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| Franconiasaurus brevispinus Sachs, Eggmaier & Madzia, 2024 (artwork by Joschua Knüppe). |
Plesiosaurs are Mesozoic reptiles fully adapted to an aquatic lifestyle. Throughout their evolutionary history exceeding 140 million years plesiosaurs dispersed globally, achieved substantial diversity, occupied a variety of ecological niches, and experienced multiple faunal turnovers. Of those, the Early/Middle Jurassic transition event (∼175–171 Mya) has recently became of increased interest because it apparently profoundly affected all three major lineages of plesiosaurs. Once dominant Rhomaleosauridae started to vanish, while Pliosauridae and Plesiosauroidea diversified and gave rise to several clades that flourished for tens of millions of years. Here, we report exquisite, three-dimensionally preserved skeletons of a new plesiosaur from the Lower Jurassic of Germany. Franconiasaurus brevispinus gen. et sp. nov. lived during the late Toarcian (∼175 Mya), near the onset of the Early–Middle Jurassic turnover. Franconiasaurus displays an intriguing mixture of features, combining characters almost uniformly distributed among early plesiosaurs with those typically observed in later-diverging members of the clade. Phylogenetic analyses firmly place Franconiasaurus as the sister taxon to Cryptoclidia, bridging an evolutionary gap between early plesiosauroids, such as Plesiosaurus-like forms and microcleidids, and later-diverging representatives of the clade, such as cryptoclidids, leptocleidians, and elasmosaurids.
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| Diagnostic skeletal elements of the type specimen of Franconiasaurus brevispinus gen. et sp. nov. (BT 011224.00). (A) main slab showing partly articulated skeleton in dorsal view. Posterior part of right mandibular ramus in (B) lateral and (C) dorsal aspect; anterior cervical vertebra (D) in articular and (E) lateral view; (F) posterior cervical vertebra in lateral view; (G) cervical rib in dorsolateral view; (H) dorsal vertebra in posterior view; (I) left scapula in medial view; (J) right coracoid in supposed dorsal view; (K) left humerus in dorsal view; (L) left femur with adjacent epipodials in dorsal view; and (M) left ilium in lateral view. |
Systematic paleontology
Plesiosauria de Blainville, 1835
Plesiosauroidea Gray, 1825
Franconiasaurus gen. nov.
Etymology—After the region of Franconia (Franken in German), located in the northern part of Bavaria in southern Germany, where the specimens were found; and “σαῦρος” (sauros), Greek for “reptile”.
Franconiasaurus brevispinus sp. nov.
Diagnosis—A plesiosauroid diagnosed through the following unique combination of characters: surangular with transversely narrow dorsal side and convex lateral side, lacking a longitudinal lateral through; cervical centra amphicoelous, being wider than long/high; cervical zygapophyses narrower than centra, having planar facets and being not connected for most of their length; anterior cervical neural spines curve posterodorsally, posterior cervical neural spines inclined straight posterodorsally; posterior cervical and dorsal neural spines only moderately higher than long (height/length ratio approximately 1.6); cervical ribs with reduced anterior processes; dorsal process of scapula with slightly convex medial side, lacking a buttress like medial enforcement; long intercoracoid symphysis, coracoids being only slightly separated posteromedially; coracoid cornu does not extend to the level of the glenoid; ilium shaft straight, dorsal and ventral ends of ilium perpendicular to one another; femora and humeri subequal in length; propodials moderately elongate (length/width ratio 1.7); radius and tibia subquadratic with concave pre- and postaxial sides.
Type specimen—BT 011224.00, nearly complete, three-dimensionally preserved, and partly articulated skeleton.
Horizon and locality—The specimens were excavated in 2005 (BT 011241.00) and between 2014 and 2018 (BT 011224.00) by one of us (S.E.) in the lower section of the Jurensismergel Formation (Grammoceras thouarsense Zone), upper Toarcian, Lower Jurassic, in the Mistelgau fossil pit, Bayreuth District, Bavaria, Germany. For detailed information on the geological and stratigraphic settings see Electronic Supplementary Material S2.
Etymology—The name brevispinus refers to the low neural spines in the posterior cervical and dorsal vertebrae.
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| Life reconstruction of Franconiasaurus brevispinus gen. et sp. nov. (artwork by Joschua Knüppe). |
Sven Sachs, Stefan Eggmaier and Daniel Madzia. 2024. Exquisite Skeletons of A New transitional Plesiosaur fill gap in the Evolutionary History of plesiosauroids. Front. Earth Sci. 12. DOI: 10.3389/feart.2024.1341470