Sierraceratops, a new horned dinosaur from New Mexico, and dinosaur endemicity- why do giant dinosaurs have small geographic ranges?

Different dinosaurs lived in different parts of North America during the Cretaceous. How can you have endemism without geographic barriers? Rather than geography, climate, or flora, competition between dinosaurs may have limited the range of species.

Sierraceratops, from the latest Cretaceous (Campanian-Maastrichtian) of New Mexico

How do you find a new dinosaur? One of the easiest ways is to look somewhere new. The Hell Creek Formation of Montana, for example, has been worked for over 100 years. So if you keep going back, you’ll tend to find the same things people have been finding for the past century— Triceratops, T. rex, Edmontosaurus, Triceratops, Torosaurus, Triceratops, Triceratops, and more Triceratops. New dinosaurs do still show up in the Hell Creek, but rarely. But if you pick up and move a few hundred miles away, you tend to find entirely different things. Head down to New Mexico for example, and you’ll find species- lambeosaurs, kritosaurs, and titanosaurs- not found in up in Montana. Recently that happened when my colleagues and I described a new horned dinosaur from New Mexico, Sierraceratops.

Part of this pattern results from the fact that the rocks in different areas differ in their ages, often by millions of years. Dinosaur species tended to be short-lived. Most only lasted a million years or so before they either died out, or evolved into something else entirely. Because rocks in different areas are often different ages, you’re basically time-traveling- moving forward and backward in time as the dinosaurs evolve around you. Sierraceratops turns out to be around 6 million years older than Triceratoops and the other Hell Creek dinosaurs, so it’s hardly surprising it turned out to be distinct from Triceratops. 6 million years is about the time it took us to evolve from apes.

The new dinosaur, Sierraceratops, from New Mexico.

That’s only part of the answer, though.

It turns out that even if you compare Sierraceratops to dinosaurs that lived around the same time further north- things like Anchiceratops and Arrhinoceratops- it’s distinct from them as well. So even at the same point in time, you have different species living in different parts of the world.

Obviously, we see this pattern today. If you want to see a kangaroo in the wild, you go to Australia. But if you wanted to see a tiger, you’d go to India, and to see a giraffe, to Africa. Different places, different species.

That regions isolated by oceans or thousands of miles would have different faunas might seem obvious.

What’s somewhat surprising about dinosaurs is that even over fairly short distances, and even without oceans in the way, you tend to get very different species.

The dinosaurs of the Hall Lake Formation of New Mexico include species not found further north, like Sierraceratops and a titanosaurian sauropod.

The dinosaurs of New Mexico were only about 1000 miles or 1500 kilometers away from those of Montana. This isn’t exactly a short distance, but when you consider that caribou, wildebeest, and zebras can migrate hundreds of kilometers per year, or that many large animals have ranges spanning thousands of miles, it’s not obvious why the two areas should have different species of dinosaurs.

The horned dinosaurs or ceratopsids- Triceratops and relatives- are a good study system to examine these patterns. Horned dinosaurs are an interesting group to study because they have a good fossil record. Being herbivores, they were fairly common, being large, their bones preserve well, and tend to survive being weathered out of the ground as fossils. We’ve got a lot of fossils to study.

What’s more, the elaborate horns and frills make them easy to tell apart. The frills were probably used in courtship- like the tail of a peacock or a bird of paradise- signaling their owner’s fitness, and used to advertise to mates. They were under intense selection, and so evolved rapidly. That means every species had a distinct horn and frill arrangement, making it easy to tell them apart. And what these abundant, easy-to-tell-apart fossils show is that the species in the north tend to be different from those in the south.

So, back to our new ceratopsid from New Mexico.  It had long been assigned to Torosaurus, but unsurprisingly- given that the animal lived millions of years earlier and more than a thousand miles away- it turned out to be something else. It’s a distinct species, and apparently more closely related to southern dinosaurs, like Bravoceratops, from Texas, and Coahuilaceratops, from Mexico.

What drives these patterns— why don’t we just find the same dinosaurs everywhere? This turns out to be a surprisingly complicated problem. A number of things might prevent dinosaurs from moving around.

Geographic barriers.

Climate.

The plants.

All of them probably play some role, sometimes major roles. But my suspicion is that one of the most important causes of dinosaur distribution in North America was the dinosaurs themselves- that competition between species kept species from moving between areas.

Geographic Barriers

Kangaroos don’t live outside Australia. The reason is pretty simple: Australia is an island, surrounded by ocean, and kangaroos don’t swim long distances. Oceans played a similar role in the Cretaceous, too. A huge seaway- the Western Interior Seaway- ran north-to-south from the Gulf of Mexico to the Arctic Ocean, splitting North America in half. Dinosaurs found in New Jersey are only distantly related to those in Montana.

But there’s no water barrier separating the northern and southern parts of North America, so that can’t explain the differences.

What about mountains, then? This seems like an obvious explanation . But it fails on two fronts. First, there seems to have been a broad coastal plain extending up along the Western Interior Seaway. It’s unlikely mountains blocked the passage of dinosaurs between North and South.

Second, mountains just don’t seem to be very effective barriers. Many modern animals extend right across the Rocky Mountains today. Coyotes, for example, go right across the Rockies, as do mule deer and grizzlies. Even little animals can readily cross mountains- red squirrels are found on either side of the Rockies. If mountains can’t even stop tiny little squirrels, how could they stop huge horned dinosaurs?

Climate

Which brings us to climate. Could climate- temperature, seasonality, rainfall- control what animals live where?

Today, certain reptiles are found in Asia but not North America, and vice versa. Asia has cobras and dragon lizards, America has rattlesnakes and iguanas. Although the two continents were connected in the recent past by the Bering Land bridge, it’s just been too cold for cobras and rattlesnakes to crawl across.

Note the conspicuous absence of reptiles in this picture of Alaska.

It’s possible this plays some sort of role for dinosaur distribution. The giant titanosaurs, for example, seem restricted to the southern part of the continent- and we don’t seem to find titanosaurs anywhere at high latitudes, anywhere in the world, like in Asia, or Antarctica. It’s possible that something about titanosaurs meant they just weren’t able to adapt to the cold.

But other dinosaurs seem to have been able to adapt to a huge range of climates, and we know this because they passed back and forth between North America and Asia, using the Bering Land Bridge inside the Arctic Circle. Mongolia’s Tarbosaurus, a close relative of T. rex, is an immigrant from North America. The common Montana duckbill Edmontosaurus is related to species in China, Japan, and Siberia, suggesting it crossed the land bridge from the other direction. And within North America, giant triceratops-like horned dinosaurs seem to have evolved in the Southwest, then migrated north into Montana, Wyoming, and Canada.

The ancestors of Mongolia’s Tarbosaurus, a close relative of T. rex, trekked across the Bering Land Bridge from North America

Clearly, many dinosaurs were able to tolerate a wide range of climactic conditions- we have similar species in the subtropical floodplains of the Western Interior, the cold Arctic, and the Gobi Desert. That’s not to say climate played no role, but it probably wasn’t the decisive factor for most species, at least not directly.

Plants

Could the flora, the ferns, flowers, and trees, control which dinosaurs lived where?

Monkeys, for example, never made it to North America from Asia. Some, like the Japanese snow monkey, are fairly cold-tolerant. But the tundra and taiga or northern regions offer little fruit or other sustenance for monkeys in winter, and the grassy tundra has no trees for them to climb. On smaller scales, tree squirrels obviously don’t do well in treeless prairies, while grass-eating ground squirrels don’t do well in forests. Likewise, elk and moose inhabit forested regions, and pronghorn like the open plains.

This might help explain some of the patterns we see- perhaps some species exploited more open habitats, others more closed habitats, or they preferred different kinds of plants to eat. But again, we see that dinosaurs were often very adaptable. In moving between North America and Asia (and vice versa), duckbills and other herbivores would have moved through distinct plant communities as they went north into the Arctic Circle and then back down into the Gobi Desert (and vice versa).

These areas must have had very different plants- for example, palm trees grew in New Mexico, but not further north in Canada. Yet dinosaurs never the less adapted to living in different environments, eating different species. Many modern species are adaptable as well- African elephants are at home in both savannahs and forests, for example.

A fossil palm tree leaf from the Late Cretaceous of New Mexico. These palms were probably food for horned dinosaurs there.

And differences in the plant communities seem unlikely to explain differences in the predators. Assuming different plants in Alberta and Utah meant different species of horned dinosaurs and duckbills lived in each habitat, how would that explain why the carnivores were different?

Meat is meat, one species of duckbill probably tasted pretty much the same as another.

The Dinosaurs Themselves

The last possibility- and the one I think most likely- is that what controlled the range of dinosaurs was other dinosaurs.

Consider the biogeography of a familiar species of mammal- Homo sapiens. I know H. sapiens is a rather unusual species, so you might argue it’s not that relevant to dinosaurs. On the other hand, it’s only become widespread fairly recently, so we know a lot about how it moved around the world. In particular, the history of humans in North America is increasingly well-understood.

After arriving in Alaska around 35,000 years ago, the ancestors of today’s American Indians spread throughout North and South America. Around 4,000 years ago, a second group, the Inuit, settled the Arctic.

America was then “discovered” again many times, but few of these “discoveries” led to successful colonizations. The Polynesians made landfall in South America around 1200 AD and mingled with the locals. But despite succesfully colonizing Tahiti, New Zealand, Hawaii, and Easter Island, Polynesian cultures were not able to successfully establish themselves in South America.

The Vikings discovered North America in around 1,000 AD, but despite having iron tools, crops, and sailing ships, they were unsuccessful in colonizing the Americas. Basques visited the New World to fish for cod in the Middle Ages, but again, never successfully settled.

The failure of these people to colonize probably comes down to the fact that when they arrived, they found the Americas were densely settled and well-defended by territorial locals. The Vikings, for example, suffered attacks by the “Skraelings”, their name for the Inuit and Indian inhabitants of the Americas.

It was only with the help of disease killing off many of the locals that later waves of colonists were able to establish a presence. The Spanish brought smallpox, which proved decisive in their battles against the Aztecs and Incas; another pandemic decimated the Northeast, giving American colonists in Massachussetts room to get a foothold and found what would eventually become part of the United States.

One of the biggest constraints on where people live is other people. Climate and plants you can adapt to, but territorial locals are harder to gain a foothold against.

The first people into the Americas had stone-age technology, but were able to spread out through a huge range of environments- grasslands, deserts, rainforests- because they had no competitors. Once it was settled, becoming established was harder, until a disturbance- pandemics- created an opening (advanced technology brought by the Europeans doubtless helped as well).

In the same way, dinosaurs and other animals might opportunistically move into an area because of some disturbance- a drought, warming, cooling, climate change, a disease- that decimates the local population. Once there, they would soon adapt to the local environment- adapting to the climate, to the plants, and (perhaps most importantly) to the local endemic diseases and parasites. This gives them a competitive advantage against competing species trying to move into the area.

At some point, however, an environmental disturbance creates an opening, or perhaps a species evolves some advantage that allows it to invade. Now, the invasives get a foothold.

The process repeats, over and over- dinosaurs spread out, evolve in different ways in different habitats, producing new species. This process- invasion, adaptation that prevents further invasion until a disturbance- could create barriers to dispersal without a geographic barrier, or a direct climate barrier. It would be a biotic barrier to dispersal. Your competitors are the biggest constraint on where you can live.

The Biotic Barrier

This idea has some important implications that go far beyond dinosaurs, I’d argue. If competitors are the main constraint on dispersal, how does this predict the movements of species through space, and over time?

First, species should move from areas of high diversity, to low diversity. And in fact, we find this. Looking at mollusks, Jablonski has found that species tend to spread from low latitudes (where there are more species) to high latitudes (where there are fewer).

We also see in recent times that invasive species tend to invade temperate areas, rather than tropical areas. Low-diversity temperate habitats have more open niches, but it’s hard to get a foothold in the species-rich jungles of the Congo.

A map of invasive species. Generally speaking, the tropical rainforests of Africa, South America, and Asia have fewer invasives than higher-latitude regions. From Turbelin et al. 2016.

Similarly, studies of Anolis lizards in the Caribbean suggest that species go from high-diversity islands to lower-diversity islands, where there’s less competition. This also helps explain why species from continents (which are diverse) tend to colonize islands (which are lower in diversity), but it rarely goes the other way.

Once again, the likelihood of species moving is determined by other species.

Another important implication is that the likelihood of successfully dispersing changes through time. When life is highly diverse, there will be fewer open niches, and it will be harder to disperse. But in periods of low diversity- such as after a mass extinction- species can more easily move. This seems to happen with both worm-lizards and snakes, which dispersed in the wake of the Cretaceous-Paleogene boundary. Furthermore, a wave of invasive mammals appeared in Montana and Wyoming immediately after the asteroid impact.

Dinosaurs in time, and space

Last, these patterns are important I’d argue because we can’t consider the diversity of dinosaurs over time independently of their diversity in space.

It’s been argued that towards the end of the Cretaceous, dinosaurs were in decline. But most of what we know about dinosaur diversity at the very end comes from a small area. We’ve sampled the Hell Creek of Montana and the Lance Formation of Wyoming pretty well.

Other dinosaur faunas- the Naashoibito in New Mexico and Black Peaks in Texas, the Moreno Formation in California, the Prince Creek Formation in the Northwest Territories- are barely known at all. Then there are whole continents- Africa, Australia- about which we know almost nothing. It’s difficult to say that dinosaurs were low in diversity at the end primarily on the basis of this one small region.

Likewise, the supposed mid-Campanian “peak” in dinosaur diversity seems to be driven almost entirely by a single locality- the Dinosaur Park Formation of Alberta. Other Campanian localities, like the Kaiparowits Formation of Utah, the Kirtland of New Mexico, and the Aguja of Texas haven’t produced nearly as many species, either because they’re less diverse, or less sampled, or both. At any rate, whatever causes it, this “peak” may be local, not a global phenomenon.

The local picture may or may not reflect what’s going on worldwide. It’s easy when you focus on one little area- perhaps getting a bit territorial, like those dinosaur species- not to see what’s going on elsewhere. But it’s a big world out there, and to understand it, we need to move beyond our narrow little provinces, and see things more broadly.

References

Dalman, S. G., Lucas, S. G., Jasinski, S. E., & Longrich, N. R. (2021). Sierraceratops turneri, a new chasmosaurine ceratopsid from the Hall Lake Formation (Upper Cretaceous) of south-central New Mexico. Cretaceous Research, 105034. 

Helmus, M.R., Mahler, D.L., Losos, J.B., 2014. Island biogeography of the Anthropocene. Nature 513, 543-546.

Klein, C.G., Pisani, D., Field, D.J., Lakin, R., Wills, M.A., Longrich, N.R., 2021. Evolution and dispersal of snakes across the Cretaceous-Paleogene mass extinction. Nature Communications.

Lehman, T. M. (1997). Late Campanian dinosaur biogeography in the Western Interior of North America. In D. A. Wolberg & E. Stump (Eds.), Dinofest International, Proceedings of a Symposium Sponsored by Arizona State University, Special Publication, Academy of Natural Sciences (pp. 223-240). Philidelphia.

Lehman, T. M. (2001). Late Cretaceous Dinosaur Provinciality. In D. H. Tanke & K. Carpenter (Eds.), Mesozoic vertebrate life (pp. 310-328). Bloomington: Indiana University Press.

Jablonski, D., Belanger, C.L., Berke, S.K., Huang, S., Krug, A.Z., Roy, K., Tomasovych, A., Valentine, J.W., 2013. Out of the tropics, but how? Fossils, bridge species, and thermal ranges in the dynamics of the marine latitudinal diversity gradient. Proceedings of the National Academy of Sciences 110, 10487-10494.

Longrich, N. R. (2011). Titanoceratops ouranos, a giant horned dinosaur from the Late Campanian of New Mexico. Cretaceous Research, 32(3), 264-276. 

Longrich, N. R. (2014). The horned dinosaurs Pentaceratops and Kosmoceratops from the upper Campanian of Alberta and implications for dinosaur biogeography. Cretaceous Research, 51, 292-308. 

Longrich, N.R., Scriberas, J., Wills, M.A., 2016. Severe extinction and rapid recovery of mammals across the Cretaceous‐Paleogene boundary, and the effects of rarity on patterns of extinction and recovery. Journal of Evolutionary Biology DOI: 10.1111/jeb.12882.

Longrich, N.R., Vinther, J., Pyron, A., Pisani, D., Gauthier, J.A., 2015. Biogeography of worm lizards (Amphisbaenia) driven by end-Cretaceous mass extinction. Proceedings of the Royal Society of London B: Biological Sciences 202, 20143034.

Turbelin, A.J., Malamud, B.D., Francis, R.A., 2017. Mapping the global state of invasive alien species: patterns of invasion and policy responses. Global Ecology and Biogeography 26, 78-92.

Sampson, S. D., Loewen, M. A., Farke, A. A., Roberts, E. M., Forster, C. A., Smith, J. A., & Titus, A. L. (2010). New horned dinosaurs from Utah provide evidence for intracontinental dinosaur endemism. PLoS ONE, 5(9), e12292.