William J. Sanders, assistant research scientist and supervising preparator at the Vertebrate Fossil Preparation Laboratory of the University of Michigan's Museum of Paleontology, explains.

Variability is a hallmark of sexually reproducing organisms, and populations of such organisms always exhibit some structural differences among their members, particularly between males and females. For the paleontologist trying to understand the ecology and biology of an ancient time and place, it is vital to know whether variations in skeletal remains reflect the presence of males and females of the same species or individuals from closely related species.

Living animals have primary sexual characteristics, such as genitalia, that differ between males and females. These are soft-tissue organs that generally do not leave marks on the skeleton and are therefore not preserved in the fossil record. There may be features associated with reproduction that do leave bony traces, however. For instance, in the female human pelvis, the angle of the bony arch beneath the pubis is much wider than it is in males and the sacrum is usually is flatter as well. Both of these features relate to the need for a larger pelvic outlet for birthing infants and are thus reliable sexual markers for forensic scientists and hominid paleontologists.

Similarly, in the dinosaur Tyrannosaurus rex, some individuals have pubic bones angled farther away from the back vertebrae, a feature that experts believe created additional abdominal space to help eggs pass through in females. In addition, the second chevron bone (the V-shaped bone under each tail vertebra) in some T. rex individuals is disproportionately larger than the first chevron bone, whereas in other individuals they are more equal in size. In modern crocodiles, females have enlarged second chevron bones, which is thought to be an adaptation to assist egg-hatching. Thus, by analogy with crocodiles, T. rex individuals with larger second chevron bones may be categorized as females.

By far the most profound differences between males and females are usually secondary sexual characteristics. In the majority of animals in which they occur, these traits are associated with competition between males for resources that either directly or indirectly lead to greater reproductive access to females. This so-called sexual dimorphism is usually most pronounced in those species in which several females mate with one male, or, less commonly, those species in which multiple males mate with a single female. (Monogamous species exhibit less sexual dimorphism.) Examples of sexually dimorphic traits include elaboration of extracranial structures, as seen in the manes of male lions, the cheek pads of male orangutans and the highly embellished antlers and horns of male deer and antelopes; and pelage and feather ornamentation, such as the extravagant tail of the male peacock. (There is increasing evidence that at least some lineages of dinosaurs had feathers. Given their close relationship to birds, they may have also developed the same form of sexually-dimorphic variation.) Although these flashy features may expose the bearer to increased risk of predation, the evolutionary advantage they bring in attracting females and hence passing along offspring evidently outweighs the burden

The best criteria for assessing sex in fossils, however, are body size and associated skeletal robustness. In sexually dimorphic populations, body size is bimodally-distributed, meaning that individuals tend to cluster in size around either a small variant or a large variant, with females usually more diminutive than males. Skeletons of large females and small males are difficult to sex in these cases. Dimorphism is distinguished from interspecific differences when morphological differences can be explained as size-related.

In the case of the early hominid Australopithecus afarensis, most famously represented by Lucy, some researchers have argued that the variable fossils from Ethiopia's Middle Awash region that are attributed to this taxon in fact represent two species. But by analogy to living apes and humans, the variation does not exceed that observed in living species, and the remains probably belong to a single, highly dimorphic species in which males competed vigorously for reproductive access to females, with little Lucy being female, and the larger individuals being males.

In Tyrannosaurus rex, although there are fewer than a dozen skeletons known, there seem to be two morphs, and the larger and more robust one consistently exhibits the features described above as female traits. Thus it seems reasonable to accept this as a dimorphic species, wherein females competed for males. Conversely, at a site in Bernissart, Belgium, the remains of 39 individuals of the dinosaur Iguanodon fall into two markedly different size classes, but also exhibit anatomical differences in the skull, hand, foot and pelvis that are greater than those usually seen within a single living species. For that reason, they are placed into two non-dimorphic species, I. bernissartensis and I. mantelli, leaving paleontologists to ponder how two closely related dinosaurs managed to ecologically occupy the same area.

More to explore:
The Descent of Man, and Selection in Relation to Sex.Darwin, C. 1871. John Murray: London.
The Illustrated Encyclopedia of Dinosaurs. Norman, D. 1985. Crescent Books: New York.
Predatory Dinosaurs of the World.Paul, G.S. 1988. Simon & Schuster: New York.
The Complete T. Rex. Horner, J.R. and Lessem, D. 1993. Simon & Schuster: New York.
Sexual Selection. Andersson, M. 1994. Princeton University Press: Princeton, New Jersey.
Evolution.Ridley, M. 1996. Second Edition. Blackwell Science: Cambridge, Massachusetts

Answer originally published Dec. 6, 2004