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December 2020

Grisly Grin

I found this skull washed up on St. Helena Island. What is it?

Answer

 
The crushing jaws of a Yellowfin Bream. Photo: Ian Martin.

This toothy grin looks quite frightening. The jaws belong to a Yellowfin Bream, Acanthopagrus australis. These fish are common in estuaries and shallow coastal waters, on reef, sand, or mud bottoms. They are found along the east coast of Australia, from Cooktown in the north to Lakes Entrance in the south.

 
Yellowfin Bream. Intact. Photo: QM, Bruce Cowell.

Yellowfin Bream have rows of teeth with different shapes; these help the fish eat its varied diet which includes a lot of 'crunchie' things like oysters, pippies, small crabs and shrimp (this type of diet is technically called ‘durophagous’, meaning “eating hard things”). The combination of the blunt canine teeth at the front of the mouth and the various sizes of rounded molars at the back are perfectly suited for dislodging encrusting molluscs off rocks and biting and grinding up the tough shells and exoskeletons of their prey. Closely related fishes, such as Snappers (Pagrus auratus) and Tarwhines (Rhabdosargus sarba), also have similar diets, and thus similar teeth. There are, however, many other species of animals that have independently evolved a similar dentition. For example, Pink-tongue Skinks (Cyclodomorphus gerrardii) have large, bulbous teeth at the back of their jaws that are used to crush the shells of land snails.

 
The palate of a Pink-tongue Skink skull, showing crushing rear teeth. Photo: Jonathan Cramb.

 
Pink-tongue Skink, seeking snails. Photo: Steve Wilson.

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