Many of the characteristics of early and modern birds appeared first in theropod dinosaurs. Feathers, wishbones, modified ‘flapping’ forelimbs and hollow bones are found in the coelurosaurs – the theropod group that includes tyrannosaurs and dromaeosaurs.
Even more bird-like in appearance and behaviour are the subgroup of coelurosaurs known as deinonychosaurs. These dinosaurs, such as Bambiraptor and Velociraptor, shared the most recent common ancestor with birds. Considering who came first, perhaps we should stop describing dinosaurs as having bird-like behaviour and start thinking of birds as behaving like dinosaurs.
The fossilised skeleton of Mei long, a non-avian coelurosaur from China, was preserved with its head ‘tucked’ under its forelimb. This posture resembles the ‘sleeping’ pose commonly used by modern birds. Such inferred behaviour strengthens the link between non-avian dinosaurs and birds.
Examples of bird-like dinosaurs include:
- Caudipteryx zoui. Fossils from China, Early Cretaceous, 130–125 million years ago. Classification: Theropoda; Oviraptorosauria. Caudipteryx means ‘tail wing’, referring to the tail plume that this non-avian dinosaur may have fanned out for display. The rest of the body was covered in short primitive feathers with longer feathers on its arms and tail.
- Sinosauropteryx prima. Fossils from China, Early Cretaceous, 130–125 million years ago. Classification: Theropoda; Compsognathidae. The discovery of Sinosauropteryx prima in 1996 was one of the most important fossil finds of the century. It was the first non-avian dinosaur found with feather-like structures, providing further evidence for the link between dinosaurs and birds. Its name means ‘first Chinese lizard wing’.
- Sinornithosaurus millenii. Fossils from China, Early Cretaceous, 130-125 million years ago. Classification: Theropoda; Dromaeosauridae. Sinornithosaurus was one of the first dinosaurs discovered with feathers.This active and agile species probably hunted in groups, and recent research (published in 2010) suggests it may even have been venomous as it had similar teeth and jaw structures to modern rear-fanged snakes. Depressions on the side of its face could have housed a venom gland, and these connected to depressions above the tooth row that fed venom to long grooved teeth in the upper jaw.
The earliest-known feathered dinosaurs
Although it is widely accepted that a small group of theropod dinosaurs gave rise to birds, finding feathered dinosaurs in the fossil record that predate the earliest known bird Archaeopteryx (150myo) has proven elusive. That was until Anchiornis huxleyi was announced in 2009. This species, found in China, dates to between 151-161 million years old and has well-developed feathers on all four limbs. Quick on its heals was the announcement of another stunning new feathered dinosaur Haplocheirus sollers, also from China and about 160 million years old. Haplocheirus is a type of alvarezsauriod, a group of dinosaurs once thought to be flightless birds, but lacks the bird-like features found in later alvarezsauriods such as fused wrist bones and a backward-facing pubis.
The dinosaur–bird transition
Modern birds, early birds and coelurosaur dinosaurs share many physical features:
- hollow and thin-walled bones
- modified shoulder and forelimb enabling hands to fold against the lower arm
- modified wrist (semilunate carpal)