frilled lizard skeleton. In reptiles with external ear structures, the tympanic membrane is visible, either nearly contiguous to the surface of the skin (as with iguanids such as the green iguana), or recessed deeper into the head (as with some scincids, such as the blue-tongue skink, and agamids, such as the bearded dragon). The tympanic membrane covers the middle ear cavity. ref: http://www.anapsid.org/reptilehearing.html
One of the most noticeable variations in external ear anatomy is that of free lobes vs. attached lobes. This is a genetic trait. Although there's not 100% consensus, it's widely believed that attached earlobes are the recessive trait. If you have attached lobes, this means you most likely have two recessive alleles for it.