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Ectocion Parvus) and the Implication of Body Size Change During the Paleocene–Eocene Thermal Maximum

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Ectocion Parvus) and the Implication of Body Size Change During the Paleocene–Eocene Thermal Maximum PALAEO-06253; No of Pages 7 Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2012) xxx–xxx Contents lists available at SciVerse ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo Northward range extension of a diminutive-sized mammal (Ectocion parvus) and the implication of body size change during the Paleocene–Eocene Thermal Maximum Benjamin John Burger ⁎ Department of Geology, Uintah Basin Campus at Vernal, Utah State University, 320 North Aggie Blvd., Vernal, UT 84078, USA article info abstract Article history: An abrupt global warming event marks the Paleocene–Eocene boundary, known as the Paleocene–Eocene Thermal Received 12 March 2012 Maximum (PETM). The event is distinguished in the strata globally by a significant negative excursion of δ13Cratio Received in revised form 6 August 2012 values. The response of the terrestrial biota to the abrupt climatic change has been well studied in northern Accepted 11 September 2012 Wyoming in the Bighorn Basin, where it has been observed that the mammalian fauna during the global warming Available online xxxx event is represented by smaller, but morphologically similar species to those found later in the Eocene. Various hypotheses have been proposed to explain the observation smaller body sizes during the global warming event. Keywords: Paleocene In this article, evidence is presented to support the hypothesis that the observed body size decrease during Eocene the PETM was influenced by the appearance of smaller southern species who extended their geographic Global warming range northward during the abnormal global warming event. Using disperse organic carbon isotopic ratios Biogeography of bulk sediment, the negative excursion of δ13C was located in the Piceance Creek Basin of western Colorado, Piceance Creek 400 km to the south of the Bighorn Basin. Below the stratigraphic level marking the negative carbon excursion Carbon isotopes in the Piceance Creek Basin are five specimens of the phenacodontid mammal (Ectocion parvus), a diminutive species of the genus Ectocion restricted to the basal Eocene (Wa-0 Biozone) in northern Wyoming. The five spec- imens of E. parvus are associated with a late Paleocene (Clarkforkian) mammalian fauna in Colorado, implying that the diminutive species extended its geographic range northward during the global warming event. This evidence supports biogeographic models that assume poleward biogeographic shifts during global warming events, and will have modern day implications for the conservation of species as global temperatures rise in the near future. © 2012 Published by Elsevier B.V. 1. Introduction meters. The Wa-M is distinguished by the additional presence of the mammal Meniscotherium priscum (hence the suffixM)andlackofpri- Stable isotopic ratios of oxygen in marine sediments indicate an mates and artiodactyls. abrupt global warming event at the Paleocene–Eocene boundary In its entirety, the Wa-0 mammalian fauna is characterized by dimin- (Kennett and Stott, 1991), referred to as the Paleocene–Eocene Thermal utive and novel species (Gingerich, 2001, 2003; Strait, 2001; Secord et al., Maximum (PETM; Aubry et al., 2003; McInerney and Wing, 2011). Esti- 2012). Diminutive species include the early equid Sifrhippus sandrae mates indicate global temperatures rose upward 8° Celsius (Zachos (Froehlich, 2002; Secord et al., 2012) (also referred to either the genus et al., 2006), within a single precession cycle of Earth's orbit (less than Eohippus or Hyracotherium), the phenacodontids Ectocion parvus 24,000 years; Röhl et al., 2007). (Gingerich and Smith, 2006)andCopecion davisi (Gingerich, 2003)as Terrestrial strata crossing the Paleocene–Eocene boundary is well well as the small hyposodontid Hapomylus zalmouti (Gingerich and documented in the Bighorn Basin of northern Wyoming, where it is rec- Smith, 2006). Novel species, such as the earliest artiodactyl Diacodexis, ognized by a significant negative excursion of δ13Cratiovalues(Koch euprimate Teilhardina, and perissodactyl S. sandrae, all make their first et al., 1992; Magioncalda et al., 2004), and an abrupt turnover of mam- appearance during this interval. Similar genera co-occur in the earliest malian species (Gingerich, 2003). In the Bighorn Basin, the interval of Eocene rock units in Asia and Europe. In fact, in the northern hemisphere the Paleocene–Eocene Thermal Maximum is represented by a unique early Eocene mammalian faunas are much more cosmopolitan and mammalian fauna referred to as the Wa-0 biozone; the first biozone of homogenous than faunas known from the late Paleocene (Krause and the Eocene Wasatchian North American Land Mammal Age (NALMA). Maas, 1990; Beard, 2002; Bowen et al., 2002; Hooker and Dashzeveg, Gingerich and Smith (2006) have further divided the Wa-0 biozone, by 2003; Beard, 2008). recognizing a Wa-M biozone which characterizes the lower several Two proposed hypotheses attempt to explain the diminutive body size of mammals during the Wa-0 biozone in the Bighorn basin. The fi ⁎ Tel.: +1 435 722 1778. rst hypothesis is that mammal species underwent a phenotypic E-mail address: [email protected]. change. Populations decreased the average body-size in response to 0031-0182/$ – see front matter © 2012 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.palaeo.2012.09.008 Please cite this article as: Burger, B.J., Northward range extension of a diminutive-sized mammal (Ectocion parvus) and the implication of body size change during the Paleoc..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2012), http://dx.doi.org/10.1016/j.palaeo.2012.09.008 2 B.J. Burger / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2012) xxx–xxx the environmental conditions that existed during the PETM global Budd et al., 2002). The influence of changing plant communities also warming event. This hypothesis favors an intrinsic response of the pop- can have an effect on the magnitude of the observed δ13C values in ulation to cope with the new selection pressure of a warmer climate, the rock record (Wing et al., 2005; Smith et al., 2007). The δ13C values such as the possible decreased availability of food, shelter and habitat calculated from DOC represent an average δ13C from numerous sources, which led to their smaller size. The second hypothesis is that the some of which may be derived from resistant carbon from older sedi- observed decrease in the average body-size was the result of extrinsic mentary rocks. Although mixing (and reworking) is possible, all of forces, such as the range extension of small species into the Bighorn these sources were directly influenced by the δ13C in the atmosphere, Basin, displacing larger species. This second hypothesis explains the and the CIE has been identified using this method elsewhere in the observation by invoking biogeographical shifts in the geographic range world (Collinson et al., 2003; Harrington et al., 2005; Domingo et al., of species during the PETM rather than an in situ evolutionary response. 2009). Organic-poor sandstone units tend to have greater variance in Both hypotheses invoke Bergmann's rule, in which smaller species occur δ13C values. This greater variance of δ13C values is likely due to the com- in warming climates (or lower latitudes), but the mechanism for the bined effects of mixing and reworking, and the smaller amounts of body-size shift in each hypothesis differs. In the first hypothesis, the organic carbon found in sandstones. In contrast, organic rich rocks, such response is evolutionary driven, while the second hypothesis the re- as coals, contain in-situ organic matter sourced from fallen plant matter sponse is ecologically driven. that was not transported and exhibit δ13Cvalueswithlowvariance. In order to test these two hypotheses, an intense fossil collecting Rock samples were collected from each rock unit at approximate- effort was undertaken in the more southerly located Piceance Creek ly one-meter intervals (from each lithologic rock unit) in Sulfur Basin, 400 km to the south of the Bighorn Basin in western Colorado. Gulch, along the southern face of South Shale Ridge near the town The purpose of the collecting effort was to build a comparative collec- of DeBeque Colorado. Samples were powdered and subjected to a tion of late Paleocene and early Eocene fossil mammals to gain a bet- treatment of 10–33% HCl for several hours to remove diagenetic carbon- ter sense of the wider biogeographical occurrences of species across ate. While most samples dried at room temperature, very fine-grained North America. samples required additional heating to remove excess moisture. Rough- ly 20 milligrams of dried powdered rock was added into each tin cap- 2. Methods and materials sule. Samples were combusted at 1050 °C in a Eurovector elemental analyzer at the Colorado School of Mines. A continuous flow system The Piceance Creek Basin flanks the western margin of the south- on the GV Instruments IsoPrime mass spectrometer then measured 2 13 ern Rocky Mountains and encompasses 26,000 km of rugged topog- the δ C value on the combusted CO2 gas, with He as the carrier gas. raphy. Following the advent of the Laramide Orogeny during the late Calibration was performed using the IAEA-C-6 sucrose stable isotope Cretaceous and early Paleocene, the basin formed as eroded sediment reference standard, with an external precision of 0.5‰. Additional was shed off the recently uplifted Sawatch and Park Mountain Ranges samples were combusted at Washington State University, with an ECS to the southeast
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