Taxonomic overview and tusk growth analyses of Ziegler Reservoir proboscideans Daniel C. Fisher a,b, ⁎, Michael D. Cherney a,b , Cody Newton c,d , Adam N. Rountrey a,e , Zachary T. Calamari a,b,f , Richard K. Stucky c , Carol Lucking c , Lesley Petrie g a Museum of Paleontology, University of Michigan, 1109 Geddes Ave., Ann Arbor, MI 48109, USA b Department of Earth and Environmental Sciences, University of Michigan, 1100 N. University Ave., Ann Arbor, MI 48109, USA c Department of Earth Sciences, Denver Museum of Nature and Science, Denver, CO 80205, USA d Department of Anthropology, University of Colorado, Boulder, CO 80309, USA e Centre for Marine Futures, Oceans Institute, University of Western Australia, M470 35 Stirling Highway, Crawley, WA 6009, Australia f Richard Gilder Graduate School, American Museum of Natural History, New York, NY 10024, USA g Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, USA abstract article info Article history: Received 19 November 2013 Available online 1 October 2014 Keywords: American mastodon Columbian mammoth Pleistocene Marine Oxygen Isotope Stage 5 Stable isotope profiles MicroCT Paleoclimate Mandibular tusks At an altitude of 2705 m in the Colorado Rockies (USA), the Ziegler Reservoir fossil site gives a rare look at a high- elevation ecosystem from the late Pleistocene (especially MIS 5) of North America. Remains of more than four mammoths and about 35 mastodons dominate the macrofossil assemblage. Mammoth remains are attributed to Mammuthus columbi, and mastodon remains are referred to the well-known, continent-wide Mammut americanum. Mastodon remains occur within and between several lake-margin slump deposits. Their deposition must therefore have occurred as events that were to some degree separate in time. We treat the mastodon assemblage in each stratigraphic unit as a source of information on environmental conditions during the lives of these individuals. Mastodon mandibular tusks are abundant at the site and represent both males and females, from calves to full-grown adults. This study presents the first attempt to use microCT, thin-section, and isotope records from mandibular tusks to reconstruct features of life-history. We recognize an up-section trend in δ 18 O profiles toward higher values, suggestive of warmer temperatures. Throughout this sequence, mastodon growth histories show low mean sensitivities suggestive of low levels of environmental stress. This work helps frame expectations for assessing environmental pressures on terminal Pleistocene populations. © 2014 University of Washington. Published by Elsevier Inc. All rights reserved. Introduction The Ziegler Reservoir fossil site (ZRFS) just outside Snowmass Vil- lage, Colorado (USA) provides a remarkable opportunity to follow a high-altitude biota associated with an alpine lake/marsh environment through about 85,000 years of the Pleistocene (Mahan et al., 2014–in this volume; Pigati et al., 2014–in this volume). Its most numerous mac- roscopically identifiable remains are of proboscideans, both mammoths and mastodons. Three partial mammoth skeletons, additional mam- moth molars, and disarticulated (mostly unassociated) bones and tusks representing about 35 mastodons constitute ~80% of more than 5000 macro-vertebrate specimens recovered during fall 2010 and sum- mer 2011 excavations. In this report, we discuss both the mammoths and the mastodons found at the site, but the bones of each taxon are as- sociated with different depositional settings. All mammoth remains were found in bog or marsh deposits near the top of the section, in what are effectively lake-center locations. In contrast, mastodon remains were mostly lower in the section, near the lake margin, in diamictic slump deposits derived from the moraine that formed the lake margin or in silty layers below and between these deposits. For this assessment of ZRFS proboscidean material, we focus on pro- visional taxonomic assignment and extraction of paleobiologically and paleoclimatically relevant data from the tusk record. ZRFS mammoths present intriguing taphonomic questions, touched on below, but they are not yet thoroughly extracted from matrix and do not present a sam- ple large enough to support comparative analysis. In contrast, the num- ber and preservational quality of ZRFS mastodon specimens offer diverse opportunities for comparison. Our main source of insight into the lives and environments of these animals is analysis of the structure and composition of their tusks. Proboscidean tusks are ever-growing in- cisors composed primarily of dentin that grows through continuous ap- position along a conical pulp cavity surface at the tusk's proximal end. Previous studies of proboscidean tusk growth records have featured premaxillary (upper) tusks (e.g., Fisher, 1996, 2009), and although multiple premaxillary tusks were found at the site, most required jacketing for safe removal and transport and are not yet opened, stabilized, and available for study. Mastodon mandibular (lower) tusks Quaternary Research 82 (2014) 518–532 ⁎ Corresponding author. Fax: +1 734 936 1380. E-mail address: dcfisher@umich.edu (D.C. Fisher). http://dx.doi.org/10.1016/j.yqres.2014.07.010 0033-5894/© 2014 University of Washington. Published by Elsevier Inc. All rights reserved. Contents lists available at ScienceDirect Quaternary Research journal homepage: www.elsevier.com/locate/yqres