Lethiscus

Ever since the earliest tetrapods crawled onto land and developed limbs and digits, some lineages have just… decided the whole “legs” thing was overrated and lost them entirely.

And the earliest known group to do this were the aïstopods. These highly elongated amphibian-like animals had specialized lightly-built skulls with large jaw muscles, and they may have filled a similar ecological niche to modern snakes, hunting small terrestrial invertebrates.

Lethiscus stocki was one of the first members of this snake-like group, living in Scotland during the Early Carboniferous about 340 million years ago. Growing to at least 50cm long (~20″), it was already a very specialized animal despite its basal position among the aïstopods, with eyes set far forward on its face and no trace of vestigial limbs.

CT scans of its skull have shown some surprisingly fish-like anatomy, especially in its braincase, features that were lost very early in tetrapod evolution. This suggests that aïstopods weren’t part of the lepospondyl amphibians like previously thought, but actually originated much much earlier in the tetrapod evolutionary tree – potentially placing them somewhere among the “fishapods” between Ichthyostega and Crassigyrinus.

Haldanodon

The docodonts were a group of mammaliaformes (close relatives of the earliest true mammals) which lived across North America, Europe, and Asia from the Middle Jurassic to the Early Cretaceous. Originally only known from teeth and jaw fragments they were traditionally thought to be fairly generic shrew-like insectivores, but more recent discoveries of better fossils have revealed they were actually much more diverse, occupying ecological niches ranging from squirrel-like tree-climbers to mole-like diggers to beaver-otter-like swimmers.

Most of the more complete fossil material of these animals comes from the mid-Jurassic of China, but one species from elsewhere is also known from a partial skeleton.

Haldanodon exspectatus here lived in central Portugal during the Late Jurassic, about 155 million years ago. Around 15-20cm long (6-8″), it had small eyes and short chunky well-muscled limbs with the front paws adapted for digging. Since it inhabited a very swampy environment it probably wasn’t a pure mole-like burrower – extensive tunnels would have constantly flooded – but it may have instead been a similar sort of semi-aquatic animal to modern platypuses and desmans, foraging for invertebrates in the water and excavating burrows in the banks.

Roughened areas of bone on its snout may also have supported a patch of tough keratinous skin, which would have helped protect its face while digging.

Mixosaurus

Mixosaurus cornalianus here was a small early ichthyosaur, only about 0.8-1m long (2’7″-3’3″) and generally considered to be transitional between the eel-like swimming style of basal forms and the more dolphin-like later forms. Living during the Middle Triassic, about 242 million years ago, it inhabited a shallow tropical sea that covered what is now the modern border between Switzerland and Italy.

It was previously thought to be a slow swimmer with a low and poorly-developed tail fin, and whether it even had a dorsal fin or not was unclear. But now new specimens with soft tissue impressions have given us a big surprise.

Not only did it actually have a fairly well-developed semilunate tail fin, but it also had a dorsal fin positioned much further forward on its body than expected, giving it a shape similar to some small sharks and representing the current earliest known dorsal fin of any amniote.

Bundles of stiffening collagen fibers inside its fins were very similar to those known from later Jurassic ichthyosaur species, indicating that this adaptation evolved much earlier in the lineage than previously thought. Along with stomach contents showing it mainly ate both cephalopods and small fish – fairly fast-moving prey – this suggests it was a capable open-water swimmer. It wouldn’t have been quite as speedy as its much more specialized Jurassic relatives, but it may have still been about as efficient as the small modern sharks it resembled.

Leithia melitensis

During the mid-Pleistocene, between about 900,000 and 500,000 years ago, the Mediterranean islands of Malta and Sicily were connected and shared a unique ecosystem made of up a mix of weird endemic species. While the tiny elephants and giant swans are probably the most famous, there were also several other unusual animals such as dwarf hippos, huge owls, large cranes, giant tortoises, and big lizards.

And also massive rodents.

Leithia melitensis, the Maltese giant dormouse, was descended from garden dormice, but thanks to the lack of large land predators on Siculo-Malta it was able to evolve a much much larger body size – about 60-70cm long (2′-2’4″), almost the size of a cat.

Recent reconstructions of its skull have shown it was also proportioned differently compared to its tiny modern relatives, more chunkily built with a shorter and wider snout, bigger teeth, and thicker cheekbones that must have anchored some incredibly powerful muscles for chewing. It may have been eating a much more herbivorous diet than other dormice, processing a lot of tough fibrous vegetation.

Ceratosuchus

“Horns” seem to have convergently evolved multiple times in crocodiles over the last few million years, including in a couple of living species. These triangular crests are formed from the squamosal bone, just above their ears, and tend to be a sexually dimorphic feature used in territorial displays between males, serving to make them look bigger when they arch their necks.

But there’s another horned crocodilian known from much earlier in the Cenozoic – and this one was an alligator!

Ceratosuchus burdoshi lived in Colorado and Wyoming in the western United States during the late Paleocene and early Eocene, about 57-56 million years ago. It was a fairly small alligator, around 1.7m long (5’6″), with a broad snout featuring sharp teeth at the front and blunter teeth further back – an arrangement that suggests it was a generalist predator eating a variety of small prey, using those teeth to first grab and then crush whatever it managed to catch.

It also had large blade-like osteoderm armor on the back of its neck, which may have been arranged in line with its “horns” to make its visual displays look even spikier.

Eons Roundup 7

It’s another PBS Eons commission roundup day!

The metatherian mammals Pucadelphysand Khasia, and lineart of the sparassodont Paraborhyaena, from “How South America Made the Marsupials”
https://www.youtube.com/watch?v=l5doyrUWFbE

The dyrosaurid crocodyliform Acherontisuchus and the bothremydid turtle Puentemys, from “How a Hot Planet Created the World’s Biggest Snake”
https://www.youtube.com/watch?v=T-hDNbM-WLk

The early penguin Waimanu and the giant penguin Anthropornis, from “When Penguins Went From The Sky To The Sea”
https://www.youtube.com/watch?v=HMArjGQwLvY

Diictodon

Even before Lystrosaurus briefly took over the world, the dicynodont synapsids were a highly successful group. These herbivorous beaked-and-tusked mammal-cousins were abundant all over the supercontinent of Pangaea during the Late Permian and occupied a wide variety of ecological niches, ranging from rat-sized to almost bear-sized. (And later during the Triassic some got even bigger.)

Towards the smaller end of that size range were species like Diictodon. Living around 259-254 million years ago in Southern Africa (but with fossils also found in northern China, suggesting a much larger geographic range) this dicynodont grew up to about 45cm long (1’6″) and was a gopher-like creature adapted for digging, with a tubular body and short muscular limbs.

It was a very common animal, making up around half of all vertebrate fossils in some locations. Numerous preserved spiral-shaped burrows have been found concentrated in small areas, going down as much as 1.5m (5′) into the ground.

Several different species have been named within the Diictodon genus, but currently they’ve all been lumped together under the single name of Diictodon feliceps. There’s a lot of anatomical variation between specimens, though, with some notably being smaller and lacking the distinctive tusks seen in others – which may be evidence of sexual dimorphism, with the tuskless individuals possibly being females. (Although differences in inner ear anatomy may also indicate they were a separate species entirely, in which case female D. feliceps might instead be represented by fossils showing smaller tusks.)

I’ve illustrated one of the tuskless forms here, since they don’t generally get as much attention as the tusked ones. It’s also speculatively fluffy and iridescent similar to modern golden moles.

Groenlandaspis

The armor-headed placoderms were the dominant fish during the Devonian period, evolving a wonderfully diverse range of shapes and sizes, and occupying ecological niches in both marine and freshwater habitats.

Groenlandaspis antarctica here lived during the mid-to-late Devonian, about 383 million years ago, in the Oates Land region of Antarctica – at that time located further north than it is today, with the local climate being warm and subtropical.

It was a moderately-sized river-dwelling placoderm, around 50cm long (1’8″), and its bony armor formed a sort of pyramid shape with wing-like projections at its sides, a structure that would have acted as a hydrofoil and made it an efficient swimmer. Most of the armor plates were rigidly fused together, except for a hinge point between its head and thorax that allowed it to open its jaws, but unlike its more famous relative Dunkleosteus it couldn’t gape its mouth open particularly wide. It may have been a bottom-feeder, grubbing around in muddy riverbeds and using its small but strong jaws to crush hard-shelled prey.

Various other species of the Groenlandaspis genus have been found all around the world, but there’s something incredibly rare and special about Groenlandaspis antarctica in particular:

We actually know what color it was.

Preserved pigment cells in its fossils indicate that it was red on top and silvery-white on its underside in a countershaded pattern, camouflaging it in the murky silty waters of the ancient Antarctic rivers.

…And also made it look a bit like a prehistoric goldfish.

Hyopsodus

Back during the early Eocene, around 50 million years ago, global temperatures were much warmer than today, and in North America tropical and subtropical rainforests extended as far as Alaska.

And one of the most abundant animals in these balmy ecosystems was a small mammal called Hyopsodus, an early type of ungulate that was probably part of the perissodactyl lineage, closely related to the ancestors of modern horses.

Many different species of this genus have been discovered, ranging from rat-sized to cat-sized. Remains of Hyopsodus account for up to 30% of fossils in some locations, with tens of thousands of specimens known – although most of them are isolated teeth and jaw fragments.

(The illustration here depicts Hyopsodus wortmani, a 30cm/12″ long species which lived about 50-46 million years ago across the Western and Southern USA.)

More substantial skeletal remains of this little mammal are very rare, and initially seemed to show a long weasel-like body that resulted in Hyopsodus being given the nickname of “tube-sheep”. But more recent specimens have given us a better idea of its proportions, and it wasn’t really tubular at all. Instead it was probably built more like a cavy or a hyrax, with a more chunky body and a spine held more strongly curved.

Its teeth suggest it was a generalist omnivore, probably mainly eating a mixture of vegetation, fruits, seeds, insects, and occasionally smaller animals, and while its limbs were proportionally short it was likely still quite an agile fast-moving animal. It also appears to have had some ability to dig, and may have sheltered in burrows similarly to modern groundhogs.

But one of the most surprising things about the “tube-sheep” comes from studies of its braincase via CT scans of its skull. Its brain was unusually large for its size, and had enlarged areas associated with good senses of smell and hearing – and notably one sound-processing region (known as the inferior colliculus) was developed to a degree similar to those seen in echolocating animals.

Analysis of its ear bones suggest it wasn’t highly specialized for echolocation like bats, but may have still been capable of a more basic shrew-like version, using it for close-range navigation.

Ophthalmothule

The cryptoclidids were fairly standard-looking plesiosaurs, with long necks and small heads – but those tiny skull bones were also rather fragile and so there’s very little good fossil material of their heads, making it difficult to figure out both their feeding ecology and their exact evolutionary relationships.

But a recently-discovered specimen from the Svalbard archipelago actually preserved a mostly-complete skeleton, including an unusually intact skull.

Given the name Ophthalmothule cryostea (meaning “frozen bones of the Northern eye”), this cryptoclidid lived about 145 million years ago, right at the boundary between the Jurassic and the Cretaceous.

It measured around 5m long (16’5″) and had proportionally huge eyes that faced upwards on its head – an adaptation for seeing in low-light underwater conditions, maximizing the amount of light reaching it from above.

Those big dark-adapted eyes suggest it may have been nocturnal, or spent a lot of time diving into very deep waters in search of food. Its skull had weak jaw muscles and delicate teeth, and its gut region contained a lot of fine gravelly sediment, so it probably mainly grubbed around for small soft-bodied prey on the sea floor.

At that point in time Svalbard would have been a little further south than it is today, at a subarctic latitude, but the area would have still experienced particularly long nights during the winter. So it’s possible Ophthalmothule also developed such big sensitive eyes to help it survive through those darker seasons.