The reproductive behavior of dinosaurs will never be fully understood because their fossil remains can only hint at the kinds of behavior that might have been natural for these extinct animals. There are no clear-cut anatomical clues for determining a dinosaur's gender that can be applied to every kind of dinosaur skeleton. Yet paleontologists remain fascinated with the family life of dinosaurs because of provocative fossil evidence about nesting behavior, parental care, and the growth of dinosaurs from hatch-lings to adults. Understanding how these animals interacted to ensure the continuance of their species is an irresistible topic for those who try to paint the complete picture of dinosaurs as living creatures.
The fossil record may not shout out obvious answers about the reproductive behavior of dinosaurs, but it does provide clues. Another instructive source of information about dinosaur reproduction comes from observing living animals and inferring from their behaviors possible behaviors in dinosaurs. In the case of the-ropods, a suite of intriguing clues and assumptions is beginning to emerge that is helping scientists piece together an informed view of their courtship, reproductive, and parenting behaviors.
Before two animals mate, they may partake in a ritual of courtship. During this time, a female is approached by one or more males from among which she chooses a mate. Whether or not dinosaurs behaved in this way cannot be known for sure, but it is reasonable to assume that some form of mating ritual took place. The motivation behind courtship and mate selection is most likely connected to an innate drive to propagate the species. Finding the healthiest or fittest mate available is a natural way to increase the likelihood that offspring will be equally healthy and will live long enough to mate and have offspring of their own. Such are the underlying instincts of all living organisms. We can safely assume that dinosaurs were equally motivated when it came to mate selection.
In the game of courtship, the first requirement for a male is to get the attention of a female. With modern birds, males generally are brightly colored, and females are not. This enables males and females to identify one another but may also give the advantage to a male with superior coloring. Just what a female bird makes of this male display is a matter of speculation. A more brightly colored bird may represent a healthier potential mate and a better chance for the continuance of the species. Or the bright colors may just happen to catch the female's eye. Whatever the case, a male bird's coloration is an example of an anatomically-based sexual-display feature.
When it comes to dinosaurs, the color of skin, hairlike filaments, feathers, and other possible nonskeletal elements of sexual display are not discernable from the fossil record. Fortunately, a variety of theropod skeletons exhibit bony anatomical features that may have served the purpose of attracting members of the opposite sex.
The variation between the males and females of a species is called sexual dimorphism. Such gender-specific features will be a recurring theme in the exploration of dinosaurs in this book and in other books of The Prehistoric Earth series. Differences between the sexes of a given dinosaur species are risky to assume with certainty unless a paleontologist is presented with a large number of specimens that most likely represent a random sampling of the population and must surely include males and females. In later dinosaurs, such as the "duck-bill" dinosaurs (hadrosaurs) and horned dinosaurs (ceratopsians)—where dozens of specimens of some species can be compared—the size, shape, and presence of head crests and horns appear to signify male and female individuals. Using knowledge of living animals, paleontologists can assume that sexually dimorphic traits such as these probably were most prominent in dinosaurs that had attained reproductive maturity. This means that hatchlings and juveniles would display only the immature hints of such characteristics.
Anatomical features of theropods that may have been used for sexual display are mainly found on the head. The ceratosaur Dilo-phosaurus ( "double-crested lizard")—perhaps best known for its wholly unsubstantiated portrayal in the movie Jurassic Park as having the ability to spit poison—was a 20-foot (6 m) long, Early Jurassic theropod from Arizona with a pair of prominent crests running the length of its skull. The crests were tall and thin and grew out of the bones at the top and front of the head. Because they were too weak to have been used for defensive purposes, American paleontologist Kenneth Carpenter surmises that the double crests would have been effective sexual-display devices, especially if the dinosaur turned sideways to give the appearance that its head was larger than it truly was. Other, less spectacular examples include the nose horn and short brow horns of Ceratosaurus, the short brow horns of Allo-saurus, and the longer, jutting brow horns of Carnotaurus. Reinforcing the idea that nasal horns and brow horns were associated with mating and courtship is the possibility that they also could have been used in sparring contests between rival males—i ntraspecies battles intended to bruise but not kill the opponent.
Except for Allosaurus, the above cases for sexual dimorphism in theropods are not supported by an abundance of fossil specimens. Most of these dinosaurs are known from only a few good specimens—not enough to sample a species population accurately for males and females. A stronger case can be made for sexual dimorphism in two small theropods, Coelophysis (Late Triassic
Epoch, New Mexico) and Syntarsus (Early Jurassic, Arizona, South Africa, and Zimbabwe). Each has been found in bone beds that consist of many individual adults.
American paleontologist Edwin Colbert, who studied Coelophy-sis, and South African paleontologist Michael Raath, who did work on Syntarsus, each noticed that the adult specimens of these dinosaurs came in two basic forms. One form was more strongly built and had such features as a larger skull, a longer neck, more muscle scars around the elbow and hip, and stronger limbs. Scientists do not always agree, however, on how to tell the males from the females. In the case of Coelophysis, Colbert took a more traditional approach and believed that the larger, stronger individuals were males. In the case of Syntarsus, Raath came to the opposite conclusion and proposed that the females were the stronger, bigger-boned individuals, a variation that can be observed in modern predatory birds.
Working independently, paleontologist Kenneth Carpenter came to the same conclusion as Raath in regard to Tyrannosaurus. Carpenter believed that the larger, more robust form of T. rex was the female. The key to Carpenter's conclusion involved egg laying. Carpenter noticed that in larger T. rex specimens, the backward prong of the hip bone known as the ischium was angled downward in relation to the tail more than it was in T. rex specimens with the less robust body type. Carpenter reasoned that the increased space between the ischium and the tail provided a more ample path for the passage of eggs during egg laying. This same anatomical clue has been observed in some modern crocodylians.
Like other soft body parts, the reproductive organs of dinosaurs did not fossilize. To explain how dinosaurs mated is clearly a speculative venture, but it can be informed by the anatomy and behavior of the living archosaurian relatives of dinosaurs—birds and crocodylians.
It appears that all dinosaurs were oviparous, or egg laying, like extant crocodylians and birds. The study of dinosaurs is replete with excellent specimens of fossilized dinosaur eggs and nests. On rare occasions, the tiny remains of dinosaur embryos have been found inside fossilized eggs. In one remarkable find in 2005, an international team of paleontologists led by Tamaki Sato of the Canadian Museum of Nature found two shelled eggs in the pelvic region— right where the oviducts (egg passages) should be—of a small the-ropod dinosaur from China, thus providing direct evidence of egg laying in this type of dinosaur.
From an evolutionary standpoint, the anatomy and physiology of dinosaurs falls along a continuum that is closer to the crocodyl-ians at one end and closer to birds at the other. This assumption can help us to understand the kinds of sex organs, or gonads, that dinosaurs possessed. More basal dinosaurs may have had gonads more like those of crocodylians. Those dinosaurs that were more closely related to birds—the later theropods—probably had sex organs most similar to those of birds. In dinosaurs, as in crocodylians and birds, eggs were probably fertilized internally.
Male crocodiles transfer sperm to the females by way of a penis. When not in use, the penis is tucked inside the body, behind an opening at the base of the tail called the cloaca. The female gonads, where eggs are produced, are located inside the female's body, behind a cloaca of her own. Similar anatomy was likely present in many dinosaurs.
Today the study of dinosaur eggs and nests is a booming discipline within dinosaur science, but this was not always the case. Although paleontologists in the 1800s had recognized that dinosaurs were related to other reptiles and birds, nobody had discovered any remains of eggs or nests that were obviously dinosaurian. Some reports of fossil eggshells from France in the 1850s went largely unnoticed, and the fossil shells were attributed to extinct birds and crocodylians, not dinosaurs. Interest in dinosaur eggs changed dramatically between 1922 and 1925, when celebrity explorer Roy Chapman Andrews (1884-1960) led three fossil-hunting expeditions to central Asia for the American Museum of Natural History.
Although Andrews' original mission was to search for the fossil origins of humans, the area in Mongolia that he happened across dated from a much earlier time, the Late Cretaceous Epoch, when dinosaurs ruled. A few fragments of fossil eggshell were discovered at the end of the 1923 mission and assumed to be from birds. But
Walter Granger (1872-1941), the lead paleontologist of the expedition, was mindful that not much was known of fossil birds from that early a time. He suspected that the eggs might be dinosaurian in origin. Granger and Andrews found the proof they needed during the next expedition, in 1924, when they excavated fossil eggs in many locations. Some were fragmentary, some were complete, and some were still lying intact in what appeared to be nest clutches. It was the first definitive discovery of dinosaur eggs in close association with dinosaur skeletons. Discoveries of fossil eggs, nests, hatchlings, and rare, intact embryos of unhatched dinosaurs have grown dramatically since and have occurred on most continents.
Dinosaurs laid hard-shelled eggs that were more like those of birds than the soft-shelled eggs of crocodylians and other reptiles. The hard shell of the egg developed as the egg passed through the oviduct of the mother, where glands secreted a calcareous protective covering—the calcium-rich outer shell.
Preserved nests of dinosaurs show that they laid large numbers of eggs on the ground. Finding between 15 and 25 eggs in an intact nest is common; some fossil dinosaur nests included more than 40 eggs. The construction of the nest as well as the pattern for laying the eggs varied widely among different groups of dinosaurs. Nest construction was influenced by the materials available in the habitat to house the eggs. For example, the nest of the duck-billed dinosaur Maiasaura (Late Cretaceous, Montana) was formed from mud as a bowl-like mound. The nests then may have been covered by scraps of vegetation and mud to protect and encourage incubation of the eggs.
Dinosaurs laid their eggs in one of two basic patterns: groups of eggs called clutches and eggs laid in rows. The pattern of egg laying was probably most influenced by the size of the dinosaur. The largest dinosaurs, such as the sauropods, needed more room to lay their eggs and appear to have laid them in semicircular rows. Small to medium-sized dinosaurs preferred to lay their eggs in tight clutches.
Clutch patterns of Asiatic dinosaur egg nests, including round, oval, and elongate-shaped eggs
The eggs that have been positively associated with the remains of theropods are elongate in shape and range from 5 to 21 inches (12 to 53.6 centimeters) long, depending on the taxon. These kinds of eggs have been found in Montana, Mongolia, and China. The largest dinosaur eggs currently known were laid by theropods.
Evidence concerning the nests of predatory dinosaurs is limited to a few celebrated cases. The fossil eggs discovered by Andrews and Granger in 1924 belonged to the small Mongolian theropod Ovirap-tor, although at the time they were thought to pertain to the horned dinosaur Protoceratops. This toothless predator laid its elongate eggs in a ring within a shallow nesting pit that it dug out of the sandy soil where it lived. Oviraptor nests contained up to 24 eggs; the eggs were arranged in a circle and rested slightly upright. The ring pattern was similar to that seen in brooding birds. A variation on
this nesting pattern was used by another small theropod, Troodon, which arranged its eggs in concentric circles—a ring within a ring. A nest of Troodon —found in Montana and initially misidentified as belonging to the small plant-eating dinosaur Orodromeus—consisted of 24 eggs arranged in this manner. Other small, meat-eating dinosaurs probably used a ring strategy similar to that of Oviraptor or Troodon. The most eggs ever found in a single nest were of this type and arranged in this manner. The nest included 40 eggs that had been layered in tiers. Forty eggs seems excessive, even for a large dinosaur; American paleontologist Gregory Paul suggested that this could imply that some dinosaurs practiced "communal nesting" or nest sharing—a behavior observed in some extant large birds.
The largest eggs found near any dinosaur remains are thought to be from theropods. The eggs are elongate and an extraordinary 21 inches (53.6 centimeters) long. In at least one case, a nest of giant eggs such as these consisted of about 26 specimens grouped in pairs in a large ring about 7 feet (2 meters) across. To which kind of the-ropod these eggs pertain is currently unknown.
Much of what is known today about dinosaur nests, babies, and parental care began with the pioneering work of American paleontologist John "Jack" Horner (b. 1946) of The Museum of the Rockies in Montana. Together with his students and coworkers from Montana State University, Horner has spent much of the past 25 years studying the stunning remains of eggs, nests, and skeletons found at numerous fossil sites in central Montana. Many of these were associated with the plant-eating ornithopod Maiasaura, a large "duck-bill" dinosaur that grew to more than 30 feet (9 m) long. Orni-thopods were a major subgroup of ornithischian dinosaurs. Among the findings of Horner's team were hatchlings and baby Maiasaura, still in the nest. These hatchlings and babies measured up to 3 feet (1 m) long. Their teeth were well worn and their limb joints were weak. These factors suggested to Horner that the hatchlings were eating but were not capable of leaving the nest to fend for themselves. It became evident to Horner that Maiasaura babies remained in the nest for as long as several months, until they could get out on their own. The implications were that the defenseless young dinosaurs needed to be cared for, and that an adult dinosaur had watched over them, protected them from predators, and brought food to the nest for them to eat, like many birds care for their young today.
Evidence for the nesting and parenting habits of theropods is not as extensive as that for ornithopods. Fossils of theropod hatchlings are rarer than those of herbivorous dinosaurs. Intact nests are even scarcer. Because known theropod hatchling specimens appear to have well-developed teeth and limb bones, it might be assumed that they were ready soon after hatching to leave the nest in search for food. At present, however, it is impossible to say whether their parents protected them and taught them how to hunt, or whether the parents abandoned them, as happens with many living amniotes.
The most revealing evidence regarding the parental behavior of theropods is found in several specimens of oviraptorids and troodon-tids in which adult specimens were found lying on top of clutches of eggs, arms (probably feathered) outstretched. This appears to show brooding behavior in which the adult dinosaur was protecting and warming the eggs with its body to facilitate incubation.
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