Despite gradual extinction, some of the mammals’ cousins grew in size during the Triassic, like reptiles.
Reconstruction of the appearance of a dicynodont of the genus Placerias, which weighed about a ton. (Illustration: Dmitry Bogdanov / Wikipedia) Skeleton of the dicynodont Lisovicia bojani. (Illustration: Uppsala University) ‹ › View full size
The ancestors of mammals were therapsids, or “beast-toothed reptiles.” Most of them went extinct during the Permian period, but some survived. True, then there was a flourishing of archosaurs – land reptiles, which include the well-known dinosaurs. It is believed that at this time the therapsids as an independent group were already dying out – they became smaller and went into the shadows so as not to catch the eye of numerous and very active dinosaurs.
However, as researchers from Uppsala University and the Institute of Paleobiology of the Polish Academy of Sciences write in Science, some of the beasts’ cousins not only did not decrease during the time of dinosaurs, but increased along with reptiles. Back in 2006, in southern Poland, paleontologists found a certain bone, which, as it turned out, belonged to one of the dicynodonts – that’s the name of one of the therapsids that survived the Permian extinction. Over the next 11 years, about a thousand more bones were found there, and for a long time it seemed that this was some kind of sauropod dinosaur – the creature looked too large. But the bones of the skull and limbs convinced everyone that this was a dicynodont, which was more than 4.5 m in length, more than 2.6 m in height and weighed about 9 tons, that is, about the same as a modern elephant.
The new species of dicynodont, named Lisovicia bojani, lived in the Late Triassic, between 240 and 210 million years ago, during the height of the dinosaurs. Overall, dicynodonts were quite diverse, including some that lived in burrows and some that lived in trees, some as large as hippopotamuses. But at the time when the newly discovered L. bojani lived, dicynodonts were said to be gradually dying out, so it was all the more surprising to find a new species the size of a large prehistoric lizard. Apparently, the same evolutionary forces that caused dinosaurs to grow in size were also at work on this endangered group.
Externally, the herbivore L. bojani resembled a hybrid of a turtle and an elephant (or a very large rhinoceros). Like all dicynodonts, it had two strongly protruding tusks growing from its upper jaw (dicynodonts had almost no other teeth) and a bony beak, similar to that of turtles. Unlike other relatives, which had straight hind legs and turned out front legs like lizards, L. bojani’s front legs were also straight, apparently to better support its heavy weight. In addition, L. bojani appears to have grown very quickly—its bones show no signs of slowing down. There is a meaning to rapid growth: the faster you grow, the less likely it is that someone will eat you; however, according to experts, for such hypotheses it is better to wait for new fossil finds and new research.