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Link between ancient arthropod groups unearthed

Agnostinids and trilobites found to be long-lost relatives
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Joseph Moysiuk led research that uncovered the origins of agnostinids. PHOTO COURTESY OF JB CARON © ROYAL ONTARIO MUSEUM
Joseph Moysiuk led research that uncovered the origins of agnostinids. PHOTO COURTESY OF JB CARON © ROYAL ONTARIO MUSEUM

Agnostinids are a group of extinct early arthropods and have long confounded scientists on where they fit in the evolutionary tree.

PhD student Joseph Moysiuk and Associate Professor Jean-Bernard Caron from the Department of Ecology & Evolutionary Biology have discovered that agnostinids may be a sister group to trilobites. The findings were published in Proceedings of the Royal Society B on January 29.

The fossils used for the study come from the Burgess Shale fossil deposit in Kootenay National Park and Yoho National Park in British Columbia and are kept at the Royal Ontario Museum.

The Burgess Shale is significant due to its age and what specimens it preserves.

This fossil deposit dates back to the Cambrian Period, which was over half a billion years ago.

During this period, the Cambrian Explosion occurred, during which “the first representatives of most major living animal groups appear in the fossil record for the first time,” wrote Moysiuk in an email to The Varsity.

Furthermore, the Burgess Shale has preserved specimens exceptionally well, beyond just bones and shells, but also soft tissues such as nervous systems and digestive tracts as well.

Modern examples of arthropods include insects, crabs, and spiders.

Trilobites lived over 250 million years ago and, physically, “can be thought of as resembling a modern horseshoe crab, although they are at most very distant ‘cousins,’” wrote Moysiuk.

Evolutionary relationships can be traced by looking at shared characteristics between species. To determine where agnostinids fit with other arthropods, their fossils were used to note the similarities in “the armor and soft tissues of trilobites and other groups,” wrote Moysiuk.

While they share some similar characteristics with trilobites, agnostinids, which lived almost 100 million years ago, are eyeless and have dumbbell-shaped bodies.

Furthermore, there are six pairs of limbs located in the head of the agnostinid. In comparison, trilobites have four pairs and the mouthparts in a modern insect’s head are actually modified limbs.

The agnostinid specimen used in the study had preserved soft tissues, specifically the limbs. Moysiuk explained that the “first pair of limbs in agnostinids are long sensory antennae. These are followed by oar-like swimming appendages and several sets of walking legs.”

The relationship between the groups was established through a phylogenetic analysis, which “resulted in an evolutionary tree in which agnostinids and trilobites are more closely related to each other than to any other group,” added Moysiuk.

This finding led to a grouping in which agonostinids are part of a sister group to certain trilobites. The study found that the shared characteristics were of soft tissues and the armour — specifically the tergites, or shield-like plate covering, and cephalic shield, which covers the head area.

“The close relationship between trilobites and agnostinids, despite the difference in [the] number of head limbs, supports the notion that the composition of the head in early arthropod groups may have been evolutionarily variable, in contrast to its general stability in many modern groups like insects or spiders,” wrote Moysiuk.

Moysiuk noted that the best preserved specimen was from Marble Canyon, an area first mentioned in 2014 in Kootenay National Park. “This study is just one of a number of exciting discoveries that have come out of this site, with many more to come!”

Further research into the head region of arthropods and the development of early trilobites is needed to determine the relationship between the groups with greater specificity.