Summary Several models explain how a complex integrated system like the rodent mandible can arise from multiple developmental modules. The models propose various integrating mechanisms, including epigenetic effects of muscles on bones. We test five for their ability to predict correlations found in the individual (symmetric) and fluctuating asymmetric (FA) components of shape variation. We also use exploratory methods to discern patterns unanticipated by any model. Two models fit observed correlation matrices from both components: (1) parts originating in same mesenchymal condensation are integrated, (2) parts developmentally dependent on the same muscle form an integrated complex as do those dependent on teeth. Another fits the correlations observed in FA: each muscle insertion site is an integrated unit. However, no model fits well, and none predicts the complex structure found in the exploratory analyses, best described as a reticulated network. Furthermore, no model predicts the correlation between proximal parts of the condyloid and coronoid, which can exceed the correlations between proximal and distal parts of the same process. Additionally, no model predicts the correlation between molar alveolus and ramus and/or angular process, one of the highest correlations found in the FA component. That correlation contradicts the basic premise of all five developmental models, yet it should be anticipated from the epigenetic effects of mastication, possibly the primary morphogenetic process integrating the jaw coupling forces generated by muscle contraction with those experienced at teeth.

Warming and Shrinking In most mammals, individual body sizes tend to be smaller in warmer regions and larger in cooler regions. Secord et al. (p. 959 ; see the Perspective by Smith ) examined a high-resolution 175,000-year record of equid fossils deposited over a past climate shift—the Paleocene-Eocene Thermal Maximum—for changes in body size. Using oxygen isotopes collected from the teeth of co-occurring mammal species to track prevailing environmental temperature, a clear decrease in equid body size was seen during 130,000 years of warming, followed by a distinct increase as the climate cooled at the end of the period. These results indicate that temperature directly influenced body size in the past and may continue to have an influence as our current climate changes.

The mammalian mandible is a developmentally modular but functionally integrated system. Whether morphological integration can evolve to match the optimal pattern of functional integration may depend on the developmental origin of integration, specifically, on the role that direct epigenetic interactions play in shaping integration. These interactions are hypothesized to integrate modules and also to be highly conservative, potentially constraining the evolution of integration. Using the fox squirrel (Sciurus niger) mandible as a model system, we test five a priori developmental hypotheses that predict mandibular integration and we also explore for correlations between shapes of mandibular regions not anticipated by any of the developmental models. To determine whether direct epigenetic interactions are highly conserved in rodents, we examine the correlation structure of fluctuating asymmetry, and compare integration patterns between fox squirrels and prairie deer mice (Peromyscus maniculatus bairdii). In fox squirrels, we find a correlation structure unanticipated by all a priori developmental models: adjacent parts along the proximodistal jaw axis are correlated whereas more distant ones are not. The most notable exception is that the shape of the anterior incisor alveolus is correlated with the shape of the ramus (FA component) or coronoid (symmetric component). Those exceptions differ between species; in prairie deer mice, the molar alveolus is connected to more parts, and the incisor alveolus to fewer, than in fox squirrels.

New World monkeys (platyrrhines) are a diverse part of modern tropical ecosystems in North and South America, yet their early evolutionary history in the tropics is largely unknown. Molecular divergence estimates suggest that primates arrived in tropical Central America, the southern-most extent of the North American landmass, with several dispersals from South America starting with the emergence of the Isthmus of Panama 3-4 million years ago (Ma). The complete absence of primate fossils from Central America has, however, limited our understanding of their history in the New World. Here we present the first description of a fossil monkey recovered from the North American landmass, the oldest known crown platyrrhine, from a precisely dated 20.9-Ma layer in the Las Cascadas Formation in the Panama Canal Basin, Panama. This discovery suggests that family-level diversification of extant New World monkeys occurred in the tropics, with new divergence estimates for Cebidae between 22 and 25 Ma, and provides the oldest fossil evidence for mammalian interchange between South and North America. The timing is consistent with recent tectonic reconstructions of a relatively narrow Central American Seaway in the early Miocene epoch, coincident with over-water dispersals inferred for many other groups of animals and plants. Discovery of an early Miocene primate in Panama provides evidence for a circum-Caribbean tropical distribution of New World monkeys by this time, with ocean barriers not wholly restricting their northward movements, requiring a complex set of ecological factors to explain their absence in well-sampled similarly aged localities at higher latitudes of North America.

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Eocene archaeocete whales gave rise to all modern toothed and baleen whales (Odontoceti and Mysticeti) during or near the Eocene-Oligocene transition. Odontocetes have asymmetrical skulls, with asymmetry linked to high-frequency sound production and echolocation. Mysticetes are generally assumed to have symmetrical skulls and lack high-frequency hearing. Here we show that protocetid and basilosaurid archaeocete skulls are distinctly and directionally asymmetrical. Archaeocete asymmetry involves curvature and axial torsion of the cranium, but no telescoping. Cranial asymmetry evolved in Eocene archaeocetes as part of a complex of traits linked to directional hearing (such as pan-bone thinning of the lower jaws, mandibular fat pads, and isolation of the ear region), probably enabling them to hear the higher sonic frequencies of sound-producing fish on which they preyed. Ultrasonic echolocation evolved in Oligocene odontocetes, enabling them to find silent prey. Asymmetry and much of the sonic-frequency range of directional hearing were lost in Oligocene mysticetes during the shift to lowfrequency hearing and bulk-straining predation.Cetacea | land-to-sea transition M ost mammals have bilaterally symmetrical skulls. Symmetrical crania characterize the artiodactyls closely related to whales, and symmetrical crania characterize mysticetes within Cetacea (1) (Fig. 1A). Odontocetes are exceptional because most odontocete crania are asymmetrical, with dorsal cranial bones shifted posteriorly and to the left side (1-8). Living odontocetes have a hypertrophied melon, nasal sacs, and phonic lips used to produce high-frequency sound (> 20 kHz) (9-11). Mysticetes lack these specializations of the nasal apparatus, use low-frequency sound (11,12), and may use the larynx (13) to produce lowfrequency sound. Coupling of high-frequency echolocation with facial and cranial asymmetry in living odontocetes, and the absence of both in living artiodactyls and living mysticetes, make it reasonable to expect that asymmetry originated in odontocetes (5-7). However, it is unresolved how the cranial asymmetry of odontocetes evolved in the transition from archaeocetes to modern whales, and the history becomes even more complex when archaeocetes themselves are considered.Eocene archaeocete whales gave rise to all modern toothed and baleen whales during or near the Eocene-Oligocene transition (14-16). Archaeocetes were previously thought to have symmetrical skulls (3, 5, 7). Asymmetry observed in fossil crania has often been assumed to be an artifact of deformation following burial, and it has been ignored or even removed in published drawings [as was done initially for three of the skulls we studied (17-19)]. ResultsHere we document and quantify asymmetry in archaeocete crania. Further observations on exceptionally well-preserved archaeocete crania and dentaries suggest a link between cranial asymmetry and the ability to locate sound sources in water.We quantified midline suture deviation, δx, from a straight rostrocaudal axis [RC, after Ness...

The global climate during the early Miocene was warmer than the present and preceded the even warmer middle Miocene climatic optimum. The paleo-CO 2 records for this interval suggest paradoxically low concentrations (<450 ppm) that are difficult to reconcile with a warmer-than-present global climate. METHODS: In this study, we use a leaf gas-exchange model to estimate CO 2 concentrations using stomatal characteristics of fossil leaves from a late early Miocene Neotropical assemblage from Panama that we date to 18.01 ± 0.17 Ma via 238 U/ 206 Pb zircon geochronology. We first validated the model for Neotropical environments by estimating CO 2 from canopy leaves of 21 extant species in a natural Panamanian forest and from leaves of seven Neotropical species in greenhouse experiments at 400 and 700 ppm. KEY RESULTS: The results showed that the most probable combined CO 2 estimate from the natural forests and 400 ppm experiments is 475 ppm, and for the 700 ppm experiments is 665 ppm. CO 2 estimates from the five fossil species exhibit bimodality, with two species most consistent with a low mode (528 ppm) and three with a high mode (912 ppm). CONCLUSIONS: Despite uncertainties, it is very likely (at >95% confidence) that CO 2 during the late early Miocene exceeded 400 ppm. These results revise upwards the likely CO 2 concentration at this time, more in keeping with a CO 2-forced greenhouse climate.

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