Yesterday we looked at a report by Sobral et al. 2016 describing the stapes in several basal reptile terrestrial clades. You may recall that the summary data on the stapes was gratefully accepted, but the cladogram they produced was not a match for the large reptile tree (LRT). Today we’ll look at the Sobral et al. report on the stapes of marine diapsid reptiles.
Figure 1. From Sobral et al. 2016, their cladogram of marine diapsid reptiles nests turtles as diapsids between Youngina and Lepidosauria. I added the pink arrows and image of Mesosaurus to show relationships the LRT found based on similar morphologies. Turtles as diapsids is ludicrous and totally without supporting morphological evidence. They are pareiasaurs. The basal split among reptiles separates lepidosauromorphs from archosauromorphs.
Sobral et al. 2016 report
The stapes in Ichthyosauria
Sobral et al. 2016 report, “The stapes of most ichthyosaurs consists of a dorsomedially broadened footplate (sometimes referred to as the occipital head) (e.g., Fischer et al. 2012) and a narrower ventrolateral shaft. In some cases, such as in the Jurassic Temnodontosaurus, thickness is more consistent throughout the shaft, resulting in overall stout stapedial morphology. The stapes structurally supports the braincase by articulating with the basioccipital and the basisphenoid medially and the quadrate laterally. A massive stapes does not necessarily imply lack of hearing, however, because it could still transduce low-frequency sound waves via a bony connection directly to the fluids of the inner ear.”
“the lack of an otic notch together with the bony contact between the otic capsules and the suspensorium led to the proposition that a tympanic membrane was completely lost in ichthyosaurs and that they were not capable of acute, directional hearing.”
The stapes in thalattosauriformes
Sobral et al. 2016 report, “in Askeptosaurus [the] stapes is an elongated rod-like bone that medially thickens into a two-headed knob reminiscent of the stapedial shapes found in various stem diapsids. The lateral concave posterior emargination of the quadrate further argues for the presence of a tympanic membrane.”
The stapes in placodonts
Sobral et al. report, “Only one stapes is known to be preserved in one specimen of the basal placodont species Placodus gigas indicating that this structure may not have been fully ossified inderived placodonts.”
The stapes in pachypleurosaurs
Sobral et al. 2016 report, “the stapes has been reported in only one pachypleurosaur genus, Neusticosaurus, consisting of a short, slender, and cylindrical rod. The tympanic membrane would have been particularly large in pachypleurosaurs as they have the unique feature of a quadrate fossa posterior to the articular facet of the retroarticular process on the lower jaw, which contributed to form part of the middle ear chamber.”
The stapes in nothosaurs, pistosaurs and plesiosaurs
Sobral et al. 2016 report, “the tympanic membrane appears to have been completely lost, possibly due to these taxa being increasingly aquatic. Narrow stapes have been described for the nothosaur Ceresiosaurus and for the pistosaur Yunguisaurus, but it remains unclear whether they were used for hearing. Plesiosaurs from the Early Jurassic, such as Plesiosaurus and Stratesaurus are commonly reported to have had narrow stapes. However, from the Middle Jurassic until the clade died out at the end of the Cretaceous, no conclusive examples of plesiosaur stapes have been described.”
The stapes in turtles (Proganochelys)
Sobral et al. 2016 report, “in all post-Triassic turtles, in which the stapedial shaft articulates with the tympanic membrane via a cartilaginous extrastapes. In Proganochelys the medial footplate of the stout, rod-like stapes articulates with the FO, and the lateral aspect of the stapes fits into an articular pocket on the medial side of the quadrate.” In many turtles the quadrate produces a lateral tunnel that supports a tympanic membrane and encloses a tiny rod-like stapes and extrastapes.
Sobral et al. 2016 report, “it is clear that early reptilians did not possess tympanic hearing” and that there were multiple appearances of tympanic hearing. In early tetrapodomorphs and in stem amniotes, the stapes served an important bracing function. The paroccipital process gradually replaced the initial bracing function of the stapes. This replacement is characterized by eventual loss of the dorsal process and reduction in the size of the footplate and in the thickness of the shaft.”
The Sobral et al. observations and summaries of the literature with regard to reptile hearing are important contributions to paleontology. Unfortunately their cladograms of interrelationships are a train wreck… and there’s no reason for that in 2016, six years after the new tree topology was first presented and expanded every since. Given a little time this weekend, I will attempt to separate support stapes from auditory stapes in the LRT.
Joyce WG 2015. The origin of turtles: A paleontological perspective. Journal of Experimental Zoology Part B: Molecular and Developmental Evolution, 324B(3), 181–193. doi:10.110. 1002/jez.b.22609
Sobral G, Reisz R, Neenan JM, Müller J and Scheyer TM 2016. Chapter 8. Basal reptilians, marine diapsids, and turtles: The flowering of reptile diversity, pp. 207–243 in Evolution of the vertebrate ear, Evidence from the fossil record, Volume 59 of the series Springer Handbook of Auditory Research. Eds. Clack JA, Fay RR and Popper AN.