Bird Evolution

One of the most famous fossils is Archaeop-teryx, the oldest known bird (Fig. 17.10). The first specimen was found in Upper Jurassic sediments in southern Germany in 1861, and was hailed as the ideal "missing link" or proof

Figure 17.7 Skull of the Late Cretaceous hadrosaur Edmontosaurus.

of evolution in action. Here was an animal with a beak, wings and feathers, so it was clearly a bird, but it still had a reptilian bony tail, claws on the hand, and teeth. Since 1861, nine more skeletons have come to light, the last two in 1992 and 2005.

Archaeopteryx was about the size of a magpie, and it fed on insects. The claws on its feet and hands suggest that Archaeopteryx could climb trees, and the wings are clearly those of an active flying animal. This bird could fly as well as most modern birds, and flying allowed it to catch prey that were not available to land-living relatives. The skeleton of Archaeopteryx is very like that of Deinony-chus (see Fig. 17.5a), especially in the details of the arm and hindlimb, showing that birds are small flying theropod dinosaurs.

Until recently, birds remained rare until the Late Cretaceous, but now numerous spectacular fossils of birds and dinosaurs, with feathers preserved, have been reported from China, and astonishing new specimens are announced every month (Box 17.3). In the Late Cretaceous, new sea birds radiated. They still had teeth, but the bony tail was reduced to a short knob as in modern birds, and they had other modern features. Modern groups of birds appeared in the latest Cretaceous and Early Tertiary, including flightless ratites and ancestors of water birds, penguins and birds of prey. The perching birds or songbirds, consisting of 5000 species today, radiated in the Miocene.

Box 17.2 Oldest dinosaur embryos

Dinosaur eggs and nests have been known since the 1860s when the first finds were made in France. The most famous finds were made by American expeditions to the Late Cretaceous of Mongolia in the 1920s, when whole nests with eggs were brought back to the American Museum of Natural History. Since then, hundreds of dinosaur nest sites have been found. There is no question that dinosaurs laid eggs, and they placed them in nests they kicked out of the dirt on the ground. Dinosaurs did not build nests in trees, for rather obvious reasons!

Maybe dinosaurs covered their nests with sand, and left them to develop, or maybe they covered them with plant debris, which formed a kind of compost to keep the babies warm, as some crocodil-ians do today. Some dinosaur mothers even incubated their eggs: a mother Oviraptor was found in the 1990s sitting on a nest in Mongolia.

Most spectacular of all are unhatched eggs containing embryos inside. The oldest examples were reported in 2005, when Robert Reisz and colleagues announced some eggs laid by the prosauropod Massospondylus in the Early Jurassic of South Africa. The researchers X-rayed the eggs, and saw the little bones inside, so Diane Scott, a skilled preparator, took a year of painstaking work to remove the eggshell on one side, and pick the mudstone off the bones with a fine needle, to reveal - a complete little baby, just about to hatch (Fig. 17.8). Reisz and colleagues believe the baby Massospondylus would have walked on all fours when it was born, but it had to be looked after. Its teeth were just too small for it to be able to tear up plants by itself. Is this the oldest evidence for parental care in the fossil record?

Read more about dinosaur eggs and nests at http://www.blackwellpublishing.com/paleobiology/. The full description is in Reisz et al. (2005).

Figure 17.8 Photograph of one of the Massospondylus eggs with a complete embryo skeleton inside, measuring some 15 cm in total length. It died just before hatching. As an adult, it would have grown to a length of 5 m. (Courtesy of Robert Reisz.)
Figure 17.10 The oldest bird, Archaeopteryx, from the Late Jurassic. (Courtesy of Andrzej Elzanowski.)
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