Tuzoia

The tuzoiids were an enigmatic group of Cambrian invertebrates known mostly just from their spiny bivalved carapaces. Although hundreds of fossils of these arthropods were discovered over the last century or so, only vague fragments of the rest of their bodies have been found even in sites usually known for preserving soft tissue impressions.

…Until late 2022, when several new specimens from the Canadian Burgess Shale deposits (~508 million years ago) were described showing tuzoiid anatomy in exceptional detail, finally giving us an idea of what they looked like and where they fit into the early arthropod evolutionary tree.

Tuzoiids like Tuzoia burgessensis here would have grown up to about 23cm long (~9″). They had large eyes on short stalks, a pair of simple antennae, a horizontal fluke-like tail fan, and twelve pairs of appendages along their body – with the front two pairs at the head end being significantly spinier, and most (or all) of these limbs also bearing paddle-like exopods.

The large carapace enclosed most of the body, and was ornamented with protective spines and a net-like surface pattern that probably increased the strength of the relatively thin chitinous structure.

Together all these anatomical features now indicate that tuzoiids were early mandibulates (part of the lineage including modern myriapods, crustaceans, and insects), and were probably very closely related to the hymenocarines.

Tuzoiids seem to have been active swimmers that probably cruised around just above the seafloor, with their stout legs suggesting they could also walk around if they flexed their valves open. The arrangement of their spiny front limbs wasn’t suited to grabbing at fast-swimming prey, but instead may have been used to capture slower seafloor animals or to scavenge from carcasses.

Ambulator

Diprotodontids were large herbivorous marsupials distantly related to modern wombats and koalas, with some species reaching body sizes comparable to rhinos.

Ambulator keanei here was a mid-sized example, closer to bear-sized at around 1m tall at the shoulder (~3’3″). It lived in South Australia during the Pliocene, about 3.9-3.6 million years ago, at a time when the climate was becoming drier and the local habitat was shifting towards open grasslands – and so it was was one of the first diprotodontids known to have specialized its limb anatomy for more efficient long-distance walking.

A bone in its wrist was modified into a heel-like structure, and skin impressions show large cushioning fleshy pads on the undersides of its feet. Its feet were also rotated to bear weight mainly on the outside edges, similar to the condition seen in some ground sloths, and its fingers and toes appear to have been held raised up off the ground while walking.

Rechnisaurus

Rechnisaurus cristarhynchus here was a member of the dicynodonts, a group of stocky herbivorous beaky-jawed synapsids that were distantly related to modern mammals. Living in what is now eastern India during the Middle Triassic, about 247-242 million years ago, it’s only known from a single fossil skull – but based on the body proportions of better-known close relatives like Kannemeyeria it was probably somewhere around 1.2m long (~4′).

It had a raised bony crest running down the middle of its snout, with deep bowl-like depressions on either side that probably served to make the crest seem visually larger it already was. (They probably didn’t house any weird soft-tissue structures, however, since these type of dicynodonts tended to have very extensive keratinous coverings over their snouts.)

It also had raised bony areas around its parietal eye, and extensive bony flanges covering most of its tusks giving its face a sort of jowly appearance. All these features were probably for visual display and may have been brightly colored in life.

And, while I usually like to reconstruct dicynodonts as extensively fluffy… recently some fossil specimens of Lystrosaurus have been found showing bumpy leathery skin impressions. This doesn’t necessarily mean that all dicynodonts were hairless (especially since there are still those Permian coprolites), but since kannemeyeriiformes like Rechnisaurus were quite closely related to Lystrosaurus, I’ve gone with no fuzz at all on this one.

Crassigyrinus

Crassigyrinus scoticus was an early tetrapod from the early Carboniferous Period, known from ancient coal swamps of Scotland, Nova Scotia, and West Virginia between about 350 and 330 million years ago.

Around 2m long (6’6″), it had an elongated streamlined body with tiny vestigial-looking forelimbs, and a pelvis that wasn’t well-connected to its spine – features that suggest it had re-evolved a fully aquatic lifestyle at a time when its other early tetrapod relatives were specializing more and more for life on land.

Fossils of its skull are all rather crushed, and traditionally its head shape has been reconstructed as unusually tall and narrow. But a more recent study using CT scanning has instead come up with a wider flatter shape more in line with other early tetrapods.

Its mouth also had a very wide gape and a strong bite, and it may have occupied an ecological role similar to that of modern crocodilians, lurking in wait to ambush passing prey.

Danielsraptor

The evolution of falcons is rather poorly understood. Thanks to genetic evidence we know that they’re closely related to seriemas, parrots, and passerines, but their fossil record is patchy and little is known about the early members of their lineage.

But a group knows as masillaraptorids are giving us a rare glimpse at what some early falconiforms were up to. Known from the Eocene of Europe, these long-legged predatory birds seem to have been caracara-like terrestrial hunters specializing in chasing down prey on foot – although their wings and tails indicate they were also still strong fliers.

Danielsraptor phorusrhacoides lived during the early Eocene, about 55 million years ago, in what is now eastern England. Although only known from partial remains, it was probably around 45-60cm long (~1’6″-2′), and it had a large hooked beak with a surprising amount of convergent similarity to those of the flightless South American terror birds.

Its mixture of falcon-like and seriema-like features may indicate that the common ancestor of both of these bird groups was a similar sort of leggy ground-hunting predator.

Araeoscelis

Araeoscelis gracilis was a superficially lizard-like animal that lived during the mid-Permian, around 275 million years ago, in what is now Texas, USA. About 60cm long (~2′), it had a slender body, proportionally long legs, and a solidly-built skull with strong teeth, suggesting that it was a fast runner that specialized in cracking open the carapaces of thick-shelled prey.

It was one of the last known members of a lineage known as araeoscelidians, which are usually considered to be very early members of the diapsid reptiles – but a recent study has proposed they might have even more ancient roots than that, possibly being a branch of stem–amniotes instead.

Neolicaphrium

Neolicaphrium recens here might look like some sort of early horse, but this little mammal was actually something else entirely.

Known from southern South America during the late Pleistocene to early Holocene, between about 1 million and 11,000 years ago, Neolicaphrium was the last known member of the proterotheriids, a group of South American native ungulates that were only very distantly related to horses, tapirs, and rhinos. Instead these animals evolved their remarkably horse-like body plan completely independently, adapting for high-speed running with a single weight-bearing hoof on each foot.

Neolicaphrium was a mid-sized proterotheriid, standing around 45cm tall at the shoulder (~1’6″), and unlike some of its more specialized relatives it still had two small vestigial toes on each foot along with its main hoof. Tooth microwear studies suggest it had a browsing diet, mainly feeding on soft leaves, stems, and buds in its savannah woodland habitat.

It was one of the few South American native ungulates to survive through the Great American Biotic Interchange, when the formation of the Isthmus of Panama allowed North and South American animals to disperse into each other’s native ranges. While many of its relatives had already gone extinct in the wake of the massive ecological changes this caused, Neolicaphrium seems to have been enough of a generalist to hold on, living alongside a fairly modern-looking selection of northern immigrant mammals such as deer, peccaries, tapirs, foxes, jaguars… and also actual horses.

Some of the earliest human inhabitants of South America would have seen Neolicaphirum alive before its extinction. We don’t know whether they had any direct impact on its disappearance – but since the horses it lived alongside were hunted by humans and also went extinct, it’s possible that a combination of shifting climate and hunting pressure pushed the last of the little not-horses over the edge, too.

Ursactis

Soft-bodied annelid worms only very rarely fossilize, so the group’s origins during the Cambrian Period are still rather poorly understood. So far about thirteen different species have been found in sites of exceptional preservation, showing that even very early on in their evolution these worms had already diversified into a wide range of ecologies including bottom-feeders, carnivores, swimmers, tube-builders, and even symbiotes sharing living space with early acorn worms.

Ursactis comosa here adds a fourteenth species to the list. Found in a newly-discovered outcrop of the 508-million-year-old Burgess Shale fossil deposits in western Canada, it’s known from nearly 600 specimens clustered together in several large groups, making it the current best-known and most numerous of all Cambrian annelids.

Up to about 1.5cm long (~0.6″), it was a polychaete-like worm bearing bundles of long bristles. There was a pair of large sensory palps on its head, and its body was made up of an unusually small number of segments – just 10, with larger individuals just increasing the size of their segments instead of adding on additional ones like most modern annelids.

Unlike other Cambrian annelids it also shows some evidence of basic tagmatization, differentiating some of the rear segments of its body with much longer bristles.

The large numbers of Ursactis found preserved in one place suggests these worms were exhibiting some sort of swarming behavior. Since ages from juveniles to fully-grown adults are represented together, and their anatomy indicates they were crawling detritivores, they were probably all taking advantage of a particularly nutrient-rich patch of seafloor at the time they were abruptly buried in a mudslide.

Dunkleosteus

With its armored head and blade-like jaws, the placoderm fish Dunkleosteus terrelli is an iconic Paleozoic animal.

Living during the Late Devonian, about 375-359 million years ago, in subtropical waters covering parts of what are now North America and Europe, this species is known mostly just from the bony plates that covered its head and thorax. The rest of its skeleton was cartilaginous and rarely ever fossilized (only a few vertebrae and the pectoral fin are currently known), so its full body shape and size is poorly understood, and previous length estimates have ranged all the way up to 10m (33′).

…Except it turns out it wasn’t nearly that big.

Based on its head proportions, along with comparisons to more complete remains of other arthrodire placoderms, recent studies instead come up with a maximum length of about 4m (~13ft) – giving Dunkleosteus a much shorter-but-heavier chunky body shape, more like a tuna than a shark.

But even after this size revision Dunkleosteus would have still been one of the largest animals around at the time, with the ability to snap its jaws open at high speed and an incredibly strong bite force. It was probably specialized to mainly prey on other heavily-armored animals such as other placoderms and shelled cephalopods, and was likely a strong swimmer with a shark-like tail fin.

Preserved stomach contents in one fossil show remains of the fast-swimming cartilaginous fish Orodus – suggesting that much like the modern tuna it resembled, Dunkleosteus was also capable of bursts of high speed.

Rhombichthys

Ellimmichthyiformes were a group of ray-finned fish known from the early Cretaceous to the mid-Oligocene, about 140-30 million years ago. For much of that time they were quite widespread, found in various marine, estuarine, and freshwater environments across Africa, Eurasia, and the Americas.

Closely related to modern clupeiformes (herrings, sardines, and anchovies), and characterized by two rows of bony scutes – one in front of the dorsal fin and the other along the belly – they’re also known by the nickname “double‐armored herrings”.

Rhombichthys intoccabilis was a rather unusual-looking ellimmichthyiform from the mid-Cretaceous, around 95 million years ago. Living in shallow reef and lagoon waters covering what is now the West Bank in the Middle East, it was about 20cm long (~8″) and had a tall narrow dorsal fin along with incredibly elongated belly scutes that gave its body a rhombus-like profile.

Juveniles of this species seem to have lacked the extended belly scutes, instead having a much more rounded body shape. This may indicate that adults and juveniles occupied very different ecological roles, or that the distinctive scutes might have been a secondary sexual characteristic involved in displaying for courtship and reproduction.