Thomas M. Kaiser

The dietary regimes of two contemporaneous populations of Hippotherium primigenium (Perissodactyla, Equidae) from the Vallesian (Upper Miocene) of Southern Germany

Abstract The first occurrence of tridactyl, hipparionine horses is presently constrained in Eurasia between 11.2 and 10.8 Myr. Hippotherium primigenium is the only species recorded up to now in the early Vallesian–early Turolian of Germany. Two Vallesian populations of H. primigenium are investigated with respect to their trophic regimes. One of these populations derives from the deposits of the Upper Miocene Rhine river exposed at the locality of Eppelsheim (Germany). The other population (Howenegg) is represented by lacustrine deposits of a former crater lake about 180 km south of Eppelsheim. Both localities represent two almost contemporaneous Vallesian (MN9) populations of a single equid taxon (H. primigenium) from different geographic regions within western Germany. Trophic regimes are reconstructed using the mesowear method [Fortelius and Solounias, Am. Mus. Novit. 3301 (2000) 1–36] and the derived ‘extended’ mesowear method [Kaiser and Solounias, Geodiversitas (2003), in press]. These analyses lead to the identification of a recent trophic reference taxon for each of the two hipparion populations. The common waterbuck (Kobus ellipsiprymnus), a grazer inhabiting reed beds and also foraging into woodlands is identified as the trophic analog for the Eppelsheim population. The reference taxon of the Howenegg population is the Sumatran rhinoceros (Dicerorhinus sumatrensis), a browser, inhabiting mainly hilly country and preferring saplings, fruit, leaves, twigs, and bark as major food items. The marked differences in the trophic regimes found for the two populations of H. primigenium are interpreted as reflecting the range of possible dietary regimes, here termed the trophic spectrum of this equid species. The trophic signal of H. primigenium is therefore interpreted as an ecological signal reflecting properties of the habitat. The paleohabitat of Eppelsheim would then be recognized as representing at least seasonally extended grassy areas such as reed flats in the proximity of the Miocene Rhine river. The paleohabitat of the Howenegg, however, was most likely an immediate part of the subtropical mesophytic forests, that covered large parts of Central and Western Europe, Central Asia and southern China during the Vallesian period.

New interpretations of the systematics and palaeoecology of the Dorn-Dürkheim 1 hipparions (late Miocene, Turolian age [MN11]), Rheinhessen, Germany

The Turolian vertebrate locality of Dorn-Dürkheim is situated near the city of Mainz, SW-Germany. The mammalian fauna is significantly younger than most other late Miocene Central European faunas that have yielded an equally rich assemblage of hipparion remains. The Dorn-Dürkheim hipparion fauna consists of isolated teeth and postcranial skeletal elements. We use the Vallesian (MN9) samples ofHippotherium primigenium from Eppelsheim and Höwenegg (Germany) as standards for comparison. Based on cheek tooth occlusal dimensions and continuous variables of the astragali, we identify two clusters of dental and postcranial specimens. We argue that these two clusters belong to two populations of hipparions that differ from one another principally in their body size. We further investigate the populations from Eppelsheim (EPhP), the total of all Dorn-Dürkheim specimens (DDpPall) and the two sub-populations from Dorn-Dürkheim (DDhPri and DDhPsm) with respect to their dietary preferences. For this we use the microwear and mesowear methods. The paleodietary signals for each sub-population are found to be quite different. The larger sized population of Dorn-Dürkheim (DDhPri) was a mixed feeder, while the small sized population (DDhPsm) is interpreted to have been a dedicated browser. In addition, we redefine the calculation of indices of hypsodonty so that they are more appropriate to equine horses. In comparing the hypsodonty indices of the hipparion populations from Eppelsheim and the total of the Dorn-Dürkheim specimens we find no differences in hypsodonty. In our comparison of Dorn-Dürkheim metacarpal III’s (MC III’s) with MC III’ from other pertinent European localities, we find two MC III’s from Dorn-Dürkheim to be similar to the Höwenegg population ofH. primigenium. However, eight Dorn-Dürkheim specimens differ from the Höwenegg population in having a relatively expanded crista sagittalis and reduced lateral and medial condyles. The only complete Dorn-Dürkheim MC III is relatively longer than the those from the Höwenegg population. This leads us to conclude that the MC III morphology of the smaller sized sub-population from Dorn-Dürkheim (DDhPsm) exhibits an adaptation for more cursorial locomotion than the Höwenegg hipparions, while the larger sized sub-population ventured into less forested habitats and was less cursorial. Based on the peculiarities of metapodial build and of cheek tooth dimensions, we recognize the population DDhPsm from Dorn-Dürkheim as belonging to a new species of hipparionine horse,Hippotherium kammerschmitti. New species:Cormohipparion n. sp.,Hippotherium kammerschmitti n. sp.KurzfassungDie turolische Wirbeltierlokalität Dorn-Dürkheim liegt nahe von Mainz in Südwestdeutschland. Die Säugetierfauna ist deutlich jünger als die meisten übrigen obermiozänen mitteleuropäischen Faunen und hat ein reiches, überwiegend aus isolierten Zähnen und postcranialen Skelettelementen bestehendes Hipparionenmaterial geliefert. Als Referenz für Vergleiche wird das vallesische Fundgut vonHippotherium primigenium von den Lokalitäten Eppelsheim und Höwenegg herangezogen. Basierend auf den occlusalen Dimensionen der Backenzähne und den Abmessungen von Astragali und Metapodien werden zwei Gruppen dentaler und postcranialer Individuen identifiziert. Diese zwei Gruppen werden zu zwei verschiedenen Unterpopulationen hipparioner Pferde gestellt, die sich im wesentlichen durch ihre Körpergröße unterscheiden. In Bezug auf ihre Nahrungspräferenzen werden ferner die Populationen von Eppelsheim (EPhP), die Gesamtheit aller Dorn-Dürkheim-Individuen (DDhPall) sowie die zwei Unterpopulationen von Dorn-Dürkheim (DDhPri and DDhPsm) untersucht. Hierfür kommen die Mikrowear- und die Mesowearmethode zum Einsatz. Die großwüchsige Population von Dorn-Dürkheim (DDhPri) wird als Mischkostfresser (mixed feeder) erkannt, während die kleinwüchsige Population (DDhPsm) als ausgesprochener Konzentratselektierer (browser) interpretiert wird. In der vorliegenden Untersuchung wird ferner die Berechnung des Hypsodontie-Index neu definiert, um den besonderen Gegebenheiten der Equiden besser gerecht zu werden. Zwischen den Hipparion-Populationen von Eppelsheim und der Gesamtheit der Dorn-Dürkheim-Individuen bestehen keine Unterschiede im Hypsodontie-Index. Im Vergleich der Metacarpalia III (MC III) von Dorn-Dürkheim mit Hipparionen verschiedener europäischer Lokalitäten sind zwei MC III’s von Dorn-Dürkheim der Höwenegg-Population vonH. primigenium im Bau sehr ähnlich. Acht MC III’s von Dorn-Dürkheim zeigen jedoch eine verhältnismäßig ausgedehnte Crista sagittalis und reduzierte laterale und mediale Condylen. Das einzige vollständig erhaltene Metacarpale III von Dorn-Dürkheim ist im Verhältnis zur Höwenegg Population relativ lang gestreckt. Hieraus wird geschlossen, dass die eine Subpopulation von Dorn-Dürkheim (DDhPsm) eine Adaptation zeigt, die gegenüber den Höwenegg-Hipparionen auf verstärkte kursoriale Lokomotion verweist. Dem gegenüber war die großwüchsigere Subpopulation weniger cursorial adaptiert. Basierend auf den Besonderheiten im Bau der Metacarpalia und in den Abmessungen der Backenzähne wird die Population DDhPsm als eine neue Art hipparioner Pferde erkannt,Hippotherium kammerschmitti.

Functional significance of ontogenetic gradients in the enamel ridge pattern of the upper cheek dentition of the miocene hipparionin horseCormohipparion occidentale (Equidae, Perissodactyla)

The cheek tooth dentition of hipparionine horses functions as a shearing system, masticating food in a single phase occlusal power stroke. The edges of enamel ridges, which are exposed on the occlusal surface by means of food abrasion act as shearing blades. In order to discern the function of enamel ridges, an algorithm is introduced which reduces occlusal enamel ridge patterns to their functional residual in respect to the function as a shearing system. A further algorithm discriminates actively functioning (leading) enamel edges from passive functioning (trailing) edges. The residuals of functional enamel edges are investigated in respect to their alignment in relation to the chewing direction. A cheek tooth dentiion of the Miocene hypsodont equidCormohipparion occidentale is investigated in respect to wear related ontogenetic changes in functional enamel edge alignment using a set of 8 sections per tooth roughly representing hypothetical occlusal surfaces in different individual ages.A high degree of optimisation is found in the cheek tooth dentition ofC. occidentale in respect to the function as a shearing system of food subdivision. A decrease in function is found in the ontogeny of P2–M2, which is compensated by an increase in function by the M3 erupting last. Since this optimisation did not involve more structural investment, it is considered a highly efficient means of restricting hypsodonty to the degree absolutely deemed necessary by selection, and saving investment in unnecessary structure. This may be one of the pre-adaptations of late Miocene hipparions, making the group a diverse and successful one in large parts of the old and new world subsequently.