Transitional forms are organisms that contain some traits of one type of organism and also traits of another type of organism which are not found in the first type of organism. It is known that reptiles don't have the 3 bones that mammals have in their middle ear. In the fossil record, in strata below that which the earliest mammals are found, there exists animals with bones from their jaws that are partially in the shape of those 3 ear bones. In most other respects the animals maintain reptilian traits. The closer to to the mammal stratum that they are found, the more defined those ear bones become. Other reptilian traits also become less pronounced.