In 1984, a paper was published in China (in Chinese) that described a new bird species from the early Cretaceous period, based on part of a fossilized left foot bone that had been discovered in northwestern China by a team of paleoichthyologists in 1981. This bone was determined to be part of an ancient tern-sized bird, later named Gansus yumenensis (for the Chinese province of Gansu, and Yumen, the nearest somewhat large town to where the fossil was found). This fossil bird appeared to be more closely related to modern (neornithean) birds than even its famous cousin, Archaeopteryx. Even though this bone gave paleontologists a tantalizing glimpse at an early neornithean bird, this fossil was too fragmentary to provide much more information than that.
Yet, despite this and several other exciting discoveries, these remote fossil beds were difficult to access so they remained virtually untouched for 20 years. Until recently, that is. Several years ago, an international team of Chinese and American scientists organized an expedition to the Xiagou Formation near Changma, in the northwestern Chinese province of Gansu, to search for more bird fossils, and their efforts were well-rewarded (map: You et al. click here for an enlarged map).
After cleaning and preparing the fossils, the scientists realized they had discovered five new partial skeletons of Gansus, which collectively include bones from every part of the bird’s body except the skull and the first few neck vertebrae. Further, the team were surprised to find that this nearly complete composite skeleton of Gansus revealed a very modern-looking bird (image, You et al., Gansus; right; also see skeleton at the bottom of this essay).
“With a few exceptions, you could put any of its bones next to those of a modern bird and you would be hard-pressed to see major differences,” remarked paleontologist Jerald Harris in a late-night telephone conversation. Harris is a co-author of the paper and the Director of Paleontology at Dixie State College in Utah.
Yet surprisingly, these fossils are approximately 110 million years old, only somewhat younger than both Archaeopteryx, which is 145 million years old, and the more primitive fossil birds found in “feathered dinosaur” quarries in northeastern China. Yet, despite its antiquity, this species neatly fits into an evolutionary gap for modern birds — a gap that had previously spanned 40 million years.
“In terms of its anatomy, Gansus appears even closer to modern birds than some of the birds known from 80 million-year-old rocks in North America that have long been known to be close to modern birds,” Harris explained, saying that the bird resembled a modern-day loon.
“This tells us that the evolution of the anatomical features that we use to characterize modern birds evolved very quickly during the Early Cretaceous — there were some powerful selective pressures acting on bird evolution at this time.”
Gansus, along with other fossilized birds and feathered dinosaurs recently found in the Liaoning Province in northeastern China, have considerably expanded our current understanding of avian evolution. The picture that is emerging reveals that, after the appearance of Archaeopteryx and a few other very primitive birds, the avian evolutionary tree split into two main branches (pictured below; also refer to this tree).
One branch, the Enantiornithes (“en-AN-tee-OR-nih-theez”; the branch on the right side in the above figure), which lived at the same time and in the same places as Gansus, is extinct today — they appear to have gone extinct along with all other non-avian dinosaurs at the end of the Cretaceous, 65 million years ago. This is ironic, noted Harris, because during the first half of the Cretaceous, beginning about 130 million years ago, the Enantiornithes were the most common avian fossils that one could find.
The Enantiornithes are nicknamed the ‘opposite birds’ because the bones in their shoulders and in their feet fit together in the opposite way from those in modern birds.
The other branch, the Ornithuromorpha (“OR-nith-YUR-oh-MOR-fuh”; the branch on the left side in the above figure), gave rise to all the modern birds. This branch has a poorer fossil record, which leads scientists to believe that these birds may genuinely have been much rarer at the time than were the ‘opposite birds.’
“In terms of where it [Gansus] seems to lie on the bird evolutionary tree, it is about two branches below the ‘cap’ of modern birds,” on the Ornithuromorpha branch, said Harris.
Another interesting feature of Gansus is that it was obviously an aquatic bird. This is important because, if a person only studied modern birds to understand how all birds are related to each another, that person would predict that all modern birds evolved from earlier birds that were primarily adapted to living on land.
However, Gansus is interesting because its physical adaptations for life in water, such as webbed feet and a big, bony crest on its knee, for example, are also seen in the knees of loons and grebes. Loons and grebes are modern birds that dive deeply and swim underwater in pursuit of fishes.
But surprisingly, most birds on the branch of the avian tree leading to modern birds possess a variety of anatomical features that strongly suggest they were also adapted for life in and around water, explained Harris. Because Gansus has these features and yet is very close to modern birds, this suggests that early modern birds evolved in an aquatic context and only later in their evolution — sometime between Gansus and the first modern birds — did they lose those anatomic features when they returned land.
This, in turn, tells scientists that terrestrial niches had to first be emptied of these ‘opposite birds’ before modern and near-modern birds could move out of the water and back into those niches, said Harris.
How and when these niches were emptied is not well understood, although it is tempting to speculate. But the team noted that 80% of the bird fossils found so far in Gansu Province belong to Gansus, which makes this the oldest known bird fauna that is dominated by the Ornithuromorpha instead of by the ‘opposite birds.’
“Previously, the dominance of near-modern birds in most faunas were known only in the latter half of the Cretaceous — Gansus may be the first step near-modern birds took toward taking over in dominance from the ‘opposite birds.’ ”
This nearly complete fossil skeleton of Gansus yumenensis
preserves the brown-black impressions of feathers around both wings
(Image: Hai-lu You/CAGS).
Note: The name of this piece came from the documentary (DVD) and the book with the same name that document middle Mesozoic fossils from Central Asia and the scientists who study them — both of which I want, and very badly! The documentary airs in the USA on 19 June 2006 on the Science channel.
Jerry Harris (telephone and email)
A Nearly Modern, Amphibious Bird from the Early Cretaceous of Northwestern China by Hai-lu You, Matthew C. Lamanna, Jerald D. Harris, Luis M. Chiappe, Jingmai O’Connor, Shu-an Ji, Jun-chang Lü, Chong-xi Yuan, Da-qing Li, Xing Zhang, Kenneth J. Lacovara, Peter Dodson, & Qiang Ji (Science, 15 June 2006, vol 312:1640). [abstract]
Another story that here appeared awhile ago, describing how the expedition to China to seek these fossils was financed. (This blog entry also includes comments from Jerry Harris that you will want to read).
A new fossil bird from the Early Cretaceous of Gansu Province, northwestern China by You Hai-lu, Jingmai O’Connor, Luis M. Chiappe & Ji Qiang (Historical Biology 2005, 17:7-14).
New Enantiornithine Bird From the Marine Upper Cretaceous of Alabama by Luis M. Chiappe, James P. Lamb, Jr., and Per G. P. Ericson (Journal of Vertebrate Paleontology 2002, 22:170-174).
A second enantiornithean (Aves: Ornithothoraces) wing from the Early Cretaceous Xiagou Formation near Changma, Gansu Province, People’s Republic of China by Jerald D. Harris, Matthew C. Lamanna, You Hai-lu, Ji Shu-an, and Ji Qiang. (Already published somewhere, but I was working from the final manuscript and will provide citation when I find it)