Christine M. Janis

The Origins of C4 Grasslands: Integrating Evolutionary and Ecosystem Science

Grassland Emergence The evolution of the C4 photosynthetic pathway from the ancestral C3 pathway in grasses led to the establishment of grasslands in warm climates during the Late Miocene (8 to 3 million years ago). This was a major event in plant evolutionary history, and their high rates of foliage production sustained high levels of herbivore consumption. The past decade has seen significant advances in understanding C4 grassland ecosystem ecology, and now a wealth of data on the geological history of these ecosystems has accumulated and the phylogeny of grasses is much better known. Edwards et al. (p. 587) review this multidisciplinary research area and attempt to synthesize emerging knowledge about the evolution of grass species within the context of plant and ecosystem ecology. The evolution of grasses using C4 photosynthesis and their sudden rise to ecological dominance 3 to 8 million years ago is among the most dramatic examples of biome assembly in the geological record. A growing body of work suggests that the patterns and drivers of C4 grassland expansion were considerably more complex than originally assumed. Previous research has benefited substantially from dialog between geologists and ecologists, but current research must now integrate fully with phylogenetics. A synthesis of grass evolutionary biology with grassland ecosystem science will further our knowledge of the evolution of traits that promote dominance in grassland systems and will provide a new context in which to evaluate the relative importance of C4 photosynthesis in transforming ecosystems across large regions of Earth.

ON THE MEANS WHEREBY MAMMALS ACHIEVE INCREASED FUNCTIONAL DURABILITY OF THEIR DENTITIONS, WITH SPECIAL REFERENCE TO LIMITING FACTORS

( I ) Types of solutions available . . . . . . . . . (2) Increased wear resistance of dental tissues . . . . . . (3) Increased tooth size . . . . . . . . . . . (4) Additional teeth, and a discussion of bilophodonty . . , . , (5) Increased tooth height . . . . . . . . . . (6) Combinations of methods . . . . . . . . . . IV. Conclusion . . . . . . . . . . . . . V. Acknowledgements. . . . . . . . . . . . VI. References . . . . . . . . . . . . . (2) Functional demands, with a note on dental wear

On the relationship between hypsodonty and feeding ecology in ungulate mammals, and its utility in palaeoecology

High‐crowned (hypsodont) teeth are widely found among both extant and extinct mammalian herbivores. Extant grazing ungulates (hoofed mammals) have hypsodont teeth (a derived condition), and so extinct hypsodont forms have usually been presumed to have been grazers. Thus, hypsodonty among ungulates has, over the past 150 years, formed the basis of widespread palaeoecological interpretations, and has figured prominently in the evolutionary study of the spread of grasslands in the mid Cenozoic. However, perceived inconsistencies between levels of hypsodonty and dental wear patterns in both extant and extinct ungulates have caused some workers to reject hypsodonty as a useful predictive tool in palaeobiology, a view that we consider both misguided and premature.

EVOLUTION OF HORNS IN UNGULATES: ECOLOGY AND PALEOECOLOGY

(1) The savanna ungulate faunas of the North American Miocene were broadly similar to those of present‐day East Africa in terms of overall morphological and taxonomic diversity. However, the predominant ungulates of the African faunas are bovids, which possess bony horns that are primitively sexually dimorphic in their occurrence. The predominant ungulates of the North American Tertiary were equids, camelids and oreodonts, which all lacked horns. A limited number of horned ruminants were present, but these were largely Miocene immigrants from Eurasia. Horns were also absent from the large‐bodied herbivores in the endemic faunas of South America and Australia.

The origins and evolution of the North American grassland biome: the story from the hoofed mammals

Abstract The North American grassland biome first appeared around 18 Ma in the mid Miocene. The familiar story of the Neogene evolution of this biome is of the replacement of ungulates (hoofed mammals) having a primarily browsing diet by the more derived grazing ungulates. However, new data show a more complicated pattern of faunal succession. There was a maximum taxonomic diversity of ungulates at 16–14 Ma, including a large number of grazers, and the subsequent decline in overall diversity was largely due to the decline of the browsers, with little corresponding increase in the grazers. Additionally the mid Miocene faunas (∼18–12 Ma) contained a much greater number of browsers than any comparable present-day habitat. We discuss possible explanations for these non-analogous grassland faunas, including the possibility that the primary productivity of the vegetation was greater in the early to middle Miocene than it is today. One possible explanation for increased primary productivity is higher Miocene levels of atmospheric carbon dioxide than in the present day. The proposed difference in vegetational productivity also may explain why horses radiated as the main grazers in North America, in contrast to the radiation of antelope in the Plio–Pleistocene African grasslands.

Characterizing complex craniodental patterns related to feeding behaviour in ungulates: a multivariate approach

This work examines whether stepwise discriminant function analysis of a suite of craniodental variables enables feeding behaviour and habitat preferences to be identified in fossil ungulates. There are several morphological features of the ungulate skull, mandible and dentition that are well correlated with dietary adaptations, and thus can be used for estimating the feeding ecology of extinct taxa. However, most studies have followed an univariate approach for characterizing the relationship between diet and craniodental

Were there mammalian pursuit predators in the tertiary? Dances with wolf avatars

Fast-running, long-legged pursuit carnivores are familiar members of the present-day ecosystem, and it has been assumed that extinct large predators took similar ecomorphological roles (i.e., were “wolf avatars”) in past faunas. While these fossil taxa may also have been meat-specialists, we present evidence from limb morphology to show that there was no modern type of pursuit predator until the latest Tertiary. In contrast, ungulates evolved longer legs similar to those of present-day cursorial taxa by the middle Tertiary, some 20 million years earlier. These data suggest the need for the reevaluation of many classical evolutionary stories, not only about assignation of fossil taxa to a wolf-like mode of predatory behavior, but also to issues such as the coevolution of long legs and fast running speeds between predator and prey, and even the implicit assumption that cursorial morphologies are primarily an adaptation for speed. We conclude that evolutionary change in ungulate limb morphologies represents an adaptation to decrease transport costs in association with Tertiary climatic changes and that the present-day predation mode of long distance pursuit is a Plio-Pleistocene phenomenon, related to the development of colder and more arid climates.

An Evolutionary History of Browsing and Grazing Ungulates

Browsing (i.e., eating woody and non-woody dicotyledonous plants) and grazing (i.e., eating grass) are distinctively different types of feeding behaviour among ungulates today. Ungulates with different diets have different morphologies (both craniodental ones and in aspects of the digestive system) and physiologies, although some of these differences are merely related to body size, as grazers are usually larger than browsers. There is also a difference in the foraging behaviour in terms of the relationship between resource abundance and intake rate, which is linear in browsers but asymptotic in grazers. The spatial distribution of the food resource is also different for the different types of herbage, browse being more patchily distributed than grass, and thus browsers and grazers are likely to have a very different perception of food resources in any given ecosystem (see Gordon 2003, for review). Grass is a relatively recent type of food resource: extensive grasslands only emerged during the later Cenozoic, within the past 25 million years (Ma), while the first ungulates date back to the early Cenozoic, around 55 Ma. Browsing, probably with the incorporation of a fair amount of fruit, is thus the primitive diet of ungulates (Bodmer and Ward 2006). The general evolutionary perspective is that when the grazing species evolved later they eclipsed the browsers (Kowalevsky 1873; Matthew 1926; Perez-Barberia et al. 2001). While this view of the later Cenozoic rise of grazing ungulates, and corresponding demise of browsing ones, is broadly correct, the actual evolutionary picture is of course much more complex. While the most familiar ungulates today (horses, cows, elephants, etc.) are mainly grazers, in fact specialised grazing is a fairly recent evolutionary adaptation: the first true grazers (i.e., animals subsisting primarily on grass year round) are no more than 10 million years old, and a predominance of tropical grazers as is familiar to us in Africa today has a history of only a couple of million years. In order to understand the evolutionary history of diets and feeding adaptations in ungulates we need to first pose several questions. Firstly, what exactly are ungulates, and how are different ungulate groups related to each other? Secondly, how can we deduce the feeding behaviour of extinct species from their fossilised remains? And thirdly, how have differences in the earth’s climate and environment

Cenozoic climate change influences mammalian evolutionary dynamics

Global climate change is having profound impacts on the natural world. However, climate influence on faunal dynamics at macroevolutionary scales remains poorly understood. In this paper we investigate the influence of climate over deep time on the diversity patterns of Cenozoic North American mammals. We use factor analysis to identify temporally correlated assemblages of taxa, or major evolutionary faunas that we can then study in relation to climatic change over the past 65 million years. These taxa can be grouped into six consecutive faunal associations that show some correspondence with the qualitative mammalian chronofaunas of previous workers. We also show that the diversity pattern of most of these chronofaunas can be correlated with the stacked deep-sea benthic foraminiferal oxygen isotope (δ18O) curve, which strongly suggests climatic forcing of faunal dynamics over a large macroevolutionary timescale. This study demonstrates the profound influence of climate on the diversity patterns of North American terrestrial mammals over the Cenozoic.

THE DIETS OF THE DROMOMERYCIDAE (MAMMALIA: ARTIODACTYLA) AND THEIR RESPONSE TO MIOCENE VEGETATIONAL CHANGE

Abstract The probable diets of members of the extinct ruminant family Dromomerycidae were determined via an assessment of gross anatomical correlates of feeding strategy, mesowear analysis, and microscopic scar topography of enamel surfaces of cheek teeth. Discriminant models derived from 108 extant ruminants of known diet were applied to fossil taxa to ascertain probable trophic habits in dromomerycids. Microwear and mesowear analyses of molar tooth wear supplemented this gross skull and tooth morphological assessment as a means of providing more direct and independent sources of evidence for the nature and potential shifts in diet within dromomerycid lineages. In general, estimations of diets obtained from the study of gross morphology correlated well with those obtained from wear patterns. However, this was not always the case, suggesting that independent means of dietary analysis are critical when attempting to reconstruct paleodiets. In addition, hypsodonty (relative molar crown height) proved to be problematic as a variable in determining the diet of these extinct taxa. Information obtained from gross morphology, microwear, and mesowear support the hypothesis that later species of the Dromomerycidae within the tribe Cranioceratini had a shift in diet toward coarser food materials as a response to a trend toward increasing aridity and a shift in vegetational structure in the late Miocene–early Pliocene of North America.