Ohmdenosaurus
Ohmdenosaurus | |
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The only known specimen (bones of the lower leg) in posterior (rear) view, on exhibit at the Urweltmuseum Hauff , Germany
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Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Clade: | Dinosauria |
Clade: | Saurischia |
Clade: | †Sauropodomorpha |
Clade: | †Sauropoda |
Genus: | †Ohmdenosaurus Wild, 1978 |
Species: | †O. liasicus
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Binomial name | |
†Ohmdenosaurus liasicus Wild, 1978
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Ohmdenosaurus ('Ohmden lizard') is a
One of the earliest known sauropods, Ohmdenosaurus was
History of discovery
The
In the 1970s, German
A chunk of rock – a gray-black, finely laminated slate containing small fragments of fish fossils – is still attached to the lower end of the fossil. This rock indicates that the fossil was found in the Unterer Schiefer ('lower slate'), the oldest part of the Posidonia Shale. It is therefore early Toarcian in age (ca. 182 million years ago).[4][3] When Ohmdenosaurus was described in 1978, it was one of the earliest sauropods known at the time and only the second fossil of a terrestrial saurian to be discovered from the Toarcian. Between the tibia and astragalus, the specimen also preserves a limestone geode that is rich in the mineral pyrite and contains fossils of the snail Coelodiscus.[3]
Description
Like all sauropods, Ohmdenosaurus was a
The tibia is 405 mm (15.9 in) long. The bone preserves projections that served as attachment sites for muscles, including the cnemial crest, which projects by about 4 cm (1.6 in) from the upper front of the bone, and the crista lateralis, which runs for about 13 cm (5.1 in) down the upper half of the shaft but is mostly broken off. The upper end of the tibia is oval in shape when viewed from above, with a width-to-length ratio of 1.4. The lower end of the tibia is rotated by 90° relative to the upper end. The lower end of the tibia is formed by two rounded, well-separated prominences, the medial (inner) and lateral (outer) condyles. The medial condyle is much larger than the (broken) lateral condyle and located c. 2–3 cm (0.8–1.2 in) lower than the latter.[3]
The astragalus is 140 mm (5.5 in) in diameter, sandal-shaped, and rotated by 90° out of its original position, exposing its upper surface. This surface has two concavities, a larger medial and a smaller lateral one; the latter was located about 2 cm (0.8 in) higher than the former. These concavities received the medial and lateral condyles of the tibia, respectively. A furrow between these concavities is thought to have been an attachment site for ligaments of the ankle. The much smaller calcaneus is 43 mm (1.7 in) in diameter and 15 mm (0.6 in) in maximal height, and circular in shape. Its probable lower surface is convex, and its probable upper surface is roughly textured, indicating the presence of a cartilaginous covering. Below the lateral condyle of the tibia, Wild noted several other small elements 5–30 mm (0.2–1.2 in) in size, which he identified as cartilage given their grainy and irregular surfaces. Unlike bone, cartilage is rarely preserved in fossils, and in this case might have been preserved thanks to the absorption of calcium salts. As these elements are located close to the attachment site of the Achilles tendon, the area of the ankle that experienced the highest stresses in life, Wild argued that they may represent sesamoids (small structures embedded within tendons). Alternatively, they could be calcified pieces of the cartilage of the astragalus.[3]
Classification
Other basal sauropods have been described since, but relationships to these forms remain vague given the incompleteness of the Ohmdenosaurus specimen. In 1990,
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.[9] Michael Simms and colleagues, in 2021, suggested that Ohmdenosaurus might be considered a nomen dubium (dubious name) due to the incompleteness of its remains.[10] 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.[11]
The following cladogram shows the possible relationships of Ohmdenosaurus according to Omar Regalado Fernández and Ingmar Werneburg in 2022:[11]
Taphonomy
Wild, in his 1978 description, attempted to reconstruct the taphonomy of the specimen – the events between the death and final deposition of the individual. Such reconstructions are important for the understanding of the formation of the Posidonia Shale as a unique fossil deposit. The tibia of Ohmdenosaurus shows two excavations caused by weathering that are 2–5 cm (0.8–2.0 in) deep. These excavations are located on the sidewards projecting upper and lower ends, but only on the medial (inner) side of the bone. This indicates that the bone must have laid on its lateral (outer) side and partly covered by sediment, only exposing the most protruding parts of the other side to the elements. Because similar weathering traces are not seen in other fossils from the Posidonia shale due to the absence of currents near the sea floor, the weathering likely took place while the specimen was still on land or near the shore, perhaps in a river delta.[3]
Based on this evidence, Wild concluded that the specimen must have been transported and deposited twice. First, it was transported from its place of death to the first site of deposition near the coast, where the weathering took place. This transport is indicated by the presumed partial sediment cover, which indicates a site where sedimentation took place. The second transport to its final site of deposition far off the coast could have happened through strong currents near the surface. Wild, however, considered it more likely that scavengers such as crocodiles or plesiosaurs brought the specimen to its final site because of the massiveness of the tibia and the considerable distance to the coast. It was probably only during this second transport that the carcass got separated: the tibia and ankle were still articulated when found, indicating that soft tissue was still in place that held these bones together when the specimen arrived at its final site. The snail Coelodiscus that was found with the specimen could have been a scavenger feeding on the decaying soft tissue.[3]
Palaeoenvironment
The Posidonia shale at Holzmaden was deposited in a subtropical inland sea at c. 30°N with a water depth of 100–600 m (300–2,000 ft).
References
- ^ ISBN 9781840766233.
- ISBN 978-3-319-77401-5.
- ^ a b c d e f g h i Wild, R. (1978). "Ein Sauropoden-Rest (Reptilia, Saurischia) aus dem Posidonienschiefer (Lias, Toarcium) von Holzmaden". Stuttgarter Beiträge zur Naturkunde, Serie B (Geologie und Paläontologie) (in German). 41: 1–15.
- ISSN 0031-0182.
- OCLC 20670312.
- from the original on 2023-10-30. Retrieved 2021-10-17.
- S2CID 85317450.
- from the original on 2023-10-30. Retrieved 2020-05-15.
- ISSN 1661-8734.
- from the original on 2023-10-30. Retrieved 2023-08-13.
- ^ .
- ^ Riegraf, Wolfgang (1985). "Mikrofauna, Biostratigraphie und Fazies im Unteren Toarcium Südwestdeutschlands und Vergleiche met benachbarten Gebieten". Tübinger Mikropaläontologische Mitteilungen (3): 1–233.
- ISSN 1432-1904.
- ^ Ansorge, J. (2003). "Insects from the Lower Toarcian of Middle Europe and England". Acta Zoologica Cracoviensia. 46: 291–310.
- ^ Wilde, V. (2001). "Die Landpflanzen-Taphozönose aus dem Posidonienschiefer des Unteren Jura (Schwarzer Jura [Epsilon], Unter-Toarcium) in Deutschland und ihre Deutung". Stuttgarter Beiträge zur Naturkunde (in German). 304.