Evolution Of Theropods Linked To Environmental Factors
November 29, 2012

Theropod Dinosaur Growth Linked To Environmental Factors, Not Diet

Alan McStravick for redOrbit.com - Your Universe Online

The heaviest flying bird tops out at between 40-42 pounds. And sure, some of the flightless birds can grow to upwards of 300 pounds. But to imagine ancestors of these modern day creatures tipping the scales at upwards of 7,000 pounds is hard to do, indeed.

A recent study out of North Carolina State University (NCSU) and the Field Museum in Chicago, Illinois, looks specifically at the feathered herbivores of the Cretaceous period and how their size and utility fluctuated over time. They wanted to learn what might have occurred to cause these theropods to abandon their meat eating in favor of a plants-only diet.

Conventional wisdom, as it applied to herbivorous dinosaurs, was that size really did matter. The idea was that a larger herbivore would benefit from being bigger due to an increase in the overall size of the digestive tract. The larger digestive tract would allow for a greater extraction of nutrition from their low-calorie and high-fiber diets. This idea led scientists to believe that animals that switch from a carnivorous to an herbivorous diet would, due to natural selection, grow larger and larger.

Lindsay Zanno, research assistant professor of biology at NCSU and director of the Paleontology & Geology Research Lab at the North Carolina Museum of Natural Sciences, along with Peter Makovicky, associate curator of paleontology at the Field Museum, focused their study on three groups of feathered theropods from the Cretaceous period whose gargantuan size seemed to back up the conventional wisdom discussed above. The researchers wanted to explore if their evolutionary super-sizing was, in fact, directly related to their leafy green diet.

“Having three closely related lineages of dinosaurs,” states Makovicky, “adapting to herbivory over the same geological time span and showing evidence of increasing size provided a near perfect test case.”

To set up their research Zanno and Makovicky estimated the overall body masses of 47 extinct species of feathered dinosaurs. These specific species came from 3 major groups that were known to have given up their carnivorous ways. The 3 groups were the ornithomimosaurs (bird-mimics), oviraptorosaurs (egg-thieves), and the therizinosaurs (sythe-lizards). The therizinosaurs were distinctly more bizarre when compared to the previous 2 groups. The therizinosaur looked especially funny due to its gawkish limbs, pot bellies, scythe-like claws on its long front hands and its abundance of feathers. Most all of the species in the studied lineages possessed a toothless beak, three-toed feet that it used for its primary form of conveyance, and shorter tails than most other dinosaurs. These shorter tails actually helped the theropods look much more like modern birds than other dinosaurs of the day.

Also, each of the studied groups exhibited an evolution to monstrous proportions. For example, the largest of the oviraptorosaurs grew to over 7,000 pounds. In the other two groups, the largest species topped out at over 13,000 pounds. “The largest feathered dinosaurs were more than 100 times more massive than your average person,” says Zanno. “The reality is that for most of us, it is downright difficult to imagine a feathered animal of gigantic proportions.”

One of the earliest findings the team was able to determine was that the average body mass of the 3 groups did, over time, increase dramatically. The earliest of the species were the smallest while the last evolution of the species produced the largest. The researchers were quick to point out that this finding did not necessarily prove there was a correlation between this increase in overall size and whether or not it was related to an evolutionary advantage for the species.

Zanno and Makovicky utilized a series of evolutionary models to the data they had collected to examine if the theropods increase in size was a product of natural selection. They wanted to see which model could best replicate the change of body mass from the ancestor species to the descendant species. What they discovered was surprising to them and to the conventional wisdom on the subject. They found that their 3 groups experimented with different body masses in their evolution. Some of the theropods actually changed to a smaller form while others in the groups got larger. This showed them that there was no clear natural selection that would drive them to increase in size. There was no evolutionary advantage for the theropods to increase to their larger sizes.

Zanno stated, “Results of our study don´t rule out diet as affecting body mass, but do seem to indicate that fluctuating environmental conditions over time were trumping the benefit of becoming a giant.” He continued, “The long and short of it is that for plant-eating theropods, bigger wasn´t always better.”

“Where resources permitted, these animals could get as big as elephants, but that clearly was not the case in all environments and time periods,” says Makovicky. “Factors such as resource abundance and competition with other herbivores likely played a more significant role.” He added that uneven sampling in the fossil record, such as preferential preservation of smaller species in earlier time periods and larger species in later ones, could also impact the results.

The researchers are publishing their results in the journal Proceedings of the Royal Society B.