Seeing ghosts: Neoproterozoic bilaterian body plans

Abstract

Abstract Bilaterians originated before 560 Ma. By 520–530 Ma, the fossil record reveals a fauna teeming with bilaterians with highly disparate body plans, in which most living phyla must have been represented. Between these dates, few bilaterian body fossil types have been found, although trace fossils indicate the presence of vermiform animals, small-bodied before the beginning of the Cambrian. The diversity of Cambrian lineages implies a branch-rich phylogenetic tree, yet organisms representing branch nodes of phyla and ancestral alliances of phyla are unknown, and most of the branches are inhabited by ‘ghosts’. The body plans of these ghosts must be precursory to the body plans of crown phyla, and their genomes must be precursory to crown genomes. Bilaterian Hox genes mediate anteroposterior regionation and are usually found in transcriptionally colinear clusters, a feature conserved even when clusters have been broken up during evolution. When body sections mediated by Hox genes are reduced during morphological evolution, the corresponding gene is sometimes deleted. It may be possible to reconstruct the anteroposterior regionation of ghost lineages from Hox clusters modifications. More information on Hox gene assemblages and functions in small-bodied crown phyla, such as acoelomorphs and others, is required to explore this possibility more fully.