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From a deer’s elaborate branching antlers to the fiddler crab’s oversize claw, the animal kingdom is full of flashy features used in combat to help secure a mate.
A team of researchers announced last week that it has found the earliest known evidence of sexual combat in the form of a trident-headed trilobite that scuttled the seafloor 400 million years ago.
Trilobites were one of the earliest arthropods, the group of invertebrates containing insects, spiders, lobsters, crabs and other organisms with exoskeletons, segmented bodies and jointed limbs. These pill bug-like sea creatures first emerged 521 million years ago and died out 252 million years ago in the mass extinction that gave way to the dinosaurs.
There were over 22,000 species of trilobite, some reaching lengths of more than 2 feet, but the type that caught the eye of paleontologist Alan Gishlick was more modest in size, around 2 to 3 inches. He recalls seeing specimens of Walliserops at fossil trade shows and marveling at the trident-shaped protrusion branching off the trilobites’ heads.
“That’s the type of structure that has to have a function. You don’t put that much biological energy into something that doesn’t do something,” said Gishlick, an associate professor of paleontology at Bloomsburg University of Pennsylvania.
Researchers have proposed various uses for these forking protrusions, including defense, hunting and attracting mates.
In a paper published January 17 in the scientific journal Proceedings of the National Academy of Sciences, Gishlick and coauthor Richard Fortey delved into these hypotheses, ruling out the trident as a means of defense or a hunting tool based on how the trilobite would have been able to move it. The trident wouldn’t be of much use against predators attacking from above or behind, and while it could have been used to spear prey, the trilobite would then be stuck with its meal just out of reach.
What made the most sense to Gishlick and Fortey, a paleontologist at the Natural History Museum in London, was that Walliserops used the trident to fight among each other.
Their thinking was bolstered by an unusual specimen of Walliserops with a deformed trident bearing four prongs instead of the usual three. If the trident was a vital part of day-to-day survival, they reasoned, then the trilobite probably wouldn’t have lasted for long with a malformed one.
Bolstered with the evidence for Walliserops’ trident being used to win mates, the researchers turned to the closest analogue they could find in the modern world. “The structure reminds me a heck of a lot of beetle horns,” Gishlick said.
The researchers used a technique called landmark-based geometric morphometrics, which Gishlick described as a means of comparing complex shapes in a statistically robust way, to analyze the surface-level similarity of trilobite tridents and horns of rhinoceros beetles. They found that the trilobite tridents’ shape had a lot in common with the horns of beetles that flip their dueling partners in a “shoveling” motion, as opposed to other species whose horns are better for fencing or grasping.
Gishlick said he believes that, like in beetles, trilobites’ tridents were “sexual weapons” used by males sparring to win mates. “This is the earliest known structure that we can point to and say, ‘Yeah, I’m pretty sure that this is an animal weapon used in reproductive competition,’ ” he said.
Furthermore, Gishlick explained: “Generally, organisms that are involved in interspecific combat over mates are highly dimorphic” — varying in appearance from one sex to the other — “because only one does the competition, and generally in the animal world that’s the male.”
Growing features such as big combat-ready horns requires a lot of energy, and female animals already have to expend lots of it to produce eggs.
If the trilobites’ tridents are the first evidence of sexual weapons, then they could also be the earliest known evidence of sexual dimorphism. There’s one problem with this hypothesis, however: Scientists have no definitive means of telling which Walliserops are male and which are female, and no trident-less Walliserops have been discovered.
That might be due to bias by fossil collectors, who Gishlick said often prioritize bigger, flashier specimens, or because the females might be labeled as entirely different species. “This to me makes it very clear that you better be looking for females,” Gishlick said.
Erin McCullough, an assistant professor of biology at Clark University in Massachusetts, said she agrees with the researchers’ conclusion that the trilobite tridents were likely used for interspecies combat. However, she’s not sold on their argument that this was a trait only possessed by males.
“In general, if there’s going to be an extravagant trait that’s used for fighting for mates, usually, it’s the males that have the extravagant trait, but biology is fun because there’s always exceptions — female reindeer have antlers,” said McCullough, who was not involved with the study (but whose beetle analyses Gishlick and Fortey drew upon for their work).
“If they are arguing that these are male weapons that are used to gain access to females, it would have been a stronger story to me if they had evidence that the females don’t have weapons.”