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Reviving a Cambrian Mystery: The Formal Description of Helmetia expansa

For more than a century, the Cambrian arthropod Helmetia expansa has puzzled scientists. Initially discovered by paleontologist Charles Doolittle Walcott in 1918 and classified as a crustacean, this species remained largely enigmatic. Despite being frequently referenced in various academic studies, it had never been provided a formal description, and only a single specimen had ever been illustrated.

A recent study published in the Journal of Systematic Palaeontology has changed that. Researchers from Harvard University, led by Sarah Losso, a postdoctoral fellow in the Department of Organismic and Evolutionary Biology, have formally described Helmetia expansa, unveiling new insights into its anatomy, behavior, and evolutionary ties.

Helmetia expansa is part of a unique class of early arthropods known as concilitergans, which are related to trilobites but differ significantly in their lack of calcified exoskeletons. Consequently, fossils of concilitergans only form under exceptional conditions, such as those found in the 508-million-year-old Burgess Shale in Canada, where even delicate structures like guts, legs, and gills have been preserved.

While several specimens were collected over the years, only one had been depicted previously. No comprehensive studies had been conducted to explore additional specimens in detail to clarify the evolution of concilitergans or to formally classify the species. “Studying multiple specimens is essential to appreciate the full extent of morphology and preservation in this group,” Losso emphasized.

The research team meticulously examined 36 specimens housed at the Smithsonian Institution and the Royal Ontario Museum, all originating from the Cambrian Period of the Burgess Shale. They documented the specimens through wet and dry photography using a polarizing filter designed to highlight minute details and variations in the fossilization process, comparing them to analogous species from the Chengjiang biota in China and one from Greenland’s early Cambrian Sirius Passet.

Helmetia featured a leaf-like exoskeleton; some specimens even retained impressions of eyes, digestive systems, and limbs. Early arthropods possessed limbs specialized for locomotion and food acquisition, alongside gills for breathing. Previous interpretations of the holotype indicated that only gills were visible, leading to the assumption that Helmetia lacked legs and relied solely on swimming. However, the newer findings revealed broad gills alongside walking legs in several specimens, suggesting that these creatures likely walked similarly to trilobites.

Perhaps the most remarkable discovery was the documentation of two specimens in the early stages of molting, a behavior that had never before been recorded in concilitergans. “We did not have evidence of molting strategies in any concilitergan,” noted Losso. “While all arthropods molt their hard exoskeletons to grow, observing this behavior in concilitergans was unprecedented. It requires capturing a specimen precisely at the right moment.” The molting specimens displayed an emerging exoskeleton at the forefront of the head, indicating that the animal exited from the anterior end, similarly to horseshoe crabs, contrasting with typical crabs that shed from the rear.

The study also revealed a diverse range of adult body sizes as Helmetia matured. The smallest documented specimen measured just 92 millimeters, whereas one specimen surpassed 180 millimeters in length. “These observations not only inform us about growth patterns in these ancient organisms but also provide insights into their potential size,” Losso remarked.

Based on new interpretations of Helmetia expansa’s anatomy, the team classified two distinct groups within helmetiids: Helmediidae, which includes Helmetia expansa characterized by segment boundaries and lateral spines, and Tegopeltidae, identified by segment fusion and absence of spines. The researchers also included Arthroaspis bergstroemi, known from Greenland since 2013, within the Conciliterga group.

“Our findings offer a more comprehensive understanding of Helmetia‘s anatomy, its lifestyle, and the relationships among concilitergans,” concluded Losso. “This is crucial for ongoing research into Conciliterga and the broader landscape of early arthropods.”

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