Crylophosaurus ellioti, an Early Jurassic carnosaur from Antarctica. Auckland Museum, Auckland, New Zealand.

remains on egg clutches (e.g., Oviraptor and Troodon) does not necessarily mean that these dinosaurs were female. Although the majority of birds that exhibit brooding behavior are female, it has also been documented in some male birds. For example, male emus (Dromaius novaehollandiaea), a large flightless bird native to Australia, sit continuously on egg clutches for about 55 days and thus serve as the main provider of warmth and protection. Interestingly, climatic conditions also should be taken into account when assessing the thermoregulatory significance of dinosaurs sitting on egg clutches. For example, tropical seabirds must sit on egg clutches to prevent them from overheating in the hot daytime sun. This means that their endothermy is actually helping to keep the eggs at lower temperatures.

The prominent cranial and vertebral processes in theropods, as well as feathers, were potential sexual displays or species identifiers. Among ceratosaurs, the Early Jurassic Dilophosaurus has twinned parietal blades, the Early Jurassic Ceratosaurus has a single nasal horn, and the Early Cretaceous Carnotaurus has horns dorsal to the orbits. Tetanurans with head ornamentation include the Early Jurassic Crylophosaurus of Antarctica and the Late Jurassic Allosaurus (Fig. 9.12). Crylophosaurus had a pompadour-like projection of bone, which resulted in some paleontologists informally dubbing it "Elvisaurus". Similarly, Allosaurus possessed prominent lachrymals. Spinosaurs, such as Spinosaurus, Baryonyx, and Suchomimus, developed long, vertically-oriented processes that emanated from their dorsal vertebrae and formed sail-like features on their backs, probably to attract potential for mates. Alternatively, these "sails" are also interpreted as thermoregulatory structures that absorbed sunlight, vented heat, or both. Finally, Cretaceous theropods with feathery integuments also argue for a display function, which is likely because most of these accouter-ments were not used for flight. One tetanuran in particular, Caudipteryx, had a caudal feather fan with differently colored, alternating bands, a common feature in modern birds who use it to attract potential mates of the same species. Another

Early Cretaceous theropod, Microraptor, has feathers on all four limbs, which may have aided in gliding, but also would have added a considerable profile to an otherwise very small dinosaur.

Eggs, embryos, and nests for a few species of theropods are documented from Late Cretaceous strata. Otherwise, little is known about the reproduction of most theropods other than speculation derived from the reproductive behavior of modern crocodilians and birds. Some dinosaur eggs have long been allied with theropods without any other corroborating evidence other than their co-occurrence in same-age strata as theropod skeletal remains. In other cases, some eggs were mistakenly assigned to non-theropod dinosaurs when actually they were of theropod origin. More theropod eggs will almost certainly be identified in the future, and they can only be reliably attributed to theropods on the basis of:

1 examination of egg interiors for embryonic remains;

2 their direct association with an adult of the same species in a nest structure;

3 better classification of eggshell types into a cladistic framework that helps to establish relationships to extant theropods (birds); and

4 eggs located within the body cavity of an adult theropod.

The last of these criteria is documented for a specimen of Sinosauropteryx, which had two egg-like bodies in its pelvic region. Yet another clue about theropod eggs may come from biochemical analyses of theropod eggs that closely resemble those of modern avians (Chapter 8).

The best example of a misidentification of a theropod egglayer and its eggs was corrected with new evidence about Oviraptor, a common theropod in Late Cretaceous deposits of Mongolia. At least two specimens have been discovered directly above nests containing egg clutches (see Fig. 8.7A). This combination of body and trace fossil evidence is nearly indisputable for its support of brooding behavior in non-avian theropods. The clutches, which normally consist of about 15 eggs, had egg forms previously assigned to the ceratopsian Protoceratops, which is in the same deposits. This mistaken identity lasted for nearly 70 years until an Oviraptor embryo was found in one of the presumed Protoceratops eggs. Unfortunately, numerous illustrations during that time graphically depicted Oviraptor crushing eggs, yolk dripping villainously from its toothless jaws. This presumption was an example of how evidence was fitted to a hypothesis, as this theropod's odd jaw apparatus was conjectured as an evolutionary adaptation for breaking eggs. In actuality, Oviraptor was a "good mother" theropod, although the functional morphology of its jaws is still subject to debate.

Similarly, in Upper Cretaceous rocks of Montana, a skeleton of Troodon was discovered on top of eight eggs attributed previously to the ornithopod Orodromeus (Chapter 11), and a predator-prey relationship was hypothesized for these two also. However, re-examination of the previously identified Orodromeus eggs revealed that they contained Troodon embryos. Accordingly, the proximity of the adult skeleton and eggs actually supports a parental relationship. Other theropod embryos include probable Velociraptor remains that were, interestingly, associated with Oviraptor eggs (discussed briefly in Chapter 8) and an unidentified species of ther-izinosaur in the Cretaceous of Mongolia.

Nest structures are only known for two theropods, Oviraptor and Troodon. A complete mound nest attributed to Troodon, with a distinctive upraised sedimentary rim, was also found in the Late Cretaceous Two Medicine Formation of Montana, which indicates that theropod nests might be recognizable as trace fossils without necessarily having accompanying body fossil evidence. The egg clutch size for Troodon was as much as 24. Eggs were laid in a spiral pattern and oriented with their long axes nearly vertical, suggesting that the mother manipulated them after they were laid in the nest structure (see Fig. 14.11). Some paleontologists have also suggested that these nests originally were covered with vegetation to aid incubation of the eggs (similar to modern crocodilian nests). However, no direct evidence of this association, such as fossil plant material, has been described so far.

The statistically significant close proximity of paired eggs within the Troodon clutch is strong evidence that favors dual oviducts in this theropod, which would have enabled laying the eggs two at a time. This is an excellent example of how indirect, non-skeletal evidence can be interrelated with theropod soft-part anatomy. Likewise, Oviraptor clutches also seem to show egg pairing, although statistical analyses on these clutches are lacking. Furthermore, the two eggs found in the pelvic region of a specimen of Sinosauropteryx also suggest the former presence of two oviducts. Consequently, dinosaur paleontologists are now considering the possibility of dual oviducts in at least some theropods.

Other than Oviraptor, Troodon, and Sinosauropteryx, no other information about egg laying or brooding behavior is currently available for theropods. Of particular interest to some paleontologists are the reproductive habits of the larger cerato-saurs and tetanurans. Questions that remain to be answered are:

1 did they build nests?

2 what did their eggs look like?;

3 what were the clutch sizes, and did they lay a few large eggs or many smaller ones, and

4 did either parent stay close to the nest after eggs were laid, or otherwise care for their young?

Hopefully, future investigations and discoveries will lend further insight into these questions and others about theropod reproduction.

Was this article helpful?

0 0

Post a comment