Some 10,500 bird species are inhabiting the
Earth today, but nearly two hundred species have already gone extinct in historic times as a result of human activities, primarily due to deforestation and sometimes hunting. Another 1,200 bird
species are considered to be under threat of extinction (link). The relationships among extant orders, which in birds are traditionally indicated by the latin suffix
-iformes, are shown in the following timetree:
Timetree showing phylogenetic interrelationships among avian orders. The timetree follows Jarvis et al. (2014), Claramunt & Cracraft
(2015), Prum et al. (2015), Ksepka et al. (2017), Reddy et al. (2017), Crouch et al. (2019), Field et al. (2019), Gilbert et al. (2019), Houde et al. (2019), and Kimball et al. (2019).
Supposed crown-group ages are indicated by blue lines, with details on crown-group ages being given in
the respective Individual orders
Modern or crown birds are represented by two principal clades, Palaeognathae and Neognathae, which diverged from each other during the Cretaceous, some 110 mya. The unresolved main radiations within Neognathae occurred around 65 mya, following the Chicxulub asteroid impact on the Yucatan peninsula in Mexico 66 mya. After the demise of non-avian dinosaurs, possibly as a result of the bolide impact, birds underwent a rapid diversification, a so-called radiation. This might explain the unresolved polytomy at the base of Neoaves. Moreover, the timetree supports radiation of all three major avian clades (Palaeognathae, Galloanseres, and Neoaves) near the Cretaceous-Paleogene (K-Pg) boundary.
Divergence time estimates are based on molecular dating techniques, which in turn depend on reliable fossil calibrations (i.e. correctly dated and taxonomically correctly placed fossils). The estimated divergence time of a given clade often differs significantly among authors, largely depending on the selection of fossils and the maximum age constraint set be the investigators (Cracraft et al., 2015).
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