Andrew B. Heckert

A new aetosaur (Reptilia: Archosauria) from the Upper Triassic of Texas and the phylogeny of aetosaurs

ABSTRACT Coahomasuchus kahleorum, gen. et sp. nov. is a small (<1 m long) aetosaur from the Upper Triassic Colorado City Member of the Dockum Formation in West Texas. The holotype consists of a nearly complete articulated skeleton, including a well-preserved dorsal and ventral carapace. Several postcranial features diagnose Coahomasuchus, including: dorsal paramedian plates considerably (3.25:1) wider than long, unflexed, lack keels or horns, and bear faint sub-parallel to radial ornamentation, and lateral scutes that are also unflexed, flat, and bear a faint radial pattern of pits and grooves. Coahomasuchus co-occurs with the aetosaur Longosuchus and the phytosaurs Angistorhinus and Paleorhinus, indicating that it is of Otischalkian (early late Carnian) age. Detailed phylogenetic analysis reveals that several North American aetosaur taxa are junior subjective synonyms of previously named taxa. Lucasuchus was named from material from the type locality of Longosuchus and is a junior subjective synonym of t...

Late Triassic dinosaursfrom the western United States

Western North America has one of the most extensive fossil records of Late Triassic dinosaurs. AllUpper Triassic strata are assigned to the Chinle Group which yields four successive, dinosaur-bearing faunas. Otischalkian (early Tuvalian) dinosaur specimens are fragmentary and indeterminate. Adamanian (late Tuvalian) dinosaurs include a herrerasaurid, Camposaurus arizonensis nov. gen. et sp., Caseosaurus crosbyensis nov. gen. et sp., Tecovasaurus murryi, a new ornithischian and an indeterminate prosauropod. Revueltian (early-middle Norian) dinosaurs include Chindesaurus bryansmalli, two new herrerasaurids, a prosauropod, Revueltosaurus callenderi, Technosaurus smalli, Lucianosaurus wildi, Protoavis texensis and the original syntypes of Coelophysis bauri. Apachean (Rhaetian) evidence for dinosaurs is principally ichnological (Grallator, Pseudotetrasauropus, Tetrasauropus), but at least three dinosaurs are known from osseous remains, including the neotype of Coelophysis bauri. Dinosaurs define three biogeographic provinces in the Late Triassic. In North America, dinosaurs are rare before the late Tuvalian (late Carnian) and appear to become increasingly numerous through the remainder of the Late Triassic.

A thin-shelled reptile from the Late Triassic of North America and the origin of the turtle shell

A new, thin-shelled fossil from the Upper Triassic (Revueltian: Norian) Chinle Group of New Mexico, Chinlechelys tenertesta, is one of the most primitive known unambiguous members of the turtle stem lineage. The thin-shelled nature of the new turtle combined with its likely terrestrial habitat preference hint at taphonomic filters that basal turtles had to overcome before entering the fossil record. Chinlechelys tenertesta possesses neck spines formed by multiple osteoderms, indicating that the earliest known turtles were covered with rows of dermal armour. More importantly, the primitive, vertically oriented dorsal ribs of the new turtle are only poorly associated with the overlying costal bones, indicating that these two structures are independent ossifications in basal turtles. These novel observations lend support to the hypothesis that the turtle shell was originally a complex composite in which dermal armour fused with the endoskeletal ribs and vertebrae of an ancestral lineage instead of forming de novo. The critical shell elements (i.e. costals and neurals) are thus not simple outgrowths of the bone of the endoskeletal elements as has been hypothesized from some embryological observations.

Redescription of Redondasuchus reseri,, a Late Triassic aetosaur (Reptilia: Archosauria) from New Mexico (U.S.A.), and the biochronology and phylogeny of aetosaurs

Abstract The Late Triassic (Rhaetian) aetosaur Redondasuchus reseri from the Redonda Formation of eastern NewMexico, U.S.A. is distinguished from all other aetosaurs except Typothorax by its paramedian scutes with ventral keels, and from Typothorax by its paramedian scutes with discontinuous ventral keels. Redondasuchus can be further distinguished from other aetosaurs by its paramedian scutes in which the lateral third is flexed downward approximately 45°. It is the only Rhaetian aetosaur currently known from the United States and is an index taxon for the Apachean land-vertebrate faunachron. A cladistic analysis of the aetosaurs, utilizing both characters derived from armor and other skeletal information, indicates that two distinct grades of aetosaurs exist. There is a basal grade consisting of the paraphyletic assemblage of Aetosaurus and (Stagonolepis + Aetosauroides)), and an advanced grade consisting of the clade Neoaetosauroides + Redondasuchus + ((Longosuchus + Desmatosuchus) + (Paratypothorax + Typothorax))). Integration of the existing aetosaur biochronology with operating cladistic definitions of aetosaur taxa produces a useful biostratigraphy of Upper Triassic strata in the western United States.

SOUTH AMERICAN OCCURRENCES OF THE ADAMANIAN (LATE TRIASSIC:LATEST CARNIAN) INDEX TAXON STAGONOLEPIS (ARCHOSAURIA: AETOSAURIA) AND THEIR BIOCHRONOLOGICAL SIGNIFICANCE

Abstract Aetosaur fossils from the Upper Triassic Ischigualasto Formation of Argentina formerly assigned to Aetosauroides scagliai Casamiquela and Argentinosuchus bonapartei Casamiquela are reassigned to Stagonolepis robertsoni Agassiz (small specimens) and S. wellesi (Long and Ballew) (larger specimens). Numerous features of the skull, vertebral column, appendicular skeleton, and particularly the armor, of these Argentinian aetosaurs are identical to those of Stagonolepis and differ from other aetosaurs. Identification of Stagonolepis in South America has important implications for the correlation of nonmarine strata across Late Triassic Pangea. Stagonolepis is an index taxon of the Adamanian land-vertebrate faunachron of latest Carnian age. Its occurrences in the Ischigualasto and Santa Maria formations, as well as at the type locality in the Lossiemouth Sandstone of Scotland and in the Blasensandstein of the German Keuper, cross-correlate with a well-established tetrapod biostratigraphy of the Chinle Group in western North America. Thus, all Stagonolepis records are of Adamanian (latest Carnian) age, not Otischalkian (early or “middle” Carnian) age, as proposed by some earlier workers. This correlation also demonstrates that the oldest known dinosaurs are not from the Ischigualasto and Santa Maria formations. Previous workers obtained radioisotopic dates of 227.8 Ma from the Ischigualasto Formation, providing a maximum numerical date for Stagonolepis-bearing units. This date must be late Carnian, not of Ladinian age, and thus supports the apparent age of the Ladinian-Carnian boundary (232 Ma) of time scales based on the Newark Supergroup in eastern North America.

Aetosauria: a clade of armoured pseudosuchians from the Upper Triassic continental beds

Abstract Aetosauria is a clade of obligately quadrupedal, heavily armoured pseudosuchians known from Upper Triassic (late Carnian–Rhaetian) strata on every modern continent except Australia and Antarctica. As many as 22 genera and 26 species ranging from 1 to 6 m in length, and with a body mass ranging from less than 10 to more than 500 kg, are known. Aetosauroides scagliai was recently recovered as the most basal aetosaur, placed outside of Stagonolepididae (the last common ancestor of Desmatosuchus and Aetosaurus). Interrelationships among the basal aetosaurs are not well understood but two clades with relatively apomorphic armour – the spinose Desmatosuchinae and the generally wide-bodied Typothoracisinae – are consistently recognized. Paramedian and lateral osteoderms are often distinctive at the generic level but variation within the carapace is not well understood in many taxa, warranting caution in assigning isolated osteoderms to specific taxa. The aetosaur skull and dentition varies across taxa, and there is increasing evidence that at least some aetosaurs relied on invertebrates and/or small vertebrates as a food source. Histological evidence indicates that, after an initial period of rapid growth, lines of arrested growth (LAGs) are common and later growth was relatively slow. The common and widespread Late Triassic ichnogenus Brachychirotherium probably represents the track of an aetosaur.

Late Triassic Aetosaurs as the Trackmaker of the Tetrapod Footprint Ichnotaxon Brachychirotherium

Brachychirotherium is the common ichnogenus of Late Triassic chirothere footprints well known from western Europe, North America, Argentina and South Africa. Although it has long been agreed by most workers that the trackmaker of Brachychirotherium was a derived crurotarsan archosaur, the trackmaker has been identified as either a rauisuchian or an aetosaur, and some workers attribute it to a primitive crocodylomorph (sphenosuchian). New knowledge of the osteology of the manus and pes of a large aetosaur, Typothorax coccinarum, indicates a close correspondence between the manus and pes structure of aetosaurs and the morphology of Brachychirotherium. Furthermore, functional analysis of complete skeletons indicates aetosaurs plausibly placed their feet in the narrow gauge, nearly the overstepped walk characteristic of Brachychirotherium. Brachychirotherium and aetosaurs have matched distributions, that is, they were Pangea-wide during the Late Triassic. The manus and pes morphology of rauisuchians and early crocodylomorphs (sphenosuchians) deviate from Brachychirotherium footprint morphology in key features, thus excluding their identification as trackmakers. Aetosaurs made Brachychirotherium footprints.

Dinosaur skin impressions and associated skeletal remains from the upper Campanian of southwestern New Mexico: new data on the integument morphology of hadrosaurs

ABSTRACT Skin impressions from the tail region of an indeterminate hadrosaur recently excavated from the Upper Cretaceous Ringbone Formation, southwest New Mexico represent the first known dinosaur specimen from New Mexico with impressions of the integument preserved in association with skeletal remains. The mid- to distal-caudal region is represented by 20 articulated centra, other disarticulated centra, a single chevron, ossified tendons and fragmentary bone, including poorly preserved neural spines. The skin impressions are preserved in negative and positive relief between two very fine-grained sandstone beds, interpreted as part of a fluvio-lacustrine facies package. The impression surface is directly below the ossified tendons, and 2.5 m from the articulated vertebral column. The skin impressions are six discrete patches characterized by predominantly apical, circular to ovate tubercles. Measurements of the long and short axes of individual tubercles demonstrate that a distribution of relatively homo...

The type locality ofCoelophysis, a Late Triassic dinosaur from north-central New Mexico (USA)

KurzfassungDer Dinosaurierfundpunkt an der Ghost Ranch ist nicht die Typ-Lokalität einer der drei benannten Arten des spättriadischen CeratosaurierCoelophysis (Dinosaurier; Saurischia: Theropoda). Neu entdeckte Fundpunkte spättriadischer Ceratosaurier in der Nähe der Ghost Ranch passen dagegen besser zu der geographischen und stratigraphischen Beschreibung des ErstentdeckersDavid Baldwin. Darüber hinaus stimmen Erhaltung und Morphologie der Ceratosaurier der neuen Fundpunkte besser mit dem Originalmaterial vonBaldwin überein als das Material der Ghost Ranch. Wir nehmen daher an, daß die neuen FundpunkteBaldwins ursprüngliche Fundpunkte sind; damit werden die neuen Funde zu Topotypen vonCoelophysis. Diese Topotypen zeigen, daß sichCoelophysis deutlich vonRioarribasaurus, einem Dinosaurier von der Ghost Ranch, unterscheidet.AbstractThe dinosaur quarry at Ghost Ranch is not the type locality of any of the three named species of the Late Triassic ceratosaurian dinosaurCoelophysis (Saurischia: Theropoda). Instead, newly discovered localities near Ghost Ranch that produce Late Triassic ceratosaurs match the geographic and stratigraphic description of two of theCoelophysis type localities provided byDavid Baldwin, the original collector. Furthermore, the preservation and morphology of ceratosaur fossils from the new localities more nearly matchesBaldwin’s original material than does the Ghost Ranch quarry material. We conclude that these new localities encompassBaldwin’s localities, so the newly collected ceratosaur fossils from these localities are probable topotypes ofCoelophysis. These topotypes preserve unique morphology that suggestsCoelophysis is a taxon distinct fromRioarribasaurus, the Ghost Ranch dinosaur.

A Large Archosauriform Tooth with Multiple Supernumerary Carinae from the Upper Triassic of New Mexico (USA), with Comments on Carina Development and Anomalies in the Archosauria

Here we report a tooth of a large archosauriform from the Upper Triassic of New Mexico, USA that displays developmental anomalies of carina formation. This tooth has two supernumerary carinae, both on the lingual side of the tooth. Previously, carina anomalies of this sort were primarily known from theropod dinosaurs, but always from the labial surface. Integrating this specimen into a reassessment of the published accounts of carina anomalies in other fossil diapsids reveals that supernumerary carinae are more widespread throughout Archosauriformes than previously reported. Our interpretation of this developmental anomaly highlights the present lack of understanding of tooth development in archosaurs, particularly carina formation, and suggests that crown morphology development in archosauriforms may be constrained differently than it is in mammals. This developmental constraint may explain the differences observed between the complexity found in mammal and archosauriform cusp morphology.