Palaeontologia Electronica palaeo-electronica.org The narial morphology of Metarhinus and Sphenocoelus (Mammalia, Perissodactyla, Brontotheriidae) Bryn J. Mader ABSTRACT The brontothere genera Metarhinus and Sphenocoelus (= Dolichorhinus) are both characterized by an unusual redirection of the air passageway in which the internal nares are effectively closed off and new openings that serve as the functional internal nares have appeared posteriorly. Rather than passing lateral to the vomer, the air- stream appears to pass over or through the bone (thus bypassing the anatomical inter- nal nares) and exits through an enlarged vomerine notch. If correctly interpreted, this morphology is unique among mammals. In the past it has been suggested that the pos- terior displacement of the internal nares is either an amphibious adaptation or an adap- tation to enhance olfaction. The current paper favors the latter interpretation. Among other reasons is that the change in morphology channels the airstream directly toward the olfactory epithelium in the area of the ethmoid. In order for the vomer to channel air, the septal cartilage would require significant modification. It is speculated here that the respiratory septum (vomer, crest of the maxilla, and crest of the palatine) might have been wholly or partially isolated from the olfactory septum (perpendicular plate of the ethmoid and septal cartilage), allowing air to be channeled through (or over) the vomer. Bryn J. Mader. Department of Biological Sciences and Geology, Queensborough Community College, 222- 05 56th Avenue, Bayside, NY, 11364-1497, USA. [email protected] Keywords: titanothere; Dolichorhinus, vomer; nasal breather; nasal cycle; evo-devo Submission: 26 August 2018 Acceptance: 18 January 2019 INTRODUCTION nares have appeared posteriorly. Riggs (1912) was the first to recognize this in Sphenocoelus (= Doli- The brontothere genera Metarhinus and chorhinus), although he assigned the attribute to a Sphenocoelus are both characterized by an single species only. Peterson (1924, p. 442-443) unusual redirection of the air passageway in which recognized the broader distribution of the trait, the internal nares are effectively closed off and regarding it as a generic rather than a specific new openings that serve as the functional internal Mader, Bryn J. 2019. The narial morphology of Metarhinus and Sphenocoelus (Mammalia, Perissodactyla, Brontotheriidae). Palaeontologia Electronica 22.1.8A 1-15. https://doi.org/10.26879/919 palaeo-electronica.org/content/2019/2417-narial-morphology Copyright: February 2019 Society of Vertebrate Paleontology. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. creativecommons.org/licenses/by/4.0/creativecommons.org/licenses/by-nc-sa/4.0/ MADER: NARIAL MORPHOLOGY character. Peterson also more fully described the many mammals (e.g., Equus, Camelus, Panthera) anatomy (p. 409) noting peculiar pouch-like struc- there is a prominent anterior-posterior channel that tures (choanal pouches of Mader, 2008, and Mihl- runs down the length of the vomer medially. The bachler, 2008) that close off the internal nares and dorsal part of this channel is open anteriorly where re-direct the air stream posteriorly to exit at a new the bone articulates with the premaxilla (thus pre- opening between the pterygoid processes. Osborn senting a u-shaped cross section), but may be (1929) identified the choanal pouches of Spheno- closed posteriorly, thus forming a hollow tube. The coelus as maxilloturbinates, as did Mihlbachler septal cartilage fits into this u-shaped channel (= (2008). septal groove) and extends up to the dorsal roof of In 1981, an extremely well-preserved Metarhi- the nasal cavity as a continuous sheet. nus skull was collected by L. Shields of the Univer- Normally the air stream passes laterally on sity of Colorado. The skull was preserved in an each side of the vomer before exiting the nasal exceptionally soft matrix, which made it possible to cavity through the internal nares (= choanae). In prepare out structures that, otherwise, could not many mammals the vomer forks posteriorly where have been examined. This included structures in it joins the presphenoid (vomerine wings or alae) the internal nares similar to those of Sphenocoe- leaving a small, roughly triangular space between lus. A bony structure projecting from the internal the two bones known as the vomerine notch (see nares between the pterygoids in the same general Eisenmann et al., 1988, figure 1). The vomerine location as the choanal pouches of Sphenocoelus notch is also known by the Latin name, incisura was interpreted by Mihlbachler (2008) as posteri- vomeris (Sisson, 1910). orly displaced turbinates. However, Mader (2009) This vomerine notch is found in equids (e.g., identified this same structure as a greatly enlarged Equus caballus), thus indicating its presence vomer. Mader noted that the internal nares were among Perissodactyla, but is also found in felids essentially closed off and that an opening posterior (e.g., Panthera leo, Felis concolor, Felis sylvestris), to the enlarged vomer was the functional exit for and canids (e.g., Canis lupus). The vomerine notch the airstream. Mader identified that opening as a in horses is rather small, but its morphology in cats highly modified vomerine notch. The purpose of and dogs may help to explain how the condition in the present paper is to review and expand upon dolichorhinine brontotheres (sensu Mader, 1989, the interpretations of these structures given by 1998, and 2008) could have evolved. It will be Mader (2008, 2009) and to further explore their noted from the lion skull pictured in Figure 1, that possible functional significance. there is a distinct gap between the presphenoid and vomer, and it is only the alae of the vomer that METHODS approach the anterior edge of the presphenoid. This large opening marks the exit of the anterior- The taxonomy employed in the present paper posterior channel of the vomer noted above. follows that of Mader (1998, 2008), which differs Hence, in osteological preparations (where the somewhat from that of Mihlbachler (2008), the septal cartilage is not preserved) there is a continu- other major recent reviser of North American bron- ous open passageway running through the interior totheres. of the vomer from the nasal cavity to the vomerine Institutional abbreviations. AMNH, American notch. It is speculated here that this passage Museum of Natural History, New York, New York, through the vomer became the functional air pas- USA; FMNH PM, Field Museum of Natural History, sageway in certain members of the Dolichorhin- Chicago, Illinois, USA; UCM, University of Colo- inae. rado Museum, Boulder, Colorado, USA. Dissection of the domestic cat (Felis catus) Narial Morphology of Typical Mammals performed for this study indicates that, in the living animal, the area of the vomerine notch lies within The following description of the narial mor- the nasopharynx, dorsal to the soft palate (the soft phology in typical mammals is an expansion of the palate must be removed in order to inspect the description given by Mader (2009). In most mam- area). The epithelial tissue covering the presphe- mals the vomer forms a thin, vertical plate that is noid/vomer area is thin and appears to be continu- part of the nasal septum and helps divide the pos- ous and uninterrupted. There is no perforation terior nasal cavity into right and left sides. The through the soft tissue allowing the vomerine notch nasal septum is also composed of the perpendicu- to communicate directly with the nasopharynx. lar plate of the ethmoid, the septal cartilage, the Therefore, if this became the air passageway in crest of the maxilla, and the crest of the palatine. In 2 PALAEO-ELECTRONICA.ORG FIGURE 1. Skull of African Lion (Panthera leo, author’s private collection) showing anatomy of vomer and vomerine notch. Turbinates in this specimen are largely missing. 1, anterior view; 2, ventral view; 3, detail of anterior nasal cav- ity; 4, detail of internal narial region. Abbreviations: IN, internal nares; Ps, presphenoid; V, vomer; VN, vomerine notch, the presumed functional internal nares (N) in all other figures. Rectangles superimposed on upper photo- graphs show areas of detail below. In 3 and 4 the vomer has been lightened for better clarity. certain dolichorhinine brontotheres, a perforation in tral pair. The ventral pair project from the lateral the soft tissue would have had to evolve. walls of the nasal cavity from the maxillae and are The turbinates (or conchae) are usually elabo- known as maxilloturbinates. The two pairs located rate scrolls of very thin bone that fill most of the above this (i.e., the middle pair and dorsal pair) interior of the nasal cavity on either side of the project from the ethmoid bone and are, thus, eth- nasal septum. There are three pairs of turbinates moturbinates. altogether: a dorsal pair, a middle pair, and a ven- 3 MADER: NARIAL MORPHOLOGY SYSTEMATIC PALEONTOLOGY body of the vomer and anterior to the presphenoid (labelled “N” in Figure 2). This opening is in the Order PERISSODACTYLA Owen, 1848 exact location of the vomerine notch found in Family BRONTOTHERIIDAE Marsh, 1873 horses and some other mammals and is identified Subfamily DOLICHORHININAE Riggs, 1912 by Mader (2009 and the present paper) as such. Genus METARHINUS Osborn, 1908 Regarding the vomer, the identity of this bone v. 1912 Rhadinorhinus Riggs, p. 36-37, pl. 11 (figs. was most evident when the skull was apart (prior to 2-3) (original description). restoration), and it could be readily viewed from v. 1914 Heterotitanops Peterson, p. 53-57, figs. 1-2, multiple angles. However, this same bone was pl. 11 (original description). identified as posteriorly shifted turbinates by Mihl- v. 2008 Fossendorhinus Mihlbachler, p. 63-69, figs. bachler (2008). That this bone does not represent 24-26 (new generic name).