"It's based on how modern horseshoe crabs mate," said Erickson. His study of the trackways in the Potsdam formation in northern New York was presented at the recent Geological Society of America meeting in Denver.
Erickson has been studying the trackways for some time, trying to make sense of what appears to be a very busy high tide with many arthropods skittering about a half-billion years ago.
The trouble is, say other "trace fossil" experts, it's awfully hard to catch ancient animals in the act of having sex.
"Sex, that's a real tough one," said paleontologist Whitey Hagadorn of Amherst College in Massachusetts. "It's theoretically possible (to find fossil evidence of sex), but the evidence needs to be rigorously examined."
About the only way ancient sex has ever been unmistakably captured in fossils is when two animals are buried alive while sexually coupled, said Hagadorn. Also, sex can be inferred when you can find body parts that are probably sexual in nature, he said. Sex is also inferred in animals when a female dies, is buried and fossilized while in the act of giving birth, said Hagadorn.
But in all those cases the actual fossilized bodies of the animals provide the evidence for sexual behavior. Trackways, on the other hand, tend to preserve less about anatomy and more about behavior.
It's also important to keep in mind that the earliest animals — and many marine animals today — have no explicit sexual behaviors, said Erickson. Instead, they release huge quantities of gametes into the open water, where fertilization happens.
Arthropods — which include crabs and insects — are among the first animals that might have been capable of sexual behaviors which, in turn, had a chance of getting fossilized in some way, said Erickson.
A lot of work remains to be done to confirm or refute that the tracks even belong to horseshoe-like crabs, said Hagadorn. Until then, the question of sex in the fossil record will remain "interesting and provocative," he said.
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