The Transitional Species of the Mammalian Ear

​The evolution of the auditory ossicles is a story that most think begins in the synapsids (early amniotes of the late Carboniferous, sometimes referred to as the "Mammal-Like Reptiles"). We picture the classic dimetrodons lumbering about the early forests, it's anterior mandible beginning to separate and differentiate into what would become the mallus and the incus.
These synapsids already had their stapes though, the most interior of the inner ear bones. When did this happen? How? What about the nature of hearing itself?
We begin our story in the genes of our deep sea relatives. Fish have what is known as a Lateral Line along both sides to detect movement, vibration, and pressure gradients in the surrounding water. The Lateral Line is composed of neuromasts (small receptors with hair-like projections which extend into a jelly-like sac ). The Lateral Line pits are found in the fossils of ancient fish as well, dating back hundreds of millions of years ago. The Lateral Line formation is controlled by the gene known as Pax 2, and the same exact gene is responsible for the formation of the inner ear in mammals and the varying levels of auditory ability in reptiles and amphibians.
The receptors for BOTH these taxa appears in amphioxus in the form of hair-like epithelial cells and connecting neurons. Coincidentally, this organism is thought to be the precursor for all chordates.
To put it all more plainly: same gene that controls the formation of the lateral line (detecting prey, orientation, schooling) controls the formation of the mammalian inner ear (modern balance/hearing organ) and the ancestor of BOTH has the genes for the receptor type's origin.
Can we go back any further though?
Box jellyfish are incredibly "primitive" animals. They have a sort of ancient eye (unique to sea jellies), but certainly lack any type of ear or lateral line.
What do their genes say? They don't have Pax 2 (balance/hearing) OR Pax 6 (sight) but have a single gene for their primitive eyes that is a genetic mosaic of BOTH Pax 2 and Pax 6.
The implication here is that perhaps ancient cnidarians hold the key to the eventual duplication or point mutation that progenated Pax 2 and Pax 6 from the precursor mosaic.
So the genetics are in place by the time we reach the Sarcopterygians like Eusthenopteron, what about the physical form? The actual inner ear bones? Eusthenopteron's stapes is nearly in place, and by the time we meet the early amphibian Tulerpeton, the first inner ear bone is in place, although hearing would have been incredibly poor.
The synapsids though, hold the key for the mallus and the incus. Through the following organisms (some species some genera) , we find the inner ear bones separating out from the ramus of the mandible and moving interior nearer to the stapes. This is an evolutionary trend, meaning there aren't known synapsids, therapsids or cynodonts in the mammalian lineage who violate this theme of separating and migrating ossicles "primitive" to the modern "derived" malus and incus.


Stapes In Modern Position
Dimetrodon: Post mandibular bones are beginning to separate and diminish in size
Sphenocodontid: Mandibular bone migration continues to the interior of the skull, size continues to diminish
Eotitanosuchus: More bone migration and diminishing, but specifically focused on the quadrate rami (incus)
Gorganops: More bone migration and diminishing, as well as an inflated vestibule a potential primitive cochlea for primitive sound detection.
Lycaenops: In addition to the previous continued bone migration and diminishing, we see some developments in the formation of the modern tympanic membrane (eardrum): "...shows that the reflected lamina covering the angular notch is extremely thin but stabilized by low, radial crests; it seems most likely that the thin bony plate covering the recessus mandibularis already functioned as an inefficient forerunner of the tympanic membrane, although the pressure ratio must have been very low."
The author continues, addressing the idea of a partiathrinl tympanic membrane: " ‘evolutionary optimization is not measured in absolute terms, but by its relation to contemporaneous and sympatric competitors, i.e. it must have been good enough for the Permian world’ (p. 316). Luo & Crompton (1994) carefully analyzed the structural and functional transformation of the quadrate into an incus in advanced cynodonts. "
Thrinaxodon: The articular bone (malus) is in or almost in position, and is the appropriate size. And in addition, we see the development of some of the outer ear as well, the tympanic membrane now being located inside the skull. " The otic region is defined by the regions surrounding the temporal fenestrae. Most notable is evidence of a deep recess that is just anterior to the fenestra ovalis, containing evidence of smooth muscle interactions with the skull. Such smooth muscle interactions have been interpreted to be indicative of the tympanum and give the implications that this recess, in conjunction with the fenestra ovalis, outline the origin of the ear in Thrinaxodon. This is a new synapomorphy as this physiology had arisen in Thrinaxodon and had been conserved through late Cynodontia."
Furthermore, more information can be gleaned in relation to the location of the tympanic membrane, including the nature of what would become the muscles which allows mammals to move their ears about: " The remainder of this pit opens to an "un-ossified" region which comes somewhat close to the cochlear recess, giving one the assumption that inner ear articulation occurred directly within this region."
Key Information Regarding Thrinaxodon
Thrinaxodon's skull looks very much like a modern mammal, the Kangaroo Rat, in the middle of it's embryological development, only to finish up as an adult with a modern mammal inner ear. This of course supports the notion that some aspects of embryology mirror evovlutionary history (at least in utero or newborn in marsupials). Fascinatingly enough, in the link below the squamate Agama lizard, newborn and adult Kangaroo-rats and Thrinaxodon are compared.
Notice anything interesting?
Stapes and Malus In Modern Position
Cynognathus: With two ear bones in place, we are now watching the incus, which is properly diminished but still migrating. Additionally, this animal's mandibular joints are evolving as well. Generally reptiles have two jaw joints while mammals have one one. Cynognathus has two joints, like reptiles, but one of these joints is mammalian in nature!
Yanucodon: We now have one mammalian jaw joint, a nearly in place incus and an in-place malus and stapes.
Stapes, Malus and Incus in Modern Position
Eomaius/Sinodelphys: True mammals from the Early-Mid Cretaceous. Inner ear is fully functional! The former, Eomaia, is a placental mammal while the latter, Sinodelphys, is a marsupial suggesting the inner ear developed prior to the placental/marsupial split!


The picture can be painted clearly: the genetics for function and the paleontology for form are easily traced through history and present day. The two show our relationship to all other animals, from box jellies to ancient synapsids.
The study of the inner ear is, in my opinion, one of the most fascinating examples of evolution and it is all encased in the sides of our skulls by three tiny bones.