Paleontologists have identified a new species of plesiosaur ancestor based on two new specimens from the Early Triassic Nanzhang-Yuan’an Fauna of China. The new species shows key features of its Middle Triassic relatives, but has a relatively short neck, measuring 0.48 of the trunk length, compared to over 0.8 from the Middle Triassic onwards. Comparative analysis shows that their neck elongation occurred over a five million period around 250 million years ago, probably driven by feeding pressure in a time of rapid re-establishment of new kinds of marine ecosystems.
Fast rates of plesiosaur evolution. Image credit: Qi-Ling Liu.
“The Early Triassic was a time of rapid evolution of life in the oceans, following the devastating end-Permian mass extinction, marked especially by the appearance of new animals and new modes of life,” said Dr. Qi-Ling Liu, a paleontologist at the China University of Geosciences in Wuhan, and colleagues.
“In particular, new benthic groups such as bivalves provided food for new predatory gastropods, malacostracans, echinoids and fishes.”
“These in turn provided food for durophagous fishes and marine reptiles and macropredatory reptiles that fed on fishes and other reptiles.”
“Long necks likely evolved as an adaptation to snapping rapidly at the faster swimming fishes of the new ecosystems or dipping for benthic prey in murky seabed sediments.”
“The long-necked plesiosaurian clades, first occurring in the latest Triassic, evolved from Early-Middle Triassic short-necked eosauropterygian predecessors.”
Chusaurus xiangensis lived in what is now China during the Early Triassic epoch, roughly 248 million years ago.
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,” Dr. Liu said.
“It’s small, less than 0.5 m long, but this was close to the ancestry of the important group of marine reptiles called Sauropterygia.”
“Our new reptile, Chusaurus xiangensis, is a pachypleurosaur, one of a group of small marine predators that were very important in the Triassic period.”
“I wasn’t sure at first whether it was a pachypleurosaur though because the neck seemed to be too short.”
“The fossils come from the Nanzhang-Yuan’an Fauna of Hubei,” added Dr. Li Tian, also from the China University of Geosciences Wuhan.
“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 radiometreic dates showing the fauna is dated at 248 million years ago.”
“The end-Permian mass extinction had been the biggest mass extinction of all time and only one in twenty species survived,” said Dr. Michael Benton, a researcher at the University of Bristol.
“The Early Triassic was a time of recovery and marine reptiles evolved very fast at that time, most of them predators on 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.”
“The pachypleurosaurs lengthened their necks mainly by adding new vertebrae,” said Professor Cheng Long, a paleontologist at the Wuhan Centre of China Geological Survey.
“Normally, vertebrates like reptiles and mammals (and us) have seven neck vertebrae. Chusaurus xiangensis already had 17, whereas later pachypleurosaurs 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.”
“Not all long-necked animals do it in the same way,” said Dr. Tom Stubbs, a paleontologist at the Open University.
“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, a paleontologist at 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 stabilised at a length just equal to the length of the trunk.”
The discovery is described in a paper published August 31, 2023 in the journal BMC Ecology and Evolution.
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QL. Liu et al. 2023. Rapid neck elongation in Sauropterygia (Reptilia: Diapsida) revealed by a new basal pachypleurosaur from the Lower Triassic of China. BMC Ecol Evo 23, 44; doi: 10.1186/s12862-023-02150-w
