Triassic beasts and where to find them
Sue Beardmore (UK)
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.
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.
Less familiar are the placodonts, such as Paraplacodus and Cyamodus, whose fossils generally comprise broad, flat teeth used to grind food items, such as molluscs found in bottom sediments. The group is still considered to have been aquatic, despite their robust, bulky armour that resembles a turtle carapace in some aspects. Downright weird is the protorosaur Tanystropheus (Fig. 4), easily recognised by its excessively long neck that comprised up to half of the 5m total body length.
Stranger still, the neck had only a limited number of vertebrae (9 to 12 in comparison to other aquatic reptiles, like plesiosaurs with around 40), several of which were twice as long as the skull. As such the flexibility and range of movement of the neck was reduced. Combined with an awkward, barrel-shaped body and short legs, there is an ongoing debate as to whether Tanystropheus was capable of moving on land or better supported by water. The presence of fossils from a second protorosaur called Macrocnemus, which shares with Tanystropheus and other protorosaurs the elongated neck, albeit to a lesser extent, increases the possibility that Tanystropheus might have been terrestrial.
However, in contrast to Tanystropheus, the body of Macrocnemus is more gracile and the limbs long and slender, suggesting an agile creature that was capable of running quickly across land. Footprints assigned to Macrocnemus are found in Triassic tidal flat sediments elsewhere in Europe. Finally on this list is the robust, crocodile-like rauisuchian, Ticinosuchus, known from only a single complete skeleton measuring 2.5m and three partial fragments, which was conclusively land-based. Illustrations show it slowly roaming coastal habitats in search of prey like Macrocnemus.
Although less well studied, the number of fish taxa exceeds that of reptile species and includes representatives of most major groups. For example, the lobe-finned Sarcopterygia, whose ancestors had crawled out of water onto land in the Devonian, are represented by rare fossils of the coelacanths Ticinepomis and Holophagus. Chondrichthyans – the sharks – are known mainly from tooth plates and rare skeletal remains of the several metre long Acrodus, and the smaller, less than a metre long Acronemus and Hybodus.
Best represented in terms of abundance and diversity are the ray-finned Actinopterygia, the majority of which are less than 10cm long, such as Eosemionotus, Peltopleurus, Peripeltopleurus, Pholidopleurus and Habroichthys. Slightly larger, 10 to 80cm-sized taxa include Ptycholepis, Eoeugnathus, Ctenognathichthys, Ticinolepis, Colobodus and Crenilepis, with larger still and presumably predatory fish like Saurichthys (30 to 90cm) and Birgeria (approximately 100cm) at the top of the scale.
The Besano Formation is followed by the Meride Limestone (early Ladinian), in which vertebrate taxa are less numerous. The unusual reptile, Tanystropheus, is still present as rare fossils in the Cava inferiore beds, the oldest of three fossiliferous horizons in the lower Meride Limestone, as are the fish Habroichthys, Eosemionotus, Ticinolepis and Saurichthys. The pachypleurosaurid Neusticosaurus, specifically of the species N. pusillus, replaces Serpianosaurus as the most abundant reptile. Other reptiles are limited to the 3m-long nothosaur, Ceresiosaurus. The second fossiliferous horizon in the Meride Limestone, the Cava superiore beds, is different again, with N. peyeri being the most abundant and Ceresiosaurus present, but rare.
Of the fish, Eosemionotus and Saurichthys are still recorded alongside the taxa, Besania and Legnonotus, known only from this bed. In the third horizon, the Cassina beds, reptiles are present in the form of Tanystropheus, Macrocnemus, Ceresiosaurus and the relatively more numerous and largest pachpleurosaurid N. edwardsii. The only fish continuously present are Saurichthys and Ticinolepis, also occurring in the most recently discovered fossiliferous horizon, the thin Sceltrich beds, at the base of the upper Meride Limestone. At the top of the Meride Limestone is the Kalkschieferzone, with small fish like Prohalecites, Caelatichthys and Felberia, and the odd reptile, such as the one-metre-long nothosaur Lariosaurus.
That each horizon has its own assemblage of fossils is an indication of changes in environment through time. One notable change is water depth, which varied among and also within each horizon. For example, the Besano Formation was deposited under water that varied between 30 and 100 m in depth, and the lower Meride Limestone only 30 to 40m in depth. When water depth was greater, ammonoids drifted into the basin and bivalves colonised the seabed, both useful for dating the rocks and correlating specific levels to other, sometimes distant localities. Under shallower water, the connections between the Monte San Giorgio Basin, surrounding platform and open seas were reduced by the many sand bars, reefs and tectonically lifted blocks in the underlying San Salvatore Limestone, leading to a stratification of the water column.
Coprolites containing bones suggest that predators were constantly present and feeding in the oxygenated surface waters, but anything that sank to the seabed was effectively protected from scavenging by the predominantly oxygen-poor bottom waters. Water circulation was also affected by a monsoonal climate, present in the Triassic due to the configuration of existing land into the supercontinent Pangaea. The location of Monte San Giorgio at 20 degrees north of the equator and in the crux of the ‘C’-shaped land mass resulted in frequent storms that became stronger still in the Late Triassic. The presence of land plants and also the terrestrial reptiles is evidence for such storms and also the water currents necessary to transport them so far out to sea.
The above description of Monte San Giorgio in the Triassic is very detailed for a locality so far back in time. However, there are still many aspects that need to be studied further. These include the relationships of the animals to each other and beyond the locality, how and where they lived (as is the case for Tanystropheus), how they came to be preserved in these sediments (as for Ticinosuchus), and the wider evolution of the animals and their surrounding environment through the window that these Middle Triassic sediments have offered.
In the absence of accessible outcrop, recent investigations have necessarily used material already in collections, notably in Zurich and Milan, but also worldwide, as would be expected for any famous locality. The largest collection is housed at the Paläontologisches Institut und Museum der Universität Zürich, where stunning examples of the many reptiles and fish are also exhibited proudly in large, glass-fronted cases. A topographic model of the mountain shows the distribution of the main rock units, all tilted to the south.
In Lugano, in the southern Canton Ticino in which Monte San Giorgio is also located, is the Museo Cantonale di Storia Naturale with a permanent display of fossils and reconstructions, including a model of Tanystropheus that shows one proposed but unlikely posture of the neck. With funding from Canton Ticino and the Swiss Government, this regional museum is the only institution with ongoing scientific excavations at the locality. On the southern slopes of Monte San Giorgio, the recently renovated museum in Meride is entirely dedicated to the locally found fossils, using the three floors to cleverly demonstrate their stratigraphic distribution. The models of several aquatic reptiles hang in the open space between the ground floor and ceiling; and a prowling Ticinosuchus stands at the front desk.
In summary, discoveries are still being made in a few excavations at Monte San Giorgio that further build, even change, our perception of this ancient landscape. For example, the discovery of the Sceltrich beds in the Meride Limestone is filling a gap in our knowledge between the other better known and studied horizons. The Sceltrich beds still have great potential as a source of further examples of known taxa, as well as for new creatures that are as exceptional and/or weird; it also hints at the possibility of finding additional fruitful horizons in the future. Work at Monte San Giorgio and, in the many museums associated with it, thus continues to be crucial for promoting as well as protecting the locality.
About the author
Sue Beardmore is a collections assistant at the Oxford University Museum of Natural History. Further information on the fossils and Monte San Giorgio can be found at www.montesangiorgio.org.