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Lend Me Your Ears

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The evolution of the mammalian ear is one of the best documented evolutionary transitions in the fossil record. Yet there are very few popular treatises on the subject available, much less explanation on what it might mean for paleoartists. After seeing one-too-many Dimetrodon incorrectly depicted with reptile-like ears, I decided to step out of my comfort zone and take a shot at explaining the whole deal myself.

Important to note off the bat: modern mammals, reptiles, and frogs all generally have a tympanic (eardrum-bearing) ear for hearing airborne sounds, but the tympanic ear is not homologous between these groups! Instead, it evolved at least three different times (and probably more) in vertebrate history.

Modern mammals have three ossicles (small bones) in their middle ear: the stapes, incus, and malleus. They also have an ectotympanic ring that helps support the eardrum. The stapes is present in most tetrapods, having been inherited from part of the hyoid arch in fish. The incus and malleus, however, are unique to mammals among modern animals. Where did they come from? The incus is actually modified from the quadrate, a bone in the upper jaw of non-mammalian bony vertebrates. The malleus is a modified articular and prearticular, bones in the lower jaw of non-mammalian bony vertebrates. The ectotympanic is a modified angular, another lower jaw bone. The quadrate and articular form the jaw hinge in non-mammalian bony vertebrates. In modern mammals, only the anteriormost bone is left in the lower jaw, the dentary (which holds the teeth), and the jaw hinge is instead formed by articulating it with the squamosal of the upper jaw.

The basalmost synapsids had a typical tetrapod skull with the quadrate, articular, and angular forming the parts of the jaw they are supposed to. There is no evidence that these bones were specialized for transmitting airborne sound in basal synapsids. As such, they likely did not have any visible ears in life: no ears for Dimetrodon! However, they may have used their lower jaws to detect ground-transmitted vibrations.

In non-mammalian therapsids, the lateral surface of the angular (homologous with the eardrum-supporting ectotympanic in modern mammals) is modified into a plate called the reflected lamina. The reflected lamina almost certainly did not originate as an adaptation for hearing; instead, its initial purpose was likely to increase the area of attachment for jaw muscles. However, it has been suggested that at least some derived therapsids (such as the anomodont Pristerodon) had specializations for receiving airborne sound, in which case the angular may have been recruited to support an eardrum. Given that their post-dentary bones were still a large part of the lower jaw, these therapsids were likely still capable of detecting ground-transmitted vibrations. However, it is perhaps not coincidental that at this point synapsids had a more upright posture than their forebears and would have spent more time with their head off the ground.

Here we descend into the netherworld where life appearance is concerned. There are no modern species with a reflected lamina to inform us what (if any) impact it would have on an animal's life appearance. If these therapsids had eardrums, a simple possibility is that they were visible on the side of the jaw, as I have depicted here. However, this is by no means the only possibility. Additionally, there is no guarantee that the eardrums would have been immediately obvious in life. I have drawn in the eardrums to highlight when they may have originated in synapsid evolution, but some modern animals like certain turtles show that eardrums can be laterally exposed and yet still fairly inconspicuous. On top of that, all of this is assuming that eardrums were present at all. Some researchers maintain that an eardrum did not evolve at all until near the base of Mammalia.

In eucynodonts, there was a dramatic increase in the size of the jaw muscles and reduction in the size of the post-dentary bones. Due to these modifications, little of the angular would have been laterally exposed. It is possible that an ear canal leading to the eardrum was present at this stage, in which case the ears may have appeared as holes or slits rather than an exposed eardrum. However, this is highly speculative and should be taken with all the aforementioned caveats in mind.

Near the base of Mammalia, some cynodonts further reduced the size of their post-dentary bones and gained a dentary-squamosal jaw articulation next to the original quadrate-articular articulation: they had a double jaw hinge. Morganucodon and Sinoconodon are examples of near-mammals with this anatomical condition.

In all modern mammals, the post-dentary bones are entirely separate from the lower jaw and they do full-time hearing duty. For the longest time it was thought that this transformation happened only once in mammalian history, but the story turned out to be not so simple. Most stem-monotremes and stem-therians indeed had post-dentary bones that were displaced medially (inward) from the lower jaw (thus, I have them outlined in gray in the diagram), but the ossicles were still partially connected to the jaw via an ossified Meckel's cartilage. The complete detachment of the ear ossicles happened separately in monotremes and in therians (as well as some extinct groups such as multituberculates). The earliest known large pinnae (ear flaps) are found in a stem-therian, the eutriconodont Spinolestes.

Of additional note for artists: modern mammals have a long ear canal. Hence, in a modern platypus, the external ear opening is much higher up on the head than the ear ossicles themselves. My personal speculation is that, given the medial displacement of the ear ossicles, a long ear canal was also present in Mesozoic mammals. However, to my knowledge little has been said about this specific topic in the literature.

The full detachment of the ear ossicles from the jaw likely allowed mammals to specialize in hearing airborne and especially high frequency sounds. However, some fossorial mammals today have secondarily developed adaptations for detecting ground-transmitted vibrations.

Skull diagrams were referenced from Allin and Hopson (1992), Barry (1967), Lautenschlager et al. (2016), Luo (2011), Ramírez-Chaves et al. (2016), Rubidge and Sidor (2001), and WitmerLab. I was unable to find good references for drawing platypus ear ossicles in situ in lateral view (likely because they wouldn't be visible under natural conditions) and had to approximate them from images in medial and ventral view.

Special thanks to Christian Kammerer for providing advice and suggestions that greatly improved this diagram and explanation!

A staggering amount of literature has been written on the evolution of synapsid ears, but some references I found particularly helpful in making this diagram include the following:
-Allin, E.F. 1975. Evolution of the mammalian middle ear. Journal of Morphology 147: 403-437. doi: 10.1002/jmor.1051470404
-Allin, E.F. and J.A. Hopson. 1992. Evolution of the auditory system in Synapsida ("mammal-like reptiles" and primitive mammals) as seen in the fossil record. Pp. 587-614 in D.B. Webster, A.N. Popper, and R.R. Fay (eds.), The Evolutionary Biology of Hearing. Springer New York; New York, NY.
-Gaetano, L.C. and F. Abdala. 2015. The stapes of gomphodont cynodonts: insights into the middle ear structure of non-mammaliaform cynodonts. PLoS ONE 10: e0131174. doi: 10.1371/journal.pone.0131174
-Laaß, M. 2016. The origins of the cochlea and impedance matching hearing in synapsids. Acta Palaeontologica Polonica 61: 267-280. doi: 10.4202/app.00140.2014
-Luo, Z.-X. 2011. Developmental patterns in Mesozoic evolution of mammal ears. Annual Review of Ecology, Evolution, and Systematics 42: 355-380. doi: 10.1146/annurev-ecolsys-032511-142302
-Luo, Z.-X., J.A. Schultz, and E.G. Ekdale. 2016. Evolution of the middle and inner ears of mammaliaforms: the approach to mammals. Pp. 139-174 in J.A. Clack, R.R. Fay, and A.N. Popper (eds.), Evolution of the Vertebrate Ear. Springer Handbook of Auditory Research 59. Springer International Publishing, Cham.
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KonskyKkt's avatar

wow, had no idea dimetrodon didnt have ears. Thanks, this is some really interesting and (for me) new info