The semi-aquatic spinosaurids were an unusual exception to this with six or seven teeth per premaxilla – and one particular member of this lineage seems to have been just a little bit weirder.
Baryonyx walkeri lived during the early Cretaceous, around 130-125 million years ago, in what is now southeast England. About 9m long (~30′), it had distinctive enlarged curving claws on the first fingers of its hands, along with a long narrow snout with a “rosette” at the tip followed by a notch (a shape convergent with the jaws of modern pike conger eels).
And that premaxillary rosette had a strangely asymmetrical arrangement of teeth.
The left side had six teeth, and the right side had seven.
Why? We don’t know!
Baryonyx skull material is rare and fragmentary, so it’s unclear if this was actually a characteristic feature of the species or if the known asymmetric rosette just represents an unusual individual.
Stegosaurs are some of the most popular and recognizable dinosaurs thanks to their unique appearances, with small heads, elaborate back plates, and spiky thagomizer tails.
Closely related to the ankylosaurs, they first appeared in the mid-Jurassic about 170 million years ago. While they lasted until at least the mid-Cretaceous (~100 milion years ago), their heyday was in the latter half of the Jurassic, ranging all across Asia, Europe, Africa, and North America – and the North American species like the eponymous Stegosaurus developed especially elaborate plates in a distinctive asymmetrical pattern, not arranged in pairs like most other stegosaurs but in alternating rows along each side of the midline of their backs.
Much like its more famous relative its plates seem to have alternated along its back, which may have been an adaptation to maximize visible surface area while minimizing the number of plates, saving on the energy needed to grow such large elaborate ornamentation.
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.
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.
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 20th 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.
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.
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 20th 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.
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 speciesCetiosaurus 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.
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“.
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.)
Initially just known from its skull domes, it was one of the first pachycephalosaurs to be discovered and was very poorly understood until more complete remains were found in the 1920s. Then it spent a couple of decades being mixed up with Troodon due to similarities in tooth shape, until the discovery of Pachycephalosaurus led to pachycephalosaurs finally being recognized as a distinct group of ornithischian dinosaurs in the 1940s.
Stegoceras validum
For much of the 20th century Stegoceras was treated as a wastebasket taxon for any small-to-mid-sized North American (and one Asian) pachycephalosaur, and multiple different species were named based on what were often rather dubious fragmentary fossils. But towards the start of the 21st century this mess did start getting cleaned up, merging some dubious species into the original Stegoceras validum, and moving others to separate genera like Sphaerotholus, Colepiocephale, Hanssuesia, and Sinocephale.
By the early 2000s just the Canadian Stegoceras validum remained – but then in 2011 the new species Stegoceras novomexicanum was named based on specimens from New Mexico, USA. The validity of this second species has been debated, since the fossils are juveniles and might instead belong to Stegoceras validum or another genus like Sphaerotholus, but if it is some sort of Stegoceras then it significantly re-extends the known geographic range of this little pachycephalosaur.
From left to right: Asfaltovenator vialidadi, Torvosaurus tanneri, Giganotosaurus carolinii, & Baryonyx walkeri
But then cladistic analysis in the 1980s and 1990s revealed that some of these theropods weren’t actually closely related at all. Carnosaurs weren’t a natural lineage but instead were highly polyphyletic, with the physical similarities between them seeming to be more due to convergent evolution than direct shared ancestry.
Some carnosaurs were split off and reclassified as more “primitive” types of theropod, while the tyrannosaurs were placed much closer to birds with the coelurosaurs. The remaining “carnosaurs” were just the allosaurids, carcharodontosaurs, and their closest relatives, and some paleontologists now prefer to use the name Allosauroidea for this group to distance it from the previous wastebasket mess.
…But Carnosauria might not be done just yet.
The discovery of Asfaltovenator in 2019 complicated matters once again, with a mixture of anatomical features linking it to both the allosauroids and the megalosauroids (megalosaurids, spinosaurids, and their relatives) – suggesting that these two groups might actually have been closely related to each other in a single lineage after all.
This would potentially return Carnosauria back to something surprisingly close to its original definition, with the various megalosauroids now forming an evolutionary grade leading to the allosauroids.
Troodontids were small bird-like theropod dinosaurs, lightly built with slender legs and sickle-shaped “raptor” claws on the second toes of their feet. They had fairly big brains proportional to their body size, rather like modern birds, and their large forward-facing eyes had good depth perception. Owl-like asymmetrical ears in some species gave them a very keen sense of hearing, suggesting they may have been nocturnal hunters using sound to pinpoint the location of small prey.
The original specimen of the namesake of the group, Troodon formosus, was a serrated tooth discovered in the 1850s, about 77 million years old and originating from the Late CretaceousJudith River Formation fossil beds in Montana, USA. It was so little to work with that it was initially mistaken for a lizard tooth, then during the 20th century it was recognized as belonging to a dinosaur and spent time classified as a megalosaurid, then a pachycephalosaur, then finally as a small theropod similar to the Mongolian Saurornithoides.
In the late 1980s it was merged together with multiple other troodontids (including Stenonychosaurus of speculative “dinosauroid” fame), and since Troodon had been the first of all of them to be named it took priority as the genus name.
And then for a while every single Late Cretaceous troodontid specimen from North America was also lumped into Troodon, turning it into a wastebasket taxon.
The problem was that all these troodontids came from locations separated by thousands of kilometers and millions of years of time, and it’s unlikely that they all actually represented just one single species. But they were only known from rare fragmentary remains, making distinguishing them from each other difficult, and the original Troodon tooth didn’t really have any distinctive features either – it turns out most troodontid teeth all look exactly the same!
It was becmoning increasingly dubious whether Troodon was even a valid name at all, and during the 2010s several paleontologists began trying to sort the mess out. The old names Pectinodon and Stenonychosaurus were revived, and some ‘Troodon’ fossils were also split off and given completely new names, becoming Albertavenator and Latenivenatrix*.
As of 2022, Troodon itself is now in a sort of taxonomic limbo, with some paleontologists abandoning it as a dubious name while others are still arguing in favor of continuing to use it. The name could potentially be properly rescued if the original tooth can be clearly linked to better fossil material, letting Troodon take over priority again from one of the other better-established troodontids, or by defining a new type species similar to what happened with Iguanodon.
…But with how incredibly generic that tooth is, both of those options would be very difficult.
