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Contemplating “what if” questions about the extinction of nonavian dinosaurs has long been a pastime of scientists and the general public alike. If the dinosaurs hadn’t died out, how would they have evolved, and could they have developed sentience? It is just those questions that paleontologist Dale Russell and model maker/taxidermist Ron Séguin attempted to address in proposing a hypothetical hominid-like dinosaur they called a “dinosauroid.” This musing appeared as an addendum to a 1982 paper that detailed the process of skeletal reconstruction and modeling behind the construction of a mount of the dinosaur Stenonychosaurus for the National Museum of Natural Sciences in Ottawa, Canada. From the beginning, Russell and Séguin frequently referred to hominid evolution, comparing Stenonychosaurus to the recently discovered hominid specimen “Lucy” in terms of completeness and importance. (This comparison overstates the case: there are more relatively complete theropod fossils than hominid fossils, and Stenonychosaurus did not add as much to paleontological knowledge.)

Stenonychosaurus is a theropod dinosaur belonging to a group called troodontids. Theropods are the group of predominantly carnivorous dinosaurs that include such famous members as Tyrannosaurus, Allosaurus, and Velociraptor; the omnivorous and herbivorous ornithomimids, oviraptorids, and therizinosaurids; and birds. Troodontids are small, gracile, possibly omnivorous theropods closely related to dromaeosaurs (such as Deinonychus and Velociraptor) and avialans (the lineage that includes Archaeopteryx and birds). Like dromaeosaurs, troodontids have enlarged claws on their second pedal digit and a semilunate carpal in the wrist. Troodontids also possessed elongate feathers on their hands and tails; in many ways, they were quite avian-like. Stenonychosaurus was about 1.2 m long and probably weighed about 70 kg. It was bipedal and had a tridactyl hand, with a somewhat opposable first digit. It also had large eyes and a narrow muzzle, allowing for stereoscopic vision, and relatively enlarged cerebral hemispheres.

Four features of Stenonychosaurus impressed Russell and Séguin: its stereoscopic vision, its high encephalization quotient, what they took to be its opposable digits, and the fact that its bipedality freed the hands from use in locomotion. These are all features that might plausibly be thought to be required for intelligence in humans. On this basis, they suggested that if dinosaurs had continued to evolve, troodontids such as Stenonychosaurus were the most likely dinosaurs to attain a human level of intelligence. Hence, their hypothetical dinosauroid.

Regarding stereoscopic vision, nonavian maniraptor dinosaurs, such as troodontids and dromaeosaurs, do indeed have narrow muzzles, and troodontids couple that with very large orbits, suggesting large eyes. Furthermore, because of the narrow muzzle, the eyes can be directed forward, allowing for some degree of stereoscopic vision. However, this vision is not equivalent to that of primates because the long snout would have still obstructed their field of vision, giving them a stereoscopic vision more akin to that of dogs than primates.

Of all the evidence cited, the encephalization quotient (EQ) is most interesting. The EQ is the ratio of an animal’s brain weight to the brain weight of a “typical” animal of the same body weight. Russell and Séguin calculated Stenonychosaurus’s EQ at about 0.34, comparable to that of galliform birds, armadillos, insectivores, and—most significant from their perspective—the Mesozoic ancestors of humans. But given the uncertainty about the weights of the body and the brain in fossil organisms, the calculations contain a large potential for error, and having a high EQ does not necessarily mean that an organism will evolve “intelligence” in a hominid sense.

Russell’s proposal that Stenonychosaurus’s first and third digits were opposable was incorrect, however. Functional studies of the hands of theropods show that they are incapable of the kind of digit opposability that we see in primates. Furthermore, the structure of the hand would make evolving true opposability unlikely, because the first metacarpal (which rotates in primate opposability) is tightly appressed to the second metacarpal and the fused distal carpals in Stenonychosaurus thus cannot move independently. The only joint free to evolve opposition would be between the first metacarpal and its first phalanx.

The most problematic part of Russell and Séguin’s argument is bipedality. Assuming that the vertically bipedal form seen in hominids is the ideal form for sentient organisms, they conclude that selection for sentience would directly lead to a hominid-like set of body proportions in dinosaurs.While bipedality and sentience may be related, it seems unlikely that such hominid-like proportions would evolve in theropod dinosaurs, which start out with a body plan fundamentally different from that of nonhominid primates. In both theropods and hominids, the form that bipedality takes is the result of the skeletal structure (long bodies with proportionally short limbs in theropods, short bodies with proportionally long limbs in hominids). Hominid bipedality is the result of the body plan and proportions of arboreal apes that hang from branches and thus takes on the shape dictated by the skeleton. In theropod bipedality, the body is cantilevered between the legs, with the pelvis acting as a fulcrum and counterbalance provided by a long tail and neck. As detailed by Russell and Séguin, in order to assume a hominid-like bipedality, a theropod would have to shorten its tail and neck, reorient its skull articulation and its pelvis, reorganize its legs, and flatten its feet. This would take a large degree of skeletal and muscular reorganization, and since the intermediaries would be ungainly at best, the idea that theropod dinosaurs would give up the highly efficient counterbalanced bipedalism for the more inefficient vertical bipedalism of humans makes little selective sense.

In the end, Russell and Séguin’s hypothesis was less science than science fiction, a congenial genre for such musings. The science fiction and fantasy literature has a number of examples of bipedal hominid-like dinosaurs, probably the most famous of which appear in Harry Harrison’s Eden series, which features humans living alongside (and in conflict with) hominid-like dinosaurs. Eric Garcia’s Anonymous Rex series features incognito dinosaurs wearing human costumes. Spacetraveling hominid-like dinosaurs descended from hadrosaurs appeared in the Star Trek: Voyager episode “Distant Origin” (Season 5). It could be argued the “Sleestacks” of Sid and Marty Croft’s 1970s television show Land of the Lost were hominid-like dinosaurs; they look a lot like Russell and Séguin’s dinosauroids.

Bibliography:

Dixon, D. (1988). The new dinosaurs: An alternative evolution. New York: HarperCollins.
Russell, D. A. (1969). A new specimen of Stenonychosaurus from the Oldman Formation (Cretaceous) of Alberta. Canadian Journal of Earth Science, 6, 595–612.
Russell, D. A. (1981). Speculations on the evolution of intelligence in multicellular organisms. In J. Billingham (Ed.), Life in the universe: Proceedings of the Conference on Life in the Universe, NASA Ames Research Center, June 19–20, 1979 (pp. 259–275). Cambridge: MIT Press.
Russell, D. A., & Séguin, R. (1982). Reconstructions of the small Cretaceous theropod Stenenychosaurus inequalis and a hypothetical dinosauroid. Syllogues, 37, 1–43.

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