The entotympanic of Equus caballus (Perissodactyla, Mammalia)

Mammalian Biology 78 (2013) 231–234

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Short Communication

The entotympanic of Equus caballus (Perissodactyla, Mammalia) Wolfgang Maier a , Adrian Tröscher b , Irina Ruf b,∗ a b

Institut für Evolution und Ökologie, Fachbereich Biologie, Eberhard-Karls-Universität Tübingen, Auf der Morgenstelle 28, 72076 Tübingen, Germany Steinmann-Institut für Geologie, Mineralogie und Paläontologie, Rheinische Friedrich-Wilhelms-Universität Bonn, Nussallee 8, 53115 Bonn, Germany

a r t i c l e

i n f o

Article history: Received 21 March 2012 Accepted 11 May 2012 Available online 12 June 2012 Keywords: Perissodactyla Equus caballus Bulla tympanica Entotympanic Ectotympanic

a b s t r a c t In the majority of extant placental mammals the bulla tympanica is composed of two skeletal elements, the entotympanic and the ectotympanic. Former studies revealed that the presence of an entotympanic in the bulla tympanica of extant Perissodactyla is restricted to Rhinocerotidae. The existence of the entotympanic in Tapiridae and Equidae remained speculative. Here we present the first evidence of an entotympanic, strictly speaking rostral entotympanic, in the domestic horse, Equus caballus. The enchondrally ossified entotympanic can be easily separated from the desmal ectotympanic by its greater thickness and by its cancellous bone texture in a late fetal stage. Both elements are separated by a suture that is in the process of coalescence. The complete fusion of the two elements and the unification of bone texture are almost accomplished at birth but the entotympanic and ectotympanic assume the same thickness obviously not until early postnatal development. Based on modern phylogenetic hypotheses we can conclude that the common ancestor of Perissodactyla must have possessed a well-developed entotympanic, probably only evident in their fetal life. This must be considered as a plesiomorphic character state of this order, because the entotympanic is a neomorphic apomorphy of placental mammals. However, the prenatal fusion of the entotympanic and the ectotympanic is an apomorphy of Equus caballus and possibly of the Equidae as a whole. © 2012 Deutsche Gesellschaft für Säugetierkunde. Published by Elsevier GmbH. All rights reserved.

The bulla tympanica of therian mammals is very differently composed (van Kampen 1905; van der Klaauw 1922, 1931; Starck 1967, 1995; Novacek 1977; MacPhee 1979, 1981). In marsupials it is formed by different processes of neighbouring skull elements (Maier 1989); in many placentals a new element, the entotympanic, participates in the ventral closure of the bulla tympanica (van Kampen 1905; van der Klaauw 1931; MacPhee 1979, 1981). The entotympanic is most probably a neomorphic feature of placentals, because it is present in both xenarthrans and afrotherians, which are now commonly considered as basal branches of the Placentalia (Murphy et al. 2001; Nishihara et al. 2006; Marcot 2007; Asher et al. 2009; Meredith et al. 2011; Hassanin et al. 2012). Ontogenetically, this element develops first as cartilage and then becomes ossified secondarily; in a few taxa it remains cartilaginous or it is calcified only (van Kampen 1905). Ontogenetic studies have further shown that the entotympanic is normally constituted of two heterogeneous elements: The rostral entotympanic appears to be derived from the cartilage of the auditory tube; the posterior entotympanic is mostly connected with the sheath of the tympanohyal (van der Klaauw 1922; Spatz 1966; MacPhee 1981). These two elements normally fuse and ossify as the os bullae (van Kampen 1905). Thus the

∗ Corresponding author. Tel.: +49 228 733102; fax: +49 228 733509. E-mail address: [email protected] (I. Ruf).

entotympanic is not essentially new but appears to be derived from pre-existing cranial structures. According to the literature only a few higher taxa of placental mammals are missing entotympanics, namely Glires, Primates and Cetartiodactyla (van Kampen 1905; van der Klaauw 1931). Based on modern cladistic hypotheses we have to conclude that the absence of this neomorphic skeletal element must be the result of parallel secondary reduction. Within Euarchontoglires, Scandentia and Dermoptera have well-developed entotympanics. The same is true for the sister-taxon of the Cetartiodactyla, as all members of the Pegasoferae (Chiroptera, Carnivora, Pholidota, and Perissodactyla; Nishihara et al. 2006) show well-developed entotympanics. Among the extant Perissodactyla van Kampen (1905) was able to show an entotympanic only in an adult specimen of Dicerorhinus sumatrensis. Here the entotympanic is a bony lamella forming the medial wall of the tympanic cavity; it is separated from the petrosal by a suture (Fig. 1). Ventrally, there remains an open gap between the ectotympanic and the entotympanic that is most probably closed by derivatives of the annulus membrane. The histological serial section of a fetal Diceros bicornis (Table 1) is far too young to provide any information about the entotympanic. The bulla tympanica of Tapirus terrestris looks very similar to that of the rhino, but van Kampen (1905) stated that a separate entotympanic is missing. The medial wall of the bulla tympanica is formed by a bony crest of the petrosal (Fig. 1), and van Kampen considered

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Table 1 Investigated specimens of Perissodactyla. Abbreviations: CRL – crown-rump length, HL – head length, SL – skull length. Species

Specimen number

Collection

Age

Data set

Diceros bicornis

–

Fachbereich Biologie, Universität Tübingen, Germany

Histological serial sections

Equus sp.

–

Fachbereich Biologie, Universität Tübingen, Germany

Equus caballus

Mamm 447

Equus caballus

M1825

Equus caballus

Collection W. Maier

Zoologische Schausammlung, Universität Tübingen, Germany Steinmann-Institut, Universität Bonn, Germany Fachbereich Biologie, Universität Tübingen, Germany

Fetal CRL 114 mm HL 37 mm Fetal CRL 170 mm HL 60 mm Late fetal SL 255 mm Neonate SL 322 mm Postnatal SL 340 mm

it possible, that this crest represents an entotympanic that is fused with that bone; he realized that only an ontogenetic study can clarify this homology, but to our knowledge no fetal or juvenile tapir has ever been studied. In a subadult Tapirus indicus Parker (1882) observed a rudimentary os bullae imbedded in fibrous tissue. Equus caballus has a closed bulla tympanica that medially contacts the petrosal by a suture (Fig. 1). Because no ontogenetic studies were then available, van Kampen (1905) speculated that in addition to the ectotympanic, an entotympanic may contribute to the formation of the bulla. In this case, the ectotympanic with the styliform process would provide the outer, the entotympanic the inner wall (Fig. 1). However, to our knowledge no entotympanic has

Fig. 1. Schematic drawings of frontal sections of the left bulla tympanica of extant Perissodactyla (after van Kampen 1905); phylogenetic arrangement according to Springer et al. (2003), lettering by the authors. An isolated entotympanic is present in a juvenile Dicerorhinus sumatrensis, whereas its existence in Tapirus terrestris and in Equus caballus is doubtful according to van Kampen (1905). Abbreviations: ect – ectotympanic, ect+ent? – dubious fusion of ectotympanic and presumed entotympanic, ent – entotympanic, ent? – dubious entotympanic, mt – membrana tympani, pe – petrosal, sq – squamosal. Not to scale.

Histological serial sections

␮CT resolution (voxel size) 0.090464 mm ␮CT resolution (voxel size) 0.057847 mm ␮CT resolution (voxel size) 0.032350 mm

yet been shown in fetal or infantile specimens of the horse (Martin 1914; Nickel et al. 1984; Schaller 2007). In order to elucidate the pattern in Equidae we investigated three skulls of late fetal to early postnatal stages of the domestic horse Equus caballus using the ␮CT scanner v|tome|x s (GE phoenix|x-ray) at Steinmann-Institut, Universität Bonn, Germany (Table 1). The software AVIZO 7.0 (Visualization Sciences Group) was used to perform virtual 3D models and virtual cross-sections of the bulla tympanica. In addition, histological serial sections of a young fetal Equus sp. (most probably a zebra) were investigated (Table 1). All examined species including those taken from van Kampen (1905) were taxonomically classified according to Wilson and Reeder (2005). The fetal stage of Equus sp. has developed neither an entotympanic nor a cartilage of the auditory tube yet. The ventral aspect of the bulla tympanica of a late fetal Equus caballus (Mamm 447) shows a clear distinction between the ectotympanic ring and the crescent-shaped entotympanic at its medial side (Fig. 2A and B). The bony entotympanic forms a flat bulge with an irregular surface; the ectotympanic shows a somewhat smoother surface. Both elements are separated by a flat sulcus with more pronounced perforations. It is evident that the contact line between the two bullar bones is in the process of coalescence. Furthermore, the entotympanic is relatively broad at its anterior end and narrower posteriorly; as it does not reach the posterior end of the ectotympanic, the observed entotympanic seems to be a rostral entotympanic only. These observations are supported by the virtual ␮CT crosssections of the same specimen (Fig. 2C). The ectotympanic is a thin and dense bony plate, whereas the entotympanic is thick and cancellous; this is not unexpected because the bony entotympanic normally is the endochondral ossification of the entotympanic cartilage, whereas the ectotympanic is a desmal ossification. The ␮CT scan of a neonate foal (M1825) reveals that the entotympanic is still distinctly thicker than the ectotympanic. However, both elements appear quite dense and they no longer show differences in their bone texture; furthermore the suture between entotympanic and ectotympanic is not visible anymore (Fig. 2D). In the skull of an older foal (collection W. Maier) the differences between the ecto- and entotympanic have completely disappeared and both have almost the same thickness as demonstrated by the ␮CT images (Fig. 2E). These few observations demonstrate that the entotympanic of Equus caballus can only be distinguished in prenatal ontogenetic stages. We do not yet know exactly when the entotympanic cartilage appears during fetal development of the horse and when it begins to ossify, but it must be relatively late in ontogeny; however, it is no more distinguishable in the newborn. Because horses have precocial young, late fetuses are very big and well differentiated, and these perinatal stages are very difficult to handle by

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Fig. 2. (A) Photo of the left bulla tympanica of a late fetus of Equus caballus (Mamm 447) in ventral view; the different surface structure of the entotympanic and ectotympanic as well as the suture in the process of fusion (arrows) are clearly visible. Insert indicates picture detail. (B) Virtual 3D reconstruction of the same region based on a ␮CT scan; entotympanic in dark grey, ectotympanic in light grey. Cutting plane indicates the level of the transversal ␮CT images of the left ear region of Equus caballus in (C)–(E). (C) late fetus (Mamm 447), (D) neonate (M1825), (E) postnatal (collection W. Maier). Abbreviations: co – cochlea, eam – external acoustic meatus, ect – ectotympanic, ent – entotympanic, ma – malleus, mtt – musculus tensor tympani, pe – petrosal, sh – stylohyal, sq – squamosal, tt – tegmen tympani.

histological techniques. Modern imaging techniques will help to close this gap of morphological knowledge as demonstrated in this study. In any case, we predict that younger stages will show a clearer distinction between the two skeletal elements, and that the entotympanic at the beginning will show up as cartilage, which probably has connections with the tubal cartilage and the rostral tegmen tympani. Our observations confirm that the complete fusion of entotympanic and ectotympanic occurs during very late fetal development and that the bone texture becomes re-organized soon after birth forming the composite bulla of the foals and adult horses. Therefore it is perhaps not unexpected that no distinct entotympanic has yet been described in fossil horses. However, an isolated entotympanic has been described in certain Chalicotheriidae (van der Klaauw 1931; Radinsky 1964). Furthermore, the entotympanic of Equus is strongly reminiscent of that in many carnivores (Hunt 1974). The find of a rostral entotympanic in Equus caballus renders it most likely that this skeletal element of the bulla tympanica also existed as a plesiomorphic structure in the groundplan of the last common ancestor of extant perissodactyls and as a distinct element present at least in late fetal stages. This conclusion is not unexpected, because all presumeable relatives within Pegasoferae, Scrotifera, Laurasiatheria, and Placentalia possess a neomorphic rostral entotympanic (van Kampen 1905). However, its prenatal fusion with the ectotympanic has to be considered as an apomorphy within the Perissodactyla.

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