What can teeth tell us about an animal that is thought to be the last link between mammals and reptiles? Quite a lot, actually. The recent discovery of fossilized teeth has cast doubt on the idea that early mammals quickly brought about the extinction of mammal-like reptiles – it seems that something survived for far longer than expected.
The tritylodontids were a family of mammal-like reptiles that were abundant during the Jurassic. They were remarkably successful, making their way to nearly every corner of the globe, feeding on the lush vegetation that flourished in the warming climate of the time. By the end of the Jurassic period, though, they had been pushed into extinction as emerging early mammal species took over their ecological roles. The discovery of a new species of tritylodontid from Japan suggests that these animals may have survived 30 million years longer than previously thought, living alongside the mammal species that would ultimately replace them.
“Tritylodontids were herbivores with unique sets of teeth which intersect when they bite,” said Hiroshige Matsuoka of Kyoto University, and lead author on the current study. “They had pretty much the same features as mammals – for instance they were most likely warm-blooded – but taxonomically speaking they were reptiles, because in their jaws they still had a bone that in mammals is used for hearing.”
The discovery of more than 250 fossil tritylodontid teeth in an outcrop of rocks from the Lower Cretaceous in Kuwajima, Japan, now suggests that some of these fascinating animals – the last link between reptiles and mammals – survived far beyond the end of the Jurassic.
“This raises new questions about how tritylodontids and their mammalian neighbours shared or separated ecological roles,” said Matsuoka.
The teeth were used to describe a new tritylodontid species, Montirictus kuwajimaensis, as well as work out its position in the evolutionary tree. Montirictus kuwajimaensis is now the youngest known species of tritylodontid.
“Usually fossils are identified as a new species only when a relatively complete set of structures like a jaw bone are found. In these cases, characteristics of teeth tend to be described only briefly,” added Matsuoka. “Tritylodontid teeth have three rows of 2-3 cusps. This time we paid attention to fine details like the size and shape of each cusp. By using this method it should be possible to characterize other species on the evolutionary tree as well.”
The fossil site from which the teeth were uncovered has revealed a startling variety of animal and plant fossils, and the team are excited about the possible revelations that ongoing excavations will bring.
“Because fossils of so many diverse families of animals are to be found in Kuwajima, we’d like to keep investigating the site to uncover things not just about individual species, but also about entire ecological dynamics,” said Matsuoka.
Matsuoka, H., Kusuhashi, N., & Corfe, I. J. (2016). A new Early Cretaceous tritylodontid (Synapsida, Cynodontia, Mammaliamorpha) from the Kuwajima Formation (Tetori Group) of central Japan. Journal of Vertebrate Paleontology, DOI:10.1080/02724634.2016.1112289
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