A Middle Triassic stem-turtle and the evolution of the turtle body plan

Area of Science:

• Vertebrate Zoology
• Paleontology
• Evolutionary Biology

Background:

• The evolutionary history of turtles is debated due to conflicting data and fossil gaps.
• Key fossil evidence includes Odontochelys (partially shelled) and Proganochelys (fully shelled).
• Eunotosaurus is a hypothesized early stem-turtle, but a temporal gap exists before Odontochelys.

Purpose of the Study:

• To describe a new fossil reptile, Pappochelys, from the Middle Triassic.
• To investigate Pappochelys's phylogenetic position relative to other early turtles and stem-turtles.
• To provide new evidence on the early evolution of the turtle shell and diapsid affinities.

Main Methods:

• Fossil discovery and excavation of Pappochelys from Middle Triassic deposits.
• Comparative anatomical analysis of Pappochelys's skeletal features, including skull, ribs, gastralia, and limb girdles.
• Phylogenetic analysis incorporating Pappochelys into existing datasets of early turtle evolution.

Main Results:

• Pappochelys is dated to approximately 240 million years ago, chronologically and structurally intermediate between Eunotosaurus and Odontochelys.
• Shared features with Odontochelys include limb girdle morphology and broad, T-shaped trunk ribs.
• Pappochelys possesses a gastralia cuirass instead of a plastron, suggesting plastron formation through gastralia fusion.
• Skull morphology supports diapsid affinities for turtles.

Conclusions:

• Pappochelys represents a critical transitional fossil in early turtle evolution.
• The findings support the hypothesis that the turtle plastron evolved through the fusion of gastralia.
• The discovery reinforces the diapsid ancestry of turtles.