mammal – Page 13 – Nix Illustration

Eons Roundup

This year I’ve been lucky enough to have some of my work featured in several PBS Eons videos – and I even recently got the opportunity to do some custom images for them! Since I didn’t show any of these off at the time, here they are now:

The basal temnospondyl amphibian Iberospondylus, from “When Giant Amphibians Reigned”
https://www.youtube.com/watch?v=rGthtRZl8B0

The flying paleognath bird Lithornis, from “When Birds Stopped Flying”
https://www.youtube.com/watch?v=M3h05ajJw0o

The ground sloth Nematherium, from “How Sloths Went From the Seas to the Trees”
https://www.youtube.com/watch?v=pt9tBtQoAHo

Happy new year, everybody!

Peltephilus

Peltephilus ferox, an armadillo from the Early Miocene of Argentina (~17-16 mya) that was similar in size to a large dog, probably around 1.5m long (5′). It had less solid armor than its modern relatives, with its bony osteoderms being arranged more like chain mail, loosely connected to each other and slightly overlapping, creating a much more flexible body covering.

Its most unusual features were the horns on its snout, convergently resembling the later horned gophers of North America. But unlike other mammals Peltephilus‘ horns were actually modified plates of its face armor, enlarged pointed osteoderms that were only connected to its skull by soft tissue membranes – meaning that after death they tended to fall off, and the exact number and position of them is still a little uncertain.

Its unusually broad snout and large teeth were originally interpreted as evidence of it being an active carnivore, but more recent studies of its anatomy have suggested that it was much more likely to have been a herbivorous or omnivorous digger, mainly feeding on underground plant matter like roots and tubers.

Enaliarctos

Enaliarctos mealsi, an early seal from the Late Oligocene and Early Miocene of California, USA (~23-20 mya).

Measuring about 1.5m long (5′), it was a transitional form between modern seals and their more otter-like ancestors. It was well-adapted for swimming with a flexible spine and flipper-like limbs, but unlike most modern pinnipeds it probably used both its front and hind flippers for propulsion.

Its teeth also still resembled those of terrestrial carnivores, with slicing carnassials at the back of its jaws. This suggests that it had to drag larger prey items back to shore in order to tear them apart and eat them, similar to the behavior of modern otters.

Diplacodon gigan

Diplacodon gigan, a brontothere from the Early Eocene of Wyoming, USA (~46-42 mya). Standing around 2.1m tall at the shoulder (~7′) it was named after the kaiju Gigan for its relatively large size – not quite as big as some later brontotheres, but still about 20% larger than other known species of Diplacodon.

It had a pair of blunt bony projections on its snout which would have been covered with skin in life, similar to the ossicones of modern giraffids, with males having larger “horns” than females.

Despite looking very similar to rhinos, brontotheres were actually much more closely related to horses, with the resemblance being a result of convergent evolution for the same sort of big-tanky-herbivore ecological niche.

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.

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 #19 – Even More Metatherians

While the opossum-like herpetotheriids and peradectids survived in the northern continents for most of the Cenozoic, a wider variety of metatherian mammals were found in the south. Alongside the true marsupials and the sparassodonts, a group known as the polydolopimorphs existed in South America for over 60 million years. Although most of the their fossil remains consist only of isolated teeth and jaw fragments, they seem to have been a very diverse group that adapted to a wide range of ecological niches including insectivores, herbivores, and fruit-eating and seed-eating specialists.

Their exact evolutionary position within the metatherians is still rather unclear and under dispute, with different studies giving different results. They were probably marsupialiformes, slightly less closely related to marsupials than the herpetotheriids, but some paleontologists instead consider them to have been true marsupials related to either the shrew opossums or the microbiotheres. (And some go with both options, proposing that they weren’t even a natural group but were polyphyletic, with some being marsupialiformes and others being true marsupials.)

The earliest definite polydolopimorph fossils come form the very start of the Paleocene in South America (~66 mya), but their lineage likely goes further back into the Late Cretaceous – possible remains from North America suggest they may have originated there at least 70 million years ago, with their ancestors migrating into South America shortly before the end-Cretaceous extinction. A few also reached Antarctica by the Late Eocene (~40-33 mya), before the continent had fully separated from its neighbors and frozen over, but it’s unclear whether any ever made it as far as Australia alongside their marsupial relatives.

They were most diverse during the first half of the Cenozoic, and in the latter half they were represented mainly by a highly specialized lineage called the argyrolagids. Known from western and southern South America (Peru, Bolivia, Argentina, and Chile) from the Early Oligocene onwards, these polydolopimorphs were convergently rodent-like desert herbivores with short forelimbs and long hopping hindlimbs that gave them a resemblance to jerboas or springhares.

Argyrolagus palmeri here lived during the Early Pliocene of Argentina (~5-3.5 mya). About 40cm long (1′4″), it had only two toes on its feet, a long pointed snout, and large eyes and ears that indicate it was probably nocturnal.

These last polydolopimorphs survived until at least the end of the Pliocene, around 2.5 million years ago. Their disappearance coincides with the time of the Great American Interchange – when South America became connected to Central and North America – and they may have been some of the victims of the extinction caused by the influx of placentals from the north.

Almost-Living Fossils Month #18 – Metatherians Everywhere

While modern marsupials are native only to Australasia and the Americas, they’re part of a larger grouping of mammals known as metatherians that were once found around most of the world. Thought to have originated in Asia, perhaps as far back as the Late Jurassic (~160 mya), the ancestral metatherians spread to Europe and the Americas during the Cretaceous and gave rise to multiple different lineages, ranging from small sabertoothed carnivores to otter-like swimmers to the main mammalian predators of Cenozoic South America.

True marsupial fossils didn’t appear in the fossil record until the start of the Paleocene (~65 mya), but some of their closest evolutionary “cousins” – the herpetotheriids – originated a little earlier in the Late Cretaceous, around 70-66 million years ago.

Herpetotheriids were small opossum-like animals found in North America, Asia, Europe, and Africa. They were more terrestrial than the opossums they resembled, adapted for running on the ground rather than climbing, but were probably similarly opportunistic omnivores eating a wide variety of foods.

They survived through the end-Cretaceous extinction and seem to have actually been quite common in the northern continents for most of the Cenozoic. They also weren’t the only metatherians in those regions, living alongside another group called the peradectids which were true marsupials closely related to opossums.

In North America the native metatherians all went extinct sometime around the early Miocene (~20 mya) – although the continent would later be recolonized by opossums from South America during the Great American Interchange. Meanwhile the African herpetotheriids disappeared around the same time, but the European and Asian metatherians continued on into a little longer into the mid-Miocene.

Amphiperatherium frequens was one of the last known members of the herpetotheriids, living in Europe during the early-to-mid Miocene (~20-13 mya). It was about the size of a small rat, around 15-20cm long (6-8″), and seems to have mainly inhabited warm humid environments.

Although the disappearance of the northern metatherians has traditionally been blamed on them being out-competed by placental mammals, the fact that they survived alongside placentals just fine for over 45 million years suggests that something else pushed them over the brink. Their extinction may in fact be more linked to the ongoing drying and cooling climate trends in the latter half of the Cenozoic.

The herpetotheriid’s extinction seems to have happened soon after the Middle Miocene disruption, a sudden shift towards cooler temperatures that may have altered their local ecosystems too quickly for them to adapt. The peradectids hung on until at least 11 million years ago, with some of the last members known from Southeast Asia, but they may have likewise fallen victim to further episodes of global cooling towards the end of the Miocene and into the Pliocene.

Almost-Living Fossils Month #15 – Digging Dryolestoids

First appearing in the mid-Jurassic, about 168 million years ago, a group of mammals called dryolestoids were some of the closest known relatives to the therians (the group that contains modern marsupials and placentals). They were found throughout North America, Eurasia, and North Africa up until the Early Cretaceous (~125 mya), but then mostly disappeared from the northern continents and instead migrated into South America – where they went on to flourish and became some of the most common mammals in the continent by the Late Cretaceous.

Although mostly known from only teeth and jaw fragments, the dryolestoids seem to have been a pretty diverse group of mammals during the Mesozoic, adapting to a variety of different diets and lifestyles ranging from small insectivores to relatively large herbivores.

Most of them died out in the end-Cretaceous mass extinction, except for the dog-sized herbivorous Peligrotherium which survived until 59 million years ago in the Early Paleocene… and one other known example from much much later into the Cenozoic.

Despite being absent from the fossil record for over 40 million years, the dryolestoids reappeared again in the Early Miocene of Patagonia (~21-17.5 mya) with a single late-surviving member: Necrolestes patagonensis.

The known Necrolestes fossils are surprisingly well-preserved compared to most other dryolestoids, with about a third of its skeleton represented. It was a small mole-like burrowing animal, about 10-15cm long (4-6″), with large canine teeth and an upturned snout. The cartilage in its nose was ossified into bone, strengthening it and probably supporting a pad of thick toughened skin – and also suggesting that it was a “head-lift digger”, using its snout like a shovel to dig through the soil.

While it superficially resembled the earlier mole-like dryolestoid Paurodon, it was actually much closer related to more generalist Mesozoic forms like the sabertoothed Cronopio.

After Necrolestes there’s no further evidence of dryolestoids living any closer to modern day. Much like the late-surviving gondwanatheres they lived alongside, these last dryolestoids may have specialized themselves so much that they couldn’t cope with sudden environmental changes, and the Middle Miocene extinction could have finished them off entirely.

Almost-Living Fossils Month #11 – The Lost Island Murderer

The modern solenodons, today found only on the Caribbean/Antillean islands of Cuba and Hispaniola, represent the last living survivors of a very ancient branch of placental mammals. Although they’re part of the eulipotyphlans – the lineage that also contains shrews, moles, and hedgehogs – their last common ancestor with the other members of that group dates back to over 70 million years ago.

And during their early evolutionary history, somewhere between the Late Cretaceous and mid-Eocene (~68-43 mya), the ancestors of the solenodons diverged into two different lineages: the solenodons themselves, and the nesophontids.

Known only from skulls and jumbled partial skeletal remains, nesophontids seem to have been fairly similar to their solenodon cousins. They would have been shrew-like in appearance, varying in total size from 5 to 20cm long (2-8″), with slender flexible snouts that they used to sniff out their small invertebrate prey. They also probably had a venomous bite like the solenodons, since their canine teeth had distinctive grooves for injecting toxic saliva.

Their remains have been found in various islands around the Caribbean, in Cuba, Hispaniola, Puerto Rico, the United States Virgin Islands, and the Cayman Islands. Between six and twelve different species are represented, all in a single genus called Nesophontes (meaning “island murderer”) – and all of the known specimens are actually subfossils from the Holocene, dating to less than 12,000 years old.

These relatively young ages mean that pieces of nesophontid DNA have been successfully recovered from bones within preserved owl pellets, which is how we could confirm their close relationship to solenodons and the dates of their ancestors’ probable divergence. But we have absolutely no idea about the rest of their family history during the entire Cenozoic, such as how and when they originally colonized the Caribbean or whether they had any other extinct relatives in the Americas.

(There is one possible partial specimen from a piece of Dominican amber, dating to the Oligocene or Early Miocene, 29-18 million years ago, but it’s not clear whether it’s a nesophontid, a solenodon, or something else entirely.)

Nesophontes edithae was the only species of nesophontid living in Puerto Rico and the United States Virgin Islands. It was the largest known member of the genus, at least 20cm long (8″), and may have been able to reach such a size due to the absence of competition from solenodons on those particular islands.

Along with its relatives around the Caribbean it seems to have survived frustratingly close to modern times, even existing alongside indigenous humans, with some remains only about 500 years old. But when the islands were colonized by Europeans from the 1490s onwards, the combination of the introduction of invasive Old World rats, new predators like domestic cats and dogs, and extensive deforestation probably killed off the nesophontids very quickly (along with all but two of the solenodon species).

Despite claims of fresh-looking remains in owl pellets and searches for surviving populations, there’s currently no convincing evidence for any living nesophontids, and radiocarbon dating has never found anything younger than 500-600 years old.