Time is unfathomably deep. Earth scientists try to get a grip on it by dividing it into 'periods'. In this series, we have already passed through the Cambrian, Ordovician, Devonian and Carboniferous. Within those periods, geologists and palaeontologists do the finer work: defining 'epochs', such as the Upper Devonian, and 'ages', such as the Famennian. But they also zoom out and group periods together by asking: what is their greatest common denominator? These are the 'eras', spanning many tens to hundreds of millions of years. At the boundaries of eras lie major turning points in geology — and these almost always coincide with a mass extinction.

In this fourth episode of Planet Belgium, we have reached the end of one such 'era' or 'major epoch': the Palaeozoic. Literally: ‘the time of ancient life’. We are stepping into the Mesozoic, ‘the era of middle life’. New players have appeared on the stage, and they will become dominant in this era: the reptiles. Think dinosaurs, mosasaurs and pterosaurs. On land, at sea and in the air, respectively, they become unavoidable.

But their dominance did not come about overnight. For that, we must go back to two P's: the Permian and Pangaea.

Pangaea was the gigantic continent that formed at the transition from the Carboniferous to the Permian, roughly 300 million years ago. It was a mighty collision: the supercontinent Gondwana (think Africa plus South America plus Antarctica plus Australia) crashing into its northern counterpart Laurussia (North America and Europe). Bang! — though continents collide in slow motion, centimetre by centimetre, over tens of millions of years. With Pangaea, all landmasses were united, surrounded by a single ocean, Panthalassa. Note for later: the next time the whole world comes together will be in roughly 250 million years.

At the collision zone between Gondwana and Laurussia, roughly straddling the equator, a long mountain range arose. It ran from the Appalachians across several well-known European ranges (the Massif Central, the Vosges, the Eifel, the Harz…) all the way to the Urals. This is the Variscan mountain belt. In our region, the Ardennes rose high (far higher than the gentle hills they are today), while Flanders remained relatively unaffected.

All the world's landmass together is cosy, but it also has consequences — especially for areas no longer bordering an ocean, like ours. Clouds deposit their rain closer to continental margins, so the near-endless interior of Pangaea dries out. And the Ardennes and other ranges to our south blocked moist oceanic air. Our boggy, lush forests of the Carboniferous, full of towering scale trees (episode 3), became desert with sand dunes. Who would have thought our rainy little country was once the Sahara?

Another, older name for Pangaea is the 'New Red Continent', by analogy with the 'Old Red Continent' from the Early Devonian (episode 2). The red colour comes from the iron minerals in the sand oxidising. On my desk lies a piece of light-red sandstone from a quarry in Vielsalm, picked up during a field trip because I found it beautiful. Now I realise how remarkable it is — because from the Permian (299 to 252 million years ago), erosion has left very few traces in our country. Of virtually every other period in the past five hundred million years, we do have fine outcrops, especially in Wallonia.

Against the bone-dry world of the early Pangaea, some vertebrates were already quite well equipped since the Carboniferous. We call them amniotes. They are the ancestors of all reptiles, birds and mammals alike. They evolved from amphibious four-limbed animals whose fins were strong enough to haul themselves out of the water, and whose swim bladders had transformed into lungs. Those amphibians still had to return to the water to lay their eggs.

Amniotes found a solution. They created a comfortable miniature pond inside the egg. How? Through a series of membranes: the amnion, the yolk sac, the allantois and the chorion. The amnion is the amniotic sac in which the embryo develops. From the ventral side of the embryo spring the yolk sac, containing food reserves, and the allantois, the waste sac. The three sacs sit within a protective membrane, the chorion. Around that chorion comes either a soft, leathery shell or a harder one made of calcium.

Amniotes were the great explorers of their time. They ventured ever further from the water and formed new communities — made possible because other organisms had preceded them in colonising dry land: seed-bearing trees (such as conifers), fungi, arachnids and insects.

The amniotes split into three fundamental groups during the Carboniferous. "For that, you need to look at the skull and count the holes behind the eye socket," says palaeontologist and science communicator Koen Stein. "The so-called temporal openings. Where openings exist, muscles can be better anchored, and a greater bite force develops."

"The anapsids have no temple window, and their simple skull most closely resembles that of amphibians. There are no anapsids alive today. The synapsids have one opening, and that group includes mammals. Feel your own temple. But the synapsid group also includes Permian animals like Dimetrodon, with its spectacular sail on the back — it's very hard to grasp that we are relatively closely related to that creature. The third group are the diapsids, with two temple windows. Within them, one subgroup will evolve to include snakes, lizards and the mosasaurs (marine reptiles). Another subgroup will give rise to crocodilians, pterosaurs (flying reptiles) and — yes — the dinosaurs and birds. We call this latter subgroup the archosaurs, literally the 'ruling lizards'." And many of them walked upright, which would prove to be another game changer.

During the Permian, the diapsid reptiles are not yet in charge, and there are no dinosaurs whatsoever. It is the synapsids (one temple window, the lineage of the mammals) that call the shots. They are adapted to the dry regions of Pangaea and refresh themselves at the lakes filled by mega-monsoons. They are the crawling apex predators with a flamboyant back sail, like Dimetrodon. Later in the Permian, they are outcompeted by other bizarre proto-mammals: the gorgonopsians.

You might describe them as large 'sabre-toothed rats' — slender predators between half a metre and three metres long, with a broad, flattened snout and impressive canine teeth. These they sank into the flesh of other mammal-like creatures, the dicynodonts: stocky herbivores resembling a wild boar with the head of a turtle. These are among the forgotten weirdos of the Permian, which Hollywood has shown little interest in.

In the archosaurs, however… Their kingdom begins after an apocalyptic period. The Great Dying is the largest mass extinction the Earth has ever known. "Most people know about the mass extinction at the end of the Cretaceous, but at the end of the Permian, about 252 million years ago, as many as 95% of marine species and 70% of land species went extinct."

This third of five great mass extinctions did not happen all at once, but in pulses over a period of roughly fifteen million years. Many animal groups could not recover their lost diversity between pulses. In the final two million years of the Permian, what is now Siberia was literally cracking apart. Gigantic quantities of lava poured through fissures in the Earth's crust. The volume of basalt (rapidly cooled magma) in the last million years of the Permian is estimated at four million cubic kilometres. If you were to spread that basalt evenly across Belgium, the layer would be 130 kilometres thick — thirteen times the cruising altitude of a commercial aircraft.

The Great Dying is the largest mass extinction the Earth has ever known

The amount of CO₂ released from the ground during that million years is estimated at 208,000 gigatons — 80 times more than humanity's total emissions since the Industrial Revolution. The eruptions also released sulphur and heavy metals. All of this led to intense warming (in a world already hot), acid rain, ocean acidification and oxygen depletion, a hole in the ozone layer and metal poisoning. The flood basalts of the Siberian Traps draw a thick line: goodbye, Palaeozoic. We shall not see these creatures again (except as fossils): trilobites (which existed for 270 million years), sea scorpions, the corals of the Devonian and Carboniferous, and on land the proto-mammals with their back sails and sabre teeth. The Earth had very nearly wiped the slate clean — but a few survivors were handed a new playing field.

A traveller to the middle of the Triassic, 230 million years ago, some twenty million years after the greatest mass extinction, would search a long time for dinosaurs. They exist, but they are no bigger than a cat. Koen Stein: "You would, however, encounter plenty of large and fearsome archosaurs. They look as though someone had fitted a crocodile with a T. rex head and asked it to walk upright. The diversity of such crocodilian-like creatures — including the feared Rauisuchia — was enormous in the Triassic. There were even herbivores among them."

The still-small dinosaurs share an important characteristic with those crocodilian archosaur cousins: upright posture. Their forelimbs and hindlimbs sit directly beneath the body, not splayed to the sides like in crocodilians and lizards — cold-blooded crawlers that have no need for speed. The dinosaur femur had a head at the top that fitted perfectly into an open hip socket. This turned the legs into natural load-bearing columns, vastly increasing stability and carrying capacity. With legs beneath your body, you become a sprinter; you can cover greater distances, catch prey more quickly, while expending less energy as your limbs move fluidly. Legs beneath the body is an evolutionary tipping point.

In Argentina and Brazil — in the wetter zones of the otherwise dry Pangaea — early Dinosauria from the mid-Triassic have been found since the 1960s, including Herrerasaurus, Eoraptor and Pisanosaurus. The differences between these three would lead to the great split into three clades. Herrerasaurus and its relatives gave rise to the lineage of carnivorous, agile, intelligent predators such as Velociraptor, T. rex and birds — the theropods. Eoraptor and kin are in all likelihood the forerunners of the plant-eating long-necks, the sauropods. And Pisanosaurus, or close relatives, are thought to have led to the lineage including Iguanodon, hadrosaurs (duck-billed dinosaurs), horned dinosaurs like Triceratops, and armoured ones like Stegosaurus and Ankylosaurus.

This last group, the ornithischians, have a pelvis in which the pubic bone is rotated backwards. In theropods and sauropods, it points forward. This division between ornithischians (bird-hipped) and saurischians (lizard-hipped), based on the position of the hip bones, has long been a classic distinction. But today palaeontologists are testing alternative family trees by processing enormous datasets of anatomical characteristics with statistical programmes. The debate about the major branches of the dinosaur family tree is very much alive.

Did those early dinosaurs also walk here in Belgium? Yes. In a bone bed at Habay-la-Vieille, in the southernmost part of our country near Arlon, dinosaur teeth have been found from the Late Triassic — from all three dinosaur groups: sauropods, theropods and ornithischians.

Around 210 million years ago, dinosaurs had not only become more diverse and numerous, but also larger. "Some had long necks, like Plateosaurus, weighing up to three tonnes. In the Europe of that time, they were as ubiquitous as cows today. They were found in abundance in Germany, France and Switzerland. Every self-respecting natural history museum has a genuine specimen on display. It is from this group that the iconic long-necks of the Jurassic will evolve."