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Why being wrong is important

Six months ago, someone posted a photo of an ungulate carcass on iNaturalist. The location is a desert mountain range in southern California, within Anza Borrego Desert State Park. I spend a lot of time in this region, which is characterized by large boulders, sparse cholla, agave, ocotillo, and ironwood. Classic bighorn sheep habitat. Clearly, the photo depicted a bighorn sheep carcass. A few of us, including a large carnivore biologist and multiple track & sign experts, all identified the carcass as a Desert Bighorn Sheep, Ovis canadensis ssp. nelsoni. We were all wrong. Identification of bones in the field can be difficult and it's even more challenging from photos. But we should have caught a detail captured by one sharp-eyed contributor. Anterior to the eye socket is the lacrimal bone. In bighorn sheep, this bone is smooth. The photo showed a large divot in this bone, not due to breakage, but an intact anatomical feature. This was a mule deer carcass (Odocoileus hemionus). I would have bet A LOT of money this wasn't a mule deer, based on location alone. I'd never bothered to look at diagnostic differences between bighorn and mule deer skulls, despite owning a copy of Elbroch's Animal Skulls: A Guide to North American Species. And despite having participated in an annual bighorn sheep count in this region for the last 3 years, in an overlap zone of bighorn and mule deer..... and always on the lookout for bones. I wasn't curious enough, rigorous enough. But I learned something new! Now it's time to look at pronghorn skulls.

The author at home with what he is sure is a mule deer skull. Below is close-up of the lacrimal bone depression anterior to the eye socket.


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