Their middle ear is enclosed by a bowl-shaped bony structure called the tympanic bulla, which can be fossilized. A fossil bulla is illustrated below.
|fossil whale ear bone (bulla), 7 cm wide; Neogene age (Miocene?), California?|
|models of modern melon-head whale bullae; each one is 3 cm wide|
For an interesting article on whale ear bones, including fossil ones, the link is http://vmnhpaleontology.wordpress.com/2007/12/19/in-the-news–whale-ancestors-and-more-ear-bones/
This link provides information on how fossil whale-ear bones help support the evolutionary concept that whales have an ancestry with artiodactyls (modern ones are cows, deer, pigs, etc.).