High-resolution chronostratigraphy of the terrestrial Cretaceous-PaleogeneEmail alerting services articles cite this article to receive free e-mail alerts when new www.gsapubs.org/cgi/alerts click SubscribeAmerica Bulletin to subscribe to Geological Society of www.gsapubs.org/subscriptions/ click Permission requestto contact GSA http://www.geosociety.org/pubs/copyrt.htm#gsa click official positions of the Society. citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect presentation of diverse opinions and positions by scientists worldwide, regardless of their race, includes a reference to the article's full citation. GSA provides this and other forums for the the abstracts only of their articles on their own or their organization's Web site providing the posting to further education and science. This file may not be posted to any Web site, but authors may post works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent their employment. Individual scientists are hereby granted permission, without fees or furtherCopyright not claimed on content prepared wholly by U.S. government employees within scope of Notes articles must include the digital object identifier (DOIs) and date of initial publication.

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The mass extinction at the Cretaceous-Paleogene boundary marks one of the most important biotic turnover events in Earth history. Yet, despite decades of study, the causes of the Cretaceous-Paleogene boundary crises remain under debate. An important tool that has the capacity to greatly improve our understanding of the events around the Cretaceous-Paleogene boundary is the geomagnetic polarity time scale (GPTS). The GPTS is used for age control in numerous Cretaceous-Paleogene boundary studies, including the timing of Deccan Traps volcanism, a majority of studies in marine sections, and studies on climate and ecological change across the Cretaceous-Paleogene boundary. The current calibration of the GPTS for circum-Cretaceous-Paleogene boundary polarity chrons (C30n-C28n) from the Geologic Time Scale draws heavily on astronomical tuning and uses a 40 Ar/ 39 Ar age for the Cretaceous-Paleogene boundary as a tie point that has since been shown to be 200 ka too old. Furthermore, complex sedimentation has been recorded in marine sections immediately following the Cretaceous-Paleogene boundary, which can possibly obscure orbital signals and complicate cyclostratigraphic interpretation. An independent test of the cyclostratigraphy for this time period is imperative for confidence in the astronomical time scale. Further, polarity reversal ages given in the GPTS do not include uncertainty estimates, making them unsuitable for quanti-

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Earliest Paleocene Purgatorius often is regarded as the geologically oldest primate, but it has been known only from fossilized dentitions since it was first described half a century ago. The dentition of Purgatorius is more primitive than those of all known living and fossil primates, leading some researchers to suggest that it lies near the ancestry of all other primates; however, others have questioned its affinities to primates or even to placental mammals. Here we report the first (to our knowledge) nondental remains (tarsal bones) attributed to Purgatorius from the same earliest Paleocene deposits that have yielded numerous fossil dentitions of this poorly known mammal. Three independent phylogenetic analyses that incorporate new data from these fossils support primate affinities of Purgatorius among euarchontan mammals (primates, treeshrews, and colugos). Astragali and calcanei attributed to Purgatorius indicate a mobile ankle typical of arboreal euarchontan mammals generally and of Paleocene and Eocene plesiadapiforms specifically and provide the earliest fossil evidence of arboreality in primates and other euarchontan mammals. Postcranial specializations for arboreality in the earliest primates likely played a key role in the evolutionary success of this mammalian radiation in the Paleocene.

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