Global long-term stability of individual dietary specialization in herbivorous mammals.

Dietary variation within species has important ecological and evolutionary implications. While theoreticians have debated the consequences of trait variance (including dietary specialization), empirical studies have yet to examine intraspecific dietary variability across the globe and through time. Here, we use new and published serial sampled δ13Cenamel values of herbivorous mammals from the Miocene to the present (318 individuals summarized, 4134 samples) to examine how dietary strategy (i.e. browser, mixed-feeder, grazer) affects individual isotopic variation. We find that almost all herbivores, regardless of dietary strategy, are composed of individual specialists. For example, Cormohipparion emsliei (Equidae) from the Pliocene of Florida (approx. 5 Ma) exhibits a δ13Cenamel range of 13.4‰, but all individuals sampled have δ13Cenamel ranges of less than or equal to 2‰ (mean = 1.1‰). Most notably, this pattern holds globally and through time, with almost all herbivorous mammal individuals exhibiting narrow δ13Cenamel ranges (less than or equal to 3‰), demonstrating that individuals are specialized and less representative of their overall species' dietary breadth. Individual specialization probably reduces intraspecific competition, increases carrying capacities, and may have stabilizing effects on species and communities over time. Individual specialization among species with both narrow and broad dietary niches is common over space and time-a phenomenon not previously well recognized or documented empirically.

An extinct north American porcupine with a South American tail.

New World porcupines (Erethizontinae) originated in South America and dispersed into North America as part of the Great American Biotic Interchange (GABI) 3-4 million years ago.1 Extant prehensile-tailed porcupines (Coendou) today live in tropical forests of Central and South America.2,3 In contrast, North American porcupines (Erethizon dorsatum) are thought to be ecologically adapted to higher-latitude temperate forests, with a larger body, shorter tail, and diet that includes bark.4,5,6,7 Limited fossils8,9,10,11,12,13 have hindered our understanding of the timing of this ecological differentiation relative to intercontinental dispersal during the GABI and expansion into temperate habitats.14,15,16,17,18 Here, we describe functionally important features of the skeleton of the extinct Erethizon poyeri, the oldest nearly complete porcupine skeleton documented from North America, found in the early Pleistocene of Florida. It differs from extant E. dorsatum in having a long, prehensile tail, grasping foot, and lacking dental specializations for bark gnawing, similar to tropical Coendou. Results from phylogenetic analysis suggest that the more arboreal characteristics found in E. poyeri are ancestral for erethizontines. Only after it expanded into temperate, Nearctic habitats did Erethizon acquire the characteristic features that it is known for today. When combined with molecular estimates of divergence times, results suggest that Erethizon was ecologically similar to a larger species of Coendou when it crossed the Isthmus of Panama by the early Pleistocene. It is likely that the range of this more tropically adapted form was limited to a continuous forested biome that extended from South America through the Gulf Coast.

Body temperatures of modern and extinct vertebrates from (13)C-(18)O bond abundances in bioapatite.

The stable isotope compositions of biologically precipitated apatite in bone, teeth, and scales are widely used to obtain information on the diet, behavior, and physiology of extinct organisms and to reconstruct past climate. Here we report the application of a new type of geochemical measurement to bioapatite, a "clumped-isotope" paleothermometer, based on the thermodynamically driven preference for (13)C and (18)O to bond with each other within carbonate ions in the bioapatite crystal lattice. This effect is dependent on temperature but, unlike conventional stable isotope paleothermometers, is independent from the isotopic composition of water from which the mineral formed. We show that the abundance of (13)C-(18)O bonds in the carbonate component of tooth bioapatite from modern specimens decreases with increasing body temperature of the animal, following a relationship between isotope "clumping" and temperature that is statistically indistinguishable from inorganic calcite. This result is in agreement with a theoretical model of isotopic ordering in carbonate ion groups in apatite and calcite. This thermometer constrains body temperatures of bioapatite-producing organisms with an accuracy of 1-2 degrees C. Analyses of fossilized tooth enamel of both Pleistocene and Miocene age yielded temperatures within error of those derived from similar modern taxa. Clumped-isotope analysis of bioapatite represents a new approach in the study of the thermophysiology of extinct species, allowing the first direct measurement of their body temperatures. It will also open new avenues in the study of paleoclimate, as the measurement of clumped isotopes in phosphorites and fossils has the potential to reconstruct environmental temperatures.

Sabertooth carcass consumption behavior and the dynamics of Pleistocene large carnivoran guilds.

Apex predators play an important role in the top-down regulation of ecological communities. Their hunting and feeding behaviors influence, respectively, prey demography and the availability of resources to other consumers. Among the most iconic-and enigmatic-terrestrial predators of the late Cenozoic are the Machairodontinae, a diverse group of big cats whose hypertrophied upper canines have earned them the moniker "sabertooths." Many aspects of these animals' paleobiology, especially their prey preferences and carcass consumption behavior, remain unsettled. While skeletal anatomy, dental morphology and wear, and isotopic profiles provide important insights, the most direct way to resolve these issues is through the fossil remains of sabertooth prey. Here, we report on a taphonomic analysis of an early Pleistocene faunal assemblage from Haile 21A (Florida, USA) that preserves feeding damage from the lion-sized sabertooth Xenosmilus hodsonae. Patterns of tooth-marking and bone damage indicate that Xenosmilus fully defleshed the carcasses of their prey and even engaged in some minor bone consumption. This has important implications for Pleistocene carnivoran guild dynamics, including the carcass foraging behavior of the first stone-tool-using hominins.

A new machairodont from the Palmetto Fauna (early Pliocene) of Florida, with comments on the origin of the Smilodontini (Mammalia, Carnivora, Felidae).

South-central Florida's latest Hemphillian Palmetto Fauna includes two machairodontine felids, the lion-sized Machairodus coloradensis and a smaller, jaguar-sized species, initially referred to Megantereon hesperus based on a single, relatively incomplete mandible. This made the latter the oldest record of Megantereon, suggesting a New World origin of the genus. Subsequent workers variously accepted or rejected this identification and biogeographic scenario. Fortunately, new material, which preserves previously unknown characters, is now known for the smaller taxon. The most parsimonious results of a phylogenetic analysis using 37 cranio-mandibular characters from 13 taxa place it in the Smilodontini, like the original study; however, as the sister-taxon to Megantereon and Smilodon. Accordingly, we formally describe Rhizosmilodon fiteae gen. et sp. nov. Rhizosmilodon, Megantereon, and Smilodon ( = Smilodontini) share synapomorphies relative to their sister-taxon Machairodontini: serrations smaller and restricted to canines; offset of P3 with P4 and p4 with m1; complete verticalization of mandibular symphysis; m1 shortened and robust with widest point anterior to notch; and extreme posterior "lean" to p3/p4. Rhizosmilodon has small anterior and posterior accessory cusps on p4, a relatively large lower canine, and small, non-procumbent lower incisors; all more primitive states than in Megantereon and Smilodon. The former also differs from Megantereon and Smilodon gracilis by having a very small mandibular flange. Rhizosmilodon is the oldest known member of the Smilodontini, suggesting that the tribe originated in North America. Two more derived, similar-sized species evolved in parallel during the Blancan, Megantereon hesperus and Smilodon gracilis. The former is rarer, known only from the north-central and northwestern US, and presumably dispersed into the Old World. The latter is known from the eastern and southern US, and dispersed into South America.