Some 10,500 bird species are inhabiting the Earth today, but almost six hundred species have already gone extinct in historic times as a result of human activities, primarily due to deforestation and hunting (Sayol et al., 2020).
Modern or crown birds are represented by two principal clades, Palaeognathae and Neognathae, which diverged from each other around 94 mya. The 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.
Dated phylogenies, in which branch lengths are proportional to time, are usually referred to as chronograms or timetrees. The relationships among avian orders, which in birds are traditionally indicated by the suffix "iformes", are shown in the following timetree:
Timetree showing phylogenetic interrelationships among the 35 extant avian orders according to the results of Kuhl et al. (2021). Two newly recognized higher clades have not been named by the authors. Crown-group ages are indicated by blue lines. For those orders that contain more than just one family, crown group ages were derived from figure 3 in Kuhl et al. (2021).
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