Champsosaurus: CT scanning reveals its ear openings really were on the bottom of its skull

At Ottawa’s Alta Vista Animal Hospital, when an animal gets a CT scan it’s usually a cat or dog and the vet is looking for broken bones or tumours.

But I brought a Champsosaurus, and I wanted a CT scan to look for its ears.

Let me explain. The up to two-meter long Champsosaurus was a genus of aquatic reptile that lived from 90 to 55 million years ago. Champsosaurus resembled modern crocodiles, but is only distantly related to them, and has no descendants alive today.

A mounted skeleton of Champsosaurus natator.

A large specimen of Champsosaurus natator, a species that lived in western North America from about 70 to 66 million years ago. On display in the Fossil Gallery at the museum. Catalogue number: CMNFV 8919. Image: Thomas Dudgeon, © Canadian Museum of Nature.

So, what we know about Champsosaurus comes from its fossil remains, and this leads to the ear problem.

Champsosaurus had a relatively delicate skull, with a long and slender snout and large arches of bone at the back of the skull that anchored jaw muscles. The skull was so fragile that few are well preserved, and thus little is known about the finer internal structures such as the braincase and the inner ear.

The skull of Champsosaurus

Top view of a well-preserved Champsosaurus skull in the Canadian Museum of Nature collection. Fun fact: Scientists call heart-shaped objects cordiform. Tilt your head to the right and see how the long snout and large arches of bone give Champsosaurus a heart-shaped skull. Catalogue number: CMNFV 8920. Image: Thomas Dudgeon, © Canadian Museum of Nature.

However, in the 1950s, museum palaeontologist L.S. Russell suggested that holes on the bottom surface of the Champsosaurus skulls were the ear openings. As you can imagine, this is a strange place for ear openings to be, and not all palaeontologists were convinced. Although some animals, like a few salamander species, have ear openings on the bottom of their skulls, most animals, including humans of course, have ears on the sides of their skulls.

So, as part of my Master’s thesis in Carleton University’s Department of Earth Sciences, I decided to tackle the mystery of whether the Champsosaurus’ ears really were on the bottom of its skull.

Fortunately, there are two well-preserved skulls of Champsosaurus in the Canadian Museum of Nature’s palaeo collection.

A Champsosaurus skull on the bed of a large CT scanner.

A Champsosaurus skull ready to go through a CT scanner at Ottawa’s Alta Vista Animal Hospital. Catalogue number: CMNFV 8919. Image: Thomas Dudgeon, © Canadian Museum of Nature.

I carefully transported one skull to the Alta Vista Animal Hospital for a much overdue check-up, and the other was sent to the University of Texas CT facility for scanning. CT scanning is a medical technique that uses X-rays to create hundreds of thin cross-sectional images, providing a detailed view of the internal structures.

When I examined the CT scans of the Champsosaurus skulls, to my surprise and excitement, I discovered that the openings on the bottom of the skull do indeed lead to the inner ear.

Schematic drawing of a Champsosaurus skull

A line drawing of a Champsosaurus skull in bottom-view, ear openings highlighted in blue. Catalogue number: CMNFV 8920. Image: Thomas Dudgeon, © Canadian Museum of Nature.

This of course raises the question of why Champsosaurus’ ear openings were on the bottom of its skull. It may have been due to the fact that the large arches of bone at the back of the skull were so prominent they displaced the ear openings from the side to the bottom.

To explore other possibilities, and how this influenced its hearing, I’m now comparing Champsosaurus ear anatomy to that of modern reptiles and amphibians.

I hope it will give me a sense of what this intriguing Cretaceous creature could hear with ears on the bottom of its skull.

This entry was posted in Fossils, Research, Species Discovery and Change and tagged , , , . Bookmark the permalink.

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