Why the Early Paleozoic was intrinsically prone to marine extinction.

The geological record of marine animal biodiversity reflects the interplay between changing rates of speciation versus extinction. Compared to mass extinctions, background extinctions have received little attention. To disentangle the different contributions of global climate state, continental configuration, and atmospheric oxygen concentration (pO2) to variations in background extinction rates, we drive an animal physiological model with the environmental outputs from an Earth system model across intervals spanning the past 541 million years. We find that climate and continental configuration combined to make extinction susceptibility an order of magnitude higher during the Early Paleozoic than during the rest of the Phanerozoic, consistent with extinction rates derived from paleontological databases. The high extinction susceptibility arises in the model from the limited geographical range of marine organisms. It stands even when assuming present-day pO2, suggesting that increasing oxygenation through the Paleozoic is not necessary to explain why extinction rates apparently declined with time.

A Pleistocene assemblage of near-modern Papio hamadryas from the Middle Awash study area, Afar Rift, Ethiopia.

OBJECTIVES: The aim of this study is to assess a new assemblage of papionin fossils (n = 143) recovered from later Pleistocene sediments in the Middle Awash study area in the Afar Rift of Ethiopia. MATERIALS AND METHODS: We collected metric and qualitative data to compare the craniodental and postcranial anatomy of the papionin fossils with subspecies of modern Papio hamadryas and with Plio-Pleistocene African papionins. We also estimated sex and ontogenetic age. RESULTS: The new fossils fit well within the range of morphological variation observed for extant P. hamadryas, overlapping most closely in dental size and proportions with the P. h. cynocephalus individuals in our extant samples, and well within the ranges of P. h. anubis and P. h. hamadryas. The considerable overlap in craniodental anatomy with multiple subspecies precludes subspecific diagnosis. We therefore referred 143 individuals to P. hamadryas ssp. The majority of the individuals assessed for ontogenetic age fell into middle- and old-adult age categories based on the degree of dental wear. Males (26%) were better represented than females (12%) among individuals preserving the canine-premolar honing complex. DISCUSSION: These new near-modern P. hamadryas fossils provide a window into population-level variation in the later Pleistocene. Our findings echo previous suggestions from genomic studies that the papionin family tree may have included a ghost population and provide a basis for future testing of hypotheses regarding hybridization in the recent evolutionary history of this taxon.