Category: science illustration

Litovoi

Litovoi tholocephalos, a multituberculate mammal from the Late Cretaceous of Romania (~70-66 mya). Living on what was at the time the large offshore Hațeg Island, this rat-sized animal (about 25cm /10″ long) was part of a lineage of insectivorous multis called the kogaionids, with the same sort of red-colored enamel on its teeth as other species like Barbatodon.

Its brain was surprisingly tiny proportional to its size – one of the smallest known brain-to-body ratios of any mammal, and more similar to those of non-mammalian cynodonts – but it also seems have been highly specialized for processing sensory input, with relatively enormous regions associated with smell, eyesight, balance, and motor control. The olfactory bulbs of its brain were so enlarged, in fact, that they caused its skull to bulge out into an unusually dome-shaped forehead.

Its reduced brain size may have been due to limited food availability on its isolated island home. Brains are very metabolically expensive organs, and some other extinct island mammals like hippos, hominids, and goats are also known to have evolved smaller brain sizes. Modern shrews even seasonally shrink their own brains during winter for similar energy-saving reasons.

Caelestiventus

Caelestiventus hanseni, a pterosaur from the Late Triassic of Utah, USA. Living about 208-210 million years ago, it was very closely related to Dimorphodon – but unlike its younger coastal-dwelling relative it instead lived in a desert environment made up of a massive sand dune sea with occasional interdunal lakes.

It’s the earliest known example of a desert pterosaur, over 60 million years older than other examples, suggesting that even fairly early in their evolutionary history these flying animals had already adapted to a much wider range of habitats than previously thought.

Although only known from a partial skull and a single wing bone, it was probably one of the largest Triassic pterosaurs with a wingspan of over 1.5m (4′11″). It had a “keel” on its lower jaw that may have supported a soft-tissue crest or a pelican-like throat pouch, and there were several different types of teeth in its mouth – large pointed fangs at the front, “leaf-shaped” blades further back in its upper jaw, and numerous much smaller teeth along its lower jaw.

The skull roof also preserved the impression of Caelestiventus’ brain shape, showing that it had very well-developed vision but a poor sense of smell.

Thanahita

Thanahita distos, a recently-named species from the mid-Silurian of the UK (~430 mya).

This little lobopodian was very closely related to the famous Cambrian Hallucigenia, but it lived over 70 million years later – giving us the first evidence that these weird worms weren’t just short-lived “evolutionary experiments”, but must have actually been a very successful lineage that thrived for quite a long time.

Measuring around 3.5cm long (1.4″), it had seven pairs of legs tipped with one or two claws each, and at least two pairs of shorter tentacles on its neck. The head region of the only known fossil specimen wasn’t preserved, so it’s unclear exactly what its front end looked like – but it would have probably been quite similar to Hallucigenia with a slender oval head, two simple eyes, and a small round mouth ringed by tiny teeth.

Unlike its spiky relative, however, Thanahita’s back was covered in rows of numerous small raised soft-tissue “tufts”. I’ve reconstructed it here with them brightly warning colored, mimicking stinging coral polyps.

Almost-Living Fossils Month #27 – Those Giant Sharks

For the final entry this month, let’s look at a particularly famous lineage: the megatooth sharks.

More formally known as the otodontids, the megatooths were a group of sharks that first appeared in the Early Cretaceous, about 115 million years ago. They were a branch of the mackerel shark lineage – making them evolutionary cousins to a variety of modern species like the great white shark, basking shark, and goblin shark – and had a near-worldwide distribution, with fossils known from every continent except Antarctica.

Early otodontids in the Cretaceous were usually small-to-medium sized, around 2-3m long (6′6″-9′10″), but after surviving through the end-Cretaceous extinction they took over the marine apex predator niches left vacant by the vanished mosasaurs and plesiosaurs and began to get very big. Species of Otodus in the Paleocene and early Eocene may have reached sizes of at least 9m long (29′6″), twice the size of an average great white.

Their teeth gradually became proportionally larger in their jaws, losing their side cusplets and taking on a chunky triangular shape with finely serrated edges. This gave them an incredibly powerful bite force, and they would have probably fed on pretty much any other large marine vertebrates they could catch, including bony fish, smaller sharks, turtles, and early penguins – and then when marine mammals like early whales and sirenians appeared in the mid-Eocene, they adapted to this new food source too.

By the Late Eocene (~35 mya) the Otudus lineage was still developing even chunkier and more serrated teeth, and by the Early Oligocene (~28 mya) Otodus chubutensis reached even larger sizes rivaling the modern whale shark at around 12m long (39′4″).

But the most well-known member of the group evolved just a few million years later in the Early Miocene (~23 mya) – the absolutely enormous “megalodon”.

There’s some debate about what genus name megalodon should be assigned to – at the moment its formal name is usually considered to be Otodus megalodon, but some paleontologists place it in Carcharocles or Megaselachus or Procarcharodon instead. Whatever you want to call it, it was a ridiculously big shark – size estimates range up to about 18m (59′), which would make it potentially the largest fish to have ever lived.

Since these huge sharks are all known mostly from just their fossilized teeth (and occasionally a few exceptionally preserved cartilaginous vertebrae), it’s hard to tell what they actually would have looked like in life. Megalodon is frequently depicted as simply a scaled-up great white, but it’s unclear how accurate that really is – it may have convergently resembled a giant great white due to their similar predatory habits, or it could have had a build more like the larger basking shark or whale shark.

A preserved megalodon skull has actually been found, but no studies of it have been published yet. It might give us some important clues about the head shape of this giant shark, but until there’s some official information all we can do is continue to speculate.

Megalodon was a highly successful species, living all around the world in warm and temperate ocean waters for around 20 million years. Its teeth have been found in association with the bones of many different smaller whale species, suggesting it frequently ate marine mammals, and the patterns of the bite marks indicate it probably used different hunting strategies than modern great whites. Some whales seem to have been heavily rammed and then had their ribcages bitten into, targeting their hearts and lungs, while others had their flippers ripped off to immobilize them.

During the Pliocene (~5-2.6 mya), however, megalodon began to struggle. Cooling oceans and changes in the abundance of the marine mammals it ate began to restrict its available prey. Baleen whales started to grow too large for it to effectively hunt, since it preferred to target smaller species, and they also shifted their ranges towards the cold polar waters that megalodon didn’t seem to be able to survive in. In addition, dropping sea levels may have destroyed most of its shallow warm-water nursery sites, making it harder for newborn young to survive into adulthood.

By the end of the Pliocene, somewhere between 3.6 and 2.6 million years ago, megalodon went completely extinct. Despite some very pseudoscientific claims, there’s definitely no living “Meg” out there anymore – if there was, we’d be constantly finding freshly-shed teeth and whales with giant bite marks on their bodies!

Almost-Living Fossils Month #26 – Angry Land-Flamingo-Ducks

The presbyornithids were an early group of waterfowl birds – relatives of modern ducks, geese, swans, and screamers – that first appeared in the Late Cretaceous, about 71 million years ago. With their long necks, long legs, and duck-like bills adapted for filter-feeding, they seem to have essentially been primitive ducks converging on the body shape and lifestyle of flamingos – and as a result they’re sometimes even nicknamed “flamingo ducks”.

They lived in shallow freshwater environments all around the world, and after surviving through the end-Cretaceous extinction they even became some of the most common waterbirds in the early Cenozoic. Some species have been found in large bonebeds containing fossils from thousands of individuals all in one place, suggesting they were very social and lived in huge flocks.

Around the mid-to-late Eocene (~40-37 mya) they seemed to disappear completely, until some fossils from Australia that were originally thought to be from a species of ancient stone-curlew were reassessed in 2016 and found to actually represent the latest-surviving members of the presbyornithids.

Named Wilaru, this bird lived in South Australia during the Late Oligocene and Early Miocene (~28-20 mya). Two different species have been identified: Wilaru tedfordi and its slightly larger and stockier descendant Wilaru prideauxi. With only partial pieces of their skeletons known it’s difficult to estimate their full life size, but based on similar presbyornithids they were probably both somewhere around 1m tall (3′3″).

As well as outliving the rest of their kind, the two Wilaru species were also rather weird compared to the other known flamingo-ducks, with adaptations that indicate they were spending much more time walking around on land than wading in water. Their feet resembled those of modern screamers (which are also more terrestrial) and may have partially or fully lost their webbing, and since they lived alongside various other species of waterfowl and early flamingos they clearly weren’t competing for the same ecological niches. It’s possible they might have also shifted away from their ancestral filter-feeding diet, perhaps becoming more herbivorous, but without any preserved skulls we can’t tell for certain.

Unlike other presbyornithids they also had large spurs on their wings – and based on the behavior of modern spurred waterfowl this suggests they were much less social. They were probably rather aggressive animals, living solitary or in pairs and fighting each other over mates and territory.

This major departure from the lifestyle of their ancestors may have been what allowed Wilaru to survive for so much longer than all the other presbyornithids. They might potentially have lasted a few more million years into the mid-Miocene, but a cooling and drying climate – especially a sudden temperature drop about 14 million years ago – may ultimately have altered their habitat and food sources too quickly for them cope with.

Almost-Living Fossils Month #25 –  Europe’s Fully Aquatic Frogs

The palaeobatrachids were a group of frogs, part of a fairly “primitive” lineage that also includes the living pipids. They first appeared in the fossil record about 70 million years ago in the Late Cretaceous, but may have actually originated much earlier, perhaps as far back as the Late Jurassic (~145 mya).

These frogs lived mainly in Europe, with a few possible remains also known from North America in the Cretaceous. They were fully aquatic, spending their entire lives in water, and fully-grown adults looked similar to modern Xenopus clawed frogs, with slightly flattened egg-shaped bodies, upwards-facing eyes, and long fingers and toes.

Some fossils preserve soft-tissue impressions, showing internal organs such as unusual bag-shaped lungs. Eggs and juveniles have also been found, and while most species’ tadpoles usually reached lengths of around 6cm (2.4″), a few were comparatively gigantic, growing to over twice that size.

The end-Cretaceous extinction (~66 mya) had little overall effect on the palaeobatrachids, and they continued to thrive in the warm wet environments of Europe during the early Cenozoic. But as climates in Western Europe gradually became drier and cooler starting in the Early Oligocene (~33 mya) they mostly disappeared from that region and instead shifted east towards Central and Eastern Europe, ranging as far as Russia.

By the Late Pliocene (~3 mya) they were struggling to cope with the ongoing cooling and drying, and the onset of the Pleistocene glaciations made things even worse for them.

Palaeobatrachus langhae was probably the last species of these frogs, known from the Early Pliocene to the mid-Pleistocene (~5 mya – 500,000 years ago). Growing to about 10cm long (4″), it lived in some of the final refuges of the palaeobatrachids in Eastern Europe, inhabiting inland temperate areas where winter temperatures weren’t too harsh.

Unfortunately the palaeobatrachids didn’t quite manage to make it through the Ice Age, ending up trapped by their fairly specialized habitat preferences. During repeated glacial periods the temperatures became too cold for them, freezing the water they depended on, but the warmer climates to the south were also too dry for them to migrate into – and with nowhere to go, they finally went completely extinct just half a million years ago.

Almost-Living Fossils Month #24 – Sabertoothed Sparassodonts

Along with the marsupials and the polydolopimorphs, the sparassodonts were one of the lineages of metatherian mammals that inhabited South America during its “great isolation” for most of the Cenozoic. And despite having to share the large carnivore niches with both the terror birds and the sebecosuchian crocs, they still managed to become the main mammalian predators of the region.

Their first definite fossils come from the start of the Paleocene (~65 mya), but they probably actually originated sometime in the Late Cretaceous before the mass extinction. A currently-unnamed skull from Mongolia (~70 mya) appears to be either an early sparassodont or a very close relative, and a North American metatherian called Varalphadon (~90 mya) may also be linked to the group. It’s possible that, like the marsupials, they may have first evolved in North America and later spread into South America before it became isolated.

Like their marsupial relatives they would have given birth to tiny undeveloped young, although we don’t know for certain if they actually had pouches or not. Their epipubic bones were highly reduced, so it’s possible they didn’t have pouches – but they also might have had mostly cartilaginous epipubics (like thylacines) that just didn’t fossilize.

Over the course of the Cenozoic the sparassodonts convergently evolved many similar features to placental carnivorans, with carnassial teeth for shearing through flesh and a wide variety of body shapes ranging from small weasel-like forms to long-snouted ambush hunters to large hyaena-like bone-crushers.

But by far the most famous members of the group were the thylacosmilids. First appearing in the Early Miocene, about 20-15 million years ago, these sparassodonts developed huge elongated canine teeth that resembled those of sabertoothed cats. Unlike the felid sabertooths, however, thylacosmilids’ fangs grew continuously and their lower jaws had long bony flanges that supported and protected their teeth when theirs jaws were closed.

Thylacosmilus atrox was the last and most highly specialized of the thylacosmilids, living from the Late Miocene to the Late Pliocene, around 9-3 million years ago. About 1.2-1.5m long (~4-5′) and standing 60cm tall at the shoulder (2′) it was similar in size to a modern jaguar – not huge compared to some placental predators, but still one of the largest of all known carnivorous metatherians.

Despite its huge fangs it actually had a fairly weak bite force, instead relying on its strong forelimbs to immobilize its prey before delivering precise deep stabs into soft body parts using powerful neck muscles. The structure of its limbs also suggests it wasn’t a fast runner, and it probably had to stalk or ambush its targets.

Although the extinction of Thylacosmilus and the other last sparassodonts is often blamed on being out-competed by similar placental carnivores arriving during the Great American Interchange, it seems like that wasn’t actually the case. Many of their northern placental equivalents such as Smilodon didn’t enter South America until the mid-Pleistocene (~1-0.7 mya), over 1.5 million years after the last record of any living sparassodonts. So it’s likely they never actually met each other, and the disappearance of the sparassodonts may be more linked to cooling climates in the Pliocene and early Pleistocene.

Almost-Living Fossils Month #23 – Enamel-Armored Lizards

The glyptosaurines were a group of lizards that first appeared in the Late Cretaceous, about 85 million years ago. They were an early branch of the anguid lineage, originating in North America, and had heavily armored bodies covered in bony osteoderms – superficially similar to those of modern beaded lizards, but structurally much more complex with the outermost layer formed from a unique enamel-like substance called osteodermine.

They were some of the few lizards to survived through the end-Cretaceous extinction 66 million years ago (which killed off over 80% of the lizard species known at the time) and went on to become quite successful in the warm climates of the early Cenozoic.

They spread across to Europe and Asia and developed much larger body sizes, going from small 10cm-long (4″) forms in the Early Paleocene (~65 mya) to over 60cm long (2′) by the mid-Eocene (~40 mya). In North America and Europe they became common enough that they were probably important parts of the local ecosystems, and their widespread distribution suggests they were able to adapt to a variety of different habitats and environmental niches.

Their teeth resembled those of modern omnivorous lizards like blue-tongued skinks, suggesting they had a similar generalist diet – although their strong jaws have also been proposed to be specializations for crushing hard-shelled invertebrates such as snails.

Helodermoides tuberculatus here was one of the largest glyptosaurines, about 65cm long (2′2″). It lived during the Late Eocene and Early Oligocene (~34-33 mya) in the northwestern and midwestern United States, with fossils known from Montana, Wyoming, and Nebraska.

One fossil shows evidence of having lost part of its tail, probably dropping it in a self-defense behavior to escape a predator. However, unlike the regenerating tails of many other lizards, the osteoderms of Helodermoides instead seem to have formed a thick rounded bony cap over the wound, preventing any significant regrowth and leaving its tail permanently stumpy.

During the Late Eocene and Early Oligocene the glyptosaurines began to disappear, probably struggling to cope with cooling and drying climates, and their last definite fossils date to about 30 million years ago. Possible fragmentary remains from as late as the Early Miocene of Central Europe (~16 mya) may indicate that a few isolated late-surviving members of the group persisted on for a while longer, but if they did hang on that long they were probably finished off by further sharp temperature drops in the mid-Miocene.

Almost-Living Fossils Month #22 – Some Marine Crocs

First appearing in the Middle Jurassic, about 175 million years ago, the tethysuchians were a group of neosuchian crocodilians – part of the same lineage that includes all living crocs, although they were probably more closely related to the highly marine thalattosuchians than to modern forms.

Their fossil remains have been found almost globally, except for in Antarctica and Australia, and they appear to have been highly aquatic animals living in both freshwater and marine environments. Most members of the group had very long and slender gharial-like snouts, indicating they were specialized for fish-eating, but some (like the enormous Sarcosuchus) developed broader or shorter snout shapes that suggest more generalized diets of whatever they could catch.

One lineage of tethysuchians known as the dyrosaurids evolved around the mid-Cretaceous (~100-90 mya) and quickly spread around most of the world. These crocs mainly inhabited coastal marine waters, with a few species also living full-time in estuaries or rivers.

They had tall vertebrae around their shoulders, giving them a slightly hump-backed appearance and anchoring large neck muscles that allowed them to quickly whip their jaws around to catch fast-moving fish. Their deep vertically-flattened tails were capable of an even more powerful swimming stroke than those of modern crocs, and their well-muscled limbs probably made them strong walkers when on land.

The dyrosaurids were some of the few marine reptiles to survive through the end-Cretaceous extinction (~66 mya) relatively unscathed, and several species are known from both sides of the K-Pg boundary. This may be because the marine dyrosaurids are thought to have seasonally migrated inland to breed in freshwater environments, with juveniles spending their early lives in rivers and only returning to the coasts as adults – and since freshwater ecosystems were much less affected by the mass extinction than marine ones, this allowed them to continue on while groups like the mosasaurs and plesiosaurs died out.

Dyrosaurus maghribensis here lived during the Late Paleocene and Early Eocene of  Morocco (~56-48 mya). It was similar in size to the largest living crocs, around 6m long (19′8″), but had thinner and less extensive bony osteoderm armor.

During the Eocene the dyrosaurids began to disappear, and by the Late Eocene (~37 mya) the last known species were found only in northern Africa. It’s not entirely clear why these once-successful tethysuchians began to decline, but they may have been struggling to deal with the cooling climate trends at the time. If they managed to persist until the end of the Eocene, sudden temperature and sea level drops during the Eocene-Oligocene extinction (~33 mya) probably finished them off entirely.

Almost-Living Fossils Month #21 – More Sharks

First appearing in the Early Permian, about 290 million years ago, the synechodontiformes were an early branch of the neoselachian lineage of cartilaginous fish, slightly closer related to living sharks and rays than to the hybodontiformes featured earlier this month.

They originated in the Paleo-Tethys Ocean and survived through the devastating end-Permian “Great Dying” mass extinction (~252 mya), then went on to quickly spread around most of the world and also survive through the Triassic-Jurassic extinction (~201 mya). During the Jurassic and Cretaceous they became quite common and diverse, taking over some of the niches previously occupied by the hybodontiformes and adapting to a range of marine environments from shallow coastal waters to open ocean.

Most known synechodontiform fossil remains are just their teeth, since cartilage skeletons don’t preserve very often, but there are a few rare body fossils that show they were varied in appearance with differing arrangements of dorsal fins and spines.

Paraorthacodus jurensis here was one of the species known from the Late Jurassic of Germany (~155-150 mya). Reaching lengths of at least 1.3m (4′2″), it had only one dorsal fin far back on its body, along with large pectoral fins and a low asymmetrical tail that gave it a superficial resemblance to the modern sixgill sharks.

Its teeth were close in shape to those of sand tiger sharks, and it may have had a similar lifestyle opportunistically hunting prey just above the sea floor in the waters around the continental shelf and slope. Remains of a chimaera in the mouth and gut contents of a couple of Paraorthacodus jurensis fossils suggest that smaller cartilaginous fish were fairly common elements of its diet.

A few synechodontiformes managed to survive the end-Cretaceous extinction 66 million years ago – but while the ancestors of moderns sharks thrived in the Cenozoic, the synechodontiformes never recovered anything close to their Mesozoic levels of success and instead began to decline.

The last known synechodontiforme was a currently-unnamed member of the Paraorthacodus genus, hanging on in the waters around Antarctica in the Late Eocene (~37 mya). If they managed to survive past that time it probably wasn’t for very much longer, and it’s likely they finally disappeared during another extinction event at the Eocene-Oligocene boundary.