If we go by the claspers in trilobites:
Claspers are special hook-like appendages often found in male arthropods. The male uses the claspers to hold onto the female during mating. Different groups have convergently evolved this appendage in different parts of the body depending on the exact mode of mating in that clade. Branchiopods and horseshoe crabs have both evolved claspers, but they function in different ways according to the female’s exoskeleton. For instance, branchiopods clasp on to the carapace, while Limulus clasps on to the spines. In O. serratus, the males claspers would line up with the spines on the female’s pygidium.
“We knew it could not be for mastication because the appendages are not near the head or mouth, they’re in the middle of the body,” Losso said. “This shows sexual dimorphism in trilobites, but in this case it is only expressed in the appendages. This tells us more about the reproduction in trilobites and how they would have mated, which previously has been hard to understand and has been very speculative based on modern analogies.”
“There are very few cases of fossils that have directly informed reproductive ecology and behavior, particularly in fossils this old. In this case, because there is a structure that is very specifically adapted for this function, it is possible to make this particular argument, and more particular of trilobites,” said Ortega-Hernández. “This really is the first time that it is possible to show these limbs so heavily modified for this function. And it provides strong evidence to suggest that a Limulus, or horseshoe crab-like behavior, already existed in the Cambrian completely by convergence. So, it really helps us to get a sense of how these animals were actually living millions of years ago.”Harvard University, “Clasper appendages discovered in mid-Cambrian trilobite show horseshoe crab-like mating behavior” at Phys.org (May 6, 2022)
Half a billion years ago, actually.
Stasis: Life goes on but evolution does not happen