FIG. 1.

Highly schematic representation of the amniote phylogenetic tree over 500 million years to illustrate the approximate time of origin of particular features of auditory systems. Amniotes arose from the earliest amphibian tetrapods early in the paleozoic and presumably inherited from them a simple hearing organ (the lower blue ring marks the latest time possible for the origin of the ancestral amniote papilla). Apart from the lineages to the turtles and the Tuatara, that remained ancestral in a number of respects, three main lineages to modern amniotes are distinguished: Mammalian ancestors, that arose first; The archosaur line that led to the dominant land organisms of the Mesozoic (only the crocodile-alligator and bird groups survived to modern times); and Lepidosaurs (mostly lizards and snakes). The tympanic middle ear (inserts) originated independently in all groups during the Triassic, initiating the evolution of elongation and unique cellular configurations of the different inner-ear papillae. Amphibians also independently evolved a single-ossicle middle ear, but it is not yet known exactly when this occurred. Monotreme mammals do not have a coiled cochlea, coiling originated at the root of the marsupial-placental lineages (original diagram provided by Ulrike Sienknecht).