A Triassic plesiosaurian skeleton and bone histology inform on evolution of a unique body plan

Rhaetian holotype LWL-MFN P 64047 preserves plesiosaur anatomy and highly vascularized long-bone tissue; its basal pliosaurid placement and implications for growth rate or metabolism remain topology- and histology-based inferences.

The early Carnian Bobosaurus holotype combines nothosaurian and plesiosaurian features and was interpreted near the plesiosaur lineage, but it is an older uncertain candidate rather than diagnostic Plesiosauria; the articulated Rhaetian Rhaeticosaurus provides the older secure comparison used here.

Plesiosauria is represented securely in the Rhaetian by the diagnostic articulated Rhaeticosaurus skeleton; older Bobosaurus remains an uncertain near-plesiosaur candidate rather than accepted diagnostic Plesiosauria.

The claim-linked older bound for diagnostic Plesiosauria is the Rhaetian 205.7–201.4 Ma interval; the younger 66 Ma bound remains a broad composite and Bobosaurus does not pull the range into the Carnian.

The oldest diagnostic occurrence highlighted here is from the Exter Formation at Bonenburg, Germany; older candidate evidence from northeastern Italy is shown separately.

Plesiosauria has a distinctive four-flipper body plan; Rhaeticosaurus preserves diagnostic skeletal characters, whereas Bobosaurus presents a less decisive mosaic.

Plesiosaurs were marine swimmers, but inferred physiology and four-flipper hydrodynamics remain separate from the directly preserved Rhaeticosaurus skeleton and histology.

Rhaeticosaurus is anchored by holotype LWL-MFN P 64047 from a specific Rhaetian Exter Formation bed at Bonenburg; this secure specimen does not establish an exact global plesiosaur or genus origin.

Rhaeticosaurus mertensi is a new small-bodied plesiosaurian recovered within Plesiosauria as a basal Pliosauridae member in the published 35-taxon, 270-character analysis; that placement is matrix-dependent rather than direct ancestry.

The Rhaeticosaurus holotype comes from clay pit #3 of the Lücking brick company, 1 km north of Bonenburg in Warburg, North Rhine-Westphalia, Germany, in Rhaetian dark marine mudstones of the Exter Formation; this is a locality record, not a complete distribution.

Holotype LWL-MFN P 64047 is a partial articulated subadult skeleton preserving the occiput, lower jaw, vertebral column, girdles and left limbs; its diagnosis includes ventrally concave V-shaped cervical neurocentral sutures and foreshortened zeugopodials.

Rhaeticosaurus preserves four similar pointed flippers and a short tail consistent with plesiosaurian hydrofoil and paraxial-locomotion comparisons, while histological growth and physiological implications remain interpretations rather than directly measured behaviour or metabolism.