Petrosal and inner ear anatomy and allometry amongst specimens referred to Litopterna (Placentalia)

Abstract

New isolated petrosals from the Itaboraí beds of Brazil (late Palaeocene or early Eocene) are here described and referred to the early diverging litoptern Miguelsoria parayirunhor, based on phylogenetic, size, and abundance arguments. Both the external and internal anatomy of these specimens were investigated, which for the first time document many details of the auditory region of a Palaeogene litoptern. Our cladistic analysis, which included our new observations, failed to recover a monophyletic Litopterna but did not exclude it. A constrained analysis for the monophyly of this order showed that several features such as a (sub)quadrangular and anteroposteriorly elongated tensor tympani fossa and a large notch in the vicinity of the external opening of the cochlear canaliculus may constitute synapomorphies for Litopterna. The evolution of several other auditory characters amongst Litopterna is discussed and the relative dimensions of the inner ear and surrounding petrosal in the group were also investigated. This allowed detection of negative allometry of the bony labyrinth within the petrosal, which was confirmed by measurements and regression analysis across a larger sample of placental mammals. This scaling effect probably has an important influence on several characters of the bony labyrinth and petrosal, amongst which are the length of the vestibular aqueduct and cochlear canaliculus. It demonstrates that many aspects of the morphological variation of the bony labyrinth need to be thoroughly investigated before being incorporated into phylogenetic analyses. © 2015 The Linnean Society of London

Digital reconstruction of the inner ear of Leptictidium auderiense (Leptictida, Mammalia) and North American leptictids reveals new insight into leptictidan locomotor agility

“…The comprehensive work of Ekdale (2013) on placental bony labyrinths, covering all living orders, clearly indicates that there are several exceptions to the simplified classification of Segall (1970), and that the stapedial ratio might represent a continuous character. Further confirmed exceptions in placentals are found in extinct notoungulates, litopterns, and in the living artiodactyl Moschiola (Macrini et al 2010;Orliac et al 2012;Billet et al 2015), suggesting that exceptions are common in reality, and that this classification may need revision. In our case, the leptictidans also fall in the range of marsupials described by Segall (1970).…”

Supplemental Material for 'Exceptional skull of Huayqueriana (Mammalia, Litopterna, Macraucheniidae) from the late Miocene of Argentina : anatomy, systematics, and paleobiological implications. (Bulletin of the American Museum of Natural History, no. 404)'

“…21F; table 3). All litopterns measured to date exhibit similar values, including Early Eocene taxa (2.25 to 2.4 turns), "Proterotherium" 2 (2.1), Diadiaphorus (2.25), and Macrauchenia (2.3) as measured by Billet et al (2015). The notoungulates Notostylops (2.25; Macrini et al, 2010) and Hegetotherium (2.5; Simpson, 1936;Macrini et al, 2010) are also similar, whereas in some other notoungulates (Altitypotherium, Pachyrukhos, and Cochilius) the degree of coiling is less, ~2 turns (Macrini et al, 2013).…”

“…A single cochlear canaliculus is present in IANIGLA-PV 29, differing from some specimens referred to Miguelsoria in which there is a small accessory aperture (probably vascular) accompanying the cochlear canaliculus (Billet et al, 2015).…”

“…This structure is very common in mammals, as the sinus is the major connector between pharyngeal venous plexuses, the cavernous sinus, and the internal jugular vein on the ventral surface of the skull. Salient crests bordering a similar sulcus have been described for Miguelsoria (Billet et al, 2015).…”

“…Protolipternidae, and Sparnotheriodontidae) are also frequently cited as true litopterns, but whether they actually belong in the same monophyletic group as the later taxa is debatable (Cifelli, 1993;Billet et al, 2015).…”