Reconstructing the Oldest Creatures

 by Rowan Reddy

(source: Smith609
Wikipedia)

The first Cambrian animal to be discovered was likely one you’ve seen before - the trilobite. Trilobites were a diverse group of early arthropods, occupying many ecological niches. Some filter-fed on plankton, some scuttled along the ocean floor, and some even began to journey onto the land. Trilobites were certainly some of the most successful and numerous animals of the Cambrian era, but what was the first complex creature?

The best candidate for this is Charnia - a kind of frondy life-form resembling the sea pens that can still be found in temperate waters worldwide. Before Charnia, the only thing living in the cold ocean were mats of bacteria and algae. It was named after the site where the first fossil of it was found: Charnwood Forest in Leicestershire. 

Charnia lived in deep, cold water, and absorbed nutrients through its soft skin. It grew like a plant, by periodically adding new segments to its body. Later on in the Cambrian came Wiwaxia, a spiked invertebrate covered with hard scales. Wiwaxia was often preyed upon by Anomalocaris, a kind of metre-long shrimp, which had an unusual disc-shaped mouth, divided into three segments. This strange divided mouth was originally mistaken for strange compound eyes. 

Since Cambrian fossils are roughly 513 million years old, it’s hard to know what some of them are, and even harder to differentiate between members of the same species. Many animals in the Cambrian were soft-bodied or very small, so they are even easier to miss.

For example, the early worm Hallucigenia was originally reconstructed walking on the long spines on its back. Some thought it was actually the spiky appendage of a bigger animal, but finally it was reconstructed correctly as a lobopod walking on many tube feet, with long tentacles coming from its neck. A similar situation happened with Anomalocaris, which was once thought to be multiple creatures because of its mixture of mineralized and unmineralized body parts. The first Anomalocaris fossils - isolated frontal appendages -  were discovered in 1886, and thought to be the abdomens of crustaceans. It was finally identified as a single enormous creature in the 1980s, and recently (in 2020) a pair of compound Anomalocaris eyes were found in Australia, confirming that Anomalocaris was in fact an early type of arthropod. 

We still know very little about early animals, but the amount of knowledge we have gained since the discovery of the first trilobite fossil is incredible.