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Non-dino papers: Jurassic sphenodontian and Dryolestes inner ear



From:  Ben Creisler
bcreisler@gmail.com


A couple of new Mesozoic-related papers in the October Zoological
Journal of the Linnean Society :

(The new taxon is not named in the abstract and I don't have access to
the text of the article at the moment to get it.)


Sebastián Apesteguía, Raúl O. Gómez and Guillermo W. Rougier (2012)
A basal sphenodontian (Lepidosauria) from the Jurassic of Patagonia:
new insights on the phylogeny and biogeography of Gondwanan
rhynchocephalians.
Zoological Journal of the Linnean Society 166(2): 342–360
DOI: 10.1111/j.1096-3642.2012.00837.x
http://onlinelibrary.wiley.com/doi/10.1111/j.1096-3642.2012.00837.x/abstract



Herein we describe a new rhynchocephalian taxon from the Middle
Jurassic of Patagonia, Argentina, representing the first Jurassic
record of the group in South America. The new taxon, consisting of a
complete dentary, is ascribed to Sphenodontia based on the presence of
a deep and wide Meckelian groove, long posterior process,
well-developed coronoid process, and acrodont teeth showing dental
regionalization including successional, alternate hatchling, and
additional series. This allocation is reinforced by a phylogenetic
analysis that places the new taxon in a basal position within a clade
of sphenodontians that excludes Diphydontosaurus and
Planocephalosaurus. Additionally, the new taxon clusters within a
Gondwanan clade with the Indian Godavarisaurus from the Jurassic Kota
Formation, sharing the presence of recurved and relatively large
posterior successional teeth that are ribbed and bear a peculiar
anterolingual groove. This sister-group relationship is intriguing
from a palaeobiogeographical viewpoint, as it suggests some degree of
endemism during the initial stages of the breakup of Pangaea. We also
discuss the ontogenetic stage of the new taxon and provide insights on
the evolution of successional dentition in rhynchocephalians.

====


Zhe-Xi Luo, Irina Ruf and Thomas Martin (2012)
The petrosal and inner ear of the Late Jurassic cladotherian mammal
Dryolestes leiriensis and implications for ear evolution in therian
mammals.
Zoological Journal of the Linnean Society 166(2): 433–463
DOI: 10.1111/j.1096-3642.2012.00852.x
http://onlinelibrary.wiley.com/doi/10.1111/j.1096-3642.2012.00852.x/abstract

Dryolestes leiriensis is a Late Jurassic fossil mammal of the
dryolestoid superfamily in the cladotherian clade that includes the
extant marsupials and placentals. We used high resolution
micro-computed tomography (µCT) scanning and digital reconstruction of
the virtual endocast of the inner ear to show that its cochlear canal
is coiled through 270°, and has a cribriform plate with the spiral
cochlear nerve foramina between the internal acoustic meatus and the
cochlear bony labyrinth. The cochlear canal has the primary bony
lamina for the basilar membrane with a partially formed (or partially
preserved) canal for the cochlear spiral ganglion. These structures,
in their fully developed condition, form the modiolus (the bony spiral
structure) of the fully coiled cochlea in extant marsupial and
placental mammals. The CT data show that the secondary bony lamina is
present, although less developed than in another dryolestoid
Henkelotherium and in the prototribosphenidan Vincelestes. The
presence of the primary bony lamina with spiral ganglion canal
suggests a dense and finely distributed cochlear nerve innervation of
the hair cells for improved resolution of sound frequencies. The
primary, and very probably also the secondary, bony laminae are
correlated with a more rigid support for the basilar membrane and a
narrower width of this membrane, both of which are key soft-tissue
characteristics for more sensitive hearing for higher frequency sound.
All these cochlear features originated prior to the full coiling of
the therian mammal cochlea beyond one full turn, suggesting that the
adaptation to hearing a wider range of sound frequencies, especially
higher frequencies with refined resolution, has an ancient
evolutionary origin no later than the Late Jurassic in therian
evolution. The petrosal of Dryolestes has added several features that
are not preserved in the petrosal of Henkelotherium. The petrosal
characters of dryolestoid mammals are essentially the same as those of
Vincelestes, helping to corroborate the synapomorphies of the
cladotherian clade in neural, vascular, and other petrosal
characteristics. The petrosal characteristics of Dryolestes and
Henkelotherium together represent the ancestral morphotype of the
cladotherian clade (Dryolestoidea + Vincelestes + extant Theria) from
which the extant therian mammals evolved their ear region
characteristics.