Parabrontopodus?

Rob Hope (France)

Ahh, fossil footprints… simultaneously tantalising, evocative and enigmatic! Trace fossils of footprints are known throughout the world, including in the Jura Mountains of both France and Switzerland. Recently, near the tiny French village of Coisia, about 30km north of Geneva, a large slice of rock has revealed hundreds of dinosaur footprints. Fantastic! I went along to investigate.

It was the Alpine orogeny (about 30 million years ago) that thrust and twisted this huge fossil-laden stratum into the vertical position seen today. Initially, it was a vast, flat, tidal seashore. The trackways have been dated to the final stratigraphic unit of the Jurassic Period, known as the Portlandian (sometimes also referred to as the Tithonian) about 141 million years ago.  During those times, giant sauropods came and went upon the mudflats. They left deep, rounded prints in the mud that baked in the tropical sun before being covered by silt, deposited by subsequent tides. In this way, they were miraculously preserved and, again miraculously, they were later (indeed much later) discovered.

In 2005, French palaeontologist, Jean Le Loeuff and colleagues investigated the site, before publishing their conclusions in the French scientific journal Palevol. Trace fossil prints have biological names that describe their morphology (palichnology) and not the name of the animal that might have left them. Le Loeuff informs us that the well-preserved prints at the Coisia site are of the ‘Parabrontopodus’type, meaning a narrow-gauged sauropod trackway (left by an Apatosaurus or maybe a European type of Diplodocus).

A short while after this discovery, news came of yet another remarkable find, again in France’s Jura range. In an abandoned, weed-ridden quarry, near the hamlet of Loulle, yet more dinosaur footprints were discovered. In all, thousands of prints can be seen there, with several individual trails running for tens of metres. Even Juvenile fossil tracks are preserved, which run parallel between larger trails. Were adult sauropods protecting a youngster as they wandered together upon these once soggy floors, an estimated several million years before the Coisia footprints were laid down? Research was carried out on the trackways, led by JM Mazin of Lyon University (see Le Loeuff et al., 2006; Mazin et al., 2016).

I attended a conference, organised by Mazin and leading geologist, P Hantzpergue on the interpretation of the Loulle quarry trackways. They explained that the bizarre cracked surface aspect of this fossil site was due to a thin, slimy algae covering the once sodden, gluey silts. This brackish ecosystem was established long before the sauropods had walked across them. Therefore, unlike the Coisia environment, this was not within a tidal environment.

One single fossil print here is truly astonishing (see Fig. 1, with beer bottle indicating rough scale). The original depression, created by the great weight of the lumbering animal, remains wholly intact. And surprisingly, this particular print survives in isolation. Why is this special fossil so exquisitely preserved, while the remainder of the trail has been totally obliterated? We don’t know.

As a result of the singular aspect of this remarkable footprint, identification of the palichthyospecies cannot be established. It may be that the animal that trod here was of a Parabrontopodus species or a brontopodus (a wide-gauged sauropod trail) species.  We will most likely never know.

Reference

May 2016, Jean-Michel Mazin Pierre Hantzpergue Joane Pouech, The dinosaur tracksite of Loulle (early Kimmeridgian; Jura, France), Geobios 49(3).