The trilobites' shells are made of calcite and are found in many fossil deposits. There is a lot of trilobite fossils, but they don't reveal much about how the Paleozoic animals reproduced across 250 million years of living on ancient Earth.

Some trilobites are most likely to have a loving grip, as revealed by a recently re-examined fossil. The appendages that helped males of one trilobite species grasp females during copulation are similar to the appendages that helped horseshoe crabs.

Several spiny Olenoides serratus trilobites were examined by the team. Most were less than four inches long. The team at the Royal Ontario Museum in Toronto looked at the Olenoides specimen and it looked more like a broken lobster tail than an intact trilobite.

Sarah Losso is a PhD candidate at Harvard and an author.

It was a lucky break that the specimen was fragmented because it revealed the insides of the trilobite. There are millions of broken trilobites and you don't see limbs in many of them.

There were two sets of limbs. A walking leg and a gill structure are connected to the body by a spiny food-processing section. There were two appendages along the trilobite. They had a smooth, rounded structure attached to a short, flexible endopodite that was half the length of the creature's other walking legs. The trilobite's appendages would not have reached the seafloor if it were standing on its other legs.

The researchers were able to deduce that the strange limbs were probably not caused by an injury or regeneration.

The modified legs served a sexual purpose. horseshoe crabs are distant relatives of trilobites that swim contemporary beaches. The crabs are used as a trilobite proxy because of their similar body forms. They use gripping appendages, called claspers, to hook themselves onto a female's spine, giving males an inside track to fertilize the female's eggs as soon as she releases them from a compartment in her head. The Olenoides trilobites may have used clasping appendages in the same way that female trilobites used external fertilization.

The structure and placement of these appendages on the trilobite are different from horseshoe crabs. Instead of being near the head in the contemporary species, the modified legs are along the trilobite's midsection, which makes them the perfect spot to grab the female's spiny backside.

Thomas Hegna, a paleontologist at the State University, said that it isn't likely that all trilobites mate this way.

Ms. Losso agrees that different species of trilobites probably used different reproductive approaches. Many trilobites don't have the spine of Olenoides, making it hard to grasp.

She believes that identifying one of these sexual methods shows how complex reproductive strategies evolved early on.

The behavior of holding onto a female to be in place for fertilization evolved in the mid-Cambrian.