Stratigraphic Analysis Report: Revised Biostratigraphy of the Sonsela Member via the Kaye Quarry Assemblage

 



1. Introduction: The Strategic Importance of the Kaye Quarry Discovery

The Sonsela Member of the Chinle Formation (Late Triassic, Arizona) represents a critical lithostratigraphic window into terrestrial evolution. High-resolution mapping of the Kaye Quarry, situated within the Jim Camp Wash beds of Petrified Forest National Park (PEFO), now serves as the strategic pivot point for correcting systemic errors in regional stratigraphic models. Historically, the reliance on "coarsely resolved" frameworks led to the misidentification of fossil localities and sampling points, primarily due to the failure to distinguish between multiple "look-alike" fluvial deposits, specifically the Flattops One sandstones. These lithological miscorrelations facilitated inaccurate narratives regarding faunal turnover and extinction events, often conflating discrete depositional sequences. The granular data provided by the Kaye Quarry allows for a rigorous recalibration of these superpositional relationships, moving the field from speculative approximations toward a high-resolution biostratigraphic architecture.

2. Systematic Paleontology and Ontogenetic Analysis of Sonselasuchus cedrus

The discovery of Sonselasuchus cedrus—a new shuvosaurid pseudosuchian—provides a high-density data source for analyzing population-level biological shifts. With over 950 skeletal elements representing a minimum of 36 individuals, this assemblage allows for an unprecedented resolution of ontogenetic trajectories. Identification was confirmed via an apomorphy-based identification approach using the Nesbitt et al. (2020) phylogenetic matrix, placing the taxon within Shuvosauridae. Sonselasuchus is diagnosed by a reduced maxillary body/anterior process and an enlarged subnarial foramen, alongside a toothless (edentulous) beak, hollow bones, and enlarged orbits. These features demonstrate a profound convergence with the ornithomimid dinosaurs of the Late Cretaceous, occupying a specialized adaptive zone nearly 100 million years before the bird-line archosaurs attained a similar morphology.

Statistical validation of the locomotory shift in Sonselasuchus was achieved through reduced major axis regression analysis. By comparing how limb elements scale relative to body size, we have identified a clear developmental transition from a quadrupedal juvenile stage to a bipedal adult stage.

Table 1: Ontogenetic Scaling and Locomotory Transition in Sonselasuchus cedrus

Limb Element

Growth Trajectory

Functional Impact

Femur

Positive Allometry

Hindlimb becomes proportionally longer and more robust, with a thicker proximal end.

Humerus

Negative Allometry

Forelimb scales below isometry, becoming proportionally smaller relative to body size over time.

Locomotion

Developmental Shift

Transition from quadrupedal (four-legged) juveniles to bipedal (two-legged) adults.

The precise placement of this social group within the Jim Camp Wash beds provides the high-resolution temporal anchor required to link biological populations with specific lithological sequences.

3. Recalibration of the Adamanian-Revueltian (A-R) Faunal Transition

The Adamanian-Revueltian (A-R) transition is a primary marker of environmental and biological flux in the Late Triassic. Previous hypotheses of "faunal overlap" are rejected based on the evidence from the Sonsela Member. High-resolution mapping demonstrates that Adamanian and Revueltian faunas are stratigraphically distinct, indicating an abrupt turnover rather than a gradual replacement.

Biostratigraphic Synthesis of the A-R Turnover:

  1. Lithostratigraphic Context: The transition is localized within the lower Jim Camp Wash beds of the Sonsela Member.
  2. Lithological Marker: The "persistent red silicrete" bed serves as the definitive physical marker for the turnover across the park.
  3. Temporal Correlation: The shift correlates temporally with the Manicouagan impact event (~214 Ma).
  4. Quantitative Turnover: The turnover is characterized by the loss of seven Adamanian taxa and the subsequent appearance of four new Revueltian taxa, providing a distinct quantitative boundary for the transition.

4. Deconstruction of the "Tr-4" Unconformity Model

Accurate contact mapping has necessitated the deconstruction of the "Tr-4" unconformity model. Previous researchers posited a widespread, area-wide gap in the geological record as the driver of faunal change, largely due to the misidentification of the Flattops One sandstones as a single erosional surface. By "walking the contacts," a methodology rigorously applied by Parker and Martz, we have demonstrated that these strata represent a more continuous record than previously modeled. Because the record is continuous, the faunal turnover can now be definitively linked to in-situ environmental stressors—specifically the collapse of the Late Triassic megamonsoon—rather than being dismissed as an artifact of missing time.

Strategic Model Comparison:

  • Old Model: Proposed a widespread "Tr-4" Unconformity as a temporal gap driving faunal turnover.
  • Revised Model: Demonstrates continuous deposition through the Sonsela Member; turnover is an observable biological response to climate-driven extinction events recorded in situ.

5. Climatological and Paleoecological Drivers of Change

The Sonsela Member records a shift from the Late Triassic megamonsoon toward increasing regional aridity. This is reflected in the transition from "humid and poorly drained" lithologies to "arid and well-drained" environments. The Kaye Quarry bonebed provides the "So What?" for this shift: isotopic evidence confirms that the collapse of the megamonsoon led to a lowering of the water table. This environmental stressor likely trapped the social group of Sonselasuchus—predominantly subadults—leading to a mass-mortality event as regional drought intensified approximately 215 million years ago.

The specialized body plan of Sonselasuchus represents an instance of Iterative Morphological Evolution, where the "croc-line" archosaurs utilized a bipedal, beaked niche that would be vacant for millions of years following the end-Triassic extinction before being reoccupied by ornithomimosaurs. This report integrates the Kaye Quarry data into a unified stratigraphic framework, demonstrating that the Adamanian-Revueltian turnover was a direct ecological consequence of a rapidly changing Triassic climate.