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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.
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 ...
One study based on DNA analysis suggests that Scandentia and Primates are sister clades, but does not discuss the position of Dermoptera. [9] Although it is known that Scandentia is one of the most basal Euarchontoglires clades, the exact phylogenetic position is not yet considered resolved, and it may be a sister of Glires, Primatomorpha or Dermoptera or to all other Euarchontoglires.
Gnathifera is a member of Spiralia. It is the sister taxon of a clade comprising all other spiralians. [1] [19] An alternative phylogeny place Gnathifera into a main spiralian clade Platyzoa s.l. as sister clade to Mesozoa and Platyhelminthes. [21]
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The clade Scrotifera is a sister group to the order Eulipotyphla (true insectivores) based on evidence from molecular phylogenetics, [1] and together they make superorder Laurasiatheria. The last common ancestor of Scrotifera is supposed to have diversified ca. 73.1 [ 3 ] to 85.5 [ 4 ] million years ago.
Willi Hennig 1972 Peter Chalmers Mitchell in 1920 Robert John Tillyard. The original methods used in cladistic analysis and the school of taxonomy derived from the work of the German entomologist Willi Hennig, who referred to it as phylogenetic systematics (also the title of his 1966 book); but the terms "cladistics" and "clade" were popularized by other researchers.