Stem Crown

Radiata

AV Simple Urbilateria

Advanced Bilaterian

AV Simple Urbilateria

Advanced Bilaterian

Figure 12.1 Conflicting phylogenies and scenarios on the nature and origin of the Last Common Ancestor (LCA) of the Bilateria, also featuring the extent of stem and crown groups. A, The complex Urbilateria scenario features a large, complex ancestor bearing most characters of present-day bilaterians (characters 1-9, and eventually characters 10-11 in Table 12.1). This ancestor originated from either an adult (polyp) or a larval radial cnidarian (archicoelomate theory, originally proposed by Sedgwick 1884). From this LCA evolved the more complex protostomes and deuterostomes. Note that all characters leading to the LCA are clumped at the stem. The large triangle indicates the diversification of crown bilaterians and its short height shows that its rate was fast (Cambrian 'explosion'?). B, The simple Urbilateria scenario features a small, simple LCA, similar to present-day acoelomorph flatworms, bearing a reduced set of characters (1-4 of Table 12.1) of extant bilaterians. This ancestor originated from radial planuloid ancestors similar to the planula larva of extant cnidarians (planuloid-acoeloid theory; for main references see Salvini-Plawen 1978 and Willmer 1990). From this ancestor evolved more complex bilaterians to be followed by the most advanced protostomes and deuterostomes. Note that the number of characters leading to the LCA are few, that time of diversification of crown bilaterians was longer and its rate slower than in the alternative scenario. A: anterior; AB: aboral; D: dorsal; O: oral; P: posterior; V: ventral.

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