Jurassic Mammal

An early mammal called Priacodon fruitaensis was well adapted for powerful and precise biting, and had a varied faunivorous diet that likely included insects and small vertebrates, according to a University of Bonn-led study.

Life restoration of the carnivorous mammal Gobiconodon, a distant relative of Priacodon fruitaensis. Image credit: Pavel Riha / CC BY-SA 3.0.

Life restoration of the carnivorous mammal Gobiconodon, a distant relative of Priacodon fruitaensis. Image credit: Pavel Riha / CC BY-SA 3.0.

Priacodon fruitaensis lived approximately 150 million years ago (Jurassic period) in what is now Colorado, the United States.

The prehistoric animal was less than 20 cm (8 inches) in length and had a mass between 41 and 61 g.

Priacodon fruitaensis was first described in 1981 from a fossilized jaw found in rocks of the Upper Jurassic Morrison Formation.

It belongs to Triconodontidae, an extinct family of early crown mammals with a fossil record from the Late Jurassic through the Late Cretaceous epoch.

In new research, Dr. Kai Jäger and his colleagues from the Institute for Geosciences at the University of Bonn and Oklahoma’s Museum of Natural History examined the molar occlusion, jaw movement and morphology of Priacodon fruitaensis.

“Until now, it was unclear exactly how the teeth in the upper and lower jaws of Priacodon fruitaensis fit together. We have now been able to answer that question,” said co-author Professor Thomas Martin, a paleontologist in the Institute for Geosciences at the University of Bonn.

The researchers used a special tomography method to produce high-resolution 3D images of Priacodon fruitaensis’ molars.

“The upper and lower jaws each contain several molars,” they said.

“In the predecessors of mammals, molar 1 of the upper jaw would bite down precisely on molar 1 of the lower jaw when chewing.”

“In more developed mammals, however, the rows of teeth are shifted against each other.”

“Molar 1 at the top therefore hits exactly between molar 1 and molar 2 when biting down, so that it comes into contact with two molars instead of one.”

Right dentition of Priacodon fruitaensis: (a) upper tooth row in occlusal view; (b) lower tooth row in occlusal view; (c) upper tooth row in lingual view (mirrored for better comparison); (d) lower tooth row in buccal view. Scale bar - 2.5 mm. Image credit: Jäger et al., doi: 10.1038/s41598-020-79159-4.

Right dentition of Priacodon fruitaensis: (a) upper tooth row in occlusal view; (b) lower tooth row in occlusal view; (c) upper tooth row in lingual view (mirrored for better comparison); (d) lower tooth row in buccal view. Scale bar – 2.5 mm. Image credit: Jäger et al., doi: 10.1038/s41598-020-79159-4.

“We compared both options on the computer. This showed that the animal bit down like a modern mammal,” Dr. Jäger said.

“Its dentition must have made it easy for Priacodon fruitaensis to cut the flesh of its prey. However, the animal was probably not a pure carnivore: its molars have cone-shaped elevations, similar to the peaks of a mountain.”

“Such cusps are particularly useful for perforating and crushing insect carapaces.”

“They are therefore also found in today’s insectivores. However, the combination of carnivore and insectivore teeth is probably unique in this form.”

“The cusps are also noticeable in other ways. They are practically the same size in all molars. This made the dentition extremely precise and efficient.”

“However, these advantages came at a price: small changes in the structure of the cusps would probably have dramatically worsened the chewing performance.”

“This potentially made it more difficult for the dental apparatus to evolve,” Dr. Jäger said.

“This type of dentition has in fact survived almost unchanged in certain lineages of evolutionary history over a period of 80 million years.”

“At some point, however, its owners became extinct — perhaps because their teeth could not adapt to changing food conditions.”

The findings were published in the journal Scientific Reports.

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K.R.K. Jäger et al. 2020. Molar occlusion and jaw roll in early crown mammals. Sci Rep 10, 22378; doi: 10.1038/s41598-020-79159-4

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