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Small coelurosaurs like this Troodon appear to have maintained stable levels of disparity during the last 12 million years of the Cretaceous. Image courtesy AMNH/J. Brougham.

Why did the non-avian dinosaurs become extinct? There’s no shortage of ideas, but no one really knows. And even though paleontologists have narrowed them down to a short list of extinction triggers—including an asteroid strike, massive volcanic outpouring, sea level changes and climate alterations—how these events translated into the extinction of entire clades of organisms remains hotly debated.

One of the most contentious questions is whether dinosaurs thrived right until the end of the Cretaceous, or whether they were already declining before the lights went out. Based on species counts, mostly from the roughly 66-million-year-old rock of western North America’s Hell Creek Formation, it might seem that dinosaurs were not quite as diverse as they were in the same area 10 million years earlier. But detecting this decline depends on how species are identified and counted—a quirk affected by how we distinguish dinosaurs and other organisms known only from fossils. If we recognize that Triceratops and Torosaurus were separate dinosaur genera, for example, there were two big ceratopsids present in western North America at the end of the Cretaceous. But if we start from the position that the dinosaurs we call Torosaurus were really the skeletally mature form of Triceratops, then ceratopsid diversity is cut in half. And even the best circumstances, the fossil record is an imperfect catalog of prehistoric life that we are only sampling a few pieces from. Determining diversity by taking species counts is not as simple as it sounds.

In a Nature Communications paper published today, paleontologists Stephen Brusatte, Richard Butler, Albert Prieto-Márquez and Mark Norell take a different approach. Rather than track species and genera, the researchers followed trends in morphological disparity—how the forms of dinosaurs varied across seven major groups, both globally and regionally. Differences in form translate to differences in lifestyle and behavior, mostly avoiding tangled taxonomic arguments, and this technique gauges how many forms of dinosaurs were present at a given time. This is a proxy to detect which groups of dinosaurs might have been thriving and which were declining over time.

Disparity trends in four dinosaur groups during the final 12 million years of the Cretaceous (North American species only). Time (from 77-65 million years ago) is shown on the x axis. The y axis shows the disparity metric: sum of variances derived from anatomical character databases. The error bars indicate whether comparisons between time intervals are significant or not (overlap of error bars means non-significance, no overlap means significance). Overall, the large-bodied bulk-feeding ceratopsids and hadrosauroids underwent a marked long-term decline, but the carnivorous coelurosaurs and small herbivorous pachycephalosaurs were stable. (AMNH/S. Brusatte)

Brusatte and co-authors tracked disparity trends among ankylosaurs, sauropods, hadrosauroids, ceratopsids, pachycephalosaurids, tyrannosauroids and non-avian coelurosaurs during the last 12 million years of the Cretaceous (from the Late Campanian age to the Maastrichtian). There was no simple pattern that held true for all dinosaurs—some groups stayed the same while others declined. The heavily armored ankylosaurs, dome-headed pachycephalosaurs, formidable tyrannosaurs and small, feathery coelurosaurs didn’t seem to show any major changes in disparity over this span. And the massive, long-necked sauropods showed a very slight increase in disparity from the Campanian to the Maastrichtian. Both locally and globally, these dinosaur groups were not dwindling away.

The shovel-beaked hadrosaurs and horned ceratopsids showed different trends. Horned dinosaurs suffered a significant drop in disparity between the Campanian and the Maastrichtian, at least partially attributable to the disappearance of an entire ceratopsid subgroup. During the Campanian, both centrosaurines (like Centrosaurus) and chasmosaurines (like Chasmosaurus) roamed North America, but by the Maastrichtian, only the chasmosaurines were left. And while hadrosaur disparity dipped slightly from a global perspective, the pattern differed between continents. In Asia, hadrosaurs appear to show very slight increases in disparity, but North American hadrosaurs suffered a sharp decline across the 12-million-year study range. What was true for North American dinosaurs was not necessarily true for the rest of the world.

“Compared with previous studies that focused on species richness or faunal abundance,” Brusatte and colleagues write, “these disparity calculations paint a more nuanced picture of the final 12 million years of dinosaur history.” The idea that dinosaurs, as a whole, were either thriving or declining is a false dichotomy. The last twelve million years were clearly a time of flux—especially in North America, where some dinosaur groups stayed stable but the largest, most abundant herbivores were not as varied as their predecessors had been.

That sauropod dinosaurs increased in disparity at the end of the Cretaceous is especially noteworthy. When I was a kid, sauropods were often cast as Jurassic titans that were replaced by dinosaurs with superior plant-shearing abilities, such as certaopsids and hadrosaurs. Yet sauropods hung on, and as the horned and shovel-beaked dinosaurs declined, sauropods might have again been expanding. We will never know what would have happened had the Cretaceous extinction been canceled. Although, if the non-avian dinosaurs had been given a reprieve from extinction, we almost certainly wouldn’t have evolved to ponder what happened so long ago.

As this study points out, it is a mistake to think of dinosaurs as a monolithic group. The pressures behind dinosaur evolution, and the reasons for their extinction, varied from group to group and place to place. The more we learn about them, the more complex their history becomes. And there’s still much we don’t know. To date, most of what we think we understand about the extinction of the non-avian dinosaurs comes from western North America—relatively accessible sites that record the transition from the last days of the dinosaurs to a world dominated by mammals. These sites, no matter how well we study them, can only be a small part of what was a global extinction, and what we find in North America may not be representative of the rest of the planet. “It may be,” Brusatte and collaborators write, “that the North American record represents a local anomaly,” with “extreme fluctuations of the inland Western Interior Sea, mountain building, and proposed biogeographic provincialism” influencing dinosaur evolution in a unique way not seen on other continents.

If we want to understand the evolution and extinction of the last dinosaurs, we need to take a more refined, localized approach and not think of dinosaurs as a uniform group. For as much ink has been spilled about dinosaur evolution and extinction, we are still only beginning to piece together a picture of what the final days of the Cretaceous were like.

Reference:

Brusatte, S., Butler, R., Prieto-Márquez, A., & Norell, M. (2012). Dinosaur morphological diversity and the end-Cretaceous extinction Nature Communications, 3 DOI: 10.1038/ncomms1815

The known skeleton of Juratyrant (black outline) compared to the dinosaur Guanlong for size. The scale bar is one meter. From Benson, 2008.

Despite belonging to one of the most famous dinosaur groups of all time, few people have heard of Stokesosaurus clevelandi. This predator, named in 1974 by paleontologist James Madsen, Jr., was a tyrannosauroid dinosaur that roamed North America tens of millions of years before Tyrannosaurus rex.

The bones of Stokesosaurus were initially discovered in the fossil-rich Cleveland-Lloyd Dinosaur quarry in eastern Utah. Although dominated by the remains of at least 46 Allosaurus, rarer traces of other theropod dinosaurs have come out of the quarry. (The mid-size carnivore Marshosaurus and possibly a distinct species of Ceratosaurus have also been recognized from bones found here.) In the case of Stokesosaurus, Madsen had identified two portions of the hip and a piece of the upper jaw, the premaxilla, as belonging to this small theropod. The complete animal probably didn’t stretch longer than 12 feet from nose to tail. While Madsen was tentative about this conclusion, the diminutive predator seemed to represent the early days of the tyrant dinosaurs in North America. Since then, one of the hips has been lost and the jaw fragment is thought to have belonged to a different dinosaur, but the primary hip Madsen relied upon for his description still indicates the presence of the tyrants in Jurassic Utah around 150 million years ago.

By now you may be wondering why I opened a post titled “England’s Jurassic Tyrant” with a note about a tyrannosauroid from Utah. The reason is because, until recently, Stokesosaurus was thought to have been present in Jurassic Europe, too. In 2008, paleontologist Roger Benson described a partial skeleton from the Late Jurassic of England that he attributed to a new species of the dinosaur, Stokesosaurus langhami. There was far more of this animal than the North American species, whose anatomy remains largely a mystery. The new species, on the other hand, was represented by numerous vertebrae, the majority of the hips, and most of a hindlimb.

But the dinosaur Benson described probably wasn’t Stokesosaurus, after all. In a paper to be published at Acta Palaeontologica Polonica, Benson and colleague Stephen Brusatte suggest that the more complete material from England represents a distinct genus of dinosaur. The change was spurred by the discovery of additional archaic tyrannosaurs in recent years. These finds indicated that some of the features Benson had used to link Stokesosaurus from Utah and the British form together were widely distributed among the tyrannosauroids and therefore might not reveal clear relationships. The more complete material from England now seems more distinct from Stokesosaurus than previously understood. Brusatte and Benson have renamed the animal Juratyrant.

But we still know very little about Stokesosaurus, Juratyrant and their close relatives. For Stokesosaurus, most of the skeleton is unknown, and significant portions of Juratyrant—such as the skull and forelimbs—have yet to be found. These tyrants are hardly unique in this respect. Other closely related dinosaurs such as Aviatyrannis are known from frustratingly incomplete remains. We know that these dinosaurs were small predators that set the stage for the later rise of more imposing tyrants, but what they looked like and how they lived remains mysterious.

References:

Benson, R. (2008). New information on Stokesosaurus, a tyrannosauroid (Dinosauria: Theropoda) from North America and the United Kingdom Journal of Vertebrate Paleontology, 28 (3), 732-750 DOI: 10.1671/0272-4634(2008)28[732:NIOSAT]2.0.CO;2

Brusatte, S., & Benson, R. (2012). The systematics of Late Jurassic tyrannosauroids (Dinosauria: Theropoda) from Europe and North America Acta Palaeontologica Polonica DOI: 10.4202/app.2011.0141

Madsen, J. 1974. A new theropod dinosaur from the Upper Jurassic of Utah. Journal of Paleontology, 48 (1), 27-31

The formidable hand claw of Dryptosaurus. From Brusatte et al., 2011.

I have a soft spot for Dryptosaurus aquilunguis. Even though this dinosaur was not as big or imposing as some of its tyrannosauroid cousins, and even though most of what we know about it comes from a frustratingly incomplete skeleton discovered in 1866, this dinosaur embodies much of what I love about paleontology. Historically, Dryptosaurus played a pivotal role in a major image shift which transformed dinosaurs from bizarre, lumbering monsters into active, bird-like creatures. This Late Cretaceous predator remains an enigmatic dinosaur that paleontologists are continuing to learn from. (Plus, Dryptosaurus was found in my home state of New Jersey, which adds to its sentimental appeal.) All of that is why I was thrilled to see paleontologists Stephen Brusatte, Roger Benson and Mark Norell take another crack of describing the known remains of this dinosaur in a recent American Museum Novitates paper.

For years no one was ever quite sure what Dryptosaurus was. One of the first dinosaurs known from a partial skeleton, at first it seemed to simply be a North American cousin of Britain’s Megalosaurus. As more fossils were found, this connection broke down, and the nature of Dryptosaurus was a mystery. Kenneth Carpenter and colleagues published a new assessment of the dinosaur in 1997. Their conclusion—based on the bits of the dinosaur’s jaw, backbone, hips and limbs—was that Dryptosaurus was so different from other theropod dinosaurs as to be given its own family, though there were hints that it might belong to the wider group of theropods called coelurosaurs.

Since 1997, though, our understanding of theropod dinosaurs and their relationships has changed drastically. Many new species have been found and family trees have been reshuffled multiple times. Within these shifting relationships Dryptosaurus has come to be regarded as a tyrannosauroid—the group to which all tyrant dinosaurs belong. But Dryptosaurus was not an eastern copy of Tyrannosaurus or any of the other stocky contemporary tyrannosaurs of western North America. The similarities between Dryptosaurus and the recently discovered Appalachiosaurus, especially, appeared to indicate that the mysterious predator from Cretaceous New Jersey was a different kind of tyrant dinosaur. This revised understanding—as well as the fact that the original specimen is quickly deteriorating—inspired Brusatte and co-authors to carry out a detailed reanalysis.

The researchers confirmed that Dryptosaurus possessed a mish-mash of archaic traits seen in early tyrannosauroids as well as specialized characteristics. Instead of falling inside the group containing many of the more famous, burly tyrannosaurs of the Late Cretaceous—such as Gorgosaurus, Daspletosaurus and Tyrannosaurus itself—Dryptosaurus likely represents a long-surviving lineage which had branched off some earlier point in time and had a unique evolutionary history in the eastern part of North America. (During the Late Cretaceous the western and eastern parts of the country were separated by a warm inland sea, and, as a consequence, dinosaurs evolved differently on either side of the aquatic barrier.) Whether both Dryptosaurus and Appalachiosaurus form a previously unknown tyrannosauroid subgroup together is presently unknown, but even if they do not, they clearly indicate that tyrannosaurs in the East were adapted in different ways than those in the West. Overall, Dryptosaurus appears to take up an “intermediate” position between the smaller, more lightly built tyrannosauroids like Dilong and the more imposing tyrannosaurids, thus representing a yet poorly known part of the tyrant family tree.

Despite the incomplete nature of the only Dryptosaurus skeleton and the degradation of the remains over the years, Brusatte and co-authors were able to ascertain a few unique characteristics. For one thing, Dryptosaurus had relatively big hands. Although this dinosaur, like other tyrannosaurs of the Late Cretaceous, had relatively short arms, the known finger elements of Dryptosaurus are very long and, in terms of proportions, more closely resemble their counterparts in early, big-handed tyrannosauroids. Dryptosaurus may have had the novel combination of short arms with big hands, and two inferences can be drawn from this.

In evolutionary terms, Brusatte and colleagues point out, the big hands of Dryptosaurus may indicate that the forelimbs of tyrannosauroids did not become downsized in a linear, uniform manner. Instead of becoming reduced in size as an entire package, perhaps tyrannosauroid arms became shorter before the gradual downsizing of the hands, meaning that Dryptosaurus might represent an earlier, big-handed condition. Further discoveries of tyrannosauroid arms will be needed to test this idea, but the big hands of Dryptosaurus hint that this dinosaur may have been catching and killing prey in a different way than other tyrannosaurs. In the paper’s abstract the authors speculate that “Dryptosaurus may have used both its skull and arms as weapons for prey acquisition and processing.”

Will more Dryptosaurus bones ever be found? Some probably have, but are isolated bones or scraps that are difficult to identify as being from the tyrannosauroid. The state of vertebrate paleontology in New Jersey also complicates matters. In addition to the fact that many sites have been closed, paved over, or are otherwise inaccessible, most of the productive fossil sites from the Late Cretaceous in the Garden State represent marine environments. The dinosaurs found there are the remnants of carcasses washed downriver and out to the coast where, in most cases, they either fell apart or were ripped apart by scavengers. (One arm bone from a juvenile hadrosaur, kept at the New Jersey State Museum, is deeply scored by many shark bite marks. The bone looks as if someone went at it with a Ginsu knife.) If a Dryptosaurus skeleton ever turns up, chances are that it is going to be elsewhere on the East Coast in deposits more amenable to quick preservation. Perhaps, someday, a more complete skeleton will turn up, but at the moment, we will have to mourn the gradual decay of the one and only Dryptosaurus skeleton known to science.

References:

Brusatte, S.; Benson, R.; and Norell, M. (2011). The Anatomy of Dryptosaurus aquilunguis (Dinosauria: Theropoda) and a Review of Its Tyrannosauroid Affinities American Museum Novitates, 3717, 1-53 DOI: 10.1206/3717.2