Reptile stapes evolution, part 1: terrestrial taxa

I saw this recent publication (Sobral et al. 2016) at
ResearchGate.net. It’s all about the stapes, tympanic membrane and cranial bones that make up the hearing apparatus in reptiles. Unfortunately the cladograms used are, once again antiquated, the product of taxon exclusion and not matched by the large reptile tree (LRT) which produces a completely different tree topology based on magnitudes more taxa, none of which are suprageneric.

From the Sobral et al. abstract
“In this chapter we revise the otic anatomy of early reptilians, including some aquatic groups and turtles. Basal members possessed a stout stapes that still retained its ancestral bracing function, and they lacked a tympanic membrane. The acquisition of tympanic hearing did not happen until later in the evolution of the clade and occurred independently in both parareptiles and diapsids.”

  1. The authors do not include synapsids (including mammals) within the clade Reptilia. They define Repitlia as: “the most inclusive clade containing Lacerta agilis Linnaeus 1758 and Crocodylus niloticus Laurenti 1768, but not Homo sapiens Linnaeus 1758.” Since Lacerta (a lepidosauromorph) and Crocodylus (an archosauromorph) do not have a last common ancestor more recent than Gephyrostegus bohemicus in the LRT, that clade thus includes Homo and the definition is invalid.
  2. The authors retain the clade “Parareptilia” members of which are paraphyletic in the LRT.
  3. The authors confess, “Because of the uncertainty in their relationships, it is difficult to understand their patterns of otic evolution.” There is no uncertainty of relationships within the LRT, online for all to see since 2010.
  4. The authors also report, “Unfortunately, there is as yet no recent, detailed analysis tackling early reptilian phylogenetic relationships.” That detailed analysis is within the LRT, online for all to see since 2010.
  5. The authors agree with Joyce 2015 that turtles are diapsid reptiles, which is not supported in the LRT. Joyce posits Eunotosaurus as a diapsid (it is not) turtle ancestor. Only by massive taxon exclusion is Eunotosaurus a turtle ancestor and only by deriving Eunotosaurus from Archosauria + Lepidosauria (not sister clades) does Eunotosaurus become, in Joyce’s vision, a diapsid.
  6. The authors report “the phylogenetic position of mesosaurs is uncertain.” The LRT has nested them firmly between basal pachypleurosaurs and thalattosaurs + ichthyosaurs for the last 6 years.
  7. The authors report correctly that millerettids are basal to procolophonids and pareiasaurs, but fail to note they are also basal to diadectids and turtles.
  8. The authors lament, “The phylogeny of millerettids is poorly understood.” In the LRT the phylogeny of millerettids is well understood.
  9. The authors do not realize the interrelationship of bolosaurs and procolophonids with diadectids and so ignore the latter or consider them a stem reptile.
  10. The authors ally Delorhynchus and Bolosaurus. The LRT separates them in distinct clades with many intervening taxa.
  11. The authors include Owenetta as a procolophonid, but it is not closely related in the LRT.
  12. The authors note, “The phylogenetic relationships of basal diapsid clades are still controversial, and their early evolutionary history remains poorly understood.” In the LRT their is no controversy and relationships are well understood. Part of their confusion stems from the fact that the authors do not yet realize the Diapsida is diphyletic, with lepidosauromorph diapsids unrelated to archosauromorph diapsids in the LRT.
  13. The authors note the exact phylogenetic position of the genus Youngina is uncertain. In the LRT several specimens are employed and every position is certain.
Figure 1. Antiquated cladogram (Sobral et al. 2016) of basal reptile relationships.

Figure 1. Antiquated cladogram (Sobral et al. 2016) of basal reptile relationships. If you’re familiar with the taxa at ReptileEvolution.com you’ll see the morphological mismatches, the nesting of derived taxa at basal nodes, the use of suprageneric taxa and worst of all, a large swath of taxon exclusion.

The stapes in Captorhinus
After describing the stapes of Captorhinus as “massive and complex with a much expanded footplate” the authors note that in more basal unnamed captorhinids, “the shafte of the stapes is long and narrow.”

The stapes in Parareptilia
The authors consider Erpetonyx “the oldest parareptile” which they date from the latest Carboniferous. The LRT nests Erpetonyx with Broomia and Milleropsis as stem diapsids. The authors claim that “Parareptilia includes groups that were among the first reptilians to evolve herbivory and associated modified feeding mechanisms,” but then they include Mesosaurs as basal parareptiles and excluded the herbivorous captorhinids. The nonsense continues unabated. The authors report, “This group shows many evolutionary novelties that parallel and predate those seen in other amniote groups. Among those novelties are the independent acquisition of tympanic hearing and impedance-matching hearing.”

The stapes in mesosaurs
The authors report, “their otic region is poorly known.”

The stapes in millerettids
The authors report, the braincase of Milleretta is very similar to that of Captorhinus. In the LRT basal captorhinids and Milleretta are separated by only a few intervening taxa. “The stapes is very different. It is stout and bears a rather narrow footplate and a very short shaft. The shaft expands significantly distally to become wider than the footplate. The stapes does not contact the quadrate.”

The stapes in bolosaurids, pareiasauromorphs and procolophonids
The authors report, “they otic morphology is not well understood. In Delorhynchus” (actually closer to Eunotosaurus and Acleistorhinus) “the stapes resembles closely that of Captorhinus.” In the LRT they are somewhat related, not close, not far. In Procolophon, “the stapes is very short, but the distance of the distal end from the deep, well-developed otic notch may indicate that it was much longer.”

The authors report, “pareiasauromorphs have very prominent otic notches indicating the undoubted presence of a large tympanic membrane.” Pareiasaurs have a notch hidden from lateral view by the quadratojugal flange. Closely related Macroleter and Emeroleter have a prominent notch, but it is framed by the supratemporal, postorbital, squamosal and quadratojugal. The authors only mention the latter two. In Macroleter, the authors report, “The stapes bears a small footplate. Although the shaft is long, it would have had no close contact with the lateral side of the skull. The shaft is also very slender.” 

In Pareiasuchus, a very well-preserved pareiasaur skull, a stapes was not preserved. In another pareiasaur, Deltavjatia, “The preserved part of the stapes is very short, and the footplate is formed by two articular surfaces separated by a sulcus.”

The stapes of basal diapsida
The authors report, “There is no evidence of a tympanic ear in these early diapsids” (Petrolacosaurus and Araeoscelis).They lack an otic notch and have a laterally or ventrally oriented stapes with a dorsal process. In fact, the stapes seems to have functioned more as a support for the jaw joint, directly or indirectly.”

In Youngina (but which one??), “The stapes is very long and robust. There is no sign of an osseous dorsal process and the shaft is perforated by a large foramen for the stapedial artery. The footplate is separated from the shaft by a poorly defined neck. It is not much larger than the shaft itself. The shaft is long and slender and appears to extend laterally toward a slight emargination of the squamosal-quadrate complex. An imperforate, ossified extrastapes has also been identified.”

The stapes in marine diapsid reptiles
we’ll look at those tomorrow.

The stapes in the basal amniotes, Gephyrostegus and Silvanerpeton
have not been identified in the literature. This is very strange if the stapes in these two is supposed to be a robust jaw-supporting bone.

References
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.

 

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