How Eggs Shaped Dinosaur Evolution

How did dinosaurs come to rule the Mesozoic world? No one knows for sure, but the way dinosaurs reproduced probably had something to do with it. Dinosaurs grew fast, started mating before they hit skeletal maturity, and laid clutches of multiple eggs—a life history that may have allowed dinosaurs to rapidly proliferate and diversify. And egg laying itself may have been critical to why many dinosaurs were able to attain gigantic sizes. By laying clutches of small eggs, dinosaurs may have been able to sidestep biological constraints that have limited the size of mammals.

But there was a catch. Consider a large dinosaur, such as Diplodocus. Infant Diplodocus hatched out of eggs roughly the size of a large grapefruit, and if they were lucky, the dinosaurs grew to be more than 80 feet long as adults. And the little sauropods were not just small copies of adults. Like many other dinosaurs, individual Diplodocus changed drastically during their lives, and young dinosaurs may have preferred different habitats and food sources from those of more mature individuals. As outlined by Daryl Codron and co-authors in a new Biology Letters paper, this peculiar life history may have been a consequence of laying eggs.

Codron’s group created a virtual dinosaur assemblage to see how intensely dinosaurs might have competed with one another as they grew. If all dinosaurs started off relatively small, then the largest species had to pass through a series of size classes and change their ecological role as they matured. This ramped up the pressure on young dinosaurs. Juvenile dinosaurs had to contend with other juveniles as well as dinosaurs that topped out at smaller sizes. In a diverse Late Jurassic ecosystem, for example, young Allosaurus, Torvosaurus and Ceratosaurus not only had to compete with one another, but also with smaller carnivores like Ornitholestes, Coelurus, Marshosaurus and Stokesosaurus. Dinosaurs would have faced the most competition at small size classes, and this may have driven some dinosaur lineages to become large.

The new paper also suggests that dinosaur life history may have played a role in the demise of the non-avian species. Competition at smaller size classes, Codron and colleagues suggest, drove dinosaurs to become bigger and bigger, and this created a lack of species that were small at maturity. Mammals and avian dinosaurs occupied those niches. This could have made dinosaurs more vulnerable to the intense pressures of the end-Cretaceous extinction. If the catastrophe targeted large animals, but was less severe among small animals, then non-avian dinosaurs would have been doomed. The big dinosaurs disappeared, and there were no small non-avian dinosaurs left to quickly proliferate in the aftermath.

As John Hutchinson pointed out in a Nature news story about this research, however, we’re going to need a lot more testing to see if this hypothesis holds up. The conclusion is based on a virtual model of ecosystems that we can’t study directly, and mass extinctions are frustratingly complicated phenomena.

Of course, a new dinosaur extinction scenario is irresistible journalist bait. Various news sources picked up the extinction hook (promoted in the paper’s press release) and pointed to the fact that dinosaurs laid eggs as the seeds of their undoing. But this isn’t quite right. After all, turtles, crocodylians and birds all laid eggs, too, and they survived. And mammals did not survive the end-Cretaceous extinction unscathed—several mammalian lineages disappeared or took major hits during the catastrophe. Likewise, not all dinosaurs alive during the final days of the Cretaceous were huge. Titans like Tyrannosaurus, Triceratops and Edmontosaurus are the most famous end-Cretaceous dinosaurs, but in western North America alone, there were also relatively small ceratopians, oviraptorosaurs and troodontid dinosaurs that topped out at about six feet in length. Were these dinosaurs still too big to survive? Was the threshold even lower? If it was, then the reason why medium-sized animals such as crocodylians survived, and why some mammals disappeared, becomes even more complicated. Why non-avian dinosaurs perished, and why so many other lineages survived, remains a mystery.

References:

Codron, D., Carbone, C., Muller, D., & Clauss, M. (2012). Ontogenetic niche shifts in dinosaurs influenced size, diversity and extinction in terrestrial vertebrates Biology Letters DOI: 10.1098/rsbl.2012.0240

Riley Black | | READ MORE

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.