Sunday 3 September 2017

Grimmenodon aureum: A new species of Pycnodont Fish from the Early Cretaceous of northeast Germany.

Pycnodont Fish were a highly successful group of Ray-finned Fish, Actinopterygii, that tended to be highly laterally compressed, with deep bodies, and lacked the evertable jaws of modern Teleosts, but often had specialised dentition, with many species apparently being durophagous, first .e. adapted to crush the shells of organisms such as Molluscs. They first appeared in the Late Triassic of Europe, and became a dominant group of Fish in many ecosystems in the Late Jurassic and Cretaceous. However Pycnodont Fish from the Early and Middle Jurassic are very rare, with only about twelve species known, suggesting that this group was very badly affected by the End Triassic Extinction.

In a paper published in the Journal of Vertebrate Palaeontology on 30 August 2017, Sebastian Stumpf and Jörg Ansorge of the Institute of Geography and Geology at the University of Greifswald and Cathrin Pfaff and Jürgen Kriwet of the Department of Palaeontology at the University of Vienna describe a new species of Pycnodont Fish from the Early Jurassic of Grimmen in Mecklenburg-Western Pomerania in northwest Germany.

The new species is named Grimmenodon aureum, where 'Grimmenodon' means 'Grimmen-tooth', in reference to the location where it was found, and 'aureum' means 'golden', due to a pyrite coating on the specimen, which gives it a golden sheen. The species is described from a single left prearticular bone (part of the lowe jaw) with four preserved tooth rows.

Grimmenodon aureum, an isolated, almost complete left prearticular with dentition from the lower Toarcian of Grimmen, Mecklenburg-Western Pomerania, Germany, in (A), occlusal, (B), ventral oblique, and (C), lateral views (anterior to the left). Stumpf et al. (2017).

Stumpf et al. also describe an isolated tooth crown from a deeper (and therefore older) bed at the same location, which they believe comes from a second Pycnodont Fish species, though they refrain from describing it as a new species, pointing out that the large number of Pyconodont species described from isolated an sometimes fragmentary teeth already makes understanding the taxonomy of the group very difficult, particularly as they are known to have heterodont dentition (different shaped teeth with different purposes in different parts of the mouth, as in modern Mammals).

An isolated tooth crown from an unknown Pycnodont Fish from the upper Pliensbachian of Grimmen, Mecklenburg- Western Pomerania, Germany, in (A), occlusal, (B), basal, and (C), lateral views. Stumpf et al. (2017).

Stumf et al. hypothesise that the discovery of two Pycnodont Fish from Early Jurassic shallow-marine deposits at the same location may indicate that this group was not as rare in this interval as previously thought, but that the low diversity known may be an artifact of low sampling at suitable sites, suggesting they may have been less impacted by the End Triassic Extinction than has previously been thought.

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