Inner ear of a notoungulate placental mammal: anatomical description and examination of potentially phylogenetically informative characters

Abstract

We provide the first detailed description of the inner ear of a notoungulate, an extinct group of endemic South American placental mammals, based on a three-dimensional reconstruction extracted from CT imagery of a skull of Notostylops murinus. This description provides new anatomical data that should prove to be phylogenetically informative, an especially significant aspect of this research given that both the interrelationships of notoungulates and the position of Notoungulata within Placentalia are still unresolved. We also assess the locomotor agility of Notostylops based on measurements of the semicircular canals. This is the best available data on the locomotion of a notostylopid because significant postcranial remains for this group have not been described. The cochlea of Notostylops has 2.25 turns, and the stapedial ratio is 1.6. The stapedial ratio is one of the lowest recorded for a eutherian, which typically have ratios greater than 1.8. The fenestra cochleae is located posterior to the fenestra vestibuli, a condition previously only reported for some stem primates. The separation of the saccule and utricule of the vestibule is visible on the digital endocast of the bony labyrinth. The posterior arm of the LSC and the inferior arm of the PSC are confluent, but these do not form a secondary crus commune, and the phylogenetic or functional significance of this confluence is unclear at this time. Locomotor agility scores for Notostylops suggest a medium or 'average' degree of agility of motion compared to extant mammals. In terms of its locomotion, we tentatively predict that Notostylops was a generalized terrestrial mammal, with cursorial tendencies, based on its agility scores and the range of locomotor patterns inferred from postcranial analyses of other notoungulates.

Fossil evidence on evolution of inner ear cochlea in Jurassic mammals

“…The agility scores derived from the sizes of the canals for Dryolestes range from 3.16 to 3.67 (electronic supplementary material). This shows that Dryolestes had a nearly average degree of agility of motion, according to scales of extant mammals [19], and interpreted for fossil mammals [20,21]. Based on extant mammals with the same range of agility scores [19] as Dryolestes, it is certain that Dryolestes did not have gliding, flying, saltatorial or fully aquatic locomotion.…”

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).…”

“…Besides postcranial anatomy, the shape and proportions of vestibular labyrinth structures also record configurations related to locomotion and posture, which in turn can be used as a reliable proxy to characterise locomotor behaviour and agility in extant, and, by extrapolation, extinct mammals (e.g., Spoor et al 2007;Walker et al 2008;Silcox et al 2009;David et al 2010;Macrini et al 2010;Malinzak et al 2012;Ryan et al 2012;Berlin et al 2013). To date, nothing is known about this pattern in Leptictida.…”

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)'

“…It projects beyond the PSC as seen in dorsal view ( fig. 21E), as in other described litopterns and Notostylops (Macrini et al, 2010;Billet et al, 2015), in contrast to Pachyrukhos in which it does not extend as far (Macrini et al, 2013). However, it should be noted that the difference may be size related, in the sense that Pachyrukhos, a very small notoungulate, does not exhibit greatly thickened capsular walls.…”

“…The semicircular canals are arranged approximately at mutual right angles (table 3). The shape, relative size, and radius of curvature of the semicircular canals vary among litopterns and notoungulates (see Macrini et al, 2010Macrini et al, , 2013Billet et al, 2015).…”