Birds are among the most extensively studied of all animal groups. Hundreds
of academic journals and thousands of scientists are devoted to bird research,
while amateur enthusiasts (called birdwatchers or, more commonly, birders)
probably number in the millions.
Birds are categorised as a biological class, Aves. The earliest known species
of this class is Archaeopteryx lithographica, from the Late Jurassic period.
According to the most recent consensus, Aves and a sister group, the order
Crocodilia, together form a group of unnamed rank, the Archosauria.
Phylogenetically, Aves is usually defined as all descendants of the most
recent common ancestor of modern birds (or of a specific modern bird species
like Passer domesticus), and Archaeopteryx. Modern phylogenies place birds in
the dinosaur clade Theropoda.
Modern birds are divided into two superorders, the Paleognathae (mostly
flightless birds like ostriches), and the wildly diverse Neognathae, containing
all other birds.
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