New sphenodontian (Reptilia: Lepidosauria) from a novel Late Triassic paleobiota in western North America sheds light on the earliest radiation of herbivorous lepidosaurs

Herbivory is a common ecological function among extant lepidosaurs, but little is known about the origin of this feeding strategy within Lepidosauria. Here we describe a sphenodontian (Lepidosauria) from the Late Triassic of western North America, Trullidens purgatorii n. gen. n. sp., that reveals new aspects of the earliest radiation of herbivorous lepidosaurs. This taxon is represented by an isolated lower jaw with robust structure bearing transversely widened dentition and extensive wear facets, suggesting a masticatory apparatus specialized for herbivory. An unusual ‘incisor-like’ tooth is present at the anterior end of the jaw; a unique feature among lepidosaurs, this tooth is convergent with the incisors of extant rodents and lagomorphs. Phylogenetic analyses support the placement of this taxon within opisthodontian sphenodontians, a group sharing derived cranio-dental morphologies specialized for herbivory. The new taxon was recovered in a recently discovered and unnamed series of Upper Triassic strata in southeastern Colorado, USA, exposed in Canyons incised by the Purgatoire River and its tributaries. These strata comprise a dominantly red-bed sequence of conglomerates, sandstones, and siltstones deposited in a fluvio-lacustrine setting, preserving a Late Triassic biota of invertebrate and vertebrate ichnofossils, plant macrofossils, bony fish, temnospondyl amphibians, and reptiles. We use aetosaur osteoderms as biostratigraphic links to the nearby Chinle Formation of Arizona, USA, establishing a middle Norian age for these strata. The presence of an opisthodontian from western equatorial Pangaea in the Norian Stage reveals a near-global radiation of this clade across the Pangaean supercontinent during the Late Triassic. UUID: http://zoobank.org/A737c03f-863a-488e-a860-5cc914548774.

Orbital pacing of redox cycles suggested by red bed color of the Late Triassic Chinle Formation, Arizona

<p>Late Triassic records of the orbital pacing of climate are well documented from the stratigraphy of lake basins and marine facies. However, fewer studies have focused on detecting orbital climate signals preserved by fluvial depositional environments, home to terrestrial life. The sedimentary Chinle Formation of the Colorado Plateau (southwestern USA) is a succession of Late Triassic largely red beds that preserves numerous vertebrate fossils, including evidence of the Adamanian–Revueltian tetrapod faunal transition. Floodplain mudstones showing pedogenic features alternate on various thickness scales with channel sandstones. We assessed the cyclostratigraphy of red bed color for a ~250-m-thick interval of the Chinle Formation dated to 209-216 Ma using a scientific drill core from the Petrified Forest National Park, Arizona, 1A. Diffuse reflectance spectroscopy demonstrates that red bed color in this core derives from the mineral hematite, probably formed in response to the wetting and drying of soils under monsoonal rainfall. The magnetochronology and high-precision U-Pb detrital zircon dates of the core, and the astrochronostratigraphic polarity time scale of the Newark-Hartford basins are used to provide an age model for our spectral analyses and cyclostratigraphy. From the red-green and yellow-blue time series, we identified evidence of the long eccentricity, Jupiter-Venus cycle (405 kyr), longer-period grand eccentricity cycles including the Mars-Earth cycle, and possibly the Mars-Earth inclination cycle. There are also hints at higher frequency cycles. Although the relative amount of 405 kyr power is a fraction of the total variability, there is significant coherence between the Newark Basin depth rank record and the Chinle color at the 405 kyr and the ~100 kyr cycles. Our findings support previous interpretations that color and hematite variations formed during the Late Triassic and are unrelated to a younger diagenetic component of the red beds. Fluvial accumulation of the Chinle sediments was not as discontinuous as other studies have suggested, allowing for a reconstruction of orbital climate changes that may have affected the development of terrestrial ecosystems in Western Equatorial Pangaea.</p>

Hematite reconstruction of Late Triassic hydroclimate over the Colorado Plateau

Hematite is the most abundant surficial iron oxide on Earth resulting from near-surface processes that make it important for addressing numerous geologic problems. While red beds have proved to be excellent paleomagnetic recorders, the early diagenetic origin of hematite in these units is often questioned. Here, we validate pigmentary hematite (“pigmentite”) as a proxy indicator for the Late Triassic environment and its penecontemporaneous origin by analyzing spectrophotometric measurements of a 14.5-My–long red bed sequence in scientific drill core CPCP-PFNP13-1A of the Chinle Formation, Arizona. Pigmentite concentrations in the red beds track the evolving pattern of the Late Triassic monsoon and indicate a long-term rise in aridity beginning at ∼215 Ma followed by increased oscillatory climate change at ∼213 Ma. These monsoonal changes are attributed to the northward drift of the Colorado Plateau as part of Laurentia into the arid subtropics during a time of fluctuating CO2. Our results refine the record of the Late Triassic monsoon and indicate significant changes in rainfall proximal to the Adamanian–Revueltian biotic transition that thus may have contributed to apparent faunal and floral events at 216 to 213 Ma.

A new non-mammalian eucynodont from the Chinle Formation (Triassic: Norian), and implications for the early Mesozoic equatorial cynodont record

The Upper Triassic tetrapod fossil record of North America features a pronounced discrepancy between the assemblages of present-day Virginia and North Carolina relative to those of the American Southwest. While both are typified by large-bodied archosaurian reptiles like phytosaurs and aetosaurs, the latter notably lacks substantial representation of mammal relatives, including cynodonts. Recently collected non-mammalian eucynodontian jaws from the middle Norian Blue Mesa Member of the Chinle Formation in northeastern Arizona shed light on the Triassic cynodont record from western equatorial Pangaea. Importantly, they reveal new biogeographic connections to eastern equatorial Pangaea as well as southern portions of the supercontinent. This discovery indicates that the faunal dissimilarity previously recognized between the western and eastern portions of equatorial Pangaea is overstated and possibly reflects longstanding sampling biases, rather than a true biogeographic pattern.

LA-ICPMS U–Pb geochronology of detrital zircon grains from the Coconino, Moenkopi, and Chinle formations in the Petrified Forest National Park (Arizona)

Uranium–lead (U–Pb) geochronology was conducted by laser ablation – inductively coupled plasma mass spectrometry (LA-ICPMS) on 7175 detrital zircon grains from 29 samples from the Coconino Sandstone, Moenkopi Formation, and Chinle Formation. These samples were recovered from ∼ 520 m of drill core that was acquired during the Colorado Plateau Coring Project (CPCP), located in Petrified Forest National Park (Arizona). A sample from the lower Permian Coconino Sandstone yields a broad distribution of Proterozoic and Paleozoic ages that are consistent with derivation from the Appalachian and Ouachita orogens, with little input from local basement or Ancestral Rocky Mountain sources. Four samples from the Holbrook Member of the Moenkopi Formation yield a different set of Precambrian and Paleozoic age groups, indicating derivation from the Ouachita orogen, the East Mexico arc, and the Permo-Triassic arc built along the Cordilleran margin. A total of 23 samples from the Chinle Formation contain variable proportions of Proterozoic and Paleozoic zircon grains but are dominated by Late Triassic grains. LA-ICPMS ages of these grains belong to five main groups that correspond to the Mesa Redondo Member, Blue Mesa Member and lower part of the Sonsela Member, upper part of the Sonsela Member, middle part of the Petrified Forest Member, and upper part of the Petrified Forest Member. The ages of pre-Triassic grains also correspond to these chronostratigraphic units and are interpreted to reflect varying contributions from the Appalachian orogen to the east, Ouachita orogen to the southeast, Precambrian basement exposed in the ancestral Mogollon Highlands to the south, East Mexico arc, and Permian–Triassic arc built along the southern Cordilleran margin. Triassic grains in each chronostratigraphic unit also have distinct U and thorium (Th) concentrations, which are interpreted to reflect temporal changes in the chemistry of arc magmatism. Comparison of our LA-ICPMS ages with available chemical abrasion thermal ionization mass spectrometry (CA-TIMS) ages and new magnetostratigraphic data provides new insights into the depositional history of the Chinle Formation, as well as methods utilized to determine depositional ages of fluvial strata. For parts of the Chinle Formation that are dominated by fine-grained clastic strata (e.g., mudstone and siltstone), such as the Blue Mesa Member and Petrified Forest Member, all three chronometers agree (to within ∼ 1 Myr), and robust depositional chronologies have been determined. In contrast, for stratigraphic intervals dominated by coarse-grained clastic strata (e.g., sandstone), such as most of the Sonsela Member, the three chronologic records disagree due to recycling of older zircon grains and variable dilution of syn-depositional-age grains. This results in LA-ICPMS ages that significantly predate deposition and CA-TIMS ages that range between the other two chronometers. These complications challenge attempts to establish a well-defined chronostratigraphic age model for the Chinle Formation.

U-Pb zircon geochronology and depositional age models for the Upper Triassic Chinle Formation (Petrified Forest National Park, Arizona, USA): Implications for Late Triassic paleoecological and paleoenvironmental change

The Upper Triassic Chinle Formation is a critical non-marine archive of low-paleolatitude biotic and environmental change in southwestern North America. The well-studied and highly fossiliferous Chinle strata at Petrified Forest National Park (PFNP), Arizona, preserve a biotic turnover event recorded by vertebrate and palynomorph fossils, which has been alternatively hypothesized to coincide with tectonically driven climate change or with the Manicouagan impact event at ca. 215.5 Ma. Previous outcrop-based geochronologic age constraints are difficult to put in an accurate stratigraphic framework because lateral facies changes and discontinuous outcrops allow for multiple interpretations. A major goal of the Colorado Plateau Coring Project (CPCP) was to retrieve a continuous record in unambiguous superposition designed to remedy this situation. We sampled the 520-m-long core 1A of the CPCP to develop an accurate age model in unquestionable superposition by combining U-Pb zircon ages and magnetostratigraphy. From 13 horizons of volcanic detritus-rich siltstone and sandstone, we screened up to ∼300 zircon crystals per sample using laser ablation−inductively coupled plasma−mass spectrometry and subsequently analyzed up to 19 crystals of the youngest age population using the chemical abrasion−isotope dilution−thermal ionization mass (CA-ID-TIMS) spectrometry method. These data provide new maximum depositional ages for the top of the Moenkopi Formation (ca. 241 Ma), the lower Blue Mesa Member (ca. 222 Ma), and the lower (ca. 218 to 217 Ma) and upper (ca. 213.5 Ma) Sonsela Member. The maximum depositional ages obtained for the upper Chinle Formation fall well within previously proposed age constraints, whereas the maximum depositional ages for the lower Chinle Formation are relatively younger than previously proposed ages from outcrop; however, core to outcrop stratigraphic correlations remain uncertain. By correlating our new ages with the magnetostratigraphy of the core, two feasible age model solutions can be proposed. Model 1 assumes that the youngest, coherent U-Pb age clusters of each sample are representative of the maximum depositional ages and are close to (<1 Ma difference) the true time of deposition throughout the Sonsela Member. This model suggests a significant decrease in average sediment accumulation rate in the mid-Sonsela Member. Hence, the biotic turnover preserved in the mid-Sonsela Member at PFNP is also middle Norian in age, but may, at least partially, be an artifact of a condensed section. Model 2 following the magnetostratigraphic-based age model for the CPCP core 1A suggests instead that the ages from the lower and middle Sonsela Member are inherited populations of zircon crystals that are 1−3 Ma older than the true depositional age of the strata. This results in a model in which no sudden decrease in sediment accumulation rate is necessary and implies that the base of the Sonsela Member is no older than ca. 216 Ma. Independent of these alternatives, both age models agree that none of the preserved Chinle Formation in PFNP is Carnian (>227 Ma) in age, and hence the biotic turnover event cannot be correlated to the Carnian−Norian boundary but is rather a mid-Norian event. Our age models demonstrate the powers, but also the challenges, of integrating detrital CA-ID-TIMS ages with magnetostratigraphic data to properly interpret complex sedimentary sequences.