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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.