Stage 4 | Troyes - Bar-Sur-Aube / Jurassic era: dangerous swimming paradise

The ladies will race their way eastwards today, and will travel from the Cretaceous deeper into geological time to arrive in the Jurassic. If you would have visited Northern France 170 million years ago, during the Jurassic era, you would have seen a tropical swimming paradise. Azure blue waters as far as the eye can see, with at the horizon the occasional island with white beaches and lush vegetation. From the Vosges to Wales, everything was submerged, a shallow sea at the margins of the mighty Tethys Ocean that extended southeastwards, as a giant tear between the continents Gondwana and Laurasia.

The dangerous sea of the Jurassic era

And while you’re sitting there on the beach of the Gulf of Luxembourg, with the warm carbonate sands in the Gulf tickling your feet, you could’ve thought you were at the Bahamas. Hammock, pina colada, and don’t forget the sunscreen. But a refreshing dive into the sea, or even paddling the shallow sea, would have been a bad idea. The Jurassic seas were full of live, and not all was cuddly and sweet. of the cephalopods (the squids), like ammonites and belemnitesm were fine, even if those were vicious predators. But outright dangerous were a large collection of sea reptiles, kike Ichthyosaurs, swimming dinosaurs with eyes as large as saucers and with an athletic and smooth body that looks a lot like today’s dolphins. A text book example of convergent evolution. These apex-predators, who find themselves near the top of the food chain, had to be careful themselves. Other sea reptiles were hunting them, such as Plesiosaurs, and Pliosaurs that could have monstrous dimensions of as much as 20 meters!

Meat-eating Theropods on land

In this Jurassic Park, it was nice and warm. Not very surprising, because the global CO2 concentration was about 5 times higher than today, well above 2000 ppmv. Highly favorable conditions for plants. At the shores of the larger landmasses like the Vosges, forests of swamp cypresses rose up, with closer to the ground ferns and related plants like palm-ferns and horsetails. We now find the pollen and microscopic spores of all these plant species as microfossils in the sediments that were deposits in the shallow Jurassic sea.

And amidst even more monstrous scaly reptiles, in this case dinosaurs like the plant-eating Sauropods and the meat-eating Theropods, these forests provided shelter for a warm-blooded little fellow, “model shrew”. It had five toes on each foot, a pointy snout, and hair. A Morganucodon. This is one of the oldest mammals, a distant ancestor of a long-extinct branch. It would take another 100 million years before a giant space rock would end the reign of all those monsters. And Morganucodon nodded in approval.

I am interested in understanding the evolution of the biosphere in deep time. I focus on major transitions in Earth history. I use a multidisciplinary approach combining paleontology with in- and organic geochemistry. Check the Geo-TdF-team-2022.

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