Pinniped lizards quickly became long-necked

In a relatively short period of time, the early representatives of the pinniped lizards developed a long neck with many vertebrae. © Qi-Ling Liu, University of Bristol

It is the trademark of the famous marine dinosaurs of the Mesozoic period: how the long neck developed in the ancestors of the plesiosaurs around 250 million years ago is now shed light on how the long neck of the plesiosaur ancestors developed. Comparisons of its characteristics with those of later species show that the necks grew very quickly in evolutionary terms: in just around five million years, their length had doubled due to the addition of vertebrae, the paleontologists report.

A long neck can be practical, as the evolutionary developments in different animals illustrate. The most striking examples include giraffes and some bird species. But there were also famous long-necks in the earlier history of development: while the sauropods nibbled on the treetops on land, the long-necked plesiosaurs skillfully hunted fish in the seas until the end of the Cretaceous period. The development of these creatures dates back to the early Triassic period. But so far little is known about how the neck developed in the early phase of evolution of the pinniped lizards (Sauropterygia).

A still short-necked representative

The discovery of an early representative of this group of animals from a fossil deposit in the Chinese province of Hubei now sheds light on this question. “We were lucky to find two complete skeletons of this creature,” says first author Qi-Ling Liu from the China University of Geosciences in Wuhan. The Chinese-British research team named the fossil species Chusaurus xiangensis. Through radiometric dating, the relics could be dated to around 248 million years old and thus to the early Triassic period. Basically, it initially became clear: “Although Chusaurus was only less than half a meter long, it was still an early representative of the marine reptiles from the Sauropterygia group,” said the paleontologist.

As further investigations showed, Chusaurus can already be assigned to the pachypleurosaurs, from which the later forms of the plesiosaurs emerged. “It belonged to this group of small sea predators that were very important in the Triassic. “At first I wasn’t sure if it was a pachypleurosaur because the neck seemed so short,” says Liu. However, this is precisely the particularly interesting aspect of this creature, as the team reports.

Rapid long-neck development

It became apparent that the neck had already lengthened: in Chusaurus it was already half the length of the animal's torso. He was already equipped with 17 cervical vertebrae. As the comparisons with later representatives of the pachypleurosaurs made clear, the neck lengthening continued quickly from an evolutionary perspective: in just five million years, the neck length in this group of finned lizards had already reached an average of 80 percent of the trunk length, the study results show. “The pachypleurosaurs lengthened their necks by adding new vertebrae,” says co-author Cheng Long from the Wuhan Center of China Geological Survey. Specifically, there was an increase to 25 cervical vertebrae, the paleontologists report.

“Not all animals have extended their necks in this way,” emphasizes co-author Tom Stubbs from the University of Bristol. “Giraffes, for example, have retained the seven cervical vertebrae common to vertebrates, but each of them has become very long, allowing these animals to reach high into the trees. In flamingos, for example, there is a combination of both aspects: They have added additional vertebrae, but each one has also become longer,” says the scientist. As the study makes clear, the pinniped lizards apparently relied on quantity. This ultimately culminated in the large plesiosaurs of the Jurassic and Cretaceous periods. Some representatives of the elasmosaurs even have 72 cervical vertebrae. Her snake-like neck was five times as long as her torso.

As far as their ancestors in the Triassic period are concerned, there is now evidence of a rapid, but still limited, increase in neck length: “Our study shows that the neck length of pachypleurosaurs doubled in five million years. However, further development then slowed down,” says co-author Ben Moon from the University of Bristol. They probably reached a neck length that was perfect for their way of life at the time and was about the same length as their torso. “We believe that as small predators they fed mainly on shrimp and small fish. A relatively long neck helped them. “Any longer would probably have caused costs that ultimately wouldn’t have been worth it,” says Moon. This was obviously different with the diet and prey of their later descendants and so Elasmosaurus and Co. eventually developed their almost snake-like necks.

Source: University of Bristol, specialist article: BMC Ecology and Evolution, doi: 10.1186/s12862-023-02150-w

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