Sex Life of One of Earth’s Earliest Animals Exposed

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Trilobites may be the most successful animal group ever to exist. These armored, pillbug-like arthropods, named for their distinctive three-lobed bodies, were the first hard-bodied animals to have ever lived on Earth. They appeared some 520 million years ago and dominated the fossil record of ancient seas for nearly 300 million years afterward. To date, paleontologists have uncovered a staggering 20,000 species, sporting every outlandish configuration of plates, spines and horns imaginable. Russell Bicknell, a paleobiologist from the University of New England, Australia, says, “It’s like Evolution, go home; it’s drunk.”

Despite their abundance, no one has been able figure out how trilobites reproduce–until now. Scientists have discovered a rare fossil that shows a tiny pair of grasping appendages which allowed the male trilobite to hold the female during mating.

Reproduction of extinct animals is notoriously hard to study, especially invertebrates. “Understanding reproductive behavior is difficult because it doesn’t preserve easily,” says Harvard University paleontologist Sarah Losso, one of the co-authors of the recent study, which was published in Geology. Trilobites’ delicate parts, such as antennae, legs, and reproductive structures, were made from soft tissues that are rarely petrified. Paleontologists can infer that legs exist based on sockets found in the outer shells of certain species and trace impressions. However, reproductive organs were difficult to find.

Another reason is scientific bias. Some researchers believed that trilobite reproduction would be the same as mammalian reproductive strategies. This led them to search for structures like a “trilobite penis,” which has yet to be discovered. Thomas Hegna, an invertebrate paleontologist from the State University of New York Fredonia, said, “That’s not appropriate.”

All that changed when Losso and her colleagues took a closer look at a 508-million-year-old trilobite fossil from the famous Burgess Shale deposit in British Columbia. The species, Olenoides serratus, is about as well-known as ancient arthropods get: it makes up many of the Burgess Shale’s fossils and the genus even lends its name to a Yu-Gi-Oh fantasy game card.

Losso had a specimen in an unusual location. It fossilized lying on its side rather than on its back or belly like most Olenoides fossils. Its appendages were preserved in incredible detail down to the joints. Researchers found two sets of grasping appendages, which looked a lot like reproductive structures known as claspers.

Claspers are a common reproductive feature in marine species. They allow male animals to keep their mate while they release sperm underwater. Claspers are found in shrimp, aquatic insects, and sharks.

Horseshoe and trilobites aren’t closely related. Scientists believe that the ancient arthropods may have occupied a niche similar to their modern counterparts, scooting along sandy bottoms like the ones found in the aqua Roombas and eating morsels that drift to seafloor. Bicknell, who wasn’t involved in the study, says that there were trilobite species which looked almost exactly like a horseshoe. Horseshoe crabs are used to model trilobites’ behavior and life cycle.

The looks of claspers in horseshoe crabs helped Losso and her team verify that the Olenoides appendages were more than just malformed feet. Claspers indicate that some species of male and female trilobites have different-looking bodies. Bicknell states that they have discovered a Rosetta stone fossil which allows us to confirm our theories about sexual dimorphism. “This is just an extremely nice, fundamental addition

Losso warns that this feature may not be universal. She says that “finding claspers in Olenoides Serratus doesn’t mean that all trilobites reproduced this way” The study is an important milestone in trilobite paleontology and will inform future research. It suggests that animals evolved more specialized limbs than previously thought, and this was done early in their evolutionary history. Hegna said, “It speaks to an extremely cool underlying modularity.” “They’re an armored Swiss Army knife.”

ABOUT THE AUTHOR(S)

    Joanna Thompson is an insect en

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