Fossil Birds since Archaeopteryx Cretaceous Birds 135 65

Fossil Birds since Archaeopteryx • Cretaceous Birds (135 -65 mya) • Dinosaurs dominant land vertebrates. • Concurrent with the dinosaurs in Cretaceous, see the rise and temporary dominance of toothed birds – Ichthyornithiformes – Hesperornithiformes – Enantiornithes

Cretaceous Birds • Ichthyornithiformes – tern-like – fish-eaters – modern flight adaptations (keeled sternum, loss and fusion of bones, etc. )

Cretaceous Birds • Hesperornithiformes – Large (up to 5 ft. long), flightless, foot-propelled divers. Fish-eaters. – Loon-like. – Vestigial wings and unkeeled sternum, but some fusion of bones indicates ancestry from flying birds. – Fossils of some forms occur in western SD.

Cretaceous Birds • Enantiornithes = the “opposite birds, ” called this because of a reversed pattern of fusion of tarsal elements relative to modern birds. – Show adaptive radiation in Cretaceous – Have well developed flight apparatus, skull with teeth and a primitive pelvic region – Formerly thought to represent early forms of many modern lineages; actually are a side branch, extinct by end of Cretaceous

Cretaceous Birds • Also present in late Cretaceous were Neornithine “transitional shorebirds” and some stem groups for modern birds (e. g. , Waterfowl and Galliformes) • Transitional shorebirds are thought to give rise to: – modern shorebirds – modern waders – modern waterfowl

Avian Phylogeny based on Feduccia (1995, 2003)

Tertiary Birds (65 -1. 5 mya) • Early to mid Tertiary (Paleocene into Miocene, 65 -15 mya) was time of major adaptive radiation of birds. • By the mid-Paleocene get first appearance of large, heavy-bodied, flightless predators to fill bipedal carnivore niche vacated by the disappearance of dinosaurs.

Tertiary Bipedal Carnivores • Gastornis (Diatryma) = Eocene of North America; 6 -7 feet tall; very heavy bodied, huge bill • Formerly classified with cranes, but appears related to Anseriformes; debated whether predatory or herbivorous – lacks pronounced hook on bill of predatory birds

Tertiary Bipedal Carnivores • Phororacids = middle Paleocene - Pliocene of S. America; 5 -8 feet tall; most with lighter build than Gastornis • Related to Gruiiformes • Note strongly hooked bill → predatory

Tertiary Bird Fossil Evidence • All present Orders of birds, except for the Passeriformes (perching birds) were present by end of Eocene (about 35 mya). • Passerines first appear unambiguously in fossil record in early Oligocene. • By middle of Miocene (about 15 mya) the majority of modern avian genera had appeared. • Molecular evidence suggests earlier radiation.

Pleistocene Period (1. 5 mya-10 Kya) • Pleistocene was period of dramatic climate change and glaciation. • In general was a period of decline and extinction for birds, but derivation of geographic races and speciation also occurred. • Some very large flightless birds also radiated during this period.

Pleistocene Flightless Birds • Moas = New Zealand; up to 9 -10 feet tall • Elephant Birds = from Madagascar; largest bird ever - up to > 1000 lbs.

Evolution of Flightlessness • Flight is expensive to maintain, both in terms of metabolism and embryology. When costs outweigh benefits, flightlessness may evolve. • Flightlessness has evolved many times in 13 avian Orders (always from flying ancestors). • Its evolution is usually associated with geographic isolation and relative absence of terrestrial predators.

Evolution of Flightlessness • Flightlessness occurs multiple times in: – Gruiformes (Phororacids, cranes, and especially rails) – Podicipediformes (grebes) – Anseriformes (Gastornis, waterfowl) – Columbiformes (pigeons, doves, dodo)

Modifications Associated with Flightlessness • Reduction of muscle and bones of wing and pectoral girdle • Loss of keeled sternum • Tendency toward large size • These modifications serve to save energy and all flightless birds show these tendencies.

Modifications Associated with Flightlessness • Flightlessness (and associated modifications) generally comes about through arrested embryonic development (Neoteny = delayed somatic development while reproductive organs mature a normal rate). • Flightless birds are characterized by a number of neotenic features.

Neotenic Features of Flightless Birds • Sternum late to form and ossify in rails, pigeons and grebes (where flightlessness has evolved multiple times). No flightless Galliforms exist these develop sternum very early in embryonic development, so can’t have arrested sternal development without profound effects on the development of the bird as a whole. • Sternum remains unkeeled in flightless birds.

Neotenic Features of Flightless Birds • Obtuse angle between scapula and coracoid. Through embryonic development in flying birds, passes through obtuse to acute angle. Arrested development fixes obtuse angle. • Unossified ilioischiatic fenestra in pelvis. • Skull bones are sutured rather than extensively fused, as in adults of flying forms.