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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.
The diagram shows three common ways of doing this. For previously defined clades A, B, and C, the clade X can be defined as: The three most common ways to define the name of a clade: node-based, branch-based and apomorphy-based definition. The tree represents a phylogenetic hypothesis of the relations of A, B and C.
A clade is characterized by one or more apomorphies: derived character states present in the first member of the taxon, inherited by its descendants (unless secondarily lost), and not inherited by any other taxa. Paraphyly: A paraphyletic assemblage is one that is constructed by taking a clade and removing one or more smaller clades. [32]
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 ...
Chrompodellids are a clade of unicellular protists containing two functionally different groups: the photosynthetic "chromerids" and the predatory phagotrophic "colpodellids". Like other Alveolata , they present tubular mitochondrial cristae and highly flattened cortical alveoli [ 8 ] with microtubules underneath.
For a given clade, any trait or feature (e.g. a specific phenotype) that appears in the clade's common ancestor; the same trait may also appear in some or all of the lineal descendants included within the clade, indicating that it has undergone little or no significant change during the clade's evolutionary history and thus retained its ...
The International Code of Phylogenetic Nomenclature, known as the PhyloCode for short, is a formal set of rules governing phylogenetic nomenclature.Its current version is specifically designed to regulate the naming of clades, leaving the governance of species names up to the rank-based nomenclature codes (ICN, ICNCP, ICNP, ICZN, ICVCN).
Their definition of Mammalia is roughly equal to the Mammaliaformes as defined by McKenna and Bell (1997) and other authors. They also define their taxonomic levels as clades and do not apply Linnean hierarchies. Mammalia †Sinoconodon - earliest and most basal of mammals; Unnamed clade 1 - a clade that contains all other mammals.