Artistic reconstruction of the Burgess Shale concilitergan Helmetia expansa. Credit: Marianne Collins.

New research has found that an ancient fossil sheds major light on the evolution puzzle by solving a hundred-year arthropod mystery.

For over a century, the Cambrian arthropod Helmetia expansa remained a mystery.

Discovered by palaeontologist Charles Doolittle Walcott in 1918, it was initially classified as a crustacean.

Despite frequent mentions in research papers, this species has never been formally described, and only one specimen has ever been illustrated.

Now, Harvard researchers led by Sarah Losso, postdoctoral fellow in the Department of Organismic and Evolutionary Biology, formally describe Helmetia expansa, offering new insights into its anatomy, behaviour and evolutionary relationships.

Holotype of Helmetia expansa USNM 83952, dorsal view. Credit: Image courtesy of Jean-Bernard Caron and Sarah Losso.

Exceptional conditions

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 fossilised 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.

Losso said: “We need to study more than one specimen to see the species’ full range of morphology and preservation.”

The team examined 36 specimens, at the Smithsonian Institution and the Royal Ontario Museum, from the Cambrian Period of the Burgess Shale of Canada.

They photographed the specimens, both wet and dry, using a polarising filter that can better reveal subtle features and extinction phases, and compared them to related species found in the Chengjiang biota in China, and one from the early Cambrian Sirius Passet in Greenland.

Concilitergan diversity and relationships. A,Arthroaspis bergstroemi, MGUH 30382, from the early Cambrian (Stage 3) Sirius Passet, Greenland. B,Helmetia expansa, USNM 83952, from the mid-Cambrian (Wuliuan) Burgess Shale, Canada. C,Kuamaia lata, CJHMD 00064, from early Cambrian (Stage 3) Chengjiang, China. D,Tegopelte gigas, USNM 189201, from the mid-Cambrian (Wuliuan) Burgess Shale, Canada. E, simplified topology based on results from phylogenetic analyses herein which resolved Arthroaspis bergstroemi as an early-branching concilitergan. Credit: Image courtesy of Jean-Bernard Caron and Sarah Losso.

Mystery

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 it likely walked like trilobites.

Even more surprising were two specimens caught in the early stages of moulting, a behaviour never before documented in concilitergans.

Losso said: “Moulting strategies have never been known in any concilitergan.

“All arthropods moult their hard exoskeletons to grow, but no one had seen this behaviour before in a concilitergan because you have to catch a specimen in the act of moulting, and it’s difficult to get just the right timing.”

The moulting specimens show the new exoskeleton closer to the edge of the head, suggesting the animal exited front 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.

Two groups

The researchers also discovered a wide range of adult body sizes as Helmetia grew. While the smallest specimen was only 92mm long, one exceeded over 180mm.

Losso said: “These patterns tell us not only how these 508-million-year-old animals grew, but how big they could get.”

Based on updated interpretations of Helmetia expansa’s morphology, the researchers confirmed two main helmetiids groups: the Helmediidae, which includes Helmetia expansa and is characterised by segment boundaries and side spines, and Tegopeltidae, which is marked by segment fusion and a lack of spines.

The researchers also assigned Arthroaspis bergstroemi, known from Greenland since 2013, to the group Conciliterga.

Losso said: “Our findings give a much fuller picture of what Helmetia looked like, how it lived, and how concilitergans are related to each other, which is very important for future studies on Conciliterga and other early arthropods.”

The study is published in the Journal of Systematic Palaeontology.