It looks more like a vertebra to me - the hole in the top images is where the spinal cord would run through.
I'm afraid I don't know enough about animal physiology to say what its from, but if it was washed up on a beach, then it is possible it is from an aquatic mammal (looking at the dimensions against the ruler, it is quite large). What species of seals, whales, dolphins or porpoises do you commonly get in the Gulf of Mexico?
A few image searches on Google to help narrow it down a bit further, perhaps, but I haven't spotted anything directly comparable:
However, the shape is dependent on where the vertebra is located in the spine, and it could also be from a large land mammal that has been washed downstream and, ultimately, up onto the beach.
Hopefully one of our experts will be able to give you a better response than I can.
This is the first two neck vertebrae of a Dolphin. From the size and morphology (shape) I would suggest a common dolphin. I was hoping to post a link to one on the internet but I cannot find an exact match. However, here is a post regarding a larger example of the neck verteba of a dolphin with a discussion