Echoes of the past: Cotylocara & the evolution of echolocation

25 March 2014 by Travis Park, posted in Cetacea

I seem to be writing about fossil cetaceans an awful lot recently; you would almost think I’m doing a PhD on them! Just to be different then, here’s a blog post about… fossil cetaceans. This post makes it three in a row on these magnificent marine mammals. This time however the focus is on the odontocetes (the toothed whales) rather than the archaeocetes or mysticetes that I’ve been talking about a bit more recently.

Odontocetes differ from modern mysticetes by possessing teeth and also by using echolocation to find their way around and detect potential meals in the ocean depths or turbid rivers. Down there, vision isn’t much use as light levels are too low. Cetaceans also have the worst sense of smell of any mammal group so their noses aren’t going to be much help either. So what those clever toothed whales have done is evolved an auditory system where they use echolocation to make sense of the environment around them. They produce high-frequency vocalizations at the phonic lips (a constriction in the nasal passages just beneath the blowhole) and then use air sinuses and the melon (the fatty structure that gives odontocetes their bulbous foreheads) to modulate their transmission.

The skull of Cotylocara macei. The hallmarks of echolocation are present on the skull, making htis one of the earliest odontocetes that would have possessed the ability. Image by James Carew & Mitchell Colgan.

The skull of Cotylocara macei. The hallmarks of echolocation are present on the skull, making htis one of the earliest odontocetes that would have possessed the ability. Image by James Carew & Mitchell Colgan.

This is a very complex behaviour and much remains unknown about it even after many decades of research on the topic. One such unknown is when it and its underlying anatomy first evolved. Fossils to the rescue! A new species of fossil toothed whale named Cotylocara macei, from the Oligocene (~28 Ma) of South Carolina has enabled palaeontologists to gain a clearer understanding of when echolocation first appeared.

The fossil, published a couple of weeks ago in the journal Nature, consists of an almost complete skull, partial jaws, three neck vertebrae and portions of the ribs. The name, Cotylocara macei, translates as ‘cavity head’ in reference to the morphology of its skull whilst the specific name honours Mace Brown for his contributions to the founding of a Natural History Museum at the College of Charleston, South Carolina, USA.

A reconstruction of Cotylocara macei by the fantastic palaeoartist Carl Buell.

A reconstruction of Cotylocara macei by the fantastic palaeoartist Carl Buell.

But how can palaeontologists tell if a fossil odontocete was using echolocation if the structures used to produce and direct the sounds are all gone? These structures leave correlates in the skeleton that scientists can search for and therefore infer that the animal was echolocating. Cotylocara possessed several of these correlates, including: a dense, thick and downturned rostrum; fossae where the air sacs would be; asymmetry of the cranium and exceptionally broad maxillae. These features together strongly indicate that Cotylocara possessed the ability to echlocate, albeit most likely in a rudimentary form.

The authors also placed Cotylocara into a phylogenetic analysis, which helped to shed further light on the early evolution of echolocation. Cotylocara was found to belong to the earliest diverging family of odontocetes, the Xenorophidae. Based on its position within the xenorophids the authors conclude that the ability to produce sounds at the phonic lips most likely evolved in the common ancestor of all odontocetes. There is also a trend within the xenorophids that sees the frontals, maxillae and premaxillae extend backwards in the skull. This has been associated with the enlargement of the facial muscles that modulate echolocation calls.

The phylogenetic analysis from the Geisler et al paper. You can see Cotylocara places within the xenorophids. Image from Geisler et al. 2014.

The phylogenetic analysis from the Geisler et al paper. You can see Cotylocara placed within the xenorophids. Image from Geisler et al. 2014.

A still unanswered question however is whether Cotylocara was actually able to hear the high frequency sounds it was producing. This can be answered by looking at the internal structure of the inner ear bone, also known as the periotic in cetaceans. Unfortunately the periotics in Cotylocara are not well enough preserved and we will have to wait to see just what these early echolocators could hear. Like me, I’m sure you’ll be ‘listening in’ to see what else palaeontology can reveal about echolocation.

Reference

Jonathan H. Geisler, Matthew W. Colbert, James L. Carew. A new fossil species supports an early origin for toothed whale echolocation. Nature, 2014; DOI: 10.1038/nature13086


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