Redondasaurus

Abstract: We document here a new taxon of sphenodontian, Whitakersaurus bermani gen. et sp. nov., that is also the most complete sphenodontian fossil from the Upper Triassic Chinle Group in the south-western USA and the first Chinle sphenodontian represented by more than a single fragmentary dentulous element. The holotype was recovered during preparation of block C-8-82 from the famous Coelophysis (Whitaker) quarry at Ghost Ranch, New Mexico, and is the most complete small vertebrate recovered from the quarry. Detailed lithostratigraphy and geologic mapping demonstrate that the Whitaker quarry is in the Rock Point Formation of the Chinle Group, so Whitakersaurus is the first sphenodontian reported from this unit. Records of the phytosaur Redondasaurus at the quarry and elsewhere in the Chinle Group demonstrate that the quarry, and thus Whitakersaurus, is of Apachean (late Norian–Rhaetian) age. The sphenodontian specimen consists of incomplete left and right dentaries, a partial left? maxilla?, and impressions of a probable palatal element, all of which preserve multiple teeth. Whitakersaurus is distinct from other sphenodontians in possessing a unique combination of the following features: marginal dentition pleurodont anteriorly and posteriorly acrodont; pronounced heterodonty in dentary, with as many as 15 smaller, peg-like teeth anteriorly and several larger, posterior teeth that are conical and striated; faint radial ornamentation of posterior tooth crowns; presence of c. 19 dentary teeth; and absence of a distinct flange on posterior teeth. Numerous other details distinguish it from both more primitive and more derived taxa. Whitakersaurus, therefore, helps to document further mosaic evolution and an extensive diversification event of sphenodontians during Triassic time. Although sphenodontian taxa are relatively easily recognized, widely distributed, and common small- or microvertebrate fossils, the long stratigraphic ranges of taxa known from multiple specimens hinders their utility as index fossils with which to correlate strata across Pangaea.

Abstract: Triassic strata in the Chama Basin of Rio Arriba County, New Mexico, pertain to the Upper Triassic Chinle Group (in ascending order, the Zuni Mountains, Shinarump, Salitral, Poleo, Petrified Forest and Rock Point formations). The base of the Chinle Group locally is the Zuni Mountains Formation (formerly "mottled strata"), a pedogenic weathering profile as much as 7 m thick, developed in the top of the Pennsylvanian-Permian Cutler Group. Where the Zuni Mountains Formation is absent, the base of the Chinle Group is the Shinarump Formation. In the Chama Basin, the Shinarump Formation (= Agua Zarca Formation of previous usage) is as much as 13 m thick and consists mostly of trough-crossbedded, quartzose sandstone and siliceous conglomerate. The Salitral Formation is as much as 31 m of mostly greenish and reddish brown, bentonitic mudstone. The Salitral Formation is divided into two members: a lower, Piedra Lumbre Member of greenish mudstone with a peristent sandstone bed (the El Cerrito Bed) at its top, and an upper, Youngsville Member, which mostly consists of reddish-brown mud- stone. The Poleo Formation is up to 41 m thick and is mostly grayish yellow, trough-crossbedded litharenitic and subarkosic sandstone with minor amounts of both intrabasinal and siliceous conglomerate. Above the Poleo Formation, as much as 200 m of strata, dominated by reddish brown, bentonitic mudstone, constitute the Petrified Forest Formation. In the Chama Basin, the Petrified Forest Formation consists of two members, the lower Mesa Montosa Member, up to 24 m of thin-bedded sandstone, siltstone and mudstone, which is overlain by up to 176 m of the bentonitic mudstone-dominated Painted Desert Member. The Rock Point Formation in the Chama Basin disconformably overlies the Petrified Forest Formation and is as much as 70 m thick and mostly laterally persistent, repetitive beds of reddish brown and grayish red siltstone and ripple-laminar sandstone that essentially lack volcanic detritus. In the Chama Basin, unionid bivalves from the Petrified Forest Formation are consistent with a Revueltian age, and paly- nomorphs from the Painted Desert Member of the Petrified Forest Formation and from the Rock Point Formation are of Norian age. Three formations of the Chinle Group in the Chama Basin contain biochronologically important vertebrate fossils, notably the aetosaur Desmatosuchus haplocerus (Adamanian) in the Salitral Formation, the aetosaurs Typothorax coccinarum and Desmatosuchus chamaensis and the phytosaur Pseudopalatus buceros (Revueltian) in the Petrified Forest Formation, and the phytosaur Redondasaurus (Apachean) in the Rock Point Formation. These fossils and lithostratigraphy allow precise correla- tion of the Chinle Group strata exposed in north-central New Mexico with other Upper Triassic strata in New Mexico.

Abstract: The skull anatomy of a new species of the phytosaur Machaeroprosopus is described for the first time on the basis of two specimens from the Upper Triassic Cooper Canyon Formation of Texas. Additional information is provided by a third specimen referred to Machaeroprosopus sp. A paranasal bone, an additional paired element of the narial region, is identified. Important new data are presented for the braincase, including the morphology of the epipterygoid and presphenoid, an anterior process of the prootic, an anteroventral process of the laterosphenoid, and a parasphenoid process. Machaeroprosopus lottorum n. sp. is characterised by four apomorphies: a supratemporal fenestra closed on the skull roof with bevelled anterior rim, a comparatively short squamosal, a flat and rugose narial rim, and medially extended palatines that come close to form an ossified secondary palate. With respect to the supratemporal fenestra, the supraoccipital-parietal complex and several features of the squamosal, Machaeroprosopus lottorum n. sp. bridges the morphological gap between species previously referred to the genera Pseudopalatus and Redondasaurus. A parsimony analysis of known species of Machaeroprosopus supports the hypothesis that the development of the rostral crest in Machaeroprosopus is a sexually dimorphic feature, and questions the validity of the genus Redon- dasaurus. Consequently, Redondasaurus is here considered a junior synonym of Machaeroprosopus.

Abstract: Upper Triassic strata exposed in the Four Corners region belong to the Chinle Group of late Carnian-Rhaetian age. Chinle Group strata can be divided into eight lithostratigraphic intervals: (1) mottled stratalTemple Mountain Formation-as much as 31 m of mostly color mottled, deeply pedoturbated siltstone, sandstone and conglomerate; (2) Shinarump Formation-up to 76 m of trough-crossbedded sandstone and siliceous extrabasinal conglomerate; (3) Monitor Butte/Cameron/Bluewater Creek Formations- up to 84 m of varied lithofacies ranging from green bentonitic mudstones (Monitor Butte) to sandstones (Cameron) to red-bed mudstones (Bluewater Creek); (4) Blue Mesa Member of Petrified Forest Formation-up to 100 m of blue, gray, purple and red variegated bentonitic mudstone; (5) Moss Back Formation/Sonsela Member of Petrified Forest Formation-up to 50 m of trough-cross bedded sandstone and intrabasinal conglomerate; (6) Painted Desert Member of Petrified Forest Formation-up to 150 m of mostly red-bed bentonitic mudstone and siltstone; (7) Owl Rock Formation-up to 150 m of pale red and orange siltstone interbedded with ledges of pedogenic calcrete limestone; (8) Rock Point Formation-up to 300 m of reddish brown, cyclically-bedded sandstone and non-bentonitic siltstone. In southwestern Colorado, the base of the Chinle Group is the Moss Back Formation resting on Lower Permian strata. We abandon the term Dolores Formation and correlate its informal members as follows: (1) lower member = Moss Back Formation; (2) middle member = Painted Desert Member of Petrified Forest Formation; and (3) upper member = Rock Point Formation. The informal term "Kane Springs strata," applied to some Chinle Group coarse-grained strata in southeastern Utah, is also abandoned. Church Rock Member (Formation) is a synonym of Rock Point Formation, and the term Church Rock should not be applied to nearly all the Chinle Group section in southeastern Utah. Palynomorphs, megafossil plants and fossil vertebrates support the following age assignments for Chinle Group strata in the Four Corners region: late Carnian = mottled strata/Temple Mountain Formation, Shinarump Formation, Monitor Butte/Cameron/Bluewater Creek Formations and Blue Mesa Member of Petrified Forest Formation; early-middle Norian Moss Back Formation/ Sonsela Member of Petrified Forest Formation, Painted Desert Member of Petrified Forest Formation and Owl Rock Formation; and Rhaetian = Rock Point Formation. The Chinle Group consists of three unconformity-bounded sequences: Shinarump-Blue Mesa sequence of late Carnian age; Moss Back-Owl Rock sequence of early-middle Norian age; and Rock Point sequence of Rhaetian age. Facies architecture and biostratigraphy support 'a genetic relationship between Chinle Group strata on the Colorado Plateau and shallow marine strata of the Mesozoic marine province of western Nevada. This relationship suggests that eustasy was the primary allochthonous control on Chinle Group sedimentation. At Big Indian Rock in the Lisbon Valley of southeastern Utah, a skull of the phytosaur Redondasaurus is in a thin, discontinuous mud-pebble conglomerate near the base of the Wingate Sandstone. Redondasaurus is an index fossil of the Late Triassic Apachean (Rhaetian) land-vertebrate faunachron. Unabraded surface texture, large size and preservation of thin, fragile bone suggest that the phytosaur skull is not reworked, so the Triassic-Jurassic boundary is stratigraphically above it. No unconformity surface is present in the lower Wingate Sandstone above the skull. Thus, at Big Indian Rock, the J-O unconformity is not at the base of the Wingate Sandstone. If the basal Wingate is of Late Triassic age, then the Moenave Formation, with which it intertongues laterally, must also include Triassic strata. This suggests the Triassic-Jurassic boundary on the Colorado Plateau is relatively transitional-not a profound unconformity-within the Wingate-Moenave lithosome.