Fun facts about dinosaurs are on today’s menu. Not everything perished in the Cretaceous-Paleogene extinction event (the big asteroid that created an Ice Age which wiped out the dinosaurs). Modern Earth’s small, elegant birds were once the largest, most meat-eating animals to walk that land. The evolutionary link between birds and dinosaurs is very strong; the origin of birds begins when dinosaurs ruled the world.
“Dinosaurs aren’t extinct; there are about 10,000 species alive today in the form of birds”
Dr. Roger Benson, Oxford University’s Department of Earth Sciences
That might be a difficult to imagine but they aren’t that different, right?
Much of the evidence for the evolution of dinosaurs to birds comes from transition fossils found in Northern China. For example, we know that many therapods (bipedal, meat-eating dinosaurs like T. rex and Velociraptor) had feathers because of incredibly detailed fossils preserved in volcanic ash. These fossils are so well preserved that even the colours of feathers can be reconstructed.
In fact, some have suggested that all dinosaurs may have been feathered as a result of some fossils unearthed in Siberia recently. I’m skeptical.
“Developmental experiments in modern chickens suggest that avian scales are aborted feathers, an idea that explains why birds have scaly legs.”
Danielle Dhouailly, Université Joseph Fourier in Grenoble, France
Evolutionary Shrink Ray
There were two main drivers in the evolution of dinosaurs to birds.
- Sustained miniaturisation
- Very fast evolution of new skeletal adaptations
The dinosaur lineage that evolved to become birds shrank in size rapidly and continuously for about 50 million years, evolving at a rate four times faster than other lineages. The most difficult thing to get to grips with is how dinosaurs are so gigantic and birds so small. But shrinking played an important role in their success. It was the huge timescale and unusually rapid evolutionary rates that made it possible.
“Birds out-shrank and out-evolved their dinosaurian ancestors, surviving where their larger, less evolvable relatives could not.”
Michael Lee, Australian Museum in Adelaide
Maintaining this rate of shrinking for so long required a sustained miniaturisation. Smaller size meant the animal developed more quickly, resulting in developmental changes such as shorter snouts, smaller teeth and insulating feathers.
Reduced size resulted in new postures causing a more bird-like gait as well as new arboreal, and later aerial, habitats to be explored. They were able to fill new ecological roles. Aerial habitats seem like a big jump (pun intended), but it is quite likely these smaller therapods were excellent climbers.
“What we found was striking. Dinosaur body size evolved very rapidly in early forms, likely associated with the invasion of new ecological niches. In general, rates slowed down as these lineages continued to diversify…but it’s the sustained high rates of evolution in the feather miniraptoran dinosaur lineage that led to birds – the second great evolutionary radiation of dinosaurs.”
David Evans, Royal Ontario Museum
Archaeopteryx lithographica, discovered in southern Germany, is one of the most important fossil discoveries ever. It is a true ancestral bird and a great demonstration of evolution from one vertebrate class to another – a skeleton similar to therapods as well as bird-like features.
It is still unclear how its wings and feathers were used for flight. It’s likely that either birds evolved from ancestors living in trees and would glide down, or birds lived on the ground and made long leaps.
This guy is the biggest four-winged dinosaur discovered yet. Pretty recently too. Its tail feathers at 30 cm long are the longest of any known dinosaur.
“Numerous anatomical features and behaviours that we have long associated with birds in fact evolved in dinosaurs before the first birds arrived on the scene.”
Alan Turner, PhD, Assistant Professor in Anatomical Sciences at Stony Brook University
These features include hollow bones, nesting behaviour, feathers and possibly flight (or more likely, gliding). Aerodynamic studies have shown the long tail to be important in decreasing descent speed for a safe landing.
One Last Attempt at Convincing You Birds are Dinosaurs
To recreate how dinosaurs might have moved, scientists fitted a weighted prosthetic tail to a chicken. This is because of the many similarities in leg structure between therapods and modern birds such as bipedalism and digitigrade movement (walking on toes).
Feathers are light and used for flight; tails are heavy and used for protection and balance, which seems inconsistent. This investigation of therapod limb positioning highlighted how similar dinosaurs and birds are by looking at their movement. Granted we only know of their movement through graphical and mechanical reconstruction, but those models, while theoretical, were constructed from careful inference.
Nevertheless, it can still be difficult to imagine is how something so massive evolved into much smaller, feathered creatures that could fly. This is why it is important to keep in mind how evolution works. It is an accumulation of many small, neutral or advantageous characteristics over millions of years.