The evolutionary history of polycotylid plesiosaurians

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.

A new plesiosaurian from the Jurassic–Cretaceous transitional interval of the Slottsmøya Member (Volgian), with insights into the cranial anatomy of cryptoclidids using computed tomography

“…However, along the remainder of their length they lack a firm midline contact and enclose a narrow and elongate slit-like opening, referred to here as the interfrontal vacuity. This vacuity is not homologous to the 'frontal foramen' observed near the anterior margin of the frontal in some polycotylids, which is not located along the midline (Carpenter, 1996;Fischer et al, 2018). However, this vacuity does bear some resemblance to the dorsomedian frontal foramen described in Brancasaurus brancai (Sachs, Hornung & Kear, 2016).…”

The macroevolutionary landscape of short-necked plesiosaurians

“…We gathered measurements for all pliosaurid and polycotylid species known to date with relevant cranial material (40 species; see Tables S1 , S2 in Supplementary Information and Supplementary Files). We chose to score the moderately long-necked polycotylid Thililua longicollis , because of its fairly derived phylogenetic position and possession of craniodental features otherwise associated with short-necked plesiosaurians such as an elongated rostrum and carinated teeth 11 . The measurements were then used to create generate 11 morphofunctional traits (8 craniodental, 3 postcranial) describing the body plan of the sampled species (Fig.…”

“…2 . Measurements were obtained via several sources: by hand (either measured on the specimens by a digital calliper to the nearest 0.1 mm, or on high-resolution laser scans, see below), from the literature, from colleagues, and from high-resolution photographs (see 11 , 19 for previous versions of this dataset). We used a Creaform HandySCAN 300 laser surface scanner to create high-precision three-dimensional models of nine important specimens (resolution from 0.3 to 0.5 mm; see Table S2 for details; all the 3D models are deposited in Morphosource: http://www.morphosource.org/Detail/ProjectDetail/Show/project_id/1018 ).…”

“…We obtained phylogenetic trees by analysing an updated version of the morphological data matrix of 11 (originating from Benson and Druckenmiller 25 ; see Supplementary Information n for the details of the modifications). We analysed the dataset in TNT v1.5 26 using the parsimony ratchet (ratchet: 200 iterations, drift and fuse activated, with 10 iterations each), within an implied weighting framework at k = 3.…”

Peer Review #1 of "A new plesiosaurian from the Jurassic–Cretaceous transitional interval of the Slottsmøya Member (Volgian), with insights into the cranial anatomy of cryptoclidids using computed tomography (v0.1)"

“…However, along the remainder of their length they lack a firm midline contact and enclose a narrow and elongate slit-like opening, referred to here as the interfrontal vacuity. This vacuity is not homologous to the "frontal foramen" observed near the anterior margin of the frontal in some polycotylids, which is not located along the midline (Carpenter, 1996;Fischer et al, 2018). However, this vacuity does bear some resemblance to the dorsomedian frontal foramen described in Brancasaurus brancai (Sachs, Hornung and Kear, 2016 The frontal-parietal suture is somewhat obscured in dorsal view due to gypsum mineralization and the presence of rugosities in the anterior portion of the parietal.…”