Domningia and other Indian sirenians

Over the last 12 years a number of fossil sirenians have been described from Eocene through Miocene deposits from India. This has not stopped as, new taxa are been discovered and described; much of the effort is spearheaded by Sunil Bajpai of the Dept. of Earth Sciences, Indian Institute of Technology and J. G. M. Thewissen of the Dept. of Anatomy and Neurobiology, NEOUCOM (Thewissen Lab). So far all the fossil sirenians from India have been collected from outcrops in the Kachchh District near the western coast of the country; this area is better known for primitive cetaceans such as remingtonocetids (Kumar & Sahni, 1986).

Eocene

The oldest Indian sirenians come from the Middle Eocene Harudi Formation; a total of three species representing two families: Protosiren sp. (Protosirenidae), Eotheroides babiae, Eosiren sp. (Dugongidae, Halitheriinae) (Bajpai et al. 2006). Other species belonging to those genera are found in Eocene deposits in northern Africa (Domning, 1996) indicating a southern Tethyan influence, better referred to as the Indo-African Region of the Tethys Realm (Harzhauser et al. 2002). There should be more on Eocene sirenians from India, sometime this year.

Oligocene

So far only one Oligocene sirenian is known from this region, Bharatisiren indica (Dugongidae, Dugonginae) from the Maniyara Fort Formation of Late Oligocene age (Bajpai et al. 2006). This is the oldest dugongine found in this region, and it is as old as the dugongines, Crenatosiren olseni and Dioplotherium manigualti from the Western Atlantic (Domning, 1989; Domning, 1997) and a new taxon from Puerto Rico. Dugongines are a group that is thought to have evolved in the Western Atlantic and Caribbean Region (Domning, 2001), the occurrence of B. indica seems to point to a latest Eocene or Early Oligocene origin for the group with subsequent trans-Atlantic dispersal to the Indo-African Region. An alternative scenario, is the origin of dugongines in the Mediterranean region of the Tethys Realm with subsequent east and west dispersal, unfortunately no Early or Late Oligocene dugongines are known from this region (the names of the paleobiogeographic areas based on Harzhauser et al. 2002).

Lateral view of the skull of B. indica from Bajpai et al. (2006).

Miocene

During the Early Miocene there seems to have been a radiation of dugongines in this region. With at least two taxa known Bharatisiren kachchhense, Domningia sodhae* from Khari Nadi Formation (Bajpai & Domning, 1997; Thewissen & Bajpai, 2009), and another currently under study, it is an good example of multispecies communities like the ones present in the Western Atlantic and Caribbean Region (Domning, 2001). As you can see in the composite picture below, a notable difference between B. kachchhense and Domningia sodhae is the rostral deflection, which probably reflects different degrees of specializations for bottom feeding (Domning, 2001). They seem to have had similar shape of their tusks, large and more-or-less oval in cross-section, this morphology most likely aided in obtaining large seagrass rhizomes (Domning, 2001; Domning & Beatty, 2007), although probably at different levels due to the differences in rostral deflection. Interestingly, it seems that there were no post-Eocene halitheriines.

*Domningia sodhae was aptly named after Dr. Daryl P. Domning of Howard University, world renowned paleosirenologist, a well deserved recognition.

Lateral view of B. kachchhense (top; from Bajpai & Domning, 1997) and Domningia sodhae (bottom; image reversed, from Thewissen & Bajpai, 2009).

Where are the halitheriines?

So far no post-Eocene halitheriines are known from the Oligocene and Miocene of India, at least two possible explanations come to mind: (1) they were present but no fossils have been found yet, or; (2) they were totally absent. An explanation for the second alternative could be provided by the invertebrate fauna. Based on gastropod fauna it seems that, during the Oligocene, changes in the geological settings in the Tethys Realm led to changes in the ocean currents and what was previously known as the Indo-African Region was subsequently divided into the Mediterranean-Iranian and Western Indian-Eastern African provinces of the Western Tethyan Region (Harzhauser et al. 2002). Subsequently, during the Early Miocene, further fragmentation of the Western Indian-Eastern African Province led to an increase of South-East Asian influence and the formation of the Proto-Western Indian Ocean Province; this separation was further augmented by the closure of the Eastern Mediterranean seaway during the latest Early Miocene (Harzhauser et al. 2002). The progressive isolation of what would become the Proto-Western Indian Ocean Province from a Tethyan influence might have prevented halitheriines from entering the region, especially after the Late Oligocene, leading to specialization and radiation of dugongines in this part of the world.

 References

Bajpai, S. & D. P. Domning. 1997. A new dugongine sirenian from the Early Miocene of India. Journal of Vertebrate Paleontology 17(1):219-228.

Bajpai, S., J. G. M. Thewissen, V. V. Kapur, B. N. Tiwari & A. Sahni. 2006. Eocene and Oligocene sirenians (Mammalia) from Kachchh, India. Journal of Vertebrate Paleontology 26(2):400-410.

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. 1996. Bibliography and index of the Sirenia and Desmostylia. Smithsonian Contributions to Paleobiology 80:1-611.

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

Harzhauser, M., W. E. Piller & F. F. Steininger. 2002. Circum-Mediterranean Oligo-Miocene biogeographic evolution – the gastropods’ point of view. Palaeogeography, Palaeoclimatology, Palaeoecology 183:103-133.

Kumar, K. & A. Sahni. 1986. Remingtonocetus harudiensis, new combination, a Middle Eocene Archeocete (Mammalia, Cetacea) from western Kutch, India. Journal of Vertebrate Paleontology 6(4):326-349.

Thewissen, J. G. M. & S. Bajpai. 2009. A new Miocene sirenian from Kutch, India. Acta Palaeontologica Polonica 54(1):7-13.