Almost-Living Fossils Month #05 – Cryptic Choristoderes

The choristoderes were a group of aquatic reptiles that mostly inhabited freshwater environments. Known mainly from North America, Europe, and Asia, they first appeared in the fossil record in the Late Triassic (~205 mya) – although their lineage could potentially go back further than that – and they varied in appearance from large long-snouted croc-like creatures to more lizard-like and miniature plesiosaur-like forms.

Many them were fully aquatic and spent their entire lives in the water, with some developing the ability to give live birth and others returning to land only to lay eggs (with only females having well-developed enough limbs to be able to haul themselves out onto shore). In some places their fossils are incredibly common, with every life stage represented from babies to adults (even one with two heads!), and yet despite having such detailed knowledge of their lives we still don’t know exactly what type of reptile they actually were.

Their evolutionary origins and relationships are very unclear, with the only real certainly being that they’re at least diapsids. They’re often classified as either archosauromorphs (closer related to crocodilians and dinosaurs/birds) or as lepidosauromorphs (closer related to lizards), but they could also be a much earlier separate branch of the reptile family tree.

Some of the large crocodilian-like neochoristoderes survived into the Cenozoic and initially did quite well for themselves – even outcompeting actual crocodilians in the northern continents for a while – but then they seem to have fallen victim to the cooling and drying climate of the Eocene-Oligocene extinction about 33 million years ago.

But that wasn’t the end of the choristodere lineage just yet.

A small number of fossils of a little choristodere named Lazarussuchus have been found in a few different places around Europe, with the youngest specimens dating to as recently as the Early Miocene (~20-16 mya). Surprisingly it wasn’t closely related to the neochoristoderes at all, but instead seems to have been part of a much older and more “primitive” branch of the choristodere family tree that must have been surviving since at least the mid-Jurassic with very little presence in the fossil record.

At about 30cm long (1′) it was less aquatic than most other choristoderes, with large claws that would have given it good traction on land and a more generalized lizard-like body plan. One specimen even preserves soft tissue impressions, showing that its toes lacked webbing and that it had a low crest running along its tail.

Its lack of specialization may have been the reason for its longer survival, being able to adapt to a wider variety of environments compared to its more water-reliant cousins.

It’s unclear exactly how much closer to present day these rare last choristoderes survived. If they managed to make it through the mid-Miocene extinction then they might potentially have persisted until the onset of the Pleistocene Ice Age 2.5 million years ago – but their fossils are scarce enough that we’ll probably never know for certain.

Almost-Living Fossils Month #04 – The Last Multis

Known almost exclusively from the southern continents of Gondwana – hence their name – the gondwanatheres were part of a widespread and very long-lived group of mammals known as multituberculates.

Although multis resembled placental rodents, and gave birth to tiny undeveloped young in a similar manner to marsupials, they originated much further back in the mammal evolutionary tree. They existed by at least the Early Jurassic (~183 mya), and their ancestry may go even further back into the Late Triassic (~220 mya) if they were descended from haramiyidans.

Multis survived the end-Cretaceous extinction and became very diverse through the first half of the Cenozoic, until a combination of factors such as climate shifts, new types of vegetation, and the evolution of new mammalian predators (and possibly also competition from placental rodents) resulted in most of them going extinct by the Early Oligocene (~33 mya) – with only the gondwanatheres surviving past that point in the then-isolated continent of South America.

Patagonia peregrina was the very last known gondwanathere in the fossil record, living just 21-17.5 million years ago in the Early Miocene of Argentina. Although only teeth and jaw fragments have been found so far, it was probably about 15cm long (6″) and would have been a burrowing herbivore similar to modern gophers or tuco-tuco. Its ever-growing rodent-like teeth were adapted for grazing on tough grasses in its savanna-like habitat, and it would have lived alongside several other now-extinct types of mammal – but we’ll be getting to those ones later in the month.

Since it seems like these last gondwanatheres had survived by retreating into a rather specialized ecological niche, they sadly probably didn’t persist for very long beyond the time of Patagonia. A wave of extinctions associated with sudden climate cooling about 14 million years ago may well have been the final blow to the once-successful lineage of the multituberculates.

Almost-Living Fossils Month #03 – The Crowned Starfish

Known mainly from around Europe – with a few records from the Atlantic coast of North America and the Caribbean – the stauranderasterids were a family of starfish that first appeared about 190 million years ago in the Early Jurassic.

Not much is known about their life ecology, although they seem to have inhabited shallow tropical seas and like many other starfish would probably have preyed on various slow-moving marine invertebrates. Complete specimens are very rare compared to just isolated elements, making their maximum size difficult to estimate, but they likely grew to at least 5-10cm across (2″-4″).

They had enlarged ossicles forming a bumpy “crown” over their central disc, with five arms that could be either narrow and elongated (such as in Stauranderaster coronatus here) or shorter and club-shaped (like Manfredaster bulbiferus). This gave them some visual similarities to modern starfish like Protoreaster, and since they were both part of a larger grouping of starfish called valvatids it’s unclear whether these features mean that they were very closely related to each other or if it was simply due to convergent evolution.

The stauranderasterids survived until at least the Late Paleocene/Early Eocene (~56 mya), but some possible remains from Cuba date to as recently as the Early Micoene (~23-16 mya).

Almost-Living Fossils Month #02 – The Saber-Toothed “Herring”

First appearing in the mid-Cretaceous, about 113 million years ago, Enchodus was a small-to-medium-sized genus of predatory fish. Different species ranged from a few centimeters to up to 1.5m in length (4′11″), with Enchodus gladiolus here being an averaged-sized example at about 60cmlong (2′).

The most distinctive feature of these fish were the enlarged fang-like teeth in both their upper and lower jaws, over 6cm (2.4″) long in the largest individuals, which may have been a specialization for feeding on soft-bodied cephalopods.

Despite having been nicknamed “saber-toothed herrings”, they weren’t actually closely related to herrings at all, instead being part of the aulopiformes – a group also containing modern lizardfish, lancetfish, and a different type of sabertooth fish.

Fossils of various Enchodus species have been found all over the world, and they seem to have been very common and important members of ancient marine ecosytems, occupying mid-level carnivore niches and in turn being eaten by other predators. Their remains have been identified within the preserved stomach contents of marine reptiles such as plesiosaurs and mosasaurs, as well as sharks and hesperornithean birds.

These toothy fish survived through the end-Cretaceous mass extinction and continued their success for almost 30 million years into the Cenozoic, with the last known fossils dating to just 37 million years ago in the Late Eocene. They probably didn’t survive much longer beyond that date, since there was an extinction event at the Eocene-Oligocene boundary (~34 mya), a period of sudden cooling that affected many marine animals at the time.

Almost-Living Fossils Month #01

It’s finally time for “Almost-Living” Fossils Month!

There are various modern species that we call “living fossils” because they’ve survived for very long periods of geological time, but there are also plenty of long-lived and successful groups that didn’t quite manage to make it to the present day.

For all of August I’ll be doing six posts on this topic a week (taking a break on Sundays), and the basic criteria for entries are that a featured organism has to be part of a group that:

• originated in the Mesozoic or earlier
• existed for at least 50 million years
• and survived through the end-Cretaceous mass extinction, only to go extinct later in the Cenozoic

So, let’s begin with…

An illustration of an extinct small mammal. It resembles a shrew, with a pointed whiskery snout, small rounded ears, and a long furred tail. It also has pointed spurs on its ankles.

The Saint Bathans Mammal

Dating to the mid-Miocene, about 19-15 million years ago, this little mammal was discovered in the Saint Bathans fossil deposits in the South Island of New Zealand, and was probably the size of a large shrew, around 10cm long (4″). It doesn’t have an official scientific name yet, but its fragmentary remains might represent something very special.

Not only was it a terrestrial mammal living at a time when none were thought to inhabit New Zealand at all, but it also doesn’t seem to fit into any known group of mammals. It’s definitely not a placental or marsupial or monotreme, and it doesn’t match any of the other “archaic” groups – so it could represent an entirely new lineage we didn’t even know existed.

At best it’s been classified as a theriiform, belonging somewhere between eutriconodonts and multituberculates in the mammal family tree. This means it would have to be part of a group with a very long ghost lineage going all the way back to the Late Triassic or Early Jurassic (~220-190 mya). Whatever it actually was, its ancestors were probably already present on New Zealand when the islands split away from Gondwana in the Late Cretaceous (~85 mya), and must have remained isolated there for tens of millions of years more.

It’s unknown how much closer to present day these mysterious archaic mammals actually survived, but it’s possible they were one of the casualties during the Middle Miocene disruption extinction about 14 million years ago, when the relatively warm climate suddenly cooled. And then land mammals were completely absent from New Zealand until the arrival of humans ~750 years ago.

(It’s also worth noting that the fossils of the Saint Bathans mammal were described over ten years ago, and our knowledge of ancient mammals and their evolutionary tree has changed a lot in that time. New studies or more fossil material might reveal the Saint Bathans mammal to actually be one of the very last survivors of an already-known group, but that would still make it an amazing and unique discovery.)

Xinpusaurus

Thalattosaurs were a weird and rather mysterious group of Triassic marine reptiles. It’s not clear where they actually fit on the reptile evolutionary tree (we know they’re diapsids, but nobody can really agree on anything more definite than that), and they had some very strange skulls that seem to have been highly specialized for something, although their actual function is still unknown.

Xinpusaurus kohi here is known from the Late Triassic of China (~232-221 mya). About 1.3m long (4′3″), with half of that being its paddle-like tail, it had an elongated upper jaw that formed a protruding pointed spear-shaped snout.

It’s not clear whether this odd snoot was an adaptation for hunting similar to the long bills of swordfish – there’s quite a bit of variation in length and shape between different individual specimens – or if it was serving some other purpose like the sexually dimorphic noses of some modern lizards.

Rebellatrix

Coelacanths are famous for being “living fossils”, completely disappearing from the fossil record at the end of the Cretaceous but then being rediscovered alive just 80 years ago. But although they’re often thought to have physically changed very little over the last 300 million years or so, more recent discoveries are starting to show that coelacanth body forms and lifestyles were actually more varied in the distant past.

Meet the wonderfully-named Rebellatrix divaricerca, from the Early Triassic of British Columbia, Canada (~251-247 mya). Measuring around 1.3m long (4′3″), its body shape and large symmetrical forked tail suggest it was adapted for fast swimming. Unlike its slow-moving deep-water modern relatives this coelacanth was a speedy oceanic active predator, convergently similar to tuna or some sharks.

Since it lived in the immediate wake of the end-Permian “Great Dying” mass extinction, Rebellatrix may have rapidly evolved from more standard-looking coelacanths to take advantage of a suddenly vacant ecological niche – or it might be part of a more extensive unusual lineage whose other members simply haven’t been discovered yet.

Barbaturex

Barbaturex morrisoni, a large herbivorous lizard which lived about 40-37 million years ago during the Eocene. Known from Myanmar in Southeast Asia, it’s estimated to have reached lengths of 1.4-1.8m (4′7″-5′10″) and was closely related to modern spiny-tailed lizards.

It had a row of bony knobs along the edges of its lower jaw, which may have supported some sort of display structure. I’ve given it some fleshy double-dewlaps here, and a spiky tail similar to its relatives, but since it’s only known from fragmentary fossils these features are pretty speculative.

Surprisingly Barbaturex was much bigger than a lot of the herbivorous ungulate mammals around at the time, and was also larger than most of the local carnivores – a very different situation to modern ecosystems, where even the biggest plant-eating lizards are still smaller than ungulates.

Eucladoceros

Eucladoceros dicranios, a deer from the Pliocene and Pleistocene of Europe (~3.5-1 mya). Close in size to a modern moose, standing about 1.8m tall at the shoulder (5′10″), the males of this species had a set of particularly large antlers – measuring up to 1.7 meters across (5′6″) and bristling with at least twelve prongs each – giving it the nickname of “bush-antlered deer”.

The more famous “Irish elk” (Megaloceros giganteus) would later develop even bigger antlers, but Eucladoceros was the earliest known deer to evolve this sort of extremely elaborate headgear.

Zby

Zby atlanticus, a sauropod dinosaur from the Late Jurassic of Portugal (~156-151 mya). While its genus name might look like a keyboard smash, it was actually named after the Russian-French paleontologist Georges Zbyszewski, who spent much of his career studying Portuguese fossils.

(As for how to pronounce it, according to the original paper it’s “zee-bee”.)

It was a close relative of Turiasaurus, the largest dinosaur currently known from Europe – and although Zby itself wasn’t quite so enormous it was still pretty big, probably measuring somewhere around 15-19m long (49′2″-62′4″).

In fact, all the sauropods known from Late Jurassic Portugal seem to have grown to very large adult sizes. The complete lack of medium or small forms suggests that other types of herbivorous dinosaurs may have dominated the region’s lower-browsing niches at the time.