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From the elaborate branching antlers of a deer to the oversized claws of the fiddler crab, the animal kingdom is full of striking features used in combat to help secure a mate.
A team of researchers announced last week that they found the oldest known evidence of sexual combat in the form of a trident-headed trilobite that sank to the seafloor 400 million years ago.
Trilobites were one of the earliest arthropods, the group of invertebrates that contains insects, spiders, lobsters, crabs, and other organisms with exoskeletons, segmented bodies, and jointed limbs. These woodlice-like sea creatures first emerged 521 million years ago and went extinct 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 type that caught paleontologist Alan Gishlick’s eye was more modest in size, around 2 to 3 inches. He remembers seeing Walliserops specimens at fossil trade shows and marveling at the forking trident-shaped bulge on trilobite heads.
“That’s the kind of structure a function has to have. You don’t put as much biological energy into something that doesn’t do somethingsaid Gishlick, an associate professor of paleontology at Bloomsburg University of Pennsylvania.
Researchers have proposed various uses for these forked protrusions, including defense, hunting, and mate attraction.
In an article published on January 17 in the scientific journal Proceedings of the National Academy of Sciences, Gishlick and co-author Richard Fortey elaborated on 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 throw prey, the trilobite would get stuck with its food just out of reach.
What made the most sense to Gishlick and Fortey, a paleontologist at London’s Natural History Museum, was that Walliserops used the trident to fight each other.
His thought was bolstered by an unusual Walliserops specimen with a misshapen trident that had 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 long with a malformation.
Bolstered by evidence of Walliserops’ use of the pitchfork to gain a mate, the researchers turned to the closest analogue they could find in the modern world. “The structure reminds me of a lot of beetle horns,” Gishlick said.
The researchers used a technique called landmark-based geometric morphometry, which Gishlick described as a means of comparing complex shapes in a statistically sound way, to analyze the surface-level similarity of trilobite tridents and rhinoceros beetle horns. . They found that the shape of trilobite tridents had much in common with the horns of beetles that flipped their dueling partners in a “shoveling” motion, unlike other species whose horns are better for wielding or grasping.
Gishlick said he believes that, like beetles, trilobite pitchforks were “sexual weapons” used by males fighting to gain a mate. “This is the oldest 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: “Generally, organisms that are involved in interspecific combat in pairs are highly dimorphic” (varying in appearance from one sex to the other) “because only one competes, and usually in the animal world that is the male.” . ”
Growing features such as large combat-ready horns require a lot of energy, and females already have to expend a lot of energy to produce eggs.
If trilobite pitchforks are the first evidence of sexual weapons, they might also be the oldest known evidence of sexual dimorphism. However, there is a problem with this hypothesis: scientists have no definitive means of knowing which Walliserops are male and which are female, and no tridentless Walliserops have been discovered.
That could be due to the bias of fossil collectors, who Gishlick says often prioritize larger, flashier specimens, or because females may be labeled as completely different species. “This to me makes it very clear that you’re better off looking for women,” Gishlick said.
Erin McCullough, an assistant professor of biology at Clark University in Massachusetts, said she agrees with the researchers’ conclusion that trilobite tridents were likely used for interspecies combat. However, she is not convinced by her argument that this was a trait that only men possessed.
“In general, if there’s going to be a quirky trait that’s used to fight in pairs, usually it’s the males that have the quirky trait, but the biology is fun because there are always exceptions: female reindeer have antlers.” said McCullough, who was not involved in the study (but whose beetles Gishlick and Fortey analyzed were based on their work).
“If they’re arguing that these are male weapons being used to gain access to women, it would have made a stronger story for me if they had evidence that women don’t have weapons.”
