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Wings over Spain - bird fossils - includes related article
Natural History, Sept, 1998 by Luis M. Chiappe
Recently I learned that The Valley of Gwangi, a 1969 "lost world" movie that featured Mexican cowboys battling vicious dinosaurs and pterosaurs, was largely filmed in and near the picturesque medieval town of Cuenca, in central Spain. What the filmmakers couldn't have foreseen was the discovery in the mid-1980s of a real and more exciting prehistoric realm buried in a small pocket of limestone at a site near Cuenca. Fossils from this site, which the locals call Las Hoyas, preserve the remains of an array of creatures that lived in and near a freshwater lake some 115 million years ago.
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Today, this region is hilly and arid, with occasional rock outcrops surrounded by pines and other trees. The thin-layered rocks of Las Hoyas, which cover an area of just a couple of acres, are known as lithographic limestones, the kind once used by printmakers; they preserve the remains of ancient animals in exquisite detail. Over the past decade, Jose L. Sanz and his associates, from the Universidad Autonoma in Madrid, have excavated an entire Mesozoic ecosystem--including insects, crustaceans, fishes, amphibians, and dinosaurs--from the Las Hoyas limestones. Among the most precious finds are fossils of primitive birds, which I have had the privilege to study with my Spanish colleagues.
The detailed fossils reveal not only the small, delicate bones of birds but also their feathers, and in one case the remains of a last meal--seafood--in the stomach. But the bird that never had the chance to digest that supper has a bigger claim to fame. It has given us new clues as to how and when in their evolutionary history early birds fine-tuned their ability to fly. The impressions of this bird's wing plumage included a small tuft of feathers attached to the "thumb." Immediately recognizable as the bastard wing, or alula, this feature is a characteristic of many modern birds but had never before been encountered in a fossil bird from the Mesozoic. The new bird was christened Eoalulavis, "early bird with alula." The presence of the little tuft shows that fully 115 million years ago, the finch-sized bird was able to fly and maneuver almost as well as its modern counterparts.
The bastard wing functions like the wing flap on an airplane. When a bird wants to reduce its speed or make a landing, it increases the angle of its wing to the horizon. The drag produced by this wing position helps the bird slow down. But when the angle between the direction of the airflow and the wing surface gets too steep, turbulence over the wing increases until the bird loses the lift necessary to maintain flight. Like an airplane under similar circumstances, the bird is in danger of stalling in midair. The bastard wing then comes to the rescue. By raising this small appendage, the bird creates a slot between it and the main part of the wing, similar to what happens when a pilot deploys a craft's wing flaps. The slot allows air to stream over the main wing's upper surface, easing turbulence and allowing the bird (or plane) to brake without stalling.
The primitive birds from Las Hoyas are not the only ones known from Spain. For the past century, bird feathers have been found in the lithographic limestones of the Montsec range of Catalonia, in the north. These deposits are older than those of Las Hoyas, and they too once formed the bottom of a freshwater lake. A recent and extraordinary find (made by an amateur fossil hunter, who presented the specimen to a Catalonian museum) proved to be the remains of a 135-million-year-old hatchling, the most ancient ever found.
Like any modern hatchling bird, this as-yet-unnamed relative of Eoalulavis has a large head with enormous eye orbits. But other aspects of its skull--such as the presence of teeth, the relative size of the braincase, and the ample space for the attachment of jaw muscles--are reminiscent of Archaeopteryx and birds' dinosaurian ancestors. Where the hatchling is far more advanced than Archaeopteryx, however, and closer to modern flying birds is in its specializations for powered flight--the design and proportions of its wings and shoulder. (As an older but close relative of Eoalulavis, it may also have had a bastard wing, although this is not evident in the fossil.) So, despite its toothy, dinosaurian-looking head, this bird, in adult form, probably flew as well as Eoalulavis.
During the early history of birds, evolution focused primarily on the development of flight. The 150-million-year-old Archaeopteryx, the earliest-known and most primitive fossil bird, was probably not an accomplished flyer. Yet by 115 million years ago, or possibly even 135 million years ago, some primitive birds not only flew but flew well, having developed novel structures they used aerodynamically.
Unprecedented numbers of Mesozoic birds discovered worldwide over the past twenty years have contributed to a revolution in evolutionary thought among paleontologists. The fossil record has shown birds to be the direct heirs of bipedal dinosaurs (see "First Came Feathers," facing page, and "Origins of the Feathered Nest," Natural History, June 1995). We also now know that birds were a large and diverse group, even in their early evolutionary history (see "A Diversity of Early Birds," Natural History, June 1995). The early birds from Spain take their place in this growing roster and show us that the airspace above the large land dinosaurs was full of flapping wings.
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