Researchers studying a 120-million-year-old fossilized bird from China discovered a compacted, football-shaped cluster of fish bones nearby. Many birds form these so-called pellets out of indigestible food items (like bones) in their digestive tract, and then regurgitate them. This discovery is the world’s oldest bird pellet. And according to a new Current Biology study, it suggests that birds in the Early Cretaceous already had a digestive system resembling that of modern birds.
In animals, indigestible matter is either expelled as feces or regurgitated through the mouth. Unlike their carnivorous dinosaur ancestors, modern birds lack both teeth and bony jaws. Instead, they’ve evolved beaks and a specialized digestive system that can process unmasticated food. A bird’s stomach is divided into two chambers, one of which is a muscular, thick-walled gizzard that often contains small pebbles to help break down food. Indigestible items remain in the gizzard, where they’re compacted into a pellet by muscular contractions and eventually regurgitated through an upward wave-like movement (called antiperistalsis).
With few exceptions, raptors and seabirds these days produce pellets. During the Mesozoic Era (which includes the Triassic, Jurassic, and Cretaceous periods), Enantiornithes was the most successful bird lineage. It’s a sister group of Ornithuromorpha, which gave rise to all living birds today. But with few fossils, we still don’t know much about their feeding habits.
Now, a Chinese Academy of Sciences team led by Min Wang has analyzed fossils belonging to an enantiornithine bird unearthed from the Lower Cretaceous Jiufotang Formation near the town of Dapingfang in Liaoning Province, northeastern China.
Curiously, a spindle-shaped cluster of fish bones overlapped with the fossils of the bird’s right wing bone (or humerus, labeled "rh" in the image to the right). Its proximity to the skeleton suggests it was a pellet (labeled "gp," near the top) that was regurgitated shortly before, or even at, the time of death. The pellet measured 22.6 millimeters (0.9 inches) by 7.1 millimeters (0.3 inches), and among the bones were vertebrae and neural spines that likely belonged to Lycoptera, the most abundant fish at this site.
The cluster wasn’t feces because it’s unlikely those items could have passed through the intestines on its way out the other end. Some of the bones were sharp and more than 2 millimeters (0.1 inches) long, and they didn’t show signs of digestive erosion. Not to mention, the elongated oval shape and the cohesiveness of the contents are characteristically pellet-like.
Not only does the pellet clearly indicate that some enantiornithines were piscivorous, it also suggests that the digestive tract of ancient birds resemble that of living ones: efficient antiperistalsis and a two-chambered stomach with a muscular gizzard. By providing a safe mechanism for eliminating bony matter, the ability to regurgitate pellets helped facilitate the evolution of piscivory – and perhaps other forms of carnivory – in both enantiornithines and ornithuromorphs during the Cretaceous.
Life reconstruction of a fish-eating enantiornithine bird. SHI Aijuan
Image in the text: M. Wang et al., Current Biology (2016)