Stem group

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In palaeontology, a stem group is a systematic grouping that is required to accommodate fossils in the classification of organisms. A stem group lies basally to a crown group, consisting of its most closely related living relatives. Stem groups are by definition paraphyletic but can be defined because of the objective definition of the crown group they give rise to. The terminology of subdivision of stem groups has proved difficult. Jefferies proposed that the smallest paraphyletic units of the stem group were plesions; and that the crown group could be successively enlarged by addition of more and more basal plesions to form larger monophyletic groups called scions. However, this terminology (especially scions) has not widely been used.