I have a skull that I picked up in a field in Texas twenty years ago. It’s simultaneously big and delicate. It was found on a range where cattle graze, but I didn’t think it was broad and squat enough for a cow skull. Those who were with me agreed that it probably belonged to a horse. While I’ve drawn and painted it many times in the years since, I’ve never questioned that decision. I just assumed we were right.

While taking an online course in natural history illustration, I got interested in looking at skeletons. Understanding how a skeleton works helps you understand how animals are put together, and why they move the way they do. While I have owned Animal Painting and Anatomy by W. Frank Calderon since I was a teenager, I’ve never really used it. Inspired by the class, I opened it up. When I got the section about horses, I realized for the first time that my skull couldn’t possibly be from a horse. It has no front teeth.

I couldn’t believe I had never noticed this before. I was horse-crazy as a kid. I know horses have teeth in the front for cropping grass. I should have realized that the skull I had found couldn’t belong to a horse, but I hadn’t given it any thought. I’d let what I thought I knew get in my way.

I looked at the other skulls in my book. It definitely didn’t belong to a cow or any of the other animals it covered. I wasn’t sure how to go about looking for other options. I couldn’t even think of what it might be.

Before I had a chance to start googling skulls, I got lucky. While watching Marley Peifer interview JP Panter live, I caught sight of a skull just like it in one of JP’s nature journals. After the interview was over, I went back to the video and copied down what she’d written on her page: Odocoileus virginianus.

Odocoileus virginianus is a white-tailed deer.

A deer?! That possibility had never crossed my mind, but it makes perfect sense. It’s much more likely I would come across a deer’s skull than a horse’s out in the scrubby grazing land where it was found. For all these years, I’d been completely wrong in what I thought I had.

I learned many lessons here. Deer have much bigger heads than I would have guessed, and I’m thinking horse heads must be much larger than I realize (or I wouldn’t have mistaken one for the other). But the deep lesson is a simple one, something that we can all fall prey to.

The greatest obstacle to discovery is not ignorance, it is the illusion of knowledge.

Daniel J. Boorstin

Sometimes what we don’t know isn’t the problem. Sometimes it’s what we think we know.

Have you ever made a mistake because you assumed you already knew the answer?