Physiology is how animals convert and transfer matter and energy in their daily lives. Metabolism is all the processes that they use to accomplish these tasks. Animals take in, use, and send out air, water, food and other nutrients that are used in ways determined by the cells of their bodies, which in turn were determined by genes, but also are affected by the surrounding environment. Because dinosaurs were living animals, undoubtedly they also converted matter and energy in accordance with their bodies and surrounding environments. The scientific questions that usually deal with these basic presumptions start with the word "How?":

■ How did dinosaurs reproduce, and how do their eggs provide information about the physiology of both mothers and offspring?

■ How did dinosaurs form bones and teeth, the most likely parts of a dinosaur to be preserved in the fossil record?

■ How quickly did dinosaurs grow, after they hatched from eggs?

■ How did dinosaur bones record clues about their metabolism, such as whether they generated their own body heat or depended on the outside environment for warmth?

■ How do other data besides bones and eggs, such as paleoenvironmental setting and trace fossils, provide insights into dinosaur physiology?

All of these pertinent questions, asked about dinosaur physiology for more than 150 years, have still not all been satisfactorily answered.

Dinosaur Reproduction: Sex and Eggs

Why Learn about Dinosaur Reproduction and Eggs?

Paleontologists have known about dinosaur eggs since early in the second half of the nineteenth century, although verifiable nests were not described and interpreted until the 1920s (Chapter 3). Despite this long history, the study of dinosaur eggs was neglected in favor of skeletal evidence and dinosaur tracks. Fortunately, the relatively lower status of these dinosaur fossils has improved in the past 20 years. This renewed study, originating from more than 200 egg and nest sites documented worldwide, includes the identification of dinosaur embryos and presumed parents associated with egg and juvenile remains. The most well-known dinosaur egg sites, some with embryos, are in Montana, France, China, Mongolia, and, most recently, Argentina.

Dinosaur eggs provide important evidence on how dinosaurs reproduced and, by extension, how egg laying and the life of an embryo in an egg reflect dinosaur physiology. Moreover, dinosaur nests lend insights into how dinosaurs ensured that their broods hatched and sometimes whether they cared for their young, which also relates to their physiology and behavioral relatedness to modern archosaurs. Dinosaur eggs are used increasingly as additional evidence for working out evolutionary relationships of different dinosaur clades. Indeed, eggs are now being classified through cladis-tics in an attempt to reconcile egg types with probable egg layers.

The oviparous trait associated with dinosaurs contrasts with giving live birth (viviparous), which is typical of most mammals and occurs in a few reptiles. In continental ecosystems, the formation of enclosed eggs, which probably happened slightly more than 300 million years ago, was an extraordinary evolutionary development in the history of vertebrates (Chapter 6). Unlike fish and amphibians, reptiles can migrate within continental interiors without depending on nearby water bodies. Thus, the architecture of fossil eggs provides clues as to how dinosaurs adapted to a variety of terrestrial ecosystems. Of biogeochemical significance is the fact that some dinosaur eggshells contain amino acids, which may tell us about the dinosaurian production of those biomo-lecules in the absence of other evidence from their skeletal remains. Eggshells also contain carbon and oxygen isotopes, as well as some trace elements. These chemical clues provide information about the possible dietary preferences of the mother dinosaurs. These elements also may reflect environmental conditions in both the pre- and post-burial history of an egg.

Like all birds, most reptiles, and a few mammals, dinosaurs reproduced through eggs formed within the body of the female and laid on land to hatch, a trait called oviparous.

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