Over a century ago, Charles Doolittle Walcott of the Smithsonian Institution embarked on an investigation of peculiar star-shaped fossils with lobes discovered in the approximately 514-million-year-old Conasauga Formation in Alabama. These fossils, known as Brooksella, became the center of a heated debate that spanned more than a century.
The crux of the controversy revolved around the true nature of Brooksella. Was it truly a jellyfish that held significance for the marine ecosystems of the middle Cambrian period, a crucial time when Earth’s animal life was originating and diversifying? Or were the fossils merely preserved gas bubbles, bulbous algae, or glass sponges composed of opaline silica?
Alternatively, some hypothesized that Brooksella might not even be a fossil.
To shed light on the matter, we conducted a comprehensive analysis using shape and chemical assessments, as well as high-resolution 3D imaging. Our aim was to determine if Brooksella was a sponge-like fossil, a trace fossil representing worm-like animal burrows, or something entirely different. Our investigations revealed that Brooksella did not exhibit the characteristic features of glass sponges, particularly the fused spicules that form their bodies. Additionally, it did not display the expected growth patterns observed in sponges over their lifetimes. Significantly, we observed that the purported excurrent canal (osculum) of Brooksella, as found in the field, was consistently oriented downward in the sediment, making it extremely challenging, if not impossible, for the organism to filter water for sustenance.
Furthermore, we found no evidence indicating that worms were responsible for creating the distinctive star-shaped lobes. In order to gain further insights, we compared the composition and internal structure of Brooksella with silica concretions found in the same middle Cambrian rock formations. Apart from the presence of lobes in Brooksella and their absence in the concretions, we discovered no discernible disparities between the two. Consequently, we concluded that Brooksella did not represent an early phase of sponge diversification during the middle Cambrian era but rather constituted an unusual type of silica concretion. Concretions, known for their diverse shapes, can occasionally mimic organic formations.
The significance of our findings is twofold. Firstly, there exist numerous puzzling Cambrian fossils that necessitate rigorous scrutiny to determine their true nature. This process is vital for refining estimates of biodiversity during the Cambrian period, which witnessed the emergence of most major animal groups on Earth. Secondly, our research underscores the importance of closely examining early fossil materials, especially utilizing advanced analytical techniques like micro-CT, in conjunction with traditional laboratory and field approaches. This is particularly relevant when investigating unusual fossils and rocks from the Cambrian, which have frequently confounded scientists throughout history.
Morrison Nolan, a geoscientist from Virginia Tech, expresses his fascination with Brooksella alternata due to the diverse and conflicting conclusions reached by scientists in their attempts to identify it. He highlights the challenges involved in distinguishing different forms of life and even differentiating life from non-life, particularly when dealing with early materials in the geological record. Nolan acknowledges the valuable contributions of amateur paleontological and geological groups like the Georgia Mineral Society in educating the public about the geological past and showcasing intriguing features.
Sally Walker, a paleontology professor at the University of Georgia, shares her intrigue with Brooksella, noting its unique 3D star-shaped structure, which is uncommon for soft-squishy animals like sponges. She highlights the puzzling aspect of the fossil’s preservation, considering that sponges tend to be flattened during the fossilization process, especially when they are over 500 million years old. Walker also points out the curious orientation of Brooksella in its natural habitat, with most lobes directed downward, which contradicts expectations for a sponge feeding on mud. Additionally, she poses the question of the physical, chemical, and possibly biological processes responsible for the formation of these enigmatic Brooksella concretions, leaving it as a challenge for future paleontologists to unravel.
James Schiffbauer, an associate professor of geological sciences at the University of Missouri, emphasizes the emerging potential of microCT technology in unraveling fossil mysteries. He mentions that while microCT has found extensive applications in materials sciences and engineering, its capacity for elucidating the fossil record is only just beginning to be explored. Schiffbauer believes that this project exemplifies the power of microCT in solving enigmas related to fossils. By scrutinizing the internal structure of Brooksella in relation to its various interpretations over time, it becomes increasingly evident that none of these interpretations truly align with the evidence.