Presenting a water bowl to a Savannah may also prove a challenge, as some will promptly begin to bat all the water out of the bowl until it is empty, using their front paws.
The thymus is involved in the development of T cells - making sure that any T cells that react to self-antigens, antigens that are normally found in the body, are promptly destroyed.
Apparently what happened was that water collected on these tar pits and the bigger animals like the elephants ventured out on to the apparently firm surface to drink, and were promptly bogged in the tar.