An Ancient Predator Used Its Small Tail to Float Motionless in the Water

Some 240 million years ago, during the time of aquatic reptilian predators, one species used its short, flat tail to balance and float motionless as it hunted for prey.

Paleontologists from Chinese Academy of Scientists, together with the Canadian Museum of Nature, have analyzed two skeletons embedded in a thin layer of limestone from two quarries in southwest China. Initially, they identified both skeletons as nothosaurs - Triassic marine reptiles characterized by its small head, flipper-like appendages, fangs, a long neck, and a longer tail widely believed used for propulsion. However, finding short and flat tails led them to identify these as a new species.

Researchers detail the discovery in the Journal of Vertebrate Paleontology.

Short-Tailed Lizard

"Our analysis of two well-preserved skeletons reveals a reptile with a broad, pachyostotic body (denser boned) and a very short, flattened tail," explained Dr. Qing-Hua Shang, an author of the study from the Beijing-based Chinese Academy of Sciences. Shang added that a long tail can be used to "flick" across the water and generate thrust for movement. However, she proposed that the new species is more fitted to living near the bottom of shallow seas, with its short and flat tail used as a "float," keeping its balance and conserving its energy.

Researchers named the new species Brevicaudosaurus jiyangshanensis: The Latin words "brevi" (short), "caudo" (tail), and the Greek "sauros" for "lizard." Its species name was taken from the location where the skeletons were discovered, in Jiyangshan quarry. The skeletons measured less than 60 centimeters long.

Further analysis of the skeleton offers additional insight into how it lived. The bones on its forelimbs are more developed compared to the hind limbs, suggesting that these forelegs helped them swim across the water. Additionally, researchers found its front bones to be shorter compared to other species, which severely limits the power it can produce.

With a Float and a Ballast

Its skeleton, especially the vertebrae and the ribs, are dense and thick, further supporting the notion that the reptile was stout and stocky in its life. While this body type does not help its swimming speed, it was invaluable in its underwater stability. This trade-off between speed and stability is thanks to the new reptile's thick bones with a relatively high mass, serving as a ballast.

The presence of dense bones, called pachyostosis, allowed the ancient lizard to steadily buoy itself in shallow waters. Dense bones, paired with the short flat tail, might have allowed the predator to float steady and motionless in the water. Its buoyancy and stocky build would have also allowed it to walk on the seabed and scout for prey.

Additionally, the presence of its equally dense ribs suggests that the reptile also had large lungs. With the lack of notable support for its estimated body weight, scientists believe that nothosaurs were mostly marine animals, coming only to the surface to breathe.

Another discovery in this new species is that they also had "stapes" - the bar-shaped bone found in the middle ear used for reception and transmission of sounds. It was later omitted in the later evolution of nothosaurs or other marine reptiles.

"Perhaps this small, slow-swimming marine reptile had to be vigilante for large predators as it floated in the shallows, as well as being a predator itself," said Dr. Xiao-Chun Wu, co-author of the study from the Canadian Museum of Nature.