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From the elaborate branching antlers of a deer to the oversized claws of a fiddler crab, the animal kingdom is full of fascinating features used in battle to help secure a mate.
A team of researchers announced last week that it had found the earliest known evidence of sexual struggle in the form of a trident-headed trilobite, which prowled the ocean floor 400 million years ago.
Trilobites were one of the earliest arthropods, an invertebrate group consisting of insects, spiders, lobsters, crabs, and other organisms with exoskeletons, segmented bodies, and jointed limbs. These pill bug-like marine creatures first emerged 521 million years ago and became extinct 252 million years ago in the mass extinction that gave way to the dinosaurs.
There were more than 22,000 species of trilobite, some exceeding 2 feet in length, but the type that caught the eye of paleontologist Alan Gislick was more modest in size, about 2 to 3 inches. He recalls seeing specimens of Walliserops at fossil trade shows and being amazed at the trident-shaped protrusion from the trilobite’s head.
“It’s the type of structure that has to have a function. You don’t put that biological energy into something that doesn’t Anythingsaid Gishlick, an associate professor of paleontology at the University of Pennsylvania’s Bloomsburg.
Researchers have proposed various uses for these forking protrusions, including defense, hunting, and attracting mates.
in A paper published on January 17 in the scientific journal Proceedings of the National Academy of Sciences, Gishlick and co-author Richard Forti delve into these hypotheses, ruling out the trident as a means of defense or a hunting tool based on how the trilobite was able to move. The trident would not be of much use against predators attacking from above or from behind, and while it could have been used for spear hunting, the trilobite would then have been trapped with its food out of reach.
What mattered most to Gishlick and Forti, a paleontologist at the Natural History Museum in London, was that Wallisopterus used tridents to fight among themselves.
His thinking was bolstered by an unusual specimen of Walliserops, which had a deformed trident, with four prongs instead of the usual three. If the trident was an important part of day-to-day existence, he reasoned, then the trilobite probably wouldn’t have lasted long with a deformed one.
With evidence that Valliserops’ trident was used to win mates, researchers turned to the closest analog found in the modern world. Gislick said, “The structure reminds me a lot of Beatle horns.”
The researchers used a technique called landmark-based geometric morphometrics, which Gishlik described as a means of comparing complex shapes in a statistically robust way, to account for the surface-level similarity of trilobite tridents and the horns of Rhinoceros beetles. can be analysed. They found that the shape of trilobite tridents had a lot in common with the horns of beetles that flip their dueling partners in a “shovel” motion, unlike other species whose horns are better suited for fencing or catching.
Gishlik said he believed that, like beetles, trilobites’ tridents were “sexual weapons” used by males to win mates. “This is the earliest known structure that we can point to and say, ‘Yeah, I’m pretty sure this is an animal weapon used in breeding competition,'” he said.
In addition, Gishlick explained: “Generally, organisms that engage in interspecific fighting over mates are highly dimorphic”—different in appearance from one sex to another—”because only one competes, and usually Normally in the animal world he is a man.
Growing features such as large combat-ready horns requires a lot of energy, and it already costs female animals a lot to produce eggs.
If the tridents of trilobites are the first evidence of sexual weapons, they may also be the oldest known evidence of sexual dimorphism. There’s a problem with this hypothesis, though: Scientists have no definitive means of telling which Walliserops are male and which are female, and no trident-less Walliserops have been discovered.
This may be due to bias by fossil collectors, who Gishlik said often prefer larger, flashier specimens, or because females may be labeled as an entirely different species. “It makes it very clear to me that you are looking for better women,” Gishlik said.
Erin McCullough, an assistant professor of biology at Clark University in Massachusetts, said she agrees with the researchers’ conclusion that the trilobite trident was probably used for interstellar warfare. However, she wasn’t sold on their argument that it was a trait only men had.
“In general, if there’s going to be an extraordinary trait that’s used to fight for mates, usually, it’s males with that extraordinary trait, but biology is funny because there are always exceptions — females. Reindeer have antlers,” said McCullough, who was not involved in the study (but whose beetle analysis Gishlik and Forti drew from for their work).
“If they’re making the argument that these are men’s weapons used to gain access to women, that would have been a stronger story to me if they had evidence that women didn’t have weapons.”