Wednesday 1 June 2022

A Thyreophoran osteoderm from the Early Jurassic of northeastern Germany.

Early Jurassic Dinosaur fossils are rare in Europe, with only a few specimens known from Dorset in England, central Portugal, and several sites in Germany. One of the German sites which produces Dinosaur material is the Grimmen Clay Pit in Mecklenburg-Western Pomerania, northeastern Germany. This site was formerly worked for clays for the aggregates industry, and comprises a detached block of Lower Toarcian clay within a Pleistocene glacial sequence. This block has been badly distorted, making it hard to establish an internal stratigraphy, but contains fossiliferous concretions, which have produced numerous Ammonites (which have been used to establish its age), as well as marine, and more rarely terrestrial, Vertebrate material. 

In a paper published in the journal Paläontologische Zeitschrift on 19 January 2022, Marco Schade of the Institute of Geography and Geology and Zoological Institute and Museum at the University of Greifswald, and the Department of Earth and Environmental Sciences at  Ludwig-Maximilians-Universität, and Jörg Ansorge, also of the Institute of Geography and Geology at the University of Greifswald, describe an osteoderm from a Thyreophoran Dinosaur from the Grimmen Clay Pit.

The osteoderm, given the catalogue number GG 504, was discovered in several pieces by Jörg Ansorge in 2017, and subsequently prepared by him. The internal structure of the bone examined by micro-computed tomography at the Department of Cytology and Evolutionary Biology at the University of Greifswald.

GG 504, osteoderm of an unknown Thyreophoran Dinosaur. (a) Top view of the slightly convex side; (b) view on the acute edge, showing the inclination of the osteoderm and its base towards the less convex side; (c) cross section of the proximodistal mid-length; d, intertrabecular spaces filled with aragonite; (e) magnified part of a cross section. Abbreviations: ae, acute edge; al, aragonitic layers; be, blunt edge; co, cortex. Schade & Ansorge (2022).

The specimen comprises five separate fragments, which can be assembled to form a single piece with a maximum dimension of 15.5 cm. From the overall shape of this piece, Schade and Ansorge calculate that it represents about 75% of the original bone. Distinct striations can be seen on both surfaces of the specimen, appearing wrinkled under a light microscope, though these are rougher and more irregular on the more convex side. The osteoderm is thickest in the middle of what Schade and Ansorge assess to be its forward edge, where it is about 2 cm thick, tapering to the rear and either side. Both sides of the bone are convex, although one more than the other, with the specimen being slightly bent and the more convex side forming the inner part of the bend. The osteoderm appears to have originally been sub-parallel along much of its length, tapering towards the rear.

GG 504, osteoderm of an unknown Thyreophoran Dinosaur. (a) Top view of the pronounced convex side; (b) base in proximal view; (c) close-up of striated cortex shown in (a); (d) magnified cortex; (e) close-up of section close to the base shown in (a). Abbreviations: co, cortex; ob, offset base; sd, slight depression. Schade & Ansorge (2022).

The bulk of the osteoderm appears to be made up of Haversian bone (bone made up of a series of parallel tubes, which in life would have held blood vessels), surrounded by a thin, laminated cortex. These laminae are separated by layers of aragonite in places, and appear to show lines of arrested growth.

GG 504, osteoderm of an unknown Thyreophoran Dinosaur. (a) Magnified section of the distal-most fragment, arrow points towards former apex; (b) close-up of cortex from the pronounced convex side shown in (a), arrows without lines hint towards potential lines of arrested growth; (c) close-up of cortex from the slightly convex side shown in (a). Abbreviations: ?ShF, potential Sharpey’s fibres; ?so, potential secondary osteons; vc, vascular cavities. Schade & Ansorge (2022).

Specimen GG 504 resembles a previously described specimen, GG 85/1 23, which has been assigned to Emausaurus, an early diverging Thyreophoran Dinosaur (that is to say, an early Thyreophoran Dinosaur that cannot be confidently assigned to either of the main Thyreophoran groups, the Stegosaurs and Ankylosaurs, and which presumably lived before those groups diverged), and which was also recovered from the Grimmen Clay Pit. However, GG 504 is significantly larger than GG 85/1 23 (15.5 cm vs 4.5 cm), and GG85/1 23 is the largest known osteoderm associated with Emausaurus. Furthermore, GG 85/1 23 has a rougher, more vascularised base than GG 504, and a less symmetrical shape.

GG 504 also shows some resemblances to osteoderms associated with Scutellosaurus lawleri, another early diverging Thyreophoran from the Early Jurassic of Arizona, although again it is significantly larger than any osteoderm associated with this species (Scutellosaurus lawleri is only thought to have reached about 120 cm in total length).

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