Pappochelys rosinae

Reconstruction of Pappochelys rosinae（圖片來源：Rainer Schoch，採用 CC BY-SA 4.0 授權）

Age

Triassic（Ladinian）

240 Ma

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Taxonomy

Kingdom: Animalia

Phylum: Chordata

Class: Sauropsida

Clade: Pantestudines

Genus: Pappochelys

Species: Pappochelys rosinae

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Morphological description

Pappochelys rosinae was a small stem-turtle, with an estimated adult total length of less than about 20 cm. Its overall body outline had already shifted clearly toward the turtle body plan, but it still lacked a true carapace and plastron. One of its principal features was the marked broadening of the trunk ribs, which were T-shaped in cross-section. On the ventral side, it possessed a full set of unusually robust gastralia that were closely packed and formed a basket-like ventral structure, but these elements had not yet fused into a true plastron.

The skull was proportionately small, with a short, pointed snout, large orbits, and a triangular outline in dorsal view. The temporal region already showed an upper temporal fenestra and a ventrally open lower temporal fenestra, indicating diapsid affinities rather than membership in a primitively anapsid reptilian group. Teeth were present in both the upper and lower jaws. The premaxilla bore about four teeth, the maxilla up to about 17, and the dentary at least about 29. Most of the teeth were small and peg-like, and the anterior dentary teeth were slightly inclined posteriorly.

The cervical vertebrae were elongate and low, the dorsal vertebrae had elongate centra, and the neural spines were low. The number of trunk vertebrae did not exceed nine. The tail was very long and slender, whip-like in form, and made up more than half of the total body length. The shoulder girdle and pelvis already displayed many features approaching those of early turtles, including a tall, straight dorsal process of the scapula, a plate-like coracoid, an ilium with a long postacetabular process, and a pubis with a thyroid foramen and a distinct lateral process. The limbs were not modified into paddle-like structures. The forelimbs and hind limbs were similar in proportion, the limbs were short, and the claws were strong and elongate.

Holotype specimen SMNS 91360 of Pappochelys rosinae（圖片來源：Rainer Schoch，採用 CC BY-SA 4.0 授權）

Skeleton of Pappochelys rosinae（圖片來源：Schoch RR et al. (2019)，採用 CC BY 4.0 授權）

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Etymology

Generic name Pappochelys is derived from the Greek pappos, meaning "grandfather", and chelys, meaning "turtle", directly referring to its position on the turtle stem.

Specific name rosinae honours Isabell Rosin, the preparator who worked on and prepared the key specimens of this taxon.

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Biological description

The discovery of Pappochelys rosinae also helped clarify a question that had long been debated: were turtles truly anapsid reptiles? Many older textbooks treated turtles as representatives of Anapsida, as though they had retained the most ancient reptilian skull condition. However, fossils such as Pappochelys rosinae disrupted that simplified interpretation. Its skull clearly retained upper and lower temporal openings, showing that the ancestors of turtles did not begin as forms entirely lacking temporal fenestrae. Instead, the temporal region was more likely reduced or remodelled later in turtle evolution. This is why turtles are now placed among diapsid reptiles.

Pappochelys rosinae lived about 240 million years ago during the Middle Triassic, and its fossils were recovered from lacustrine deposits at Vellberg in Germany. At first glance, this setting makes it easy to interpret the animal as aquatic or semi-aquatic. In addition, its thickened ribs and gastralia were at one time taken as possible ballast-like adaptations for buoyancy control in water. Later histological study, however, showed that Pappochelys rosinae had clear osteosclerosis, meaning that the cortical bone was thick, the medullary cavity was small, and the skeleton as a whole was relatively heavy and compact. This condition does not match the typical pattern seen in strongly aquatic reptiles. Many highly aquatic reptiles show extensive internal spongy bone development, flattening of the limbs, lateral compression of the tail, and other coordinated adaptations for swimming. Pappochelys rosinae lacked these clear features, and its limbs remained short and robust.

Instead, a number of its traits are more consistent with an animal adapted for digging in soil or similar substrate, and it also possessed long, strong claws. Even so, the relevant studies did not define it as a strictly terrestrial burrower. The more cautious interpretation is that Pappochelys rosinae probably also frequented lakeshores or shallow-water margins, representing a mode of life centred on land but also associated with wetland or shoreline habitats.

The fossils of Pappochelys rosinae come from a dark grey lacustrine claystone layer in which they were preserved together with fishes, temnospondyl amphibians, and various terrestrial diapsid reptiles. This indicates an ecosystem in which a lake and the surrounding land formed an interconnected environmental mosaic.

Some fossils of Pappochelys rosinae preserve traces of predation and digestion, suggesting that at least part of the material may represent the remains of individuals that had been eaten by large local predators and later regurgitated or passed as coprolitic material.

（Author: Shui-Ye You）

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References

1. Schoch RR and Sues HD. (2015). A Middle Triassic stem-turtle and the evolution of the turtle body plan. Nature.

2. Schoch RR and Sues HD. (2019). The origin of the turtle body plan: evidence from fossils and embryos. Palaeontology.

3. Schoch RR et al. (2019). Microanatomy of the stem-turtle Pappochelys rosinae indicates a predominantly fossorial mode of life and clarifies early steps in the evolution of the shell. Scientific Reports.

49.25, 9.88

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