Two ancient dog-toothed species discovered

Two new species of cynodonts – some of the first animals to have mammal-like qualities – have been discovered hiding in plain sight in Brazil museums.

Cynodonts, meaning “dog teeth”, mark the evolutionary transition between reptiles and mammals. They’re thought to be the some of the oldest direct ancestors to modern mammals, including humans.

“Studying cynodonts is very interesting because it is important to know more about the beginnings of the group we and other mammals belong to,” vertebrate palaeontologist and lead author Agustín Martinelli from the Federal University of Rio Grande do Sul in Brazil says.

Cynodonts are thought to have diversified on the brink of the end-Permian extinction event that wiped out the vast majority of species some 250 million years ago – but they flourished afterwards.

They’re made of two known major groups: plant-eating cynognathians and flesh-eating probainognathians.

The fossils belong to the probainognathian group and couldn’t be classified with any known species within. So the researchers made their own, naming them Bonacynodon schultzi and Santacruzgnathus abdalai, after respected South American paleontologists.

The fossil remains were reported in the journal PloS ONE

Skull and teeth fossils from the B. schultzi species indicate the animals had uncharacteristically large, dog-like teeth.

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B. schultzi‘s upper right teeth.
Credit: PLOS One

But the S. abdalai fossil, which is only of a lower jaw, suggests the animals in this species were as little as 15 centimetres in length.

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Parts of S. abdalai‘s lower jaw.
Credit: PLOS One

They’ll help scientists refine the probainognathian group’s relationships, according to Flinders University palaeontologist John Long, who was not involved in the study.

In the paper, the researchers write that some characteristics of cynodonts in South America are poorly explored, despite the continent having the richest diversity of cynodont fossils in the world.

Martinelli says the study “shows how diverse this group is in South America, and the relevance of these fossils to study mammal origins”.

But fossil discoveries aren’t limited to brushing away dirt on an expedition – these species were hidden in public collections.

The B. schultzi fossils sat idle in a museum in Rio de Janeiro for 60 years, never formally studied until Martinelli blew away the dust.

And the S. abdalai fossil was discovered mixed with material from an entirely different cynodont species, collected eight years prior.

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