Did Dinosaurs Sit on Their Nests? A Fossil Caught in the Act

A small number of skeletons have been recovered sitting directly on top of arranged clutches of eggs, with the hind limbs folded beneath the body, the arms spread outward and backward around the clutch and the eggs laid in a ring or pairs rather than scattered. The posture is not a chance burial position, since the limbs are articulated and the body is centred on the eggs in several independent specimens from different localities. The arrangement closely matches the brooding posture of modern birds, with the forelimbs positioned where wings would cover the clutch, and it is one of the clearest cases in palaeontology of a behaviour being preserved directly rather than inferred. The animals were apparently buried rapidly, most likely by collapsing sand, while on the nest.

The reading of these fossils reversed completely, which is a useful cautionary tale:

• •The first such animal was found near a clutch in the 1920s and was assumed to be raiding it
• •It was given a name meaning egg thief on that basis, which it still carries
• •The eggs were assumed to belong to a different dinosaur found in the same beds
• •Decades later an embryo was found inside one of those eggs and belonged to the supposed thief
• •The animal was therefore on its own nest rather than robbing somebody else's
• •Further specimens in the same posture confirmed the reinterpretation

The clutches themselves carry information independent of the adults found with them. Eggs arranged in a ring with a gap in the centre suggest the adult sat in the middle with the eggs around rather than directly beneath, which would let a heavier animal brood without crushing them. Pairs of eggs laid together indicate two functioning oviducts producing simultaneously, which differs from birds, where one has been lost, and matches what the pelvic anatomy suggests. Eggshell structure, including thickness and the arrangement of pores, indicates whether a clutch was buried in sediment or exposed to air, since buried eggs need different gas exchange, and the evidence differs between groups. Nesting grounds with many clutches at the same horizon indicate colonial nesting, and repeated horizons at the same site indicate return over years.

Nearly all of these fossils come from a small number of localities in the Gobi desert, and the reason is geological rather than biological. Those deposits preserve animals buried rapidly in dry sand, either by collapsing dune faces or by sand-laden flows after rain, which catches animals in life position and preserves delicate arrangements that ordinary burial destroys. The same beds produced the specimen of two animals locked together mid-combat and several individuals preserved in a sleeping posture with the head tucked, which is the same kind of evidence about behaviour. Expeditions have worked the area since the 1920s with interruptions for war and politics, and the material is the single richest source anywhere for how small dinosaurs behaved. The dependence on one region is a real limitation, since it is not known how typical those animals were.

The significance goes beyond the individual behaviour. Brooding is one of a set of characteristics shared between birds and their closest extinct relatives, alongside feathers, wishbones, hollow bones, sleeping postures preserved in other specimens and the arrangement of the respiratory system, and each such shared feature strengthens the conclusion that the behaviours of modern birds were largely inherited rather than invented. The question of whether brooding was primarily for warming the eggs or for protecting and shading them is not settled, and it matters for the debate about body temperature in these animals. The evidence is also limited in an important way, since it comes from one group of small feathered predators and says nothing directly about whether much larger dinosaurs, which could not have sat on a clutch without destroying it, cared for eggs at all.

Several skeletons have been found crouched on arranged clutches with the arms spread around them, matching the brooding posture of modern birds, in independent specimens from different sites. The first such animal was named for being an egg thief and was later shown to be on its own nest. The behaviour is one of a set of features linking these animals to birds.