The comings and goings of lepidosauromorph ears

Lepidosauromorph (lizard and turtle) eardrums
Sometimes the turtle or lepidosaur eardrum is flush with the surface. Sometimes there’s a deeper eardrum just below the surface or deeper still, almost invisible in a hole or in a slit. In snakes and other burrowing and marine reptiles there’s typically no trace of an external ear.

[We’ll talk about the archosauromorph (mammals, birds and crocs) eardrums in a later post.]

Back, back, back to the otic notch
The famous “otic notch”  has traditionally been considered the site of the eardrum of prehistoric amphibians like Silvanerpeton and Gephyrostegus. That’s where frog eardrums are located and it is assumed the same held true for many other prehistoric amphibians with this trait. (More below.)

Reptile eardrums - here the reduction of the otic notch is demonstrated from Silvanerpeton to Gephyrostegus and Cephalerpeton, a basal lepidosauromorph reptile.

Reptile eardrums – here the reduction of the otic notch is demonstrated from Silvanerpeton to Gephyrostegus and Cephalerpeton, a basal lepidosauromorph reptile.

Some prehistoric lepidosauromorphs also have an otic notch. Among other traits, this notch has caused some experts to consider, Diadectes, for instance, to be a pre-reptile. Now we know that Diadectes nests well within the Reptilia in the large reptile tree. Ironically a similar notch is found in the related and much later appearing Procolophon, which paleontologists universally consider a reptile. But the connection between the two has never been recognized except here.

Reptile eardrums in Diadectes and Procolophon

Figure 2. Reptile eardrums in Diadectes and Procolophon

Earliest reptile eardrums?
2007 news event following publication of Bashkyroleter, a macroleterid (not procolophonid!) with a very large eardrum frame, prompted this post.

In that case, Müller and Tsuji (2007)  reported, “the presence of true tympanic ears has never been recorded in a Paleozoic amniote, suggesting they evolved fairly recently in amniote history. The configuration of the tympanic ear in these parareptiles is unique for amniotes in that it is not the quadrate as in other reptiles, or the tympanic (angular) as in mammals, but the squamosal and the quadratojugal to which the main parts of the tympanum are connected.

To their point,
one of the hallmarks of early reptiles is the disappearance of the otic notch, as seen tentatively in Cephalerpeton and more fully in captorhinids. The squamosal posterior rim changes from concave with an overhanging supratemporal in Gephyrostegus to straight to slightly convex at the top in Concordia. Such a change signals the diminution of the large surface eardrum in taxa without an otic notch.

Reptile eardrums. Basal lepidosauromorphs in phylogenetic order.

Figure 3. Reptile eardrums. Basal lepidosauromorphs in phylogenetic order.

A tiny notch reappears at the base of the squamosal where it meets the rising quadratojugal in Milleretta and its descendants, Acleistorhinus and Eunotosaurus.

A more Concordia-like arrangement extends to TseajaiaSolenodonsaurus, and their chroniosuchid kin, along with Orobates and its descendants. An indentation of the squamosal reappears in Diadectes and the lineage of turtles and also in Macroleter and the above mentioned Bashkyroleter and kin.

Reptile eardrums Mid-lepidosauromorpha in phylogenetic order.

Figure 4. Reptile eardrums Mid-lepidosauromorpha in phylogenetic order.

Strangely a sister to Macroleter, Saurorictus, had virturally no identation. This appears to be an exception based on its small size, not the basal condition, contra Müller and Tsuji (2007). The phylogenetic tree provided by Müller and Tsuji (2007) does not match that of the large reptile tree.

Thereafter Macroleter and kin the embayment of the squamosal and quadratojugal remained deep moving through Emeroleter, Coletta, Sauropareion and other owenettids (not procolophonids!)

At the next step up the lepidosauromorph tree, with Paliguana the squamosal was greatly reduced and stayed that way within the clade of Kuehneosaurus through Xianglong, two of the gliding lizards. In these taxa the quadrate was concave posteriorly, perhaps taking over the function of the squamosal and quadratojugal.

Figure 5. Reptile eardrums among basal Lepidosauriformes

Figure 5. Reptile eardrums among basal Lepidosauriformes

Another lepidosauromorph branch leading to rhynchosaurs, trilophosaurs and sphenodontids retained a concave squamosal + quadratojugal and sometimes just a quadrate framing an eardrum (this is known only in the living Sphenodon, the rest are all extinct).

Reptile ears, living representatives

Figure 6 Reptile ears, living representatives.

A third branch of lepidosaurs leads to the tritosaurs and squamates including the living Iguana and Varanus. Living lizards demonstrate a wide variety of surface eardrums, slits, holes and the absence of external indications of ears (in Lanthanotus, snakes and burrowing lizards, for instance). In the Tritosauria, megalancosaurs and pterosaurs had straight quadrates so we imagine the external ear opening could only be a tall slit, perhaps bounded by the jaw adductor.

Figure 6. Reptile eardrums, Tritosaurs

Figure 6. Reptile eardrums, Tritosaurs, including tanystropheids and pterosaurs

A little more background from Wever 1978 on salamanders:
In vertebrates the auditory hair cels are immersed in fluid and are sensitive to displacements; in all vertebrates above the fishes, sound stimuli involve these cels only by mobilizing the inner ear fluids. This fluid mobilization ordinarily is achieved in one of two ways.

1. In birds and mammals and in a number of reptiles one wall of the inner ear capsule contains a round window, an opening covered by a thin membrane beyond which lies an air cavity. This window provides a place of pressure relief when vibratory sound pressures are exerted from the outside, usually by way of an external ear opening and a middle ear apparatus applied to the oval window of the cochlea. The fluid then is set in oscillation between oval and round windows: an inward displacement at the oval window is accompanied by an outward displacement of equal volume at the round window.

2. A second method of fluid mobilization is utilized in many of the reptiles, including turtles, snakes, amphisbaenians, Sphenodon, and a few species of lizards, which lack a round window. These use a reentrant fluidcircuit: a pathway leads inward from the footplate of the oval window and takes a roundabout course to the outer surface of this same foot plate, and the fluid surges back and forth along this pathway.

These two modes of fluid mobilization are both effective for their purpose, though the reentrant fluid circuit, because of the increased mass of fluid that must be set in motion, serves best for low tones and restricts the ear’s sensitivity to high tones.

To the point of the Wever 1978 paper: The Anura (frogs and toads) posses a round window and use the first solution mentioned. The Gymnophiona (caecilians) use a reentrant fluid circuit. In Caudata (salamanders) sound literally goes in one ear and out the other, unique among tetrapods. Sounds applied to the oval window of one ear produce a path of vibratory motion that passes through the brain cavity to the oval window on the opposite side.

But this is a blog about reptiles, so we’ll stop this discussion on that interesting note.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Müller J, Tsuji LA 2007. Impedance-Matching Hearing in Paleozoic Reptiles: Evidence of Advanced Sensory Perception at an Early Stage of Amniote Evolution. PLoS ONE 2(9): e889. doi:10.1371/journal.pone.0000889
Wever EG 1978. Sound transmission in the salamander ear. PNAS 75(1):529-530.

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