Allometric and Phylogenetic Aspects of Stapes Morphology in Ruminantia (Mammalia, Artiodactyla)

Coraline Guignard,Loïc Costeur,Georg Schulz,Bastien Mennecart,Bert Müller,Laura Dziomber

doi:10.3389/feart.2020.00176

Abstract

The stapes is the smallest bone of the mammalian skeleton. Being the innermost middle ear ossicle, it is in contact with the inner ear and is directly responsible for sound transmission into it. Today, Ruminantia are one of the most diversified groups of large mammals with more than 200 species. However, their stapes has been very little studied. Here we investigate the shape of 66 stapes from 44 species of extant and extinct Ruminantia, including intra-individual and intra-specific observations, based on 3D tomographic data. Shape differences and similarities are quantitatively discussed thanks to 3D geometric morphometrics. The overall size of the stapes scales with a negative allometry in comparison to body mass. Moreover, the overall shape of the stapes informs about phylogeny. A trend is observed from a concave posterior crus with an enlarged stapes capitulum in Antilocapridae to a relatively straight posterior crus with a little reduced stapes capitulum in Cervidae, Bovidae being intermediate. In addition, the stapes of Antilocapridae is relatively trapezoid in lateral view; that of Cervidae is more triangular in lateral view; and that of Bovidae is relatively rectangular in lateral view. The stapedial footplate shape also gives phylogenetic information. The Tragulidae stapedial footplate is antero-posteriorly asymmetrical. The stapedial footplate is ovoid in stem Pecora. It is asymmetrical in Bovidae, while it is more symmetrical in Cervidae. This is in agreement with previous studies on the ruminant bony labyrinth showing that the oval window, the counterpart of the stapedial footplate on the inner ear, bears a strong phylogenetic signal already distinguishable in early Miocene ruminants.

Highlights

The middle ear bones compose a chain of three bones forming a complex structure only present in mammals
A strong negative allometry is observed between these elements and body mass (α = 0.1794 and α = 0.1658, respectively, a isometry values being 1 and 1.5, respectively, Figure 2) indicating that larger species have relatively small stapes and smaller species have relatively large ones
The intercrural foramen centroid size possesses a negative allometry in comparison to the stapes centroid size (α = 0.6762, a isometry values being 1.5) indicating that the intercrural foramen grows slower than the stapes (Figure 2)

Summary

Introduction

The middle ear bones (ossicles) compose a chain of three bones (stapes, incus, and malleus) forming a complex structure only present in mammals. The evolution of the incus and the malleus from the “reptilian grade” jaw bones to the smallest bones of the mammalian skeleton has been intensively studied (e.g., Rich et al, 2005; Luo, 2007; Meng et al, 2011; Maier and Ruf, 2016a,b). Their morphology and size are often associated to hearing sensitivity (Fleischer, 1978; Rosowski and Graybeal, 1991; Stoessel et al, 2016a; Bernardi and Couette, 2017). The three dimensional (3D) structure of this bone is complex and can barely be described based on 2D images only (Stoessel et al, 2016b)

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