mammal – Page 7 – Nix Illustration

It Came From The Wastebasket #08: Stem-Carnivoramorphs Do What Creodon’t

Creodonts were some of the earliest predatory placental mammals to evolve after the extinction of the non-avian dinosaurs, first appearing in the mid-Paleocene about 60 million years ago. Represented by two main lineages – the oxyaenids and the hyaenodonts – they ranged across North America, Eurasia, and Africa, and were the dominant large carnivorous mammals until the end of the Eocene (~34 million years ago), with forms like Sarkastodon being some of the biggest mammalian land predators of all time.

After that point they started to decline over most of their range, gradually being replaced by early carnivorans – but the hyaenodonts retained their dominance for a while longer in Africa, diversifying during the Oligocene and early Miocene and producing more giant apex predators. The last known representatives of these animals survived in Asia until the late Miocene, just 9 million years ago, ending an impressive run that had lasted for most of the Cenozoic.

This grouping was originally named in the 1870s to encompass just the oxyaenids and Didymictis (a genus now considered to be a viverravid). Just a few years later hyaenodonts, miacids, arctocyonids, leptictids, and mesonychids were all lumped in, too – and at one point creodonts were even a part of the massive insectivoran mess before instead being classified as ancestors of the carnivorans.

During the first half of the 20^th century creodonts were recognized as actually being a loose collection of mostly-unrelated mammals, and over the next few decades various groups were gradually removed and reassigned to other parts of the mammal family tree. Towards the end of the century most of the creodont wastebasket had been cleared, and just the oxyaenids and the hyaenodonts were left as two branches of one seemingly distinct creodont lineage.

An illustration of two "creodonts". On the left is Patriofelis, an oxyaenid that looks like a mix between a weasel and a cat, with a short boxy snout, a low-slung body, a long tail, and a greyish color scheme with faint darker spots in its pelt. On the right is Hyaenodon, a hyaenodont that looks like a mix between a dog and a tiger, with a long boxy snout, a heavyset body, a cat-like tail, and a striped coat. — The cougar-sized oxyaenid *Patriofelis ferox* (left) & the bear-sized hyaenodont *Hyaenodon gigas* (right)

…But their evolutionary relationships were still a problem.

They’d been traditionally considered to be early carnivorans, but although they had flesh-slicing carnassials the creodonts’ versions of these teeth weren’t quite right. Different teeth in their jaws had been specialized for this function compared to those of true carnivorans – with oxyaenids and hyaenodonts having slightly different arrangements compared to each other, too – suggesting a lot of convergent evolution rather than shared ancestry.

By the 1990s it wasn’t clear anymore if the oxyaenids and hyaenodonts were even closely related to each other, or what type of mammal they actually were.

But over the last couple of decades the consensus seems to have become that creodonts weren’t a single natural group, but that they were still related to carnivorans – oxyaenids and hyaenodonts were actually two separate offshoots of the Ferae, forming an evolutionary grade of stem lineages between pangolins and the carnivoramorphs.

A cladogram showing the classification of oxyaenids and hyaenodonts within the group Ferae. They're shown as two separate lineages branching off between pangolins and the ancestors of modern carnivorans. A bracket marking indicates that they both traditionally used to be classified as "creodonts".

It Came From The Wastebasket #06: Messy Miacids

Most modern meat-eating placental mammals are carnivorans, a group that contains two distinct lineages: the feliforms (cats, hyenas, mongooses, viverrids, civets, linsangs, and euplerids) and the caniforms (dogs, bears, seals, raccoons, and mustelids).

The closest living relatives of these animals are pangolins, and their last common ancestor probably lived sometime between the Late Cretaceous and early Paleocene. But the actual early evolutionary history of the carnivorans themselves is rather murkier.

The earliest known carnivoran-like forms – known as carnivoramorphs – all looked vaguely-genet-like and were an ecologically diverse bunch of small predators, ranging from weasel-sized tree-climbers to fox-sized ground-based hunters, found all across North America and Eurasia during the Paleocene and Eocene. They lacked most of the anatomical specializations of true carnivorans, and didn’t quite fit into either the feliforms or caniforms, but their distinctive carnassial teeth make it obvious they were still very closely related.

From their initial discovery in the late 19^th century, through to the late 20^th century, these carnivoramorphs were traditionally all lumped together under the name “miacids“. As a result the group quickly turned into a big wastebasket taxon of similar-looking animals, all united more by just not being true carnivorans than by any shared characteristics between themselves.

An illustration of Miacis, an extinct mammal related to early carnivorans. It's a somewhat weasel-like animal with a small triangular head, small rounded ears, a long tubular body, cat-like limbs, and a long bushy tail. It's depicted with brownish fur, with raccoon-like black-and-white markings on its face and a stripey tail. — *Miacis parvivorus*

But during the last couple of decades this mess has finally started to get cleared up. One distinct lineage of miacid-like animals called viverravids were split off, now thought to be the one of very earliest branches of the carnivoramorph evolutionary tree. Several other “miacids” have also been reassessed and renamed, reclassified as falling into various points in an evolutionary grade between viverravids and true carnivorans, and a couple of species even turned out to actually be caniforms.

A cladogram showing the classification of carnivoramorphs. Miacis is shown as just one of several different branching lineages originating between viverravids and modern carnivorans. A bracket marking indicates that everything before the true carnivorans traditionally used to be considered to be "miacids".

The true carnivorans arose from somewhere within the “miacids” during the mid-Eocene, but it’s still unclear where exactly to draw the taxonomic line between them. Forms like Quercygale and Tapocyon might be very close to the ancestral carnivoran – but they might instead be early feliforms – and some studies have also proposed that nimravids (“false sabertooth cats”) may actually be “advanced” carnivoramorphs instead of early feliforms.

There are also quite a few remaining “miacids” that still need sorting out, especially in the genus Miacis. There have to be other distinct lineages of these carnivoramorphs still hidden in the remaining wastebasket pile, and if we can eventually distinguish them from each other it might help to make early carnivoran relationships a bit clearer.

It Came From The Wastebasket #03: The Gomphothere In The Room

The three living species of elephants are the last surviving members of the proboscidean lineage – but up until the end of the last ice age about 11,000 years ago their relatives were much more numerous and widespread, found on every continent except Australia and Antarctica. Mammoths are probably the most famous of these recently-extinct proboscideans, closely related to modern Asian elephants, but there were also the more distantly-related stegodonts and mastodons…

…And also the gomphotheres.

A cladogram showing the traditional classification of gomphotheres as a poorly-defined group evolutionarily between mastodons and modern elephants.

Traditionally any proboscideans that fell into the evolutionary grade between mastodons and elephants-and-stegodonts were all labelled as gomphotheres. As a result by the late 20^th century this group ended up as a wastebasket full of elephant-like forms that didn’t easily fit anywhere else, defined more by what they weren’t rather than by any features they all had in common.

This big collection of gomphotheres was highly diverse. Some species independently evolved similar convergent features, and there was also considerable individual physical variation within species, making the actual taxonomy of these animals very difficult to figure out. But over the last few decades there’s been a lot of revision of proboscidean evolutionary relationships, and gradually the gomphothere wastebasket has been clearing up. Groups like the choerolophodontids, amebelodontids, and anancids have been split off, leaving a more defined lineage of gomphotheres that do have shared anatomical characteristics – distinctive three-lobed trefoil-shaped wear patterns on their molar teeth.

A cladogram showing a more modern classification of gomphotheres, with the gomphotheriids as just one of several different lineages originating between mastodons and modern elephants.

These gomphotheriids were widespread, found across Africa, Europe, Asia, and the Americas – and they were especially successful in the latter. They arrived in North America during the Miocene (~16 million years ago) via the Beringia land bridge, and rapidly spread across the continent and down into Central America. They went on to become the only proboscideans to disperse into South America during the Great American Biotic Interchange, with two different lineages arriving at separate times – Notiomastodon around 2.5 million years ago, and Cuvieronius around 750,000 years ago.

An illustration of Cuvieronius, an extinct elephant-like gomphothere. It has a longer flatter head than modern elephants, and its tusks have a spiral twist to them. — *Cuvieronius hyodon*, a 2.3m tall (7’7″) South American gomphotheriid with distinctive spiraled tusks.

The exact relationships of the gomphotheriids to other elephant-like proboscideans are still a little uncertain. Both protein sequences and mitochondrial DNA have recently been recovered from 35,000-13,000-year old Notiomastodon specimens, but these studies have given different taxonomic conclusions – with the protein results suggesting gomphotheriids were most closely related to mastodons, and the DNA results suggesting they were much closer to true elephants.

Spectember 2022 #04: Aquatic Brontotheres

Squeezing in one last bonus #Spectember post this year!

This one isn’t based on a specific prompt, but instead is a companion piece to a previous one.

While North American brontotheres were adapting to the spread of grasslands, some of their Asian cousins took a very different evolutionary path through the rest of the Cenozoic.

Spectember 2022 #02: ‘Modern’ Brontotheres and Paraceratheres

Today’s #Spectember concept is a combination of a couple of anonymous submissions:

A digital illustration of two speculative hoofed mammals, descended from extinct brontotheres and paraceratheres. One resembles a hairy rhinoceros with an odd U-shaped horn on its nose and a fork-like bony "horn" on the back of its head. The other looks like a chunky camel with a moose-like bulbous nose and short downward-pointing protruding tusks. — Crowned brontothere (left) and woolly paracerathere (right)

These two animals are the descendants of brontotheres and paraceratheres, almost the last living representatives of their kinds, hanging on in the equivalent of modern-day times in a world similar to our own.

Palaeosinopa

Cimolestans were one of the major mammal lineages that survived through the K-Pg mass extinction 66 million years ago. Closely related to early placentals, they had a burst of diversification during the first half of the Cenozoic and rapidly evolved into a wide range of specialized forms – some uniquely weird, and others convergently resembling more familiar modern animals like squirrels, bears, ground sloths, and hippos.

And one group known as the pantolestids were incredibly otter-like.

(Because synapsids love them some lutrinization.)

Palaeosinopa didelphoides here lived during the mid-Eocene, about 52 million years ago, in what is now the Mountain West region of the USA. It was similar in size to a small otter, about 1m long (3’3″), and had a streamlined body with a well-muscled neck, short powerful forelimbs, slightly longer hindlimbs, and a very long tail.

Inhabiting a subtropical lake ecosystem, it probably swam using both hindlimb paddling and otter-like tail undulations. Its strong jaws and teeth suggest it was specialized for crunching hard shellfish prey, but so far preserved gut contents have only shown fish bones and scales. Fairly large claws indicate it was also able to dig out burrows similarly to modern otters and beavers.

Although pantolestids were never particularly common animals they were quite widespread, expanding their range from their evolutionary origins in North America across to Europe and eventually into Asia. A cooling and drying climate at the end of the Eocene seems to have driven most of the group into extinction alongside all their other cimolestan relatives – but a few of the Asian species clung on slightly longer as the very last of their kind, with the last known fossils dating to about 28 million years ago in the early Oligocene.

Tsaidamotherium

Tsaidamotherium hedini was a ruminant ungulate living around 11 million years ago during the late Miocene, in the northeastern part of the Tibetan Plateau in what is now Northwestern China. Although it’s known only from partial skull remains it was probably similar in body size to a large sheep, about 80cm tall at the shoulder (2’7″).

Since its discovery in the 1930s it’s traditionally been classified as part of the muskox lineage, but in 2022 it was proposed to actually be a giraffoid very closely related to the newly-discovered Discokeryx.

Tsaidamotherium had some extremely unusual headgear, with highly asymmetrical “horns” (actually ossicones if was a giraffoid). The left one was small and positioned above the eye, while the right one was shifted back and towards the middle of the forehead, and was expanded out into a wide bony disk that would have supported a large helmet-like domed keratin covering.

Its skull also had a very large nasal cavity resembling that of the modern saiga antelope, suggesting it may have convergently evolved a similar sort of complex air-filtering snout to deal with dry cold air in its mountainous habitat.

Casatia

Modern beluga whales and narwhals are the only living representatives of the monodontid lineage, found only in cold Arctic and sub-Arctic waters. But this whale family actually first evolved in much warmer climates – and some of them were downright tropical.

Casatia thermophila lived about 5 million years ago during the early Pliocene, in the Mediterranean Sea around Tuscany, Italy. Although known only from a couple of partial skulls and a few vertebrae it was probably similar in size to its modern relatives, around 5m long (16’4″).

It seems to have had a larger number of functional teeth than modern monodontids, and probably didn’t suction feed like its modern close relatives. Instead it may have fed more like most porpoises and dolphins, relying more on speed and snapping jaws to capture prey.

It inhabited the Mediterranean at a time not long after the sea there had mostly dried up and then been rapidly refilled. The presence of warm-water marine species such as bull sharks, tiger sharks, and dugongs in the same fossil beds as Casatia indicates the local climate at the time was hotter than it is today, with tropical temperatures – and suggests that this whale’s ancestors must have originally moved into the replenishing Mediterranean from lower latitudes alongside these other warmth-adapted animals.

This tropical monodontid was also much closer related to modern belugas than modern narwhals are, which raises the possibility that the two living monodontid species actually specialized for colder conditions completely independently of each other rather than descending from a cold-adapted common ancestor. Instead modern belugas and narwhals may have originated from separate warm-water monodontid ancestors who evolved similar cold-tolerant adaptations in parallel as the climate cooled during the onset of the Quaternary ice age, while the rest of their relatives all went extinct.

Bathyergoides

Blesmols, or African mole-rats, are a group of rodents adapted for mole-like burrowing. Closely related to the more famous naked mole-rat, these little mammals have reduced eyes and ears along with incisors that protrude out even when their mouths are closed, allowing them to excavate tunnels primarily using their teeth.

One of the earliest known fossil blesmols is Bathyergoides neotertiarius here, from the early Miocene of Namibia about 20 million years ago. For almost a century this species was known only from teeth and partial skull remains, but recently a partial skeleton was described giving us a better idea of its overall appearance and lifestyle.

Bathyergoides was a fairly large blesmol, around 25cm long (~10″), and was already specialized for tooth-digging with a skull very similar to modern forms. It had powerful muscular forelimbs that would have been used to push back loose soil while burrowing, but unlike its living relatives it also had a long tail and relatively slender hindlimb bones – with anatomy suggesting its legs were used more for stabilizing its posture than for actively digging.

It may have had a less subterranean lifestyle than modern blesmols, digging out extensive burrows but still foraging for food above ground in a similar manner to modern semi-fossorial rodents like giant pouched rats.

Retro vs Modern #16: Uintatherium anceps

Discovered in the Western United States during the early 1870s, Uintatherium anceps was part of one of the earlier major conflicts in the the Bone Wars. Nearly 30 different scientific names were applied to various fossil specimens of this mammal in under two decades, and the taxonomic tangle wasn’t properly sorted out until nearly a century later in the 1960s when they were recognized as actually all being the same species.

1870s

Paleontologist Edward Cope considered Uintatherium (under the name “Loxolophodon”) and its close relatives to be proboscideans – part of the elephant lineage – due to some of the similarities in their anatomy. The first reconstruction of these animals showed this version, depicting elephant-like animals with downward-pointing tusks, short tapir-like trunks, and the multiple bony projections on their skulls speculatively shown as attachment points for large antler-like horns.

Cope’s rival Othniel Marsh heavily criticised that interpretation of Uintatherium, arguing that these huge mammals were instead a separate group within the ungulates named dinoceratans – although this wasn’t really as huge of a classification difference as it seems today, since at the time proboscideans were also considered to be ungulates!

The dinoceratan ungulate interpretation quickly won out, and for a while in the 20^th century Uintatherium actually became a fairly popular and well-known prehistoric mega-mammal, commonly included in collections of cheap plastic “dinosaurs” and usually depicted as more of a knobbly-headed sabertoothed rhino.

2020s

In recent years the dinoceratans seem to have fallen into obscurity and some degree of paleontological neglect, with little modern work on the group and no major studies for the last couple of decades – although this might be starting to change.

Despite the early ideas about them being ungulates, the evolutionary relationships of dinoceratans have become much more murky over the last century or so. Due to different elements of their anatomy being highly convergent with various other mammals it’s easy to find “false positives” in morphological comparisons, and they’ve been proposed as being connected to a wide variety of groups including “condylarths“, “insectivores“, rodents, and cimolestans. But some mid-2010s research suggests they were in fact ungulates after all, closely related to early South American forms like Carodnia – a lineage whose own evolutionary relationships are murky, but may have close affinities with modern horses, rhinos, and tapirs.

We now know Uintatherium anceps lived across the Western and South Central USA during the mid-Eocene, about 46-40 million years ago, at a time when warm wet climates extended up into the Arctic and lush tropical-style rainforests covered much of the continent.

It was similar in size and build to a modern white rhino, about 4m long (13′) and stood around 1.7m tall at the shoulder (5’7″). It had three distinctive pairs of “horns” on its forehead, snout, and nose, that were similar in structure to the ossicones of giraffids, probably covered in skin and hair rather than keratin. Its elongated canine teeth were protected by bony flanges on its lower jaw, and seem to have been a sexually dimorphic feature that was much more prominent in males.

It also had an oddly concave skull, with its forehead dipping inwards, and an unusually tiny braincase for its size. It probably wasn’t a particularly intelligent animal, but it didn’t really need to be – as one of the first types of herbivorous mammal to get truly huge in the early Cenozoic, a fully-grown Uintatherium probably had no natural predators at all.