Understanding how ears develop in frogs and toads is key to understanding how and why these structures are so easily lost and regained.
Many amphibians lose structures or never develop them due to a change in development rate or by never completing development. When adult structures are not formed and instead juvenile features are retained it is called paedamorphosis. Paedamorphosis could be involved in ear loss in frogs and toads.
Frog and toad ears develop throughout the tadpole stages and post metamorphosis. The inner ear develops completely before metamorphosis, whereas the outer and middle ear elements develop after the tadpole develops limbs. Since the inner ear structures are never lost in earless frogs or toads it is likely that development throughout metamorphosis and post metamorphosis is key to understanding ear loss.
The few frog and toad species that have been studied for outer and middle ear development show a wide range in development rate for these structures. Some frogs take only a few days post metamorphosis to fully develop their outer and middle ear structures, other frogs take a month, and one toad took an entire year!
Already my data from one species of toad has confirmed that there is variation in developmental timing amongst the family bufonidae and Jenny Stynoski's research on the development of ears and skulls across the other 11 focal species is bound to answer more important questions about the evolution of ear loss.
A brood of Rhinella marina eggs were raised from hatchling to 15 weeks post metamorphosis.
Individuals were preserved throughout this time span to track ear and skull development.
Preserved specimens were then photographed, measured, and either sliced for morphological analysis of ear development or cleared and stained for analysis of bone development.
A middle ear!
This means Bufo marinus is developing an ear faster than previously recorded bufonid species.
The tissue was stained with Eosin and Toluidine Blue after plastic sectioning (see histology site for more details).
The developing skeleton of a juvenile Rhinella marina 12 weeks post metamorphosis.
The body was cleared and stained.
Blue - cartilage
Red - bone