Urbilaterian - Characteristics of The Urbilaterian

Characteristics of The Urbilaterian

The urbilaterian must have possessed all those traits common to living bilateria that are thought only to have evolved once. The defining features are a three-layered (triploblastic) embryo and two main axes of symmetry — a top-to-bottom axis and a front-to-back axis.

A traditional approach to reconstructing the urbilaterian considers it as a "roundish flatworm" consisting of as few features as possible. This concept of a very simple animal was abandoned when it was realised that the vast array of traits common to bilaterians would produce quite a complex animal, but has had a resurgence with the inclusion of a phylum of flatworms within the bilateria.

The urbilaterian is often considered to have possessed a gut and internal organs, a segmented body and a centralised nervous system, as well as a biphasic life cycle (i.e. consisting of larvae and adults) and some features of embryonic development. However, this need not necessarily be the case. If these features were of key importance to the evolution of large size, it is possible that bilaterian lineages each stumbled upon them independently, and perhaps even co-opted the same underlying genetic machinery from a different role.