Walker quaternary dating
Birdsbelonging to various lineages have been found in Cretaceous deposits of Asia, Europe, and North and South America. Prominent and well-known Cretaceous bird taxa included the Enantiornithes, a fairly diverse group of birds, mostly flying forms; Hesperornithiformes, toothed birds (see drawing below) which were mostly flightless swimmers; and Ichthyornithiformes, toothed flying birds that probably fed on fish. The fossils, in a new genus (Waimanu), provide a lower estimate of 61–62 Ma for the divergence between penguins and other birds and thus establish a reliable calibration point for avian evolution. Combining fossil calibration points, DNA sequences, maximum likelihood, and Bayesian analysis, the penguin calibrations imply a radiation of modern (crown group) birds in the Late Cretaceous. Cooperate or speciate: new theory for the distribution of passerine birds. However, recent evidence indicates that this scenario is likely incorrect: The prevailing view has been that ratites are monophyletic, with the flighted tinamous as their sister group, suggesting a single loss of flight in the common ancestry of ratites. (2008) conducted phylogenetic analyses of 20 unlinked nuclear genes and found a genome-wide signal that unequivocally places tinamous within ratites, making ratites polyphyletic and suggesting multiple losses of flight.
Yellow areas indicate the subclass Ornithurae; blue areas indicate the subclass Sauriurae (From: Feduccia 2003). Birds of tropical rainforest: comparative biogeography and ecology. 215-228 in Biogeography and ecology of forest bird communities (A.
The question of whether relatives of living birds co-existed with non-bird dinosaurs has evoked controversy.
Some investigators, using “molecular clock” models and DNA sequence data as well as the distribution of living birds, have concluded that relatives of living birds must have existed alongside non-avian dinosaurs and survived the mass extinction of dinosaurs at the K/T boundary.
The most plausible hypothesis requires at least three losses of flight and explains the many morphological and behavioral similarities among ratites by parallel or convergent evolution. Credibility intervals (95%) are indicated by grey bars at numbered internal nodes. Periods when Antarctica was ice-covered (black continuous bars) are indicated by shaded grey rectangles.
Finally, this phylogeny demands fundamental reconsideration of proposals that relate ratite evolution to continental drift. Ocean temperature is based on high-resolution deep-sea oxygen isotope records. Origin and expansion of penguins -- Classic problems in historical biogeography are where did penguins originate, and why are such mobile birds restricted to the Southern Hemisphere?
“We now know that duck and chicken relatives coexisted with non-avian dinosaurs.