Towards the end of the Cretaceous, about 69 million years ago, the most diverse and numerous mammals in the northern hemisphere were the metatherians, close relatives of modern marsupials.
About the size of a modern mouse, around 10-15cm long (4-6″), and with teeth that suggest it was a shrew-like insectivore, this little metatherian lived in northern Alaska in what’s known as the Paaŋaqtat Province – a region with a distinctive population of endemic polar animals. At the time this area was located at an even higher latitude than it is today, around 80-85ºN, but due to a greenhouse climate it was also warmer, with no permanent ice and the average temperatures staying above freezing.
Unnuakomys was by far the most common mammal species in the Paaŋaqtat Province, represented by numerous fossil teeth and a few jaw fragments, and it also seems to have been the only metatherian living in the whole region. This may just be a preservation bias in the fossil record, but it might also indicate that Unnuakomys was uniquely specialized to endure the several months of continuous darkness each winter in its polar woodland environment, while other North American metatherians were restricted to more southerly latitudes.
Cabarzia trostheidei here lived during the early Permian in what is now Germany, about 295 million years ago.
Despite its very lizard-like appearance it was actually part of the varanopid lineage, a group of scaly amniotes traditionally classified as early synapsids (distant relatives of modern mammals), but which more recently have been proposed to instead be sauropsid reptiles closer related to early diapsids.
It was around 50cm long (1’8″), and its short arms, long legs, slender body, and long tail suggest it was capable of shifting into a bipedal posture when running at high speeds, similarly to some modern lizards – probably mainly to escape from larger predators, but possibly also used to pursue fast-moving prey like flying insects.
And whether varanopids were actually synapsids or sauropsids, this makes Cabarzia the earliest known example of an animal running on two legs.
Halszkaraptorines were a group of small dromaeosaurids known only from the Late Cretaceous of Mongolia. They were odd little raptors with flattened snouts, long necks, and flipper-like arms – features that suggest they were specialized for swimming, making them the second known lineage of semi-aquatic non-avian dinosaurs after the spinosaurids.
Natovenator polydontus lived in what is now the Gobi Desert in southern Mongolia, around 72 million years ago. The size of a small duck, about 45cm long (18″), it had jaws full of many needle-like teeth, a long flexible goose-like neck, and a streamlined body with a wide flattened ribcage convergently shaped like those of modern diving birds.
Although it had long strong legs, these don’t show much in the way of aquatic specializations and would have been used more for walking and running on land. Instead it may have used its flipper-like arms to propel itself through the water, like modern penguins or auks.
It probably had a lifestyle similar to modern mergansers, swimming and diving in lakes and rivers, and preying on fish, amphibians, and aquatic invertebrates.
Most of them also had jaws full of round flat teeth used to crush hard-shelled prey, but some may instead have been herbivorous grazers similar to parrotfish.
And a couple of lineages even became carnivores.
Serrasalmimus secans lived in what is now Morocco during the late Paleocene, about 59 million years ago. Although only known from its jaws, the size of the fossil material suggests it was fairly large for a pycnodont, possibly around 80cm long (~2’8″).
It had sharp flesh-cutting teeth similar to those of modern piranha, but with a surprising evolutionary twist. Unlike any other known ray-finned fish, Serrasalmimus‘ teeth were true shearing carnassials anchored into bony sockets, with new replacement teeth forming directly below each current tooth – a very specific arrangement of features previously only known in mammals.
This is an especially remarkable example of convergent evolution because on land placental carnivorans were developing their own carnassials at the same time, just a few million years after the K-Pg mass extinction. Both mammals and pycnodonts were simultaneously taking advantage of the vacant predatory roles in their respective ecosystems, and ended up with incredibly similar tooth adaptations as a result.
A single partial skeleton discovered in the 1880s is the only known record of this species, and represents a juvenile that would have been around 15-20cm long (6-8″). We don’t know exactly what it would have looked like as an adult, but it was probably quite similar to other closely-related members of the brachyopid family – mostly-aquatic salamander-like animals with short but wide toothy jaws, eyes set towards the front of the head, small limbs, and paddle-like tails.
A recent re-analysis of the Platycepsion specimen found evidence of soft-tissue preservation of external gills, showing that it wasn’t just a juvenile but a true larva, a sort of temnospondyl “tadpole”.
Distinct larval stages have been found in a few other types of temnospondyls, but this is the first definite example from the stereospondyls, a major Mesozoic lineage that survived all the way into the Early Cretaceous.
And yet despite the huge diversity of gliding mammals, and their particular prevalence in tropical forests, there’s an entire continent famous for its rainforests that’s somehow completely lacking any modern examples: South America.
It’s not clear why the gliding lifestyle never took off in South America, but the continent is surprisingly devoid of any other gliding vertebrates, too. The only exceptions are a few species of flying frogs in the northwestern tropical forests around Colombia.
But back in the early Eocene, about 53-50 million years ago, there was at least one South American gliding mammal. Some fossil limb bones found in the Itaboraí Formation in southeastern Brazil look very much like those of a gliding mammal – long and thin, with a locking elbow joint, knees adapted for jumping, and flexible ankles typical of tree-climbers.
These remains haven’t been given a new scientific name, however, because there’s a good chance they belong to an already-described species. Fossils from Itaboraí are found disarticulated, broken, and with bones of multiple different species jumbled together, so most fossil mammals named from the site have been based on their more easily distinguishable teeth and jaw fragments.
The problem is matching those teeth with these bones.
Currently the best identity guess based on size is Gaylordia macrocynodonta. This mammal would have been around 30cm long (1′), about the size of a modern rat, and had distinctive large canine teeth. It used to be classified as a marsupial related to opossums, but more recent studies have found it to have actually been a marsupialiform metatherian instead, much more closely related to Pucadelphys and sparassodonts than to any modern true marsupials.
Gaylordia‘s crushing molars suggest it was carnivorous, able to crunch through bones or hard-shelled invertebrate prey. This would be a very unusual diet for a gliding mammal, since most other mammalian gliders are herbivores or omnivores – the only other known predatory examples were the volaticotheres over 110 million years earlier.
The first definite ichthyosaur fossil found in Colombia was a single well-preserved skull, found in Early Cretaceous deposits dating to between 130 and 112 million years ago.
Although first discovered in the 1970s, this marine reptile wasn’t described until the late 1990s, at the time being named as a species of Platypterygius. But since then more pieces of the skeleton have been recovered, and the Platypterygius genus has been found to be a wastebasket taxon in need of revision, so in 2021 the Colombian ichthyosaur got a more detailed redescription and its own distinct name: Kyhytysuka sachicarum.
Kyhytysuka was a mid-sized ichthyosaur, about 5.5m long (18′) – about the size of a small modern orca – with a large head and a long robust snout. Its teeth varied in size, shape, and spacing along its jaws, with several different regions that were specialized to catch, slice, and crush its prey.
It could also open its jaws very widely, possibly up to an angle of 75°, suggesting it was able to tackle particularly large prey such as other marine reptiles. Possible soft tissue preservation around its lower jaw might also be evidence of elastic connective tissue that would have allowed its throat to expand out while swallowing big prey items.
This makes Kyhytysuka the first known example of a Cretaceous-aged ichthyosaur with an apex predator lifestyle, convergently evolving a similar ecological role to some earlier Triassic and Jurassic species.
First discovered in the early 1970s, in the mid-CarboniferousBear Gulch Limestone deposits (~324 million years ago) of Montana, USA, it was initially mistaken for the long-sought-after “conodont animal” due to the presence of numerous conodont teeth inside its body. But just a few years later well-preserved eel-like conodont animals were found elsewhere, and it became apparent that the conodont teeth inside Typhloesus had actually just been part of its last meal.
But if it wasn’t a conodont… then what was it?
Up to about 10cm long (4″), Typhloesus had a streamlined body with a vertical tail fin and paired “keels” along its sides. It had a mouth and a gut cavity, but no apparent anus, and it also didn’t seem to have any eyes or other sensory structures. And in the middle of its body there was something very weird – a pair of “ferrodiscus” organs, disc-shaped structures which contained high concentrations of iron but whose function was completely unknown.
This anatomy just didn’t match any other known animals, so much so that it gained the nickname of “alien goldfish”.
For the next few decades it remained a bizarre enigma, at best tentatively considered to represent an unknown lineage of some sort of metazoan that left almost no other fossil record due to being entirely soft-bodied.
But now, 50 years after its initial discovery, we might just finally have a clue about Typhloesus’ true identity.
It’s not a definite identification yet, and even if it was a mollusc it was an incredibly strange one, with features like the ferrodiscus still lacking any explanation. But this discovery at least shows that there are still new details waiting to be found in the “alien goldfish” fossils, and gives us a start towards bringing its classification back down to earth.
A wastebasket taxon is what happens when species can’t be easily classified and instead get hurled into a “catch-all” category.
…But that’s not the only kind of taxonomic tangle that can befall a new discovery.
When a scientific name is assigned to a new species, but it isn’t given a corresponding formal description and type specimen, it becomes a nomen nudum – a “naked name”. Without a proper description and assigned holotype the name isn’t valid, and the new species isn’t technically accepted by the wider scientific community.
This has even happened to some surprisingly famous names. In the 1920s Velociraptor mongoliensis was briefly given the nomen nudum“Ovoraptor djadochtari” before getting its much more familiar name when it was officially described. Meanwhile the giant pterosaur Quetzalcoatlus northropi was stuck as a nomen nudum for decades, only finally getting a proper published description in 2021.
And there’s another particular long-standing nomen nudum that became mildly infamous – “Thotobolosaurus”, the “trash heap lizard”.
Kholumolumo ellenbergerorum
Discovered next to a literal trash pile in the village of Maphutseng in Lesotho, a few scattered and broken bones of this “prosauropod” sauropodomorph dinosaur were first found in 1930. But it wasn’t until the mid-1950s that a more extensive bonebed began to be unearthed at the site, and over the next decade over 1000 fossil fragments were collected.
In the mid-1960s the remains were initially classified as belonging to Euskelosaurus browni (which is now considered to be a wastebasket taxon), but just a few years later in 1970 the “Maphutseng Beast” was re-evaluated as a species new to science. It was referred to as “Thotobolosaurus mabeatae” – based on the local name of the discovery site, “Thotobolo ea ‘Ma-Beata” (trash heap of Beata’s mother) – but this name was never actually formally published.
Despite “Thotobolosaurus” being an undescribed nomen nudum it nonetheless went on to be repeatedly referenced in scientific literature over the next few decades, and appeared in several popular dinosaur books (even as recently as 2020!).
In the mid-1990s it was alternatively named “Kholumolumosaurus ellenbergerorum” in a Ph.D. dissertation, with this name derived from the kholumolumo, a reptilian creature in Sotho mythology, and the Ellenberger brothers who worked on the site. But this also didn’t count as a formal publication and instead became a second nomen nudum for the species.
Eventually, 90 years after the first bones were found and 50 years after the debut of the name “Thotobolosaurus”, this long-neglected sauropodomorph was finally given a proper published full anatomical description in 2020.
And it also got a third name, this time officially valid, based on the second one from the 1990s: Kholumolumo ellenbergerorum.
For something associated with trash for so long, Kholumolumo is actually now one of the most completely-known prosauropods. At least five different individuals were present in the collected fossil material, possibly as many as ten, and between them most of the full skeleton is represented – with the exception of the skulls, which are only known from a couple of small fragments.
We now know Kholumolumo was rather heavily-built, with chunky limb bones and unusually short shinbones. It would have been one of the biggest animals around in the Late Triassic (~210 million years ago), measuring at least 9m long (~30′) and weighing around 1.7 tonnes (1.9 US tons), but despite its size it seems to have still been bipedal.
Due to the highly disarticulated nature of the bones the fossil site may have been a “bone accumulation area”, a place where dismembered bits and pieces of different carcasses were regularly carried to be eaten by a predator or scavenger – essentially a trash heap, fittingly enough. A couple of “rauisuchian” teeth have actually been found among the remains, which might indicate what was chomping on these particular Kholumolumo.
“Insectivora” was a wastebasket taxon so bad it had to be revised multiple times, but there’s another particularly infamous case in mammal taxonomy that’s still in the process of being resolved – the “condylarths“.
This group was first created in the early 1880s, during the Bone Wars, and initially was just a subgroup of odd-toed ungulates containing only the phenacodontids. But just a few years later Condylarthra was promoted up to its own order, and groups like the periptychids and hyopsodontids were added in too.
Then over the next few decades various groups were added and removed from the condylarths, most notably with the mesonychids and arctocyonids being brought in from their previous position with the creodonts.
By the mid-20th century the condylarths had become a big convenient dumping ground for any and all “primitive” ungulate-like mammals that didn’t easily fit into any modern groups, ranging in age from the early Paleocene through to the early Oligocene. But it soon became apparent that they had the same problem as the “insectivores” – there weren’t really any unique anatomical features that united all these animals together.
They generally had rounded-cusped molar teeth and hoof-like toes, but they also had rather generalized “primitive mammal” features and a diverse range of ecologies. Some were small herbivores, but others were coati-like or dog-like omnivores, and some were even bear-sized carnivores.
From left to right, top row: Hyopsodus lepidus (hyopsodontid), Meniscotherium chamense (phenacodontid), Arctocyon primaevus (“arctocyonid”). Bottom row: Ectoconus ditrigonus (periptychid), Mesonyx obtusidens (mesonychid)
It wasn’t even clear how the various different condylarth groups were actually related to each other. The best guess was that arctocyonids had arisen from within the “insectivores”, with a Protungulatum-like form as the common ancestor of all the other condylarths. Where exactly modern ungulates had then evolved from within the condylarths was also still uncertain.
Cladistic analysis in the 1980s began to tackle the confusing pile of assorted condylarths, and showed that they weren’t the single ancestral source of all modern ungulates, but instead a loose collection of several unrelated groups from all over the ungulate evolutionary tree. Arctocyonids, periptychids, and hyopsodontids were placed as early “primitive” lineages, phenacodontids were loosely linked with the ancestors of odd-toed ungulates once again, and mesonychids were considered to be the ancestors of whales.
And, once again paralleling the mess of the “insectivores”, it wasn’t until genetic methods became available in the late 1990s that larger-scale ungulate relationships began to be properly resolved. The paenungulates (elephants, hyraxes, and sirenians), which had been traditionally considered to be a major branch of ungulates, were removed entirely and reclassified as afrotheres. And, along with some new fossil discoveries, whales were recognized as having actually evolved from within the even-toed ungulates instead of from mesonychids.
This shake-up threw the still-problematic “condylarth” classifications back into question – with some “condylarths” turning out toalso be afrotheres instead of true ungulates.
Today the actual relationships of the main “condylarth” ungulate families are still in the process of being figured out, and there’s a lot of remaining uncertainty and disagreement about them.
Phenacodontids seem to have mostly maintained their traditional position as early odd-toed ungulates, and hyopsodontids may potentially be part of this group too – possibly as members of the hippomorph lineage, closely related to horses and brontotheres. Arctocyonids might be a wastebasket themselves, with some studies finding them to be a mix of several different archaic ungulate lineages. Periptychids may have links to the even-toed ungulates. The mesonychids, meanwhile, are now generally considered to be a separate order from the traditional “condylarths”, and may be either an early branch of the even-toed ungulates or much more basal ungulates closely related to the “arctocyonids”.
Since the term “condylarth” no longer has any real taxonomic meaning some paleontologists have proposed replacing it with “archaic ungulate” to distance from the historical messiness of the old name. But this hasn’t really caught on, and many papers still use “condylarth” in a very loose sense to refer to an “evolutionary grade” of early ungulates of unclear evolutionary affinities.
And while that’s the last main entry for this month, we’re not quite done yet. There’s still one weekday left in October, and after digging through so many taxonomic garbage cans there’s only one place we can go now.
