Saturday, December 16, 2017

[Paleontology • 2017] Rhaeticosaurus mertensi • A Triassic Plesiosaurian Skeleton and Bone Histology Inform on Evolution of A Unique Body Plan

Rhaeticosaurus mertensi 
Wintrich, Hayashi, Houssaye, Nakajima & Sander, 2017

Secondary marine adaptation is a major pattern in amniote evolution, accompanied by specific bone histological adaptations. In the aftermath of the end-Permian extinction, diverse marine reptiles evolved early in the Triassic. Plesiosauria is the most diverse and one of the longest-lived clades of marine reptiles, but its bone histology is least known among the major marine amniote clades. Plesiosaurians had a unique and puzzling body plan, sporting four evenly shaped pointed flippers and (in most clades) a small head on a long, stiffened neck. The flippers were used as hydrofoils in underwater flight. A wide temporal, morphological, and morphometric gap separates plesiosaurians from their closest relatives (basal pistosaurs, Bobosaurus). For nearly two centuries, plesiosaurians were thought to appear suddenly in the earliest Jurassic after the end-Triassic extinctions. We describe the first Triassic plesiosaurian, from the Rhaetian of Germany, and compare its long bone histology to that of later plesiosaurians sampled for this study. The new taxon is recovered as a basal member of the Pliosauridae, revealing that diversification of plesiosaurians was a Triassic event and that several lineages must have crossed into the Jurassic. Plesiosaurian histology is strikingly uniform and different from stem sauropterygians. Histology suggests the concurrent evolution of fast growth and an elevated metabolic rate as an adaptation to cruising and efficient foraging in the open sea. The new specimen corroborates the hypothesis that open ocean life of plesiosaurians facilitated their survival of the end-Triassic extinctions.

Systematic paleontology
Reptilia Linnaeus, 1758
Diapsida Osborn, 1903

Plesiosauria de Blainville, 1835

Phylogenetic definition: We offer the following apomorphy-based definition of Plesiosauria: Sauropterygians with a short, wide trunk–bearing four flippers of even structure and subequal size, the flippers consisting of long, straight propodials combined with very short and dorsoventrally flattened zeugopodials.

Diagnosis: Plesiosauria is diagnosed (see Materials and Methods) by two unique and unambiguous synapomorphies: tooth enamel surface, striations present (character, 106; state, 0; see comment in table S2); orientation of cervical zygapophyses, dorsomedially facing (128, 1). An unambiguous but not unique synapomorphy is as follows: dorsal half of ilium, subequal anterior and posterior expansion (174, 0).

Rhaeticosaurus mertensi gen. et sp. nov.

Etymology: The genus name is based on rhaeticus, latinized adjective meaning “from the Rhaetian stage,” and sauros (Greek), meaning lizard or saurian. The specific epithet honors the discoverer of the holotype, Michael Mertens of Schwaney, Westphalia, Germany.

Holotype specimen: LWL-Museum für Naturkunde (Münster, Germany), LWL-MFN P 64047.

Locality and horizon: Clay pit #3 of Lücking brick company, 1 km north of the village of Bonenburg, city of Warburg, North Rhine-Westphalia, Germany (Fig. 1A). The specimen derives from Rhaetian dark marine mudstones of the Exter Formation, 21 m in the section below the Triassic-Jurassic boundary and about 3.5 m below a bonebed containing a vertebrate fauna of Rhaetian age.

Diagnosis: Small-bodied plesiosaurian with an estimated total length of 237 cm (Fig. 2, A and B). The new taxon has two autapomorphies (Fig. 2C): a modified V-shaped neurocentral suture in the anterior and middle cervical vertebrae. In Rhaeticosaurus, the sides of the “V” are ventrally concave, and the tip of the “V” almost reaches the ventral margin of the centrum. In other plesiosaurians with a V-shaped neurocentral suture, the sides of the “V” are straight, and the tip only extends to the middle of the centrum. The second autapomorphy is greatly foreshortened zeugopodials with a humerus/radius ratio of 3.8 and a femur/tibia ratio of 4.3 (Fig. 2, B, D, and E, and table S4). In addition, there are 10 unambiguous but not unique synapomorphies (tables S2 and S3).

Phylogenetic relationships: To assess the systematic position of the Triassic plesiosaurian skeleton, we coded it for a recently published phylogenetic data matrix aimed at clarifying plesiosaurian interrelationships (data file S1) (4). Rhaeticosaurus was found to be nested within Plesiosauria as a basal member of the Pliosauridae, with Anningasaura as the most basal plesiosaurian (Fig. 3A). As a consequence, six nodes in the cladogram are of Triassic age, indicating pre-Jurassic diversification of plesiosaurians into their major clades (Fig. 3A).

This is the skeleton of Rhaeticosaurus on exhibit at the LWL-Museum für Naturkunde in Münster (Germany). The disintegrated skull and neck can be seen on the left.
Photo: Georg Oleschinski

  Tanja Wintrich, Shoji Hayashi, Alexandra Houssaye, Yasuhisa Nakajima and P. Martin Sander. 2017. A Triassic Plesiosaurian Skeleton and Bone Histology Inform on Evolution of A Unique Body Plan. Science Advances. DOI: 10.1126/sciadv.1701144

The Oldest Plesiosaur Was a Strong Swimmer via @unibonn @EurekAlert

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