Hypsodonty

Based on comparative mandibular anatomy, observations of chewing behaviour, chewing forces and dental microwear, it has been suggested that an additional effect of the ruminant digestive strategy could be a reduction of both the required chewing load and tooth wear ruminants are exposed to. This effect is hypothesized to be the result of digestion, mixing, and digesta sorting prior to regurgitation for rumination, which might both soften the material and wash off external abrasives such as sand, grit and dust. Putatively, these external abrasives would thus be trapped in the (fore)stomach and excreted via the faeces. We investigated the location of sand in the stomach of goats fed diets containing phytoliths and sand for several months. The contents of the stomach section from where rumination material is recruited were comparatively depleted of sand. Sand mainly accumulated in another stomach section, the abomasum, without causing clinical problems. A certain phytolith content should hence affect ruminants and non-ruminant herbivores somewhat alike; however, a certain external abrasives content should affect ruminants less than non-ruminants. Results from feeding experiments as well as tooth wear studies support this hypothesis, and caution against the default use of dental anatomy and wear as taxon-free environmental proxies in paleobiology.

Ecometric patterning is community-level sorting of functional traits along environmental gradients that arises historically by geographic sorting, trait evolution, and extinction. We developed a stochastic model to explore how ecometric patterns and clade dynamics emerge from microevolutionary processes. Strong selection, high probability of extirpation, and high heritability led to strong ecometric patterning, but high rates of dispersal and weak selection do not. Phylogenetic structuring arose only when selection intensity, dispersal, and extirpation are all high. Ancestry and environmental geography produced historical effects on patterns of trait evolution and local diversity of species, but ecometric patterns appeared to be largely deterministic. Phylogenetic trait correlations and clade sorting appear to arise more easily in changing environments than static ones. Microevolutionary parameters and historical factors both affect ecometric lag time and thus balance between extinction, adaptation, and geographic reorganization as responses to climate change.

The abrupt spread of grasslands using C4 photosynthesis, sometimes referred to as the rapid increase in C4 ecosystems (RICE), occurred in the late Miocene in North America. While fossil plant specimens from the Miocene Dove Spring Formation of California as well as data from phylogenetic studies and molecular clocks show that C4 grasses evolved prior to the RICE, most isotopic data from paleosols and mammal tooth enamel suggest that its abundance on the landscape was minimal. However, a few recent studies from the Great Plains suggest that C4 grasslands may have been more prominent prior to the RICE event. Here we examine stable carbon isotope values from ungulate tooth enamel from the Barstow Formation of southern California, which is geographically and temporally close to the Dove Spring Formation, and records a diverse and abundant paleofauna of medial Miocene age. Tooth enamel δ13C values were examined in seven ungulate genera including the hypsodont equids; Acritohippus sp. and Scaphohippus sp.; the camelids, Aepycamelus sp., Hesperocamelus sp., and Procamelus sp.; the antilocaprid, Merycodus sp.; and the proboscidean, Gomphotherium sp. More positive δ13C values observed within the equids, camelids, and antilocaprids are suggestive of C4 grasses being included in the diets of these taxa. The equids exhibited the most positive mean δ13C values, which indicate a higher component of dietary C4 grasses (up to 18%) when compared to the other sampled ungulate taxa. The tooth enamel isotope values presented in this study show the presence of C4 grasses as a significant component of ungulate diets millions of years before the RICE. The abundance of C4 plants earlier in the Miocene may imply a more significant role in the major ungulate diversification events than previously suspected. The few pre-RICE localities showing evidence of C4 abundance suggests that these grasslands were geographically restricted which limits, but may not exclude, the possibility that a world-wide mechanism controlled its spread.

A herbivore's diet can affect its teeth by causing different types of wear. Browsers typically have sharper, higher cusps, while grazers show lower, blunter cusps, presumably due to the more abrasive nature of their diet. On the macroscopic scale, this allows the reconstruction of herbivore diets based on the shape of the tooth's profile, using the mesowear method. However, the timeframe involved in constituting a stable dietary signal represented by mesowear has not been precisely defined. To obtain a more precise delimitation of this timeframe, sheep (Ovis aries, n = 39) were fed pelleted diets containing external abrasives of different sizes (⌀ 4 μm, ⌀ 50 μm, and ⌀ 130 μm), and concentrations (4%, or 8%) for 17 months. Mesowear was scored on the skulls at the end of the experiment, as well as on computer-tomographic scans and on resin casts of the teeth taken at different time-points along the experiment. These datasets were compared, and changes in score were calculated using the CT and cast data from the beginning and end of the experiment. Overall, even though a visual trend appeared of more wear on higher abrasion diets, it was of an extremely small magnitude, and the dietary effect on the mesowear scores or the change of the mesowear scores in these animals was never significant. This leads us to conclude that, at least in small ruminants, mesowear is more of a general signal than a seasonal one, and needs to be considered as such for tooth wear-based palaeodietary reconstructions. Experiments with natural forages are required to corroborate this conclusion.

Diet affects many factors of an animal's anatomy, but teeth are a specific focus of dietary research, as their durability lends them to record information on a large variety of scales. Abrasive diets like those of grazing herbivores are known to wear down teeth, but how that wear affects tooth growth and the relations between its different morphological components is rarely investigated. Seven pelleted diets varying in abrasive size and concentration were fed over a 17-month period to 49 sheep (Ovis aries), of which n = 39 qualified for morphology measurements. Using computed tomography, scans of the skulls were made over the course of the experiment, and the impact of diet-related wear was observed on tooth volume and morphology, including the position of dental burr marks, over time. Digital caliper measurements were applied to 3D renderings of the teeth, and the volume of crown and root segments were investigated separately. We aimed to detect a signal of root growth compensating for wear, and test if this mechanism would be affected by dietary abrasives. Crown-segment volume loss was correlated to root-segment volume gain. Height and burr mark measurements indicated a much higher experimental tooth wear than that previously reported for free-ranging animals. The reason for this is unclear. There was no relationship between tooth height and dentine basin depth. For all parameters, there was no effect of diet; hence, while the measurements corroborate general understanding of tooth wear and compensatory processes, these methods appear not suitable to assess subtle differences between feeding regimes.

K E Y W O R D S:  3D imaging, dietary signal, incipient hypsodonty, root growth, ruminant, tooth volume, tooth wear