23 Dianne’s beautiful dolphin had an unusual smile

Cetacea, Platanistoidea: Aureia rerehua

The seas surrounding Zealandia during the Oligocene were rich in species that were broadly similar to the biodiversity living in today’s oceans, but sometimes possessed obvious differences. We can see clear examples of this amongst the ancient toothed-whales such as Waipatia (see Archaic dolphin connects oceans of Zealandia to endangered river resident). These animals had the general body plan we would recognise from dolphins living today but had procumbent incisors.

Dianne Nyhof preparing Aureia rerehua (specimen OU 22553). The top right image shows the under-side or ventral surface of the skull after it had been prepared out from the limestone rock. Image credit: RE Fordyce.

The ecology of toothed-whales with procumbent teeth has been a challenge to understand because of the absence of living examples. Fossils of these ancient toothed-whales have been discovered in North Otago and South Canterbury for many decades, and the diversity and ancient ecology of these animals has formed a strong research theme for the Department of Geology at the University of Otago. The hunting behaviour of ancient toothed-whales with procumbent teeth was a focus of Shane Meekin’s Master of Science research. Shane and two colleagues explored these insights further when they described Aureia rerehua, a new genus and species of ancient toothed-whale discovered in South Canterbury.

The partial skeleton of Aureia rerehua was discovered in a limestone quarry in the Hakataramea Valley. The bones included the skull and lower jaws, teeth, ear bones, vertebrae and ribs. The fossil had been collected from Otekaike Limestone and a small sample of the sediment around the fossil helped to confirm that the animal had lived between 22 and 23 million years ago.

Aureia rerehua is described as having ‘splayed’ teeth. This means that the teeth along both the upper and lower jaws curved outwards before pointing down. And although the procumbent front teeth weren’t found attached to the skull, the researchers concluded that they nevertheless protruded horizontally forward (i.e. were procumbent).

The lower jaw of Aureia rerehua was slender and the vertebrae of the neck allowed the head to be quite mobile. Taken together with the unusual dentition, the researchers proposed that Aureia rerehua hunted by snapping at prey to seize it in a basket of teeth, before sucking that prey deeper into its mouth. A close analogue today might be the endangered living Ganges river dolphin Platanista gangetica.

Aureia rerehua was referred to by the name “Dianne’s Dolphin” during the preparation and study stages of research, ahead of receiving its scientific name. The name Aureia is drawn from aurei, the Te Reo Māori word for cloak pin, referring to the shape of the teeth. In Te Reo Māori rerehua is beautiful, referring to its well-preserved face.

—Written by Daniel B Thomas