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The term sister group is used in phylogenetic analysis, however, only groups identified in the analysis are labeled as "sister groups".. An example is birds, whose commonly cited living sister group is the crocodiles, but that is true only when discussing extant organisms; [3] [4] when other, extinct groups are considered, the relationship between birds and crocodiles appears distant.
The crown age of a clade refers to the age of the most recent common ancestor of all of the species in the clade. The stem age of a clade refers to the time that the ancestral lineage of the clade diverged from its sister clade. A clade's stem age is either the same as or older than its crown age. [15] Ages of clades cannot be directly observed.
Parahoxozoa was defined by the presence of several gene (sub)classes (HNF, CUT, PROS, ZF, CERS, K50, S50-PRD), as well as Hox/ParaHox-ANTP from which the name of this clade originated. It was later proposed [ 12 ] [ 13 ] and contested [ 14 ] that a gene of the same class (ANTP) as the Hox/ParaHox, the NK gene and the Cdx Parahox gene, is also ...
Branches down to the divergence to the next significant (e.g. extant) sister are considered stem-groupings of the clade, but in principle each level stands on its own, to be assigned a unique name. For a fully bifurcated tree, adding a group to a tree also adds an additional (named) clade, and a new level on that branch.
In this diagram, the clade labelled "Neornithes" is the crown group of birds: it includes the most recent common ancestor of all living birds and its descendants, living or not. Although considered to be birds (i.e. members of the clade Aves), Archaeopteryx and other extinct groups are not included in the crown group, as they fall outside the ...
In terms of mtDNA, however, it appears that modern humans and Neanderthals form a sister clade, with Denisovans as basal outgroup. The split of Neanderthal and modern human mtDNA is dated to about 498–295 kya, i.e. significantly younger than the date estimated based on nuclear DNA.
Phylogenetic studies originally recovered Silesauridae as a clade sister to Dinosauria, including a variety of Triassic taxa related to Silesaurus. Sometimes the genera are recovered as successive sister taxa to dinosaurs rather than a clade, but often a group is recovered as possibly quadrupedal herbivorous taxa. [13]
Obazoa is a proposed sister clade of Amoebozoa (which together form Amorphea). The term Obazoa is based on the OBA acronym for O pisthokonta , B reviatea , and A pusomonadidae , the group's three constituent clades .