The first fossil of a bilateral animal, Vernanimalcula, was discovered in China in 2005. It had a mouth, intestines, and sensory pits, and was likely tripoblastic and coelomate. Its discovery sheds light on the evolution of bilateral animals.
The first fossil body of a bilateral animal is Vernanimalcula (“small spring animal”), a simple metazoan that lived between 580 and 600 million years ago, during the Ediacaran period. Vernanimalcula was discovered in 2005 in a phosphate fossil deposit in Guizhou Province, China called the Doushantuo Formation. Phosphate fossils are extremely detailed, with microscopic-level features, allowing for analyzes not possible with other types of fossils. A minority of scientists argue that Vernanimalcula is a taphonomic artifact, caused by the intrusion of phosphate into a spherical inorganic or unicellular object, but the majority consider it a true fossil of a bilateral animal.
Most animals are bilateral, meaning they are symmetrical about a central axis. Exceptions are sponges, which have no symmetry, cnidarians (jellyfish and relatives), which have radial symmetry, and some echinoderms (starfish and relatives) with pentaradial symmetry, but evolved from bilateral ancestors. The rest of the animals are bilateral.
Vernanimalcula is a small (0.1 – 0.2 mm) oval-shaped fossil. It has a mouth, intestines, and several superficial pits which may be sensory organs. If it were indeed a real animal and not a taphonomic artifact, Vernanimalcula would have had a few hundred cells. It may have been eutelic, like many modern microfauna, meaning that each adult individual had the same number of cells and only grew in size as the cells expanded. Vernanimalcula is thought to have been tripoblastic, meaning that like most modern animals, its body consisted of three layers, an ectoderm, mesoderm, and endoderm.
The discovery of the first bilateral animals like Vernanimalcula has taught scientists some interesting facts about the evolution of bilateral animals in general. First, Vernanimalcula is a coelomate, meaning it possesses a body cavity. This is evidence that coelomates were the first multicellular animals, and acoelomates (such as nematodes) evolved from coelomates rather than vice versa.
There is some circumstantial evidence of bilateral tripoblastic animals even earlier than 600 million years ago, in the form of trace fossils from India and the decline of microbial towers called stromatolites over a billion years ago, suggesting the possible presence of grazing organisms, which they may have been bilateral animals.
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