Smaller dinos were harder to wipe out, and they're still around

The asteroid that wiped out dinosaurs 66 million years ago didn’t get them all. Some of them survive to this day in the form of birds, and they may have made it because they got smaller.

For about 170 million years, dinosaurs were the dominant life-forms that lived on land. In this period, spanning the Triassic, Jurassic and Cretaceous periods, they evolved to acquire a variety of traits. One of them was size. As different lineages competed for different resources, some of them became very big: the t-Rex weighed almost eight tonnes. The dinosaurs that would evolve to become birds, on the other hand, weighed a few kilograms or less. The lightest, Qiliania graffini, weighed 15 grams.

A new data analysis has revealed that their small size helped them continue to survive after the mass-extinction event that was to follow. “There is increasing evidence that this extinction event was ecologically selective, and that large animals in particular suffered the most,” said Roger Benson, a palaeobiologist at the Department of Earth Sciences, Oxford University.

Dr. Benson and his colleagues conducted the analysis, published in PLoS Biology on May 6. First, they compiled a list of dinosaurs to study. Then, the team used the results of a previous study to estimate these reptiles’ masses. That study was conducted by two members of the same team who had established that the thickness of the leg bones have a strong relationship with body mass, “allowing the masses of extinct animals to be estimated”.

Their analysis found that once some dinosaurs evolved to giant sizes, they had it harder to invade new ecological niches and didn’t evolve new body forms. According to Dr. Benson, they were effectively ‘locked in’ to their niches. After the extinction event, these dinosaurs had a hard time adapting to their new environment.

I've gotten too big, haven't I?
I’ve gotten too big, haven’t I?

However, why the smaller ones had it easier is not immediately clear. Nick Longrich, from the Department of Biology and Biochemistry, University of Bath, who wasn’t involved in the analysis, thinks the crashing asteroid’s impact would have altered the environment drastically enough to favor smaller animals over bigger ones.

“There probably wasn’t much food when the asteroid hit,” he said. “It would have put a lot of dust and debris in the air, covering the sun, so there would be no photosynthesis going on. It would have been a global famine.” As a result, the available food would have been insects and invertebrates, the smaller animals that eat them, and their predators.

The dust and debris would also have caused a global cooling, and smaller animals will have had it easier to find shelter. These animals also reproduce rapidly, according to Dr. Longrich, implying that they would have been harder to wipe out.

Dr. Benson’s analysis also found the early birds were able to survive because they were able to maintain a rapid rate of evolution. The other dinosaurs also evolved rapidly in their early years, but as their respective ecological niches became saturated, the rate at which they diversified slowed down. The birds however continued to produce ecological diversity and adapted better.

One such study was published in the International Journal of Organic Evolution in February 2014 which noted, “The high evolutionary rates arose primarily from a reduction in body size.”

However, it is not immediately clear why birds and their close relatives became small in the first place. “The explanation must be specific to the ecology of these animals, because many other dinosaur lineages existing at the same time did not become as small,” Dr. Benson explained.

So although the odds are 70 million-to-1 that a big asteroid will impact Earth and threaten life, birds and insects will have it easier if it does.


References:

  1. Rates of Dinosaur Body Mass Evolution Indicate 170 Million Years of Sustained Ecological Innovation on the Avian Stem Lineage. PLoS Biol 12(5): e1001853. doi:10.1371/journal.pbio.1001853
  2. High Rates of Evolution Preceded the Origin of Birds. International Journal of Organic Evolution. doi:10.1111/evo.12363