A necessary antecedent of dinosaur eggs was dinosaur sexual activity. The certainty of dinosaur sex as a prelude to their laying eggs is supported by the numerous observations of how mating in all egg-laying vertebrates is a necessary precursor to egg development. Fertilization of an egg without the help of a male, known as parthenogenesis, is common in nearly every major invertebrate clade but is known in only a few vertebrates (some amphibians and lizards). As a result, this process probably does not apply to dinosaurs.
Dinosaur sexuality has been the subject of much debate on the basis of little scientific evidence. In fact, paleontologists are still uncertain about which dinosaurs in the fossil record represent male and female specimens (Chapters 5 and 6). A few researchers have recently promoted the case for sexual dimorphism in what are anatomically very similar dinosaur species from the same stratigraphic intervals. For example, some paleontologists have proposed that more robust forms of some dinosaurs (such as Tyrannosaurus) are females, based on a similar disparity of size seen in the sexes of modern large reptiles. Larger size of female reptiles, which is the opposite expectation for sexual dimorphism in most mammals, corresponds to a capacity to hold many eggs in reptilian body cavities.
Before mating occurs in modern vertebrates, males and females both use methods of sexual attraction in which they undergo sexual selection, or the choosing of their mates on the basis of preferred traits. When done by enough individuals within a population, this process ultimately affects the evolutionary history of a species by causing genetic change of that population over time (Chapter 6). Some birds that have colorful or prominent plumage in one gender, such as peacocks, provide examples of sexual selection, in which they cause a visual stimulus for a potential mate. In this respect, the elaborate and prominent head shields and horns of ceratopsians (Chapter 13) and skull crests of hadrosaurs (Chapter 11) have been proposed as possible display structures. In modern animals, displays are sometimes accompanied by other sensory signals, such as mating calls. Male crocodilians, for example, will bellow for attention and are sometimes answered by vocalizations from a nearby female. In at least some hadrosaurs, vocalization structures, also associated with cranial crests, have been postulated (Chapter 11).
In some instances visual and auditory stimuli are not the only cues to mating. For example, some animals use pheromones, which are complex biomolecules emitted by an organism into a water body or air for the purpose of causing a response in another individual of the same species. Pheromones can elicit numerous responses in organisms, such as triggering silent alarms, providing a trail for others to follow (seen readily in ants), or signaling aggregation, but they are often recognized for their sexually attractive qualities. Whether dinosaurs used pheromones or not is unknown. Many modern reptiles use olfactory sensations for mate attraction, but pheromones have not yet been detected in birds. Consequently, paleontologists have little basis for inferring this physiological function in dinosaurs through their hypothesized closest living relatives.
In many cases, once a female vertebrate shows receptivity to a male, mating will occur quickly relative to time spent in attraction and courtship. In fish and amphibians, the male's sperm is simply deposited in the water near a female rather than through more proximal association. Amniotes presumably developed sex organs that worked more effectively for getting gametes together through direct bodily contact. After all, internal fertilization was necessary before an egg could be developed internally. In some reptiles (crocodilians and turtles) the delivery of sperm into a female's oviduct, which also functions as the "birth canal" for egg laying, is sometimes facilitated by the insertion into the oviduct of a male's penis. Snakes and lizards have similar but smaller structures called hemipenes. Most modern bird species lack a penis, although some flightless birds and ducks, geese, and swans do possess such organs. Birds without penises mate through close contact of their cloacae, which are openings that double in function as outlets for gametes (sex cells, such as sperm and eggs) and excretion of bodily wastes in males and females, as well as egg-laying in females.
Because the preservation potential for non-mineralized dinosaur tissues is so low (Chapter 7), no direct evidence of dinosaur reproductive organs is known. Nevertheless, the arrangement of bones in the pelvic region of some dinosaurs may relate to muscles associated with penile organs in males. Furthermore, at least one example of a theropod was found with probable eggs in its body cavity (Chapter 9). This helped to corroborate the presence of an oviduct and (of course) its gender. Moreover, the statistically-defined pairing of eggs in the nest of another theropod strongly suggests that it laid the eggs through paired oviducts (Chapter 9), a trait seen in some modern crocodilians.
Dinosaur mating was probably achieved through the male approaching the female's caudal region and positioning its posteroventral anatomy in close association with the female's posterior. Such positioning was relatively straightforward for bipedal dinosaurs, but probably necessitated a temporarily bipedal posture for quadrupedal dinosaurs. A similar posture is demonstrated by modern elephants, which are the heaviest land animals available for such a comparison (Fig. 8.1). The rather lengthy tails of some dinosaurs, such as sauropods, seemingly would have been impediments to mating. Unfortunately, no trackways or other trace fossils have yet provided evidence of dinosaur mating habits. Likewise, no dinosaur skeletons of the same species have been found in what might be construed as a compromising position, even after applying much imagination to them. Documentation of trace fossils indicating dinosaur mating in particular is an area yearning for further, in-depth research, considering the amount of sedimentary disturbance that must
FIGURE 8.1 African elephants (Loxodonta africanus) mating, providing a model for mating positions of some large quadrupedal dinosaurs. The male is located posterodorsally with respect to the female. W. M. Colbeck/OSF/Animals Animals.
have been caused by the mating of some of these animals, which together comprised tens of metric tons.
Fertilization in modern vertebrates is the result of the uniting of gametes from a male and a female. The gamete from each parent contains half the number (hap-loid) of chromosomes (genetic material) of a typical body (somatic) cell from each parent, thus both parents together create a diploid cell. The formation of gametes by each parent is accomplished through a splitting of a diploid cell into haploid cells, a process called meiosis. The division of body cells into more body cells (diploid to diploid) is mitosis. A fertilized egg (zygote) subsequently results in mitotic division of cells through cleavage.
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