A new species of Tyrannosaurus from the Late Cretaceous of New Mexico
Tyrannosaurus mcraeensis. Reconstruction by Sergei Krasovsky
A fossil from New Mexico shows that giant tyrannosaurs evolved millions of years earlier than thought— in southern North America.
Suddenly, Tyrannosaurus
Tyrannosaurus AMNH 5027, one of the most famous dinosaur specimens in the world. From Osborn, 1917.
66 million years ago, the giant Tyrannosaurus rex appeared suddenly in the Great Plains of western North America. 40 feet long and weighing 8 tons or more, it was among the largest terrestrial predators of all time, and the last of the dinosaurian superpredators, huge carnivores weighing multiple tons. The biology and evoluytion of T. rex have been the subject of extensive study, and is known from dozens of skeletons- and yet we know very little about where it came from.
Tyrannosaurus seems to appear suddenly in the late Maastrichtian, at the very end of the Cretaceous, without any obvious ancestors or close relatives in the fossil record. Earlier predators in the region- Albertosaurus, Gorgosaurus, Daspletosaurus- aren’t that closely related to T. rex. The closest relatives of T. rex, Tarbosaurus bataar and Zhuchengtyrannus magnus, come from the Gobi Desert of Mongolia and Northern China, respectively. That’s led to speculation Tyrannosaurus’ ancestors lived in Asia, then migrated over the Bering Land Bridge into North America.
Now restudy of long-overlooked fossils from New Mexico suggests another possibility. Tyrannosaurus may have evolved in North America after all, but may come from the southern part of the continent, where we have fewer fossils. After evolving to giant size over millions of years, Tyrannosaurus finally spread north into Montana, Wyoming and Canada around 67 million years ago. Somewhat surprisingly, this discovery isn’t the result of newly collected fossils. Neither were the fossils hidden away in some dusty storeroom. They were on display in a museum in the middle of downtown Albuquerque, and had been described several times and- mistakenly, it now turns out- assigned to T. rex
The location of the new tyrannosaur in southern New Mexico. From Dalman et al. (2024)
The bones come from outcrops near Elephant Butte in western New Mexico. Here, along the banks of the Rio Grande, the rocks of the Hall Lake Formation slowly weather away, exposing dinosaurs from the end of the Cretaceous. These outcrops aren’t terribly productive, but that also means they haven’t seen the sort of intensive study that other parts of North America. Over the course of a century or so, a handful of dinosaurs have emerged from the badlands here. Most of them are scraps— a few vertebrae, limb bones, that sort of thing— and they haven’t attracted a lot of attention.
Outcrops of the Hall Lake Formation of New Mexico. Photo by Spencer Lucas.
The bones come from outcrops near Elephant Butte in western New Mexico. Here, along the banks of the Rio Grande, the rocks of the Hall Lake Formation slowly weather away, exposing dinosaurs from the end of the Cretaceous. These outcrops aren’t terribly productive, but that also means they haven’t seen the sort of intensive study that other parts of North America. Over the course of a century or so, a handful of dinosaurs have emerged from the badlands here. Most of them are scraps— a few vertebrae, limb bones, that sort of thing— and they haven’t attracted a lot of attention.
A duckbill dinosaur femur weathers out of the Hall Lake Formation.
In 1983, an enormous tyrannosaur jaw was discovered weathering out of these outcrops, and then excavated by Dave Gillette of the New Mexico Museum of Natural History. Along with the jaw, a few other bones- a prearticular, a scrappy palatine, a chevron from the tail, and a few teeth- were recovered. Gillette assigned the jaw to Tyrannosaurus, although this interpretation wasn’t universally accepted. In a 1990 paper, Tom Lehman and Kenneth Carpenter suggested it likely represented a new genus. However, in a review of New Mexico fossils, Carr and Williamson argued that “NMMNH P- 3698 does not differ in any significant way from other material identified as T. rex, contra Lehman and Carpenter, (1990), and we uphold the original identification of Gillette et al. (1986).” I’d argue the word significant does a lot of work here— significant is often in the eye of the beholder. Observations that seem insignificant to one person can be highly significant to another.
The dentary is rather unusual— it’s got a long, low profile unlike any other Tyrannosaurus, and the back of the dentary is narrow and upturned, is again unlike anything seen in Tyrannosaurus. Are these differences significant, or not? Lehman and Carpenter— who are fairly conservative paleontologists and not known for naming species based on trivial differences— seemed to think so; Carr and Williamson didn’t.
The Age of the Hall Lake Formation
Another important piece of the puzzle is the age of the Hall Lake Formation. Referral of the Hall Lake tyrannosaur to T. rex is based in part on an argument that, if not perfectly circular, tends to spiral back on itself in places with self-reinforcing conclusions. Why is the animal T. rex? Well, one reason we tend to assume it’s T. rex is the age of the rocks. The sediments date to the late Maastrichtian, when T. rex lived. So how do we know they’re late Maastrichtian in age?
Well, because of the fossils from the Hall Lake are typical of the late Maastrichtian—after all, they include Tyrannosaurus and Torosaurus. How do we know these animals represent Tyrannosaurus and Torosaurus?
Well, they’re late Maastrichtian in age…
And so on.
Sierraceratops
In the end, what finally forced a rethink of the age problem wasn’t the tyrannosaur at all, but the ceratopsian - the so-called Torosaurus. A few years ago I helped out on a study of this dinosaur, which had never been described in detail before. The animal was long assumed to be a Torosaurus (because it was late Maastrichtian in age… again, that circular reasoning) and also because it had holes in the frill, while the frill in Triceratops is solid. But most horned dinosaurs have openings in the frill, so this tells you nothing, except it isn’t Triceratops. A close look at the animal showed that it had a narrow midline bar of the frill- unlike the wide bar in Torosaurus. It was something more primitive, maybe related to Coahuilaceratops from the late Campanian of Mexico. This was exciting, at least for me, since I’ve done a lot of work on ceratopsians.
The frill of Sierraceratops. Not a Triceratops, but not a Torosaurus either.
There’s a lot of debate dinosaur diversity just before the asteroid impact, and how many lineages actually go up to the end of the Cretaceous. Finding a primitive ceratopsian lineage in the late Maastrichtian would be a really interesting story, since I’ve always suspected we’re underestimating the diversity of dinosaurs in the Maastrichtian. But in science you need to be skeptical, especially when the facts seem to fit neatly into your pet theory. Did we actually have any good evidence that these rocks were late Maastrichtian?
In fact, some of the volcanic ash beds in the rocks had been dated using uranium dating. And the radiometric dates were surprisingly old— just below the tyrannosaur were a series of dates- 73, 75, 74, 75 million years ago. These dates aren’t perfect- they have error bars of up to a million years (that’s why you get a date of 75 million years sitting above a date of 74). Still, it’s unlikely all of them are off by six or seven million years.
Unfortunately, we don’t have radiometric dates above the bones. So in theory, the tyrannosaur could be significantly younger than the ashes. But it doesn’t lie that far above them. Based on the rates of deposition calculated for the sediments, it’s unlikely you could pack six or seven million years into the 100 feet or so separating the tyrannosaur from the ashes - not unless we’re missing hundreds of feet of sediment, or sedimentation rates slowed dramatically- neither of which do we any evidence to think is true.
So our Torosaurus isn’t really a Torosaurus, it’s similar to a Campanian species dating to 73 million years ago. Our rocks aren’t late Maastrichtian, they’re either latest Campanian or early Maastricthian. If our tyrannosaur is substantially older than Tyrannosaurus rex, it can’t be a T. rex. Dinosaur species don’t last that long, typically just a million or a half-million years. So what do we have?
The Devil is in the Details
At this point we took a close look at the skeleton. A very close look. It turns out that the bones are different from T. rex. In fact, every single bone is slightly different from the corresponding element in T. rex. Since we have lots of T. rex fossils, we have a reasonable idea of what sort of variation exists in a T. rex, and this animal consistently lies outside of that range of variation, in every single bone of its skeleton .
- The dentary, for example, is slender and upturned.
- The postorbital lacks the prominent bump above the eye seen in T. rex
- The squamosal is shorter, and has a bar of bone forming the border of the recess in its ventral surface
- The prearticular is gently curved, not bent like a hockey-stick like in T. rex
- The articular lacks the big facet for the splenial
- The splenial is straighter, and the apex points up instead of back
And so on. I won’t bore you with the details (that’s what the paper is for).
Are these differences subtle? yYes, but so are the differences between, say, T. rex and Tarbosaurus. You need to spend a fair amount of time studying the fossils to be able to reliably tell the difference between the two. Likewise, the differences between humans and Homo erectus are subtle- we have more bulbous heads, a stronger chin, and lack brow ridges- but nobody would doubt we’re separate species.
And that’s how evolution proceeds- subtle differences build up over thousands and millions of years. The differences between say, a lynx and a cougar skull are subtle, as are the differences between human skulls and neanderthal skulls (although we are here with cities and nuclear weapons and the internet, and they’re extinct, so clearly there are some pretty significant differences reflected in the subtle variations in skull shape).
It’s not a T. rex, it’s something older, and more primitive.
The origins of giant tyrannosaurs
It seems then we’ve solved the mystery of where Tyrannosaurus comes from. The giant tyrannosaurs are in the southern part of North America, where fossils tend to be rarer, so we’ve largely overlooked them. This is further supported by the next-oldest Tyrannosaurus, a maxilla from the Javelina Formation of Texas, which probably dates to the middle of the Maastrichtian. And there’s a high diversity of tyrannosaurines- relatives of Tyrannosaurus- in the southwest, including animals like Lythronax, Teratophoneus, and Bistahieversor. The southern part of the North America seems to be epicenter of tyrannosaurine evolution.
It turns out that the dinosaurs in different parts of North America are doing very different things. Despite the relatively short distances separating them, the dinosaurs of Montana, Wyoming and Canada are different species than the ones in Utah, New Mexico, and Mexico. The giant tyrannosaurs were part of a distinct, southern fauna.
A Land of Giants
What’s interesting here is that Tyrannosaurus isn’t the only giant dinosaur from the south. A number of lineages appear to either be restricted to the south, or originate there. The giant ceratopsid Titanoceratops, for example, shows that the triceratopsins- Triceratops, Torosaurus, and kin- are evolving in the south. We also have the giant duckbill Magnapaulia from Baja California. Meanwhile, the hadrosaurs and tyrannosaurs known from the far north, in the Arctic Circle in Alaska, seem to be fairly small animals.
Something about the south may have driven the evolution of large dinosaurs. The evolution of large predators could be fairly easily explained by the evolution of large herbivores- but if so, why did the herbivores get so big? Its possible something about the climate may have favored the evolution of large size, or perhaps dinosaur physiology. Maybe less seasonal environments, with fairly high growth rates year round, favored the evolution of big dinosaurs. We don’t really know- whether this even is a pattern needs to be studied in more detail.
Just how many tyrannosaur species are there?
It now appears that fossils assigned to T. rex will end up representing multiple species. A number of small tyrannosaurs once identified as juvenile T. rex seem to represent an additional genus, or even several genera- Nanotyrannus, and possibly a second taxon, Stygivenator. The remaining Tyrannosaurus may represent two or more species, T. rex, T. imperator and maybe even a third, T. regina (two seems plausible to me, three stretches my credulity, but who knows- more study is needed here). The Javelina tyrannosaur, from Texas, may represent another Tyrannosaurus, and then we have the New Mexico specimen.
Remarkably, what we used to call “T. rex” could end up being four, five, or six different species. As incredible as this might seem, studies of modern animals suggest that we’ve been underestimating diversity. In recent years, we’ve recognized new species of elephant, giraffe, beaked whale, and even baleen whale. Even working with modern organisms- where we can potentially collect large numbers of specimens and study every bit of their anatomy in detail.
People may ask, why do we care about tyrannosaur species?
Because species identification is a basic but fundamental problem. So much is downstream of alpha taxonomy. Speciation rates. Biogeographic patterns and diversity gradients. Evolutionary trees. Extinction rates. Mass extinction. T. rex is just one species, but it’s well known and well studied. If we have it wrong, what else have we gotten wrong- about ammonites, trilobites, foraminifera, fossil pollen? If we’re underestimating the diversity of one of the most intensely studied fossil species, just how many other things are we missing?
References
Dalman, Sebastian G., Mark A. Loewen, R. Alexander Pyron, Steven E. Jasinski, D. Edward Malinzak, Spencer G. Lucas, Anthony R. Fiorillo, Philip J. Currie, and Nicholas R. Longrich. "A Giant Tyrannosaur from the Campanian–Maastrichtian of Southern North America and the Evolution of Tyrannosaurid Gigantism." Scientific Reports (2024). https://dx.doi.org/https://doi.org/10.1038/s41598-023-47011-0.
Dalman, Sebastian G., Spencer G. Lucas, Steven E. Jasinski, and Nicholas R. Longrich. "Sierraceratops Turneri, a New Chasmosaurine Ceratopsid from the Hall Lake Formation (Upper Cretaceous) of South-Central New Mexico." Cretaceous Research 130 (2022): 105034.
Longrich, Nicholas R, and Evan T Saitta. "Taxonomic Status of Nanotyrannus Lancensis (Dinosauria: Tyrannosauroidea)—a Distinct Taxon of Small-Bodied Tyrannosaur." Fossil Studies 2, no. 1 (2024): 1-65.
Longrich, Nicholas R. "Titanoceratops ouranos, a Giant Horned Dinosaur from the Late Campanian of New Mexico." Cretaceous Research 32, no. 3 (2011): 264-76.
Paul, Gregory S, W Scott Persons, and Jay Van Raalte. "The Tyrant Lizard King, Queen and Emperor: Multiple Lines of Morphological and Stratigraphic Evidence Support Subtle Evolution and Probable Speciation within the North American Genus Tyrannosaurus." Evolutionary Biology 49, no. 2 (2022): 156-79.