The story of turtle evolution has long been a puzzle for paleontologists, with significant gaps in the fossil record. A discovery in southwestern China has provided a piece of that puzzle, challenging long-held ideas about how turtles came to be. This finding revealed an ancient, extinct relative of modern turtles that lived during the Late Triassic period. The creature, named Eorhynchochelys sinensis, offers a new window into the deep past, prompting a re-evaluation of the steps that led to the familiar shelled reptiles we know today.
Anatomy of an Ancient Turtle Relative
Eorhynchochelys sinensis presented a combination of features that surprised scientists. Its most notable characteristic was a distinct, toothless beak, structurally similar to that of modern turtles. The name Eorhynchochelys sinensis translates to “first turtle with a beak from China,” highlighting the importance of this feature.
Contrasting with its modern-looking beak was the complete absence of a carapace, the upper, domed part of a turtle’s shell. However, the fossil did show the presence of a fully formed plastron, the flat, bony plate that protects the underside of a turtle. The animal’s body was wide and disc-shaped, measuring over six feet in length, and it possessed a long, whip-like tail.
The ribs of Eorhynchochelys were broad and flat, unlike the slender ribs of most other reptiles, representing a preliminary stage of shell formation. These flattened ribs did not fuse together to form a solid carapace, leaving its back vulnerable. It also had large, clawed feet, which may have been used for digging in mud or navigating its environment.
The World of Eorhynchochelys sinensis
Fossils of Eorhynchochelys sinensis were unearthed from deposits dating to the Late Triassic period, approximately 228 million years ago. The geological evidence from the site in Guizhou province, China, indicates that the region was once a shallow marine environment. This suggests that Eorhynchochelys was an aquatic creature, spending much of its life in coastal waters.
The environment would have been a warm, subtropical sea, teeming with a variety of marine life. Scientists believe it foraged for food in the soft sediment of the seafloor. Its powerful, clawed limbs would have been suited for maneuvering in these muddy, near-shore habitats, possibly digging for invertebrates like mollusks and shrimp.
Rethinking Turtle Evolution
The discovery of Eorhynchochelys changed the scientific understanding of turtle origins. Its existence provides strong evidence that a beak evolved before a complete shell, a sequence that counters previous theories. For years, another early turtle relative, Odontochelys, which had a partial shell but retained teeth, suggested that the shell came first. Eorhynchochelys demonstrates that the evolutionary path was not a simple, linear progression of traits.
The presence of a well-developed plastron without a carapace supports the “belly-first” model of shell evolution. This theory posits that the turtle shell began as a ventral shield, likely for protection from predators attacking from below in an aquatic environment. The broadening of the ribs seen in Eorhynchochelys represents an intermediate step toward the formation of the carapace, which would appear in later turtle ancestors.
Furthermore, the skull of Eorhynchochelys helped to resolve a long-standing debate about where turtles fit on the reptile family tree. Its skull possessed two openings behind the eyes, a feature characteristic of diapsid reptiles, which include lizards, snakes, and crocodiles. Modern turtles have anapsid skulls, meaning they lack these openings, which had previously made their ancestry difficult to place.
The diapsid skull of Eorhynchochelys confirms that turtles are not a primitive lineage of reptiles but are instead related to other advanced diapsids. It indicates that the solid skulls of modern turtles are the result of these openings closing over millions of years. This discovery provided a clear anatomical link, cementing the place of turtles within the diapsid branch of the reptile evolutionary tree.