High diversity, low disparity and small body size in plesiosaurs (Reptilia, Sauropterygia) from the Triassic-Jurassic boundary - PubMed (original) (raw)

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High diversity, low disparity and small body size in plesiosaurs (Reptilia, Sauropterygia) from the Triassic-Jurassic boundary

Roger B J Benson et al. PLoS One. 2012.

Abstract

Invasion of the open ocean by tetrapods represents a major evolutionary transition that occurred independently in cetaceans, mosasauroids, chelonioids (sea turtles), ichthyosaurs and plesiosaurs. Plesiosaurian reptiles invaded pelagic ocean environments immediately following the Late Triassic extinctions. This diversification is recorded by three intensively-sampled European fossil faunas, spanning 20 million years (Ma). These provide an unparalleled opportunity to document changes in key macroevolutionary parameters associated with secondary adaptation to pelagic life in tetrapods. A comprehensive assessment focuses on the oldest fauna, from the Blue Lias Formation of Street, and nearby localities, in Somerset, UK (Earliest Jurassic: 200 Ma), identifying three new species representing two small-bodied rhomaleosaurids (Stratesaurus taylori gen et sp. nov.; Avalonnectes arturi gen. et sp. nov) and the most basal plesiosauroid, Eoplesiosaurus antiquior gen. et sp. nov. The initial radiation of plesiosaurs was characterised by high, but short-lived, diversity of an archaic clade, Rhomaleosauridae. Representatives of this initial radiation were replaced by derived, neoplesiosaurian plesiosaurs at small-medium body sizes during a more gradual accumulation of morphological disparity. This gradualistic modality suggests that adaptive radiations within tetrapod subclades are not always characterised by the initially high levels of disparity observed in the Paleozoic origins of major metazoan body plans, or in the origin of tetrapods. High rhomaleosaurid diversity immediately following the Triassic-Jurassic boundary supports the gradual model of Late Triassic extinctions, mostly predating the boundary itself. Increase in both maximum and minimum body length early in plesiosaurian history suggests a driven evolutionary trend. However, Maximum-likelihood models suggest only passive expansion into higher body size categories.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1

Figure 1. Holotype of Stratesaurus taylori (OUMNH J.10337).

A–B, skull in dorsal view, C–E, anterior cervical vertebrae in left lateral (C), ventral (D) and anterior (E) views, F, H–I, left ilium in dorsal (F), lateral (H) and posterior (I) views, G, ‘pectoral’ vertebra; in left lateral view. In line drawing (B), grey tone indicates damage. Abbreviations: bs, basisphenoid; exof, exoccipital facet of basioccipital; jug, jugal; fr, frontal; hy, hyoid; lpmx, left premaxilla; mx, maxilla; occ, occipital condyle; pal, palatine; par, parietal; po, postorbital; popr, posterolateral process of prezygapophysis; prz, prezygapophysis; pt, pterygoid; qua, quadrate; rmx, right maxilla; rpmx, right premaxilla; scler, sclerotic ring; sq, squamosal. Scale bars equal 50 mm (A–B, F, H–I) and 20 mm (C–E, G).

Figure 2

Figure 2. Holotype of Avalonnectes arturi (NHMUK 14550).

A–B, skull in dorsal view; C–E, postcranial skeleton; in left dorsolateral (C) and left lateral (D–E) views. In line drawings (B, E) dark grey tone indicates damage and light grey tone indicates the palate. Abbreviations: ca, caudal vertebra [number following indicates order in preserved series]; ce, cervical vertebra; d, dorsal vertebra; depr, depression; ecto, ectopterygoid; epip, epipterygoid; exp, expanded neural spine apex; fr, frontal; jug, jugal; l., left [followed by name of element]; mx, maxilla; p, ‘pectoral’ vertebra; par, parietal; pmx, premaxilla; po, postorbital; pofr, postfrontal; prfr, prefrontal; qua, quadrate; r., right [followed by name of element]; s, sacral vertebra; sq, squamosal; unexp, unexpanded neural spine apex. Scale bars equal 50 mm (A–B), 20 mm (C), and 200 mm (D–E).

Figure 3

Figure 3. Holotype of Eoplesiosaurus antiquior (TTNCM 8348) in right lateral view.

Image in A is a composite made from four photographs (divisions are indicated by black and white lines), with enlargement of anterior cervical vertebrae (B; magnified portion is enlarged x2.0 times). Gastralia are not shown in line drawing (C). Abbreviations: ca, caudal vertebra; ce, cervical vertebra [number following indicates order in preserved series]; chv, chevron; l., left [followed by name of element]; pro, lateral projection; prz, prezygapophysis; r., right [followed by name of element]; trp, transverse process. Scale bars equal 200 mm (A, C) and 50 mm (B).

Figure 4

Figure 4. Phylogeny of Lower Jurassic plesiosaurians.

Temporally-calibrated strict consensus of 42 MPTs recovered from our phylogenetic analysis. Triangular symbols represent non-neoplesiosaurian plesiosaurians (mainly rhomaleosaurids), squares represent pliosaurids and circles represent plesiosauroids. Unfilled shapes represent British taxa whereas grey-filled shapes represent German and French taxa. Key localities yielding abundant remains from four narrow horizons are indicated by grey bands, although contemporaneous specimen are known from other localities: A, Street, Somerset, UK (lowermost Hettangian); B, Lyme Regis and Charmouth, Dorset, UK (late Hettangian–Sinemurian); C, Holzmaden and vicinity, Baden-Württemberg, Germany (H. falciferum Chronozone; lower Hettangian); D, Yorkshire, UK (H. bifrons Chronozone; lower Hettangian). Dashed lines indicate polytomy at base of Pistosauria prior to deletion of Pistosaurus from the set of MPTs.

Figure 5

Figure 5. Early evolution of Plesiosauria.

Plots of A, phylogenetic diversity ; B, disparity (main pairwise dissimilarity and sum of variances of PCO axes with 95% confidence intervals [64]); C, body size, based on the proxy trunk length (in metres); D, body proportions, based on the proxy neck:trunk length ratio. In C–D, triangles represent non-neoplesiosaurian plesiosaurians (mainly rhomaleosaurids), squares represent pliosaurids and circles represent plesiosauroids; unfilled shapes represent earliest Hettangian taxa, grey-filled shapes indicate late Hettangian–Pliensbachian taxa, and black shapes indicated Toarcian taxa.

Figure 6

Figure 6. Early plesiosaurian morphospace.

First three principal coordinate axes of dissimilarity among Lower Jurassic plesiosaurians. A, PCo2 versus PCo1, B, PCo2 versus PCo3.

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