taxonomy – Nix Illustration

It Came From The Trash Heap (We Don’t Talk About Kholumolumo)

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”.

An illustration of Kholumolumo (previously known as "Thotobolosaurus"), an extinct prosauropod dinosaur. It's a bipedal dinosaur with a small head, long neck, chunky arms, thick bird-like legs, and a long counterbalacing tail. It's colored dark blue, with lighter striping on its underside and a speckled red-and-orange pattern along its back. Its face is brighter teal-blue with a mask-like streak of orange than continues down onto a fleshy throat dewlap. There are also reddish quill-like filaments on the back of its neck and in a tuft at the tip of its tail. — *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.

It Came From The Wastebasket #20: Colossally Convoluted Condylarths

“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-20^th 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.

An illustration showing five different "condylarths". On the top row is Hyopsodus, a small guinea-pig-like animal with a long horse-like head; then Meniscotherium, a slightly larger animal that somewhat resembles a capybara; then the much larger Arctocyon, which has a slightly bear-like body, hoofed towes, and a dog-like head. On the bottom row is Ectoconus, a small animal with a tapir-like body and a blunt rectangular snout; then Mesonyx, a much larger dog-like animal with hoofed toes and a long tail. — 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.

An image of a diagram from a 1967 academic paper, showing a complicated attempt to figure out the evolutionary relationships of "insectivores", with many different group names linked by arrows. The presence of the "condylarths" Hyopsodontidae and Arctocyonidae is highlighted by red circles. For comparison next to it is the "Pepe Silvia" conspiracy wall meme. — This looks familiar
(Image sources: http://hdl.handle.net/2246/358 & https://knowyourmeme.com/memes/pepe-silvia)

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.

An image of a diagram from a 1994 academic paper, showing the proposed evolutionary relationships of various "condylarths" with main ungulate groups. It differs majorly from modern understanding by its inclusion of elephants, sirenians, hyraxes, and their extinct relatives, along with showing whales as descending from mesonychids instead of artiodactyls. — …And if you know modern mammal phylogeny you’ll probably see some big problems here. 🐘
(Image source: https://doi.org/10.1017/S2475263000001343)

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 to also 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.

A cladogram showing the modern classification of several different "condylarth" families. They're shown as potentially occupying positions throughout the branches of the even-toed and odd-toed ungulates.

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.

…See you in the trash heap.

It Came From The Wastebasket #19: The Pterrible Fate Of Ptychopariida

The Ptychopariida were some of the earliest known trilobites, first appearing in the early Cambrian about 521 million years ago and surviving until the end of the Ordovician about 444 million years ago. They included some of the most numerous and common trilobite species, and were probably ancestral to multiple other major lineages – including the very last trilobites at the end of the Permian – making them incredibly important in understanding the overall evolution of trilobites as a whole.

…But this group is also one of the biggest wastebaskets in paleontology.

First established in the early 20^th century, the ptychopariids seemed to have some fairly good defining characteristics based on their facial sutures, large thoraxes, and relatively small pygidia. But the group quickly became a dumping ground for a massive amount of Cambrian trilobites, eventually containing numerous different families, hundreds of genera, and many more individual species.

Actually figuring out their internal evolutionary relationships also turned out to be extremely difficult – so much so that some paleontologists working on them just gave up trying and arranged the genera names alphabetically instead!

Even cladistic studies from the 1970s onward struggled to make sense of these highly “problematic” trilobites, and any larger-scale analysis was a daunting task due to how huge and diverse the ptychopariid wastebasket had become over the years. Worse, some of the anatomical features the group had been based around were starting to look more like the result of a lot of convergent evolution across multiple lineages than any actual shared ancestry.

Efforts were still made at breaking up the mess, however, with better-understood sub-groups like the Proetida, Harpida, Asaphida, Trinucleida, and Olenida being gradually split off into their own separate orders over the course of the last few decades.

An illustration of Ptychoparia, an extinct trilobite from the Cambrian period. It has a large semicircular head with a pair of antennae, small eyes, and a bulbous "forehead" region. Its body has 13 segements, and its "tail" is small and shaped like a blunted triangle. — *Ptychoparia striata*

But even by the early 2010s what remained of the Ptychopariida was still paraphyletic at best, more of an “evolutionary grade” of early trilobites than a single lineage, with most of its constituent families also rather poorly defined. There was even a proposal to abandon the group entirely, stating that “it serves no scientific purpose” and that its orphaned contents should be considered “order uncertain” until their actual relationships can be untangled.

Today the “ptychopariids” are in dire need of a full revision – since they were the ancestors of many other major groups they’re still crucial for understanding early trilobite evolution. There may be a salvageable single lineage somewhere in the remains of this wastebasket, even if it’s restricted to just close relatives of the genus Ptychoparia, but until somebody tackles them properly they’re stuck in taxonomic limbo with their name only being used in a loose sense.

It Came From The Wastebasket #18: Lots And Lots of Lepidotes

Lepidotes was a ray-finned fish that lived during the Mesozoic, found in both freshwater and shallow marine environments. It was a member of the ginglymodian fish lineage, related to modern gars, and along with distinctive thick enamelled scales on its body it was also one of the earliest types of fish able to protrude its jaws for suction feeding.

First discovered in Jurassic-aged fossil deposits in Europe in the 1830s, this genus was quickly turned into a notorious wastebasket taxon for any similar-looking fossil fish. Over time dozens of different Lepidotes species were named, many of them rather dubious, from locations all around the world and spanning a time period of over 100 million years.

Lepidotes gigas

But despite Lepidotes being a wastebasket for almost two centuries, it wasn’t until surprisingly recently that any real progress began to be made on cleaning it all up.

In the early 2010s a large-scale review of ginglymodian relationships found that many “Lepidotes” species were either invalid or polyphyletic, belonging in completely different genera or families. True Lepidotes were restricted down to just the original type species Lepidotes gigas and a few of its closest relatives, all from the early Jurassic of Europe, while some other forms were moved into the newer genera Scheenstia and Callipurbeckia. Since then some other “Lepidotes” have also been reclassified, creating new names like Macrosemimimus, Occitanichthys, and Quasimodichthys.

There’s still work needing to be done on untangling all these Lepidotes-like fish – Scheenstia might actually now represent several different lineages, for example – but at least Lepidotes itself is now in a much better situation than it was just a couple of decades ago.

It Came From The Wastebasket #17: Getting Ornithomimus In Order

The ostrich-like “bird-mimic” dinosaur Ornithomimus was named in 1890, based on some hand and foot bones from Late Cretaceous-aged fossil beds in Colorado, USA.

The first ornithomimid known to science, it was initially thought to be a ornithopod, but then a few years later more fossil material revealed it was actually a theropod – and then it spent some time classified as a “megalosaur” before ornithomimids were finally recognized as being coelurosaurs in the early 20^th century.

And for nearly a century after its discovery it was treated as a wastebasket taxon for any similar-looking fossil material from North America and Asia, with around 17 different species named within the genus. One of these was split off into Struthiomimus in 1917, but it wasn’t until much later that the rest began to get sorted out.

A review of known Ornithomimus fossils in the early 1970s renamed a couple more species into the new genera Archaeornithomimus and Dromiceiomimus, and dismissed most of the remaining species as dubious or invalid. Just two valid species now remained: the original Ornithomimus velox from Colorado, and Ornithomimus edmontonicus from Alberta, Canada.

An illustration of Ornithomimus, an extinct feathered dinosaur. It has a a small beaked head atop a long slender neck, two wing-like arms with three clawed fingers, long ostrich-like legs, and a counterbalancing tail with longer feathers towards the tip. — *Ornithomimus edmontonicus*

Since then opinions have gone back and forth about some of the other Ornithomimus species. For a while Dromiceiomimus was merged back into Ornithomimus, but more recently it’s been found to have distinct limb proportions and was probably actually a separate genus after all. Another species that’s usually considered to be part of Struthiomimus is also sometimes instead classified as an Ornithomimus instead.

Really all of the North American ornithomimids are in need of a modern taxonomic revision – especially since Ornithomimus edmontonicus shows enough anatomical variation that it might actually represent a species complex of multiple very similar forms, which might get split apart in the future if anyone can figure out how to reliably distinguish them.

It Came From The Wastebasket #16: Catopsalis Catastrophe

The rodent-like multituberculates were a major lineage of mammals that were only distantly related to modern marsupials and placentals. They originated around the time of the mid-Jurassic (~168 million years ago), survived through the end-Cretaceous mass extinction, and went on to become one of the most diverse and successful types of mammal in the Paleocene. After that point they began to decline, and after anw over-130-million-year-long run they went extinct* in the early Oligocene (~33 million years ago).

(* Except, possibly, in South America, where an enigmatic fossil known as Patagonia peregrina may represent a multi surviving as recently as about 18 million years ago in the early Miocene.)

First discovered in North America in the 1880s, Catopsalis foliatus was part of a group of multituberculates called taeniolabidoids. These multis got significantly larger than the rest of their kind – averaging beaver-sized but with some species getting up to at least capybara-sized – and were some of the first mammals to evolve into relatively big herbivores after the extinction of the non-avian dinosaurs.

An illustration of Catopsalis, an extinct multituberculate mammal. It resembles a rodent, with a whiskery nose, large eyes, small rounded ears, short clawed legs with spurs on its ankles, and a long tufted tail. Its colored mostly brown with pale spotted markings along its sides. — *Catopsalis foliatus*

Catopsalis was named based on a partial jawbone and a few teeth, and over the next century or so various other similar-looking fossils from both North America and Asia were added into the genus as additional species. Eventually Catopsalis contained eight different species, ranging over about 10 million years from the late Cretaceous to the early Eocene – not especially big compared to some other wastebaskets we’ve looked at this month, but it was still a problem, muddying up attempts to understand the actual evolutionary relationships and biogeography of the taeniolabidoids.

Cladistic studies in the 1980s showed that Catopsalis was paraphyletic, made up of at least five separate lineages, and a few of them were subsequently renamed and reclassified. The Cretaceous Asian forms became Djadochtatherium and Catopsbaatar, and are now considered to be part of a different lineage of multis known as djadochtatherioids, while one of the remaining North American species then became Valenopsalis.

…But a couple of other new Catopsalis species have also been named in the meantime (one as recently as 2018), so there are still seven different species that need sorting out in this particular wastebasket.

It Came From The Wastebasket #15: Rauisuchian Revolution

Pseudosuchians, or “croc-line archosaurs”, are one of the two major lineages of archosaur reptiles, alongside the avemetatarsalians (pterosaurs and dinosaurs). Although today they’re represented only by crocodilians, they were especially successful and diverse back in the Triassic – and it was only after a mass extinction took out most of them that the dinosaurs were able to rise to prominence for the rest of the Mesozoic Era.

A grouping of pseudosuchians traditionally known as “rauisuchians” had upright limbs in a distinctive “pillar-erect” hip arrangement. Many of these croc-relatives were large quadrupedal predators, but others developed bipedal theropod-like postures, with some so remarkably convergent that they were initially misidentified as ornithomimosaurs.

The first rauisuchians were discovered in the 1930s, represented only by fragmentary remains, and while they were initially recognized as being pseudosuchians their exact evolutionary relationships within that group were poorly understood for a long time. Over the next several decades they were classified with aetosaurs (early armored pseudosuchians), then ornithosuchids (even earlier pseudosuchians), and then erythrosuchids (not even pseudosuchians but an earlier type of archosauriform).

More complete fossil discoveries and better cladistic analysis methods in the 1980s led to them being classified as being very closely related to crocodylomorphs, with three main lineages recognized: the prestosuchids, the rauisuchids, and the poposauroids.

An illustration of three different "rauisuchians", extinct relatives of modern crocodiles. At the top is Prestosuchus, a quadrupedal reptile with a boxy theropod-dinosaur-like head, platigrade bear-like legs, and a long tapering tail. It's colored yellow-brown with lighter underbelly and cat-like darker spots-and-stripes. In the middle of Postosuchus, a bipedal reptile that convergently resembles a tyrannosaur, with a boxy head, small arms, plantigrade legs, and a long counterbalancing tail. it's colored brown on top and white underneath, with irregular splotches of black and white across its body. At the bottom is Effigia, a bipedal reptile that convergently resembles a featherless ornithomimosaur, with a beaked bird-like head, a long neck, small arms, bird-like legs, and a counterbalancing tail. It's dark-colored with faint reddish and yellowish stripes and a paler underside. — The “prestosuchid” *Prestosuchus chiniquensis*, the rauisuchid *Postosuchus kirkpatricki*, & the poposauroid *Effigia okeeffeae* (not to scale)

But even by the end of the 20^th century “Rauisuchia” had never actually gotten a formal definition, and it had very much become a wastebasket taxon for a variety of paracrocodylomorph pseudosuchians that didn’t easily fit into any other major lineages.

In the 2000s renewed interest in rauisuchians’ anatomy and evolutionary relationships led to increasing recognition that they weren’t even a single defined group, with various species instead falling into different points along an “evolutionary grade“. The poposauroids and rauisuchids still seem to be distinct lineages, but the “prestosuchids” were found to be polyphyletic, with some forming a grade between the other two “rauisuchid” groups and others turning out to not even be paracrocodylomorphs.

A cladogram showing the classification of poposauroids, Prestosuchus, and rauisuchids within the group Pseudosuchia. They're shown as three separate lineages branching off between aetosaurs and the ancestors of modern crocodilians. A bracket marking indicates that all three traditionally used to be classified as "rauisuchians".

And although the taxonomic concept of “Rauisuchia” as a distinct group has now been abandoned, the term “rauisuchians” does still remain in common use as an informal name for these animals – probably because it’s much more concise than saying “non-crocodylomorph paracrocodylomorphs”.

It Came From The Wastebasket #14: The Protorthoptera Puzzle

Protorthoptera was a group of fossil insects created in the early 20^th century to categorize “primitive” neopterans – some of the earliest insects to have evolved the ability to fold their wings down over their backs. Known mostly from just fossilized forewings, they first appeared around 320 million years ago in the late Carboniferous, and after heavy losses during the Great Dying mass extinction they eventually disappeared in the mid-Triassic about 240 million years ago.

And this group was a massive wastebasket taxon.

As early as the mid-20^th century the protorthopterans were recognized as being a general taxonomic dumping ground, containing a mixture of early members of multiple different “orthopteroid” insect lineages. But invertebrate paleontologists at the time considered this collection of “primitive” insects to lack enough distinctive features to confidently separate them out from each other, and so the highly paraphyletic grouping continued to be used well into the 1990s.

An illustration of Ctenoptilus elongatus, an extinct insect. It has long thin antennae, a small grasshopper-like head, six legs each ending in two small claws, a cylindrical abdomen, and two pairs of large wings folded over its back. It's colored red, tan, and dark brown, with striped markings on its wings. — *Ctenoptilus elongatus*

But in the early 2000s this situation finally changed. Proper cladistic analysis of protorthopteran fossils identified defining features of the wing vein patterns, and many species were reclassified into various lineages within the Archaeorthoptera – which includes modern grasshoppers, crickets, and locusts along with several closely related fossil groups like the titanopterans and caloneurodeans.

“Protorthoptera” is still sometimes used in a loose sense for fossil neopteran insects that still can’t be confidently classified anywhere else, so the wastebasket isn’t entirely cleared here.

And there are some alternate classification systems (mainly proposed by Russian paleontologists) that instead consider many protorthopterans to be notopterans closely related to modern ice-crawlers, and place others as part of other modern neopteran lineages such as webspinners and true bugs.

Hopefully better fossil discoveries and future studies will eventually help clear things up, and give us a better overall picture of the evolution of these insects.

It Came From The Wastebasket #13: Certifying Cetiosaurus

Discovered in England in the early 1840s, Cetiosaurus was one of the first sauropod dinosaurs known to science – although its scrappy remains were initially mistaken for a massive crocodile-like marine reptile, hence its name meaning “whale lizard”.

It wasn’t even identified as being some sort of dinosaur until a couple of decades later, and then in the 1870s discoveries of much more complete sauropods like Brontosaurus and Diplodocus in North America led to it finally being recognized as a similar long-necked form.

Like some other early dinosaur discoveries it quickly became a wastebasket taxon, with vaguely-similar fragmentary fossils found in England, France, Switzerland, and Morocco all being lumped under its name. By the end of the 20^th century nearly 20 different Cetiosaurus species had been created, most of them highly dubious and based on poor fossil material without any distinctive anatomical features.

An illustration of Cetiosaurus, a sauropod dinosaur. It has a small head atop a long neck, a chunky four-legged body with mitten-like forefeet and three-clawed hindfeet, and a long tapering tail. It's colored pale teal with darker red-brown stripes and a lighter underside, with brighter yellow-green towards its head. There are also speculative red quill-like structures on its throat, cheeks, neck, and tail tip. — *Cetiosaurus oxoniensis*

It wasn’t until the early 2000s that the mess was finally cleaned up. A major revision and redescription of Cetiosaurus determined that just one species was actually valid: Cetiosaurus oxoniensis from the mid-Jurassic of England, known from fairly complete remains and dating to about 170 million years ago.

But this did create a new problem – it meant that the original type species Cetiosaurus medius wasn’t actually based on anything distinctive. So, much like what happened with Iguanodon, the type species was officially changed to Cetiosaurus oxoniensis in 2014, ensuring that this historically significant genus was defined by decent fossil material rather than by dubious fragments.

It Came From The Wastebasket #12: Coelurosaur Confusion

Historically Coelurosauria was the counterpart to the Carnosauria, with both of them representing two major lineages of theropod dinosaurs.

Created as a group in the early 20^th century, coelurosaurs quickly became a dumping ground for all small-bodied theropods – including coelophysoids, compsognathids, ornithomimids, oviraptorosaurs, dromaeosaurids, and troodontids– and for a while this wastebasket taxon also included the large-bodied ceratosaurids and tyrannosauroids, before they were moved over into the carnosaurs.

But during the 1960s and 1970s this arrangement began to break down. A better understanding of groups like dromaeosaurs revealed a confusing mixture of traditional “carnosaur” and “coelurosaur” anatomical features, and paleontologists struggled to figure out where these sorts of theropods actually fit in.

The development of cladistic methods from the 1970s onwards led to efforts to clean up the coelurosaur wastebasket, trying to figure out a more accurate version of these animals’ evolutionary relationships. After briefly collapsing Coelurosauria down to just coelophysoids and “coelurids“, the growing recognition of modern birds as living theropod dinosaurs eventually resulted in the group being properly redefined in the 1980s as “birds, and all theropods closer related to them than to carnosaurs“.

An illustration showing four examples of coelurosaurs, theropod dinosaurs closely related to birds. One the left is Citipati, a bird-like feathery oviraptorosaur that has a cassowary-like crest on its head, a short beak, a long neck, feathered wings, long slender legs, and a short tail with a feather fan at the end. It's mostly colored black and white, with bright blue and yellow face markings and small eyespots on its wing and tail feathers. At the top is Albertosaurus, a tyrannosaur with short bony crests above its eyes, tiny two-fingered hands, long bird-like legs, and a long thick counterbalancing tail. It's colored with a blotchy striped pattern of yellow, red, and dark brown, with bright blue on the crests over its eyes. On the right is Yi, a small bird-like scansoriopterygid with a fluffy feathery coat, four long tail feathers, and large membranous wings supported by a bony extension from its wrist that make it look like a dino-bat or a wyvern. It's colored brownish-grey with a yellow snout and striped markings on its neck, wings, and tail feathers. At the bottom is Sinosauropteryx, a compsognathid with a typical theropod body plan – triangular head, S-shaped neck, three-clawed hands, bird-like legs, and a long tail – but it's covered in fluffy feathers which are especially bushy on its tail. it's colored ginger-and-white, with a black raccoon-like mask marking over its eyes and a stripey tail. — Clockwise from the left (not to scale): *Citipati osmolskae*, *Albertosaurus sarcophagus*, *Yi qi*, *Sinosauropteryx prima*

The coelophysoids were finally removed entirely, reclassified as a much earlier branch of theropods – but quite a few of the other groups from earlier concepts of Coelurosauria survived this reshuffling, with the compsognathids, ornithomimids, oviraptorosaurs, dromaeosaurs, and troodontids all proving themselves to have really been closely related the whole time. Meanwhile the tyrannosauroids were brought back in, along with the therizinosaurs, alvarezsauroids, and a whole bunch of paravian and avialan lineages.

(Megaraptorans might belong somewhere in the coelurosaurs, too – possibly being tyrannosauroids – but their classification is currently being disputed.)