10-3 Birds

The archosaurs ruled the world after the Great Permian extinction (251 Ma). Scientists divide the archosaurs into two groups, archosaurs closer to dinosaurs and birds, and archosaurs closer to crocodiles. This section describes the evolution of the birds (Orthnithodira) within the Dinosauromorpha clade of the Archosauria.

Alfred Romer found the oldest dinosauromorph in the fossil record in Mid Triassic (236 – 234 Ma) strata in Argentina, Lagerpeton chanarensis (Figure 10‑10). It was 70 cm long and weighed 4 kg. Lagerpetonidae were also found in Arizona and New Mexico. Dromomeron was similar to Lagerpeton and is found in the Late Triassic strata (220 Ma) in Petrified Forest National Park in Arizona.

Figure 10‑10. Left. Lagerpeton chanarensis from the Mid Triassic. (236 – 234 Ma) in Argentina. Right. Dromomeron (220 Ma) from Chinle Formation in Petrified Forest. Image credits: Nobu Tomura. Used here per CC BY-SA 2.5

Figure 10‑11. Ornithischian “bird hip” (left) and early Saurischian “nonbird hip” (right). Image credits: Fred the Oyster. Used here per CC BY-SA 4.0

The Dinosauria included the Ornithischia and the Saurischia. The first species with a pelvic structure like birds were the Ornithischia (bird-hip on the left side of Figure 10‑11); however, they did not look like birds. For example, Stegasaurus was an Ornithischian. Some scientists think that the bird hip evolved four separate times in the Mesozoic Era. These scientists do not think that the fact that the Ornithischian hip is like a bird hip places them in the evolutionary line of birds.

Figure 10‑12. Skeletal restoration of Eodromaeus (230 Ma). Image credit: Kabacchi. Used here per CC BY-SA 2.0

The Saurischians evolved within the Dinosauroformes. The Theropods were a suborder of the Saurischians with hollow bones and three toes. They were light and fast. Some theropods grew extremely large, such as Tyrannosaurus Rex. Eodromaeus (Figure 10‑12) was an early Theropoda found in 231 Ma formation in Chile and has been called the “Eve” of dinosaurs. The forelimbs were small, and locomotion was purely from the hind legs (Figure 10‑13). It had the characteristic “bird hip.” It was a small carnivore and was able to run 30 MPH.

Figure 10‑13. Artist’s rendition of Eodromaeus (230 Ma). Image credit: Nobu Tomura. Used here per CC BY-SA 2.5

The Neotherapoda clade of the theropods began to exhibit birdlike features such as hollow bones and an S-shaped neck. The Coelophysoids within the Neotherapoda lived between 220 and 183 Ma. Thousands of skeletons of Coeleophysis bauri (Figure 10‑14) are in the late Triassic Chinle and Moenave formations of the Colorado Plateau.

Figure 10-16a. Coelophysis bauri (209 – 201 Ma) fossil from late Triassic of New Mexico. Credit: James St. John. Used here per CC BY-SA 2.0.

Figure 10-16b. Coelophysis bauri and human. Credit: Dr. Jeff Martz/National Park Service.

Figure 10‑15. Coelophysis kayentakatae (196 Ma). Credit: Dmitry Bogdanov. Used here per CC BY-SA 3.0

Coelophysis kayentakatae (196 Ma, Figure 10‑15) was in the Early Jurassic Kayenta formation (Figure 10‑33) on the Colorado Plateau. Dilophosaurus fossils (Figure 10‑16) were in the Early Jurassic Kayenta Formations (193 Ma) on the Colorado Plateau. They were large predatory dinosaurs (up to 7 m long), approximately the size of a brown bear, but they probably had feathers and looked like birds. They were between Coleophysoids and later theropods.

Figure 10‑16. Artist’s conception of Dilophosaurus wetherilli with hypothetical feathers. Credit: Leandra Walters. Used here per CC BY 2.5

Figure 10‑17. Scansoriopterygidae Yi qi (165 Ma) was a climbing and gliding Maniraptora. Credit: Emily Willoughby. Used here per CC BY 4.0

While archosaurs continued to evolve on the Colorado Plateau, the next phase of bird evolution was the maniraptora, which began with the Scansoriopterygidae Yi qi (Figure 10‑17) in the Late Jurassic Jiaojishan Formation in China. It would climb up trees and glide down. Its wings formed from membranes between elongated fingers and arms. Fossils left impressions of down-like feathers. There were long ribbon-like feathers in the tail. The head and neck feathers were long and thick. Based on this fossil, flight probably began with climbing and gliding.

Figure 10‑18. Eosinopteryx (160 Ma) Credit: El fosilmaniaco. Used here per CC BY 3.0

Eosinopteryx brevipenna (160 Ma) from a Late Jurassic formation in China looked like a bird (Figure 10‑18), but the bone structure indicates that it could not flap its wings. More detailed life reconstructions (images) are online.

Figure 10‑19. Aurornis (160 Ma or 125 Ma) Credit: El fosilmaniaco. Credit: El fosilmaniaco. Used here per CC BY 3.0

The nextstep in bird evolution was Aurornis, which was probably from a 160 Ma Late Jurassic formation in China. It was the same size as a pheasant (Figure 10‑19). Downy feathers covered the tail, chest and neck. It flew by flapping its wings. It is tentatively classified as the first bird.

The flying birds are within the Avialae clade. The earliest possible members (160 Ma) are in Late Jurassic strata in the Tiaojishan Formation in China. There are hundreds of fossils of Anchiornis Huxley, which had four feathered wings (Figure 10‑20, Figure 10‑21, and Figure 10‑22). The adults were too heavy to fly based on the ratio of weight to wing area so they must have glided. This causes some scientists to place them outside the Avialae clade.

Figure 10‑20. Anchiornis Huxley fossil. Beijing Museum of Natural History. Image credit: Jonathon Chen. Used here per CC BY-SA 4.0.

Figure 10‑21. Anchiornis Huxley Life Reconstruction (160 Ma) Credit: Nobu Tomura. Used here per CC BY-SA 2.5

Figure 10‑22. Skeletal restoration of Anchiornis Huxley (160 Ma) Credit: Scott Hartman. Used here per CC BY-SA 4.0

Archaeopteryx (150 Ma) is the first flying bird in the fossil record. Archaeopteryx (Figure 10‑23) was the ancestor of modern birds but was also similar to the velociraptors, which were feathered dinosaurs classified as dromaeosaurs. Archaeopteryx is so similar to the feathered dinosaurs that it has been classified as a dromaeosaur or vice-versa. They are also both classified as Maniraptora. The Compsognathidae (elegant jaw) lived in the Late Jurassic – Early Cretaceous (150.8 – 108 Ma) at the same time and general location as Archaeopteryx. Figure 10‑24 shows Archaeopteryx hunting Compsagnathus, which is sometimes classified as a maniraptor or as close to maniraptors, but did not fly.

Figure 10‑23. Left. Archaeopteryx reconstructed skeleton. Image credit. Jim the Photographer. Used here per CC BY-SA 2.0. Right. Achaeopteryx fossil. Berlin Museum of Natural History. Image credit. H. Raab. Used here per CC BY-SA 3.0

Figure 10‑24. Restoration of Archaeopteryx chasing a juvenile Compsognathus. Credit: Durbed. Used here per CC BY-SA 3.0.

The Enantiornithes (opposite birds) appear (145.5 – 66 Ma) at the beginning of the Cretaceous, four million years after Archaeopteryx. They are called opposite birds because their wing joints are reversed from that of modern birds. Although their overall appearance was similar to modern birds, they had clawed fingers, and they had teeth (Figure 10‑25).

Figure 10‑25. Reconstruction of opposite bird, Iberomesornis expuesto. Madrid Museum of Natural Science. Reconstruction by Jose-Manuel Benito Alvarez. Image credit. Locutus Borg. Public domain.

The first bird in the fossil record that did not have teeth was Confuciusornis sanctus, which appeared 125-120 Ma (Figure 10‑26).

Figure 10‑26. Life reconstruction of Confuciusornis. Credit: Velizar Simeonovski. https://peerj.com/articles/5831/. Used here per CC BY 4.0

The first bird that paleontologists classify as a modern bird is Gansus yumenensis (Figure 10‑27), which was found in a 120 Ma former lake bed.[1] Gansus had modern wing joints and evidence of webbed feet like a diving duck or loon, but unlike modern birds it had clawed wings. All birds in the modern world probably descended from Gansus and other birds in this period.

Figure 10‑27. Gansus yumenensis fossil specimen. Beijing Museum of Natural History. Credit Jonathan Chen. Used here per CC BY-SA 4.0.

The bird cousins, the dromaeosaurs, were dominant hunters during the Cretaceous period. Some of their fossil arms and tails have preserved long feathers, and some show short feathers covering the body. Others have evidence of the bumps where feathers were attached. Although they had wings, they could not fly. Rather, they ran and then glided to attack prey or evade predators (Figure 10‑28).

Figure 10‑28. The Dromeosaurs of the Cretaceous: Microraptor, Velociraptor mongolensis, Austroraptor cabazai, Dromeaosaurus albertensis, Utahraptor ostrommaysorum, and Deinonychus antirrhopus. Credit: Fred Wierum. Used here per CC BY-SA 4.0.

References

[1] 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. 2006. A Nearly Modern Amphibious Bird from the Early Creataceous of Northwestern China Science 312 (2006) no. 5780: 1640 - 1643

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Confucious Sanctus, early Cretaceous bird fossil. Credit: James St. John. Used here per CC SA 2.0.

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