Journal of Paleontology, Volume 95, Memoir 82, 2021, p. 1–115 Copyright © The Author(s), 2021. Published by Cambridge University Press on behalf of The Paleontological Society. All rights reserved. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/ by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. 0022-3360/21/1937-2337 doi: 10.1017/jpa.2020.74 Carnivorous mammals from the middle Eocene Washakie Formation, Wyoming, USA, and their diversity trajectory in a post-warming world Susumu Tomiya,1,2,3 Shawn P. Zack,4 Michelle Spaulding,5 and John J. Flynn2,6 1Center for International Collaboration and Advanced Studies in Primatology, Kyoto University Primate Research Institute, 41-2 Kanrin, Inuyama, Aichi 484-8506, Japan <[email protected]> 2Negaunee Integrative Research and Gantz Family Collections Centers, Field Museum of Natural History, 1400 South Lake Shore Drive, Chicago, Illinois 60605, USA 3University of California Museum of Paleontology, Berkeley, California 94720, USA 4Department of Basic Medical Sciences, University of Arizona College of Medicine, 425 N Fifth Street, Phoenix, Arizona 85004, USA <[email protected]> 5Department of Biological Sciences, Purdue University Northwest, 1401 S US Highway 421, Westville, Indiana 46391, USA <mspauldi@ pnw.edu> 6Richard Gilder Graduate School and Division of Paleontology, American Museum of Natural History, 200 Central Park West, New York, New York 10024, USA <jfl[email protected]> Abstract.—The middle Eocene Washakie Formation of Wyoming, USA, provides a rare window, within a single deposi- tional basin, into the faunal transition that followed the early Eocene warming events. Based on extensive examination, we report a minimum of 27 species of carnivorous mammals from this formation, more than doubling the previous taxic count. Included in this revised list are a new species of carnivoraform, Neovulpavus mccarrolli n. sp., and up to ten other possibly new taxa. Our cladistic analysis of early Carnivoraformes incorporating new data clarified the array of middle Eocene taxa that are closely related to crown-group Carnivora. These anatomically relatively derived carnivoraforms collectively had an intercontinental distribution in North America and east Asia, exhibiting notable variations in body size and dental adaptation. This time period also saw parallel trends of increase in body size and dental sectoriality in distantly related lineages of carnivores spanning a wide range of body sizes. A new, model-based Bayesian analysis of diversity dynamics accounting for imperfect detection revealed a high probability of substantial loss of carnivore species between the late Bridgerian and early Uintan North American Land Mammal ‘Ages’, coinciding with the dis- appearance of formerly common mammals such as hyopsodontids and adapiform primates. Concomitant with this decline in carnivore diversity, the Washakie vertebrate fauna underwent significant disintegration, as measured by patterns of coordinated detection of taxa at the locality level. These observations are consistent with a major biomic transition in the region in response to climatically induced opening-up of forested habitats. UUID: http://zoobank.org/9162f1a6-a12c-4d55-ba1d-dc66e8cda261 Introduction 2013; Silvestro et al., 2015; Slater, 2015; Balisi et al., 2018). Given the general rarity of mammalian carnivore fossils, Early discoveries of middle Eocene mammalian carnivores in sustained efforts to uncover, document, and reconstruct the North America stimulated development of ideas about the pro- diversity of fossil carnivore taxa are fundamental to research cesses of carnivore evolution, and were essential components in this area. of the new evolutionary synthesis in the middle of the twentieth The middle Eocene Washakie Formation of south central century (Simpson, 1944). Matthew’s(1909) systematic work on Wyoming and northwestern Colorado have produced a wealth middle Eocene-aged (Bridgerian and Uintan North American of vertebrate fossils since the late nineteenth century (Black Land Mammals ‘Ages’ [NALMAs]) mammals from Wyoming and Dawson, 1966; Roehler, 1973; Turnbull, 1978, 2002). envisioned opportunities for differential dietary adaptations as A classic collection of Washakie fossil vertebrates curated at a major driver of diversification in various groups of primarily the American Museum of Natural History (AMNH) paved the carnivorous mammals, and conceptualized the advantage of way for the early works of Granger (1909) and Matthew ecological incumbency in their evolutionary histories. More (1909), which established the basic biostratigraphic framework than a century later, these hypotheses continue to be explored for the formation. In the early 1950s, a major research program and refined in various systems at different scales (Van Valken- centered on the Washakie fauna was initiated under the direction burgh, 1999; Van Valkenburgh et al., 2004; Wesley-Hunt, of William D. Turnbull of the Field Museum of Natural History 2005; Friscia and Van Valkenburgh, 2010; Sandom et al., (FMNH), building on earlier work by Rainer Zangerl, and 1 Downloaded from https://www.cambridge.org/core. IP address: 170.106.33.14, on 27 Sep 2021 at 11:06:02, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/jpa.2020.74 2 Journal of Paleontology 95(S82):1–115 extended by John Flynn (who filled the FMNH fossil mammal hyaenodontans, and carnivoramorphans. For convenience, we curator position after Turnbull retired). Over five decades, Turn- informally refer to them collectively as ‘carnivores’ while recog- bull, Flynn, and colleagues collected extensively in the Washa- nizing that some of their middle Eocene members may have kie Basin, building one of the largest collections of vertebrate been omnivorous or frequently consumed invertebrates, and fossils from the Washakie Formation. This collection has served are thus more accurately described as a broad set of animalivores as the basis for a number of publications (Turnbull, 1978, 1991, (Friscia et al., 2007; Friscia and Van Valkenburgh, 2010). We 2002; Turnbull and Martill, 1988; McCarroll et al., 1996a, b), also note that non-mammalian carnivores such as crocodiles, but the vast majority of the fossils remain undescribed. snakes, and large birds, all of which are known from the Washa- A recent three-year project at FMNH reorganized and kie Formation, may have played important ecological roles as expanded knowledge of this collection through the cataloging predators of mammals. of ∼3,000 additional specimens (see Acknowledgments), mak- ing the time ripe for a renewed examination of the vertebrate Geological and paleoenvironmental setting diversity in the Washakie Formation. This paper aims to advance our knowledge of the Bridgerian–Uintan faunal transition in the Geology and NALMA biochronology.—Exposures of the central Rocky Mountain region during an early phase of carni- Washakie Formation cover an area of ∼1,600 km2 in the voran evolution and dynamic turnover in other carnivore clades Washakie Basin of south-central Wyoming, and are present in (Eizirik et al., 2010; Tomiya, 2011; Tomiya and Tseng, 2016). the Sand Wash Basin of northwestern Colorado (Roehler, Our objectives are as follows: (1) we thoroughly reevaluate pre- 1973; Turnbull, 1978; Stucky et al., 1996; Fig. 1). Within the viously reported occurrences of carnivorous mammals from the Washakie Basin, the Washakie Formation unconformably Washakie Formation, and describe taxonomically significant overlies the Green River Formation and is divided into the late specimens that have yet to be reported in the literature; (2) we Bridgerian Kinney Rim Member (Twkk) and the late present a revised phylogenetic hypothesis for early carnivora- Bridgerian to Uintan Adobe Town Member; the latter is forms to provide the evolutionary context for interpreting the further divided, informally, into lower (Twka1), middle morphological diversity of middle Eocene taxa from the Washa- (Twka2), and upper (Twka3) units based on stratigraphy kie Basin; (3) we investigate whether the sharp drop in diversity (Turnbull, 1978; Flynn, 1986), with auxiliary information on of carnivorous taxa from ca. 49 to ca. 45 Ma in western North characteristic mammalian assemblages (Roehler, 1973, 1992; America (from 13 to 5 genera, or a 62% loss; Wesley-Hunt, McCarroll et al., 1996b; Robinson et al., 2004; Murphey 2005) is detected within a single depositional basin after et al., 2018; Fig. 2). We generally follow Roehler’s(1973) accounting for sampling incompleteness. Such an assessment— designation of lower (Twka1 of later authors) and middle made possible by the substantial temporal span of the Washakie Basin sequence and the availability of specimen-level taxon-occurrence data—is essential for elucidating the mechan- isms of diversity dynamics at larger geographic scales. This study is concerned with taxa from the lower and mid- dle units of the Adobe Town Member of the Washakie Forma- tion (abbreviated as Twka1 and Twka2, respectively, in which “wk” and “a” denote Washakie Formation and Adobe Town Member [Turnbull, 1978]), because no ordinally identifiable carnivore specimen has been recovered from the underlying Kinney Rim Member, and only two are known from the upper unit of the Adobe Town Member (Twka3). We intend to describe