Plesiosaurs were marine reptiles that existed during the Mesozoic era. They were fully adapted to aquatic life. Their evolution lasted for over 140 million years. During this time, they spread across the seas and oceans worldwide.
During the early/middle Jurassic period (175–171 million years ago), the previously dominant lineage of plesiosaurs, Rhomaleosauridae, began to decline. They disappeared around 161 million years ago (Callovian), while Pliosauridae and Plesiosauroidea became increasingly diverse, giving rise to several groups that thrived for tens of millions of years thereafter.
Life reconstruction of Franconiasaurus brevispinus. ©Joschua Knüppe.
Plesiosaur Franconiasaurus combines the features of early plesiosaurs with those characteristic of later members of the group. It belonged to the clade Plesiosauroidea, which existed from the late Triassic to the very end of the Cretaceous period. Phylogenetic analysis confidently identifies plesiosaur Franconiasaurus as a sister taxon to Cryptoclidia. This demonstrates the evolutionary connection between Plesiosaurus-like forms and microcleidids, which later evolved into cryptoclidids, leptocleidians, and elasmosaurids.
This is Sven Sachs studying the Franconiasurus holotype during his first visit to the collections of the Urweltmuseum Oberfranken in 2017.
Sven Sachs from the Naturkunde-Museum Bielefeld and colleagues described a new plesiosaur that lived in the seas at the end of the Early Jurassic (late Toarcian) in Germany. His name is Franconiasaurus brevispinus. The generic name “Franconiasaurus” combines Franconia (Franken in German), after the Bavarian region where the holotype comes from, and the Greek “sauros”, meaning “reptile”. The species name “brevispinus” refers to the low neural spines of some cervical and dorsal vertebrae.
The phylogenetic placement of Franconiasaurus brevispinus gen. et sp. nov. showed on the non-xenopsarian segment of Plesiosauroidea. A tree based on strict consensus trees reconstructed through weighted parsimony analyses.
The scientists described this new genus based on two well-preserved skeletons, which retained the three-dimensional structure of the elements. Researchers found the fossils in an abandoned pit in Mistelgau, Bayreuth District, Bavaria (Jurensismergel Formation), where people had previously extracted clay.
Some anatomical details of plesiosaur Franconiasaurus
Diagnostic skeletal elements of the type specimen of Franconiasaurus brevispinus. The main slab shows a partly articulated skeleton in dorsal view. ©Sachs et al.
The plesiosaur Franconiasaurus holotype consists of a nearly complete skeleton. It includes part of the mandible, including a left dentary, but not the majority of the skull. The second partially articulated specimen is a larger individual and includes vertebrae, ribs, and some girdle and limb elements.
Overview of the referred specimen of plesiosaur Franconiasaurus brevispinus. ©Sachs et al.
All cervical centra are amphicoelous. Neurocentral sutures are completely fused in both specimens, which means that these were adult individuals. The cervical ribs bear reduced anterior processes, similar to the condition in the basal pliosaurid Arminisaurus. Dorsal and gastric ribs are present in both specimens. All propodials (brachium and femur) are preserved in the holotype and partially in the second specimen. Both specimens also retain epipodials (antebrachium and crus). There are some isolated metapodial elements and phalanges. The phalanges are elongated and slender.
Plesiosaur Franconiasaurus brevispinus.
The researchers published their paper online in the journal Frontiers in Earth Science. Sven Sachs et al. 2024. Exquisite skeletons of a new transitional plesiosaur fill gap in the evolutionary history of plesiosauroids.