First North American fossil monkey and early Miocene tropical biotic interchange.

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.

Cranial asymmetry in Eocene archaeocete whales and the evolution of directional hearing in water.

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 low-frequency hearing and bulk-straining predation.

Modularity of the rodent mandible: integrating bones, muscles, and teeth.

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.

Integrated Chronology, Flora and Faunas, and Paleoecology of the Alajuela Formation, Late Miocene of Panama.

The late Miocene was an important time to understand the geological, climatic, and biotic evolution of the ancient New World tropics and the context for the Great American Biotic Interchange (GABI). Despite this importance, upper Miocene deposits containing diverse faunas and floras and their associated geological context are rare in Central America. We present an integrated study of the geological and paleontological context and age of a new locality from Lago Alajuela in northern Panama (Caribbean side) containing late Miocene marine and terrestrial fossils (plants, invertebrates, and vertebrates) from the Alajuela Formation. These taxa indicate predominantly estuarine and shallow marine paleoenvironments, along with terrestrial influences based on the occurrence of land mammals. Sr-isotope ratio analyses of in situ scallop shells indicate an age for the Alajuela Formation of 9.77 ± 0.22 Ma, which also equates to a latest Clarendonian (Cl3) North American Land Mammal Age. Along with the roughly contemporaneous late Miocene Gatun and Lago Bayano faunas in Panama, we now have the opportunity to reconstruct the dynamics of the Central America seaway that existed before final closure coincident with formation of the Isthmus of Panama.

Evolutionary Patterns among Living and Fossil Kogiid Sperm Whales: Evidence from the Neogene of Central America.

Kogiids are known by two living species, the pygmy and dwarf sperm whale (Kogia breviceps and K. sima). Both are relatively rare, and as their names suggest, they are closely related to the sperm whale, all being characterized by the presence of a spermaceti organ. However, this organ is much reduced in kogiids and may have become functionally different. Here we describe a fossil kogiid from the late Miocene of Panama and we explore the evolutionary history of the group with special attention to this evolutionary reduction. The fossil consists of cranial material from the late Tortonian (~7.5 Ma) Piña facies of the Chagres Formation in Panama. Detailed comparison with other fossil and extant kogiids and the results of a phylogenetic analysis place the Panamanian kogiid, herein named Nanokogia isthmia gen. et sp. nov., as a taxon most closely related to Praekogia cedrosensis from the Messinian (~6 Ma) of Baja California and to Kogia spp. Furthermore our results show that reduction of the spermaceti organ has occurred iteratively in kogiids, once in Thalassocetus antwerpiensis in the early-middle Miocene, and more recently in Kogia spp. Additionally, we estimate the divergence between extant species of Kogia at around the late Pliocene, later than previously predicted by molecular estimates. Finally, comparison of Nanokogia with the coeval Scaphokogia cochlearis from Peru shows that these two species display a greater morphological disparity between them than that observed between the extant members of the group. We hypothesize that this reflects differences in feeding ecologies of the two species, with Nanokogia being more similar to extant Kogia. Nanokogia shows that kogiids have been part of the Neotropical marine mammal communities at least since the late Miocene, and gives us insight into the evolutionary history and origins of one of the rarest groups of living whales.

Evolution of the earliest horses driven by climate change in the Paleocene-Eocene Thermal Maximum.

Body size plays a critical role in mammalian ecology and physiology. Previous research has shown that many mammals became smaller during the Paleocene-Eocene Thermal Maximum (PETM), but the timing and magnitude of that change relative to climate change have been unclear. A high-resolution record of continental climate and equid body size change shows a directional size decrease of ~30% over the first ~130,000 years of the PETM, followed by a ~76% increase in the recovery phase of the PETM. These size changes are negatively correlated with temperature inferred from oxygen isotopes in mammal teeth and were probably driven by shifts in temperature and possibly high atmospheric CO(2) concentrations. These findings could be important for understanding mammalian evolutionary responses to future global warming.