Depositional environment and taphonomy of some fossil vertebrate occurrences in Lower Permian redbeds in Archer County, Texas
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The Lower Permian Admiral Formation redbeds in north-central Texas are famous for their well-studied vertebrate fauna. Taphonomical and paleoecological aspects, however, are inadequately understood. The prerequisite for taphonomical interpretations is an analysis of the depositional environments. Low relief and low regional dip expose extensive paleoslopes in western Archer County. Three major depositional systems may be recognized: a fine-grained meanderbelt, a low sinuosity fine-grained fluvial system, and a tidal flat. The small scale of the sedimentation (average sandstone thickness 1. 5 m) is remarkable. Four types of vertebrate occurrences can be distinguished: Type 1: Mass death bonebeds are situated in a floodbasin facies comprised of gray and red mudstones with abundant Psaronius roots (a swamp-dwelling tree fern) which is associated with the fluvial systems. Such basins were covered by a dense swamp forest with a high diversity of vertebrates. This type is exemplified by the Geraldine Bonebed, which has yielded at least 45 partly articulated skeletons representing 4 genera of tetrapods, and remains of another 8 vertebrate taxa. The bones were found on a layer of fern, seed fern, and conifer foliage and wood. This occurrence was formed by a single catastrophic event, possibly a forest fire, which drove the animals of the swamp forest into a pond, where they died of suffocation and were concentrated into a bonebed by physical processes (wind). Type 2: Lag bonebeds, situated on the landward margin of tidal flat environments, are represented by the Rattlesnake Canyon Bonebed which consists mainly of a calcareous concretion conglomerate, which contains fragmentary bone, serpulid worm colonies (brackish water!), and calamitelean wood. The diversity of forms represented by articulated material is low. The ubiquitous predator Dimetrodon and an amphibian, Trimerorachis, which tolerates brackish water, are common. This type was deposited as lag in a storm washover deposit. Type 3: Ponds (abandoned channels, etc.) which contained a fauna dominated by aquatic forms (the fishes Xenacanthus and Ectosteorachis, and the amphibian Archeria) were gradually filled by fine-grained sediment and organic debris (vertebrates, plants). These oxbow lakes were probably rimmed by stands of Calamites. Four examples are described. Type 4: Single, complete skeletons examplified here by Diadectes are occasionally found in red floodplain mudstones.