Why is a stumpy-legged, pig-sized reptilian cousin of mammals such an interesting fossil genus? There are actually several reasons, which you will learn about in this lesson.
A (Very) Distant Relative
The name Lystrosaurus (pronounced liss-tro-sore-us) means ‘shovel lizard,’ a reference to what is believed to be its burrowing habit. The genus belongs to a group of reptiles called therapsids, which also included the ancestors of mammals. They lived during the late Permian Period. (The Permian is the interval of geologic time between 299 and 251 million years ago. Geologists refer to the last half as the ‘late’ Permian.)Although we can trace our mammalian evolutionary path back to the therapsid lineage, lystrosaurs (as the collection of all species of the genus is known) are not our direct ancestors.
More like cousins, many times removed.Lystrosaurs were tetrapods, which means they walked around on four feet. They also walked with their legs slightly splayed out to their sides, similar to alligators, and had five toes on each foot.The vertebrae at the base of their spine (called the sacral vertebrae) were not fused like they are in mammals, but they were large and probably not as flexible as those found in modern lizards. When they walked, they probably didn’t sway as much from side-to-side as lizards do.They weren’t monsters, but they weren’t exactly dainty, either.
Fossils indicate they averaged about one meter in length, and estimates are that they weighed in the vicinity of 50 to 90 kilograms (100 to 200 pounds). That’s about the size of an average pig.Their front legs had large bones and were bigger than their rear ones. Paleontologists interpret that to mean that lystrosaurs were powerful diggers and burrowers. The difference is clear in the skeletal reproduction shown in the following photograph.
Lystrosaurs were plant-eaters, although we don’t know exactly what types they preferred.
They sported a horny, beak-like jaw, like a turtle, a feature adapted to grabbing and ripping plants. Instead of teeth, their jaws contained bony plates used for grinding. But they also had two upper, canine-like tusks that may have been used for digging roots.
One Hardy Reptile
Because Lystrosaurs evolved during late Permian time, they are an older reptile than the dinosaurs. They lived all across the southern hemisphere portion of the supercontinent Pangaea.The sheer abundance of Lystrosaurus fossils led to considerable disagreement over exactly how many species there actually were.
At one time, over twenty species were identified, but that number has fallen as fossils have been more closely studied and paleontologists realized that many fossils actually were of the same species. Currently, the number of recognized species is somewhere between four and six (obviously, there is still some discussion).
The genus was one of the few large terrestrial animals that survived the greatest extinction event in geologic history. At the end of the Permian Period, 95% of marine species and 70% of terrestrial vertebrates died out. But at least one species of Lystrosaurus survived, for reasons no one has yet figured out (maybe it was just luck).
Since most other land animals were gone, there was less competition for food and living space, and likely many fewer predators. So Lystrosaurus spread out across southern Pangaea (the part known as Gondwana), and they became the most abundant terrestrial animal during the early Triassic Period.
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