How Do You Give an Eye Exam to a Fossil?

Aug 4, 2014

A giant sea scorpion that lived 400 million years ago was thought to be a top predator. Now, it's not a sure thing.

How do you give an eye exam to a creature that's been extinct for hundreds of millions of years? First, you need a fossil -- a really well-preserved fossil.

"If you think about an animal today, you might cut it open, dissect it, and you might look at the internal structures of the eyes," said Ross Anderson, a graduate student at Yale University. "With a fossil, which is now basically rock, you can't do that."

Anderson is studying the eyes of Acutiramus cummingsi, a giant sea scorpion about the length of a crocodile that lived 400 million years ago. For a while, it was thought to be a top predator but, "recently, there was a study which looked at the strength of the claws, and showed that they might not have been able to tear anything that was armored," Anderson said.

Anderson began thinking about the animal's eyesight, and wondering if the creature had good vision, or couldn't really see its prey. If not, it would "suggest they weren't the big predators that we think they were," Anderson said.

The animal had compound eyes. Think of the eyeballs you'd see on a dragonfly: large, forward-facing, with lots of tiny little lenses. The general idea is the more lenses in an eye, the better the vision. 

Compound eyes, like the ones found in this portrait of Calliphora vomitoria, feature lots of lenses, which each function like tiny magnifying glasses, focusing light on a point so an animal's brain can make sense of an exterior image.
Credit Wikimedia Commons
The giant sea scorpion's eyes were more like what you'd find in a Long Island horseshoe crab.

Anderson's fossil was so good that many of those lenses were preserved in the rock. After looking at the fossil with an electron microscope, Anderson said, he found that the number of lenses was not that large, compared to other organisms that lived at the time, and modern predatory arthropods.

Anderson also found the angle between the lenses to be large, which meant weaker vision. It's nothing, he said, like what you would see in modern predators like dragonflies or mantis shrimp. The giant sea scorpion's eyes were more like what you'd find in a Long Island horseshoe crab. "Maybe our sea scorpions had a very similar lifestyle to the modern horseshoe crab," he said, " in that they were these kinds of scavengers."

That finding could force scientists to re-imagine the predator-prey relationships thought to have existed in ancient oceans.

The research was published in the journal Biology Letters