The evolutionary history of polycotylid plesiosaurians

Area of Science:

• Paleontology
• Marine Reptile Evolution
• Cretaceous Ecosystems

Background:

• Polycotylidae plesiosaurs radiated significantly in the Late Cretaceous, but their evolutionary history remains unclear.
• The enigmatic taxon *Thililua longicollis* exhibits unusual cranial features and neck length, complicating polycotylid phylogeny.

Purpose of the Study:

• To re-examine the holotype of *Thililua longicollis* and clarify its cranial anatomy.
• To establish the phylogenetic relationships of *Thililua* within the Polycotylidae.
• To investigate the evolutionary patterns of polycotylid radiation and disparity.

Main Methods:

• Reinterpretation of the holotype specimen of *Thililua longicollis*.
• Detailed analysis of cranial and postcranial anatomy.
• Phylogenetic analyses using a new cladistic dataset based on direct observation.
• Analysis of polycotylid Bauplan (body plan) to assess evolutionary disparity.

Main Results:

• *Thililua longicollis* possesses a distinct jugal, premaxilla-parietal contact, and carinated teeth.
• Phylogenetic analyses place *Thililua* and *Mauriciosaurus* at the base of the early Late Cretaceous polycotyline radiation.
• Polycotylid radiation showed an early burst of disparity in neck length during the Cenomanian-Turonian, followed by constraint.
• Two clades, Occultonectia and Polycotylinae, persisted until the Maastrichtian with limited diversity.

Conclusions:

• The reinterpretation of *Thililua longicollis* refines understanding of early polycotylid evolution.
• Polycotylid disparity followed a pattern of early experimentation and late constraint, not directly linked to extinctions of other marine reptile groups.
• The long-term survival of specific polycotylid clades indicates evolutionary resilience despite a later reduction in diversity.