Reproduction

Among higher animals, reproduction consists of a number of separate phases of activity. Courtship by the male is often preceded by the establishment of a territory and conflict with competing males. This agonistic or aggressive behaviour may consist primarily or even entirely of display (Sect. 9.2.3), and actual combat may be highly ritualised. Agonistic behaviour among dinosaurs probably resulted in the establishment of hierarchies, as it does in extant vertebrates. Interactions between conspecifics may thus result in the avoidance of potentially injurious battles, although fights to the death do sometimes take place (Coombs 1990).

Agonistic Behaviour

Peter Dodson (1996) emphasised that the ceratopsians would have had excellent eyesight and possessed colour vision, as do many modern reptiles including cro-codilians. This can be inferred from the fact that visual signals can be found all round fossilised ceratopsian skulls. For example, not only were there conspicuous frills, but these were sculptured with various knobs and processes that would have enhanced an already striking appearance (Fig. 95). These were probably also brightly coloured. Casts of the braincases of many other dinosaur taxa suggest that they, too, would have enjoyed highly developed visual acuity.

Horns Spikes Frills

■ Fig. 95a-f. Typical Upper Cretaceous ceratopsian heads with horns and frills (not to scale). a Styracosaurus (total body length 5.2 m), b Chasmosaurus (length ca. 5.2 m), c Pentaceratops (total body length ca. 6 m), d Torosaurus (length ca.7.6 m), e Triceratops (total body length ca. 9 m), f Pachyrhinosaurus (length ca. 5.5 m)

■ Fig. 95a-f. Typical Upper Cretaceous ceratopsian heads with horns and frills (not to scale). a Styracosaurus (total body length 5.2 m), b Chasmosaurus (length ca. 5.2 m), c Pentaceratops (total body length ca. 6 m), d Torosaurus (length ca.7.6 m), e Triceratops (total body length ca. 9 m), f Pachyrhinosaurus (length ca. 5.5 m)

There seems to be little doubt that Tyrannosaurus and the smaller Nano-tyrannus (length ca. 5 m) from Montana had some degree of overlap in their fields of vision. (This does not appear to have been the case among other carnosaurs so far discovered.) Although overlapping visual fields have been thought to indicate stereoscopic vision, this is not necessarily the case: some birds have strongly overlapping fields of vision yet it has been shown experimentally that they do not possess stereoscopic vision (see discussion in Molnar and Farlow 1990). The same argument could well apply to some other carnivorous dinosaurs such as the ornithomimosaurians and the bird-like Troodontidae.

Some dinosaur horns were indeed dangerous weapons. Their bony cores were often lengthened by keratin sheaths - hard,but weighing less than bone. These might have functioned not only as weapons and in aposematic threat (Sect. 9.2.3) but also in intraspecific display (Fig. 95). Intraspecific display was probably the primary function of the floppy nose bone of Einiosaurus from Montana, although this might also have been used in aposematic bluff against potential enemies. So, too, would have been the shield and frill of Chasmosaurus (Fig. 95b) from Canada and New Mexico. Unlike the solid bone neck shield and frill of Triceratops (Figs. 72,94,95e), the shield of Chasmosaurus was widely fenestrated. It cannot have been strong enough to have served as protection from predatory attack but, since it was undoubtedly covered with thick skin, it would have been an excellent device for agonistic display, especially if brightly coloured. No doubt its design was the result of a compromise between size, weight and strength. Although usually less dramatic than the horns and frills of ceratopsians (Fig. 95), different types of ornamentation have, not surprisingly, been found in many other taxa of dinosaurs. Its functions usually lie in signalling its possessor's species and in agonistic display (Sect. 10.6.2).

Agonistic behaviour among dinosaurs has been discussed in detail by Coombs (1990) and Sampson (1997) among others. According to the well-documented account by Coombs, the dinosaurian features reasonably to be interpreted as visual or vocal threat display - and which could sometimes also have been actual weapons - included:

1. Bright colours: the presence of these can be inferred from the excellent colour vision of crocodiles and birds, as well as from the presumably well-developed bird-like eyes of dinosaurs.

2. Flamboyant tails: dinosaur tails could have been used for display, especially when unusually long as in Diplodocidae (Fig. 66) or decorated with armour plates, as in stegosaurs (Fig. 71) and ankylosaurs (Fig. 90). Tails are used for display in many modern vertebrate taxa including reptiles, birds and mammals. The tails of most dinosaurs were carried clear of the ground, as already mentioned, and tail 'drags' are seldom to be seen in association with fossil trackways. Some genera clearly used their tails as weapons (Sect. 9.2.2).

■ Fig. 96a-c. Upper Cretaceous crested hadrosaur heads (not to scale). a Parasaurolophus (total body length ca. 10 m), b Corythrosaurus (total body length ca. 9 m), c Tsinatasaurus (total body length ca. 10 m)

Reptiles Iguanodon

■ Fig. 97. Iguanodon (Iguanodontidae; Lower Cretaceous; length ca. 9 m) shown here in an upright posture. (After Augusta 1956)

3. Upright posture, long necks: sauropods might well have indulged in giraffe-like sparring with their long necks: they probably reared up on their hind legs, propped by the tail, when they were using their large claws in combat. Some genera (e.g. Brachiosaurus; Fig. 68; and Camarasaurus) might have had an inflatable proboscis for display purposes - like those of elephant seals (Mirounga spp.) - but this hypothesis has been disputed.

4. The dorsal plates, flattened bodies and elongated hind legs of the stego-saurs would have been very suitable for lateral display, while the tail spikes were undoubtedly used in combat (Sect. 9.2.2).

5. The crests and complex narial passages of the duckbilled hadrosaurs (Fig. 96) were almost certainly used for visual and sometimes vocal display. The thumb spikes of Iguanodon (Fig. 97) might possibly have been used both for display and combat, as might the canine tusks of the vegetarian Heterodontosaurus (Fig. 98).

6. Ankylosaurids could have used their tail clubs for combat, and their broad, triangular skulls for thrusting head to head in competition for dominance (Fig. 99).

■ Fig. 98. Heterodontosaurus (Heterodontidae; Lower Jurassic; length ca. 90 cm). (After Norman 1985)
Heterodontosaurus Head
■ Fig. 99. Possible styles of grappling combat in Centrosaurus (left) and Triceratops (right). (Based on a drawing by Bill Parsons in Sampson 1997)

7. The frills and horns of the ceratopsians (Fig. 95) were probably used for both display and combat, as we have seen. Fossil skulls with punctured wounds on the frills and damaged horns are proof that the animals indulged in tests of dominance.

8. It is generally accepted that the thickened crania of Stegoceras and other Pachycephalosauria were an adaptation for sexual combat (Fig. 100; Galton 1970b).

9. The sabre-like claws of Deinonychus (Figs. 86, 87) and other Dromaeo-sauridae might well have been used in sexual combat like the spurs of fighting cocks, their reinforced tails forming a tripod with the rear legs when the animals were fighting.

10. Various unrelated genera such as the iguanodont Ouranosaurus (Fig. 101a) from Niger, the duckbilled Hypacrosaurus (Fig. 101b) from North America, as well as the carnosaurs Acrocanthosaurus (Fig. 101c) from North America and Spinosaurus (Fig. 101d) from North Africa possessed sail-like extensions supported by elongated neural spines, which were almost certainly used for display.

■ Fig. 100. Stegoceras (Pachycephalosauria; Upper Cretaceous; length ca. 2 m) in sexual combat. (Cloudsley-Thompson 1994 after Halstead and Halstead 1981)

■ Fig. 100. Stegoceras (Pachycephalosauria; Upper Cretaceous; length ca. 2 m) in sexual combat. (Cloudsley-Thompson 1994 after Halstead and Halstead 1981)

Images Late Cretaceous Reptiles

■ Fig. 101a-d. Dinosaur display structures supported by neural spines. a Ouranosaurus (Igua-nodontidae; Lower Cretaceous; length ca. 7 m), b Hypacrosaurus (Spinosauridae; Upper Cretaceous; length ca. 9 m), c Acrocanthosaurus (Spinosauridae; Lower Cretaceous; length ca. 12 m), d Spinosaurus (Spinosauridae; Upper Cretaceous; length ca. 12 m). (After Palmer 1999)

■ Fig. 102. Theropods with cranial crests. Left Dilophosaurus (Megalosauridae; Lower Jurassic; length ca. 6 m). Right Ceratosaurus (Ceratosauridae; Upper Cretaceous; length ca. 6 m)

11. Finally, many carnivorous dinosaurs would have displayed their formidable teeth with open jaws. Several genera of carnivorous dinosaurs had cranial crests that would almost certainly have been brightly coloured and used for display. Examples include Dilophosaurus, Ceratosaurus (Fig. 102), and possibly Ornitholestes, all from North America.

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