Tag: convergent evolution

Stylinodon

In the early Cenozoic mammals were rapidly diversifying and evolving. And while it was the placental mammals that would end up being the most successful across much of the world, they weren’t the first mammal lineage to take advantage of all the ecological niches left vacant in the wake of the end-Cretaceous mass extinction.

The cimolestans were a group of non-placental eutherians – mammals closer related to modern placentals than to marsupials – that very quickly evolved into a wide range of niches during the Paleocene and Eocene, becoming some of the largest mammals of their time and producing forms as varied as squirrel-like, otter-like, ground sloth-like, and hippo-like.

But some of the weirdest of them all were the taeniodonts. Originating back in the late Cretaceous, these herbivorous cimolestans were characterized by short blunt snouts with large front teeth, and limbs with long claws.

Stylinodon mirus here was one of the largest taeniodonts, standing around 70cm tall at the shoulder (2’4″), and was also one of the last of its kind, living during the mid-Eocene about 50-40 million years ago in western North America.

It took the specializations of its lineage to the extreme, with a odd-looking boxy skull with enormous chisel-like ever-growing front teeth similar to those of a rodent – but derived from its canine teeth rather than its incisors.

Stylinodon skull | photograph by Yinan Chen | CC0

Its powerful front limbs and large claws were clearly specialized for digging, and for a long time it was thought to be obvious what its diet was – clearly it must have been unearthing roots and tubers from underground, right?

However, closer looks at its teeth raise a problem with that interpretation. That sort of food source should have left numerous telltale marks on the chewing surfaces of its teeth, scratches and gouges and abrasions from dirt and grit mixed in with the roots being eaten.

Yet Stylinodon barely shows any of those wear marks, suggesting that it rarely actually ate those food items. Its tooth surfaces were instead worn very smooth, indicating that it was eating something particularly tough that was constantly “polishing” them as it chewed — but what exactly that food source was is still unknown.

It may also have used its forelimbs to help pull down branches down towards its mouth, stripping off leaves and bark similar to ground sloths, chalicotheres, and therizinosaurs – but it probably did mostly use those big claws to actually dig, just perhaps mainly to construct large burrows rather than to find food.

Furcacetus

The two living subspecies of the South Asian river dolphin are the last surviving members of a lineage known as the Platanistoidea, an early evolutionary branch of the toothed whales. This group was once much more diverse and widespread than their modern representatives, found in oceanic habitats around the world from the Oligocene to the mid-Miocene.

Many of them had forward-pointing protruding teeth at the tips of their snouts, resembling those of some plesiosaurs or pterosaurs, suggesting they were a convergent adaptation used for snagging hold of slippery soft-bodied aquatic prey.

Furcacetus flexirostrum is one the newest additions to this group, named and described in late March 2020. It lived in Pacific coastal waters around Peru during the early Miocene, about 19-18 million years ago, and was about the same size as modern South Asian river dolphins at around 2.3m long (7’7″).

And it had a uniquely-shaped snout for a cetacean, curving upwards for most of its length but then turning downwards right at the tip, which along with large forward-pointing teeth gave its jaws a vaguely crocodilian appearance.

A closeup view of the jaws of Furcacetus.

Much like slender-snouted crocodilians and spinosaurids, this arrangement would have allowed Furcacetus to make quick bites at small-fast-moving prey like fish and crustaceans.

Weird Heads Month #29: Giant Saw-Toothed Birds

The pelagornithids, or “pseudotooth birds”, were a group of large seabirds that were found around the world for almost the entire Cenozoic, existing for at least 60 million years and only going completely extinct just 2.5 million years ago.

Their evolutionary relationships are uncertain and in the past they’ve been considered as relatives of pelicaniformes, albatrosses and petrels, or storks, but more recently they’ve been proposed to have been closer related to ducks and geese instead.

Whatever they were, they were some of the largest birds to ever fly, and many of the “smaller” species still had wingspans comparable to the largest modern flying birds.

But their most notable feature was their beaks. Although at first glance they look like they were lined with pointy teeth, these structures were actually outgrowths of their jaw bones covered with keratinous beak tissue. While these bony spikes would have been useful for holding onto slippery aquatic animals like fish and squid, they were actually hollow and relatively fragile so pelagornithids must have mainly caught smaller prey that couldn’t thrash around hard enough to break anything.

The serrations also only developed towards full maturity, and the “toothless” juveniles may have had a completely different ecology to adults.

Pelagornis chilensis here was one of the larger species of pelagornithid, with a wingspan of 5-6m (16’4″-19’8″), known from the western and northern coasts of South America during the late Miocene about 11-5 million years ago.

Like other pelagornithids it was highly adapted for albatross-like dynamic soaring, with long narrow wings that allowed it to travel huge distances while expending very little energy – but with its proportionally short legs it would have been clumsy on the ground and probably spent the vast majority of its life on the wing, only returning to land to breed.

Weird Heads Month #28: Pig-Nosed Tanks

There’s already been quite a few Triassic weirdos in this series, so it’s probably not much of a surprise that we’ve got one more before the end of the month.

Desmatosuchus spurensis here was part of a group called aetosaurs, a lineage of heavily-armored herbivorous archosaurs which convergently resembled the later ankylosaurs but were more closely related to modern crocodilians.

Living in the Southwestern and South Central United States during the late Triassic, about 221-210 million years ago, Desmatosuchus measured around 4.5m long (14’9″) and was covered in thick interlocking bony osteoderms that protected its back, sides, belly, and tail, with longer spines over its neck and shoulders.

It had a triangular skull with a few blunt teeth at the back of its jaws and a toothless snout at the front. Its pointed lower jaw probably had a keratinous beak, while its upper jaw had an odd upturned flared tip. What exactly was going on with that snoot is uncertain, but it may have anchored a shovel-shaped upper keratinous beak – or, since there was a little bit of flexibility between its snout bones, possibly even a pig-like nose!

It probably mostly ate soft vegetation, using its shovel-like snout to dig up roots and tubers, although similarities with the skulls of modern armadillos suggest it may also have fed on insect grubs.

Weird Heads Month #27: The Weirdest Wildebeest

Earlier in this series we saw some ruminants with bizarre-looking headgear, but there was another species in that group that evolved a completely different type of strange head.

Rusingoryx atopocranion was a close relative of modern wildebeest that lived during the late Pleistocene, around 100,000 to 50,000 years ago. Its fossil remains are known from the Kenyan part of Lake Victoria, on Rusinga Island – an area which wasn’t actually an island at the time due to lower lake levels, and was instead part of a hot dry grassland environment.

Standing about 1.2m at the shoulder (~4′), it had an oddly-shaped skull with a pointed snout and a highly domed forehead. But this wasn’t the thick bony dome of a headbutting animal – this structure was narrow and fairly fragile, and had looping nasal passages running through it.

Instead it was something never before seen in any mammal: a bony nasal crest convergently similar to those of hadrosaurid dinosaurs.

Juveniles had less developed crests, developing them as they matured, and one skull that may represent an adult female also has a smaller crest, suggesting that this feature was sexually dimorphic.

Based on just the anatomy of the nasal passages Rusingoryx may have honked at a frequency similar to a vuvuzela, but the added length of its vocal tract could have lowered this pitch even further, closer to infrasound ranges – so more like a tuba! Such low frequencies can travel very long distances and are also below the hearing range of many carnivores, and would have effectively allowed Rusingoryx to shout at each other in “stealth mode”.

Weird Heads Month #25: The Case of the Missing Trunk

The rhino-like toxodontids from earlier in this series weren’t the only weird-headed South American ungulates. Another group known as the litopterns evolved in a different direction, becoming long-legged fast-moving animals convergently filling the same sort of ecological niches as modern horses, deer, bovids, camelids, and giraffids.

Macrauchenia patachonica was one of the strangest members of this lineage, living from the Late Miocene to the end of the Pleistocene, between about 7 million years ago and just 12,000 years ago.

It stood around 1.8m at the shoulder (5’11”) and resembled a large camel or llama with thee-toed hoofed feet, but its head was… confusing.

Its skull had a bizarre combination of features, with a shape closer to a sauropod dinosaur than a mammal, a cartoonish-looking set of teeth, and its nostrils set up high above its eyes, more like a cetacean blowhole than a terrestrial herbivore.

Due to its retracted nostrils it’s commonly been restored with an elephant-like or tapir-like trunk. And while a trunk gives Marauchenia a wonderfully weird and memorable appearance, there’s just one problem with that interpretation.

There’s no evidence for it.

Aside from its nostrils being far back on its head, it didn’t have any other features associated with anchoring the complex musculature of a trunk. In fact, a recent study found that its skull characteristics were much closer to those of moose than tapirs!

It seems more likely that it had a moose-like bulbous fleshy nose – possibly giving it an enhanced sense of smell or functioning as a resonating chamber – perhaps with slightly retracted external nostrils like a giraffe or sauropod to prevent it from being stabbed in the nose when feeding on spiky vegetation.

Whatever it was doing with its weird schnoz, it was clearly a highly successful species, since it was found across most of South America in a wide range of habitats.

Weird Heads Month #23: Dome-Headed Claw-Horses

Much like Platybelodon from a few entries back, chalicotheres look like a fictional creature design rather than something that actually existed.

These animals were odd-toed ungulates related to modern horses, tapirs, and rhinos, who ranged across Africa, Eurasia, and North America for a large chunk of the Cenozoic. Instead of hooves they had large claws on their feet, and they appear to have occupied the same sort of ecological niche as ground sloths or therizinosaurs – sitting or rearing up on their hind legs to browse on high vegetation, using the hook-like claws on their forelimbs to pull down and strip branches.

There were two different lineages of chalicotheres which developed along slightly different evolutionary paths: the knuckle-walking gorilla-like chalicotheriines and the more goat-like schizotheriines.

Tylocephalonyx skinneri here was one of the latter group, known from the Miocene of North America about 16-13 million years ago. Standing about 2m tall at the shoulder (6’6″), it had the same sort of chunky body as other schizotheriines and walked around with its large front claws held up to keep them raised away from the ground.

But there was also an unusual feature on its otherwise rather horse-like head – a large bony dome on top of its skull, like a mammalian version of a pachycephalosaur.

It probably used its dome in the same way as the dinosaurs it convergently resembled, headbutting or flankbutting in fights with each other.

Weird Heads Month #16: Big Honking Snoots

The dinoceratans featured here a few days ago were some of the first large mammalian herbivores to evolve in the Cenozoic, but during the Eocene they were joined by another group: the even bigger brontotheres.

Part of the odd-toed ungulate lineage, brontotheres convergently resembled rhinos but were actually much more closely related to horses. And much like the dinoceratans they also had some unusual heads, with some species evolving concave foreheads and sexually dimorphic ossicone-like pairs of blunt horns on their noses.

But others went really weird.

Embolotherium andrewsi lived in Mongolia during the late Eocene, around 37-34 million years ago. Standing around 2.5m tall at the shoulder (8’2″), it was one of the largest brontotheres and also one of the oddest-looking.

It had a large bony “battering ram” at the front of its snout, formed from modified nasal bones – and while some reconstructions tend to shrinkwrap this structure as a horn, the fact that the nasal cavity appears to have extended all the way to its tip suggests that it was actually supporting a huge bulbous nose.

Since Embolotherium also doesn’t seem to have been sexually dimorphic like other brontotheres, its enormous ridiculous-looking snoot may instead have been a resonating chamber used for sound production and communication.

Weird Heads Month #13: Many-Horned Mammals

The dinoceratans were a lineage of hoofed herbivorous mammals whose evolutionary affinities are a little uncertain, but may have been related to the South American meridiungulates. Found in Asia and North America from the late Paleocene to the late Eocene, they had bulky rhino-like bodies and were some of the largest terrestrial animals of their time.

Eobasileus cornutus was one of the biggest of them all, measuring around 2.1m tall at the shoulder (~7′) and living in the Western United States during the early Eocene, about 46-40 million years ago.

And it had a very odd-looking head, with six blunt ossicone-like horns, large sabre-like fangs, bony flanges on its lower jaw, a concave forehead, and a proportionally tiny brain for its body size. The horns and fangs were sexually dimorphic, much smaller in females, suggesting they were mainly used for display or combat between males.

Weird Heads Month #12: Double-Crested Dinosaurs

Dilophosaurus wetherilli is a fairly recognizable dinosaur thanks to its memorable appearance in the Jurassic Park franchise – but unfortunately that also means the popular image of it is completely wrong.

Rather than a small frill-necked venom-spitting creature, this early theropod was actually rather large, reaching around 7m long (~23′), and along with its distinctive double crests it also had a narrow snout with large teeth and a distinctive notch at the front of its lower jaw.

It lived in North America during the early Jurassic, about 196-183 million years ago, and while it wasn’t venomous its notched jaws were probably capable of delivering powerful bites to small struggling prey, much like the similar-looking ornithosuchids in the Triassic. Some structural similarities to the skulls of spinosaurids suggest it may have primarily eaten fish.

Its two bony crests were probably used for visual display, with juveniles only having small crests that fully developed as they matured. They also may have had a more extensive keratinous covering, so it’s not clear what their actual shape and full extent was in life.