Friday Fossil Feature – A Closer Look at the Echo Hunter

By Robert Boessenecker (@CoastalPaleo) and Sarah Boessenecker (@tetrameryx)

Happy Fossil Friday!

Echovenator sandersi, the Echo Hunter. Image Source.

We all know that odontocetes (toothed whales and dolphins) use echolocation – this bio-sonar allows them to find their way under water and hunt their prey. They send a series of ‘pings’ out and listen for the echo with specialized organs and facial structures that allow them to be successful predators, and even differentiate shapes!

Recently, a new paper was published describing a fossil dolphin from the Charleston area that supports an earlier origin than previously thought for ultrasonic hearing & echolocation in toothed whales. This dolphin, named Echovenator sandersi, is a xenorophid dolphin from the Chandler Bridge Formation, which is late Oligocene in age (26-23 million years old).

Xenorophidae are the oldest and most primitive known group of Odontoceti; they have an anteriorly placed blowhole/melon, heterodont teeth, and a mostly symmetrical skull and also large brain size.

Skull of Echovenator.

Other fossils like Cotylocara macei (CCNHM collections) demonstrate that xenorophids already had ability to produce sounds for echolocation, owing to a series of unique sinuses and other structures in the facial region of the skull.

In a separate recently published paper, a single xenorophid periotic (inner earbone) was recently analyzed using micro-CT (computed tomography) and found capable of hearing ultrasonic frequency sounds; however, identification of this periotic is unclear and not associated with a skull.

Periotic of Echovenator.

Echovenator preserves earbones with loosely coiled cochlea and other features indicative of ultrasonic hearing as in modern Odontoceti. New analysis suggests that high frequency hearing is the primitive condition for all cetaceans, and that baleen whales (Mysticeti) and odontocetes diverged early from this ancestral condition, with baleen whales evolving low frequency hearing and toothed whales evolving ultrasonic hearing.

Phylogenetic tree demonstrating the evolution of cetacean hearing.

This indicates that toothed whales have had the ability to echolocate since the inception of the group at around 34 million years ago, and is a key innovation in whale evolution

Two skulls in CCNHM collections also represent Echovenator, and are either juveniles or perhaps a second species in the newly described genus.

Echovenator specimens in CCNHM Collections. Photo by R. Boessenecker

Further Reading:

Churchill, M., Martinez-Caceres, M., Muizon, C. de, Mnieckowski, J., and J.H. Geisler. 2016. The origin of high-frequency hearing in whales. Current Biology, Online Early [Article Link]