Dryptosaurus’ Surprising Hands

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

Riley Black | | READ MORE

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.