Squaloraja

Discovered in the late 1820s by pioneering paleontologist Mary Anning, the odd-looking fossil of the cartilaginous fish Squaloraja polyspondyla seemed to have characteristics of both sharks and rays.

It was initially thought to be a “missing link” transitional form between those two groups, but later it was identified as being something else entirely – it was actually part of the chimaera lineage, much closer related to modern ratfish, and its ray-like features were due to convergent evolution for a bottom-feeding lifestyle.

Living during the early Jurassic period, about 200-195 million years ago, Squaloraja fossils are now known from the south coast of England, southern Belguim, and northern Italy. Around 30cm long (1’), this weird fish had a massive wide flat snout that looked like an even more extreme version of the long noses seen in some of its modern relatives, and this enormous snoot would have been absolutely packed with sensory receptors to help it locate small aquatic prey hidden in the muddy seafloor.

Some specimens also have a distinctive long horn-like spine on their foreheads, and since these individuals also have claspers it seems like this was a sexually dimorphic feature. Much like the smaller head claspers on modern chimaeras, male Squaloraja probably used this “horn” to hang onto females’ pectoral fins during mating – and with it being such a large elaborate structure it may also have been used for visual display purposes, too.

Diasparactus

A group of early tetrapods known as diadectids were some of the first land animals to become specialized herbivores, developing adaptations for the bulk processing of tough fibrous vegetation. They had powerful jaws, chisel-like front teeth, and grinding cheek teeth, and they grew to relatively large sizes for their time with bulky bodies supporting voluminous plant-fermenting guts.

Although usually considered to be reptilomorphs – “amphibian-grade” animals more closely related to amniotes than to modern amphibians – some studies have instead placed these early plant-eaters as being true amniotes related to the synapsids. Fossil trackways show they may have had amniote-like claws on their feet, and that their highly flexible lizard-like ankle joints allowed them to walk much more efficiently than other early tetrapods, possibly using a semi-upright gait, but these may be convergently evolved features. Since we don’t know whether they laid amniote-like eggs or if they instead spawned amphibian-style in water, it’s currently hard to tell for certain just what they really were.

Diasparactus zenos (sometimes alternately known as Diadectes zenos) was a diadectid that lived during the early Permian in New Mexico, USA, about 296 million years ago. Around 1.3m long (4’3”), it was only about half the size of its largest relatives, but it’s notable for having unusually high neural spines on its vertebrae – not quite long enough to be considered a sail, but more of a “high back” that may have supported powerful musculature or fatty deposits.

Perplexisaurus

If there’s any equivalent to carcinization in mammals, it’s turning into an otter-beaver-like semi-aquatic form.

Because it just keeps happening.

Modern examples alone include otters, beavers, muskrats, giant otter shrews, desmans, aquatic genets, yapoks, lutrine opossums, and platypuses – and in the fossil record there were early pinnipeds, remingtonocetids, pantolestids, stagodontids, and Liaoconodon going as far back as the early Cretaceous. Even outside of the true mammals there were also Castorocauda, Haldanodon, and Kayentatherium during the Jurassic, and much further back in the late Permian there was the early cynodont Procynosuchus.

So a non-cynodont synapsid doing the exact same thing really isn’t all that surprising.

Perplexisaurus foveatus was a member of the therocephalians, a group of synapsids that were close evolutionary “cousins” of the cynodonts-and-true-mammals lineage. Similar in size to a modern rat, about 20cm long (8″), it lived in Western Russia during the Late Permian about 268-265 million years ago.

At the time this region was a river plain with a tropical climate, experiencing seasonal floods that turned the whole area into what’s known as “viesses” (a name based on the abbreviation “V.S.S.” standing for “very shallow sea”), vast shallow lake-seas that persisted for weeks or months at a time.

So this little animal has been interpreted as being semi-aquatic, swimming around and feeding on aquatic invertebrates and tiny fish and amphibians. Its skull had numerous pits around the front of its face, suggesting that it had a highly sensitive snout – probably whiskery, allowing it to hunt entirely by touch in dark murky water, but it’s also been proposed to have possibly had an electroreceptive sense similar to modern platypuses.

Brontornis

Brontornis burmeisteri was one of the largest flightless birds known to have ever existed, standing around 2.8m tall (9’2″) and estimated to have weighed 400kg (~880lbs).

Known from the early and mid-Miocene of Argentina, between about 17 and 11 million years ago, it’s traditionally considered to be one of the carnivorous terror birds that dominated predatory roles in South American ecosystems during the long Cenozoic isolation of the continent.

But Brontornis might not actually have been a terror bird at all – it may have instead been a giant cousin of ducks and geese.

The known fossil material is fragmentary enough that it’s still hard to tell for certain, but there’s some evidence that links it to the gastornithiformes, a group of huge herbivorous birds related to modern waterfowl.

If it was a gastornithiform, that would mean it represents a previously completely unknown lineage of South American giant flightless galloanserans. And, along with the gastornithids and the mihirungs, it would represent a third time that group of birds convergently evolved this sort of body plan and ecological role on entirely different continents during the Cenozoic.

Tarrasius

The spinal column in tetrapods is made up of five different regions of distinctly-shaped vertebrae: cervical (neck), thoracic (upper back attached to ribs), lumbar (lower back without ribs), sacral (pelvic) and caudal (tail).

Non-tetrapod vertebrates like fish have spines that are much less differentiated, with just body and tail segments. So for a long time multiple distinct spine regions were thought to be something completely unique to tetrapods – a specialization developed early in their evolutionary history that served to better support their weight when moving around on land.

But one little fossil fish makes this idea… problematic.

Tarrasius problematicus lived during the early Carboniferous, about 345 million years ago, in shallow tropical marine waters in what is now southern Scotland. Around 9cm long (3.5″), it was an early type of ray-finned fish with a scaleless body and a long scaled eel-like tail with a single continuous dorsal fin.

And it also had some very unusual vertebrae for a non-tetrapod fish.

Its spine shows five different regions all corresponding to those seen in tetrapods, despite it not being closely related to them. But unlike early tetrapods Tarrasius was no land-walker, with its lack of hind fins indicating it was instead a streamlined fully aquatic fast swimmer.

It’s not clear why this fish developed such an incredibly convergent backbone, but it may have helped to stiffen its body so its more flexible tail could provide more efficient thrust, swimming like a modern tadpole.

It also suggests that a pre-existing genetic basis for regionalization – specific patterns of Hox gene expression – was actually an ancestral trait for all bony fish or jawed vertebrates. Tarrasius and early tetrapods may have just happened to specialize their spines in the same way for different purposes, with only the tetrapods going on to see long-term evolutionary success with it.

Adalatherium

Even for a fossil species from an isolated island, Adalatherium hui is very weird.

This mammal was part of an enigmatic group known as gondwanatheres, which were probably early members of the theriiform lineage – slightly closer related to modern marsupials and placentals than to monotremes. Found in the southern continents of Gondwana between the Late Cretaceous and the Miocene, these animals were adapted for herbivory with convergently rodent-like ever-growing front teeth that helped them chew through tough plant matter.

They were previously known mainly from isolated teeth and jaw fragments, with some rare full skull material, but Adalatherium is remarkable for being represented by a complete skeleton.

And it’s turned out to be far stranger than anyone expected.

Living in northwestern Madagascar during the Late Cretaceous, about 70-66 million years ago, Adalatherium was one of the larger known Mesozoic mammals at around 60cm long (2′) – although the one known specimen seems to have been a juvenile, so mature individuals were probably slightly larger.

(And based on its body proportions, its close relative Vintana may actually have been even bigger than previously thought. Whether this sort of large size was common in Cretaceous gondwanatheres or if this was just island gigantism is still unknown, though.)

It was probably a marmot-like digging animal, excavating burrows with its large claws and powerful limbs, and since it likely evolved from ancestors that had become isolated on Madagascar over 20 million years earlier it had developed a very unusual mixture of both “primitive” and highly specialized anatomical features. It had more back vertebrae than any other known Mesozoic mammal, upright forelimbs, sprawling hind legs with bowed-out tibias, strong back and leg musculature, and a therian-like pelvis with epipubic bones.

And then there’s the snoot.

The snout region of Adalatherium‘s skull was pockmarked with a large number of foramina, holes that allow the passage of nerves and blood vessels through the bone. It had more of these than any other known mammal, and their presence suggests that it probably had a very sensitive upper lip and whiskery snout. Most mammals with a lot of whiskers just have one very big foramina, but Adalatherium seems to have evolved a different solution to the same problem.

It also had one other bizarre feature – a hole in the top of its nose. A large “internasal vacuity” between its nasal bones is a unique feature not known in any other mammal, and its function is a total mystery.

Since this hole was also surrounded by many foramina it may have supported some sort of soft-tissue sensory structure on top of its nose. So I’ve speculatively depicted it here with a leathery horn-like “shield”.

Adalatherium skull
From fig 2 in Krause, D. W. et al (2020). Skeleton of a Cretaceous mammal from Madagascar reflects long-term insularity. Nature 581, 421–427. https://doi.org/10.1038/s41586-020-2234-8

Aquilarhinus

Aquilarhinus palimentus here was an early hadrosaurid dinosaur known from the Late Cretaceous of Texas, USA, living about 80 million years ago. Around 5m long (16″5″), it had a prominent humped nose that seems to have been an evolutionary prelude to the larger and much more elaborate crests found in later hadrosaurs.

It also had an unusually wide and shovel-like beak, unlike any other known hadrosaur, which was probably a specialization for a different diet than its relatives. Since it lived along coastal marshlands it may have used its broad jaws to scoop up large mouthfuls of soft vegetation – or, much like the “shovel-tusker” proboscideans that were once thought to have a similar lifestyle, it may actually have been doing something else entirely with that beak.

Eons Roundup 10

Time for some more PBS Eons commission work!

The radiodonts Lyrarapax and Tamisiocaris, from “How Plankton Created A Bizarre Giant of the Seas”
https://www.youtube.com/watch?v=G0oKBPZODhM


The rhynchocephalians Sphenotitan, Clevosaurus, and Kawasphenodon, from “When Lizards Took Over the World”
https://www.youtube.com/watch?v=peeX3PKOE_w

April Fools 2021: Time for T. rex

Did you know that in all the time I’ve been posting paleoart here, I’ve never actually done a proper full illustration of the most popular name in all of paleontology?

I think it’s finally time to fix this glaring oversight.

It’s time for T. rex.

Continue reading “April Fools 2021: Time for T. rex”

Cambrian Explosion Month #31: Phylum Brachiopoda

While modern brachiopods superficially resemble clams, they’re not actually very closely related to each other. Clams are bivalve molluscs, related to snails and squid, while brachiopods are lophophorates related to bryozoans and horseshoe worms.

Their two shell valves are also arranged very differently – while bivalve shells originate from the left and right sides of their bodies, brachiopods grow theirs on the top and bottom.

They first appear in the fossil record in the early Cambrian, about 530 million years ago, but they may have actually diverged from a tommotiid-like ancestor as far back as the late Ediacaran. Only around 300 species survive today, but during the Paleozoic they were some of the most abundant filter-feeding and reef-building animals with tens of thousands of fossil species known. Different species tended to have strict habitat and temperature preferences, and so their fossils are also useful indicators of how ancient climates changed over time.

Continue reading “Cambrian Explosion Month #31: Phylum Brachiopoda”