Miocene of north-central Chile: biogeographic and ecological implications. Journal of Vertebrate Paleontology 28(3):918-922.

Domning, D. P. 1989. Fossil Sirenia of the West Atlantic and Caribbean region. II. Dioplotherium manigaulti Cope, 1883. Journal of Vertebrate Paleontology 9:415-428.

Domning, D. P. 1997. Fossil Sirenia of the West Atlantic and Caribbean region. VI. Crenatosiren olseni (Reinhart, 1976). Journal of Vertebrate Paleontology 17:397-412.

Domning, D. P. 2001. Sirenians, seagrasses, and Cenozoic ecological change in the Caribbean. Palaeogeography, Palaeoclimatology, Palaeoecology 166:27-50.

Domning, D. P. & O. A. Aguilera. 2008. Fossil Sirenia of the West Atlantic and Caribbean region. VIII. Nanosiren garciae, gen. et sp. nov. and Nanosiren sanchezi, sp. nov. Journal of Vertebrate Paleontology 28:479-500.

Domning, D. P. & B. L. Beatty. 2007. Use of tusks in feeding by dugongid sirenians: observations and tests of hypotheses. Anatomical Record 290:523-538.

Domning, D. P., C. E. Ray & M. C. Mckenna. 1986. Two new Oligocene desmostylians and a discussion of Tethytherian systematics. Smithsonian Contributions to Paleobiology 59:1-56.

Inuzuka, N., D. P. Domning & C. E. Ray. 1994. Summary of taxa and morphological adaptations of the Desmostylia. Island Arc 3(4):522-537.

Muizon, C. de & D. P. Domning. 1985. The first records of fossil sirenians in the southeastern Pacific Ocean. Bulletin du Muséum National d’Histoire Naturelle (Paris) (4)7, Sect. C, no. 3:189-213.

Muizon, C. de & H. G. McDonald. 1995. An aquatic sloth from the Pliocene of Perú. Nature 375:224-227.