The sturdy, calcite-infused exoskeletons of trilobites and their segmented shells are nearly ubiquitous in fossil deposits from the Cambrian Period to the Permian. But this trove of trilobite fossils has revealed frustratingly little when it comes to how the Paleozoic animals reproduced across 250 million years of living on ancient Earth.
A recently re-examined fossil from the Burgess Shale is pulling back the shroud of mystery over the ancient arthropods’ sex lives, and revealing that some trilobites most likely sported a loving grip. In a study published Friday in Geology, paleontologists at Harvard University identified a pair of modified appendages that probably helped males of one trilobite species grasp females during copulation in a similar manner to that of modern horseshoe crabs.
The team examined several spiny Olenoides serratus trilobites collected from the Cambrian site. Most were just under four inches long. While the Burgess Shale is known for its detailed preservation of even delicate tissue, one of the Olenoides specimens the team examined at the Royal Ontario Museum in Toronto looked more like a badly broken lobster tail than an intact trilobite.
“It’s a sad looking specimen – it’s missing most of the head and half of the body,” said Sarah Losso, a Ph.D. candidate at Harvard and an author of the study.
The fragmented nature of the specimen was a lucky break because it revealed the anatomy often hidden under a trilobite’s shell. Importantly, nine of the arthropod’s rarely fossilized appendages were preserved in detail. “There are millions of broken trilobites and you don’t see limbs in many of them,” said Javier Ortega-Hernández, a co-author on the new study and Ms. Losso’s Ph.D. adviser. “Having this specimen that is broken in just the right way that you can see these limbs is a one in a million sort of situation.”
Two sets of limbs stood out. The standard trilobite limb is segmented into three distinct portions – a walking leg, or endopodite, and a gill structure, the exopodite, are connected to the body by a spiny food-processing section, the protopodite. But two appendages along the trilobite’s midsection had a noticeably warped anatomy. Instead of having a spiny, triangular protopodite for food processing, they had a smooth, rounded structure attached to a short, flexible fingerlike endopodite that was just half the length of the creature’s other walking legs. When the trilobite was standing on its other legs, these modified appendages would not have reached the seafloor.
The researchers were able to deduce that, based on their growth patterns, the strange morphology of these shriveled limbs was probably not caused by an injury or regeneration.
Instead, Ms. Losso and Dr. Ortega-Hernández concluded that these modified legs served a sexual purpose. They based their hypothesis on horseshoe crabs, distant relatives of trilobites that swim contemporary beaches today. The crabs are often used as a trilobite proxy because of their similar body forms. They use gripping appendages, called claspers, to hook themselves onto a female’s spines, giving males an inside track to fertilize the female’s eggs as soon as she releases them from a compartment in her head. Because female trilobites also probably practiced external fertilization, emitting their eggs into the swirling tide for males to fertilize, the Olenoides trilobites may have used clasping appendages in the same way.
The structure and placement of these clasper-like appendages on the trilobite are slightly different from those of living horseshoe crabs. Instead of being near the head in the contemporary species, the modified legs are along the trilobite’s midsection, which places them in the perfect spot to latch onto the female’s spiny backside.
But it isn’t likely that all trilobites mated this way. “I think there’s probably a genuine variation in how trilobites mate,” said Thomas Hegna, a paleontologist at the State University of New York at Fredonia who was part of a team that described the first cluster of trilobite eggs ever found. “I don’t think everybody was clasping.”
Ms. Los agrees that diverse species of trilobites probably used a variety of reproductive approaches beyond clasping. Many trilobites lack the spines of Olenoides, making them difficult to grasp.
But she believes identifying one of these sexual methods underscores that complex reproductive strategies evolved early on.
“This behavior of holding onto a female to be in place for fertilization evolved by the mid-Cambrian,” Ms. Losso said. “Only a few million years after the first trilobites, they’ve already developed this unexpectedly complex behavior.”