In 1995, a palaeontologist called Mark Norrell reported an amazing discovery - the fossilised remains of a dinosaur called Troodon, sitting on top of a large clutch of eggs. The fossil was so well-preserved and its posture so unmistakeable that it provided strong proof that some dinosaurs incubated their eggs just as modern birds do. And since then, two other small predatory species - Oviraptor and Citipati - have been found in brooding positions on top of egg clutches. But a subtler look at these fossils reveal much more about dinosaur parenting than the simple fact that it existed. To David Varricchio from Montana State University, they also tell us which parent took more responsibility for the young. Based on the size of the egg clutches and the bones of the parent, Varricchio thinks that it was the males that cared for the babies. ...So there you have it: stay-at-home dads are as ancient as the dinosaurs!
The team noted that the clutches so delicately incubated by Troodon, Oviraptor and Citipati contained a substantial number of eggs, about 22 to 30 eggs apiece. Compared to most of the 433 living birds and crocodilians whose clutch sizes have been studied, the dinosaurs were sitting on far more eggs than animals of their size normally do. The team found that species where both parents chip in, or where mum takes the lead, usually settle for smaller clutches. Only those where dad does almost all of the work tend to rear such large broods. ...
With daddy standing watch and foraging, females can devote more of their energy to actually producing and laying eggs. That allows individuals to lay more eggs, which can then be pooled together to form mega-clutches. Varricchio thinks that the large nests of dinosaurs like Troodon were also communal ones that included eggs laid by many different females. Troodon and Oviraptor eggs are also relatively larger than those of bigger predatory dinosaurs like Allosaurus, and Varrichio suggests that the energetic demands of producing and incubating bigger eggs may have driven the evolution of male-only parenting among small predatory dinosaurs in the first place.
Another clue suggests that the dinosaurs atop the fossil eggs were male. When the time comes to reproduce, female birds lay down a special layer of spongy tissue called medullary bone in the long bones of their limbs. Rich in calcium and phosphorus, medullary bone provides birds with the minerals they need to lay strong-shelled eggs. As they lay and these minerals are used up, the tissue is slowly absorbed back into the body.
Many dinosaurs had medullary bone too, including Tyrannosaurus, Allosaurus, and Tenontosaurus, all of which are more distantly related to birds than Troodon, Oviraptor or Citipati. This means that the females of these smaller species should also have traces of this special tissue during the reproductive season. But none of the fossils that Varricchio looked at did, nor did they have the large cavities needed to reabsorb the tissue after eggs are laid.
Alone, the absence of medullary bone doesn't guarantee that the incubating individuals were male - they could have been females that had either gone past sexual maturity or hadn't reached it yet. A few modern birds also lack medullary bone in both genders, but their food provides them with all the minerals they need to produce eggs. But the fossilised eggs were so large and numerous that it's difficult to envisage how a female could have found enough mineral-rich food without help from a partner.
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