24 Feb. () –
Star-shaped markings and lobes from the Conasauga Formation of Alabama, about 514 million years ago and believed to be Cambrian fossils, they are actually accumulations of silica.
More than 100 years ago, Charles Doolittle Walcott of the Smithsonian Institution was asked to examine these pieces, known as Brooksella. He described them as jellyfish that likely floated in the mid-Cambrian seas of what is now the southeastern United States. Little did he know that the Cambrian fossil he named would spark more than 100 years of controversy.
The controversy revolved around the interpretation of what Brooksella really was: Was it really an important jellyfish to marine ecosystems in the Middle Cambrian, a time when animals first originated and diversified on Earth? Or was Brooksella nothing more than preserved gas bubbles? Or maybe it was a type of bulbous algae? Or a glass sponge made of opaline silica? Or, as he had thought, maybe Brooksella wasn’t a fossil at all.
Through chemical and shape analysis combined with high-resolution three-dimensional images, scientists have now assessed whether Brooksella was a fossil, like a sponge, a trace fossil, representing the burrows of worm-like animals, or not. “We found that Brooksella lacked the characteristics of glass sponges, specifically the opaline-fused spicules that make up the body. It also did not grow as one would expect a sponge to do throughout its life. More importantly, on the ground, its presumed excurrent channel (osculum) was always oriented downward in the sediment, which would make it very difficult – if not impossible – the percolation of water in search of food”, they explain in an article published in Peer J.
There was also no indication that the worms They will make the iconic star-shaped earlobes.
Brooksella’s composition and internal structure were then compared to silica concretions (accumulations) from the same Middle Cambrian bedrocks. “We found no difference between Brooksella and the concretions, Other than that Brooksella had earlobes and the concretions didn’t.”
“Thus, we concluded that Brooksella was not part of the early diversification of sponges in mid-Cambrian seas, but was instead an unusual type of silica accumulation. Concretions can come in all sorts of shapes, to the point that some appear to be organically formed.“, they point out.
The authors — led by Virginia Tech paleontology Ph.D. candidate Morrison R. Nolan — understand their find is twofold: First, there are numerous enigmatic Cambrian fossils that need to be examined to determine if they are indeed fossils and to help paleontologists to refine estimates of biodiversity for the Cambrian, when most of the major animal groups on Earth originated.
Secondly –they specify– This is not the first time that unusual fossils and rocks from the Cambrian have baffled scientists.and our findings highlight the need for close examination of early fossil materials, especially using powerful new analytical techniques such as microcomputed tomography in combination with classical laboratory and field methods.
