Before we talk about them in specific, let’s first determine their classification.
A synapsid is a member of the group that contains both mammals and all vertebrates more closely related to mammals than reptiles. They are distinguished by the presence of zygomatic arches, a trait possessed by all mammals, including humans (although we call ours “cheek bones”).
The group of synapsids that includes modern mammals is called Therapsida. Members of this group (called ”therapsids”) are distinguished by the vertical positioning of their legs beneath their bodies, and the differentiation of their teeth into incisors, canines, and molars.
Therapsida evolved in the Early Permian, and its members quickly diversified. One of the major clades of therapsids that lived during the Permian period was called Theriodonta. It is this group to which the gorgonopsids belonged.
Above: Gorgonops, the O.G. (Original Gorgonopsid). Illustration by Dmitry Bogdanov.
Gorgonopsids first arrived on the scene about 260 million years ago, and are likely some of the most iconic Permian animals, second only to Dimetrodon. Their “saber-tooth” canines - an adaptation to a carnivorous lifestyle that has evolved independently multiple times - are instantly recognizable, and made them the dominant carnivores of the Late Permian.
Species include Gorgonops, a six-foot-long species from South Africa; Lycaenops, which lived alongside Gorgonops and was about half its size, the fox to Gorgonops’s wolf; and Inostrancevia, a Russian species that grew to the largest size of any known gorgonopsid, up to twelve feet long.
The gorgonopsids were a casualty of the mass extinction that occurred at the end of the Permian period, 252 million years ago. However, some of their theriodont relatives survived. It is this group that eventually gave rise to modern mammals.
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).
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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.
Hey! So I am a huge fan of Gorgonopsids, but the speculative art for them always leans towards two depictions. They're either fur-covered and highlight their mammalian features, or they're scaly or naked and classically more reptilian. I'm more biased towards the mammalian depictions, but I'm unsure which one is more accurate from a scientific perspective? You used both examples in your post about them, and wanted to hear your opinion.
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.
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, 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.
I saw someone claim that, since early reptilian synapsids had epipubic bones like marsupials, they couldn't have had any sort of long-term pregnancy, but since we also have no fossil evidence of baby synapsids or even synapsid eggs, that they made have birthed tadpole-like young. which, I don't know enough about early synapsids to know if that's bullshit or plausible. whats your take on that?
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.
Could you stop using the term "mammal-like reptile" in reference to synapsids? All modern reptiles are on the Sauropsida branch of Amniota, so animals on the Synapsida branch should not really be considered reptiles at all. So so sphenacodonts shouldn't be considered "in transition between reptiles and mammals" because synapsids were never reptiles in the first place.
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.
Trilobites are pretty much a staple of paleontology and yet they're almost never talked about. What can you tell me about them?
Trilobites were one of the first major groups of animals to establish themselves on the planet Earth, but they’ve never really gotten their due - and their story isn’t a particularly happy one
Trilobites are classified in their own unique subphylum of Arthropoda, the group that includes modern-day insects, arachnids, crustaceans, and myriapods. None of these animals are descended from trilobites, who have no direct descendants alive today. (The modern-day horseshoe crab, while sometimes mistaken for a descendant of trilobites, is actually more closely related to spiders.)
Some of Earth’s earliest known animals, trilobites originated in the Early Cambrian, some 540 million years ago. The first known trilobites show all the features of later specimens, indicating that even earlier transitional forms exist, but have not yet been discovered.
The trilobites are named for their “three lobes” - the left, right, and central sections into which the thorax of the exoskeleton is divided. Like modern-day insects, trilobites were divided into three distinct parts - the cephalon (or “head”), the thorax, and the pygidium, a fused segment that may or may not have borne legs.
Wait, they had legs?
I genuinely never knew this. I thought the “lobes” were how they got around, that they kind of crept around on the ground by wiggling their– oh my God. This is mind-blowing. And look, at the top you can see preserved antennae! I never knew they had those, either!
Holy crap, you can even see the outlines of the legs on the specimen preserved at the bottom! Am I the only one who never knew this? Was I the only person who was imagining trilobites wrong his whole life?
Well, regardless, the vast majority of trilobite fossils are shells discarded after molting, composed of only the larger pieces that would have stuck together for a while after such a process. I guess it’s understandable that I had such a plebeian understanding of trilobites for so long.
Almost all trilobites had large and complex eyes - some of the first to develop in animals. They likely had very good eyesight, allowing them to spot predators on the approach - or their own prey.
Trilobites inhabited a wide variety of lifestyles. Some were bottom-dwelling crawlers who scavenged from fallen carcasses; others filter-fed from the silt; still others were predators; and others were active swimmers who fed on plankton. Some of them, like Phacops rana, defended themselves from predators by rolling into balls of impenetrable armor; others developed more elaborate defenses, like the “trident” of Walliserops trifurcatus.
You might recognize some of the examples in the above list as the roles and adaptations taken on today by crustaceans, the dominant group of marine arthropods in our modern seas. You might also remember that earlier in this post, I said that trilobites have no living descendants. That’s not a coincidence.
Trilobites survived through the Cambrian, Ordovician, and Silurian, undergoing explosions of diversity and dominating the prehistoric seas. Then, the Devonian mass extinction occurred. It’s uncertain why this extinction occurred, but a great deal of Earth’s life was wiped out, including every order of trilobite - except one. The order Proetida managed to struggle on into the Carboniferous, but their foothold was already lost; other groups of arthropods quickly filled all the niches that trilobites had abandoned, and the trilobites were completely extinct by the end of the Permian period.
While the trilobites may be long extinct, their strange and alien anatomy still fascinates paleontologists today - even if the public may not be as enthused. I think it has a great deal to do with the fact that there are just so damn many trilobite species, most of which are pretty indistinguishable at first glance. However, as you can see from my reaction above, there’s still a lot to learn about these ancient arthropods - and a lot of it may be right under our noses, waiting for us to see it.
Koolasuchus sure was one huge amphibian, wasn't it?
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.