(Lystrosaurus murrayi)

Lystrosaurus was a member of Dicynodontia, one of the many groups of non-mammalian therapsids that inhabited Earth during the Permian and Triassic periods (and possibly up until the Cretaceous).  Dicynodonts were beaked herbivores, toothless save for a pair of tusks.  These tusks likely had many uses that varied between species, including sexual dimorphism and display, a form of defense against predators, and potentially even digging in the ground for food.

Lystrosaurus was probably an avid digger, due to the large size and powerful build of its forelimbs; it may have dug for food, or even nested in burrows underground.  

(Another species of Lystrosaurus: L. georgi.  Both images above by Dmitry Bogdanov.)

Lystrosaurus is notable for surviving the Permian-Triassic mass extinction, which drove the vast majority of life on Earth at the time extinct, and became one of the world’s most common animals for a short time after the extinction event.  In some locations, Lystrosaurus fossils up more than 95% of the total fossils.

Lystrosaurus fossils were also used as proof of plate tectonics.  The presence of Lystrosaurus fossils in Africa, India, and Antarctica proved that the continents were once connected (since Lystrosaurus obviously could not have swam across the ocean).

Several members of the group Theriocephalia - a group of therapsids, or “mammal-like reptiles*”, related to both gorgonopsids and modern mammals - are believed to have been venomous!

Euchambersia, a theriocephalian from Late Permian South Africa, had large openings behind each canine tooth that are believed to have held venom glands.  Grooves on each tooth would have routed the venom into the bitten prey’s body.

(Above: Dmitry Bogdanov’s restoration of the animal in question.)

Another African theriocephalian, Ichibengops, had similar grooves on its teeth, and may have been venomous as well.  Euchambersia and Ichibengops belonged to two different families of theriocephalians, implying that many more of these animals may have been venomous as well.

*”Mammal-like reptiles” is not a scientifically accurate term, as even pre-mammalian therapsids cannot be called “reptiles”.  It’s used here for clarity’s sake.

No skin impressions have been discovered from any non-mammalian therapsids - at least not to my knowledge.  As a result, their actual integuments are unknown.  However, if you want my theories, I can wildly speculate for a moment.

The first unambiguously furred mammals were the Mammaliaformes, a group of animals descended from the cynodont therapsids, and which eventually gave rise to modern mammals.  Castorocauda, an aquatic mammaliform from 164 million years ago, is the earliest mammal preserved with impressions of fur.  However, earlier pre-mammalian therapsids, such as the 205-million-year-old Morganucodon, have been preserved with Harderian glands.  In modern mammals, these glands secrete a substance used for grooming fur, meaning that fur may have been present in these animals as well.

(Furry Morganucodon, by FunkMonk.)

Gorgonopsids are even older than Morganucodon - the earliest known gorgonopsid fossils are from 262 million years ago - and so they likely did not have a full pelt of advanced mammalian hair.  However, they were quite divergent from reptiles at that point, and probably weren’t scaly, either.  It seems more likely to me that gorgonopsids were lightly furred, if at all, and were otherwise covered by bare mammalian skin.  However, based on the presence of small “pits” in their facial bones, they may have had sensory whiskers.  (Perhaps these whiskers predated fur, and eventually gave rise to it… but don’t quote me on that.)

Dimetrodon!  One of the most famous prehistoric creatures, and yet one of the most frequently misunderstood.

(Above image by Alphynix.)

Dimetrodon, contrary to popular belief, was not a dinosaur.  It didn’t even coexist with the dinosaurs; it lived from 295 to 270 million years ago, predating the dinosaurs by over 40 million years.

Dimetrodon was actually a sphenacodont.  Sphenacodonts were synapsids, members of the group that contain modern mammals.  As a result, sphenacodonts like Dimetrodon are frequently described as “mammal-like reptiles”, along with other varieties of non-mammalian synapsids, such as the gorgonopsids and the therocephalians - even though none of these animals were truly reptiles.

While initially considered sluggish and reptilian (as in the above reconstruction, done by Charles Knight in 1897), Dimetrodon has since undergone a paleontological brand overhaul, and is now often depicted as a more active, potentially even mammalian creature - one of the biggest predatory animals of the Permian period.

There are currently thirteen accepted species of Dimetrodon, most of which were native to the southwestern United States; one species is known from Canada, and another is known from Germany.  These species ranged greatly in size, from five to fifteen feet in length.  Some of these species may or may not represent growth stages of the same species of animal.

That’s all well and good - but what’s up with the fin on its back?

Dimetrodon is most famous for its sail.  Long, thin neural spines growing from the animal’s back were connected by blood vessel-filled soft tissue, forming not just a sail of skin, but a veritable icon of prehistory – a visual shorthand for “primordial beast” that has since been erroneously applied to numerous dinosaurs over the years. However, Dimetrodon was not unique in its possession of a sail; numerous other reptiles and amphibians have sported similar sails, frills, ridges, bumps, and lumps over the eons.

The purpose of Dimetrodon’s sail is not certain, but it definitely looked quite different than the usual depiction.  To quote from the linked source:

Although traditionally depicted with a fully skin-covered sail, the bone structure of Dimetrodon’s neural spines suggests differently. The base seems to have anchored strong back muscles… with soft-tissue webbing only extending partway up the bones. Meanwhile the upper portions of the “sail” were probably free-standing, and many fossils show crooked bends where the unsupported spines didn’t grow straight.

A prevalent theory is that the sails were an important factor in the recognition of individuals, and that they played a role in visual communication.

Whatever they were used for, it doesn’t change the fact that Dimetrodon was quite a visually striking animal.

(Dimetrodon grandis, by John Conway.)

I don’t actually know very much about early reptilian synapsids, but that seems pretty unlikely.  Reptiles had been laying hard-shelled eggs for at least 30 million years by then; it seems unlikely that they would have reverted to a more amphibian method of birth.  I don’t think giving birth to live young became commonplace until synapsids became much more mammal-like, but if there’s paleontological evidence proving me wrong, I’d be excited to hear it.

I use the term “mammal-like reptiles” because it’s a term that a lot of people are familiar with, and it helps to explain the phylogenetic position of synapsids.  I’m going to continue to use it for those reasons.  However, I have added clarifications to the posts that use the term, explaining that “mammal-like reptile” is not an actual scientific term, and I have removed the offending phrase from the Dimetrodon post and replaced it with a more accurate description of the phylogenetic position of non-mammalian synapsids.

It sure was - but that’s not necessarily the most interesting thing about it!

Koolasuchus was a salamander-like animal that grew to lengths of over 15 feet.  It lived in southern Australia, and is believed to have been an aquatic ambush predator similar to a crocodile.  It’s perhaps most famous for its appearance on Walking With Dinosaurs, where it’s shown preying on the small ornithopod Leaellynasaura.

Despite its salamander-like appearance, Koolasuchus has no direct living relatives.  It belongs to an extinct family of amphibians called Chigutisauridae.  The chigutisaurs belonged to Temnospondyli, a primitive order of amphibians that originated during the Carboniferous period.

The temnospondyls were a massively diverse group of animals.  Some were fully aquatic, while others were adapted to life on land; some were as small as modern frogs and salamanders, while others were the biggest amphibians in Earth’s history.  Unfortunately, they began to decline around the Permian/Triassic boundary, as Earth’s climate grew more arid and reptiles began to diversify on land.

Most temnospondyls went extinct by the Jurassic period, except for one small group - the brachyopoids, all of which were large Koolasuchus-like ambush predators.  The brachyopoids were divided into two families - Brachyopidae, and Chigutisauridae (the group to which Koolasuchus belongs).

Koolasuchus is the latest known brachyopoid (and temnospondyl in general), living 120 million years ago, during the Early Cretaceous period - 80 million years after the extinction of all other temnospondyls.  What allowed it and the other brachyopoids to survive for so long after all their relatives went extinct?

Jurassic and Cretaceous brachyopoids all dwelled in very cold regions - either in China, or in the southern parts of Gondwana (South America, Africa, Antarctica, and Australia).  They likely had some sort of adaptation to survive in cold weather - an adaptation lacked by the crocodilians that occupied the same niches in warmer parts of the world.  As the continents drifted, climates changed, and these parts of the world became warmer, crocodilians spread to new areas, and the brachyopoids became extinct.

I’m not aware of any paleontological evidence for how Koolasuchus and its relatives survived in the cold, so I’m going to toss out a piece of wild speculation: Like the modern wood frog, Koolasuchus might have been capable of surviving extreme winter conditions, even being able to tolerate the freezing of its blood.  It would hibernate until the spring thaw - possibly underwater, in order to avoid the notice of winter-active predators, like dinosaurs.