Flying Dinosaurs And Hopeful Monsters

I believe there were no flowers then,

In the world where the humming-bird flashed ahead of creation. I believe he pierced the slow vegetable veins with his long beak. Probably he was big

As mosses and little lizards, they say, were once big, Probably he was a jabbing terrifying monster.

We look at him through the wrong end of the long telescope of Time, Luckily for us.

D. H. LAWRENCE. 1929. "Humming-Bird"

Ukhaa Tolgod gave us dinosaur eggs and embryos, theropod skeletons, and mammals galore. Back at the museum's laboratories, the treasures from the locality fired us up. Research on this material would provide new answers—and doubtless new questions—concerning important evolutionary problems. One of these certainly concerned the pathway between dinosaurs and full-fledged birds. Mark, Jim, and Luis made up the investigative trio that would tackle the problem, with a careful review of

anatomy in different theropods, including the archaic bird lineages. Now the cornucopia of evidence from Ukhaa Tolgod provided a new reference system for making these comparisons. Even the newly discovered embryo could be drawn in, as it represented remains of some kind of theropod. Besides, any data on juvenile, newborn, or embryonic dinosaurs were keys to tracing the evolutionary changes through the dinosaur genealogy.

It is important to dispel a common misconception about flying dinosaurs. Pterosaurs, those strange winged creatures, many with large crests and long beaks, that lived at the time of dinosaur dominance were not dinosaurs at all. As shown in the cladogram in Chapter 2, dinosaurs are clearly a member of a major radiation of terrestrial vertebrates, the Ar-chosauria, which includes living crocodiles, fossil pterosaurs, as well as a number of other extinct groups. Pterosaurs are, then, a side branch of ar-chosaurs outside the dinosaurs. The only flying dinosaurs are the birds.

For the question of bird origins we turn to the theropods. Within the theropods are several important branches, including the Coelurosauria and the giant stalkers like Tarbosaurus and Tyrannosaurus. One member of the coelurosaur radiation, the ostrich-like ornithomimids, is represented well by the gracile Gallimimus from the Nemegt. The fossils found in the Barun Goyot and Djadokhta Formations, however, belong to the other complex branch of coelurosaurs, the Maniraptora. Of the smallish, agile manirap-torans, dromaeosaurs like Velociraptor are the best known; they were the epitome of the lean and mean predator. In the sense of evolutionary endurance, dromaeosaurs, however, are overshadowed by another clade of maniraptorans, the Avialae. These are the specialized forms that take flight in the form of birds. Despite their extraordinary diversity and the radical alterations, maniraptorans share an unusual design in the wrist. A small wristbone or carpal has a distinctive semilunate, or crescent, shape that affects the way the wrist bends and articulates with the hand. In addition, the three fingers of the hand have a characteristic profile. Although these features are highly modified in living birds, they are found in the oldest bird, the Jurassic Archaeopteryx.

To get our bearings, let us reflect for a moment on the evolutionary organization of the maniraptorans. This limb of the dinosaur tree gives rise to at least five major branches: (1) the enigmatic Therizinosaurids; (2) the toothless, high-crested Oviraptoridae; (3) the highly specialized Troodon-tidae, with their elongate serpentlike skulls and rows of tiny, sharp, recurved teeth; (4) the predaceous Dromaeosauridae, including Velociraptor and kin, characterized by the retractable sickle claw on the hind foot and very sharp and formidable claws on the front hands (also in troodontids), as well as a stiff tail; and (5) the Avialae, the most specialized of the theropods, characterized by highly modified front appendages, shoulder and pelvic girdles that indicate the basic blueprint for flying birds.

A formal classification scheme designates modern birds as members of the group Aves within the group Avialae. As noted throughout this book, there is decisive evidence that, technically speaking, dinosaurs are not extinct. At least one of its lineages stays with us in the form of birds, a baroquely elaborate and diverse worldwide group of perchers, migrators, hoverers, and even flightless forms. There are a few detractors from this idea, scientists who still argue that birds and other dinosaurs do not share a close kinship. But these workers are clearly in the stubborn minority, and, as discussed below, their arguments are unconvincing. They remind me of a group of geologists who for years rejected the notion of plate tectonics because of a few problematic observations. No theory in science can be irrefutably proven, but the theory of bird-dinosaur affinity is one of the strongest connections in vertebrate evolution.

Aves is one of the most successful of all the vertebrates, with at least 8,600 living species. Doubtless much of the staging for the evolution of birds took place in the Mesozoic. In the Gobi, birds are not extravagantly represented by fossils. A smallish delicate form, Gobipteryx, has been collected in several sites by Mongolians, Russians, and our own field parties. Small ovoid eggs of this bird are particularly abundant in Kheerman Tsav, the brilliant chromatic and multicolored canyon in the frontier west of the Nemegt Valley. Many of these eggs have embryonic remains.

Those small eggs and skeletons of Gobipteryx are not much evidence of an avian renaissance during the Mesozoic. There are other regions that do better in providing these clues. The Cretaceous of Argentina, Spain, and China produces the short-winged enantiornithines, which vary from the sparrow-sized Sinornis to the turkey-vulture-sized Enantiornis. The Cretaceous badlands of Patagonian Argentina also gave us, in the mid

Cladogram Dinosaurs Birds Simple
Cladogram for theropods (EdHeck)

1980s, the stubby-winged flightless Patagopteryx. Since the 1880s, the later Cretaceous of North America has been generous in providing skeletons of the flightless loonlike Hesperornis, and the tern-sized, big-headed flier, Ichthyornis ("fish bird"). These birds populated ancient shorelines and doubdess hunted over the waters, nabbing fish between their long "beaks" armed with tiny, sharp teeth. The Mesozoic diversity and relations of birds are superbly summarized by Luis Chiappe in the June 1995 "dinosaur issue" of Natural History magazine.

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