Cambrian fossils reveal new clues about arthropod evolution

The Cambrian explosion, over 500 million years ago, gave rise to many bizarre and complex life forms. Some were short-lived and mysterious, and vanished without a trace. Others left behind fossils so rare that generations of scientists could only speculate about their nature.

Among these elusive creatures was Helmetia expansa, an arthropod discovered in 1918 by Charles Doolittle Walcott.

Glimpse into a lost world

For more than a century, Helmetia remained an enigma. Only a single specimen had ever been illustrated. It appeared in research papers but was never formally described.

With no follow-up studies and no modern analysis, Helmetia existed more as a fossil footnote than a central figure in Cambrian biology.

Now, after 107 years, a new study has brought Helmetia back to life – figuratively speaking. Led by Sarah Losso at Harvard University, the research team has provided the first complete description of this strange animal. And what they found offers a detailed glimpse into a lost world.

Examining the Cambrian fossils

The new work is based on 36 specimens. They come from two major collections: the Smithsonian Institution and the Royal Ontario Museum.

These fossils were collected between 1975 and 1998 from Canada’s Burgess Shale, a site that is famous for preserving delicate body structures in extraordinary detail.

The team used high-resolution photography, cross-polarized light, and detailed morphological comparisons. For the first time, they could examine Helmetia’s anatomy with clarity.

This new approach revealed more than just exoskeletons and spines. It uncovered preserved guts, eyes, antennae, and even animals caught mid-molt.

“We need to study more than one specimen to see the species’ full range of morphology and preservation,” said Losso. This is especially important for species known only from flattened, soft-bodied fossils.

Surprises in the Cambrian fossils

Helmetia expansa had a broad, leaf-like body. It was oval in shape and reached lengths of 92 to 183 millimeters (3.6 to 7.2 inches). The body consisted of a cephalon, six thoracic segments, and a large pygidium.

Each thoracic segment bore spines that increased in size toward the rear. The exoskeleton’s edge showed fine serrations, a feature that is rarely preserved in such fossils.

At the front, Helmetia sported large anterolateral spines extending from the head. Between them sat a rounded sclerite – a hardened plate – beneath which lay two large, medial eyes.

Two lateral eyes were tucked under small domes on either side of the head. These features give Helmetia an alien appearance but also hint at visual complexity.

Beneath the surface, the researchers found further surprises. The gut was a narrow, straight tube with a J-shaped foregut and five pairs of digestive glands – oblong and ultrastructurally complex. These internal organs add depth to our understanding of Cambrian arthropod biology.

Movement of early arthropods

Early interpretations of Helmetia suggested a floating animal, perhaps drifting above the seafloor. Some even argued it lacked legs entirely.

But the new fossils proved otherwise. Several specimens preserved both gills and walking limbs, which were narrow, segmented appendages hidden under the body.

The limbs were biramous, meaning they branched into two parts. One acted as a leg for walking and grasping, the other functioned as a gill. This arrangement mirrors that of many early arthropods and suggests a crawling lifestyle, not a pelagic one.

The team concluded that Helmetia walked on the ocean floor, using its limbs to move and its gills to exchange gases.

Early molting strategies

Among the most remarkable finds were two Helmetia individuals preserved mid-molt. The fossils showed a second, fainter outline beneath the primary exoskeleton. This indicated that the animals were forming a new cuticle before shedding the old one.

“Molting strategies have never been known in any concilitergan,” said 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 team noted that the new exoskeletons were closer to the front of the body. This suggests Helmetia exited its molt through the head, much like modern horseshoe crabs. Most crabs exit from the rear, so this anterior strategy marks an important evolutionary detail.

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

Tiny and complex digestive glands

The internal anatomy of Helmetia included more than just a gut. Researchers found five pairs of digestive glands – small, ovoid structures with submillimetric lamellae. These paired organs are rare among concilitergans and were only previously known in Arthroaspis bergstroemi, a distant relative.

The researchers believe these glands helped process complex food. Their lamellar structure resembles those seen in other marine species that are predators or scavengers. In this light, Helmetia may have fed on soft-bodied prey or organic debris on the seafloor.

These gut glands also serve a deeper purpose. Their similarity to those in Arthroaspis helped confirm a close evolutionary relationship between the two species. Both are now considered part of the same broader group.

Significance of the Cambrian fossils

The new study also revised the family tree of the Conciliterga, the order of arthropods to which Helmetia belongs. Previous studies placed trilobites as close relatives. But with better data and a broader comparison set, the researchers now propose a new structure.

Arthroaspis emerged as the most primitive concilitergan. Helmetia, with its well-defined segments and spines, sits within the newly redefined family Helmetiidae. This group includes other species like Kuamaia, Haifengella, and Rhombicalvaria. These helmetiids share features like visible tergite boundaries and spiny bodies.

In contrast, a second group – Tegopeltidae – includes concilitergans with fused segments and no spines. Species like Tegopelte and Saperion fall into this category. The revised tree provides a cleaner classification and suggests that body segment fusion evolved multiple times.

Evidence of epimorphic growth

All studied Helmetia specimens had six thoracic segments. This means they were adults, fully developed in body structure. Yet, their sizes varied significantly, with the largest individuals being twice the length of the smallest.

This pattern suggests that Helmetia continued to grow during adulthood without adding new body segments – a process known as epimorphic growth. Whether they also experienced an earlier phase of segment addition, like many trilobites, remains unknown due to the lack of juvenile fossils.

The growth pattern seen here mirrors that of Arthroaspis, which also showed size doubling in adulthood. The consistency between the two strengthens their evolutionary connection.

Fossils of Cambrian arthropods

The resurrection of Helmetia expansa through detailed study marks a major leap in understanding Cambrian arthropods. Once an obscure name, Helmetia now stands as one of the best-documented concilitergans.

The study sheds light on its structure, growth, molting, and feeding. It redraws concilitergan evolution and confirms shared features among related species. Most importantly, the research reanimates a species long trapped in the pages of forgotten paleontology.

“Our findings give a much fuller picture of what Helmetia looked like, how it lived, and how concilitergans are related to each other,” Losso said.

Thanks to careful analysis, imaging advances, and renewed interest, Helmetia expansa has finally taken its rightful place in the story of life’s ancient past.

The study was published in the Journal of Systematic Palaeontology.

Image Credit: Marianne Collins

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