Did these ancient marine reptiles evolve short necks twice to conquer the prehistoric seas?

Nature · Published 3 October 2026 Swipe more like this →

In short: Polycotylids were once thought to be close relatives of pliosaurs due to their short necks and elongated skulls. However, modern research suggests they are actually derived plesiosauroids that independently evolved these features. This striking example of convergent evolution remains a subject of intense debate among paleontologists studying their complex history.

Polycotylidae represents a distinct group of marine reptiles that emerged during the Early Cretaceous. While they are now classified as plesiosauroids—a lineage typically characterized by long necks—polycotylids secondarily developed shorter necks. This physical transformation mirrors the body plan of pliosaurids, leading many early researchers to group these two clades together. Today, most experts recognize this as a classic instance of convergent evolution, where unrelated organisms develop similar traits to adapt to comparable environmental pressures.

The evolutionary path of these creatures is marked by a period of rapid experimentation. During the Cenomanian–Turonian interval, polycotylids displayed significant variety in their neck lengths. Contrary to some theories, this burst of anatomical diversity was not a direct result of filling ecological niches left vacant by the extinction of ichthyosaurs or pliosaurids. Instead, this phase of high disparity was followed by a period of relative constraint, where the range of body shapes narrowed significantly as the group matured.

Understanding the family tree of polycotylids is complicated by enigmatic taxa like Thililua longicollis. Found in Middle Turonian deposits in Morocco, this species possesses an unusually long neck and a unique cranial structure, including a reduced jugal bone. Because of these bizarre features, its exact position in the evolutionary hierarchy has shifted repeatedly across different studies. Some analyses place it as a basal member of the group, while others suggest it sits outside the clade entirely, highlighting the ongoing difficulty in mapping the precise relationships of these ancient predators.

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