fossil birds since archaeopteryx cretaceous birds (135-65 mya) dinosaurs dominant land vertebrates....

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

• 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 Bipedal Carnivores

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 (1.5mya-10Kya)

• 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.