Vernanimalcula

Vernanimalcula has been described as the earliest known member of the Bilateria (animals with bilateral symmetry). It lived some 580 to 600 million years ago. It was between 0.1 and 0.2 mm across (roughly the width of one or two human hairs), and probably fed on microbes on the sea floor. It may have moved over the sea floor by flexing its body. Vernanimalcula means "small spring animal", referring to its appearance in the fossil record at the end of the Marinoan Glaciation (see also Snowball Earth).



The Vernanimalcula fossils were discovered in the Doushantuo Formation in China. This formation is a Lagerstätte, one of the rare places where soft body parts and very fine details are preserved in the fossil record. The Vernanimalcula fossils show triploblastic structure, a coelom, a differentiated gut, a mouth, an anus, and paired external pits that could be sense organs.

The appearance of Vernanimalcula so early in the fossil record has important implications. It greatly reduces the likelihood that animals without coelom (acoelomates), such as flatworms, developed before animals with coeloms. The radiation of animals into many phyla may have occurred before any animal became much larger than microscopic size. The sudden appearance of many animal phyla in the Cambrian Explosion may be an illusion. The Cambrian Explosion may instead represent a (geologically) sudden increase in size and the development of easily fossilized body parts by species in existing phyla.

Criticism
The description of Vernanimalcula was not without controversy. Other workers (Bengtson and Budd) in the field have claimed that it is largely a taphonomic artefact generated by growth of phosphate within a spherical object such as an acritarch. Chen et al. have defended their interpretation of Vernanimalcula against the claims of Bengtson and Budd.