Dinosaur eggs laid two by two

by Henry Gee

The home life of nesting dinosaurs is startlingly revealed by a team of researchers from Montana, writing in the 16 January 1997 Nature. David Varricchio of the Old Trail Museum in Choteau, Montana and colleagues [including Jack Horner of the Museum of the Rockies in Bozeman, Montana]. They show how the carnivorous dinosaur Troodon laid its eggs two-by-two, in sequence. This pattern led inescapably to parental care similar to that found in modern nesting birds.

Egg Mountain in Montana is known as the site of many well-preserved dinosaur nests, many with eggs. Late last year, the presence of embryos revealed that many nests thought to have been made by the plant-eating dinosaur Orodromeus belonged to Troodon instead. This caused immediate interest, as the group of carnivorous dinosaurs to which Troodon belonged is widely believed to have been ancestral to birds. Could harbingers of birdlike nesting behaviour be traced in fossil Troodon nests? Varricchio and colleagues say 'yes'. In the Nature report, they show that Troodon maternal behaviour was a bridge between that of birds and reptiles.

The key was a statistical analysis of one of the best-preserved of eight known Troodon clutches. This showed how the 22 eggs were laid in a sequence of 11 pairs. Troodon laid its eggs two by two.

Crocodiles, relatives of both dinosaurs and birds, lay their eggs all at once, in a clump, and bury them in vegetation. This is usually the sum total of parental care lavished on the young. The eggs, warmed by burial, hatch into precocious crocodiles, able to cope with the world immediately after hatching. A female crocodile, like reptiles in general, has two functional oviducts, both of which churn out the eggs like a factory production line.

Modern birds are different. Apart from pathological cases, only one egg is produced per oviduct at a time, and the eggs are laid in sequence, over a period. This behaviour requires the parent to be present at least until the last egg in the clutch has. Birds brood their eggs themselves, rather than burying them in vegetation or earth and abandoning them to the fates.

The Troodon evidence suggests a mixture of both. The eggs were clearly laid half-buried in mud. This explains the common occurrence of dinosaur nests consisting of the bottom halves of the eggs only. So dinosaurs were like crocodiles, in the sense that they buried their eggs. And yet there is no sign of vegetation, and the occasional preservation of adult Troodon bones in the nests suggests some degree of parental brooding. The spectacular preservation in Mongolia of the adult dinosaur Oviraptor sitting on its eggs, reported in Nature at the end of 1995, supports this view.

But the pairing is the clincher. If Troodon laid its eggs in pairs, it suggests that each oviduct harbored just one egg at a time. This implies that the mother had to remain in or near the nest until at least the last pair of eggs had been laid. Whether the parents remained around to care for the infants is moot: embryological evidence suggests that even newly hatched dinosaurs were fully capable of running about on their own. And yet the seeds had been sown. Many aspects of modern bird behaviour have deep roots, being found among their dinosaur ancestors.

Macmillan Magazines Ltd. -- NATURE NEWS SERVICE 1997

Note: This item from the Nature News Service is mounted on this Web page by special permission of Nature.

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