Trilobites armed with triplets may be the first known example of sexual combat
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From the elaborately branching antlers of a deer to the massive claw of a fiddler crab, the animal kingdom is full of flashy features used in combat to help secure a mate.
A team of researchers announced last week that they’ve found the oldest known evidence of sexual combat in the form of a three-lobed, three-lobed-headed trilobite that ravaged the sea floor 400 million years ago.
Trilobites were among the first arthropods, the group of invertebrates that includes insects, spiders, lobsters, crabs, and other organisms with exoskeletons, segmented bodies, and jointed limbs. These cereal-like sea creatures first appeared 521 million years ago and vanished 252 million years ago in the mass extinction that gave way to the dinosaurs.
There were more than 22,000 species of trilobites, some reaching lengths of more than 2 feet, but the one that caught paleontologist Alan Jeslik’s attention was much more modest in size, about 2 to 3 inches. He remembers seeing specimens of Walliserops at fossil trade fairs and marveling at the trident-shaped outgrowth branching off the heads of trilobites.
“This is the kind of structure that has to have a function. You don’t put that much biological energy into something that doesn’t work Somethingsaid Jeslik, an associate professor of paleontology at Bloomsburg University in Pennsylvania.
Researchers have suggested various uses for these forked outgrowths, including defense, hunting, and attracting mates.
in A research paper published Jan. 17 in the scientific journal Proceedings of the National Academy of SciencesGeslik and co-author Richard Forte commented on these hypotheses, ruling out the trident as a defense or hunting tool based on how the trilobite was able to move it. A trident would be of little use against predators attacking from above or from behind, and while it could have been used to kill prey with a spear, a trilobite would be stuck with its meal out of reach.
What made the most sense to Jeslik and Forte, a paleontologist at the Natural History Museum in London, was that Walliserops used its trident to fight among themselves.
Their reasoning is reinforced by an unusual specimen of Walliserops with a deformed trident bearing four prongs instead of the usual three. If the trident is such a vital part of day-to-day survival, they reasoned, it’s likely that the trilobite wouldn’t last long with one deformity.
Bolstered by evidence that Walliserops used its trident to win mates, the researchers turned to the closest modern-world analogue they could find. “The structure reminds me of a large group of beetle antlers,” Jeslik said.
The researchers used a technique called feature-based geometric morphometry, which Gishlick described as a way to compare complex shapes in a statistically robust way, to analyze the surface similarity of trilobites and the horns of rhinoceros beetles. They found that the trilobite shape has a lot in common with horned beetles that flip their partners over in a “shoveling” motion, unlike other species whose horns are better for fencing or gripping.
Gishlick said he believed that, as in beetles, trilobites were “sex weapons” used by males in sparring to win over mates. “This is the first known structure that we can point to and say, ‘Yeah, I’m pretty sure this is an animal weapon used in reproductive competition,'” he said.
Furthermore, Gishlick explained, “In general, organisms that engage in interspecies combat over mates are largely dimorphic” — differing in appearance from one sex to another — “because only one does the competition, and generally in a world The animal is the male.”
Growing features such as large, battle-ready horns require a lot of energy, and female animals must actually expend a lot of it to produce eggs.
If trilobites are the first evidence of sexual arms, they may also be the first known evidence of sexual dimorphism. However, there is one problem with this hypothesis: scientists have no definitive means of telling which Walliserops is male and which is female, and Walliserops was never discovered without its trident.
This may be due to bias by fossil collectors, who Jeslik said often prioritize larger and flashier specimens, or because females may be classified as different species entirely. “It shows me that it’s better to look for females,” Jeslik said.
Erin McCullough, assistant professor of biology at Clark University in Massachusetts, said she agreed with the researchers’ conclusion that trilobites were likely used in interspecies combat. However, they were not sold on their argument that this was a trait that only males possessed.
McCullough said, “Generally, if there’s an extravagant trait that is used to fight for mates, it’s usually the males who have that extravagant trait, but biology is interesting because there are always exceptions—female reindeer have antlers.” He was not involved in the study (but the beetle analyzes that Jeslik and Forte benefited from in their work are).
“If they were arguing that these are male weapons used to get access to females, it would be a stronger story to me if they had evidence that females did not have weapons.”