Total group

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Total group (also total clade, stem clade, or panstem clade) is a term used in systematics to refer to a crown clade plus its stem group. It thus refers to all organisms sharing more recent ancestry with a group of living organism (extant members of the crown clade) than with any other living organisms. Basal members of total groups show few distinguishing features from related groups, and may thus be hard to assign to the total group. More derived members, however, show increasing numbers of synapomorphies of the crown group.

[edit] Terminology

When referring to a systematic grouping, there is a danger of confusion between the crown and total groups, although they mean different things. For example, simply referring to a fossil as a "bird" does not tell you whether it is a familiar example derived from the last common ancestor of all living forms, or whether it is a more basal form. (By definition, all living forms fall into the crown group.)

Since all total groups correspond to a crown group, some systematists have suggested using a standardized prefix (e.g., Pan-) to form names of total clades from the names of their corresponding crown clades. For example, Panmammalia (or Pan-Mammalia or pan-Mammalia) would be the total clade including Mammalia but no living non-mammals. Others have pointed out that there are already names for many total clades; for example, Synapsida and Theropsida have both been used to refer to the total mammalian clade.