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Session Schedule & Abstracts




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Sunday 3rd July, 2016

EAR1
Symposium: Show me your ear - The inner and middle ear in vertebrates 1

Room: Salon G   9:30 am–11:00 am

Moderator(s): C. Pfaff, J. A. Schultz, & R. Schellhorn
EAR1-1  9:30 am  The utility of the shark inner ear as a 'landmark' for hyoid arch position. Bronson A.*, American Museum of Natural History; Hutchins R., University of Montana; Denton J., American Museum of Natural History; Maisey J., American Museum of Natural History   abronson@amnh.org
Abstract: The inner ear of sharks (a group whose skeletal anatomy is generally poorly sampled) has been studied using CT scanning technology. Apart from its obvious functional interest, the skeletal labyrinth of the inner ear can provide 'landmark' points of reference that help describe the position of other anatomical structures, such as the location and extent of the craniohyoid attachment connecting the hyoid arch to the neurocranium. Few external features of the braincase otic region are available to define the extent of this articulation. However, by isolating the labyrinth endocast via tomographic segmentation, we were able to utilize morphological features of the skeletal labyrinth to qualitatively describe hyomandibular attachment in 25 modern elasmobranch species. After constructing a preliminary phylogeny based on hyoid attachment characters, potential synapomorphies were identified for some groups. The supposed anterior location of the craniohyoid articulation in galeomorphs is critically evaluated. In the Orectolobiformes the articulation extends farther anteriorly than in many other modern elasmobranchs, and is so far forward in Eridacnis that it extends beneath the utriculus. Our preliminary findings suggest that further investigations (including morphometric analyses) will clarify anatomical descriptions and character definitions, while also providing novel phylogenetic characters.

EAR1-2  9:45 am  Inner ear morphology in early neopterygian fishes (Actinopterygii: Neopterygii). Giles Sam*, University of Oxford; Rogers Molly, University of Oxford; Friedman Matt, University of Oxford   sam.giles@earth.ox.ac.uk
Abstract: Endocasts of the osseous labyrinth have the potential to yield information about both phylogenetic relationships and ecology. Although inner ear morphology is well documented in many groups of fossil vertebrates, little is known for early Neopterygii, the major fish radiation containing living teleosts, gars and the bowfin. Here we reconstruct endocasts of the inner ear for a sample of Mesozoic neopterygian fishes using high-resolution computed tomography. Our sample includes taxa unambiguously assigned to either the teleost (Dorsetichthys, "Pholidophorus", Elopoides) and holostean ("Aspidorynchus", "Caturus", Heterolepidotus) total-groups, as well as examples of less certain phylogenetic position (an unnamed parasemionotid and Dapedium). Our models provide a test of anatomical interpretations for forms where inner ears were reconstructed based on destructive tomography ("Caturus") or inspection of the lateral wall of the cranial chamber (Dorsetichthys), and deliver the first detailed insights on inner ear morphology in the remaining taxa. With respect to relationships, traits apparent in the inner ear broadly support past phylogenetic hypotheses concerning taxa agreed to have reasonably secure systematic placements. Inner ear morphology supports placement of Dapedium with holosteans rather than teleosts, while preserved structure in the unnamed parasemionotid is generalized to the degree that it provides no evidence of close affinity with either of the crown neopterygian lineages. This study provides proof-of-concept for the systematic utility of the innear ear in neopterygians that, in combination with similar findings for earlier-diverging actinopterygian lineages, points to the substantial potential of this anatomical system for addressing the longstanding questions in the relationships of fossil ray-finned fishes to one another and living groups.

EAR1-3  10:00 am  Mesosuchus browni (Rhynchosauria: Archosauromorpha) and the early evolution of the archosaur ear. Sobral G*, Museu Nacional do Rio de Janeiro; Butler RJ, University of Birmingham; Müller J, Museum für Naturkunde Berlin   gabisobral@gmail.com
Abstract: Rhynchosauria is a clade of herbivorous stem-archosaurs that were abundant in many Triassic terrestrial ecosystems. Mesosuchus browni represents the basalmost rhynchosaur taxon and its morphology has been important to understand the phylogenetic relationships and early evolution of the clade. However, the braincase anatomy remained largely unknown. To shed more light on its morphology, we scanned the braincase of Mesosuchus for the first time using high-resolution micro-computed tomography. In comparison to the stem-diapsid Youngina, the semicircular canals of Mesosuchus are more slender and more rounded, with a substantial elongation of the posterior canal. Together with the enlargement of the floccular lobe, this is indicative of a more active lifestyle and a more upright posture, since these structures are responsible for ensuring balance control during locomotion. The borders of the small fenestra ovalis and metotic foramen are also more well-defined. The thickening of the ventral ramus of the opisthotic more effectively separates these structures, resulting in an enhanced sense of hearing. It avoids sound transmission along non-sound-detecting routes and increases the pressure-relief function of the metotic foramen. However, the inner ear of the more derived archosauriform taxon Euparkeria shows further improvements of these mechanisms such as a larger metotic foramen and more elongate cochlea and semicircular canals. This places the morphology of Mesosuchus as intermediate between stem-diapsids and stem-archosaurs and points to a gradual acquisition of derived hearing characters. In contrast, the stapes of Mesosuchus resembles more that of Youngina than of other closely-related taxa. The anatomy of the stapes seems to be at odds with these general trends, possibly indicating a unique evolutionary history of hearing within rhynchosaurs.

EAR1-4  10:15 am  Ear ossicle morphology of the Jurassic euharamiyidan Arboroharamiya and evolution of mammalian middle ear. Meng J.*, American Museum of Natural History; Bi S.-D., Indiana University of Pennsylvania; Zheng X.-T., Shandong Tianyu Museum of Nature, Shangdong, China; Wang X.-L., Linyi University, Linyi, Shandong, China   jmeng@amnh.org
Abstract: Here we report the stapes and incus from the early Middle Jurassic (~160 Ma) euharamiyidan, Arboroharamiya, from northern China, which represent the earliest known mammalian middle ear ossicles. Both bones are greatly reduced in size in relation to those of non-mammalian cynodonts, and the stapes-skull length ratio falls in the range of extant mammals. The stapes is "rod-like" and has a large stapedial foramen and a distinct posterior process. The process is unique in shape and size among known mammals and interpreted as for insertion of a sizable stapedius muscle. The incus differs from the quadrate of non-mammalian cynodonts, such as morganucodontids, in having a small size and a slim short process. Coupled with lack of the postdentary trough/Meckelian groove on the medial surface of the dentary, these ossicles indicate that the postdentary unit (articular, prearticular, and angular) must have been completely detached from the dentary and the definitive mammalian middle ear (DMME) had developed in euharamiyidans. Among various higher-level phylogenetic hypotheses of mammaliaforms, the one we prefer shows that allotherians (containing "haramiyidans" and multituberculates) form a clade that is nested within Mammalia. This hypothesis implies that detachment of the middle ear ossicles from the dentary bone took place once in allotherians, from an ancestral condition represented by Haramiyavia to the common ancestor of euharamiyidans and multituberculates and that acquisition of the DMME in allotherians was independent to those of monotremes and therians; thus, the DMME evolved at least three times independently in mammals. Other hypotheses that place "haramiyidans" outside of Mammalia but leave multituberculates within mammals would require independent acquisition of the DMME as well as parallel evolution of numerous dental, cranial and postcranial similarities in euharamiyidans and multituberculates, respectively.

EAR1-5  10:30 am  Inner ear morphology in gondwanatherian mammals and implications for ear evolution in mammaliaforms. Hoffmann S*, Stony Brook University   simone.hoffmann@stonybrook.edu
Abstract: Therians are exceptional among vertebrates in detecting high-frequency sounds. This ability has been associated with the evolution of an ossified cribriform plate, ossified primary and secondary laminae, and elongation of the cochlear canal following the loss of a lagena. The temporal sequence of acquisition of these features remains largely unknown. A lagena is present in most extant non-mammalian vertebrates and monotremes, but absent in therians. Based on this distribution it is assumed that a lagena was present in basal mammaliaforms and lost in stem therians. Fossil evidence supporting this hypothesis is ambiguous. In fossils a lagena is assumed to have been present if the apex of the cochlear canal is expanded and a separate canal for the lagena nerve is present, as in monotremes. As such, a lagena is assumed to be present in Haldanodon and absent in cladotherians. Osseous laminae are equally ambiguous, being present in cladotherians and possibly in some multituberculates. Here I present the inner ear of two gondwanatherian mammals, Vintana and an undescribed genus, from the Cretaceous of Madagascar. Both taxa have a cochlear canal that is short and slightly curved at the apex, and preserve a modern innervation of the cochlea (primary and secondary osseous laminae, cribriform plate, cochlear ganglion canal). Whereas a lagena is absent in Vintana, a separate canal to the apex of the cochlea is present in the new Malagasy taxon. The new taxon is unique among extinct and extant mammaliaforms in preserving a lagena and osseous laminae. These taxa demonstrate the plasticity within gondwanatherian ear evolution and, if future analyses support placement of Gondwanatheria within Allotheria, within Mesozoic mammals. A lagena may have been lost independently in some multituberculates, Vintana and cladotherians. Osseous laminae seem to have evolved either at the base of Gondwanatheria and Cladotheria or independently within these two clades.

EAR1-6  10:45 am  New study of the membranous labyrinth of monotremes and comparative morphology of mammalian inner ears. Schultz JA*, University of Chicago; Zeller U, Humboldt Universität zu Berlin, Germany; Luo ZX, University of Chicago   jaschultz@uchicago.edu
Abstract: Extant monotremes are distinctive from therians (i.e., marsupials and placentals) in many structures of the membranous inner ear labyrinth. We re-examined the membranous labyrinth and innervation in the platypus (Ornithorhynchus anatinus) and short beaked echidna (Tachyglossus aculeatus), using histological sections and 3D reconstruction. Our investigations revealed inner ear features that were not described previously, or poorly understood in monotremes. For instance, the membranous scalae (scala vestibuli, scala media and scala tympani) holding the organ of Corti and macula lagena have different lengths. Scala tympani is shorter than scala media and scala vestibuli, and is not involved in the apical coil of the bony labyrinth. The helicotrema, a conduit between scala vestibuli and scala tympani, is in sub-apical position near the isthmus of the apically coiled scala media. In contrast, the therian cochlea shows all three scalae in equal lengths, coiling to the same degree, and the helicotrema in apical position. In monotremes, scala tympani bridges over scala media to connect to scala vestibuli where the organ of Corti ends. Scala vestibuli and scala media extend beyond the helicotrema forming coiled blind sacs holding the macula lagena. Bridging of scala tympani and scala vestibuli proximally to the blind ending of scalae media and vestibuli is common in some extant non-mammalian amniotes. Thus monotremes retain an ancestral condition of the helicotrema. However, in monotremes the cochlear apex that contains the lagena is enlarged and coiled. Histosections of subadult ear regions show a thin bony wall separating lagenar nerve fibers from cochlear nerve fibers. In CT scans this separation seems less prominent and fibers of cochlear and lagenar nerve are interwoven. Overall, the membranous labyrinth is more coiled than the external bony cochlear canal. The newly documented features have broad implications for the evolutionary morphology of inner ears of early mammals.



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