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The Chinle Formation of Arizona is a highly fossiliferous terrestrial unit from the Late Triassic Period. Fossil vertebrate collections from the Chinle Formation date back to the late 1890s and document a diverse fauna that is correlative with other Late Triassic faunas worldwide. No fewer than 18 holotype specimens including metoposaurids, phytosaurs, aetosaurs, crocodylomorphs, dinosauromorphs, and other archosaurs have been described from Chinle exposures in Arizona. A faunal overview of the most important localities and a review of the vertebrates is provided.

Precise field relocation and documentation of fossil localities in Petrified Forest National Park allows for better resolution of the stratigraphic ranges of taxa thought to have biochronological significance. Palynomorph data suggests that the Sonsela Member of the Chinle Formation is Latest Carnian-Early Norian in age and that the boundary between the Carnian and Norian Stages lies in the lower-middle portion of the member. There is no evidence for a significant unconformity at this boundary and whereas the Sonsela Member fauna appears to be transitional, there appears to be an abrupt change in the flora in this section.

We describe a partial phytosaur skull (PEFO 31207) from the Sonsela Member of the Upper Triassic Chinle Formation of Petrified Forest National Park, Arizona as a new species of Pseudopalatus, P. jablonskiae sp. nov. Although the holotype
specimen is incomplete, the new taxon is diagnosed by at least one autapomorphy and a unique suite of character-states. A
phylogenetic analysis of pseudopalatine phytosaurs recovers P. jablonskiae as the most basal species of Pseudopalatus. The
holotype of P. jablonskiae is especially important because it includes a well-preserved braincase; this portion of the skull is not preserved or described in detail for most phytosaur specimens. Additionally, P. jablonskiae is of biochronological significance because it is the stratigraphically lowest occurrence of Pseudopalatus within the Chinle Formation of Petrified Forest National Park, and provides strong evidence for the overlap of the Adamanian and Revueltian land-vertebrate faunachrons.

Note: Recent stratigraphic reinvestigations have shown that this specimen is not the lowest occurrence of Pseudopalatus, nor is there an Adamanian-Revueltian faunal overlap in PEFO.

Study of aetosaurian archosaur material demonstrates that the dermal armour of Desmatosuchus chamaensis shares almost no characters with that of Desmatosuchus haplocerus. Instead, the ornamentation and overall morphology of the lateral and paramedian armour of ‘D.’ chamaensis most closely resembles that of typothoracisine aetosaurs such as Paratypothorax. Autapomorphies of ‘D.’ chamaensis, for example the extension of the dorsal eminences of the paramedian plates into elongate, recurved spikes,warrant generic distinction for this taxon. This placement is also supported by a new phylogenetic hypothesis for the Aetosauria in which ‘D.’ chamaensis is a sister taxon of Paratypothorax and distinct from Desmatosuchus. Therefore, a new genus, Heliocanthus is erected for ‘D.’ chamaensis. Past phylogenetic hypotheses of the Aetosauria have been plagued by poorly supported topologies, coding errors and poor character construction. A new hypothesis places emphasis on characters of the lateral dermal armour, a character set previously under-utilised. Detailed examination of aetosaur material suggests that the aetosaurs can be divided into three groups based on the morphology of the lateral armour.Whereas it appears that the characters relating to the ornamentation of the paramedian armour are homoplastic, those relating to the overall morphology of the lateral armour may possess a stronger phylogenetic signal.

Reanalysis of known armor of the stagonolepidid (aetosaur) Desmatosuchus has shown distinct differences in the lateral
armor among individuals known from Carnian and Norian age rocks in the southwestern United States. Combined with previously reported cranial differences it is apparent that Norian specimens of Desmatosuchus represent a species distinct from the type species Desmatosuchus haplocerus and are described herein as Desmatosuchus smalli n. sp. D. smalli is known solely from Norian age rocks, whereas D. haplocerus is known only from Carnian age rocks. The separation of Desmatosuchus into distinct Carnian and Norian species suggests some biostratigraphic utility for these taxa.

One of the main methods for correlating Late Triassic terrestrial strata is through the use of land-vertebrate faunachrons (LVFs). Use of LVFs is widespread because of their supposed global application and ability to be correlated with the marine stages of the timescale. New magnetostratigraphic and radioisotopic data indicate that the traditional correlation of Late Triassic LVFs requires revision, although some authors maintain that these original correlations are sound, and that the new correlations of the marine stages to the numerical timescale are in error. Here, we examine the available evidence for cross-correlation of Late Triassic LVFs with the marine stages and numerical timescale. We conclude that the biostratigraphic links between the LVFs and marine stages are not robust; they are based on nondiagnostic specimens and/or taxonomically controversial specimens, endemic taxa, and/or ambiguously correlated assemblage zones. Given the available data, new correlations of the LVFs, marine stages, and the numerical timescale using magnetostratigraphy and radioisotopic ages that support a “long Norian” are preferential to those using largely vertebrate biostratigraphy that support a “long Tuvalian.” We also outline a framework for improving the accuracy and relevance of Late Triassic vertebrate biostratigraphy going forward in the near future.

We describe a new trilophosaurid, <i>Trilophosaurus dornorum</i> sp. nov., from the Upper Triassic (Late Carnian – Early Norian) Sonsela Member of the Chinle Formation in the Petrified Forest National Park, Arizona. Referred specimens of the new taxon are also known from the Cooper Canyon Formation (Norian) of west Texas. This large, robust trilophosaurid possesses distinct cingula on the transversely expanded, tricuspid teeth. Despite earlier reports to the contrary, this is the first well-preserved material of <i>Trilophosaurus</i> from Petrified Forest National Park. The discovery of the new taxon increases the known diversity of the trilophosaurids.

Two new partial skeletons from the Petrified Forest Member (Chinle Formation) of Petrified Forest National Park are referable to <i>Vancleavea campi</i> Long and Murry 1995. Although lacking good skull material, the new specimens possess postcranial elements that are either absent or poorly preserved in the holotype specimen. A phylogenetic analysis tentatively suggests that <i>Vancleavea campi</i> represents a basal archosauriform more derived than <i>Erythrosuchus, Proterosuchus</i>, and possibly <i>Euparkeria</i>. Comparisons with undescribed material from the Chinle Formation near St. Johns, Arizona, and Ghost Ranch, New Mexico, are needed to confirm this placement and also to determine the taxonomic validity of the genus <i>Vancleavea</i>.

A new discovery of skeletons of <i>Revueltosaurus callenderi</i> from the Upper Triassic Chinle Formation of Petrified Forest National Park, Arizona clearly shows that <i>Revueltosaurus</i> is not an ornithischian dinosaur as previously supposed. Features such as the presence of a postfrontal, crocodile-normal ankle and paramedian osteoderms with anterior bars place <i>R. callenderi</i> within the Pseudosuchia, closer to crocodylomorphs than to dinosaurs. Therefore, dental characters previously used to place <i>Revueltosaurus</i> within the Ornithischia evolved convergently among other archosaur taxa, and cannot be used to diagnose ornithischian dinosaur teeth. As a result, all other putative North American Late Triassic ornithischians, which are all based exclusively on teeth, are cast into doubt. The only reasonably well-confirmed Late Triassic ornithischians worldwide are <i>Pisanosaurus mertii</i> and an unnamed heterodontosaurid from Argentina. This considerably changes the understanding of early dinosaur diversity, distribution and
evolution in the Late Triassic.

Background: Recent revisions to the Sonsela Member of the Chinle Formation in Petrified Forest National Park have presented a three-part lithostratigraphic model based on unconventional correlations of sandstone beds. As a vertebrate faunal transition is recorded within this stratigraphic interval, these correlations, and the purported existence of a depositional hiatus (the Tr-4 unconformity) at about the same level, must be carefully re-examined.

Methodology/Principal Findings: Our investigations demonstrate the neglected necessity of walking out contacts and mapping when constructing lithostratigraphic models, and providing UTM coordinates and labeled photographs for all measured sections. We correct correlation errors within the Sonsela Member, demonstrate that there are multiple Flattops One sandstones, all of which are higher than the traditional Sonsela sandstone bed, that the Sonsela sandstone bed and Rainbow Forest Bed are equivalent, that the Rainbow Forest Bed is higher than the sandstones at the base of Blue Mesa and Agate Mesa, that strata formerly assigned to the Jim
Camp Wash beds occur at two stratigraphic levels, and that there are multiple persistent silcrete horizons within the Sonsela Member.

Conclusions/Significance: We present a revised five-part model for the Sonsela Member. The units from lowest to highest are: the Camp Butte beds, Lot’s Wife beds, Jasper Forest bed (the Sonsela sandstone)/Rainbow Forest Bed, Jim Camp Wash beds, and Martha’s Butte beds (including the Flattops One sandstones). Although there are numerous degradational/ aggradational cycles within the Chinle Formation, a single unconformable horizon within or at the base of the Sonsela Member that can be traced across the entire western United States (the ‘‘Tr-4 unconformity’’) probably does not exist. The shift from relatively humid and poorly-drained to arid and well-drained climatic conditions began during deposition of the Sonsela Member (low in the Jim Camp Wash beds), well after the Carnian-Norian transition.

One of the main methods for correlating Late Triassic terrestrial strata is through the use of land-vertebrate
faunachrons (LVFs). Use of LVFs is widespread because of their supposed global application and ability to be correlated
with the marine stages of the timescale. New magnetostratigraphic and radioisotopic data indicate that the traditional
correlation of Late Triassic LVFs requires revision, although some authors maintain that these original correlations are
sound, and that the new correlations of the marine stages to the numerical timescale are in error. Here, we examine the
available evidence for cross-correlation of Late Triassic LVFs with the marine stages and numerical timescale. We
conclude that the biostratigraphic links between the LVFs and marine stages are not robust; they are based on nondiagnostic
specimens and/or taxonomically controversial specimens, endemic taxa, and/or ambiguously correlated
assemblage zones. Given the available data, new correlations of the LVFs, marine stages, and the numerical timescale
using magnetostratigraphy and radioisotopic ages that support a “long Norian” are preferential to those using largely
vertebrate biostratigraphy that support a “long Tuvalian.” We also outline a framework for improving the accuracy
and relevance of Late Triassic vertebrate biostratigraphy going forward in the near future.

The Otis Chalk quarries in the Upper Triassic Dockum Group of West Texas have produced aetosaur material that most workers have suggested represents two distinct morphotypes. We use characters from aetosaur specimens with articulated or semi-articulated carapaces in which the anteroposterior placement of osteoderms can be established with certainty to compare homologous osteoderms in the Otis Chalk material. This study confirms that the genera Longosuchus and Lucasuchus are distinct morphotypes, which differ in that the former taxon has paramedian osteoderms with random pitted
ornamentation and low pyramidal bosses that contact the posterior margin, and spines on the lateral osteoderms that are posteriorly emarginated, whereas the latter taxon has paramedians with a strongly radial ornamentation and large conical eminences, and spines on the lateral osteoderms that are not posteriorly emarginated. Both taxa also have paramedians that are overlapped anteriorly by the laterals, a character that may be a synapomorphy of desmatosuchine aetosaurs. The arguments that these morphotypes represent ontogenetic stages or sexual dimorphs of a single biological species cannot be corroborated using either comparisons with modern pseudosuchians, other aetosaur taxa, or stratigraphic ranges. Longosuchus is known only from the type area and has no utility as an index taxon of the Otischalkian land-vertebrate faunachron, although Lucasuchus
suggests a tentative correlation between part of the Dockum Group of Texas and the Pekin Formation of North Carolina.

Recent collecting of vertebrate fossils in Petrified Forest National Park as the result of an ongoing inventory of fossil localities has produced numerous important new specimens. These include phytosaur skulls and partial skeletons, aetosaur partial skeletons and isolated, but complete, osteoderms, as well as new dinosaur material that contributes new information to a currently poor dinosaur record
from the Late Triassic of Arizona. Stratigraphic placement of phytosaur and aetosaur fossils from the park shows that several index taxa used for Late Triassic land vertebrate faunachrons overlap and that revision of these faunachrons is needed.

Two teeth collected from the Upper Triasssic Chinle Formation of northeastern Arizona are described here
and named Kraterokheirodon colberti gen. et sp. nov. These teeth are novel in having an occlusal ridge with six cusps
and a posterior shelf lacking dentine. Evidence for thecodont implantation of the root suggests amniote affinities for
these teeth. They do not match any teeth known for basal vertebrates or basal tetrapods. Although the teeth display
some affinities with “traversodont” cynodonts, there are significant differences that preclude a referral to this group.
These teeth most probably represent an unknown tetrapod clade and document the presence of a large amniote previously unknown from Late Triassic terrestrial faunas.

Ornithischian dinosaurs are one of the most taxonomically diverse dinosaur clades during the Mesozoic, yet their origin and early diversification remain virtually unknown. In recent years, several new Triassic ornithischian taxa have been proposed, mostly based upon isolated teeth. New discoveries of skeletal material of some of these tooth taxa indicate that these teeth can no longer be assigned to the Ornithischia using unambiguous synapomorphies. The Triassic record of ornithischian dinosaurs now comprises only three probable occurrences: Pisanosaurus and an unnamed heterodontosaurid from Argentina, and an unnamed specimen from the uppermost Triassic of South Africa. This revised Triassic record suggests that ornithischians were not very diverse or abundant through the Triassic, and there are large gaps in the Triassic ornithischian fossil record. Moreover, traditional living analogues for interpreting the feeding ecology of early ornithischians from their tooth morphology are generally inappropriate, and “herbivorous” archosaur teeth such as those found in early ornithischians are not necessarily
diagnostic of herbivorous feeding.

The North American Triassic dinosaur record has been repeatedly cited as one of the most complete early dinosaur assemblages. The discovery of Silesaurus from Poland and the recognition that Herrerasaurus and Eoraptor may not be theropods have forced a re-evaluation of saurischian
and theropod synapomorphies. Here, we re-evaluate each purported Triassic dinosaur from North America on a specimen by specimen basis using an apomorphy-based approach.We attempt to assign specimens to the most exclusive taxon possible. Our revision of purported Late Triassic dinosaur material from North America indicates that dinosaurs were rarer and less diverse in these strata than previously thought. This analysis concludes that non-dinosaurian dinosauriforms were present in North America in the Late Triassic. Most of the proposed theropod specimens are fragmentary and/or indistinguishable from corresponding elements in the only well-known Triassic theropod of North America, Coelophysis bauri. No Triassic material from North America can be assigned to Sauropodomorpha, because none of the purported ‘prosauropod’ material is diagnostic. Recent discovery of the skull and skeleton of Revueltosaurus callenderi from Arizona shows that it is a pseudosuchian archosaur, not an ornithischian dinosaur. As a result, other purported North American ornithischian teeth cannot be assigned to the Ornithischia and therefore, there are no confirmed North American Triassic ornithischians. Non-tetanuran theropods and possible basal saurischians
are the only identifiable dinosaurs recognised in North America until the beginning of the Jurassic
Period.

Recent stratigraphic revisions of the Upper Triassic Chinle Formation of Petrified Forest National Park, in conjunction with precise and accurate documentation of fossil tetrapod occurrences, clarified the local biostratigraphy, with regional and global implications. A significant overlap between Adamanian and Revueltian faunas is rejected, as is the validity of the Lamyan sub-land vertebrate faunachron. The Adamanian Revueltian boundary can be precisely placed within the lower Jim Camp Wash beds of the Sonsela Member and thus does not occur at the hypothesised Tr-4 unconformity. This mid-Norian faunal turnover, may coincide with a floral turnover, based on palynology studies and also on sedimentological evidence of increasing aridity. Available age constraints bracketing the turnover horizon are consistent with the age of the Manicouagan impact event. The rise of dinosaurs in western North America did not correspond to the Adamanian–Revueltian transition, and overall dinosauromorph diversity seems to have remained at a constant level across it. The paucity of detailed Late Triassic vertebrate biostratigraphic data and radioisotopic dates makes it currently impossible to either support or reject the existence of
globally synchronous Late Triassic extinctions for tetrapods.