Why Some Dinosaurs Got Smaller

The past week has been a total goldmine for dinosaur enthusiasts. Last Monday, a team led by paleontologist Steve Brusatte published a comprehensive survey revealing that dinosaurs might have survived that fateful meteorite, if only it had hit a few million years earlier or later.

Hot on the heels of that study comes a new paper from the University of Adelaide, which features a similarly impressive sample size of fossils. The Australian team cataloged 1,549 dinosaur traits derived from 120 different specimens. This massive database was then used to chart the evolutionary process that led a branch of theropod dinosaurs to shrink down to become the world's first birds.

The team, led by paleontologist Mike Lee, discovered that over the course of 50 million years, the progenitors of birds decreased in size by an average factor of 12 until they had reached an ideal flying weight.

Obviously, most of the theropods we are familiar with today—like T. rex or Allosaurus—took the opposite tack from their avian cousins, and expanded to the terrifying sizes associated with the entire dinosaurian group. But selecting the megafauna option is a risky strategy over the long term, as the large dinosaurs discovered firsthand.

"Being smaller and lighter in the land of giants, with rapidly evolving anatomical adaptations, provided these bird ancestors with new ecological opportunities, such as the ability to climb trees, glide and fly," Lee told BBC News. "Ultimately, this evolutionary flexibility helped birds survive the deadly meteorite impact which killed off all their dinosaurian cousins."

Birds ended up being the only dinosaurs that pulled through the Cretaceous-Paleogene extinction event, but they were far from the only ones that bucked the dinosaurian trend of becoming giants. Though few dinosaurs shrunk with the rapidity of the early bird clan, many different species needed to pare down to survive.

Easily the most adorable of these species was Europasaurus, among the smallest sauropods ever discovered. The miniature long-neck was only about 10 feet from head to tail, a true dwarf compared to more typical sauropods like Brachiosaurus, which was about 85 feet long, or the recently discovered Titanosaur that topped 120 feet in length.

Why was Europasaurus so small compared to its giant brethren? The conventional guess is that it was a product of insular dwarfism, which can be observed in action in many isolated ecosystems today.

Megafauna stranded on islands or other confined environments commonly decrease in size, though the exact mechanism behind these shifts is still debated (as are the mechanisms behind island gigantism, the exact opposite of insular dwarfism).

Europasaurus was not the only dinosaur that underwent a little island shrinkage. Romania's Haţeg basin was a scenic archipelago during the late Cretaceous, and many of the species found in the region skewed small, including the hadrosaur Telmatosaurus and the sauropod Magyarosaurus.

Other dinosaurs decreased in size when they were pushed into unconventional climates: for example, the new species of pygmy tyrannosaur Nanuqsaurus hoglundi. The carnivore was an Arctic dweller that grew to be only half as big as its southern tyrannosaur family, and is the latest of many cold-climate dinosaurs with smaller bodies.

Interestingly, these dinosaurs buck the mammalian trend of gigantism at the poles and smaller animals nearer the equator (called "Bergmann's rule").

It goes to show that we need more studies like those Brusatte and Lee have produced this past week if we're going to understand what drove some dinosaurs to trade in their largesse for a more petite frame, and why that ended up working out so well for the birds.