Plesiosaurs developed their distinctive long necks rapidly, evolving within a span of five million years, as revealed by a recent study introducing a new ancestor, Chusaurus xiangensis.
Plesiosaurs developed their iconic long necks in a remarkably short span of time, research indicates.
These elongated necks, believed to have been utilized to chase fast-moving fish, evolved quickly over a five-million-year period approximately 250 million years ago.
The findings, published recently in the journal BMC Ecology and Evolution, and carried out by scientists in China and the UK, show that a species known as pachypleurosaur lengthened their necks mainly by adding new vertebrae. This species had 25 vertebrae, while some Late Cretaceous plesiosaurs such as Elasmosaurus had as many as 72, and its neck was five times the length of its trunk.
Origins and New Discoveries
These creatures first appeared in the Early Triassic, a mere four million years post the end-Permian mass extinction, which led to the disappearance of nearly 90% of Earth’s species. This period was characterized by swift transformations following this catastrophic event.
In the study, the researchers describe a new, short-necked plesiosaur ancestor called Chusaurus xiangensis from the Early Triassic of Hubei Province, China. Its neck has begun to lengthen, but it is only half the length of the trunk of its body compared to 80% or higher in its later relatives.
“We were lucky enough to find two complete skeletons of this new beast,” said Qi-Ling Liu from the China University of Geosciences in Wuhan, who led the project. “It’s small, less than half a meter long, but this was close to the ancestry of the important group of marine reptiles called Sauropterygia.
“Our new reptile, Chusaurus, is a pachypleurosaur, one of a group of small marine predators that were very important in the Triassic. I wasn’t sure at first whether it was a pachypleurosaur though because the neck seemed to be too short.”
Context and Comparisons
“The fossils come from the Nanzhang-Yuan’an Fauna of Hubei,” said Dr. Li Tian, also of China University of Geosciences Wuhan, who co-supervised the project. “This has been very heavily studied in recent years as one of the oldest assemblages of marine reptiles from the Triassic. We have good quality radiometric dates showing the fauna is dated at 248 million years ago.”
Collaborator Professor Michael Benton of the University of Bristol’s School of Earth Sciences said: “The end-Permian mass extinction had been the biggest mass extinction of all time and only one in twenty species survived.
“The Early Triassic was a time of recovery and marine reptiles evolved very fast at that time, most of them predators of the shrimps, fishes, and other sea creatures. They had originated right after the extinction, so we know their rates of change were extremely rapid in the new world after the crisis.”
“Normally, vertebrates like reptiles and mammals (and us) have seven neck vertebrae. Chusaurus already had 17, whereas later pachypleurosaur had 25. Some Late Cretaceous plesiosaurs such as Elasmosaurus even had 72, and its neck was five times the length of its trunk. With so many vertebrae, these long necks must have been super-snakey and they presumably whipped the neck around to grab fishy prey while keeping the body steady.”
Varied Evolutionary Tactics
Dr. Tom Stubbs of the Open University UK added: “Not all long-necked animals do it in the same way. Giraffes for example keep the standard seven neck vertebrae, but each one is very long, so they can reach high into the trees. Flamingos also have long necks so they can reach the water to feed, because of their long legs, and they have extra vertebrae, up to twenty, but each one is also long.”
“Our study shows that pachypleurosaurs doubled the lengths of their necks in five million years, and the rate of increase then slowed down,” added Dr. Ben Moon, also of the University of Bristol. “They had presumably reached some kind of perfect neck length for their mode of life.
“We think, as small predators, they were probably mainly feeding on shrimps and small fish, so their ability to sneak up on a small shoal, and then hover in the water, darting their head after the fast-swimming prey was a great survival tool. But there might have been additional costs in having a much longer neck, so it stabilized at a length just equal to the length of the trunk.”
Reference: “Rapid neck elongation in Sauropterygia (Reptilia: Diapsida) revealed by a new basal pachypleurosaur from the Lower Triassic of China” by Qi-Ling Liu, Long Cheng, Thomas L. Stubbs, Benjamin C. Moon, Michael J. Benton and Li Tian, 31 August 2023, BMC Ecology and Evolution.