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The age of the Kayenta Formation of northeastern Arizona: overcoming the challenges of dating fossil bone.
Abstract
The Glen Canyon Group spans a period of Earth history that records the Triassic-Jurassic boundary and a biotic response to the end-Triassic mass extinction. Owing to the absence of dateable ash beds or useful biostratigraphic invertebrate fossils, the Early Jurassic has been poorly constrained in western North American and its chronology has been constructed upon difficult superpositional relationships and the presence of vertebrate taxa. The Kayenta Formation is host to a number of important taxa such as some of the first frogs, caecilians, turtles, crocodyliformes, ornithischians, and large-bodied saurischian dinosaurs in North America. However, the that unit has been assigned to either the Triassic or Jurassic nineteen different times since it was first mapped in 1882.
Here, we attempt to directly date fossil bone from the Kayenta Formation from a single quarry near Gold Spring, AZ that produced the skeletons of the theropod Dilophosaurus wetherilli and early sauropodomorph Sarahsaurus aurifontanalis. Electron-dispersive spectroscopy of bones from this quarry shows that they comprise recrystrallized hydroxylapatite with pore spaces permineralized by calcite, iron-rich calcite, and barite. We analyzed two bones from the Gold Spring quarry using a multicollector high-resolution laser ablation inductively-coupled plasma mass spectrometer (LA-ICP-MS) but the calculated apatite ages are too young to corroborate the stratigraphic evidence suggesting that the Kayenta Formation is either Late Triassic or Early Jurassic.
Further investigations into the spatial distribution of elements in the bones shows that concentrations of major and trace elements decreases moving away from the bone surface. Lead concentrations are highly variable within the fossil bone suggesting that some regions may have been subjected to Pb loss. However, the anomalously-young apatite ages are mostly a result of U enrichment, where U concentrations are as high as 1,100 ppm in the fossil bone.
We collected detrital zircons from matrix removed from the Gold Spring quarry and four other localities within the Kayenta Formation and calculated U-Pb ages using high-resolution LA-ICP-MS. One of these dates, 183.7 +/- 2.7 Ma, is the first deposition-age radiometric date from the Kayenta Formation and shifts the age designation for this Early Jurassic unit from the Sinemurian-Pliensbachian to the Pliensbachian-Toarcian. Future efforts will sample the rest of the Glen Canyon Group in order to construct the first chronology of the Early Jurassic in western North America.
... Nonetheless, this time period is important for crocodylomorph evolution because it documents the transition from early-branching crocodylomorph to mesoeucrocodylian-dominated ecosystems. Formations that bear crocodylomorph fossils and that are potentially coeval with the Clarens Formation are the Navajo Sandstone (Lower Jurassic, Sinemurian-Pliensbachian: Clark & Fastovsky 1986) and the Kayenta Formation (Lower Jurassic, Pliensbachian-Toarcian: Marsh et al. 2014) from the Glen Canyon Group in North America (Sues et al. 1994), the Zhanjia'ao Member of the Lufeng Formation (Lower Jurassic, Sinemurian : Bien 1941;Luo & Wu 1994) and the Forest Sandstone Formation (Lower Jurassic, Sinemurian-Pliensbachian: Sciscio et al. 2020) in Zimbabwe (Raath 1981 (Tykoski et al. 2002), and both Eopneumatosuchus (taxonomically uncertain) and UCMP 97638 are earlybranching crocodyliforms (Crompton & Smith 1980;Clark 1986), and Kayentasuchus is a non-crocodyliform crocodylomorph (Clark & Sues 2002). Crocodylomorphs recovered from the Lufeng Formation are the non-crocodyliform crocodylomorphs Dibothrosuchus and Phyllodontosuchus and the early-branching crocodyliforms Platyognathus (Wu & Sues 1996) and Dianosuchus (Young 1982). ...
... Uranium-lead (U-Pb) analyses of carbonate deposits in the Navajo Sandstone date the formation between 200.5 ± 1.5 Ma (Hettangian) and 195.0 ± 7.7 Ma (Hettangian-Sinemurian: Parrish et al. 2019), which is older than the Clarens Formation. Radiometric dating from the Kayenta Formation in northern Arizona places it as no older than 183.7 ± 2.7 Ma (Marsh et al. 2014), which is latest Pliensbachian and younger than the Clarens Formation. U-Pb ages of detrital zircons from the Kayenta Formation are dated as $12 Ma older than that (Dickinson & Gehrels 2009), which would make the Kayenta Formation (or at least areas of the formation) Sinemurian in age. ...
... Furthermore, Steiner & Helsley (1974) indicate that the Kayenta Formation in the Moab region is Late Triassic. For the purpose of this research we will agree with the dates presented by Marsh et al. (2014) for the Kayenta Formation (183.7 ± 2.7 Ma, Early Jurassic, late Pliensbachian-Toarcian), because these dates are also supported by biostratigraphy. The vertebrate fossils recovered from the Kayenta Formation include relatives of later-branching crocodyliforms (e.g. ...
The early-branching crocodylomorph species Notochampsa istedana is known from a single South African specimen collected more than 100 years ago. This species is potentially important for understanding early crocodylomorph evolution, but it is of uncertain taxonomic status and its stratigraphic position is poorly constrained. Here we reinvestigate the anatomy, taxonomy, systematics and biostratigraphy of the holotype specimen, SAM-PK-4013. SAM-PK-4013 has a unique suite of features that distinguish it from the closely related taxa Orthosuchus and Protosuchus. These features include the length and shape of the dentary symphysis, the number and shape of dentary teeth, and the number of dorsal ribs with expanded intercostal ridges. Notochampsa is therefore a valid taxon, and our phylogenetic analysis recovers it as sister to Orthosuchus, in a monophyletic Notochampsidae. Notochampsa and Orthosuchus share a ventrally expanded squamosal flange and expanded intercostal ridges on the dorsal ribs. Notochampsidae is in turn sister to Protosuchidae, forming the monophyletic group Notochampsoidea. Fieldwork and stratigraphic revisions show definitively that SAM-PK-4013 is from the Clarens Formation, approximately ∼65 m above the Elliot contact, ageing Notochampsa to the Pliensbachian stage, a period of vertebrate body fossil scarcity. Thus, Notochampsa istedana is the youngest known occurrence of a crocodylomorph (and vertebrate body fossil) from the Karoo Basin of South Africa.
... (b) Idealized outcrop lithostratigraphy and chronostratigraphy of Ward Terrace. Lithostratigraphy after Harshbarger et al. [21] and chronostratigraphy after Marsh et al. [22], Suarez et al. [23] and Marsh [24]. ...
... Without a sustained fluvial system, the Moenave Formation preserved very few vertebrate fossils [37,38]. The Triassic-Jurassic boundary probably lies near the middle of the Moenave Formation [22,23,38]. The upper boundary of the Kayenta Formation interfingers with the upper Lower Jurassic aeolian Navajo Sandstone [39,40]. ...
... During laboratory preparation of the Sarahsaurus aurifontanalis holotype specimen from its encasing sandstone matrix, Adam Marsh extracted a large sample of detrital zircon crystals for dating. Preliminary laser ablation ICP-MS detrital zircon U-Pb results yielded a date of 183.7 ± 2.7 Ma [22]. This date establishes this part of the Kayenta Formation (recorded in most field notes as the informal 'middle third' of the Kayenta Formation) in the Pliensbachian and Toarcian stages of the Lower Jurassic. ...
The armoured dinosaurs, Thyreophora, were a diverse clade of ornithischians known from the Early Jurassic to the end of the Cretaceous. During the Middle and Late Jurassic, the thyreophorans radiated to evolve large body size, quadrupedality, and complex chewing mechanisms, and members of the group include some of the most iconic dinosaurs, including the plated Stegosaurus and the club-tailed Ankylosaurus ; however, the early stages of thyreophoran evolution are poorly understood due to a paucity of relatively complete remains from early diverging thyreophoran taxa. Scutellosaurus lawleri is generally reconstructed as the earliest-diverging thyreophoran and is known from over 70 specimens from the Lower Jurassic Kayenta Formation of Arizona, USA. Whereas Scutellosaurus lawleri is pivotal to our understanding of character-state changes at the base of Thyreophora that can shed light on the early evolution of the armoured dinosaurs, the taxon has received limited study. Herein, we provide a detailed account of the osteology of Scutellosaurus lawleri , figuring many elements for the first time. Scutellosaurus lawleri was the only definitive bipedal thyreophoran. Histological studies indicate that it grew slowly throughout its life, possessing lamellar-zonal tissue that was a consequence neither of its small size nor phylogenetic position, but may instead be autapomorphic, and supporting other studies that suggest thyreophorans had lower basal metabolic rates than other ornithischian dinosaurs. Faunal diversity of the Kayenta Formation in comparison with other well-known Early Jurassic-aged dinosaur-bearing formations indicates that there was considerable spatial and/or environmental variation in Early Jurassic dinosaur faunas.
... B, idealized outcrop litho-and chronostratigraphy of Ward Terrace. Lithostratigraphy after Harshbarger et al. (1957) and chronostratigraphy after Marsh et al. (2014) and Suarez et al. (2017). Abbreviations: AZ, Arizona; FM, Formation; Jk, Kayenta Formation; Jn, Navajo Sandstone; SS, Sandstone; TrJm, Moenave Formation. ...
... Age and Stratigraphic Range-Early Jurassic (Sinemurian-Toarcian: Marsh et al., 2014), 'middle third' of the Silty Facies of the Kayenta Formation, Glen Canyon Group (Fig. 1B). ...
... More recently, the ages of the Kayenta Formation, the upper part of the Elliot Formation of South Africa (McPhee et al., 2014), and the Lufeng Formation of China (Fang et al., 2000;Mao et al., 2019) were reassigned to the Lower Jurassic (e.g., Welles, 1954;Harshbarger et al., 1957;Welles, 1970;Padian, 1989;Rowe, 1989;Rowe et al., 2010;Olsen et al., 2011;Marsh et al., 2014;Marsh, 2018). Recent U-Pb ages of detrital zircon from matrix removed from field jackets from the Kayenta Formation (Marsh et al., 2014;Marsh and Rowe, 2018) provided the first radiometric evidence that its age is late Early Jurassic (Sinemurian to Toarcian). ...
We describe new specimens of the ornithischian dinosaur Scutellosaurus lawleri Colbert, 1981 Colbert, E. H. 1981. A primitive ornithischian dinosaur from the Kayenta Formation of Arizona. Museum of Northern Arizona Bulletin Series 53:1–60. [Google Scholar], from the Lower Jurassic Kayenta Formation of Arizona and discuss their systematic importance. The new specimens represent at least 46 individuals and include two associated skeletons that preserve regions that were poorly known until now, including the skull and pelvis. Computed tomography (CT) assisted our interpretation of these specimens. Using an ornithischian data matrix, we first tested whether the two associated skeletons were justifiably assigned to Scutellosaurus lawleri and found that they group unequivocally with the holotype and paratype specimens. This enabled scoring of 35 character states that were previously unknown, raising the scoring completeness of Scutellosaurus lawleri from 52% to 67%. The results recovered Lesothosaurus diagnosticus as the basal-most member of Neornithischia, while corroborating the monophyly of Thyreophora and Scutellosaurus lawleri as its most basally branching member. In terms of numbers of specimens, Scutellosaurus lawleri is now the most abundant dinosaur known in any Early Jurassic vertebrate fauna. The presence of a second thyreophoran in the Kayenta Formation, along with the presence of Early Jurassic thyreophorans in Europe and Asia, suggests that Thyreophora may have originated in the northern hemisphere. The ornithischians from the Kayenta Formation support a pattern of dinosaurian diversification after the end-Triassic extinction in North America, if not a broader area, that was fueled by independent northward dispersals from the southern hemisphere, supporting dispersal as an early driver of dinosaurian evolution.
... Because it was collected a few meters above the base of the Kayenta Formation, the holotype specimen of Dilophosaurus wetherilli was considered an important biostratigraphic indicator. More recent discoveries of Dilophosaurus from the 'middle third' of the Kayenta Formation (Sues et al., 1994;Rowe et al., 2011) yielded radiometric dates that confirm an Early Jurassic age for some or all of the formation (see below; Marsh et al., 2014). This confirmation is significant because it means that Dilophosaurus wetherilli, Syntarsus kayentakatae Rowe, 1989, Sarahsaurus aurifontanalis Rowe et al., 2011, andScutellosaurus lawleri Colbert, 1981 in the Kayenta Formation of western North America are separated in time by as many as 30 Myr from non-dinosaur dinosauromorphs and small-bodied theropods, such as Coelophysis bauri Cope, 1887(Colbert, 1989, Tawa hallae Nesbitt et al., 2009b, andChindesaurus bryansmalli Long andMurry, 1995, which were living in the Late Triassic (Irmis et al., 2007). ...
... Fig. 1). Referred to as the Type Sarahsaurus Quarry (Marsh, 2014), Rowe and a small group of graduate students found and excavated this quarry between 1997 and 2000. A partially articulated Dilophosaurus wetherilli was collected initially and the holotype and paratype specimens of Sarahsaurus aurifontanalis were found directly underneath. ...
... A partially articulated Dilophosaurus wetherilli was collected initially and the holotype and paratype specimens of Sarahsaurus aurifontanalis were found directly underneath. Preliminary laser ablation ICP-MS detrital zircon U-Pb results indicate a date of 183.7 ± 2.7 Ma (Early Jurassic, Pliensbachian; Marsh et al., 2014). ...
Dilophosaurus wetherilli was the largest animal known to have lived on land in North America during the Early Jurassic. Despite its charismatic presence in pop culture and dinosaurian phylogenetic analyses, major aspects of the skeletal anatomy, taxonomy, ontogeny, and evolutionary relationships of this dinosaur remain unknown. Skeletons of this species were collected from the middle and lower part of the Kayenta Formation in the Navajo Nation in northern Arizona. Redescription of the holotype, referred, and previously undescribed specimens of Dilophosaurus wetherilli supports the existence of a single species of crested, large-bodied theropod in the Kayenta Formation. The parasagittal nasolacrimal crests are uniquely constructed by a small ridge on the nasal process of the premaxilla, dorsoventrally expanded nasal, and tall lacrimal that includes a posterior process behind the eye. The cervical vertebrae exhibit serial variation within the posterior centrodiapophyseal lamina, which bifurcates and reunites down the neck. Iterative specimen-based phylogenetic analyses result in each of the additional specimens recovered as the sister taxon to the holotype. When all five specimens are included in an analysis, they form a monophyletic clade that supports the monotypy of the genus. Dilophosaurus wetherilli is not recovered as a ceratosaur or coelophysoid, but is instead a non-averostran neotheropod in a grade with other stem-averostrans such as Cryolophosaurus ellioti and Zupaysaurus rougieri. We did not recover a monophyletic ‘Dilophosauridae.’ Instead of being apomorphic for a small clade of early theropods, it is more likely that elaboration of the nasals and lacrimals of stem-averostrans is plesiomorphically present in early ceratosaurs and tetanurans that share those features. Many characters of the axial skeleton of Dilophosaurus wetherilli are derived compared to Late Triassic theropods and may be associated with macropredation and an increase in body size in Theropoda across the Triassic-Jurassic boundary.
... A key fossil archive documenting this evolutionary phase of early crocodyliforms is the uppermost Triassic-Lower Jurassic Glen Canyon Group, exposed on the Colorado Plateau of the southwestern United States (e.g., Sues et al., 1994;Tykoski, 2005). In particular, the Lower Jurassic Kayenta Formation preserves a remarkable assemblage of crocodylomorphs and other nonmarine tetrapods, including lissamphibians, tritylodont mammaliamorphs, early mammaliaforms, lepidosauromorphs, turtles, pterosaurs, and saurischian and ornithischian dinosaurs (Breeden III & Rowe, 2020;Curtis & Padian, 1999;Hoffman & Rowe, 2018;Jenkins et al., 1983;Marsh et al., 2014;Marsh & Rowe, 2018Padian, 1984;Sterli & Joyce, 2007;Sues, 1986;Sues et al., 1994). The vast majority of these specimens are from the "silty facies" exposed in northern Arizona within the Navajo Nation. ...
... Moreau et al., 2002;Yang et al., 1996), and paleomagnetic data from the silty facies of the Kayenta Formation near the type locality of E. colberti (see Bazard & Butler, 1991) were not placed in stratigraphic context, so cannot be used for magnetostratigraphic correlation. New laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb detrital zircon ages from the Kayenta Formation of northern Arizona indicate the top of the unit could be as late as Toarcian in age, but are consistent with a Sinemurian-Pliensbachian age for the lower part of the formation, including the E. colberti type locality (Marsh, 2015(Marsh, , 2018Marsh et al., 2014). That said, these data should be interpreted cautiously, because the geologic uncertainty of LA-ICPMS U-Pb zircon ages is an order of magnitude larger than their analytical uncertainty (e.g., Herriott et al., 2019;Rasmussen et al., 2020;von Quadt et al., 2014). ...
Eopneumatosuchus colberti Crompton and Smith, 1980, known from a single partial skull, is an enigmatic crocodylomorph from the Lower Jurassic Kayenta Formation. In spite of its unique morphology, an exceptionally pneumatic braincase, and presence during a critical time period of crocodylomorph evolution, relatively little is known about this taxon. Here, we redescribe the external cranial morphology of E. colberti, present novel information on its endocranial anatomy, evaluate its phylogenetic position among early crocodylomorphs, and seek to better characterize its ecology. Our examination clarifies key aspects of cranial suture paths and braincase anatomy. Comparisons with related taxa (e.g., Protosuchus haughtoni) demonstrate that extreme pneumaticity of the braincase may be more widespread in protosuchids than previously appreciated. Computed tomography scans reveal an endocranial morphology that resembles that of other early crocodylomorphs, in particular the noncrocodyliform crocodylomorph Almadasuchus figarii. There are, however, key differences in olfactory bulb and cerebral hemisphere morphology, which demonstrate the endocranium of crocodylomorphs is not as conserved as previously hypothesized. Our phylogenetic analysis recovers E. colberti as a close relative of Protosuchus richardsoni and Edentosuchus tienshanensis, contrasting with previous hypotheses of a sister group relationship with Thalattosuchia. Previous work suggested the inner ear has some similarities to semi‐aquatic crocodyliforms, but the phylogenetic placement of E. colberti among protosuchids with a terrestrial postcranial skeletal morphology complicates paleoecological interpretation.
... Radioisotopic ages, magnetostratigraphy, and palynomorphs from the underlying Moenave Formation in southwestern Utah and northern Arizona provide maximum age constraints for the Kayenta Formation indicating it is no older than Sinemurian (Litwin, 1986;Cornet and Waanders, 2006;Downs, 2009;Donohoo-Hurley et al., 2010;Suarez et al., 2017). New U-Pb zircon ages from the 'silty facies' of the formation in northern Arizona suggest at least part of the Kayenta is late Pliensbachian to early Toarcian in age (Marsh et al., 2014;Marsh, 2015). This is consistent with magnetostratigraphic data from the Kayenta Formation and interfingering Tenny Canyon Tongue of the Navajo Sandstone in southwestern Utah, which Steiner and Tanner (2014) correlated with the lower-middle ...
... The main difficult with non-marine records is that their geochronology is poorly constrained (e.g.,Mundil et al., 2010;Irmis et al., 2010). In North America, the only exception is the tetrapod footprint record from the Newark Supergroup along the east coast (e.g.,Olsen et al., 2002), which is tied to theNewark-Harford Astrochronostratigraphic Time-Scale (Kent et al., 2017) and now verified by high-precision U-Pb ages from the Chinle Formation (Kent et al., 2018).The uppermost Chinle Formation and Glen Canyon Group on the Colorado Plateau has potential to compliment the Newark record, because it preserves an abundant footprint and body fossil record (e.g.,Sues et al., 1994;Irmis, 2005b;Lucas et al., 2005a;Tykoski, 2005;Milner et al., 2012) and contains a much longer post-extinction record (cf.Marsh, 2015;Marsh et al., 2014). However, the main limitation has been the lack of geochronologic constraints, with considerable debate over the stratigraphic placement of the Triassic-Jurassic boundary (e.g.,Lucas et al., 2005bLucas et al., , 2006bLucas et al., ,c, 2011Kirkland and Milner, 2006;Lucas & Tanner, 2007; Donohoo-Hurley et al., 2010;Milner et al., 2012; Kirkland et al., 2014;Suarez et al., 2017).Nonetheless, it has become clear that in a number of areas across the Colorado Plateau(Lucas et al., 2006c;Sprinkel et al., 2011a;Martz et al., 2014;Irmis et al., 2015; Britt et al., 2016;Suarez et al., 2017), and specifically in BENM and the surrounding area, the Triassic-Jurassic transition is preserved without significant gaps in deposition(Molina-Garza et al., 2003; Lewis et al., 2011). ...
Bears Ears National Monument (BENM) is a new, landscape-scale national monument jointly administered by the Bureau of Land Management and the Forest Service in southeastern Utah as part of the National Conservation Lands system. As initially designated, BENM encompasses 1.3 million acres of land with exceptionally fossiliferous rock units. These units comprise a semi-continuous depositional record from the Pennsylvanian Period through the middle of the Cretaceous Period. Additional Quaternary and Holocene deposits are known from unconsolidated river gravels and cave deposits. The fossil record from BENM provides unique insights into several important paleontological periods of time, including the Pennsylvanian-Permian transition from fully aquatic to more fully terrestrial tetrapods; the rise of the dinosaurs following the Triassic-Jurassic extinction; and the response of ecosystems in dry climates to sudden temperature increases at the end of the last ice age and across the Holocene. While the paleontological resources of BENM are extensive, they have historically been under-studied. Here we summarize prior paleontological work in BENM and review the data used to support paleontological resource protection in the 2016 BENM proclamation.
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.
Bears Ears National Monument (BENM) is a new landscape-scale national monument in southeastern Utah, jointly administered by the Bureau of Land Management and the U.S. Forest Service as part of the National Conservation Lands system. As initially designated in 2016, BENM encompassed 1.3 million acres of land with exceptionally fossiliferous rock units. Subsequently, in December 2017, presidential action reduced BENM to two smaller management units (Indian Creek and Shash Jáá). Although the pa-leontological resources of BENM are extensive and abundant, they have historically been under-studied. Herein we summarize prior paleontological work within the original BENM boundaries to provide a more comprehensive picture of the known paleontological resources, which are used to support paleontological resource protection. The fossil-bearing units in BENM comprise a nearly continuous depositional record from approximately the Middle Pennsylvanian Period (about 310 Ma) through the middle of the Cretaceous Period (about 115 Ma). Pleistocene and Holocene deposits are known from unconsolidated fluvial terraces and cave deposits. The fossil record from BENM provides unique insights into several important paleontological intervals of time including the Carboniferous-Permian icehouse-greenhouse transition and evolution of fully terrestrial tetrapods, the rise of the dinosaurs following the end-Triassic mass extinction, and the response of ecosystems in dry climates to sudden temperature increases at the end of the last glacial maximum.
Phase 1 of the Colorado Plateau Coring
Project (CPCP-I) recovered a total of over 850 m of stratigraphically
overlapping core from three coreholes at two sites in the Early to Middle
and Late Triassic age largely fluvial Moenkopi and Chinle formations in
Petrified Forest National Park (PFNP), northeastern Arizona, USA. Coring took
place during November and December of 2013 and the project is now in its
post-drilling science phase. The CPCP cores have abundant detrital
zircon-producing layers (with survey LA-ICP-MS dates selectively resampled
for CA-ID-TIMS U-Pb ages ranging in age from at least 210 to 241 Ma), which
together with their magnetic polarity stratigraphy demonstrate that a
globally exportable timescale can be produced from these continental
sequences and in the process show that a prominent gap in the calibrated
Phanerozoic record can be filled. The portion of core CPCP-PFNP13-1A for
which the polarity stratigraphy has been completed thus far spans ∼ 215
to 209 Ma of the Late Triassic age, and strongly validates the longer
Newark-Hartford Astrochronostratigraphic-calibrated magnetic Polarity
Time-Scale (APTS) based on cores recovered in the 1990s during the Newark
Basin Coring Project (NBCP).Core recovery was ∼ 100 % in all holes (Table 1). The coreholes were
inclined ∼ 60–75° approximately to the south to ensure azimuthal
orientation in the nearly flat-lying bedding, critical to the interpretation
of paleomagentic polarity stratigraphy. The two longest of the cores
(CPCP-PFNP13-1A and 2B) were CT-scanned in their entirety at the University
of Texas High Resolution X-ray CT Facility in Austin, TX, and subsequently
along with 2A, all cores were split and processed at the CSDCO/LacCore
Facility, in Minneapolis, MN, where they were scanned for physical property
logs and imaging. While remaining the property of the Federal Government, the
archive half of each core is curated at the NSF-sponsored LacCore Core
Repository and the working half is stored at the Rutgers University Core
Repository in Piscataway, NJ, where the initial sampling party was held in
2015 with several additional sampling events following. Additional planned
study will recover the rest of the polarity stratigraphy of the cores as
additional zircon ages, sedimentary structure and paleosol facies analysis,
stable isotope geochemistry, and calibrated XRF core scanning are
accomplished. Together with strategic outcrop studies in Petrified Forest
National Park and environs, these cores will allow the vast amount of surface
paleontological and paleoenvironmental information recorded in the
continental Triassic of western North America to be confidently placed in a
secure context along with important events such as the giant Manicouagan
impact at ∼ 215.5 Ma (Ramezani et al., 2005) and long wavelength
astronomical cycles pacing global environmental change and trends in
atmospheric gas composition during the dawn of the dinosaurs.
Sarahsaurus aurifontanalis, from the Kayenta Formation of Arizona, is one of only three sauropodomorph dinosaurs known from the Early Jurassic of North America. It joins Anchisaurus polyzelus, from the older Portland Formation of the Hartford Basin, and Seitaad reussi, from the younger Navajo Sandstone of Utah, in representing the oldest North American sauropodomorphs. If it is true that sauropodomorphs were absent from North America during the Late Triassic, the relationship among these three dinosaurs offers a test of the mechanisms that drove recovery in North American biodiversity following the end-Triassic extinction event. Here we provide the first thorough description of Sarahsaurus aurifontanalis based on completed preparation and computed tomographic imaging of the holotype and referred specimens. With new anatomical data, our phylogenetic analysis supports the conclusion that Sarahsaurus aurifontanalis is nested within the primarily Gondwanan clade Massospondylidae, while agreeing with previous analyses that the three North American sauropodomorphs do not themselves form an exclusive clade. A revised diagnosis and more thorough understanding of the anatomy of Sarahsaurus aurifontanalis support the view that independent dispersal events were at least partly responsible for the recovery in North American vertebrate diversity following a major extinction event.
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