In lieu of an abstract, here is a brief excerpt of the content:

603 Reproductive Biology of Dinosaurs Terry D. Jones and Nicholas R. Geist Discoveries of the nests and eggs of a variety of dinosaur taxa, combined with recently described fossils of baby dinosaurs, have captured the imagination of the public and stimulated professional interest in the reproductive biology of dinosaurs (a.k.a. nonavian dinosaurs). These fossils offer tantalizing glimpses into the biology of dinosaurs and bring them to life in a way that individual bones can’t. As is often the case with dinosaur biology, this area is contentious. Filling the gaps that remain in our understanding of dinosaur reproduction and, at least partially, reconciling the differences in opinion that emerge requires supplementing the intrinsic limitations of the fossil record by applying knowledge of the biology of the living relatives of dinosaurs. Some researchers, pointing to the close relationship between birds and dinosaurs, have interpreted the behavior of many dinosaurs to have been much like that of living birds. Conversely, others discern more conservative crocodilian or “reptilian” patterns of behavior from the same fossils. Phylogenetic modeling based on the biology of extant archosaurs (crocodilians and birds) notwithstanding, there is no single key to unlock the details of dinosaur reproduction, and in many cases this methodology falls short of a definitive explanation. In a number of instances the fossils seem to indicate that the reproductive patterns of dinosaurs may have been neither particularly crocodilian nor birdlike, but uniquely dinosaurian. This realization should not be too surprising considering the long evolutionary history and great diversity of the dinosaurs as well as the unique selective pressures of the Mesozoic environment. Here we utilize both neontological and paleontological data to reconstruct the reproductive anatomy, physiology , development, and behavior of dinosaurs. Anatomy and Development of Reproductive Tracts Development of the reproductive tract in extant archosaurs (crocodilians and birds) follows the generalized pattern of gnathostomes (jawed vertebrates ), in which two complete sets of ducts associated with the primordial gonads form within the dorsal body wall at an early, undifferentiated developmental stage: the archinephric (Wolffian, or mesonephric) and the paramesonephric (Müllerian) ducts (Austin 1989; Feguson 1985; Kardong 2006; Lofts and Murton 1973) (Fig. 29.1). The sperm ducts (ductus deferens) of male vertebrates develop from the archinephric ducts; the paramesonephric ducts degenerate. At sexual maturity, gametes (sperm) are conveyed via the ductus deferens from the testes into the cloaca, a common chamber Reproductive Anatomy and Physiology of the Dinosaurs 29 Jones and Geist 604 that also receives feces and urine (Austin 1989; Feguson 1985; Lofts and Murton 1973. Conversely, in females the archinephric ducts are reduced and the paired oviducts develop from the paramesonephric ducts (Austin 1989; Feguson 1985; Kardong 2006; Lofts and Murton 1973). Although development of the reproductive tract is fairly conservative in vertebrates, there are differences in the adult female reproductive tracts of crocodilians and birds. Female crocodilians, like most gnathostomes, possess paired ovaries and oviducts with eggs passing into the cloaca (Ferguson 1985). However, in the birds the right side of the female reproductive tract tends to be vestigial, leaving a single functional ovary and oviduct (Clinton and Haines 1999; Lofts and Murton 1973). This asymmetrical reproductive system represents a derived condition, results in reduced body mass, and is probably an adaptation for flight and/or production of few, relatively large eggs (Witschi 1935). The development of the male dinosaurian reproductive system, like that of extant archosaurs, likely exhibited the basic gnathostome pattern of paired gonads and sperm ducts. Regardless of the differences in the reproductive systems of adult female crocodilians and birds, fossil evidence makes determining the anatomy of female dinosaurs fairly straightforward. The apparent paired pattern of egg deposition in the theropod dinosaurs Troodon and Oviraptor implies the presence of paired female reproductive tracts (Clark et al. 1999; Dong and Currie 1998; Norell et al. 1995; Varricchio et al. 1997). Additionally, a recently described fossil of an oviraptorosaur with two eggs retained within the pelvic region provides direct evidence that confirms the primitive pattern of paired oviducts (Sato et al. 2005). One specimen of Sinosauropteryx was also described as having had two eggs fossilized within the oviduct (Chen et al. 1998). However, given the ventral location of these structures, their identification as eggs is questionable. Regardless, given the embryological similarity of the reproductive tract and the fossil evidence, it is reasonable to infer the presence of paired ovaries and oviducts for all dinosaurs. Reproductive Functional Morphology Numerous aspects of the physiology and many behaviors associated with reproductive function (e.g...

Share