Courtship and Mating

In animals living today, courtship and mating involve a whole range of visual, acoustic, tactile and chemical signals. Fossils, by their very nature, can only provide unequivocal evidence for the first of these. It is generally assumed, however, that many of the morphological structures and ornaments, such as horns, frills, crests, spikes, thickened skulls, bosses, large teeth, big eyes and so on (Sect. 9.3.1), were not only used in combat but also in eliciting behavioural responses (Horner 1997).

Adaptations are seldom the response to a single factor, as already emphasised. In most cases, several selective factors are involved. For example, frills and crests may have been exploited in both combat and visual display - when their presumed bright colours would have been important (Sect. 1.4). These adornments could also have played a role in cooling the brain. If the internal surfaces were lined with moist membrane, evaporation from this would have reduced the temperature of the surrounding tissues. Some of the Late Cretaceous duckbilled hadrosaurs had small, solid crests, but in others the crests were larger and more conspicuous. Specimens with larger crests were probably males, those of apparently the same species with smaller crests quite likely females. In such cases (Sect. 2.3.1), almost certainly both visual and vocal display would have been used. In Parasaurolophus from North America (Fig. 96a), for example, the hollow crest was probably used as a vocal resonating chamber. It would have produced a sound quite different from that of, say, Corythrosaurus (Fig. 96b) also from North America, and other genera and species of duckbilled hadrosaurs. Females that had shorter crests must have emitted sounds pitched higher than those of males of the same species. The suggestion has also been made that the hollow crest of Corythrosaurus might have enhanced the animal's sense of smell, thereby helping it to find food, detect approaching enemies and keep up with the herd. Tsintasaurus (Fig. 96c) from China had a horn on its head between the eyes. This gave it a bizarre unicorn-like appearance. If a flap of skin were anchored between the tip of the horn and the beak, it could have been inflated for use as a signalling device in courtship, as a threat to rivals, or both. Alternatively, if the horn was mounted incorrectly in the original specimen and actually pointed backwards like that of Parasaurolophus (Fig. 96a), it would have served the same purpose as that of other hadrosaurs and acted as a vocal resonator. This really seems to be the most likely explanation!

Ornamentation in fossil vertebrates has been defined by Vickaryous and Ryan (1997) as "any unique or modified anatomical feature (whether skeleton or pre served/inferred soft-tissue) that suggests a use in social behaviour, including taxon recognition (inter- or intraspecific), gender identification, and/or the recognition of the state of sexual maturity" Many types of ornamentation have been described in the previous section; others are listed by Vickaryous and Ryan (1997). Details of mating behaviour can only be inferred by comparison with the known behaviour of extant reptiles and birds.

Nesting Behaviour

Probably most, if not all dinosaurs, were oviparous - but this supposition has been confirmed only in a comparatively small number of taxa. Those examples that are known and have been described include Troodon (Theropoda), Oviraptor (Theropoda),Maiasaura (Ornithopoda), and Hypacrosaurus (Orni-thopoda) among others. All of these date from the Upper Cretaceous and all were discovered in North America - except for Oviraptor, which was found in southern Mongolia (Horner 1997). Prosauropod, sauropod, ornithopod and ceratopsian eggs of less well known provenance, however, have been found in almost every continent (Lambert 1992).

Dinosaur eggs had hard shells, but they could not have exceeded about 7 kg in weight. They were therefore very small in comparison with the sizes of the dinosaurs that produced them. For instance, the eggs of the sauropod Hypselo-saurus are among the largest dinosaur eggs known, yet they measured only 30.5x25.5 cm (Fig. 103). They were, therefore, comparable in size with the eggs of Dinornis maximus - the largest of the moas - and of the elephant bird Aepy-ornis titan of Madagascar, which measured 33x24 cm. These eggs have been described as the largest single cells to have evolved in the animal kingdom!

Although the shell of an egg may be hardened to protect the developing embryo within, it must remain sufficiently porous to allow for the passage of air from the outer world. It cannot, therefore, be inordinately thick or it will prevent respiration of the embryo from taking place. Moreover, it cannot be excessively hard or it will prevent the young animal from hatching. Since the thick-

■ Fig. 103. Comparative sizes of the eggs and adult of Hypselosaurus (Titanosauridae; Upper Cretaceous; length ca. 12 m) The eggs would normally have been buried in the nest (see text)

ness of eggshells is limited in this way, there is a geometric limit to the sizes of eggs. This limit is approached when eggs reach such volume that the pressure of the internal fluid exceeds the strength of the shell that contains it (Colbert 1962b).

It seems probable, therefore, that the eggs of Aepyornis titan and of Hypselo-saurus priscus were at about the maximum size to which any terrestrial eggs, even those of Apatosaurus or Brachiosaurus, could reach. Consequently, even the largest of the dinosaurs would have started life as comparatively tiny hatchlings. The same occurs in the largest of modern reptiles, such as the giant saltwater crocodile (Crocodylus porosus) and the great leathery turtle (Der-mochelys coriacea). The rate of growth in dinosaurs from egg to adult must have been impressive. A newly hatched Hypselosaurus weighed about 0.9 kg and grew to an adult that weighed perhaps 10 tonnes. Consequently, there must have been an increase in weight in the order of not less than 10,000 times. In Brachiosaurus spp. there may have been an increase during life of about 50,000 times! In view of the relatively small sizes of their offspring, it is to be expected that many of the dinosaurs would have tended their eggs and young, as most birds do. Colonial nesting was probably not uncommon, and dinosaurs evidently had favoured nesting sites. Some of the larger dinosaurs, especially sauropods, may well have been ovoviviparous.

Different species of dinosaur laid rounded or oval eggs on bare ground, in sandy hollows, or in nests with raised mud walls. Known dinosaur nests and eggs exhibit surprising diversity in their forms. Those of sauropods found in Europe and the Gobi Desert of Mongolia were large and subspherical in shape with a highly developed system of pores in the shell. The orifices of the pores occupied about half of the egg surfaces. These eggs probably developed in moist substrates, not far from water. The eggs of hadrosaurs, hypsilophodonts, proto-ceratopsians and theropods, on the other hand, were laid in nests composed of soil and vegetation, like those of alligators and some crocodiles. The eggs of hadrosaurs were rounded, whereas those of protoceratopsians, hypsilophodonts and theropods were elongated (see relevant entries in Currie and Padian 1997).

Each female Protoceratops laid a dozen or more eggs, sometimes as many as 18,in a sandy hollow where they formed a spiral with the narrow ends pointing inward (see below). Apparently several females sometimes laid their eggs communally in the same nest. The adults of colonial species might well have combined to protect the eggs and young from marauding theropods, forming an outwardly facing ring to shield them as musk oxen deter wolves today, as already mentioned (Sect. 9.2.2; Lambert 1992).

Although there is no evidence to suggest that dinosaurs were other than invariably oviparous, and large numbers of fossil eggs have been unearthed,very few can be shown to have been laid by any particular species. A notable exception, however, is the discovery in Mongolia of a fossil theropod Oviraptor lying on a nest which contained about 22 eggs that had probably been overwhelmed by a sand storm. On the other hand, the animal in question could have been robbing the nest of another dinosaur at the time of its death. Oviraptor lacked

■ Fig. 104. Oviraptor (Oviraptoridae; Upper Cretaceous; length ca. 1.8 m) on its nest shortly before death. (Based on Norrell et al. 1995)

teeth for biting prey, but two short teeth, pointing down from the roof of its mouth, may have been used to open the shells of other dinosaurs' eggs in the same way that a sharp vertebral process from the neck enables egg-eating snakes (Dasypeltis spp.) to cut through the shells of the birds' eggs they swallow. It appears more likely, however, that the Mongolian Oviraptor was incubating its own eggs because the front limbs were directed posteriorly, with both arms wrapped around the nest (Fig. 104). Another possibility is that the animal perished whilst in the act of ovipositing, not incubating. This seems to be obviated, however, both by the absence of eggs within its body cavity and by the neat arrangement of the eggs (Norrell et al. 1995). The later discovery of an egg containing a developing embryo proved that the Oviraptor had not been robbing the nest of another species of dinosaur (Chiappe and Dingus 2001). The fact that the eggs were comparatively large, were deposited in pairs and partly buried, indicates a degree of environmental heating. The position of some adult dinosaurs with the small forelimbs draped over their eggs, suggests that they possessed a degree of insulating pelage (Paul 1997).

Among the oldest and smallest dinosaur eggs yet discovered are some that measured only 25 mm on the longest axis, while the skeletons of the babies found with them did not exceed 20 cm. These animals from the Upper Triassic of Argentina were named Mussaurus (mouse lizard).Since no other specimens have been found, the size of the adults is not known, but it is generally believed that they could have grown to a length of 3 m with body proportions typical of prosauropods. The babies had large heads and short necks as are typical of very young animals. They clearly could not have survived without parental care. There is evidence, therefore, that the young both of hadrosaurs and of prosauropods were fed and looked after by their parents. So, too, were the young of Protoceratops (Fig. 105), already referred to, whose eggs were about 25 cm long with thin, wrinkled shells. Some of the eggs, found in the Upper Cretaceous of Mongolia, contained fossilised fragments of embryonic bones.

The care of eggs and young of dinosaurs probably ranged from nil at one extreme to the feeding and guarding of the young at the other. Eggs laid in simple holes were probably abandoned after being buried, whereas open nests were more likely to have been guarded. The incubation temperatures of nests in mounds of fermenting vegetation may well have been regulated by the parents,

■ Fig. 105. Clutch of Protoceratops hatching eggs (Protoceratopsidae; Upper Cretaceous). (Cloudsley-Thompson 1994)

by adding or removing nest material as required. This behaviour is found in me-gapode birds today. In contrast, the extreme size of the largest dinosaurs would probably have rendered any form of brooding impossible. Very large numbers of eggs would then have been laid in each clutch, however (Sect. 7.3.4). Post-nestling dinosaurs may well have lived together in groups, possibly on their own or perhaps guarded by their parents. When they grew larger they would have joined adult herds without the risk of being trampled to death. Hadrosaurs built mound nests in colonies that were probably inhabited by several successive generations. This is indicated by the large number of broken egg shells found in such places.

It has been argued that the ossified pelves of baby hadrosaurs indicate that they enjoyed well-developed locomotor ability and consequently had precocial habits. On the other hand, the shafts of the leg bone were so poorly ossified that the babies might have been immobile and therefore altricial (helpless at birth). The subject has been reviewed by Paul (1997) who suggested that hadrosaur nestlings might have been semi-altricial with just sufficient locomotor ability to flee from their nests if a predator penetrated the defences of their parents. When provided with large quantities of food by their relatively enormous parents, the tiny juveniles would have grown rapidly and soon become large enough to move in the company of adults without the risk of being inadvertently trodden on. Hadrosaurs probably foraged over considerable distances and would thus have been able to obtain vegetation when grazing had reduced to desert the region around the nesting ground.

The reproductive strategies of dinosaurs, like those of large birds and mammals today, embraced rapid growth and high rates of population expansion. Mammals differ, however, in that their rates of reproduction are low; but mammalian young benefit from lactation (Paul 1997).

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