William C. Clyde

Evidence for rapid climate change in North America during the latest Paleocene thermal maximum: oxygen isotope compositions of biogenic phosphate from the Bighorn Basin (Wyoming)

Oxygen isotope records of Cenozoic sea water temperatures indicate that a rapid warming event known as the Latest Paleocene Thermal Maximum (LPTM) occurred during the otherwise gradual increase in world temperatures during the Late Paleocene and Early Eocene. Oxygen isotope analysis of the carbonate and phosphate components of hydroxyapatite found in mammalian tooth enamel and body scales of river-dwelling fish from the Bighorn Basin in Wyoming were made to investigate corresponding changes in the terrestrial climate. A comparison of carbonate and phosphate isotope data from modern and fossil material indicates that some diagenetic alteration of the fossil material has occurred, although systematically larger intra-tooth ranges in the oxygen isotope composition of carbonate indicate that it is more likely to have been affected than phosphate. Carbonate and phosphate from the ecologically diverse mammals and fishes both record a shift to higher oxygen isotope ratios at the same time and of the same duration as the LPTM. These shifts reflect a change in the isotopic composition of regional precipitation, which in turn provides the first evidence for continental climate change during the LPTM. Assuming the present-day relation between the oxygen isotope composition of precipitation and temperature applies to conditions in the past, and that animal physiology and behavior is relatively invariant over time, the isotopic shift is equivalent to an increase of surface temperature in western North America of several degrees. This result is consistent with the magnitude of high-latitude ocean warming, and provides a basis for relating marine and terrestrial oxygen isotope records to records of terrestrial biotic change.

Intra-tooth variations in δ18O (PO4) of mammalian tooth enamel as a record of seasonal variations in continental climate variables

Abstract Regular intra-tooth variations in the δ 18 O value of mammalian tooth enamel phosphate (δ 18 O p ) have been considered a potential measure of seasonal changes in continental climate variables since they were first observed. In order to investigate this possibility in more detail, analyses were made of teeth from a number of mammalian herbivores (sheep, cattle, elk, and pigs) that lived over a wide range of geographic locations, ecological settings, and climatic conditions (Iowa, Florida, Wyoming, Iceland, England, Croatia, and the Philippines). The lack of intra-tooth δ 18 O p variations in teeth of cattle that were given tap water to drink provides strong evidence that the underlying cause of observed intra-tooth variations is primarily a change in the isotopic composition of ingested water. In concert with this interpretation, the range of intra-tooth δ 18 O p values and their absolute values from each locality mirror observed differences in the range and absolute δ 18 O values of local precipitation (δ 18 O pt ) and in climate variables. Thus intra-tooth δ 18 O p values can indeed be considered a qualitative measure of seasonal climate change in continental settings. Quantitative use of intra-tooth δ 18 O p values as a climate proxy is possible, but is hindered by lack of detailed information on aspects of mammalian physiology, behavior, and perhaps local hydrology that may also play a role in influencing δ 18 O p . This problem is exemplified by the different range in δ 18 O p values measured for sheep and cattle from the same locality around York, UK (3.4 vs. 2.6‰, respectively). The observed difference most likely reflects a difference in the relative amount of leaf water ingested by the two species. Future studies of well-constrained samples are required to test physiological models and to develop empirical relations that accurately relate δ 18 O p to δ 18 O pt . In addition to their use as indicators of seasonality, intra-tooth variations in δ 18 O p values provide valuable information for longer-term climate change and paleobiological investigations.

Mammalian community response to the latest Paleocene thermal maximum: An isotaphonomic study in the northern Bighorn Basin, Wyoming

New stratigraphic and paleontological information from the McCullough Peaks, northern Bighorn Basin, Wyoming, is incorporated into an isotaphonomic faunal database and used to investigate the impact of the latest Paleocene thermal maximum and coincident earliest Wasatchian immigration event on local mammalian community structure. Surface collections from Willwood Formation overbank deposits provide taphonomically consistent and stratigraphically resolved samples of the medium- to large-sized components of underlying mammalian communities. Rarefaction shows that the immigration event caused an abrupt and dramatic increase in species richness and evenness. After this initial increase, diversity tapered off to more typical Wasatchian levels that were still higher than those in the preceding Clarkforkian. Wasatchian immigrants were rapidly incorporated into the new community organization, representing ∼20% of the taxa and ∼50% of the individuals. Immigrant taxa generally had larger body sizes and more herbivorous and frugivorous dietary habits compared to endemic taxa, causing significant turnover in body-size structure and trophic structure. There was a significant short-term body-size decrease in many lineages that may have been prompted by the elevated temperatures and/or decreased latitudinal thermal gradients during the latest Paleocene thermal maximum. Rapid short-term climatic change (transient climates) and associated biotic dispersal can have abrupt and long-lasting effects on mammalian community evolution.

Linking the Wasatchian/Bridgerian boundary to the Cenozoic Global Climate Optimum: new magnetostratigraphic and isotopic results from South Pass, Wyoming

New paleomagnetic and stable isotopic results from the northeastern margin of the greater Green River Basin (South Pass, Wyoming) provide a refined geochronological context for the Wasatchian/Bridgerian Land Mammal Age boundary and suggest the existence of large amplitude Milankovich-scale carbon and oxygen isotopic oscillations in this area during the early Eocene. Analysis of 55 paleomagnetic sites through a 310 m section of Wasatch, Green River, and Bridger Formations indicates several reversals that can be correlated to the Geomagnetic Polarity Time Scale using radiometric age constraints. This correlation places the Wasatchian/Bridgerian boundary in Chron C23r at about 52 Ma, approximately two million years older than previous estimates. This new correlation suggests that the mammalian turnover which characterizes the Wasatchian/Bridgerian boundary is coincident with the onset of the Cenozoic Global Climate Optimum, the warmest interval of the entire Cenozoic. Intrabasinal magnetostratigraphic correlation supports earlier claims that community composition and biostratigraphic datums of basinmargin faunas can differ significantly from coeval basin-center faunas. Oxygen and carbon isotopic composition of paleosol carbonates show in-phase cyclic variations on the order of 6 and 3.5‰, respectively. Based on the preferred magnetostratigraphic correlation, the four best-defined cycles represent 397 thousand years, indicating potential forcing by variations in orbital eccentricity. Oxygen isotopic variations may be tracking wet/dry cycles amplified by adjacent changes in levels of paleolake Gosuite. Carbon isotopic variations may be tracking the vegetative response to these climate variations. Alternative interpretations involve cyclic changes in pedogenesis, driving correlated variations in isotopic inheritance from parent (Paleozoic) carbonate material and the possibility that C4 plants existed in marginal habitats earlier in geological time than previously thought. q 2001 Elsevier Science B.V. All rights reserved.

Comparing the fit of stratigraphic and morphologic data in phylogenetic analysis

Stratigraphic data are compared to morphologic data in terms of their fit to phylogenetic hypotheses for 29 data sets taken from the literature. Stratigraphic fit is measured using MacClades stratigraphic character, which tracks the number of independent discrepancies between observed order and the order of occurrence that would be expected on the basis of a given phylogenetic hypothesis. Acceptance of a phylogenetic hypothesis despite such discrepancies requires ad hoc hypotheses concerning differential probabilities of preservation and recovery. These stratigraphic ad hoc hypotheses are treated as logically equivalent to morphologic ad hoc hypotheses of homoplasy. The retention index is used to compare the number of stratigraphic and morphologic ad hoc hypotheses required by given phylogenetic hypotheses. Each data set is subjected to five analyses, varying in the constraints imposed on the structure of the phylogenetic tree against which fit is measured. Analyses 1–4 compare the stratigraphic and morphologic retention indices using phylogenetic trees consistent with the morphologically most-parsimonious cladogram reported in the original study. Analysis 5 compares retention indices using the overall (stratigraphically and morphologically) most-parsimonious phylogenetic tree, which may be, but is not necessarily, consistent with the reported cladogram. Proceeding from Analysis 1 to Analysis 5, stratigraphic data are allowed greater influence in determining the structure of phylogenetic trees, with the trees in Analysis 1 derived without reference to the stratigraphic character and the trees in Analysis 5 derived from full interaction of stratigraphic and morphologic characters. Morphologic and stratigraphic retention indices for these 29 studies cannot be statistically distinguished in comparisons 3–5, suggesting very similar degrees of fit. The values of these retention indices are high, indicating a generally high level of congruence under these phylogenetic hypotheses. Significant gains (49%) in stratigraphic fit can be realized without significant loss (4%) in morphologic fit as the stratigraphic and morphologic evidence are both allowed to participate in constraining the structure of phylogenetic hypotheses. These results suggest that arguments based on alleged “noisiness” of stratigraphic data offer inadequate grounds for ignoring stratigraphic order in phylogenetic analysis. In terms of congruence, stratigraphic and morphologic data perform about equally well.

Stratigraphic response and mammalian dispersal during initial India-Asia collision: Evidence from the Ghazij Formation, Balochistan, Pakistan

Initial continental collision between India and Asia is thought to have caused significant changes to global climate and biota, yet its timing and biogeographic consequences are uncertain. Structural and geophysical evidence indicates initial collision during the early Paleogene, but sedimentary evidence of this has been controversial owing to the intense deformation and metamorphism along the suture zone. Modern orders of mammals that appeared abruptly on northern continents coincident with the global warming event marking the Paleocene-Eocene boundary are hypothesized to have originated on the Indian subcontinent, but no relevant paleontological information has been available to test this idea. Here we present new paleomagnetic, sedimentologic, and paleontologic evidence to show that the lower Eocene Ghazij Formation of western Pakistan records continental sedimentation and mammalian dispersal associated with initial India-Asia collision. Our results are consistent with the initial collision occurring near the Paleocene-Eocene boundary, but modern orders of mammals appeared later in Indo-Pakistan and thus did not likely originate on the Indian subcontinent.

New age constraints for the Salamanca Formation and lower Río Chico Group in the western San Jorge Basin, Patagonia, Argentina: Implications for cretaceous-paleogene extinction recovery and land mammal age correlations

The Salamanca Formation of the San Jorge Basin (Patagonia, Argentina) preserves critical records of Southern Hemisphere Paleocene biotas, but its age remains poorly resolved, with estimates ranging from Late Cretaceous to middle Paleocene. We report a multi-disciplinary geochronologic study of the Salamanca Formation and overlying Rio Chico Group in the western part of the basin. New constraints include (1) an 40Ar/39Ar age determination of 67.31 ± 0.55 Ma from a basalt flow underlying the Salamanca Formation, (2) micropaleontological results indicating an early Danian age for the base of the Salamanca Formation, (3) laser ablation HR-MC-ICP-MS (high resolution-multi collector-inductively coupled plasma-mass spectrometry) U-Pb ages and a high-resolution TIMS (thermal ionization mass spectrometry) age of 61.984 ± 0.041(0.074)[0.100] Ma for zircons from volcanic ash beds in the Penas Coloradas Formation (Rio Chico Group), and (4) paleomagnetic results indicating that the Salamanca Formation in this area is entirely of normal polarity, with reversals occurring in the Rio Chico Group. Placing these new age constraints in the context of a sequence stratigraphic model for the basin, we correlate the Salamanca Formation in the study area to Chrons C29n and C28n, with the Banco Negro Inferior (BNI), a mature widespread fossiliferous paleosol unit at the top of the Salamanca Formation, corresponding to the top of Chron C28n. The diverse paleobotanical assemblages from this area are here assigned to C28n (64.67–63.49 Ma), ∼2–3 million years older than previously thought, adding to growing evidence for rapid Southern Hemisphere floral recovery after the Cretaceous-Paleogene extinction. Important Peligran and “Carodnia” zone vertebrate fossil assemblages from coastal BNI and Penas Coloradas exposures are likely older than previously thought and correlate to the early Torrejonian and early Tiffanian North American Land Mammal Ages, respectively.

Chronology of the Wasatchian Land-Mammal Age (Early Eocene): Magnetostratigraphic Results from the McCullough Peaks Section, Northern Bighorn Basin, Wyoming

The McCullough Peaks section of the Willwood Formation in the northern Bighorn Basin, Wyoming, has produced a rich fossil record of early Eocene mammals spanning much of the Wasatchian land-mammal age. The Wasatchian is an especially significant period in mammalian evolution since it marks the first appearance of several modern orders of mammals including Perissodactyla, Artiodactyla, and true Primates. Magnetostratigraphic analysis of paleosol horizons in the McCullough Peaks region is used to correlate Wasatchian land-mammal zones to the geomagnetic polarity time scale. A total of 135 paleomagnetic samples were analyzed from 37 levels in a 1480 m section that ranges from Wasatchian zone Wa-0 (earliest Sandcouleean subage) to the base of zone Wa-7 (earliest Lostcabinian subage). Progressive thermal demagnetization of samples from red (B type) and mottled red/gray (AB type) soil horizons provides the most reliable results. The characteristic magnetization is carried by hematite with unblocking temperatures between 400°C and 680°C. Fine-grained hematite and magnetite are also present and carry a strong present-day overprint. Magnetostratigraphic correlation indicates that Wasatchian zone Wa-0 (earliest Sandcouleean subage) to middle zone Wa-5 (Bunophorus interval-zone) correlate with Chron C24r, middle zone Wa-5 to middle zone Wa-6 (Lysitean subage) correlate with Chron C24n.3n, late zone Wa-6 correlates with Chron C24n.2, and early zone Wa-7 (Lostcabinian subage) correlates with Chron C24n.lr. These results indicate that the last zone, Wa-7 (Lostcabinian subage), may represent as much as one-half of the Wasatchian land-mammal age, while earlier zones Wa-0 to Wa-6 (Sandcouleean-Lysitean subages) together span only 2.4 m.y. of early Eocene time.

Geology, Paleoenvironment, and Age of Birket Qarun Locality 2 (BQ-2), Fayum Depression, Egypt

Vertebrate paleontological research in the Fayum Depression began in 1879, with Georg Schweinfurth’s recovery of whale and fish fossils on the island Geziret el-Qarn in Birket Qarun (Dames, 1883; Schweinfurth, 1886). In later years Schweinfurth worked north of the lake, in part within the uppermost levels of the Birket Qarun Formation that are exposed to the south of (and stratigraphically below) the site of the Qasr el-Sagha Temple, but he is not known to have collected any vertebrate fossils from those beds. Subsequent exploration by Hugh Beadnell, and later Richard Markgraf, led to the discovery of fragmentary remains of the cetacean Basilosaurus isis from near this stratigraphic horizon on the northwest side of Birket Qarun (Andrews, 1904; Stromer, 1908; see also Gingerich, 2008), but no vertebrate fossils were reported from sediments exposed on the northeast side of the lake until late in the 20th century, when a single premolar of the proboscidean Moeritherium was described (Holroyd et al., 1996). In the year 2000, paleontological reconnaissance in the sediments exposed along the ‘‘plain of Dime’’ led to the identification of a number of new vertebrate fossil localities (Fig. 1), most of which preserve fragmentary remains of proboscideans (Barytherium and Moeritherium), sirenians, and whales. One locality situated on the northeast side of Birket Qarun, now called Birket Qarun Locality-2 or BQ-2, initially produced surface finds of creodont postcrania, a basicranium and partial mandible of Barytherium, a partial mandible and postcrania of Moeritherium, and a small placental petrosal. Subsequent quarrying anddry screening at BQ-2 from2001 to 2005 has revealed craniodental and postcranial remains of numerous small mammals, including primates, hyracoids, herodotiines, ptolemaiids, anomaluroid and hystricognathous rodents,

Basin-wide magnetostratigraphic framework for the Bighorn Basin, Wyoming

New paleomagnetic data from six different sections in the Bighorn Basin are combined with previously published results to construct a basin-wide magnetostratigraphic framework. Geomagnetic polarity reversals between chrons C26r, C26n, C25r, C25n, C24r, and C24n have been identifi ed among multiple stratigraphic sections in different parts of the basin. Using the new magnetostratigraphic framework, paleontological, paleobotanical, and isotopic information from these varied locations in the basin can now be correlated and compared to similar records from elsewhere in the world. These new data resolve previous uncertainty concerning the timing of an important episode of faunal turnover known as Biohorizon B, which is slightly below the chron C24r-C24n boundary, close to the position of the Elmo isotope excursion in marine records. Backstripping analysis using these new magnetostratigraphic data helps defi ne the time-transgressive onset of basin formation and shows the different subsidence histories of the northern and southern parts of the basin.