Ohmdenosaurus

History of discovery

The Posidonia Shale at Holzmaden in southwestern Germany is a major fossil Lagerstätten deposited within an inland sea, containing abundant marine reptiles such as ichthyosaurs, plesiosaurs, and crocodyliforms, sometimes with soft tissue preservation. Quarried for over 400 years, first for roofing and paving and later for oil, its fossils were made famous by Bernhard Hauff, who began collecting and preparing specimens from his father's quarry. In 1892, Hauff presented an ichthyosaur preserving its original body outline, revealing dorsal fins.

With his son, he opened the Urweltmuseum Hauff in Holzmaden in 1936/37 to display the finds. In the 1970s, German palaeontologist Rupert Wild visited the museum and noticed a fossil labeled as a plesiosaur humerus. Recognizing it as a dinosaur, he borrowed it for study and further preparation. The specimen, lacking a number, consists of a right tibia with the astragalus and calcaneus.

It had long been in the museum's collection, collected from an early quarry near Ohmden that was later refilled; the exact site is unknown. In a 1978 publication, Wild named it Ohmdenosaurus liasicus, the generic name derived from Ohmden and the Greek σαῦρος (sauros), and the specific name from the Lias, an old name for the Lower Jurassic. A chunk of gray-black, finely laminated slate with fish fragments attached to the fossil's lower end indicates it came from the Unterer Schiefer, the oldest part of the Posidonia Shale, making it early Toarcian in age.

When described, it was one of the earliest known sauropods and only the second terrestrial saurian fossil from the Toarcian. Between the tibia and astragalus, a limestone geode rich in pyrite contains fossils of the snail Coelodiscus.

Description

Like all sauropods, Ohmdenosaurus was a quadrupedal herbivore with a long neck and tail. Wild estimated its total body length at 3–4 m, small for a sauropod. The tibia is 405 mm long and preserves muscle attachment sites, including a projection that extends about 4 cm from the upper front and the crista lateralis, which runs about 13 cm down the upper half of the shaft but is mostly broken. The upper end of the tibia is oval when viewed from above, with a width-to-length ratio of 1.4.

The lower end is rotated 90° relative to the upper end and formed by two rounded, well-separated condyles: the medial condyle is much larger than the broken lateral condyle and located about 2–3 cm lower. The astragalus is 140 mm in diameter, sandal-shaped, and rotated 90° out of position, exposing its upper surface, which has two concavities—a larger medial and a smaller lateral one—that received the tibial condyles. A furrow between them likely served as a ligament attachment site.

The calcaneus is 43 mm in diameter and 15 mm in maximal height, circular, with a convex probable lower surface and a roughly textured probable upper surface indicating a cartilaginous covering. Below the lateral condyle, Wild noted several small elements 5–30 mm in size, identified as cartilage due to their grainy surfaces; these may be sesamoids or calcified cartilage from the astragalus.

Classification

Saurischian dinosaurs are divided into the herbivorous sauropodomorphs and mostly carnivorous theropods. Sauropodomorpha includes quadrupedal sauropods with straight, columnar limbs and early, mostly bipedal forms with angled limb bones. In his 1978 description, Wild argued Ohmdenosaurus was a sauropod because its limb was columnar: the tibia is more massive than in basal sauropodomorphs, and its upper joint surface is perpendicular to the bone's long axis, indicating a straight standing limb.

The astragalus lacks the ascending process typical of bipedal forms but reduced or absent in sauropods due to weight distribution from columnar limbs. However, Wild noted basal features such as the sandal-shaped astragalus and stepped lower articular surface of the tibia, typical of basal sauropodomorphs but absent in other sauropods. The oval upper end of the tibia was intermediate between the circular shape in Plateosaurus and the strongly elliptical shape in later sauropods like Cetiosaurus.

Wild concluded that Ohmdenosaurus shows a mosaic of primitive and derived features and may belong to a new family of sauropods.

Taphonomy

Wild reconstructed the taphonomy of the specimen in his 1978 description. The tibia shows two excavations 2–5 cm deep caused by weathering, located on the sidewards projecting upper and lower ends only on the medial side. This indicates the bone lay on its lateral side, partly covered by sediment, exposing only the most protruding parts to the elements. Similar weathering traces are absent in other Posidonia Shale fossils due to the lack of seafloor currents, so the weathering likely occurred on land or near the shore, perhaps in a river delta.

Wild concluded the specimen was transported and deposited twice. First, it was moved from its death site to a coastal deposition site, where partial sediment cover and weathering occurred.

The second transport to its final offshore site may have been by strong surface currents, but Wild considered scavengers such as crocodiles or plesiosaurs more likely due to the tibia's massiveness and the distance to the coast. During this second transport, the carcass likely separated, but the tibia and ankle were still articulated when found, indicating soft tissue held them together. The snail Coelodiscus found with the specimen may have been a scavenger feeding on decaying soft tissue.

Did You Know?

• The Posidonia Shale was deposited in a subtropical inland sea at about 30°N with a water depth of 100–600 m.
• The nearest landmass to the Posidonia Shale was probably the Black Forest Massif about 100 km to the southwest.
• Other terrestrial evidence from the shale includes pterosaurs, dragonflies, net-winged insects, and driftwood.