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Crotalus atrox
Western Diamondback Rattlesnake (AKA: Western Diamond-Backed Rattlesnake, Adobe Snake, Arizona Diamond Rattlesnake, Coon Tail, Desert Diamond-Back, Desert Diamond Rattlesnake, Fierce Rattlesnake, Spitting Rattlesnake, Texan Rattlesnake, Texas Diamond-Back, or Texas Rattler) spotted in the Wichita Mountains Wildlife Preserve, OK. I heard this little guy rattling when I walked by him on a hiking trail, then took a quick photo and moved along to avoid provoking this venomous creature.
If you look at photos of those 5 species, you will notice right away timber and pygmy look different, so you can count them out. Then that leaves you with massasauga, prairie, and dback. Now you can look at the tail. Massasauga and prairie have brown/tan bands right in front of the rattle, and dback has black and white bands in front of the rattle. You can see the black and white bands in your photo, also gives them the nickname coon tail :)