Archaeopteryx

In 1861, two years after the publication of Charles Darwin’s On the Origin of Species, a fossil filling the role of “missing link” between reptiles and birds was discovered in the Upper Jurassic limestones of Solnhofen, Bavaria. In fact, what had been found was the first known bird Archaeopteryx lithographica. The fine grained limstone was excavated to produce limestone slabs that could be etched with acid to make lithographic plates used to print illustrations, thus the species name lithographica (Prothero, 2007, p. 257). The Solnhofen limestone is a fossil lagerstatten that represents a Jurassic-aged shallow, saline lagoon community situated in a subtropical climate (Selden & Nudds, 2004, p. 107). Archaeopteryx is perhaps the most famous fossil organism from this deposit and represents an excellent transition fossil in time and structure between reptiles and birds.

Archaeopteryx is now known from eight skeletons and one feather impression (Benton, 2005, p. 257). As an intermediate, Archaeopteryx exhibits both reptilian and bird features. The reptilian features include: thecodont teeth, a long bony tail, a dinosaurian hand with three clawed fingers, and a dinosaurian foot. The bird-like characteristics include a furcula (collarbones fused into a wishbone) and the presence of asymmetric feathers (Prothero, 1998, pp. 374-375). Archaeopteryx was a pigeon-sized bird that lived in a subtropical environment. These birds had long legs for running and wings formed from forelimbs. Although Archaeopteryx did not have a keeled sternum like modern birds reconstructions of the flight muscles suggest that it possessed the power for takeoff and sustained flapping flight (Benton, 2005, p. 264). Many think that Archaeopteryx hunted for insects and could glide and climb among the trees.

Dinosaurs & Birds

Cladistic analyses favor that birds are derived theropod dinosaurs, most closely related to dromaeosaurids or deinonychosaurs (Benton, 2005, p. 261). The difficulty encountered in determining the proper taxonomic position of possible basal Cretaceous birds seems to only reinforce the theropod/bird connection. Rahonavis was a raven-sized dinosaur/bird from the Upper Cretaceous of Madagascar that had a reversed hallux (backwards pointing first digit) and papillae on the ulna for the insertion of wing feathers. It still retained a long tail like Archaeopteryx and possessed an enlarged sickle claw on the second toe like the dromaeosaurid Velociraptor. Jeholornis from the Lower Cretaceous of China is a turkey-sized bird that possessed a tail with feathers arranged in a fan. It had broad wings with asymmetrical wing feathers and the structure of the hand was more advanced than Archaeopteryx. The type specimen of Jeholornis has seeds of the conifer Carpolithus preserved in its crop. Rahonavis and Jeholornis represent the most basal Cretaceous birds.

Confuciusornis is a primitive crow-sized bird from the Early Cretaceous of China. The genus was named for the Chinese philosopher Confusius. Confuciusornithids may be the first birds to have a toothless beak. Confuciusornis had a slight keel and a more flexible wrist than Archaeopteryx. The tail was modified with the caudal vertebrae fused forming a pygostyle. The wing retained the three long fingers with claws like those of Archaeopteryx.

Enantiornithes

The order Enantiornithes represent the most diverse bird clade of the Cretaceous. These primitive birds were distributed worldwide and ranged form sparrow size to birds with wingspans of 1.5 meters. Enantiornithines were more advanced than Archaeopteryx and Confuciusornis but more primitive than modern birds. Most birds in this clade had teeth and retained the three-clawed fingers on the hand. Sinornis of China was a sparrow-sized bird with a larger ossified sternum and a pygostyle tailbone. Sinornis was capable of sustained flight as it hunted for insects. Sinornis had a toothed beak and retained the three-clawed fingers on its wing. Sinornis possessed a wrist joint that allowed it to fold the wings against its body and an opposite first toe for perching. Enantiornthines went extinct at the end of the Cretaceous.

Ornithurines

A second major clad of Mesozoic birds was the Ornithurines. Ornithurines are a sister taxa to the radiation that gave rise to the modern birds. Members of the order Hesperornithiformes were strong swimming predatory birds. These birds were flightless and propelled themselves through the water by kicking their feet. Hesperornithiformes had teeth lining their jaws, which helped secure the fish they captured. Coprolites of these organisms show their diet consisted of sea fish. Hesperornis and Baptornis are found in the Upper Cretaceous Niobrara Chalk Formation of Kansas, USA. Members of the order Ichthyornithiformes were strong fliers that also fed on fish. Ichthyornis of the Niobrara Chalk Formation of Kansas was a gull-sized bird. Like modern birds Ichthyornis had a deeply keeled ossified sternum, unlike modern birds it had jaws lined with teeth. It is thought that Ichthyornis caught fishes in the Great Interior Seaway by diving into the water from the wing (Benson, 2005, pp. 267-274).

Neornithes (Modern Birds)

The palaeognaths (Division Palaeognathae) are represented today by the small tinamous birds of South and Cental America and the ratites. Ratites are flightless birds including the ostriches of Africa, rheas of South America, emus of Australia, Cassowaries of New Guinea, and the kiwis of New Zealand. All of these birds evolved from a flying ancestor. The tinamous birds are the only living palaeognath representatives that retain flight. Some interesting extinct forms of the ratites, like the elephant bird of Madagascar, and the various moas of New Zealand are thought to have gone extinct due to human hunting.

The majority of living birds are neognaths (Division Neognathae). Most modern orders make their first appearance during the Eocene. Ducks and swans date back to the Eocene. One unusual flightless ancestor to ducks and game birds is the Diatryma, a flightless ratite-like bird that may have been a flesh eater. Diatryma lived in North American and Europe during the Palaeocene and Eocene. Cranes and rails date back to the Eocene. Pelicans and their close relatives date from the Eocene. Gulls and auks are known from the Eocene. Flamingos are known from the Eocene, while the oldest grebe is Miocene in age. Falcons, eagles, and vultures date back to the Eocene. Albatrosses are known from the Miocene, while penguins date to the Eocene. Hummingbirds date to the Pleistocene. Pigeons and doves are known from the Eocene. The dodo bird is a famous pigeon from the island of Mauritius that sailors hunted to extinction during the 1600’s. Parrots date back to the Eocene. Woodpeckers and toucans date to the Miocene. Finally, songbirds make their first appearance during the Oligocene.

Fossil evidence suggests that modern birds underwent two major adaptive radiations. The first, occurred during the early Paleogene, especially in the Eocene. Songbirds represent the second major radiation of modern birds, which occurred during the Miocene (Benson, 2005, pp. 277-287).

Science Olympiad Fossil Event

The 2016 Science Olympiad Fossil List includes the genus Archaeopteryx under the class Aves (Birds).

Benton, M.J. (2005) Vertebrate Palaeontology [3rd Edition]. Blackwell Publishing: Main, USA.

Prothero, D.R. (1998). Bringing Fossils to Life: An Introduction to Paleobiology. New York: McGraw-Hill.

Prothero, D.R. (2007). Evolution: What Fossils Say and Why It Matters. New York: Columbia University Press.

Selden P. & Nudds, J. (2004). Evolution of Fossil Ecosystems. Chicago: The University of Chicago Press.