Rise of the Cetacea: Part II – The Ambulocetids

28 January 2014 by Travis Park, posted in Cetacea, Rise of the Cetacea

After a bit of a blogging hiatus it feels good to be back writing again. I thought I would get back into the swing of things by giving you the next post in my series on the evolution of whales, Rise of the Cetacea. In the previous post we met the earliest known cetacean group, the pakicetids. This time, we’ll be looking at another group of early semiaquatic whales, the ambulocetids.

The family is known from the middle Eocene (~45 Ma) of India and Pakistan (Uhen 2010) and contains three known genera, the relatively fragmentary Gandakasia and Himalayacetus and the better known Ambulocetus, from which the majority of information on this group is derived.

This very simplified cladogram shows the position of the ambulocetids relative to other cetaceans.

This very simplified cladogram shows the position of the ambulocetids relative to other cetaceans.

The ambulocetids were larger than the pakicetids; in particular the head and mandible are massive compared to that of more primitive cetaceans such as Pakicetus. The sacrum of Ambulocetus was the same as that of Pakicetus, possessing four fused vertebrae (Madar et al. 2002). Ambulocetus also shows features that indicate that these early cetaceans were becoming more adapted to spending time in the water, possessing an enlarged mandibular foramen similar in size to that found in modern odontocetes. This suggests that a fat pad was present inside the foramen which would have been used for sound transmission to the middle ear bones. However the lateral mandibular wall remained relatively thick suggesting that these animals could still only hear lower frequency sounds (in modern odontocetes, who have high frequency hearing, the lateral mandibular wall can be less than a millimetre thick). Despite the potential presence of a fat pad the unique contact between the tympanic bulla and the mandible suggests that instead Ambulocetus may have used bone conduction to detect sounds, both on land and in the water (Nummela et al. 2007).

This is a reconstruction of the skeleton of Ambulocetus in the Tokyo Museum. Image taken by Dave Hone. To check out Dave's fantastic palaeo blog, Archosaur Musings, just follow the link.

This is a reconstruction of the skeleton of Ambulocetus in the Tokyo Museum. Image taken by Dave Hone. To check out Dave's fantastic palaeo blog, Archosaur Musings, just follow the link.

In terms of their feeding the species in the group differ. Based on isotope data Ambulocetus and Gandakasia ate mostly terrestrial prey whereas Himalayacetus possessed a marine diet despite still drinking freshwater. The potential use of bone conduction, the placement of the orbits high on the skull and the general morphology of the skull have led to some workers postulating a crocodilian like hunting process for ambulocetids (Thewissen et al. 1996).

This reconstruction of Ambulocetus by the fantastic palaeoartist Carl Buell shows just how much these animals really did resemble furry crocodiles! Image from Gatesy et al. 2013.

This reconstruction of Ambulocetus by the fantastic palaeoartist Carl Buell shows just how much these animals really did resemble furry crocodiles! Image from Gatesy et al. 2013.

Cetaceans had taken their first steps to becoming the animals we know and love today. Yet at this stage in their evolution they resembled furry crocodiles more than modern whales! We need to explore further along the evolutionary path to see these changes and indeed we shall in the forthcoming posts in this series.

References

Gatesy, J., Geisler, J.H., Chang, J., Buell, C., Berta, A., Meredith, R.W., Springer, M.S. & McGowen, M.R., 2013. A phylogenetic blueprint for a modern whale. Molecular Phylogenetics and Evolution 66, 479–506.

Madar SI, Thewissen JGM, Hussain ST. 2002. Additional holotype remains of Ambulocetus natans (Cetacea, Ambulocetidae), and their implications for locomotion in early whales. J. Vertebr. Paleontol. 22(2):405–22

Nummela, S., Thewissen, J.G.M., Bajpai, S., Hussain, T., Kumar, K., 2007. Sound transmission in archaic and modern whales: anatomical adaptations for underwater hearing. Anatomical Record 290, 716–733.

Thewissen JGM, Madar SI, Hussain ST. 1996. Ambulocetus natans, an Eocene cetacean (Mammalia) from Pakistan. Cour Forsch-Inst
Senckenberg 191:1–86.

Uhen, M.D., 2010. The origin(s) of whales. Annual Review of Earth and Planetary Sciences 38, 189–219.

 


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