Evolutionary Whiplash: Plesiosaurs doubled their neck length by acquiring new vertebrae

 

Evolutionary Whiplash: Plesiosaurs doubled their neck length by acquiring new vertebrae

Plesiosaurs, renowned for their distinctive long necks, surprisingly evolved these iconic features in a relatively short timeframe, according to recent research. The study introduces a novel ancestor, Chusaurus xiangensis, shedding light on the rapid development of plesiosaur necks approximately 250 million years ago.

The findings, recently published in the journal BMC Ecology and Evolution, resulted from collaborative efforts between scientists in China and the UK. They reveal that a species known as pachypleurosaur achieved neck elongation primarily by adding new vertebrae. While this species had 25 vertebrae, some Late Cretaceous plesiosaurs like Elasmosaurus boasted up to 72 vertebrae, with necks five times longer than their trunks.

Origins and Remarkable Discoveries

Plesiosaurs first emerged during the Early Triassic, a mere four million years after the catastrophic end-Permian mass extinction, which wiped out nearly 90% of Earth's species. This period was marked by swift transformations in the wake of this devastating event.

In this study, researchers unveil a new short-necked plesiosaur ancestor, Chusaurus xiangensis, hailing from the Early Triassic in Hubei Province, China. Although its neck had begun lengthening, it measured only half the length of its body trunk, in stark contrast to its later relatives, which boasted necks that were 80% or more of their trunk length.

Qi-Ling Liu, who led the project at China University of Geosciences in Wuhan, expressed excitement about the discovery: “We were lucky enough to find two complete skeletons of this new beast. 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."

Context and Comparative Insights

Dr. Li Tian, also from China University of Geosciences Wuhan and a project co-supervisor, explained the significance of the fossil findings: “The fossils come from the Nanzhang-Yuan'an Fauna of Hubei. 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 emphasized the rapid evolution of marine reptiles during the Early Triassic: “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."

Professor Cheng Long, from the Wuhan Center of China Geological Survey, noted the unique method of neck elongation in pachypleurosaurs: “The pachypleurosaur lengthened their necks mainly by adding new vertebrae. Normally, vertebrates like reptiles and mammals (and us) have seven neck vertebrae Chusaurus already had 17, while 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."

Diverse Evolutionary Strategies

Dr. Tom Stubbs of the Open University UK highlighted the variation in how long-necked animals achieve their distinctive features: “Not all long-necked animals do it in the same way. Giraffes, for example, keep the standard seven neck vertebrae, but each one It 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. "

Dr. Ben Moon, also from the University of Bristol, concluded, “Our study shows that pachypleurosaurs doubled the lengths of their necks in five million years, and the rate of increase then slowed down. They had apparently 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.