Triassic beasts and where to find them

Located amid the scenic Southern Alps, on the Swiss-Italian border, is Monte San Giorgio, a mountain that rose up like many across Central Europe as a result of continental collision between Africa and Europe during the Alpine Orogeny. It is not particularly big or distinct by alpine standards but it is special, a status emphasised by the designation of its slopes as a UNESCO World Heritage site initially in 2003 for the Swiss part with the neighbouring Italian area added in 2010. To begin, the rocks outcropping on the mountain form an almost complete stratigraphic sequence from the Permian through to the Jurassic (Fig. 1), not only an extended interval of time but an important one around the massive Permo-Triassic extinction. The same rocks provide a context for the equally important Middle Triassic vertebrate, invertebrate and plant fossils, now numbering more than 20,000, that have been found at the locality over the last 170 years.

Fig. 1. A stratigraphic section of the rocks at Monte San Giorgio. © Commissione Scientifica
Transnazionale Monte San Giorgio, 2014.

In particular, it is the diversity, relative abundance and excellent preservation of the vertebrate fossils that has thrown the locality into the spotlight. These occur in six main fossiliferous horizons deposited in a shallow marine basin, the Monte San Giorgio Basin, one of many depressions on a carbonate platform between the Eurasian continent to the north and west, and the open waters of the vast Tethys Ocean to the south and east. Each horizon is visually similar, comprising alternating layers of black and grey sediments. The black layers represent background material with high organic content (the ‘bituminous shales’) that continuously, but very slowly, rained down onto the seabed. The grey layers are calcareous (event bed) material, washed into the basin from the surrounding platform. The horizons were all deposited between 243 and 239 million years ago, an age range derived from the radiometric dating of ash deposited in thin layers during intermittent volcanic activity.

In the oldest horizon, the Besano Formation or “Grenzbitumenzone” (late Anisian to early Ladinian) vertebrate diversity is greatest. Most abundant was the small (30 to 80cm), lizard-like pachypleurosaurid Serpianosaurus (Fig. 2) that spent all its life swimming in the tropical surface waters. Hunting reptiles like Serpianosaurus were the similarly shaped, but much larger and faster thalattosaurs – the metre long Clarazia and Hescheleria, the 2 to 3m Askeptosaurus and 3m plus Helveticosaurus (whose membership in this group is still to be determined). Larger still was the 4m-long nothosaur Nothosaurus, its body broader and teeth more obvious in the rare fossils. More familiar are the dolphin-like ichthyosaurs, represented by the abundant, metre-long Mixosaurus (Fig. 3), the two skeletons of the 4 to 6m Besanosaurus, found only at Monte San Giorgio, and Cymbospondylus known only from a single, partial specimen estimated at about 4m long, based on similar fossils from Nevada in the USA. The ichthyosaurs Mikadocephalus, Wimanius and Phalarodon have also been described.

Fig. 2. Two skeletons of the pachypleurosaurid, Serpianosaurus.

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