Tyrannosaurus rex’s small arms have been the punchline of many a joke. But do we know why the “tyrant lizard” and other dinosaurs have developed some unusual appendages? Throughout their evolutionary history, dinosaurs occupied a wide range of body sizes, ecological roles, and anatomical peculiarities with functions paleontologists cannot quite put a finger on. Many dinosaurs were bipedal, allowing their forelimbs to evolve more freely into different forms that go hand-in-hand with exploring new functions. A classic example is the dramatic transformation from the typical theropod arm into the wings that we see in modern birds. However, other evolutionary trends are harder to grasp, such as the purpose of Iguanodon’s thumb spike or why T. rex had such puny arms. Here is a handy list that showcases the variety of ways dinosaurs tinkered with the form and function of their forelimbs:
(Depéret, 1896; Krause et al., 2007; Burch and Carrano, 2012)
Majungasaurus crenatissimus belongs to a bizarre group of theropod dinosaurs known as abelisaurids, a group of theropods that terrorized the southern continents. Majungasaurus is one of the best-known abelisaurids and is from the latest Cretaceous Maevarano Formation of Madagascar. In many ways, abelisaurids drew many parallels with tyrannosaurs of the northern hemisphere. For example, they had reduced forelimbs—but in a drastically different way. The forelimb of Majungasaurus shows an exaggerated shortening of the radius, ulna, and even the hand itself, relative to the already small humerus. Unlike T. rex, which is always flashing a peace sign with its two fingers, Majungasaurus and other abelisaurids buck this trend by retaining four (really stubby!) fingers. This unusual forelimb reduction pattern makes functional interpretations just as enigmatic as this group.
(Osmólska and Roniewicz, 1970; Lee et al., 2014)
Deinocheirus mirificus earns its name: Deinocheirus is Greek for “terrible hands.” This ostrich-like dinosaur hails from the Late Cretaceous Nemegt Formation in the Gobi Desert of Mongolia, and for almost half a century, this ornithomimosaur theropod remained a mystery. Back in 1965, paleontologists uncovered this mysterious dinosaur based on a set of intimidatingly large forelimbs that measured well over six feet in length. Then, in 2014, another team of paleontologists described newly discovered and more complete specimens of Deinocheirus—and finally were able to put these strange arms to a body. Deinocheirus turned out to be even weirder than imagined: the body was heavily built, had an elongated and deep snout, tall neural spines of the back vertebrae that gave the creature a humpback, a pygostyle (in birds, these are fused tail vertebrae that typically support tail feathers), and stocky legs.
(Perle et al., 1993)
Another strange dinosaur from the Late Cretaceous Nemegt Formation of Mongolia, Mononykus olecranus takes a note from T. rex and then goes a step further. As the name implies, Mononykus had one large finger on its short and stubby hand. The purpose of this single digit remains uncertain, as it could have been used for digging, scratching, or clasping. However, we do know that the forelimb was relatively powerful given the small size of this dinosaur. Mononykus and relatives belong to a group of theropods known as Alvarezsauridae, a curious group of theropod dinosaurs that historically blurs the line between avian and non-avian dinosaurs.
Opisthocoelicaudia skarzynskii is a titanosaurian sauropod dinosaur from the Late Cretaceous Nemegt Formation in Mongolia. One of the more complete specimens of this peculiar group of sauropods, the Opisthocoelicaudia skeleton did not have phalanges (fingers) or carpal bones (the wrist bones in your hand). Instead, Opisthocoelicaudia and other titanosaurs relied on their metacarpals (the bones between your carpal bones and fingers) to help support their enormous weight. The metacarpals of titanosaurs became vertical and tightly bound, forming a strong pillar of support. We see this pattern of adapting the hand into a stout and column-like morphology in other large-bodied animals (for example, elephants), but titanosaurs and their large stocky body sizes demanded extreme modifications.
(Charig and Milner, 1986)
Baryonyx walkeri belongs to the Spinosauridae, a group of theropod dinosaurs with modified skulls that resembled modern-day alligators and crocodiles rather than other meat-eating dinosaurs. The initial discovery of Baryonyx occurred in 1983 when an unusually large claw (ungual) was recovered from the Lower Cretaceous Wealden deposits near Surrey, England. This Baryonyx claw measures 30 cm in length and is part of one of the more complete spinosaurid skeletons recovered to date. Combined with a strongly developed humerus, the large claw may have aided in prey capture or even supported and stabilized the creature’s weight. There’s evidence that spinosaurids may have been able to occasionally walk on all four limbs while living a semi-aquatic, fish-eating lifestyle.
Eric Gorscak is a postdoctoral researcher at The Field Museum focusing on African dinosaurs and paleobiogeography of the southern continents.
Borsuk-Bialynicka, M. 1977. A new camarasaurid sauropod Opisthocoelicaudia skarzynskii, gen. n., sp. n. from the Upper Cretaceous of Mongolia. Palaeontologia Polonica 37:5.
Burch, S. H., and M. T. Carrano. 2012. An articulated pectoral girdle and forelimb of the abelisaurid theropod Majungasaurus crenatissimus from the Late Cretaceous of Madagascar. Journal of Vertebrate Paleontology 32:1–16.
Charig, A. J., and A. C. Milner. 1986. Baryonyx, a remarkable new theropod dinosaur. Nature 324:359–361.
Depéret, C. 1896. Note sur les Dinosauriens Sauropodes et Théropodes du Crétacé supérieur de Madagascar. Bulletin de la Société Géologique de France 21:176–194.
Krause, D. W., S. D. Sampson, M. T. Carrano, and P. M. O'Connor. 2007. Overview of the history of discovery, taxonomy, phylogeny, and biogeography of Majungasaurus crenatissimus (Theropoda: Abelisauridae) from the Late Cretaceous of Madagascar. Journal of Vertebrate Paleontology 27:1–20.
Lee, Y.-N., R. Barsbold, P. J. Currie, Y. Kobayashi, H.-J. Lee, P. Godefroit, F. Escuillie, and T. Chinzorig. 2014. Resolving the long-standing enigmas of a giant ornithomimosaur Deinocheirus mirificus. Nature 515:257–260.
Osmólska, H. and Roniewicz, E. 1970. Deinocheiridae, a new family of theropod dinosaurs. Acta Palaeontologica Polonica 21:5–19.
Perle, A., M. A. Norell, L. M. Chiappe, and J. Clark. 1993. Flightless bird from the Cretaceous of Mongolia. Nature 362: 623-626.