500-million-year-old 'star-like' mystery may never have been alive

In the latest research, high-resolution 3D imaging and chemical analysis have revealed Brooksella as a "pseudofossil." These are inorganic remnants, imprints, or markings that are commonly mistaken for real fossils. Pseudofossils have the ability to resemble living things by producing what appears to be intricate or well-organized structures.

While the previously-assumed ancestors lived on Earth throughout the middle Cambrian, no evidence of their production of star-shaped lobes was discovered. Experts' comparison of Brooksella to other concrete silica structures found in different Cambrian rock strata throughout the world was the only explanation that made sense.

"We did not find any difference between Brooksella and the concretions, other than Brooksella had lobes and the concretions did not," the researchers wrote.

"We thus concluded that Brooksella was not part of early sponge diversification in middle Cambrian seas, but rather, was an unusual type of silica concretion. Concretions can be all kinds of shapes to the point some look like they were organically formed," the paper also stated.

It's spongy and, unlike other fossils

Sally Walkers from the University of Georgia explained that Brooksella possessed a three-dimensional soft sponge shape, unlike the other fossils.

"A sponge usually gets flattened like roadkill during the fossilization process—especially a fossil more than 500 million years old! Also puzzling was the fact that no one inspected Brooksella where it lived and its orientation; if they did, they would find that most lobes were oriented downward, which does not make sense for a sponge to be eating mud," she added.

Researchers still try to solve the mystery of Brooksella. The study was published in PeerJ on February 24.

Study abstract:

First described as a medusoid jellyfish, the “star-shaped” Brooksella from the Conasauga shale Lagerstätten, Southeastern USA, was variously reconsidered as algae, feeding traces, gas bubbles, and most recently hexactinellid sponges. In this work, we present new morphological, chemical, and structural data to evaluate its hexactinellid affinities, as well as whether it could be a trace fossil or pseudofossil. External and cross-sectional surfaces, thin sections, X-ray computed tomography (CT) and micro-CT imaging, revealed no evidence that Brooksella is a hexactinellid sponge or a trace fossil. Although internally Brooksella contains abundant voids and variously orientated tubes consistent with multiple burrowing or bioeroding organisms, these structures have no relation to Brooksella’s external lobe-like morphology. Furthermore, Brooksella has no pattern of growth comparable to the linear growth of early Paleozoic hexactinellids; rather, its growth is similar to syndepositional concretions. Lastly, Brooksella, except for its lobes and occasional central depression, is no different in microstructure to the silica concretions of the Conasauga Formation, strongly indicating it is a morphologically unusual endmember of the silica concretions of the formation. These findings highlight the need for thorough and accurate descriptions in Cambrian paleontology; wherein care must be taken to examine the full range of biotic and abiotic hypotheses for these compelling and unique fossils.