Introduction

The shiny enamel of mammalian teeth is the hardest and most resistant material mammals can produce. The functional quality of the enamel is determined by the hardness of the material and by its internal structure. The decussating layers of prisms serve as a crack stopping mechanism (Pfretzschner 1988). Although the internal structures are of functional significance, they are genetically controlled, since the tooth is formed in the crypt where no mechanical forces are active. Despite the common occurrence of parallel and convergent evolution, variation in enamel microstructure may still provide insights into the phylogeny of particular groups of mammals, such as perissodactyls (Koenigswald 2004).

Most internal structures in the enamel are only visible using high magnification. The Hunter-Schreger Bands we discuss here for perissodactyls are visible under low magnification, as light and dark bands. They were originally figured by Hunter (1778) and Schreger (1800) for humans and bovids two centuries ago and accordingly are named Hunter-Schreger Bands (HSB). These bands occur in the enamel of most large mammals (Koenigswald et al. 1987).

The configuration of the HSB in perissodactyls shows an unusual extent of variation compared to other mammalian orders, though there is general consistency within perissodactyl clades. HSB are oriented vertically in rhinos (Rensberger and Koenigswald 1980; Fortelius 1984; Boyde and Fortelius 1986), a phenomenon originally observed and described in some detail by Quenstedt (1852). He illustrated the vertically oriented bands in molars of rhinoceroses (Fig. 1). The specific HSB configuration of chalicotheres and brontotheres was described as U-shaped (Koenigswald 1994). As shown below, different configurations occur in other perissodactyl clades. In this paper we concentrate on two aspects of HSB: the phylogenetic significance and the functional value of reoriented HSB.

The complex structure of the enamel can be studied on various levels (Koenigswald and Clemens 1992). The schmelzmuster describes the occurrence of different enamel types within a tooth. In this study, we concentrated on one aspect of the schmelzmuster, namely the configuration of HSB (as defined in the next section), and we surveyed the variability of this character in a wide diversity of perissodactyls. We focused on cheek teeth (molars and premolars), because these are best represented for the greatest number of taxa, but we also made observations on the anterior dentition (incisors and canines) whenever possible, since the schmelzmuster may differ in the various teeth at the dentition level. We then used these observations to make inferences regarding the evolution of HSB configurations in perissodactyls.

Institutional abbreviations.—AMNH, American Museum of Natural History, Department of Paleontology, New York, New York; BSPG, Bayerische Staatsammlung für Paläontologie und Geologie, München, Germany; CM, Carnegie Museum, Section of Vertebrate Paleontology, Pittsburgh, Pennsylvania; GMH, Geiseltalmuseum, Halle, Germany; HHZM, Zoologisches Museum der Universität Hamburg, Germany; HLMD, Hessisches Landesmuseum, Darmstadt, Germany; KOE, Koenigswald enamel collection of the STIPB, Bonn, Germany; MB, Museum für Naturkunde Berlin, Germany; MCZ, Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts; MR, Michael Rummel collection, Natur-Museum, Augsburg, Germany; NHMB, Naturhistorisches Museum, Basel, Switzerland; NHMW, Naturhistorisches Museum, Wien, Austria; PMM, Palaeontological Museum, Moscow, Russia; SDSM, South Dakota School of Mines, Museum of Geology, Rapid City, South Dakota; STIPB, Steinmann Institut, Paleontology, University of Bonn, Germany; UM, University of Michigan Museum of Paleontology, Ann Arbor Michigan; USGS, United States Geological Survey, Denver, Colorado, specimens now housed at USNM; USNM, National Museum of Natural History, Smithsonian Institution, Department of Paleobiology, Washington, D.C.; ZFMK, Zoologisches Museum Alexander Koenig, Bonn, Germany; ZSTÜ, Zoologische Sammlung Universität Tübingen, Germany.

Other abbreviations.—Br, Bridgerian; EDJ, enamel-dentine junction; HSB, Hunter-Schreger bands; if, interface between fields of HSB; OES, extend almost to the outer enamel surface; SEM, Scanning Electron Microscope; Wa, Wasatchian; Incisors, canines, premolars, and molars are designated as I, C, P, M for uppers, i, c, p, m for lowers.

The nature of Hunter-Schreger Bands

Hunter-Schreger Bands are an optical phenomenon produced by refraction of light due to the internal structure of the enamel. The term HSB is not only used for this phenomenon but is traditionally also used for the underlying structure formed by layers of decussating prisms. In the terminology related to enamel structures we follow Koenigswald and Sander (1997b). Enamel is a highly mineralized material. Several structural levels must be differentiated to understand the phenomenon of HSB. The mineral hydroxyapatite forms very thin crystallites that are organized into rods, called enamel “prisms”, although these structures are not prisms in the mineralogical sense. These prisms have a diameter on the order of 5 µm. Many studies have been devoted to the changing shape of the cross-section of the enamel prisms (e.g., Shobusawa 1952; Boyde 1965; Boyde and Martin 1984; Wood and Stern 1997). The enamel prisms start at the enamel-dentine junction (EDJ) and extend almost to the outer enamel surface (OES). They are normally arranged in groups with the same prism orientation or in a very symmetrical arrangement. Occasionally the orientation of the prisms is irregular, e.g., in proboscideans (Koenigswald 1997a; Tabuce et al. 2007; Ferretti 2008). The orientation of the prisms defines the various enamel types (Koenigswald and Sander 1997a, 1997b). The enamel cap of a tooth may be formed by two or three different enamel types. These can be arranged in layers within the thickness of the enamel band or occur in specific areas of the teeth. The arrangement of enamel types in a tooth was defined as schmelzmuster (Koenigswald 1980). The different tooth families within the same dentition may show differences in the schmelzmuster (Koenigswald and Clemens 1992).

Fig. 1.

Vertical HSB configuration as figured by Quenstedt (1852: pl. 2: 1 [A]; pl. 3: 35 [B]). A. Occlusial view with HBS in the extoloph. B. Orienation and bifiurcation of the HSB in tangential aspect.

The visual phenomenon of HSB (Fig. 2) is caused by the optical properties of the crystallites within the prisms. This effect would hardly be visible in single crystallites, but it is magnified when crystallites are packed in prisms and it is strongly increased when bodies of prisms are oriented in parallel. If the direction of the light coincides more or less with the long axis of the prisms it will disappear in the depth of the enamel. Thus the cross-section appears dark. Light falling perpendicular or at an angle to the prisms is reflected, and thus the prisms appear bright. Thus prisms effectively act in the same manner as fiber optic light guides.

The enamel type defined as HSB is formed by layers of enamel prisms which decussate at a high angle—often about 90°—with the prisms of the adjacent layer. The general biomechanical function of the layers of decussating prisms is to serve as an important crack-stopping mechanism, comparable to plywood structure (Koenigswald and Pfretzschner 1987; Koenigswald et al. 1993; Pfretzschner 1988, 1994).

Fig. 2.

Appearance of the HSB in variable light. A. Scheme of the fiber optic light guide effect of enamel prisms. Those prisms illuminated perpendicular to the long axis reflect brightly, while those illuminated parallel to the long axis appear dark. B. Transverse HSB illuminated from the left side in an incisor of the artiodactyl Myotragus. Note the regular bifurcation of the light bands to the left, of the dark bands to the right. C–E. Vertical HSB in an etched ground section of a molar of Coelodonta antiquitatis (Blumenbach, 1799) (Upper Pleistocene, Germany KOE 59) illuminated from different sides. Light source on the left (C), light source on the right (D), light source perpendicular to the direction of the bands (E), highlighting the transitional zones. White dotted lines connect identical spots.

The light or dark appearance of the bands depends on where the light source is situated. If this is changed from one side to the other, light bands become dark and vice versa (Fig. 2A, C). The thickness of these bands varies and is traditionally measured in the number of prisms per band. In rodent incisors the bands may be only one prism thick (Korvenkontio 1934). In most mammals with HSB, thickness is about 6 to 15 prisms, with larger mammals tending to have thicker HSB (Kawai 1955). In the vertical HSB of Coelodonta antiquitatis we counted 11 to 13 prisms per band.

The optical effect—the light and dark banding—is most obvious when the angle between prisms of adjacent bands is large and when the bands are thick and can be observed under low magnification. Quenstedt (1852) observed and figured vertical HSB in rhinocerotid molars using low magnification. He even correctly observed that these bands bifurcate and that intermediate layers occur between the light and dark bands. These layers are formed by prisms changing from one band to the next by a more or less sharp turn (Rensberger and Koenigswald 1980). Under proper illumination this narrow transitional zone of turning prisms between two bands is visible as a thin reflecting line between the vertical bands (Fig. 2B).

Fig. 3.

Typical cross-ridges on the occlusal surface of the enamel band caused by vertical HSB in a lower molar of Rhinoceros sondaicus Desmarest, 1822 (KOE 71); Pleistocene, Saniran, Java. The vertical HSB are here superimposed by thick radial enamel.

The different prism orientation in HSB might cause differential wear resulting in a structure defined here as cross-ridges in the occlusal surface of the enamel band. They are most obvious in vertical HSB as in rhinocerotids, where they can often be felt when running a fingernail (Fig. 3) along the enamel band. Cross-ridges are not restricted to vertical HSB; they may occur wherever the HSB intersect the occlusal surfaces of the enamel band. On an oblique portion of the enamel band, even transverse HSB might create cross-ridges. Therefore, cross-ridges are not unequivocal evidence of vertical HSB.

Hunter-Schreger Bands have another important feature that differentiates them from other structural elements. The HSB bifurcate in a regular fashion, as was figured by Quenstedt (1852: table 3). One group of bands (such as the light ones) always bifurcates to one side while the other (the dark ones) bifurcates in the other direction. If the light comes from the other side the optical effect is reversed (Fig. 2B). This regular bifurcation of the HSB is related to the primary direction of prisms from the EDJ towards the OES (Koenigswald and Pfretzschner 1987: fig. 14).

We introduce the term “configuration of HSB” to refer to the course of HSB (as it intersects the OES or a tangential plane parallel to the EDJ) when viewed through the side of a tooth (buccal, lingual, mesial, or distal). This describes the orientation of HSB in relation to the growing direction of the tooth as seen from the outside or from tangential sections. The configuration often can be studied directly from the outside, but the layer of HSB is sometimes overlain by a thick layer of a different enamel type (such as radial enamel) that obscures the HSB. In such cases, a tangential section through the enamel (i.e., a section parallel to the OES and EDJ), passing through the layer with HSB, is required to see the HSB configuration. Specific types of configuration are described below. Configuration does not refer to the fact that HSB can run relatively straight or can be wavy in appearance, nor does it describe the angle of inclination defined by Korvenkontio (1934), which relates to the angle that the plane of the HSB forms with the EDJ in vertical sections.

Based on external observation and examination of tangential sections, we found four different configurations of HSB (Fig. 4):

Transverse HSB (Figs. 4, 5).—The HSB are generally parallel to the occlusal surface and the base of the enamel crown. This is essentially horizontal in premolars and molars. The HSB often undulate or show other minor differentiations but otherwise remain transverse. When the worn occlusal surface cuts obliquely through the enamel, these HSB intersect the enamel band.

Curved HSB (Fig. 4; see also Figs. 5–7, 11).—In several groups the basically transverse HSB curve toward the occlusal surface of the enamel band and thus intersect the shearing blades at a large angle. The curved HSB configuration is strictly related to specific areas of the tooth morphology and is always combined with transverse HSB in other parts of the teeth. Where HSB curve from different directions (what we refer to here as different “fields of HSB”), they may meet at a characteristic interface. The interface may be indicated by a distinct groove on the OES. These curved HSB occur mostly in prominent crests, such as well-developed ectolophs and crosslophs in upper and lower cheek teeth. Koenigswald (1994) described the most strongly curved HSB in chalicotheres and brontotheres and referred to them as U-shaped HSB. Fortelius (1985) mentioned curved HSB in several groups as “horizontal (concave)” HSB but did not describe them in detail.

Compound HSB (Fig. 4; see also Fig. 8).—We found a specific type of HSB configuration with transverse HSB as an inner zone and vertical HSB in an outer zone. The occurrence of vertical HSB may be related to enamel thickness but not to specific areas in the tooth morphology. The relative thickness of the inner and outer zones may differ, and this in turn affects how easily one can detect the corresponding HSB.

Vertical HSB (Fig. 4; see also Fig. 9).—In this configuration HSB are vertically oriented and are more or less straight, rather than wavy or curving, on all sides of the tooth from the basal cingulum (where present) up to the occlusal surface. The occurrence of vertical HSB is not related to the tooth morphology. Like transverse HSB, they exhibit unidirectional bifurcation. Since the HSB do not converge, no interfaces are present, as are found regularly in the curved HSB configuration. Cross-ridges caused by the intersection of the HSB with the occlusal surface are very characteristic.

Fig. 4.

Schematic illustration of the four configurations of Hunter-Schreger Bands (HSB) found in Perissodactyla: A. Transverse HSB configuration. B. Curved HSB configuration with interface. C. Compound HSB configuration with transverse HSB in an inner layer and vertical HSB in an outer layer. D. Vertical HSB configuration.

Fig. 5.

Transverse HSB configuration in Equoidea. A. Lower molar of Mesohippus sp. (KOE 1509); late Eocene-early Oligocene, Toadstool Park area, Nebraska, USA. The transverse HSB are to be seen in the tangential section. Note also the transversely oriented perikymata on the outer enamel surface. Both structures are independent from each other. B. Upper molar of Palaeotherium sp. (KOE 4050); upper Eocene, Frohnstetten, Germany; transverse HSB visible in the translucent enamel of the paracone from the outside.

The configuration of the HSB may differ among the different tooth categories within the dentition, especially when dentitions are heterodont (Koenigswald and Clemens 1992). Thus incisors and canines may show a different HSB configuration than premolars and molars. Our survey shows that the presence or absence of differences between incisors and molars makes the characterization of different groups easier.

Methods of investigation

Appendix 1 lists the specimens investigated for this study. The specimens studied were selected to represent a wide spectrum of perissodactyl diversity, emphasizing the earliest members of the order and sampling the various lineages. In addition, several phenacodontid condylarths were examined as an outgroup.

Derived mammalian enamel is often composed of two or three different enamel types forming separate superimposed layers parallel to the EDJ (Koenigswald and Clemens 1992). The three-dimensional prism orientation is studied best from various oriented sections under the scanning electron microscope (SEM). The sections are polished and etched following procedures outlined by Koenigswald (2004). The configuration of HSB can be studied under the binocular microscope under low magnification and often sections are not necessary. Thus specimens do not need to be sacrificed. Most taxa studied here were examined in this way. However, if the layer with HSB is covered by another enamel type, or the enamel is not translucent, non-destructive methods may not be adequate for revealing HSB configuration. In such cases, whenever possible, sections were made. These sections were oriented tangentially—that is, parallel to the EDJ and the OES. Small teeth, such as those of several early perissodactyls, had to be sectioned as well, since the HSB are more difficult to identify in uneven and small areas.

HSB close to the OES can be investigated using the described fiber optic light guide effect of the prism layers. One of the various techniques is to hold the tooth in one hand with the enamel surface in focus under the binocular lens with varying magnification. The light source is held in the other hand. It is best to direct the narrow opening of a light tube of a fiber optic lamp from one side immediately beside the enamel. The light should be applied tangentially to a corner or fracture of the enamel. Most of the light should penetrate into the enamel tangentially. HSB show their light and dark pattern when the light falls at a very low angle onto the cross-section of the enamel band. It takes practice to turn the specimen until the best view is available. It is not easy, however, to document the results photographically.

A magnification of 10 to 20 times is most effective. In relatively clear enamel the bands may become visible at slight cracks within the enamel. There is no single way to make the HSB visible due to the modification of the enamel during fossil diagenesis, but with experience and persistence the orientation of HSB usually can be identified.

All tooth surfaces were studied in order to follow HSB throughout the enamel. HSB are best seen when they are close to the outer enamel surface. Transverse HSB often run parallel to the perikymata, superficial ridges on the OES that usually develop perpendicular to the growth axis, and which are related to time intervals of enamel formation. In contrast to perikymata, HSB of all configurations show very distinct unidirectional bifurcations (Koenigswald and Pfretzschner 1987). In rhinos, where HSB are vertical, the two structures may form a grid pattern (Koenigswald 2002).

Observations

Our observations are arranged using the classification sequence given in Rose (2006), with the following exceptions. Lophiodontidae are placed after Tapiroidea based on the results of Holbrook (2009) that called into question a relationship with chalicotheres. Similarly, Deperetellidae are placed with rhinocerotoids because of the similarities of deperetellid schmelzmuster to that of Hyrachyus. We include Colodon within Tapiridae, following Colbert (2005). The results of our observations are summarized in the next section in the conclusions.

Phenacodontidae.—Only transverse HSB were observed in cheek teeth of Phenacodus, Ectocion, and Meniscotherium, as noted earlier for Tetraclaenodon (Koenigswald et al. 1987). Transverse HSB were observed in incisors of Meniscotherium and canines of Meniscotherium and Phenacodus.

Equidae.—Pfretzschner (1994) observed transverse HSB overlain by radial enamel in molars of Hyracotherium and Palaeotherium. Our own observations on various incisors, premolars and molars of Hyracotherium (sensu lato) confirm this orientation, but HSB are partially developed only weakly. In Mesohippus all investigated teeth show only transverse HSB (Fig. 5A). The outer layer of radial enamel presumably has been lost. Prominent perikymata may hide the HSB, but the refraction on cracks confirms the transverse orientation. In Hippotherium and Equus the transverse orientation of the HSB was confirmed.

In addition to the predominant transverse orientation of the HSB, some genera, e.g., Anchitherium aurelianense, present a specific convex bending in upper molars (Fortelius 1985). In the ectoloph HSB bend downwards from the tips of paracone and metacone to the parastyle and mesostyle, but do not form distinct interfaces. By this bending the HSB are almost parallel to the oblique shearing blades—in contrast to the curved HSB in brontotheres or chalicotheres. We regard this as a variation of the transverse HSB, as Fortelius (1985) used the classification “horizontal (convex)”. Thus all studied members of the Equidae share a similar, transverse orientation of the HSB, regardless of whether they are hypsodont or brachydont. It occurs in upper and lower incisors, as well as premolars and molars.

Fig. 6.

Curved HSB configuration in Heptodon calciculus Cope, 1880 (KOE 4035, 4036); early Eocene, Willwood Formation, Bighorn Basin, Wyoming, USA. A. Tangential section of the posterior loph of a lower molar with two fields of curved HSB with the typical interface. B. Tangential section of the protoconid of a lower molar with the transverse HSB. Abbreviation: if, interface between fields of HSB.

Palaeotheriidae.—The Eocene Propalaeotherium from the Geiseltal and Palaeotherium from Frohnstetten, Germany, show well-developed transverse HSB in molars (Fig. 5B), premolars, partially combined with radial enamel. In canines and incisors HSB are dominantly transverse but we observed in one upper incisor a slight upward curvature at the lateral margin so that the HSB intersect the occlusal edge obliquely. In the pronounced ridge of the mesostyle and parastyle the HSB are slightly curved downwards. Fortelius (1985) listed the HSB of palaeotheres as “horizontal (concave),” as he did for brontotheres and chalicotheres. We did not observe a similar curvature in palaeotheres as in the other two groups, which we classified as curved HSB because of the distinct interface.

Isectolophidae.—Homogalax protapirinus was difficult to investigate externally because most of the material available was very dark. Thus the enamel of lower molars had to be sectioned tangentially. From these sections it is clear that the HSB are transversely oriented on the lingual and buccal sides. The cross-lophs of the lower molars were studied with special emphasis. No deviation from the transverse orientation could be identified in any of the several sections. Lower incisors and canines also exhibit transverse HSB.

Lower molars of Cardiolophus showed transverse HSB. Specimens of Isectolophus annectens have molars with transverse HSB that do not appear to exhibit the curved HSB.

Helaletidae.—The lower molars of Heptodon calciculus show curved HSB. The HSB are more transverse on the lingual and buccal sides of the cusps. In the cross-lophs the HSB rise diagonally towards the cutting edge, forming a clear interface in the middle of the loph. This is the typical curved HSB configuration as defined above, and was corroborated in a tangential section (Fig. 6). North American specimens of Helaletes exhibit the same curved HSB. Dashzeveg and Hooker (1997) erected the genus Irdinolophus for specimens previously referred to Helaletes mongoliensis, and they assigned another Mongolian species, Helaletes fissus, to the genus Desmatotherium. They considered Irdinolophus to be closely related to deperetellids, but we include it here in our discussion of helaletids, because its HSB configuration is more similar to that of helaletids than that of deperetellids. Fortelius (1985) listed “horizontal (concave)” HSB for Helaletidae.

Lower molars of Selenaletes scopaeus, which was originally placed in Helaletidae by Radinsky (1966), have only transverse HSB and do not exhibit the curved HSB. Therefore, the placement of this genus in Helaletidae may be questioned.

Lophialetidae and other endemic Asian “tapiroids” (except Deperetellidae).—Lophialetids (including Lophialetes, Schlosseria, and Breviodon) all exhibit transverse HSB in their cheek teeth without any of the curving toward the occlusal surface that is characteristic of the tapiroid condition. This is also true of Rhodopagus and Pataecops, two genera of Asian Eocene “tapiroids” of uncertain affinity.

Tapiridae.—Extant and Pleistocene Tapirus were studied from complete and partial dentitions. In the lower molars the buccal and lingual sides are characterized by curved HSB, which start as transverse on the cusps but curve upward in the cross-lophs from both lingual and buccal sides to meet the cutting surface almost vertically (Fig. 7A). Since the HSB of both fields curve in different directions, an interface between these two fields is present in the middle of the loph. The same structure is found on the mesial as well as on the distal side of the loph. In molars where the loph is heavily worn, this interface is less obvious. In upper molars the ectoloph also shows curved HSB, with an interface just mesial to the point where the ectoloph meets the metacone. The curved HSB are most evident closer to the occlusal edge, and the HSB are transverse near the root. Thus, wear may obscure the curved HSB on the ectoloph, giving the appearance of only transverse HSB. Upper and lower incisors and canines show transverse HSB only. In the upper molars the ectoloph and buccal sides of protocone and metacone show transverse HSB. In the cross-lophs the HSB curve upwards from the buccal side. In the protoloph they cover the entire loph until it meets the ectoloph. In the metaloph the curved HSB do not extend as far to the ectoloph and meet the transverse HSB in the ectoloph before both lophs join, but there is no distinct interface between the two fields. Incisors and canines show dominantly transverse HSB, as do p2 and p3. In unworn incisors, however, curved HSB were evident at the occlusal tip.

Fig. 7.

Curved HSB configuration with the typical interface in the transverse lophs of lower molars in of tapirs. A. Tapirus sinensis Owen, 1870 (MB Ma 33219); Pleistocene, Junnan, China. B. Lophiodon remensis Lemoine, 1878 (KOE 4052); middle Eocene, Geiseltal, Germany. Abbreviation: if, interface between fields of HSB.

In Colodon occidentalis the curved configuration is well developed in the lower molars. The p2 does not show two lophs like the other molariform premolars. Nevertheless a slight indication of the curved configuration was seen at the distal wall of the trigonid. Upper molars of Colodon exhibit curved HSB on both the cross lophs and the ectoloph, with a distinct interface present on the ectoloph. In Colodon cingulatum the incisors have transverse or slightly curved HSB. Molars of Haagella peregrina show very typical curved HSB with an interface in the cross-lophs of lower molars and one-sided curved HSB in the upper molars.

The curved configuration is also present in molars of Tapiravus from the upper Eocene and in lower molars and premolars of Protapirus from the lower Oligocone.

Lophiodontidae.—All studied species of Lophiodon show the typical configuration of curved HSB in the molar lophs (Fig. 7B). In the buccal and lingual sides of the lower premolars and molars the HSB are transverse; but in the lophs the bands curve upward at the ends, forming an interface between the fields of HSB from both sides. In the cutting edges of the lophs the cross-ridges are often visible. In the upper molars the ectoloph shows transverse HSB at the base and slightly curved HSB in the upper part. There an interface was found between paracone and metacone. In the lophs the transverse HSB from the buccal side of the protocone and metacone bend upwards into the lophs to intersect the cutting edge, where they occur as cross-ridges. The field of curved HSB reaches almost to the ectoloph. In the thick part of the enamel, especially in Lophiodon rhinocerodes vertical elements originating from zigzag HSB were observed. Thus the transverse HSB partly are transitional to a compound HSB configuration. Lophiodontids are listed by Fortelius (1985) as having “horizontal (concave)” HSB.

Fig. 8.

Compound HSB configuration in Hyrachyus minimus (Fischer, 1829) (KOE 4050); middle Eocene, Geiseltal, Germany. Tangential section of the protoconid of a lower molar in three sequential levels. A. Outer layer with vertical HSB. B. Middle level with a transitional orientation of the HSB. C. Inner layer with transverse HSB.

In incisors and canines the HSB are oriented transversely. This is visible although the enamel has a rough surface texture.

Hyrachyidae.—Hyrachyus is characterized by the newly recognized compound HSB configuration in all tooth positions (Fig. 8). A superficial investigation of the outer surface of molar enamel indicated that European Hyrachyus minimus and North American H. eximius and H. modestus have vertical HSB. The vertical HSB are seen in all sides and lophs of the upper and lower molars. On the occlusal surface of the enamel band even the cross-ridges formed by the intersecting vertical HSB are visible. However, a more detailed investigation showed that these vertical HSB are only present in an outer layer, whereas transverse HSB form an inner layer. This combination was corroborated by a series of sections made parallel to the OES in molar teeth of Hyrachyus. It is difficult to observe this condition with low magnification and a light source, because the inner and outer layers can vary in thickness to a degree that one layer may be easily observed but not the other. Compound configuration was also observed in incisors and canines of Hyrachyus, with the vertical component appearing most strongly. Consequently, our observations do not confirm the classification of Fortelius (1985) as “horizontal (concave)” HSB.

Deperetellidae.—Deperetella was listed as having vertical HSB by Fortelius (1985). As reported by Holbrook (2007), specimens of Deperetella and Teleolophus exhibit cross-ridges on the occlusal surface of the enamel band that are characteristic of rhinocerotoids and their vertical HSB. Closer examination of AMNH specimens of Deperetella and Teleolophus confirm the presence of vertical HSB, but as part of the compound HSB configuration. Cheek teeth in these specimens clearly possess vertical HSB, but there also is evidence of horizontal HSB deep to the vertical layer, as observed in Hyrachyus. The horizontal component is most evident in specimens of Teleolophus and in the enamel closest to the root. Thus cheek teeth of both genera are characterized by the compound HSB configuration.

Hyracodontidae.—Hyracodon nebraskensis was represented by all tooth positions. The vertical HSB are present in the ectoloph as well as in the buccal side of the protocone and the metacone (Fig. 9A). The HSB tend to continue to the outer surface. There vertical ridges are visible which correlate with the HSB. In some areas—especially on the buccal side— these ridges in the outer surface are crossed by transverse perikymata. Special care was taken to examine incisors and canines available from a juvenile mandible; vertical HSB showing the typical bifurcations were found in all tooth positions. The same is true for all tooth positions of the genus Ardynia from Mongolia.

Fig. 9.

Vertical HSB configuration in Rhinocerotoidea. A. Hyracodon nebrascensis (Leidy, 1850) (STIPB M 1772); late Eocene—early Oligocene, White River Group, Toadstool Park area, Nebraska, USA. Vertical HSB in the shearing blade of the ectoloph in the right Ml. Note the bifurcations of the HSB. B. Floridaceras whitei Wood, 1964 (KOE 357); early Miocene, Gilchrist County, Florida, USA. Strictly vertical HSB in the shearing blade of the ectoloph of the upper P3. C. Detail of B.

In Eggysodon from the Oligocene of Germany and Switzerland the molars show vertical HSB, but—in contrast to Hyracodon—in the lower incisor only transverse HSB were detected. Triplopus proficiens exhibits vertical HSB in the molars; other teeth could not be assessed. Cheek teeth assigned to Epitriplopus uintensis, as well as a specimen assigned to Triplopus sp. (CM 11955), exhibit the combination of transverse and vertical HSB observed in Hyrachyus, described as compound HSB above.

Indricotheriidae.—Forstercooperia exhibits vertical HSB on the molars and premolars. An isolated incisor assigned to Forstercooperia totadentata (AMNH 20118) appears to have transverse HSB. Juxia sharamurenense also has vertical HSB in the cheek teeth, but on incisors certainly vertical HSB occur (AMNH 20286 and AMNH 20287). The HSB configuration of the canines varies between these two specimens. The first one has transverse HSB, whereas the second one appears to have vertical HSB. One other anterior tooth of AMNH 20287, possibly a P1 or p1, also exhibits transverse HSB.

Fig. 10.

HSB configuration in the incisors of the rhinocerotid Menoceras arikarense (Barbour, 1906) (USNM 412981); early Miocene, Arikaree Formation, Agate, Nebraska, USA. A. The lower incisor with an almost vertical shearing blade has transverse HSB. B, C. In the upper incisor with a shearing blade oblique to the growing axis the HSB are almost vertical.

Amynodontidae.—In Metamynodon and Cadurcodon the massive canines of the mandible show transversely oriented HSB, whereas vertical HSB are evident in the worn tip of an isolated il (AMNH 1092). Vertical HSB are found in the enamel of all upper and lower cheek teeth. They form an inner layer of the schmelzmuster, covered by an outer layer of radial enamel. On the occlusal surface of the enamel band the characteristic cross-ridges of the HSB, formed as the vertical HSB intersect the occlusal plane, are evident to the naked eye. Molars of Amynodon advenum and Caenolophus promissus also exhibit vertical HSB.

In the Asian Armania asiana, we found vertical HSB, although the enamel is partially corroded in the available material. The outer enamel surface is characterized by slight vertical ridges, indicating that the vertical HSB extend to the outer enamel surface (and therefore radial enamel is absent). Perikymata observed on the lingual side are oriented transversely, forming a grid pattern with the vertical HSB.

Rhinocerotidae.—Details about the various rhinocerotids studied are summarized in Table 1. All rhinocerotids are characterized by vertical HSB in premolars and molars (Fig. 9B, C; Rensberger and Koenigswald 1980), including very strong cross-ridges in most specimens. The milk dentition of Rhinoceros unicornis also showed vertical HSB in the deciduous premolars and transverse HSB in the incisors.

Incisors, however, differ in the HSB configuration and may have transverse, vertical, or compound HSB configuration. Compound HSB were found in Trigonias and in Subhyracodon lower and upper incisors, especially where the enamel is thin. In thicker parts of the enamel, especially in the enlarged upper incisors, the HSB tend to change from undulating into a zig-zag structure. Since these zig-zag HSB are parallel, the aligned crests and troughs give the appearance of vertical structures, which are different from the strictly vertical HSB in the cheek teeth. In Menoceras from Agate Springs in Nebraska, both upper and lower incisors exhibit differentiation in the compound HSB configuration. In I1, the vertical component is dominant, whereas in the i2 the transverse component is strongest (Fig. 10).

Table 1.

The HSB configuration in molars and incisors of the studied Rhinocerotidae.

Uintaceras radinskyi differs from other rhinocerotids in its schmelzmuster. The molars of the type specimen, CM 12004, display clear vertical HSB, but the upper molars also clearly exhibit an inner enamel layer that displays transverse HSB, and thus exhibit compound HSB configuration. The lower molars are not clear with regard to any horizontal HSB. The anterior teeth are isolated and their exact loci are not clear, but some that appear to be upper incisors clearly have vertical HSB. The other conical teeth that might represent the canines and lower incisors are either inconclusive or show some evidence of horizontal HSB.

Fig. 11

Curved HSB configuration in Moropus elatus. A. Buccal aspect of M2. B. detailed mapping of visible HSB in the paracone (modified from Koenigswald 1994).

Fig. 12.

Cladogram summarizing relationships among major lineages of Perissodactyla and the evolution of various HSB configurations. Boxes on the right indicate HSB configurations in various perissodactyl taxa. Boxes on the tree itself indicate changes in HSB configuration, as inferred from the distribution of HSB configurations given this phylogeny. The phylogeny is a conservative estimate of perissodactyl relationships drawn from Hooker (1989, 1994), Froehlich (1999), and Holbrook (1999, 2009).

Eomoropidae.—In Eomoropus and Litolophus, there is evidence of transverse HSB, but in no specimen could the HSB be followed into the lophs to ascertain whether it is curved. The m3 of the holotype of Eomoropus amarorum has what could be interpreted as a groove representing the interface between two U-shaped fields of HSB, but even this is questionable.

Chalicotheriidae.—The curved configuration of HSB in the molars of Moropus elatus from Agate Springs (Fig. 11) was described in detail by Koenigswald (1994) and was confirmed by the investigation of additional material of Chalicotherium, Ancylotherium pentelicum, Nestoritherium sinense, and Metaschizotherium from Europe. The thick protocone has transverse HSB, whereas in the ectoloph the HSB are strongly curved on either side of the paracone. In the upper molars, one interface on the ectoloph occurs on the pre-paracrista close to the parastyle, and another occurs distal to the mesostyle. In the lower molars, two interfaces are very close, on either side of the twinned metaconid. In addition, there are interfaces in the middle of the protolophid and hypolophid, as mentioned by Koenigswald (1994). He used the term “U-shaped HSB” for this extreme form of curved HSB. In contrast to the premolars and molars, the incisors show only transverse HSB. Fortelius (1985) listed chalicotheres as having “horizontal (concave)” HSB, which in this case is a synonym of our curved configuration.

Lambdotheriidae.—Lambdotherium, from the late early Eocene (biozone Wa-7) of Wyoming, has clearly transverse HSB in various upper and lower molars and premolars. In some upper molars, however, a slight curvature of the HSB was observed. Thus these bands intersect the occlusal facet at a high angle. No interface as in the more derived brontotheriids could be detected. No information on incisors was available, but the canines have transverse HSB.

Brontotheriidae.—The lower molars of Eotitanops show transverse HSB on the lingual side. The HSB on protoconid and hypoconid are transverse but curve upwards in the lophids, forming the typical curved HSB configuration. In upper premolars and molars of Palaeosyops transverse HSB were seen on the lingual side. The buccal side of the upper molars is formed by two cones with an angled cutting edge. Here the HSB are clearly curved with a distinct interface. Fortelius (1985) listed brontotheres as having “horizontal (concave)” HSB.

The large brontothere Megacerops from the upper Eocene, including several genera synonymized by Mihlbachler (2008), has curved HSB in upper and lower molars with distinct interfaces. The canines and incisors both show transverse HSB.

Discussion

Perissodactyl phylogeny in the light of HSB configuration

In addition to documenting the HSB configuration in fossil and Recent Perissodactyla, a major goal of this paper is to trace the evolution of HSB configuration in this order of mammals (Fig. 12). Because this goal is obviously dependent on our understanding of perissodactyl phylogeny, a brief review is given here. Schoch (1989) provided a more detailed review of the history of perissodactyl systematics prior to 1989, and we refer the reader to that paper and to Hooker (2005) for more information.

Table 2.

Generalized HSB configuration in cheek teeth, canines and incisors of the various perissodactyl families.

Wood (1934) divided perissodactyls into two suborders: Ceratomorpha, including “tapiroids” (sensu lato) and rhinocerotoids; and Hippomorpha, including equoids, chalicotherioids, and brontotherioids. While the general concept of a close relationship between the tapir and rhinoceros clades relative to the horse clade has been generally accepted (and even corroborated by molecular studies; Norman and Ashley 2000), the relationships among the so-called “hippomorph” taxa, as well as the relationships of certain putative fossil members of Ceratomorpha, are unclear or controversial.

Hooker (1989, 1994) published the first computer-generated cladistic analyses of early perissodactyl interrelationships, based primarily on Eurasian taxa and dental characters. Hooker concluded that (i) what was then called Hyracotherium actually represents multiple genera, and that the holotype of the type species, Hyracotherium leporinum, is actually a palaeotheriid, (ii) lophiodontids, historically classified as “tapiroids” sensu lato, are closely related to chalicotherioids, and are grouped together with them in the clade Ancylopoda (Hooker 1984), and (iii) Hooker's Ancylopoda formed a clade with an emended Ceratomorpha and Isectolophidae, which he called Tapiromorpha. Froehlich (1999) came to similar conclusions in a study that focused on North American taxa, but Holbrook (1999, 2001) was not able to recover Hooker's concept of Tapiromorpha from data emphasizing characters from the cranial and postcranial skeleton. Hooker (Hooker and Dashzeveg 2003, 2004; Hooker 2005) subsequently modified his view of Ancylopoda, retaining the lophiodontid-chalicotherioid relationship, but allying this group with a ceratomorph-equoid clade he called Euperissodactyla; Euperissodactyla and Ancylopoda form a larger clade, Lophodontomorpha, to the exclusion of Brontotherioidea.

Within specific groups of perissodactyls, there are other phylogenetic issues relevant to this study. Lambdotherium, known from the early Eocene of North America, has historically been interpreted as the earliest brontotherioid, but some studies have challenged this and suggested that this taxon is more closely allied with palaeotheriids (Mader 1989; Lucas and Holbrook 2004).

Tapiroidea is a term that was historically applied to all non-rhinocerotoid ceratomorphs (Radinsky 1963), but more recently this taxon has been restricted to a more exclusive monophyletic group including tapirids and helaletids (Colbert and Schoch 1998; Holbrook 1999, 2001). As a result, “isectolophids,” including the Wasatchian Homogalax and Cardiolophus as well as the Bridgerian Isectolophus, have been removed from the Tapiroidea and probably do not represent a monophyletic family (Holbrook 1999, 2001). In addition, the relationships of a variety of endemic Asian “tapiroids”—specifically lophialetids and deperetellids—to other perissodactyls are no longer clear (Holbrook 1999).

Within Rhinocerotoidea, the main phylogenetic issues concern the relationships among various taxa assigned to the Hyracodontidae. Radinsky (1966) broadened Hyracodontidae to include any rhinocerotoids that could not be assigned to Rhinocerotidae or Amynodontidae. These included small, cursorial forms like Hyracodon from North America and Eggysodon from Eurasia, as well as the large to gigantic indricotheres of Asia. Though Hyracodontidae was clearly a wastebasket taxon, others adopted Radinsky's (1966) composition of the family when attempting to establish it as monophyletic (Prothero et al. 1986, 1989). Some recent studies have cast doubt on the monophyly of this concept of Hyracodontidae, especially with regards to the inclusion of indricotheres (Holbrook 1999, 2001). Thus, we here treat indricotheres as a separate family (Indricotheriidae) and consider North American hyracodontines and Eurasian eggysodontines separately.

Given the current state of knowledge of perissodactyl phylogeny, and considering the distribution of HSB configuration observed in this study, several of the unresolved phylogenetic issues impact our ability to trace the evolution of HSB configuration. In particular, the phylogenetic positions of lophiodontids, chalicotherioids, and brontotherioids are important for interpreting HSB evolution. Even with these limitations, we can still make some inferences regarding HSB evolution, and we summarize these below (Fig. 13).

Fig. 13.

Schematic hypothesis of the evolutionary interrelationship of the four configurations of Hunter-Schreger Bands (HSB) found in Perissodactyla. From the basal transverse HSB configuration evolved the curved HSB configuration on the one hand. On the other hand the compound HSB configuration evolved and gave rise to the vertical HSB configuration in Rhinocerotidae.

The ancestral HSB configuration.—Phenacodontids exhibit the tranverse HSB configuration, which is consistent with the notion that this configuration is the ancestral condition for perissodactyls. Equids and palaeotheriids consistently exhibit the transverse configuration throughout their history, effectively retaining the ancestral condition from bunolophodont forms in the early Eocene through hypsodont forms right up to Recent times (Pfretzschner 1993). Other taxa that appear to have retained the ancestral condition include Lambdotherium, Homogalax, Cardiolophus, lophialetids, Rhodopagus, and Pataecops. Unfortunately, the retention of the ancestral condition does not clarify the relationships of these taxa to other perissodactyls.

The evolution of curved HSB.—Curved HSB generally occur together with transverse HSB and thus presumably originated from transverse HSB. They are evident in chalicotheriids, brontotheriids, lophiodontids, and tapiroids sensu stricto (i.e., helaletids and tapirids) (Figs. 13, 14). The nature and extent of the curving differs among these groups, though this is at least partly due to the differences in the development of specific lophs. Chalicotheriids and brontotheriids, for instance, have very strong ectolophs and weaker cross lophs, whereas the opposite is true for tapiroids. Lophiodontids have strong cross lophs and fairly strong ectolophs. Thus, it is much easier to detect curved HSB in the ectolophs of chalicotheriids and brontotheriids than in the cross lophs, and vice versa for tapiroids. Consequently, it is difficult to ascertain whether observed differences in HSB are due to distinct HSB patterns or to molar morphology. In any case, it seems certain that curved HSB arose independently from the transverse HSB configuration at least twice, since no recent phylogenies place all of these taxa together. However, on the basis of these observations, we cannot rule out either of the two proposed affinities of lophiodontids (either with tapiroids, or with chalicotheres). The number of times that the curved HSB arose could be more easily determined if we could determine (i) the HSB configuration of eomoropids and (ii) the position of some putative basal members of these lineages, such as Lambdotherium, Homogalax, and Cardiolophus.

The compound HSB configuration and the origin of vertical HSB.—Arguably the most interesting issue arising from these data is the evolution of HSB configurations. In rhinocerotoids we see a close linkage between the compound HSB configuration (transverse HSB in the inner zone and vertical HSB in the outer zone) and the vertical HSB configuration. These configurations are similar in that they are not restricted to specific functional areas of the dentition. The evolution of the more derived vertical configuration probably occurred by reducing the inner layer of transverse HSB (Fig. 13). The previous interpretation (Rensberger and Koenigswald 1980)— that vertical HSB derived from a tapir-like configuration— must be rejected.

Vertical HSB have long appeared to be a distinctive feature of rhinocerotoids and a synapomorphy uniting all members of the superfamily, from the basalmost Hyrachyus to modern forms. The observation of vertical HSB in deperetellids (Fortelius 1985; Holbrook 2007) suggested that these unusual endemic Asian “tapiroids” might actually be rhinocerotoids as well. Recognition of the compound HSB configuration, however, calls into question any simple interpretation. The compound HSB configuration is found in Hyrachyus, Uintaceras, and deperetellids.

Holbrook and Lucas (1997) described Uintaceras radinskyi from the Uintan of North America as the sister-group to Rhinocerotidae, and later analyses considered it to be the basalmost member of the family (Holbrook 1999, 2001; Prothero 2005). The presence of compound HSB in Uintaceras is interesting because, if Uintaceras is a basal rhinocerotid, it suggests that one of the following is true: (i) The vertical HSB configuration evolved from the compound condition independently in rhinocerotids and in the other main families of rhinocerotoids (Hyracodontidae, Amynodontidae, and Indricotheriidae) or (ii) the vertical HSB configuration characterizes all of these families, and a reversal to the compound configuration occurred in the lineage leading to Uintaceras (Figs. 12, 14).

The presence of the compound HSB configuration in deperetellids may very well be evidence of a unique relationship with rhinocerotoids. Considering that deperetellids share very few other characters with rhinocerotoids and are otherwise quite derived dentally, this may indicate that deperetellids are a derived offshoot of the lineage that led to conventional Rhinocerotoidea. It would also indicate an Asian origin for the superfamily.

Fig. 14.

Schematic diagram of the stratigraphic occurrence of the four configurations of Hunter-Schreger Bands (HSB) found in the various perissodactyl families. The range data of the families are taken from McKenna and Bell (1997). All four types occurred during the Paleogene in several families. The compound HSB configuration did not reach the Neogene. The three other types are represented by one family each in the extant fauna: the transverse HSB configuration in Equidae, the curved HSB configuration in Tapiridae, and the vertical HSB configuration in Rhinocerotidae.

In contrast to the vertical HSB in rhinocerotoid cheek teeth, some genera with enlarged incisors modified the compound HSB configuration in a very different way. Functional reasons, as described below, led to the reduction of the vertical HSB in the outer layer and gave preference to the transverse HSB of the inner layer.

Conclusion

Early in their evolution, perissodactyls modified the orientation of the transverse HSB inherited from phenacodontids. The four types of HSB configuration were fully developed during the Eocene, but several of the perissodactyl families vanished at the end of the Eocene or during the Oligocene. In the extant fauna the transverse HSB configuration is retained in Equoidea, the curved HSB configuration is preserved only in Tapirus, and the vertical HSB configuration characterizes the Rhinocerotidae. The compound HSB configuration is not present in any living perissodactyls (Fig. 14).

Of the various HSB configurations found in Perissodactyla the transverse HSB are the least derived form. These transverse HSB are retained in equoids, palaeotheriids, and isectolophids. Most other perissodactyl lineages modified this basal pattern. From the transverse HSB configuration the curved HSB configuration arose in helaletids, tapirs, lophiodontids, chalicotheres, and brontotheres. It is characterized by curved fields of HSB with distinct interfaces. The fields of curved HSB are closely correlated with tooth morphology. The evolution of the curved HSB configuration may have occurred several times independently.

A second way of reorganising the transverse HSB is found in the compound HSB configuration, where a second layer of vertical HSB is superimposed on an inner layer of transverse HSB. These vertical HSB characteristically occuron all sides of the teeth and are not related to the tooth morphology. We found this HSB configuration in hyrachyids, deperetellids, and Uintaceras. We infer that the vertical HSB configuration typical for the cheek teeth of hyracodontids, amynodontids, rhinocerotids, and indricotheriids evolved from the compound HSB configuration.

The analysis of the HSB configuration provides new evidence for the phylogenetic position of some genera and higher taxa of perissodactyls, but it also leaves some of these questions still very much unanswered. Lophiodontids show similarities in HSB to both tapiroids and chalicotheres, though all three groups are distinct from rhinocerotoids and equoids. The presence of compound HSB in deperetellids is very suggestive of a close relationship between this family and rhinocerotoids, since the compound and vertical HSB configurations are likely closely related.

This study also raises a number of new phylogenetic questions. Are the various instances of curved HSB homologous, and, if so, does this indicate a close relationship between brontotheres, chalicotheres, lophiodontids, and tapiroids? Since rhinocerotoids are thought to be closely related to tapiroids, how are the evolution of curved HSB on the one hand and compound and vertical HSB on the other related? Better sampling should provide more insights, especially data on Eocene chalicotherioids and endemic Asian “tapiroids”.

The adaptative value of the reorganization of the HSB configuration in perissodactyls is interpreted functionally as an efficient reduction of the abrasion during mastication, assuming that prisms intersecting the actual grinding surfaces almost perpendicularly resist wear better than prisms parallel to the occlusal surface. It should be mentioned that the enamel microstructure is formed in the crypt, an area free of external stresses. Enamel is not remodelled under stress like the structure of bones. In Perissodactyla various types of HSB configurations were selected for in different clades, but all of them share this specific relationship between the prisms and the occlusal surface. The various HSB configurations are not related to specific diets, but rather to phylogenetic lineages.