Article

Revised age constraints for Late Cretaceous to early Paleocene terrestrial strata from the Dawson Creek section, Big Bend National Park, west Texas

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Abstract

We analyzed samples for paleomagnetism, ⁴⁰Ar/³⁹Ar detrital sanidine ages, and mammalian fauna to produce a precise chronostratigraphic framework for the Upper Cretaceous to Lower Paleocene Dawson Creek section of Big Bend National Park, west Texas. Prior to this work, the absolute ages and durations of the Upper Cretaceous Aguja and Javelina Formations and Paleocene Black Peaks Formation were relatively poorly constrained. The documented polarity zones can be correlated to C32n-C31n, C29r, and C27r of the geomagnetic polarity time scale, with three hiatuses spanning more than 1.5 m.y. each. Rock magnetic analyses indicate that the dominant magnetic carrier in the Aguja and Black Peaks Formations is titanomagnetite, while the Javelina Formation has varying magnetic carriers, including hematite, magnetite, and maghemite. An overprint interval surrounding the Cretaceous-Paleogene boundary suggests the primary magnetic carrier, titanohematite, was likely reset by burial and/or overlying basaltic flows. These are the first independent age constraints for the Cretaceous-Paleocene strata at the Dawson Creek section that determine the age and duration of deposition of each formation in the section, as well as the age and duration of multiple unconformities through the succession. As a result, these age constraints can be used to reassess biostratigraphic and isotopic correlations between the Big Bend area and other Cretaceous-Paleogene basins across North America. Based on this new data set, we reassign the age of the mammalian fauna found in the Black Peaks Formation from the Puercan to the Torrejonian North American Land Mammal age. Our age constraints show that the dinosaur fauna in the Javelina Formation in the Dawson Creek area is latest Maastrichtian and restricted to chron C29r. Thus, the Javelina dinosaur fauna is correlative to the Hell Creek Formation dinosaur fauna from the Northern Great Plains, indicating differences between the faunas are not due to differences in age, and providing support for the hypothesis of provinciality and endemism in dinosaur communities in the late Maastrichtian. Further, the age constraints indicate that the previously documented mid-Maastrichtian and late Maastrichtian greenhouse events were rapid ( < 200 k.y.) and correlate closely with climate events documented in the marine record.

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... During the 2000s, the formation contacts within the Tornillo Group were revised, and these strata were mapped more accurately throughout Creek (Leslie et al., 2018a) Tornillo Flat (Schiebout et al., 1987) -Cretaceous vertebrate fossil site -Paleocene vertebrate fossil site TERLINGUA MONOCLINE * * * the Big Bend region (Lehman, 2002(Lehman, , 2004(Lehman, , 2007. The Javelina-Black Peaks formational contact was revised to coincide with more easily recognizable lithologic criteria, rather than fossil content. ...
... Most recently, Cobb (2016) documented an unusual conglomerate bed at the K-Pg contact on Rough Run Creek; he interpreted this bed as a tsunami deposit resulting from the Chicxulub impact, and Wiest et al. (2018) purported to identify the K-Pg boundary at the exposure on Dawson Creek on the basis of a stratigraphic change in the diameter of burrows within the fluvial sediments. Leslie et al. (2018a) obtained a paleomagnetic polarity sequence for the section on Dawson Creek and provided 40 Ar/ 39 Ar ages for detrital sanidine from sandstone beds throughout the section there. They also determined that the mammalian fossils from both the "Tom's Top" and "Dogie" localities are actually middle Torrejonian rather than late Puercan in age, and that at least on Dawson Creek, a disconformity separates middle Torrejonian strata from underlying strata thought to span the K-Pg boundary. ...
... The best exposed and most thoroughly studied outcrop of the K-Pg contact interval is along the western edge of the Park, on the south side of Dawson Creek (Fig. 3). Of the seven areas where the K-Pg contact interval can be constrained with vertebrate fossil sites, the Dawson Creek area is the only one that has been well documented geologically (Lawson, 1972;Standhardt, 1986;Lehman, 1990;Leslie et al., 2018a). The section here spanning the K-Pg contact has been a subject of a variety of studies including fluvial sedimentology (Atchley et al., 2004), paleoclimatology (Nordt et al., 2003), and paleosol geochemistry (Nordt et al., 2011). ...
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The Cretaceous-Paleogene (K-Pg) contact interval is constrained by vertebrate fossil sites at seven sites in the Tornillo Group and lies within an 80–100-m stratigraphic section between the top of the Javelina Formation and the base of the “log jam sandstone” marker bed in the Black Peaks Formation. In western exposures of this interval, the highest occurrence of in situ dinosaur specimens and the lowest occurrence of Paleocene mammal specimens are separated by an unusual conglomerate bed. This thin conglomerate bed coincides with the contact between Cretaceous and Paleogene strata and contains reworked Cretaceous fossils. It is superficially similar to conglomerate beds elsewhere attributed to the effects of tsunamis generated by the Chicxulub impact; however, the maximum depositional age of ca. 63 Ma based on detrital zircons indicates that the conglomerate was deposited about three million years after the K-Pg boundary event. Paleocene mammalian fossils from immediately above the conglomerate bed represent a fauna that can be no older than the middle Torrejonian (To2 interval zone). The contact between Cretaceous and Paleocene strata is therefore disconformal and represents a hiatus of at least three million years. A condensed section occurs at the westernmost exposure of the K-Pg contact, where at least 80 m of strata are absent below the conglomerate bed; these strata are present in exposures farther east. This condensed section likely records an erosional event resulting from uplift and deformation of the nearby Terlingua monocline. Although the 80 m of strata below the conglomerate bed are poorly fossiliferous, several clearly in situ dinosaur specimens indicate that this entire interval is Late Cretaceous in age. There is no compelling evidence for preservation of the K-Pg boundary event horizon at any of the seven sites in the Tornillo Group, and so the hiatus represented at the Cretaceous/ Paleocene contact here likely also includes some part of latest Cretaceous time. Mammalian specimens from sites in the “log jam sandstone,” ~40 m above the middle Torrejonian sites, represent an early Tiffanian fauna (Ti1 interval zone). Latest Torrejonian (To3) sites have not been recognized, and therefore a second disconformity likely coincides with the base of the “log jam sandstone” marker horizon in the Black Peaks Formation.
... Table 1 documents the 14 vertebrate fossil-bearing intervals, the generalized area, the NALMA interval zone (Lofgren et al., 2004), San Juan Basin biostratigraphic zone (Williamson, 1996), and the age of the vertebrate horizon calculated in this study using sediment accumulation rates. For vertebrate horizons 15-23, their generalized area, the associated biochronologic interval zone (Lofgren et al., 2004), San Juan Basin biozone (Williamson, 1996), and the calculated age of the vertebrate horizon are from Leslie et al. (2018a). The stratigraphic position and locality number(s) within each vertebrate horizon is included in Table DR6. ...
... However, much like in the Puercan, it is difficult to determine if the patterns of faunal change documented in the SJB during the Torrejonian are representative of local or regional phenomena because the SJB is the only basin in which faunas representing the entire Torrejonian occur in superposition (Fig. 10). Although there are collections of middle and late Torrejonian mammals from outside the SJB (e.g., Butler et al., 1987;Leslie et al., 2018a;Buckley, 2018), these faunas typically occur in isolation and are difficult to correlate precisely to the SJB record. Furthermore, in some cases, faunas have been recognized as being Torrejonian in age based on the occurrence of typical Torrejonian taxa but not the diagnostic index species for the biochrons (e.g., Hunter and Hartman, 2003). ...
... Our magnetostratigraphic results indicate relatively low sediment accumulation rates in C29r and C29n and a rough doubling of sedimentation rates in C28r, which remained consistently high through C27r and were similar to sedimentation rates reported by Leslie et al. (2018a) for C27n. We amend the SJB basin evolution model of Cather (2004), and instead hypothesize that the onset of the middle phase of relatively slow subsidence started in the Maastrichtian. ...
Article
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The lower Paleocene Ojo Alamo Sandstone and Nacimiento Formation from the San Juan Basin (SJB) in northwestern New Mexico preserve arguably the best early Paleocene mammalian record in North America and is the type location for the Puercan (Pu) and Torrejonian (To) North American land mammal ages (NALMA). However, the lack of precise depositional age constraints for the Ojo Alamo Sandstone and lower Nacimiento Formation has hindered our understanding of the timing and pacing of mammalian community change in the SJB following the Cretaceous−Paleogene mass extinction. Here we produced a high-resolution age model for the Ojo Alamo Sandstone and lower Nacimiento Formation combining magnetostratigraphy and 40Ar/39Ar geochronology spanning the first ∼3.5 m.y. of the Paleocene. Mean sediment accumulation rates during C29n were relatively low (<50 m/m.y.) and equalized from basin center to basin margin indicating an accommodation minimum; sediment accumulation rates approximately double (>90 m/m.y.) during C28r and are highest in the basin center and lowest on basin margin, which indicates high accommodation and an increase in basin subsidence near the C29n/C28r boundary (ca. 64.96 Ma). Puercan fossil localities were restricted to C29n, Torrejonian 1 localities to C28n, and lower Torrejonian 2 localities to C27r. Our revised age model for the SJB suggests that the first appearance of To1 mammals may have been diachronous across North America, with the Torrejonian 1 mammals first appearing in the north (Montana and North Dakota) during C29n, then in middle latitudes (Utah) in C28r, and lastly in southern North America (New Mexico) in C28n.
... It was found in a fluvial channel deposit of the upper Javelina Formation as part of a conglomeratic lag within a sandstone interval (Lehman et al., 2018:supplemental material). The Javelina Formation has been dated to the Maastrichtian based on biostratigraphy (see summaries in Lehman et al., 2018;Leslie et al., 2018), radiometric dating (Lehman et al., 2006;Leslie et al., 2018), and magnetostratigraphy (Leslie et al., 2018). A tuff bed in the middle of the formation in the nearby Grapevine Hills dates to 69 Ma (Lehman et al., 2006), and detrital sanidine dates from the western part of the park indicate the upper part of the formation is no older than 68 Ma (Leslie et al., 2018). ...
... It was found in a fluvial channel deposit of the upper Javelina Formation as part of a conglomeratic lag within a sandstone interval (Lehman et al., 2018:supplemental material). The Javelina Formation has been dated to the Maastrichtian based on biostratigraphy (see summaries in Lehman et al., 2018;Leslie et al., 2018), radiometric dating (Lehman et al., 2006;Leslie et al., 2018), and magnetostratigraphy (Leslie et al., 2018). A tuff bed in the middle of the formation in the nearby Grapevine Hills dates to 69 Ma (Lehman et al., 2006), and detrital sanidine dates from the western part of the park indicate the upper part of the formation is no older than 68 Ma (Leslie et al., 2018). ...
... It was found in a fluvial channel deposit of the upper Javelina Formation as part of a conglomeratic lag within a sandstone interval (Lehman et al., 2018:supplemental material). The Javelina Formation has been dated to the Maastrichtian based on biostratigraphy (see summaries in Lehman et al., 2018;Leslie et al., 2018), radiometric dating (Lehman et al., 2006;Leslie et al., 2018), and magnetostratigraphy (Leslie et al., 2018). A tuff bed in the middle of the formation in the nearby Grapevine Hills dates to 69 Ma (Lehman et al., 2006), and detrital sanidine dates from the western part of the park indicate the upper part of the formation is no older than 68 Ma (Leslie et al., 2018). ...
Article
Osteoderms (ossified dermal elements) are among the most distinctive features of Titanosauria, a spe-ciose clade of Cretaceous sauropods. Though osteoderms are often described as armor, hollow osteo-derms have also been interpreted as mineral storage sites. A new specimen (TMM 45888-1) from the Upper Cretaceous Javelina Formation, Big Bend National Park, Texas, provides insights into osteoderm structure and function. It is oval with a cusp at one end and grooved marginal ornamentation, resembling certain specimens from Argentina but without a keel. It is not divided into bulb and root regions. TMM 45888-1 is the first titanosaur osteoderm from Texas. It cannot be referred to a species but is similar to the osteoderms of Alamosaurus, the only known titanosaur from North America. X-ray CT scans reveal that the interior of TMM 45888-1 is not hollow. Large neurovascular channels connect foramina on the deep, superficial, and marginal faces. The channels are concentrated along the deep face, at the ends, and passing through the center from deep to superficial. The osteoderm was potentially capable of providing mineral storage, localized defense, and display functions based on its vasculature, material properties, and shape. Whole-body defense, vertebral stabilization, and thermo-regulation are rejected as functions based on apparent osteoderm scarcity, lack of articulations, and limited superficial vasculature. Osteoderms vary in gross and interior morphology among lithostrotians, necessitating caution when generalizing functions across the clade.
... The part of the section at Dawson Creek delimited here as Javelina Formation was also measured by Lawson (1972; 134 m, section 1, Lawson figure 4), Standhardt (1986; 130 m, section DC-W, Standhardt's figure 8), and Lehman (1990;122 m), and all recognized the same basic lithologic units. More recently, Atchley et al. (2004) and Leslie et al. (2018) measured the same section; the Javelina Formation as recognized here extends from their 42 m mark up to their 175 m mark, and so the total thickness they determined (133 m) is also comparable. A thickness of 123 m was obtained for the type section of the Javelina Formation in the present study (Fig. 6). ...
... Nordt et al. (2003) theorized on the basis of stable isotope excursions detected in the Dawson Creek section that two disconformities are present within the Javelina Formation. Atchley et al. (2004) also identified a stratigraphic sequence boundary, and based on magnetic polarity zonation, Leslie et al. (2018) interpreted an unconformity within the same section. However, it has not been possible to identify the purported isotopic excursions in other sections of the Javelina Formation (e.g., Schmidt, 2009), and the positions of unconformities hypothesized by Nordt et al. (2003), Atchley et al. (2004), and Leslie et al. (2018) do not correspond with one another or with any prominent stratigraphic or petrologic characteristics that would allow for their identification or correlation in other sections of the Javelina Formation. ...
... Atchley et al. (2004) also identified a stratigraphic sequence boundary, and based on magnetic polarity zonation, Leslie et al. (2018) interpreted an unconformity within the same section. However, it has not been possible to identify the purported isotopic excursions in other sections of the Javelina Formation (e.g., Schmidt, 2009), and the positions of unconformities hypothesized by Nordt et al. (2003), Atchley et al. (2004), and Leslie et al. (2018) do not correspond with one another or with any prominent stratigraphic or petrologic characteristics that would allow for their identification or correlation in other sections of the Javelina Formation. ...
Article
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Fluvial strata of the Tornillo Group preserve a succession of Late Cretaceous, Paleocene, and early Eocene continental faunas and floras and provide a record of the Laramide orogeny in the southern part of the North American Cordillera. Contacts between units in the Tornillo Group (Javelina, Black Peaks, and Hannold Hill formations) have proven difficult to identify, but minor adjustments to the stratigraphy allow each to be readily mapped and provide a means to assess intraformational thickness variation and syndepositional deformation within the Tornillo Basin. The Javelina Formation is thin in the southwestern part of the basin, and the Black Peaks Formation thins toward both southwestern and northeastern sides, suggesting that development of the monoclines that bound the basin began in latest Cretaceous through Paleocene time. An obscure structure extending southeastward from Grapevine anticline divides the basin into northeastern and southwestern segments. The Javelina Formation thins southwest of this structure and lacks lacustrine facies found to the northeast. The upper half of the Black Peaks Formation is absent southwest of this line, and northeast- facing monoclinal folds that affect the Hannold Hill Formation in the same vicinity are truncated at the base of the overlying Canoe Formation. Depositional limits of the Hannold Hill Formation probably did not extend to the southwest. The middle Eocene Canoe Formation is largely unaffected by contractive deformation that affects the Tornillo Group. Although incipient Laramide-age deformation broadly defined the Tornillo Basin during latest Cretaceous through Paleocene time, deformation here occurred mostly during the early Eocene.
... Co, county; km, kilometer; m, meter; mi, miles; S, section; asterisk ( * ) indicates the resolution is almost certainly an underestimate. (Fowler, 2017;Gates, Gorscak & Makovicky, 2019;Harrison et al., 2013;Horner, Goodwin & Myhrvold, 2011;Leslie et al., 2018), aside from the specimen (DDM 344.1) whose position was obtained from museum records. ...
... Using this approach, the Frenchman Formation of Saskatchewan is equivalent to the upper HCF (Fowler, 2017). The radiometric dates and stratigraphic revisions of Leslie et al. (2018) were followed to correlate the Tornillo Formation of Texas with the upper HCF. Given the imprecise dating of the Scollard and Willow Creek Formations of Alberta, and ambiguous correlation between them and the upper and middle members of the HCF, specimens from those units were excluded. ...
Article
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Background During the growth of complex multicellular organisms, chronological age, size and morphology change together in a hierarchical and coordinated pattern. Among extinct species, the growth of Tyrannosaurus rex has received repeated attention through quantitative analyses of relative maturity and chronological age. Its growth series shows an extreme transformation from shallow skulls in juveniles to deep skulls in adults along with a reduction in tooth count, and its growth curve shows that T. rex had a high growth rate in contrast to its closest relatives. However, separately, these sets of data provide an incomplete picture of the congruence between age, size, and relative maturity in this exemplar species. The goal of this work is to analyze these data sets together using cladistic analysis to produce a single hypothesis of growth that includes all of the relevant data. Methods The three axes of growth were analyzed together using cladistic analysis, based on a data set of 1,850 morphological characters and 44 specimens. The analysis was run in TNT v.1.5 under a New Technology search followed by a Traditional search. Correlation tests were run in IBM SPSS Statistics v. 24.0.0.0. Results An initial analysis that included all of the specimens recovered 50 multiple most parsimonious ontograms a series of analyses identified 13 wildcard specimens. An analysis run without the wildcard specimens recovered a single most parsimonious tree (i.e., ontogram) of 3,053 steps. The ontogram is composed of 21 growth stages, and all but the first and third are supported by unambiguously optimized synontomorphies. T. rex ontogeny can be divided into five discrete growth categories that are diagnosed by chronological age, morphology, and, in part, size (uninformative among adults). The topology shows that the transition from shallow to deep skull shape occurred between 13 and 15 years of age, and the size of the immediate relatives of T. rex was exceeded between its 15th and 18th years. Although size and maturity are congruent among juveniles and subadults, congruence is not seen among adults; for example, one of the least mature adults (RSM 2523.8) is also the largest and most massive example of the species. The extreme number of changes at the transition between juveniles and subadults shows that the ontogeny of T. rex exhibits secondary metamorphosis, analogous to the abrupt ontogenetic changes that are seen at sexual maturity among teleosts. These results provide a point of comparison for testing the congruence between maturity and chronological age, size, and mass, as well as integrating previous work on functional morphology into a rigorous ontogenetic framework. Comparison of the growth series of T. rex with those of outgroup taxa clarifies the ontogenetic trends that were inherited from the common ancestor of Archosauriformes.
... The highest Cretaceous-specific remains recovered from this section are disarticulated Quetzalcoatlus (Pterosauria, Azhdarchidae) from a fluvial body between paleosol 21 (P21) and P22 within the Javelina Formation ( Fig. 2; Lawson, 1972Lawson, , 1975Lehman, 1990). Paleocene mammals occur within the Black Peaks Formation at 20 m above the last occurrence of Cretaceous fossils in a channel complex (superjacent to P28) that is interpreted to be unconformable at its base (Atchley et al., 2004;Lehman and Busbey, 2007;Leslie et al., 2015;Cobb, 2016). Reanalysis of the Black Peaks mammalian faunas, coupled with revised magnetostratigraphy for the section, indicate that the mammals occur within the Torrejonian North American Land Mammal Age and were deposited in chron 27r (Leslie et al., in press). ...
... Dawson Creek exposure within the Tornillo Basin is an extensively studied continental K-Pg section in the southern United States. The framework of this particular outcrop includes biostratigraphy (Wilson 1970;Schiebout 1974;Standhardt 1986;Schiebout et al. 1987; Lehman 1991), magnetostratigraphy ( Rapp et al. 1983;Leslie et al. 2015, in press), terrestrial sequence stratigraphy ( Atchley et al. 2004), paleopedology (Nordt et al. 2003), and paleoenvironmental reconstruction (Lehman 1990). Previous studies of the paleosols occurring throughout the stratigraphic succession suggested that paleoclimate conditions were subhumid to semiarid, vegetation was predominantly dry woodland, and the paleosols resemble modern Entisols, Inceptisols, and Vertisols ( Atchley et al. 2004;Nordt et al. 2011). ...
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The assemblage of Pleistocene megafauna at Waco Mammoth National Monument (WMNM) and the terrestrial Cretaceous-Paleogene (K-Pg) boundary at Big Bend National Park are just two examples that demonstrate the peril effects of an extreme environmental perturbation on ancient ecosystems. Ichnology is the study of traces that are generated by organismal behavior; therefore, analyzing major die-offs through an ichnological lens can provide a unique perspective to understand the ethology of the survivors. At WMNM trace-fossil analysis was combined with taphonomy, which revealed that the Columbian mammoth herd was subjected to extensive vertebrate and invertebrate scavenging. These findings necessitated a re-evaluation of the causal mechanisms responsible for the death of the herd, as well as demonstrated that scavenging organisms out-survived, at least for some time, the large herbivores in this case. The application of ichnology to the terrestrial K-Pg boundary revealed new findings about the surviving organisms in response to the end-Cretaceous extinction. Herbivorous, soil-dwelling insects, as evidenced from analogous traces, were significantly reduced in body size (Lilliput effect) following the aftermath of the event. These findings, in conjunction with research on marine-organism responses, provide empirical evidence that the Lilliput effect was a phenomenon that affected surviving organisms across highly disparate trophic levels and ecosystems. Lastly, a hybrid approach of ichnology and zooarchaeology was used to develop morphological criteria for taphonomic analysts to differentiate between carnivore traces and unintentional, preparator air-scribe marks, which can be more similar than one might imagine. Utilization of an ichnological perspective to study survival behaviors from the ancient past may one day help address some of the decisions regarding our current mass extinction.
... The highest Cretaceous-specific remains recovered from this section are disarticulated Quetzalcoatlus (Pterosauria, Azhdarchidae) from a fluvial body between paleosol 21 (P21) and P22 within the Javelina Formation ( Fig. 2; Lawson, 1972Lawson, , 1975Lehman, 1990). Paleocene mammals occur within the Black Peaks Formation at 20 m above the last occurrence of Cretaceous fossils in a channel complex (superjacent to P28) that is interpreted to be unconformable at its base (Atchley et al., 2004;Lehman and Busbey, 2007;Leslie et al., 2015;Cobb, 2016). Reanalysis of the Black Peaks mammalian faunas, coupled with revised magnetostratigraphy for the section, indicate that the mammals occur within the Torrejonian North American Land Mammal Age and were deposited in chron 27r (Leslie et al., in press). ...
Preprint
Recent research has demonstrated that the Lilliput effect (reduction of body size in response to a mass extinction) affected all trophic levels in the marine realm following the Cretaceous-Paleogene (K-Pg) event. However, it is unclear if this size change was strictly a marine signal, or a global phenomenon that also affected continental ecosystems. Herein we present the results of an ichnological proxy for body size of soil-dwelling insects across the K-Pg boundary in Big Bend National Park, Texas, U.S.A. Quantitative efforts focused on Naktodemasis isp., which are characterized as unbranching burrows composed of ellipsoidal packets of backfill menisci. These traces were likely produced by beetle larvae or cicada nymphs based on previous comparison with structures generated in modern soils and laboratory experiments. As an approximation for the body size of the subterranean insects, this dataset indicates that a smaller Naktodemasis diameter (DN) is statistically correlated (α < 0.05) with several edaphic factors including poor drainage and weak development (Entisols). Additionally, the DN in strata immediately superjacent to the highest Cretaceous-specific taxa is smaller by 23% (5.6 ± 1.8 mm) in comparison to DN within the subjacent Cretaceous interval (7.3 ± 2.7 mm). This abrupt shift occurs in a well-drained Inceptisol, and cannot be attributed to facies changes, drainage, or paleosol maturity. Furthermore, a reduced DN (6.6 ± 2.3 mm) persists above this anomalous shift for at least 20 stratigraphic meters within chron 29r. The cause for this negative response in body size within soil-dwelling biota may be attributed to plant-community shifts in taxonomic composition and ecological strategies, which would have caused fundamental alterations to the diet of the herbivorous, subterranean insects. This study provides empirical evidence that the Lilliput effect was not restricted to marine environments during the aftermath of the K-Pg event.
... removed a large portion of the Puercan strata at Tornillo Flat. This uncon formity was coeval with the eustasydriven or tectonically driven lull in sedimentation inter preted for other parts of the basin (Lehman, 1991;Atchley et al., 2004;Leslie et al., 2018). Additional work is required in the lowermost Black Peaks Formation to identify and localize the exact stratigraphic level of the Cretaceous Paleogene boundary within levels 40 m to 84 m of the stratigraphic section. ...
Article
The early Paleogene was a dynamic period marked by long-term climatic trends and rapid climate events superimposed upon a hot greenhouse state. The response of the terrestrial hydrological cycle to these climate variations can be investigated at the continental scale by analysis of Paleogene strata in Laramide basins and the Gulf of Mexico. New U-Pb geochronologic data from detrital zircons in Paleogene fluvial strata of the Tornillo Basin in west Texas indicate relatively continuous deposition from 63 to 47 Ma. The combination of detrital zircon ages with existing carbon isotope chemostratigraphic data allows us to revise the age model of these strata and to correlate stratigraphic changes with Paleogene climate variations. Sedimentologic data indicate that, during the Paleocene (63-55.5 Ma), high-sinuosity, fluvial systems meandered on a forested floodplain with well-developed soils and swamps. The fluvial regime changed abruptly to lowsinuosity, braided fluvial systems with floodplains characterized by intermittent wetting at the Paleocene-Eocene thermal maximum (55.5 Ma). Braided fluvial regimes persisted thereafter and culminated during the early Eocene climate optimum (52-50 Ma) with deposition of a coarse sheet of sand and gravel. We interpret the transition from meandering to braided fluvial systems as a shift in discharge and sediment supply associated with the more seasonal precipitation of the hot, subtropical monsoonal climate of the early Eocene. After 54 Ma, this shift in fluvial regime is superimposed by an upwardcoarsening grain-size trend probably driven by enhanced regional Laramide tectonism. These data provide the first reconstruction of Paleogene subtropical fluvial dynamics in western North America and correlate with regional and global changes in Paleogene climate and landscapes.
... Identifiable paleosols have been measured and described from these sections (Figures 2 and S1), and some of the pedogenic horizons yield carbonate nodules. An age model for these sediments was developed using carbon isotope stratigraphy, magnetostratigraphy, and sparse vertebrate fossils (Bataille et al., 2016(Bataille et al., , 2018Rapp et al., 1983;Schiebout et al., 1987) and agrees well with a paleomagnetically constrained age model for a section with overlapping stratigraphic coverage that is exposed about 38 km to the west (Leslie et al., 2018). ...
Article
Records of subtropical climate on land from the early Paleogene offer insights into how the Earth system responds to greenhouse climate conditions. Fluvial and floodplain deposits of the Tornillo Basin (Big Bend National Park, Texas, USA) preserve a record of environmental and climatic change of the Paleocene and the early Eocene. We report carbon, oxygen, and clumped isotopic compositions (δ¹³C, δ¹⁸O, and Δ47) of paleosol carbonate nodules from this basin. Mineralogical, geochemical, and thermal modeling evidence suggests that the measured isotopic values preserve primary environmental signals with a summer bias with the exception of data from two nodules reset by local igneous intrusions. The unaltered nodules record Δ47 temperatures of 25 ± 4 and 32 ± 2 °C for the Paleocene and early Eocene nodules, respectively, showing an increase in average summer temperatures of 7 ± 3 °C. Calculations of δ¹⁸O of soil water are −2.8 ± 0.7‰ and −0.8 ± 0.4‰ (standard mean ocean water) for the early-mid-Paleocene and late Paleocene-early Eocene, showing an increase of 2.0 ± 0.9‰. The increase in temperature and δ¹⁸O values likely relates to a rise in atmospheric pCO2, although we cannot rule out that changes in paleosol texture and regional precipitation patterns also influence the record. Comparison with Δ47 estimates of summer temperature from the Green River and Bighorn Basins (WY) highlights that terrestrial surface temperatures are heterogeneous, and latitudinal temperature gradients on land remain undetermined. Previously published paleoclimate models predict summer temperatures that are 2 to 6 °C higher than our estimate; discrepancies between climate models and proxy data persist at lower latitudes.
... The presence of modern-magnitudes of MART in the Late Cretaceous also has implications for the geographic distribution and lifestyles of dinosaur and non-dinosaur faunal communities in western North America, and our understanding of vertebrate speciation patterns. There has been a long-standing debate regarding the degree of provincialism (regional partitioning) of northern and southern faunal communities along the western margin of the WIS during the Late Cretaceous (e.g., Leslie et al., 2018;Lucas et al., 2016;Morrone, 2014;Nydam et al., 2013;Gates et al., 2010;Sampson et al., 2010;Lehman et al., 2006;Lehman, 2001). Faced with a lack of known geographic barriers to animal migration, studies in favor of faunal provincialism have often cited latitudinal changes to unspecified climate parameters (e.g., temperature and/or precipitation) as the driving force behind regionally discrete animal populations (e.g., Gates et al., 2010;Sampson et al., 2010). ...
... Ma) vertebrate assemblages of the Western Interior Basin (especially dinosaurs) were endemic to distinct northern and southern communities originated in the 1970s (Sloan, 1970(Sloan, , 1976Lehman, 1987;Wolfe and Upchurch, 1987). Attempts to explain the potential presence of distinct biomes within the Western Interior Basin have been the subject of intensifying research since that time (e.g., Lehman, 2001;Sampson and Loewen, 2005;Lehman et al., 2006;Gates et al., 2010Gates et al., , 2012Sampson et al., 2010;Loewen et al., 2013;Thomson et al., 2013;Longrich, 2014;Leslie et al., 2018;Sankey, 2008;Nydam et al., 2013). These studies suggest that the evolving physiogeography of the Western Interior Basin, coupled with changing climate during the Late Cretaceous, resulted in periodic physical or ecological barriers to vertebrate movement. ...
... Leaf gas-exchange: Kowalczyk et al. (2018); Milligan et al. (2019, replaces estimates from Beerling & Royer, 2002); stomata: Beerling et al. (2002, updated by Beerling et al., 2009); Kowalczyk et al. (2018); Royer et al. (2001, updated by Beerling et al., 2009); Royer (2003, updated by Beerling et al., 2009); Steinthorsdottir et al. (2016); Wang et al. (2020); paleosols: Huang et al. (2013, updated by Foster et al., 2017); Nordt et al. (2003, updated by Foster et al., 2017); Zhang et al. (2018); liverworts: Kowalczyk et al. (2018); boron: Henehan et al. (2019). Age estimates from Nordt et al. (2003) have been updated here based on magnetostratigraphy ofLeslie et al. (2018). All ages are updated to the 2020 geomagnetic polarity timescale(Ogg, 2020). ...
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Plain Language Summary Throughout Earth history, temperature and atmospheric CO2 usually track each other. During the Late Cretaceous to early Paleocene (∼70–60 Ma), global mean surface temperature was ∼8–12.5°C higher than today. However, CO2 estimates are variable, with some estimates equal to or less than present‐day CO2 (∼415 ppm) and at odds with the estimated global temperatures. Here we use a proxy based on leaf gas‐exchange principles (i.e., photosynthesis) to estimate early Paleocene CO2 from fossil Platanites leaves (family Platanaceae) of four early Paleocene localities from the San Juan Basin, New Mexico (65.66–64.59 Ma). We first test and calibrate the proxy on two modern Platanus species, Platanus occidentalis and P. × acerifolia. We find the leaf gas‐exchange model accurately predicts present‐day CO2. Applying the leaf gas‐exchange model to the early Paleocene, we find CO2 varies between ∼660 and 1,140 ppm. Our results are consistent with other proxies, such as boron, paleosol, leaf gas‐exchange, and liverwort proxies, that all suggest moderate to elevated levels of CO2 during the Late Cretaceous and early Paleocene. These levels of atmospheric CO2 are more in line with elevated temperature during this period, consistent with most observations of CO2 and temperature throughout the Phanerozoic.
... Detrital sanidine geochronology has the potential for utilization for many Phanerozoic sedimentary deposits (Copeland and Harrison, 1990;Chetel et al., 2011). Recent applications have mostly focused on Paleocene/Late Cretaceous chronostratigraphic studies and river terrace dating in the southwest United States (Hereford et al., 2016;Karlstrom et al., 2017;Leslie et al., 2018aLeslie et al., , 2018bAslan et al., 2019;Walk et al., 2019). The power of the method lies in the robustness of sanidine to produce ultraprecise and accurate dates by single-crystal total fusion. ...
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The 40Ar/39Ar dating method is among the most versatile of geochronometers, having the potential to date a broad variety of K-bearing materials spanning from the time of Earth’s formation into the historical realm. Measurements using modern noble-gas mass spectrometers are now producing 40Ar/39Ar dates with analytical uncertainties of ∼0.1%, thereby providing precise time constraints for a wide range of geologic and extraterrestrial processes. Analyses of increasingly smaller subsamples have revealed age dispersion in many materials, including some minerals used as neutron fluence monitors. Accordingly, interpretive strategies are evolving to address observed dispersion in dates from a single sample. Moreover, inferring a geologically meaningful “age” from a measured “date” or set of dates is dependent on the geological problem being addressed and the salient assumptions associated with each set of data. We highlight requirements for collateral information that will better constrain the interpretation of 40Ar/39Ar data sets, including those associated with single-crystal fusion analyses, incremental heating experiments, and in situ analyses of microsampled domains. To ensure the utility and viability of published results, we emphasize previous recommendations for reporting 40Ar/39Ar data and the related essential metadata, with the amendment that data conform to evolving standards of being findable, accessible, interoperable, and reusable (FAIR) by both humans and computers. Our examples provide guidance for the presentation and interpretation of 40Ar/39Ar dates to maximize their interdisciplinary usage, reproducibility, and longevity.
... More recently, a multi-collector mass spectrometer fitted with four ion-counting electron multipliers deployed in the WiscAr Laboratory, in most cases, can produce 40 Ar/ 39 Ar dates with a significant improvement in precision compared to those obtained using a single-collector mass spectrometer (e.g., Jicha et al., 2016;Andersen et al., 2017). Multi-collector mass spectrometers have more stable electronics and commonly have exceptionally low hydrocarbon background levels (e.g., Schaen et al., 2020), both of which can result in 40 Ar/ 39 Ar dates for single Cretaceous sanidine crystals with analytical uncertainties of less than ± 100 ka (Jicha et al., 2016;Leslie et al., 2018). Thus, a small population of dates can have resulting weighted mean depositional ages with analytical uncertainties of ± 30-50 ka, i.e., better than 0.5 per mil. ...
Article
Since the publication of 40Ar/39Ar dates from Cretaceous bentonites in the Western Interior Basin by J.D. Obradovich in 1993 and in Japan by J.D. Obradovich and colleagues in 2002, improvements in the 40Ar/39Ar method have included a shift to astronomically calibrated ages for standard minerals and development of a new generation of multi-collector mass spectrometers. Thus, the 40Ar/39Ar chronometer can yield results that are synchronous with U-Pb zircon dates and astrochronologic age models for Cretaceous strata. Ages determined by Obradovich have ± 2σ analytical uncertainties of ± 400 ka (excluding J value or systematic contributions) that have been used to discriminate stratigraphic events at ca. 1 Ma resolution. From among several dozen sanidine samples, 32 of which were dated by Obradovich in 1993, we present new multi-collector 40Ar/39Ar ages that reduce the average analytical uncertainties by nearly an order of magnitude. These new ages (where the uncertainties also include the contribution of the neutron fluence J value) include: • Topmost Bentonite, Mowry Shale, Kaycee, Wyoming, USA, 97.52 ± 0.09 Ma • Clay Spur Bentonite, Mowry Shale, Casper, Wyoming, 98.17 ± 0.11 Ma • Arrow Creek Bentonite, Colorado Shale, Montana, USA, 99.12 ± 0.14 Ma • Upper Newcastle Sandstone, Black Hills, Wyoming, 99.49 ± 0.07 Ma • Middle Newcastle Sandstone, Black Hills, Wyoming, 99.58 ± 0.12 Ma • Shell Creek Shale, Bighorn Basin, Crow Reservation, Wyoming, 99.62 ± 0.07 Ma • Shell Creek Shale, Bighorn Basin, Greybull, Wyoming, 99.67 ± 0.13 Ma • Shell Creek Shale, Bighorn Basin, Lander, Montana, 100.07 ± 0.07 Ma • Muddy Sandstone, Wind River Basin, Wyoming, 101.23 ± 0.09 Ma • Thermopolis Shale, Bighorn Basin, Wyoming, 101.36 ± 0.11 Ma • Vaughn Member, Blackleaf Formation, Sweetgrass Arch, Montana, 102.68 ± 0.07 Ma • Taft Hill Member, Blackleaf Formation, Sweetgrass Arch, Montana, 103.08 ± 0.11 Ma • Base of the Skull Creek Shale, Black Hills, Wyoming, 104.87 ± 0.10 Ma • Thermopolis Shale, Bighorn Basin, Wyoming, 106.37 ± 0.11 Ma A new U-Pb zircon age of 104.69 ± 0.07 Ma from the Skull Creek Shale at Dinosaur Ridge, Colorado, USA, is close to the new 40Ar/39Ar age of the Skull Creek Shale in the Black Hills, Wyoming, but 5 m.y. is missing in the unconformity between the Skull Creek Shale of the Black Hills and the overlying Newcastle Sandstone. Considering the average total uncertainties that include decay constant and standard age or tracer composition for the 40Ar/39Ar (± 0.19 Ma) and the U-Pb (± 0.13 Ma) ages does not alter this finding. Moreover, the lower Thermopolis Shale in the Bighorn Basin is 1.5 Ma older than the Skull Creek Shale in the Black Hills. The 100.07 ± 0.07 Ma Shell Creek Bentonite in Montana is close to the Albian−Cenomanian boundary age of 100.2 ± 0.2 Ma of Obradovich and colleagues from Hokkaido, Japan, and 100.5 ± 0.5 Ma adopted in the 2012 geological time scale of J.G. Ogg and L.A. Hinnov. Our findings indicate that correlations based on similarity of lithology, without independent radioisotopic ages or detailed biostratigraphic constraints, can be problematic or invalid. There is much more time missing in unconformities than has been previously recognized in these important, petroleum-bearing reservoir strata.
... Ma) vertebrate assemblages of the Western Interior Basin (especially dinosaurs) were endemic to distinct northern and southern communities originated in the 1970s (Sloan, 1970(Sloan, , 1976Lehman, 1987;Wolfe and Upchurch, 1987). Attempts to explain the potential presence of distinct biomes within the Western Interior Basin have been the subject of intensifying research since that time (e.g., Lehman, 2001;Sampson and Loewen, 2005;Lehman et al., 2006;Gates et al., 2010Gates et al., , 2012Sampson et al., 2010;Loewen et al., 2013;Thomson et al., 2013;Longrich, 2014;Leslie et al., 2018;Sankey, 2008;Nydam et al., 2013). These studies suggest that the evolving physiogeography of the Western Interior Basin, coupled with changing climate during the Late Cretaceous, resulted in periodic physical or ecological barriers to vertebrate movement. ...
Article
The Upper Cretaceous Western Interior Basin of North America provides a unique laboratory for constraining the effects of spatial climate patterns on the macroevolution and spatiotemporal distribution of biological communities across geologic timescales. Previous studies suggested that Western Interior Basin terrestrial ecosystems were divided into distinct southern and northern communities, and that this provincialism was maintained by a putative climate barrier at ∼50°N paleolatitude; however, this climate barrier hypothesis has yet to be tested. We present mean annual temperature (MAT) spatial interpolations for the Western Interior Basin that confirm the presence of a distinct terrestrial climate barrier in the form of a MAT transition zone between 48°N and 58°N paleolatitude during the final 15 m.y. of the Cretaceous. This transition zone was characterized by steep latitudinal temperature gradients and divided the Western Interior Basin into warm southern and cool northern biomes. Similarity analyses of new compilations of fossil pollen and leaf records from the Western Interior Basin suggest that the biogeographical distribution of primary producers in the Western Interior Basin was heavily influenced by the presence of this temperature transition zone, which in turn may have impacted the distribution of the entire trophic system across western North America.
... We do not consider this reversed interval to be the overlying magnetochron C28r. This is based on its occurrence in what is paleontologically characterized as earliest Pu3, which has been shown to occur in C29n (e.g., Leslie et al., 2018b;Sprain et al., 2018;Lyson et al., 2019;Flynn et al., 2020). The earliest Pu3 designation of this part of the section, in turn, is based on the presence of the large multituberculate Taeniolabis taoensis, the species that defines the onset of Pu3 (Lofgren et al., 2004), at University of Wyoming (UW) locality V-91022. ...
Article
Recent studies of early Paleocene stratigraphic sections across the U.S. Western Interior are refining our understanding of the biotic recovery in the aftermath of the Cretaceous–Paleogene (K–Pg) mass extinction event. Herein we present magnetostratigraphic data from an approximately 600-m-thick section of strata of the Ferris Formation in the Hanna Basin, south-central Wyoming, that spans the K–Pg boundary and includes in conformable superposition the three subdivisions of the earliest Paleocene Puercan North American Land Mammal Age: interval zones Pu1, Pu2, and Pu3. Prior studies on early Paleocene stratigraphic sections in Montana to the north of Hanna Basin and in Colorado and New Mexico to the south have been only marginally successful in defining the temporal boundaries of these divisions. This earlier work typically restricted Pu1 entirely within reversed magnetochron C29r, Pu2 entirely within normal magnetochron C29n, and at least the early part of Pu3 in C29n, as well. Results of the present study confirm what has been only tentatively suggested previously: that interval zone Pu2 begins in the youngest part of C29r, with later Pu2 fauna extending into C29n. Although Pu3 is known to begin in C29n, its younger limit remains unknown in our Hanna Basin section, because the top of the local Puercan is well above the stratigraphic level of our sampling for this project. We estimate a date for the Pu1–Pu2 boundary in the Hanna Basin section of approximately 65.82 Ma—nearly 70 k.y. earlier than prior estimates. This boundary marks the transition from the Pu1 survival fauna to later phases of the mammalian recovery characterized by a significant increase in diversity, appearance of considerably larger-bodied mammals, and greater dental and dietary specializations.
... The highest Cretaceous-speci c fi fi remains recovered from this section are disarticulated Quetzalcoatlus (Pterosauria, Azhdarchidae) from a uvial body between paleosol 21 fl (P21) and P22 within the Javelina Formation ( ;Fig. 2 Lawson, 1972, 1975Lehman, 1990). Paleocene mammals occur within the Black Peaks Formation at 20 m above the last occurrence of Cretaceous fossils in a channel complex (superjacent to P28) that is interpreted to be unconformable at its base (Atchley et al., 2004;Lehman and Busbey, 2007;Leslie et al., 2015;Cobb, 2016). Reanalysis of the Black Peaks mammalian faunas, coupled with revised magnetostratigraphy for the section, indicate that the mammals occur within the Torrejonian North American Land Mammal Age and were deposited in chron 27r (Leslie et al., in press). ...
Conference Paper
We present the results of an ichnological proxy for ecosystem stress applied to a continental Cretaceous-Paleogene (K-Pg) mass-extinction interval at Big Bend National Park, which reveals an ecological perturbation that is otherwise not apparent in the lithology or traditional paleontology. The trace-fossil record of soil-dwelling organisms within alluvial deposits and associated paleosols from the Dawson Creek section contains Planolites, Celliforma, Skolithos and Naktodemasis (adhesive meniscate burrows; AMB), however our quantitative efforts focused on AMB (n=801) due to the pervasiveness of this trace throughout the study interval. Previous studies demonstrate AMB were likely generated by burrower bugs or cicada nymphs. As a proxy for the body size of the tracemaker, our dataset indicates that a smaller burrow diameter is statistically (α < 0.05) correlated with poor drainage (mean = 6.4±0.2 mm) in comparison to moderately (7.1±0.2 mm) or well-drained conditions (7.1±0.1 mm) within paleosols (n=15). Soil order also has a correlation to body size within the soil-dwelling community - Entisols contain smaller-diameter burrows (6.5±0.1 mm) than Inceptisols (7.2±0.1 mm) or Vertisols (7.1±0.2 mm). Whereas the precise stratigraphic placement of the K-Pg boundary is unknown at Big Bend due to the lack of diagnostic geochemical indicators and sparsity of age-specific body fossils, a previous approximation of the position based on a combination of absolute ages and rates of pedogenesis has tentatively placed the boundary at 5 m above a channel sandstone containing the highest dinosaur remains and 26 m below a Pg assemblage. Our dataset reveals an abrupt decrease in AMB diameter (5.6±0.3 mm) within a paleosol (referred to as P22) immediately superjacent to the highest dinosaur remains. P22 is a well-drained Vertisol, therefore the anomalous shift cannot be attributed to drainage or paleosol order. Furthermore, AMB diameters remain smaller on average above P22 (6.6±0.1 mm) in comparison to the burrows constructed during the interval known to be Cretaceous in age (7.3±0.1 mm). We interpret the trace fossils in P22 to signify the post-extinction recovery community and propose that the stratigraphic position of the K-Pg boundary in Big Bend is represented by the dinosaur-bearing channel sandstone subjacent to P22.
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Currently, there is only one paleo-CO 2 record from plant macrofossils that has sufficient stratigraphic resolution to potentially capture a transient spike related to rapid carbon release at the Cretaceous-Paleogene (K-Pg) boundary. Unfortunately, the associated measurements of stomatal index are off-calibration, leading to a qualitative interpretation of >2,300-ppm CO 2 . Here we reevaluate this record with a paleo-CO 2 proxy based on leaf gas exchange principles. We also test the proxy with three living species grown at 500- and 1,000-ppm CO 2 , including the nearest living relative of the K-Pg fern, and find a mean error rate of ~22%, which is comparable to other leading paleo-CO 2 proxies. Our fossils record a ~250-ppm increase in CO 2 across the K-Pg boundary from ~625 to ~875 ppm. A small CO 2 spike associated with the end-Cretaceous mass extinction is consistent with many temperature records and with carbon cycle modeling of Deccan volcanism and the meteorite impact.
Article
Part of a paleo-forest comprised of eight in situ permineralized tree stumps is preserved in the Javelina Formation (middle Maastrichtian, c. 69 Ma) near Alamo Creek in western Big Bend National Park. The stumps are rooted in an immature organic-rich paleosol resembling a modernday Aquept, developed in a landscape depression subject to gleization with a shallow fluctuating groundwater table. Seven of the stumps pertain to the dicotyledonous angiosperm Metcalfeoxylon, and one is a cuppressoid/podocarpoid conifer, indicating that is part of a mixed conifer-dicot forest. The site exhibits a woodland architecture, with stands of closely spaced trees separated on average by 10 m, and open gaps over 100 m in between. Trunk diameters vary from 33 to 97 cm, with estimated crown heights from 23 to 48 m, and represent trees of varied ages in a mature forest. Irregular growth rings are developed in the conifer wood, but absent in the Metcalfeoxylon woods. The local climate may have had one or more annual dry seasons, and the conifers were either more susceptible to moisture stress, or genetically predisposed to express growth rings more markedly than the contemporaneous dicots. Metcalfeoxylon is also the dominant tree type at another paleo-forest preserved near McKinney Spring in the underlying Aguja Formation (late Campanian, c. 75 Ma). Metcalfeoxylon may have grown preferentially in low-lying riparian environments subject to flooding and burial by alluvial overbank deposits, thus favorable to preservation of the stumps in situ. A deficiency of precipitation, along perhaps with browsing pressure exerted by dinosaur herbivores, may have been responsible for maintaining the open woodland physiognomy evident at both sites.
Article
The Maastrichtian Javelina Formation of southwestern Texas comprises a thick sequence of stream channel and floodplain deposits accumulated in a broad southeast-trending valley, several hundred kilometers inland from the Late Cretaceous shoreline. Three pterosaur species are found here. Remains of Quetzalcoatlus lawsoni, sp. nov., are concentrated in deposits of shallow alkaline lakes that developed in abandoned reaches of stream channels. Areas surrounding the lakes were vegetated with fan palms, and the higher floodplain supported a subtropical forest dominated by the dicot tree Javelinoxylon and araucariacean conifers. The shallow lakes were inhabited by a diverse invertebrate fauna of arthropods, gastropods, and bivalves, a likely food source for the slender-beaked Quetzalcoatlus lawsoni, sp. nov., which may have had a lifestyle similar to modern large gregarious wading birds. In contrast, remains of the giant Q. northropi are rare and found instead only in stream channel facies. It may have had a more solitary lifestyle and preferred riparian habitats. The warm, dry, subtropical but nonseasonal conditions of the region may represent a preferred climatic regime for azhdarchid pterosaurs generally.
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F ossil vertebrates have been described from the Paleocene of Big Bend National Park, Brewster County, Texas, by Wilson et al. (1952), Wilson (1967), Schiebout (1973, 1974), and Standhardt (1986). These studies focused on the mammalian component of the fauna, although crocodylian and chelonian remains are known. Schiebout (1973) referred a partial dentary to Diplocynodon. Teeth in the Black Peaks Formation were referred to Allognathosuchus by Straight (1996); Standardt (1986) discussed isolated crocodyliform teeth from across the Cretaceous-Tertiary Boundary in this area, but did not assign any of them to named taxa. Brochu (1996) briefly reviewed more complete remains, which were found to include an alligatorid and a member of Borealosuchus. The giant crocodylian Deinosuchus and a goniopholidid are known from underlying Cretaceous sediments (Rowe et al., 1992), and overlying Eocene units are known to preserve remains of the ziphodont taxon Pristichampsus (Busbey, 1986).
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The expression of global climate trends (106 to 107 years) and events (105 years) in terrestrial sedimentary sections can only be assessed from long, continuous continental records. Such records are rare due to the paucity of well-dated terrestrial deposits covering millions of years. This study uses isotope chemostratigraphy to develop an age model for fluvial sedimentary rocks of the Tornillo Group (Tornillo Basin, TX) and to decipher the potential expression of global climate variations in this sub-tropical paleo-fluvial system. The carbon isotope ratio (δ13C) of Tornillo Group pedogenic carbonate correlates well with δ13C variations from benthic foraminifera and suggests deposition between ~ 69 Ma to ~ 52 Ma for the studied stratigraphic interval. Higher sediment accumulation rates occurred in the greenhouse period of the early Eocene than during the cooler middle Paleocene. Carbon isotope excursions associated with Eocene hyperthermals were not identified, but their predicted stratigraphic positions coincide with thick sand bodies. We interpret the long-term sediment accumulation rates and rapid shifts in facies distribution as reflecting change in sediment supply in the basin, possibly driven by fluctuations in precipitation seasonality and intensity driven associated with global temperature changes. Carbon and oxygen isotope ratios from pedogenic carbonates display a strong correlation throughout the section, interpreted as an expression of coupling between the hydrological cycle and soil processes. Those observations suggest that global climate variations, along with tectonic and eustatic sea-level changes, may be an important control of stratigraphic variations of the Tornillo Group at multiple timescales.
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North-south provinciality among Campanian and/or Maastrichtian vertebrates, especially dinosaurs, in the Western Interior basin of North America (specifically, between West Texas and southern Alberta, Canada) has been accepted by many vertebrate paleontologists for about 30 years. However, a critical review indicates that the case for provinciality based on non-dinosaurian vertebrates is weak to nonexistent, and that the case based on dinosaurs is problematic, resting solely on a few taxa of dinosaurs, most notably the chasmosaurine ceratopsids, which have also been used to identify extreme dinosaur endemism. Paleobiogeographic provinces can be rejected because of: (1) problems and biases in sampling; (2) the lack of topographic barriers in the Western Interior basin that would divide provinces; (3) the lack of significant climatic or vegetational differences and/or gradients to provincialize vertebrates; (4) how taxonomic (largely cladotaxonomic) decisions have been intimately involved in the perception of endemism and provinciality; (5) how the demonstrable diachroneity of most fossil assemblages undermines the ability to include them in biogeographic analyses; and (6) how the non-uniformitarian conclusions of those who argue for dinosaur provinciality and endemism undermine their own arguments. Not only do we demonstrate the biological and geological implausibility of dinosaur-based biogeographic provinces and high degrees of endemism in the Western Interior basin during the Late Cretaceous, but the arguments and analyses that have been marshalled to support such concepts are questionable. Consequently, there is no compelling evidence that there was any discrete biogeographic separation of the Campanian (or Maastrichtian) dinosaur-dominated vertebrate assemblages from north to south between Texas and Alberta in the Western Interior basin. Also, there is no compelling evidence of high degrees of dinosaur endemism in the Western Interior basin during the Campanian.
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While centrosaurines and ceratopsids in general are abundant in the Late Campanian of northern Laramidia, they are much less commonly found in southern Laramidia. This has supported hypotheses of dinosaur provinciality and endemism in the Late Cretaceous with the delineation of at least two separate faunal zones, north and south Laramidia. There have been 12 genera of centrosaurines recognized from northern Laramidia while two genera, Diabloceratops and Nasutoceratops, have been named from southern Laramidia. We present an osteological description and taphonomic outline for a new centrosaurine ceratopsid from the Aguja Formation of northern Coahuila, Mexico that is not currently diagnosable to the generic level, but likely represents a new taxon. Further, we have included three-dimensional surface scans of all material attributed to this animal. Considering the large number of centrosaurines from northern Laramidia, it is likely that cladistic analyses are biased towards this faunal zone. New findings of southern centrosaurines are needed to correct this bias. This discovery expands the range of centrosaurines south to Coahuila, Mexico and adds new information to better characterize the morphology and taxonomy of centrosaurines from southern Laramidia and their evolution in comparison to their northern counterparts.
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Caenagnathid theropods are a relatively common part of the theropod fauna in the Late Cretaceous of Asia and North America, but have not previously been described from the southernmost United States. Here, we describe caenagnathid fossils from the late Campanian Aguja Formation of West Texas, and revise the systematics of caenagnathids from the Campanian of North America. Caenagnathids from the late Campanian of Canada represent three species in three genera: Caenagnathus collinsi, Chirostenotes pergracilis and Leptorhynchos elegans gen. nov. Leptorhynchos is diagnosed by its small size, its short, deep mandible, and the upturned tip of the beak. A single caenagnathid is known from the late Campanian of Utah, Hagryphus giganteus. Two caenagnathid species occur in the Aguja Formation, ?Chirostenotes sp. and Leptorhynchos gaddisi sp. nov. L. gaddisi differs from L. elegans in that the tip of the beak is narrower and less upturned. Phylogenetic analysis recovers Caenagnathidae and Oviraptoridae as monophyletic sister taxa. Within Caenagnathidae, the North American species seem to form a monophyletic assemblage, the Caenagnathinae, within which Chirostenotes and Caenagnathus form a clade to the exclusion of Leptorhynchos. The discovery of Chirostenotes gaddisi provides more evidence for the existence of a distinct dinosaurian fauna in southern North America during the Campanian. Furthermore, the Aguja fossils show that caenagnathids were widespread and highly diverse in the Late Cretaceous of North America. This diversity was maintained in two ways. First, variation in body size and beak shape suggests that diversity within formations is maintained by niche partitioning, in a way analogous to Darwin's finches. Second, diversity is maintained by high degree of endemism, with different species of caenagnathids occurring in different habitats. © 2013 Peabody Museum of Natural History, Yale University. All rights reserved.
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Two partial pachycephalosaurid skulls, from the upper Fruitland and lower Kirtland formations (Upper Cretaceous), are recognized as belonging to a new species of Stegoceras Lambe, S. novomexicanum, n. sp. Stegoceras novomexicanum differs from the only other recognized species of Stegoceras (sensu Sullivan, 2003) in possessing: a reduced and sub-rectangular posteromedial extension of the parietal; parallel squamosal sutural surface contacts of the posteromedial extension of the parietal; enlarged and medially positioned supratemporal fenestrae; and a small (adult) size. Fusion of the frontal and parietal in one specimen, coupled with a smooth dorsal surface of the frontoparietal dome, is consistent with an adult ontogenetic stage. Gross histology reveals four histomorphs, the fourth (outer-most layer) indicates arrested growth, further attesting to its mature state. Stegoceras novomexicanum is known from, and restricted to, the upper Fruitland Formation (Fossil Forest Member) and lower Kirtland Formation (Hunter Wash Member); the collective vertebrates from these contiguous strata make up the Hunter Wash local fauna. Contrary to previous reports, the Prenocephale (= Sphaerotholus)-like pachycephalosaurids are not known from the early Kirtlandian, but are restricted to the Willow Wash local fauna of the upper Kirtland Formation (De-na-zin Member). Stegoceras novomexicanum is temporally younger (Kirtlandian) than the well-known S. validum from the Judithian of Alberta, Canada. A reassessment of the newly named taxon Texacephale langstoni demonstrates that it is not based on diagnostic material and, therefore, is a nomen dubium.
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New specimens of a titanosaur sauropod from Maastrichtian strata in Big Bend National Park, Texas, include well-preserved dorsal vertebrae in association with pelvic elements. Anterior dorsal vertebrae are characterized by postspinal and centroprezygapophyseal laminae, with spinodiapophyseal laminae convergent on the anterior base of the neural spine. Posterior dorsal vertebrae are characterized by postzygodiapophyseal laminae and divided spinodiapophyseal laminae, with spinoprezygapophyseal laminae contributing to the prespinal lamina. The ilium and pubis differ from specimens previously collected, indicating a greater morphological disparity among titanosaurs in the Maastrichtian of West Texas than previously recognized. This disparity may be attributable to intraspecific variation or to the presence of multiple taxa. The new material is compatible with the only presently known North American titanosaur, Alamosaurus sanjuanensis, but is not formally referred due to lack of overlap with the hypodigm of that species. Comparison with other derived titanosaurs finds the closest affinities to Trigonosaurus pricei, Uberabatitan riberoi, and Baurutitan britoi of the Upper Cretaceous Brazilian Bauru Group, consistent with a South American immigration event at the end of the North American sauropod hiatus.
Article
Latest Cretaceous (Maastrichtian) terrestrial vertebrates from western North America occur in two faunal provinces. The Triceratops fauna is found in Canada, Montana, and Wyoming; and the Alamosaurus fauna occurs in Utah, New Mexico, and Texas. Although the two faunas are thought to have been contemporaneous, only the Northern Province contains intercalated volcanic units that have been isotopically dated. The first isotopic age from within the southern province is presented. A single outcrop of distal tuff within the Upper Cretaceous Javelina Formation in northern Big Bend National Park, Texas, contains monazite with a U-Pb age of 69.0 ± 0.9 Ma (2 sigma). The age is from a ²⁰⁷Pb/²⁰⁴Pb vs. ²³⁵U/²⁰⁴Pb isochron, an approach chosen to avoid the effects of ²³⁰Th-derived excess ²⁰⁶Pb. The age falls within the boundary interval between the poorly calibrated Edmontonian and Lancian North American Land Mammal Ages. The tuff bed occurs approximately in the middle of the fluvio-lacustrine Javelina Formation, about 90 m stratigraphically below the position of the Cretaceous-Tertiary boundary. This position is within the local range of the sauropod Alamosaurus, below two sites that have yielded remains of the pterosaur Quetzalcoatlus, and above a site with petrified logs of the dicotyledonous tree Javelinoxylon. The range zones of all three taxa span the full thickness of the Javelina Formation elsewhere in the Big Bend region. The Alamosaurus fauna is therefore Lancian to late Edmontonian in age.
Article
This report is the result of a working group on continental sequence stratigraphy that was set up at the 1991 NUNA conference in Banff, Canada. To date, sequence stratigraphic concepts have been applied mainly to the marine realm, but unconformity-bounded units have long been recognized in nonmarine strata. Successful application of sequence stratigraphic concepts to continental strata requires careful consideration of controls on base level and sediment supply. As with shallow marine environments, relative sea level can be considered as the stratigraphic as well as the geomorphic base level for coastal nonmarine settings. Farther inland, stratigraphic base level, which determines accommodation space, is more complex and takes various forms, such as the graded profile for fluvial strata, groundwater tables for some eolian strata, and lake level for some intermontane sediments. Sediment supply is also generally a more complex variable for nonmarine environments than in the marine realm because of the proximity to the source area. The influence of climate and tectonism on sediment supply can clearly be seen in many continental sediments. The fact that the major controls of climate, tectonism, and eustasy are somewhat interdependent, and that a change in one parameter will most likely be reflected in others, is also more readily apparent in continental strata. Although in its infancy, sequence stratigraphic concepts have been applied to a wide variety of continental settings in attempts to explain variations in facies architecture. Of particular interest are studies that have linked fluvial architecture on coastal plains to variations in relative sea level as indicated by coeval marine strata, and studies of lacustrine environments that show marked variation in facies between highstand and lowstand deposits. The application of sequence stratigraphy to continental strata will likely result in the development of better correlation techniques and models that better predict the location and nature of fluvial and eolian reservoirs.
Article
A juvenile specimen of the titanosaurid sauropod Alamosaurus sanjuanensis, recovered from just below the Cretaceous/Tertiary boundary horizon in Big Bend National Park, Texas, is from an individual less than half the size of adult specimens referred to this species. The disarticulated skeleton was preserved in deposits of a shallow flood-plain pond and includes elements not previously described, allowing for an improved diagnosis for this species. The elongate opisthocoelous cervical vertebrae have non-bifid posteriorly deflected neural spines with deep postspinal fossae. The dorsal vertebrae have wide spatulate neural spines with strong prespinal laminae, and lack hyposphene-hypantrum articulations. Alamosaurus sanjuanensis exhibits a unique morphology of the ischium, evident even in this juvenile specimen. Comparison with other titanosaurid species suggests that A. sanjuanensis is most closely related to an unnamed titanosaur from Peiropolis, Brazil and Neuquensaurus australis from Argentina.
Article
A bone bed in the middle part of the Javelina Formation (Maastrichtian) in Texas yielded parts of about 37 identifiable ceratopsid dinosaur bones, mostly appendicular and limb girdle elements belonging to one juvenile and two adult individuals of Torosaurus cf. utahensis. The bone bed is a lag assemblage comprising large immobile parts of the skeletons accumulated in an abandoned stream channel. In general form and proportions the postcranial bones are similar to those in Pentaceratops sternbergi and are not as robust as those in Torosaurus latus or Triceratops horridus. A few cranial elements are preserved, including parts of a parietal, squamosal, maxilla, and two dentaries. The form of the parietal fragment is comparable to that of a more nearly complete specimen of Torosaurus cf. utahensis collected nearby at about the same stratigraphic level. The bone bed material provides a basis for the first skeletal reconstruction of this enigmatic horned dinosaur. Most characters used in diagnoses of T. utahensis and T. latus are inadequate. Only the raised bar along the squamosal/parietal suture, present in T. latus; and the midline epiparietal, absent in T. latus , may discriminate the two species.
Article
One of the southernmost North American late Campanian microvertebrate assemblages was collected from the upper Aguja Formation, Big Bend National Park, Texas. The dinosaurs provide additional evidence that distinct southern and northern terrestrial vertebrate provinces occurred contemporaneously during this time due to latitudinal differences in temperature and rainfall. Southern areas, such as west Texas, were warm dry, with non-seasonal climates, and with open-canopy woodlands; they appear to be less fossil-rich and less diverse than northern areas. Nine dinosaurs are present, based on isolated teeth: pachycephalosaurid; hadrosaurid; ceratopsian; tyrannosaurid; Saurornitholestes cf. langstoni (Sues, 1978); Richardoestesia cf. gilmorei (Currie et al., 1990); a new species of Richardoestesia , which is named here; and a undetermined theropod unlike any previously described. Previous reports of Troodon sp. from the Talley Mt. and Terlingua microsites are mistaken; they are a pachycephalosaurid. Many of the dinosaur teeth are small, and are probably from juveniles or younger individuals, evidence that dinosaurs nested in the area. Paleoecologically, the upper Aguja was probably more similar to the lower and more inland faunas of the Scollard Formation (~66 Ma) of Alberta than to contemporaneous northern faunas: both had drier, open environments and lower dinosaur abundance. This connection between climate and dinosaur abundance suggests that climatic factors were important in the Late Cretaceous dinosaur extinctions.
Article
The sauropod dinosaur Alamosaurus sanjuanensis Gilmore, 1922 has been known from Maastrichtian deposits of south-western North America for nearly a century. Alamosaurus is the youngest sauropod taxon known in North America. Originally described from an isolated scapula and ischium from New Mexico, more of Alamosaurus was revealed by an incomplete skeleton from southern Utah. Additional referred specimens from western Texas provided the first few examples of cervical, dorsal and sacral vertebrae known for the taxon, but these came from relatively small and immature individuals. Here we describe an articulated series of cervical vertebrae of a large, mature titanosaur from Big Bend National Park, Texas, and provide evidence that the specimen can be referred to A. sanjuanensis. The specimen represents the first articulated cervical vertebral series described for the taxon, which clarifies aspects of cervical vertebral anatomy and provides at least one new diagnostic character for Alamosaurus. Many previous cladistic analyses found Alamosaurus to be a saltasaurid titanosaur, sometimes closely related to the Asian taxon Opisthocoelicaudia skarzynskii. We present cladistic analyses incorporating new data from this and other specimens from Big Bend National Park. The first places Alamosaurus as a lithostrotian titanosaur outside Saltasauridae. The second analysis, with greater focus on South American titanosaurs, finds Alamosaurus allied to Lognkosauria, a clade of South American titanosaurs notable for giant size and exceptionally robust necks. This relationship may be more congruent with the fossil record than hypotheses of phylogeny that would require saltasaurid titanosaurs to inhabit northern Laramidia, Beringia and Central Asia through the late Campanian and Maastrichtian while leaving no record of their presence.
Article
Intermediate composition titanohematite, Fe2-yTiyO3 with 0.5 ≤ y ≤ 0.7, is seldom the focus of paleomagnetic study and is commonly believed to be rare in nature. While largely overlooked in magnetostratigraphic studies, intermediate titanohematite has been identified as the dominant ferrimagnetic mineral in an array of Late Mesozoic and Early Cenozoic Laramide clastic deposits throughout the central United States. Intermediate titanohematite is ferrimagnetic and has similar magnetic properties to titanomagnetite, except its unique self-reversing property. Due to these similarities, and with detrital remanent magnetizations masking its self-reversing nature, intermediate titanohematite is often misidentified in sedimentary deposits. Past studies relied upon non-magnetic techniques including X-Ray diffraction and electron microprobe analysis. While these techniques can identify the presence of intermediate titanohematite, they fail to test whether the mineral is the primary recorder. To facilitate the identification of intermediate titanohematite in sedimentary deposits, we characterize this mineral using low-temperature magnetometry and high-temperature susceptibility experiments, and present a new identification technique based on titanohematite’s self-reversing property, for sediments that span the Cretaceous-Paleogene boundary (Hell Creek region, Montana). Results from the self-reversal test indicate that the majority of remanence is held by minerals that become magnetized parallel to an applied field, but that intermediate, self-reversing titanohematite (y = 0.53–0.63) is an important ancillary carrier of remanence. While earlier literature suggests that intermediate titanohematite is rare in nature, reanalysis using specialized rock magnetic techniques may reveal that it’s more abundant in the rock record, particularly within depositional basins adjacent to calc-alkaline volcanics, than previously thought.
Article
An exceptionally large skeleton of Pentaceratops sternbergi (Ornithischia:Ceratopsia) provides for the first time an accurate impression of the body proportions in this horned dinosaur. This is the only known specimen with a skull associated with most of the postcranial skeleton. The restored skull is nearly three meters in length and is thus the largest of any ceratopsian and the largest skull thus far known in any terrestrial vertebrate. The elongate rostrum, elaborate premaxillary structure, and complex frontal and cornual sinuses demonstrate a high level of intraspecific variability in these features. Such features are therefore likely not useful in providing species-level discrimination in other chasmosaurines. The function of the intricate chasmosaurine premaxillary narial structures is probably associated with an advanced level of herbivory.
Chapter
Periodically it is worthwhile to assess our knowledge and understanding of mammalian phylogeny and one of its expressions, classification. This short paper is yet another attempt to do so, taking into account the results of recently published paleontological research and drawing heavily on work in progress by many researchers in many fields and in various parts of the world. Concepts of mammalian phylogeny and classification have changed markedly during the last few years. A good many of the ideas expressed here are frankly speculative, but they are presented anyway in order to determine how well they will stand scrutiny, especially by nonpaleontologists. A few years ago I prepared a paper with a similar aim (McKenna, 1969), but that paper is now outdated. In the present offering I attempt to update certain aspects of my previous review by taking into account research published since 1969, as well as work being incorporated into a new classification of the Mammalia now being prepared which wall deal with all taxonomic levels down to the subgeneric level in essentially the same style as Simpson’s (1945) classification.
Chapter
The patterns of marine magnetic anomalies for the Late Cretaceous through Neogene (C-sequence) and Late Jurassic through Early Cretaceous (M-sequence) have been calibrated by magnetostratigraphic studies to biostratigraphy, cyclostratigraphy, and a few radiometrically dated levels. The geomagnetic polarity time scale for the past 160 myr has been constructed by fitting these constraints and a selected model for spreading rates. The status of the geomagnetic polarity time scale for each geological period is summarized in Chapters 11–22 as appropriate. PRINCIPLES Magnetic field reversals and magnetostratigraphy The principal goal of magnetostratigraphy is to document and calibrate the global geomagnetic polarity sequence in stratified rocks and to apply this geomagnetic polarity time scale for high-resolution correlation of marine magnetic anomalies and of polarity zones in other sections. The basis of magnetostratigraphy is the retention by rocks of a magnetic imprint acquired in the geomagnetic field that existed when the sedimentary rock was deposited or the igneous rock underwent cooling. The imprint most useful for paleomagnetic directions and magnetostratigraphy is recorded by particles of iron oxide minerals. Most of the material in this chapter is updated from summaries in Harland et al. (1990) and Ogg (1995). Excellent reviews are given in Opdyke and Channell (1996) for magnetostratigraphy and McElhinny and McFadden (2000) for general paleomagnetism.
Article
A theropod tooth assemblage from the lower shale member of the Aguja Formation in West Texas is part of a diverse microvertebrate fauna, designated the Lowerverse local fauna, of early Campanian age (c. 80-82 Ma). The fauna includes as many as nine distinct theropod taxa along with several indeterminate archosaurs and birds. Theropod tooth types (indeterminate tyrannosaurids, cf. Saurornitholestes, cf. Richardoestesia, cf. Paronychodon) are similar to those found in the upper shale member of the Aguja, as well as in other Campanian theropod assemblages from western North America. However, the most abundant tooth morphotype is unique, and attributed to a new varanoid lizard with remarkably theropod-like dentition, herein designated Dryadissector shilleri (gen. et sp. nov.). The presence of many unique theropod tooth morphotypes in the Lowerverse local fauna suggests that there remains significant undiscovered diversity among small theropods in southern latitude faunas, and accords with recognition of distinct latitudinal biomes during Campanian time in western North America. Due to their similar dentition, small theropods, along with varanoid lizards, may have served similar ecological roles as competitive mesopredators in the Campanian tropical predator guild. Published by Elsevier BV.
Article
Non-avian dinosaurs went extinct 66 million years ago, geologically coincident with the impact of a large bolide (comet or asteroid) during an interval of massive volcanic eruptions and changes in temperature and sea level. There has long been fervent debate about how these events affected dinosaurs. We review a wealth of new data accumulated over the past two decades, provide updated and novel analyses of long-term dinosaur diversity trends during the latest Cretaceous, and discuss an emerging consensus on the extinction’s tempo and causes. Little support exists for a global, long-term decline across non-avian dinosaur diversity prior to their extinction at the end of the Cretaceous. However, restructuring of latest Cretaceous dinosaur faunas in North America led to reduced diversity of large-bodied herbivores, perhaps making communities more susceptible to cascading extinctions. The abruptness of the dinosaur extinction suggests a key role for the bolide impact, although the coarseness of the fossil record makes testing the effects of Deccan volcanism difficult.