An artist’s reconstruction of Sapeornis, one of the two ancient birds from the Lower Cretaceous of China whose fossils preserved a crop full of seeds, indicates that early birds had a crop and digestive system similar to those of modern birds.
Credit: Art by Lida Xing and Yi Liu.
LONDON: Ancient birds may have had a modern seed-eating organ, according to a new study that might change the way we understand avian evolution.
The study, published today in the Proceedings of the National Academy of Sciences, uses fossil evidence to address two fundamental mysteries in bird history: the evolution of seed eating - or 'granivory' - mechanisms, and the loss of teeth.
"The most exciting implication of this discovery is that the digestive system of [modern] birds had appeared since the Early Cretaceous, only shortly after the earliest bird," said Zhonge Zhou, co-author of the study and a palaeontologist at the Chinese Academy of Sciences in Beijing, China. "Feeding adaptation [thus] played a vital role in the evolution and diversification of early birds."
Crops and Cretaceous
In bird history, the Early Cretaceous period - stretching from approximately 100 to 145 million years ago - is a critical interval. It is where many primitive birds first appeared and diversified. The predecessors to these birds were likely toothed insect eaters; the pointed snout (or beak) of bird skulls resulted from this selective feeding on small prey.
However, recent research has shown that seed eating was common amongst ancient, Mesozoic birds. But little is known of how these primitive birds digested their seed-based diet.
The crop - a food-storing pouch near the mouth - is the most common way for modern birds to soften seeds before passing them through a specialised stomach, or 'gizzard'.
Signs of seed-eating
Zhou and colleagues investigated hundreds of Early Cretaceous fossils from China and identified crops in the Hongshanornis and Sapeornis species, in roughly the same anatomical location as modern birds. Both species are evolutionarily distant, implying that their crops evolved independently from one another. They further identified a gizzard in the Hongshanornis specimen, meaning that this early species effectively had modern avian digestive system.
The two lineages also had reduced or completely absent teeth, causing the scientists to suggest that seed eating might be correlated with the loss of teeth in birds. Although reduced body weight has often been used to explain the loss of teeth in birds, Zhou and colleagues suggest that dietary adaptation was probably an equally or more important factor in early avian evolution.
It appears, then, that birds played an important role as seed dispersers shortly after angiosperms (the plants that produce seeds) evolved, and that this seed eating was an influential factor in evolving and diversifying the species.
Questioning the crop
Though the Chinese fossils partially fill an important evolutionary gap, Zhou and colleagues are wary of overstating the significance of seed eating in the evolution of the crop. Many birds - such as hawks and hummingbirds - also have a crop, but it does not seem to be linked with seed eating. There is, moreover, speculation among some palaeontologists that a crop could have first appeared in non-avian dinosaurs.
"There are numerous unanswered questions," said Gerald Mayr, a palaeornithologist at the Senckenberg Museum in Germany. He questioned the authors' suggestion that their study changes the broad picture of avian evolution. "The Chinese fossils [are] certainly proof that [fruit and seed eating] evolved early in some avian lineages, but I do not think that they show that granivory was primitive for birds and responsible for the loss of teeth," he said. "A large crop evolved within several modern lineages, but appears to have been absent in the last common ancestor of crown clade [still existing] birds."
"We further know of a number of avian fossils more closely related to modern birds than Hongshanornis, which had teeth and were certainly not seed-eaters ... I thus consider it more likely that the crop in the early Mesozoic birds evolved independently as that in extant birds (which itself, as noted, evolved independently more than once)," said Mayr.
Zhou is now focused on reconstructing the palaeoecology of North-eastern China in the Early Cretaceous, including diets, habitat and the food web.
