Some 75 million years ago, heavy rains in Mongolia’s Gobi Desert apparently triggered a sand dune collapse that froze two dinosaurs together forever. Today, they are two of the most spectacular fossils on display in the exhibition Dinosaurs Among Us—an exploration of the unbroken line between one group of ancient dinosaurs and modern birds.
Their species, Khaan mckennai, belongs to a group of theropod dinosaurs known as oviraptorids, relatively small dinosaurs with bird-like traits including toothless beaks, wishbones, and skulls filled with air pockets. Cousins of Tyrannosaurus rex and even closer cousins to today’s birds, some oviraptorids have even been found sitting on eggs, in a brooding posture similar to that of modern birds. The Khaan mckennai fossils on display in Dinosaurs Among Us have been housed at the Museum since their discovery, and are on loan from the Paleontological Center of the Mongolian Academy of Sciences.
These two fossils are so exquisitely preserved, it’s likely the animals died suddenly where they were standing or sitting, only inches apart. Their proximity hints at interaction of some sort. Based on other fossils in the same group that were also found in pairs, paleontologists are beginning to suspect that oviraptorids were social animals.
Recent research offers more clues. A study published in the journal Nature last year by Mark Norell, chair and Macaulay Curator of the Division of Paleontology, and three Canadian colleagues suggests that Khaan mckennai exhibited sexual dimorphism, differences in the appearance of males and females of the same species. Though the fossils are very similar, their tails vary in ways that indicate that one may have been a female, capable of laying eggs, and the other, a male, with a tail that could have supported muscles for spreading out feathers to attract a mate. Even before the latest evidence that one animal might be a male and the other a female, paleontologists casually referred to this pair as Sid and Nancy, after the star-crossed punk-rock couple of the 1970s.
A version of this story originally appeared in the Summer 2016 issue of the Member magazine Rotunda.