May 23, 2016

by David Bressan

The geological record is notoriously imperfect and geologists may face great difficulties when reconstructing long lost environments. However, fossils and tracks of animals can provide crucial clues.

In 1917, during the last years of World War I, the Italian army built a mule track onto the 712m (2,330 feet) high Pasubio Massif, including 50 galleries in the hard rocks of itsTriassic Hauptdolomite formation.

Almost 70 years later during an excursion, geologist Marco Avanzini noticed on the roof of one of the galleries some strange bulges. He immediately recognized the bulges as natural casts of dinosaur footprints.

Dinosaur tracks were not new to the Hauptdolomit formation of the Dolomites. The Hauptdolomit consists of dolostone, a chemically altered limestone, deposited 220 million years ago in a shallow sea with sandy islands. Dinosaurs used these islands to cross over the sea, possibly to reach larger, rocky islands. However, all of the previously discovered dinosaur tracks were found in large boulders of landslides, so the exact position in the sequence of the Hauptdolomit formation was therefore unknown. So what made these eleven recognized natural casts of the Pasubio Massif special is that they were the first found in situ and that they represented three distinct types of footprints of animals.

One type of footprint is attributed to a sauropod-like trackmaker and the other two types are attributed to bipedal theropods of variable sizes. The exact known stratigraphic position enabled to date the surface with the imprints to the Norian age (211 to 206 million years). Some time later, more trace fossils of dinosaurs were reported in the area.

The geological evidence alone suggests that the northern area of the Tethys-Ocean, a large oceanic basin of which the modern Mediterranean Sea is a remain, was mostly a shallow marine environment that included the occasional deeper basin.

The landscape found today in the archipelago of the Bahamas is a good modern example how geologists imagined the ancient Tethys – mostly water surrounding some very small, almost barren, islands.

The many sites of dinosaur tracks found in all Italy, from the Dolomites in the north to the Gargano peninsula in the South, have ended up telling a different story. Namely, that there were large areas of dry land to be found. The fossil tracks were found in sediments that were only temporarily dry, perhaps during low tide. During this time, dinosaurs crossed the flat marine plains to reach other islands. These islands, of which no geological record survives, were large enough to support populations of large herbivores, as the presence of sauropod-like tracks suggests.

During a general low-stand of the sea-level, these islands acted probably as bridges between the two large landmasses of Laurasia in the North and Gondwana in the South, allowing a faunal exchange between the continents. During times of high sea-level the islands became isolated and endemic “Italian” dinosaur species evolved, like Scipionyx samniticus, an extraordinary well preserved small theropod discovered in 1998. And just recently another Italian dinosaur, a sauropod, was discovered – almost to vindicate the previously discovered tracks.