Ohmdenosaurus

Extinct species of reptile

Ohmdenosaurus is a genus of sauropod dinosaur that lived during the Early Jurassic epoch in what is now Germany. The only specimen – a tibia (shinbone) and ankle – was discovered in rocks of the Posidonia Shale near the village of Ohmden. The specimen, which was originally identified as a plesiosaur, is exhibited in a local museum, the Urweltmuseum Hauff. In the 1970s, it caught the attention of German palaeontologist Rupert Wild, who recognised it as the remains of a sauropod. Wild named Ohmdenosaurus in a 1978 publication; the only known species is Ohmdenosaurus liasicus. One of the earliest known sauropods, Ohmdenosaurus was quadrupedal (four legged) and already had the columnar limbs typical for the group. It was small for a sauropod, with an estimated length of Its relationships to other sauropods remain uncertain due to the incompleteness of its remains, though one study concluded it was a eusauropod. The Posidonia Shale was deposited within a shallow inland sea and contains abundant and well preserved fossils of marine reptiles, including ichthyosaurs and plesiosaurs. Ohmdenosaurus was a terrestrial animal, and the specimen must have been transported by predators or water currents at least from the shoreline to its site of burial. It is the only dinosaur fossil known from the shale.

The Posidonia Shale at Holzmaden in southwestern Germany is one of the world's major fossil Lagerstätten (fossil deposit of exceptional importance). Deposited within an inland sea, it contains abundant fossils of marine reptiles such as ichthyosaurs, plesiosaurs, and crocodyliforms, sometimes with soft tissue preservation. This organic rich shale has been quarried for more than 400 years, first for roofing and paving, and later to also extract oil. Its exceptionally preserved fossils were made famous by Bernhard Hauff, who started to collect and prepare specimens found in a quarry owned by his father. In 1892, Hauff presented an ichthyosaur specimen that preserves the original body outline, revealing that ichthyosaurs possessed dorsal fins. Together with his son, he opened a local museum in Holzmaden in 1936/37, the Urweltmuseum Hauff, to display the finds. In the 1970s, German palaeontologist Rupert Wild was visiting the Urweltmuseum Hauff when he noticed a fossil in a display labelled as the humerus (upper arm bone) of a plesiosaur. Wild recognised the specimen as a dinosaur fossil, borrowed the specimen for study and carried out further preparation. The fossil, which has no specimen number, consists of a right tibia (shinbone) together with the astragalus and calcaneus (the upper bones of the ankle). It had long been part of the museum's collection, having been collected from one of the early quarries near the village of Ohmden that were later refilled; the exact discovery site is unknown. In a 1978 publication, Wild determined that the dinosaur fossil belonged to a new genus and species, which he named Ohmdenosaurus liasicus. The generic name Ohmdenosaurus is derived from the village of Ohmden and from the Ancient Greek σαῦρος (sauros), meaning lizard or reptile. The specific name liasicus refers to the Lias, an old name for the Lower Jurassic of Europe. Later, the Vulcanodontidae was demonstrated to be polyphyletic (does not form a natural group) and therefore fell out of use. Jay Nair and colleagues, in 2012, compared Ohmdenosaurus to the Australian genus Rhoetosaurus, noting that the tibiae of both genera are relatively slender as seen in later sauropods, unlike the more robust tibiae of other early genera. As Rhoetosaurus is geologically younger than Ohmdenosaurus, the latter would have been the earliest known sauropod with a slender tibia. The astragalus of Rhoetosaurus was found to be more similar to Ohmdenosaurus than to other sauropods. Sebastian Stumpf and colleagues, in 2015, reported fragmentary sauropod remains from the Toarcian of Grimmen in northeastern Germany, including four elements of the pelvic girdle and part of a vertebra. Although roughly contemporaneous with Ohmdenosaurus, they cannot be directly compared to the latter because they do not include elements of the hind limbs. The Grimmen remains do, however, resemble the early sauropod Tazoudasaurus from Morocco, while Ohmdenosaurus appears to be closer to Rhoetosaurus. Stumpf and colleagues therefore suggested that Ohmdenosaurus and the Grimmen sauropod were not closely related to each other. In 2020, Oliver Rauhut and colleagues included Ohmdenosaurus in a phylogenetic analysis, but found it to be unstable as it was placed in different positions in the tree by different variants of the analysis, both within and outside of Sauropoda. Michael Simms and colleagues, in 2021, suggested that Ohmdenosaurus might be considered a nomen dubium (dubious name) due to the incompleteness of its remains. In 2022, Omar Regalado Fernández and Ingmar Werneburg included Ohmdenosaurus in a phylogenetic analysis that placed it within Eusauropoda – a group that comprises most sauropods except some very basal forms such as Tazoudasaurus. Within Eusauropoda, Ohmdenosaurus forms a clade with Amygdalodon, Spinophorosaurus, and Volkheimeria in this analysis. The following cladogram shows the possible relationships of Ohmdenosaurus according to Omar Regalado Fernández and Ingmar Werneburg in 2022: Ohmdenosaurus is the only known dinosaur fossil from this formation, and other evidence for terrestrial life in the shale is scarce. Although driftwood is frequently found, other plant remains are rare and include horsetails, conifers, and the now extinct bennettitales. Since these remains are fragmented and sorted by water action, they provide limited information on the floral composition of their place of origin.