Abstract
Polycotylidae is a clade of plesiosaurians that appeared during the Early Cretaceous and became speciose and abundant early in the Late Cretaceous. However, this radiation is poorly understood. Thililua longicollis from the Middle Turonian of Morocco is an enigmatic taxon possessing an atypically long neck and, as originally reported, a series of unusual cranial features that cause unstable phylogenetic relationships for polycotylids. We reinterpret the holotype specimen of Thililua longicollis and clarify its cranial anatomy. Thililua longicollis possesses an extensive, foramina-bearing jugal, a premaxilla–parietal contact and carinated teeth. Phylogenetic analyses of a new cladistic dataset based on first-hand observation of most polycotylids recover Thililua and Mauriciosaurus as successive lineages at the base of the earliest Late Cretaceous polycotyline radiation. A new dataset summarizing the Bauplan of polycotylids reveals that their radiation produced an early burst of disparity during the Cenomanian–Turonian interval, with marked plasticity in relative neck length, but this did not arise as an ecological release following the extinction of ichthyosaurs and pliosaurids. This disparity vanished during and after the Turonian, which is consistent with a model of ‘early experimentation/late constraint’. Two polycotylid clades, Occultonectia clade nov. and Polycotylinae, survived up to the Maastrichtian, but with low diversity.
Highlights
Polycotylidae is a peculiar clade of xenopsarian plesiosaurs whose members typically exhibit short necks and elongated skulls [1,2,3,4,5]
We have problems resolving the phylogenetic position of Manemergus anguirostris, but find it likely that it is closely related to Thililua longicollis and possibly congeneric, if all of their shared features were added as new characters in the phylogenetic dataset
Thililua longicollis lacks a fontanelle between the parietals and the squamosals, unlike in M. anguirostris
Summary
Polycotylidae is a peculiar clade of xenopsarian plesiosaurs whose members typically exhibit short necks and elongated skulls [1,2,3,4,5]. Thililua longicollis is an enigmatic taxon that so far was only described in brief [20] and deviates from the general body proportions of other polycotylids in having a long neck, composed of 30 elongated cervical vertebrae bearing lateral ridges, similar to those of long-necked plesiosaurs from several other clades (Microcleididae, Cryptoclididae and Elasmosauridae). This taxon may provide critical information on the early diversification of polycotylids, because its phylogenetic position has varied intensely since its creation and because of its highly unusual anatomy. Two phylogenetically distant clades of polycotylids survived to at least the Late Campanian, severely diminished in diversity and disparity
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