For a long while, paleontologists have been trying to figure out what was up with Helmetia expansa, a prehistoric creature from before the dinosaurs that seemed like a crustacean but not. As PhysOrg explains, the first specimen was discovered in 1918, but it had never been illustrated or formally described … until now:
Helmetia expansa belongs to a rare group of early arthropods called concilitergans, close relatives of trilobites. Unlike trilobites, concilitergans lacked calcified exoskeletons, so their remains only fossilized under exceptional conditions—like those in the 508-million-year-old Burgess Shale of Canada, where even soft tissues like guts, legs, and gills were preserved.
Although more specimens were collected, only one individual of Helmetia expansa had been figured and no study examined additional material in detail to formally describe the species or clarify concilitergan evolution.
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Helmetia had a leaf-like exoskeleton, with some specimens preserving eyes, medial eyes, digestive systems and limbs. Early arthropods had limbs with a walking leg for locomotion and food capture, and a gill used for respiration. On the holotype, only the gills are visible—leading past researchers to believe Helmetia lacked legs and swam exclusively. But, the team found broad gills and walking legs in several specimens, showing that they likely walked like trilobites.
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“Molting strategies have never been known in any concilitergan,” said [Harvard evolutionary biologist Sarah] Losso. “All arthropods molt their hard exoskeletons to grow, but no one had seen this behavior before in a concilitergan because you have to catch a specimen in the act of molting, and it’s difficult to get just the right timing.”
The molting specimens show the new exoskeleton closer to the edge of the head, suggesting the animal exited from the front of the body—similar to horseshoe crabs, which use an anterior exit strategy, unlike most crabs that exit from the rear of the body.
The researchers also discovered a wide range of adult body sizes as Helmetia grew. While the smallest specimen was only 92 millimeters long, one exceeded over 180 millimeters.
“These patterns tell us not only how these 508-million-year-old animals grew, but how big they could get,” Losso said.
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You can read more about these ancient creatures here, in the Journal of Systematic Palaeontology.