Sunday, June 23, 2024

[Paleontology • 2024] Oldest southern Sauropterygian reveals early Marine Reptile Globalization

  Morphology and biogeographic context of GNS CD 540, the oldest Southern Hemisphere sauropterygian.

in Kear, Roberts, Young, Terezow, Mantle, Barros & Hurum, 2024. 
Artwork: Johan Egerkrans

Sauropterygians were the stratigraphically longest-ranging clade of Mesozoic marine reptiles with a global fossil record spanning ∼180 million years1. However, their early evolution has only been known from what is now the Northern Hemisphere, extending across the northern and trans-equatorial western margins of the Tethys paleo-ocean after the late-Early Triassic (late Olenekian, ∼248.8 million years [Ma] ago), and via possible trans-Arctic migration to the Eastern Panthalassa super-ocean prior to the earliest Middle Triassic (Olenekian–earliest Anisian, ∼247 Ma). Here, we describe the geologically oldest sea-going reptile from the Southern Hemisphere — a nothosaur (basal sauropterygian) from the Middle Triassic (Anisian, after ∼246 Ma) of New Zealand. Time-scaled ancestral range estimations thus reveal an unexpected circum-Gondwanan high-paleolatitude (>60° S7) dispersal from a northern Tethyan origination center. This coincides with the adaptive diversification of sauropterygians after the end-Permian mass extinction8 and suggests that rapid globalization accompanied their initial radiation in the earliest Mesozoic.

  Morphology and biogeographic context of the oldest Southern Hemisphere sauropterygian.
(A) mCT image of the GNS CD 540 dorsal vertebra in posterior view.
(B) Time-scaled Bayesian phylogeny (Figure S1G) of Nothosauroidea (silhouettes) with estimated ancestral ranges (pie charts), dispersal (orange circles) and vicariance (blue circles) events (Table S1). Node numbers indicate geographic ranges (red) and percent (>50%) support (black) for ancestral range estimations.
(C) Middle Triassic global map showing ancestral ranges (solid arrows) and possible dispersal routes (dashed arrows; modified from maps compiled by Colorado Plateau Geosystems Inc.
(D) Middle Triassic southern polar map with occurrence of GNS CD 540 (red star).
Anatomical abbreviations: as, centrum articular surface; le, laterally expanded neural arch contact; ns, neural spine; tp, transverse process; zg, zygantrum; zy, postzygapophysis. 
Geographic ranges: (1) Northeastern to Northwestern Tethys; (2) Northwestern Tethys to Eastern Panthalassa; (3) Northern Tethys to Southern Polar Panthalassa; (4) Northern Tethys to Southwestern Tethys.

Reconstruction of the New Zealand nothosaur. The oldest sea-going reptile from the Southern Hemisphere.
Artwork: Johan Egerkrans

 Benjamin P. Kear, Aubrey J. Roberts, George Young, Marianna Terezow, Daniel J. Mantle, Isaias Santos Barros and Jørn H. Hurum. 2024. Oldest southern Sauropterygian reveals early Marine Reptile Globalization. Current Biology. 34(12);  R562-R563. DOI: 10.1016/j.cub.2024.03.035