Dinosaurian Hominid

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, andStenonychosaurus 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 [Page 749]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.

...