Turbellarian taxonomic database

Abstract / Notes

"Bilateria are currently subdivided into three superclades: Deuterostomia,
Ecdysozoa, and Lophotrochozoa. Within this new taxonomic frame, acoelomate
Platyhelminthes, for a long time held to be basal bilaterians, are now
considered spiralian lophotrochozoans. However, recent 18S rDNA [small subunit
(SSU)] analyses have shown Platyhelminthes to be polyphyletic with two of its
orders, the Acoela and the Nemertodermatida, as the earliest extant
bilaterians. To corroborate such position and avoid the criticisms of
saturation and long-branch effects thrown on the SSU molecule, we have searched
for independent molecular data bearing good phylogenetic information at deep
evolutionary nodes. Here we report a phylogenetic analysis of DNA sequences
from the myosin heavy chain type II (myosin II) gene from a large set of
metazoans, including acoels and nemertodermatids. Our study demonstrates, both
for the myosin II data set alone and for a combined SSU+myosin II data set,
that Platyhelminthes are polyphyletic and that acoels and nemertodermatids are
the extant earliest bilaterians. Hence, the common bilaterian ancestor was not,
as currently held, large and complex but small, simple, and likely with direct
development. This scenario has far-reaching implications for understanding the
evolution of major body plans and for perceptions of the Cambrian evolutionary
explosion."

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