The robust, calcite-infused exoskeletons of trilobites and their segmented shells are nearly ubiquitous in fossil deposits from the Cambrian to Permian period. But this treasure trove of trilobite fossils has revealed frustratingly little about how Paleozoic animals reproduced over 250 million years of life on ancient Earth.
A recently re-examined fossil from the Burgess Shale lifts the mystery of the sex life of ancient arthropods and reveals 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 likely helped males of a species of trilobite grasp females during copulation in a manner similar to that 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 tissues, one of the Olenoides the 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 specimen – missing most of the head and half of the body,” said Sarah Losso, Ph.D. Harvard candidate and author of the study.
The fragmented nature of the specimen was a stroke of luck as it revealed the anatomy often hidden beneath the shell of a trilobite. Importantly, nine of the arthropod’s rarely fossilized appendages have been preserved in detail. “There are millions of broken trilobites and you don’t see limbs in a lot of them,” said Javier Ortega-Hernández, co-author of the new study and Dr. Losso’s Ph.D. to advise. “Having this specimen broken in the right way so you can see these limbs is a one-in-a-million situation.”
Two sets of limbs stood out. The standard trilobite limb is segmented into three distinct parts – 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 noticeably distorted anatomy. Instead of having a spiny triangular protopodite for processing food, they had a smooth, rounded structure attached to a short, flexible, finger-like endopodite that was only half the length of the creature’s other walking legs. When the trilobite stood 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 odd morphology of these shriveled limbs was probably not caused by 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 the trilobites that swim on contemporary beaches today. Crabs are often used as a trilobite proxy due to their similar body shapes. They use grasping appendages, called clasps, to cling to a female’s spines, giving males an inside pathway to fertilize the female’s eggs as soon as she releases them from a compartment in her head. Because female trilobites probably practiced external fertilization as well, releasing their eggs into the swirling tide for males to fertilize, Olenoide trilobites may have used clamping appendages in the same way.
The structure and placement of these clasp-like appendages on the trilobite are slightly different from those of living horseshoe crabs. Instead of being near the head in contemporary species, the modified legs are along the midsection of the trilobite, which puts them in the perfect place to cling to the spiny back of the female.
But it is unlikely that all trilobites mated this way. “I think there’s probably real 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 trilobite egg group ever found. “I don’t think everyone was hanging on.”
Losso agrees that various species of trilobites likely used a variety of reproductive approaches beyond hugging. Many trilobites lack the spines of Olenoides, making them difficult to grasp.
But she thinks the identification of one of these sexual methods underscores that complex reproductive strategies evolved early on.
“This behavior of holding a female to be in place for fertilization evolved in the mid-Cambrian,” Ms Losso said. “Just a few million years after the first trilobites, they have already developed this surprisingly complex behavior.”