The Complete Dinosaur

30. Dinosaur Eggs

613 Dinosaur Eggs Darla K. Zelenitsky, John R. Horner, and François Therrien The earliest discoveries of dinosaur eggshells were made thousands of years ago during Paleolithic or Neolithic times (Carpenter et al. 1994). Evidence of these prehistoric discoveries comes from archeological sites in the Gobi Desert of Mongolia, sites that have yielded dinosaur eggshell fragments that had been cut and carved into pieces of jewelry (Andrews 1932; Pauc and Buffetaut 1998). Although the Stone Age discoverers may have recognized the fossil eggshells as unusual or unique objects, their identity and connection to dinosaurs awaited pioneering paleontologists of the nineteenth and twentieth centuries. Dinosaur eggshells were first collected for scientific study by a priest and self-taught paleontologist named Jean-Jacques Pouech in southern France during the 1850s (Pouech 1859). He identified these specimens as fossil eggshells of extinct giant birds, probably because dinosaurs were still poorly known at the time. Subsequent comparison of the microstructure of Pouech’s eggshells to that of modern eggshells, however, led to the conclusion that they had been laid by a large reptile (Gervais 1877). At the time, the egg-layer was hypothesized to have been Hypselosaurus, which was then thought to be a large crocodile (Matheron 1869), but is now known to be a titanosaur. Shortly thereafter, Pouech’s enigmatic eggshells were forgotten, not to be recalled for several decades until the discovery of dinosaur eggs in a remote part of the world (Straelen and Denaeyer 1923). The first fossil eggs ascribed to dinosaurs were found in 1923 during the Central Asiatic Expedition of the American Museum of Natural History to the Mongolian Gobi Desert (Andrews 1932). The size of the eggs, as well as their occurrence in Cretaceous rocks containing abundant dinosaur remains, led to the conclusion that they had a dinosaurian origin. The popularization of these spectacular finds by the American Museum of Natural History paved the way for future discoveries of dinosaur eggs and eggshells worldwide. Dinosaur egg remains are currently known from more than 200 localities distributed over five continents (Carpenter and Alf 1994), only a few of which are known from pre-Cretaceous times. The oldest known dinosaur eggs are those of prosauropods from the Early Jurassic of South Africa (Kitching 1979; Reisz et al. 2005). The only other known Jurassic dinosaur eggs are those of theropods from the Late Jurassic of the United States and Portugal (Hirsch 1994; Mateus et al. 1997; Mateus et al. 2001). Eggs of sauropods, ornithopods, and stegosaurs–dinosaurs well represented by skeletal remains from the Jurassic –have yet to be identified from this period. Early Discoveries Distribution 30 Zelenitsky, Horner, and Therrien 614 Overall, the diversity of known Jurassic eggs is unexpectedly low considering the relatively high diversity of dinosaurs from this time (Hirsch 1994). The vast majority of dinosaur eggs have been recovered from terrestrial sediments of Cretaceous age, with the majority of specimens found in Upper Cretaceous deposits. Rocks of this age have yielded impressive examples of dinosaur nesting sites and of eggs associated with skeletal remains. Exquisitely preserved adult theropod skeletons sitting atop egg clutches, as well as embryonic skeletons curled within eggs, have been found in ancient eolian (windblown) deposits of China and Mongolia (Norell et al. 1994, 1995; Dong and Currie 1996). Spectacular hadrosaur and sauropod nesting sites from North America and Argentina, respectively, have been found in ancient floodplain deposits, sometimes spanning areas of over 1 km2 (Horner 1982; Horner and Currie 1994; Chiappe et al. 1998). Although eggs of several different dinosaur taxa are known from Upper Cretaceous rocks, eggs of some well-known dinosaurs from this time, such as tyrannosaurids, ornithomimosaurs, ankylosaurs, and ceratopsids, have yet to be identified. Until recently, the parentage of the various types of dinosaurs eggs remained uncertain. Prior to discoveries of eggs found in close association with skeletal remains, the parentage of some of these eggs had been hypothesized , often incorrectly, based on the predominant dinosaur species found in the same rock layer or locality. In the past 20 years, however, a number of discoveries of eggs closely associated with embryos or adults has allowed for the accurate determination of the parentage of several types of dinosaur eggs. Nonavian Theropods Although nonavian theropods comprised a relatively small percentage of overall dinosaur populations, this group is well represented in terms of specimens found closely associated with eggs. To date, the parentage of nonavian theropod eggs has been determined on the basis of eggs associated with skeletal remains from several major...