Evolution of dinosaur body size through different developmental mechanisms

The meat-eating dinosaurs known as theropods ranged in size from the bus-sized T. rex, to the dog-sized Velociraptor, to the birds we know today. The puzzle of how such wildly different dinosaur sizes evolved has been unanswered until now. A study published today in the journal Science revealed that, surprisingly, smaller and larger theropod dinosaurs like these didn’t necessarily get that way by just growing slower or faster, respectively.

“Most animals are thought to evolve to be larger or smaller by growing faster or slower than their ancestors, but this study shows that it’s just as likely that bigger or smaller animals grew for longer or shorter periods of time during growth spurts,” says Michael D. D’Emic, a paleontologist at Adelphi University and lead author of the study published today in the journal Science. 

The bones of many animals, including dinosaurs, slow or pause growth every year, leaving marks like tree rings that indicate the animal’s age and growth rate. “Widely spaced rings indicate faster growth and narrowly spaced rings tell us that an animal was growing more slowly,” said D’Emic.

D’Emic and a team of international researchers measured about 500 such growth rings in about 80 different theropod bones.“We found that there was no relationship between growth rate and size,” said D’Emic. “Some gigantic dinosaurs grew very slowly, slower than alligators do today. And some smaller dinosaurs grew very fast, as fast as mammals that are alive today.” 

This made sense to co-author Thomas Pascucci, who contributed to the project as a graduate student at Adelphi: “Extinct animals like dinosaurs inspire awe because of how different they seem from those in our modern world, but they were animals that grew under constraints and environmental factors similar to those that exist today.”

The team found that it was just as common for meat-eating dinosaurs to evolve changes to how fast they grew as it was to evolve changes to how long they grew.

“This has really important implications because changes in rate versus timing can correlate to many other things, like how many or how large your offspring are, how long you live, or how susceptible to predators you are,” D’Emic said. “Hopefully this research spurs investigations into other groups, both alive and extinct, to see what developmental mechanisms are most important in other types of animals.” 

Co-author and Ohio University graduate student Riley Sombathy hopes to take up some of those investigations, adding, “One of the things that interests me about the results of our project is the apparent decoupling between growth rate and body size."

This study opens the door to future investigations of how animals regulate their growth. According to study co-author and professor at Ohio University Patrick O’Connor, “Alteration of different growth control mechanisms, at molecular or genetic levels, probably account for the range of developmental strategies our team observed in theropod dinosaurs. Future studies of   living organisms provide an opportunity to elucidate mechanisms related to the evolution of body size in vertebrates more generally.”

The paper, “Developmental strategies underlying gigantism and miniaturization in non-avialan theropod dinosaurs,” was co-authored by Patrick O’Connor and Riley Sombathy of Ohio University; Ignacio Cerda of Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and Universidad Nacional de Río Negro in Argentina, David Varricchio of Montana State University; Diego Pol of CONICET-Museo Paleontológico Egidio Feruglio in Argentina, Rodolfo Coria of Museo Carmen Funes in Argentina; and Kristina A. Curry Rogers of Macalester College in Minnesota.