Jurassic Earth – Land of Giants

Jurassic Earth

201.3 to 145 Million Years Ago

Jurassic landscapeGerhard Boeggemann

This painting illustrates a typical Jurassic scene based on fossils found in what is now Lower Saxony in Germany. The largest dinosaur is a brachiosaurus, accompanied by several iguanadons and two composthagnus. Notice the tiny archaeopteryx perched on the deadwood to the lower right?

The Jurassic Period began 201.3 million years ago following the evolution of the first dinosaurs in the last era of the Triassic. However, now was the time when the reign of the dinosaurs would truly reach its golden age. The Jurassic was a time when the largest and most magnificent animals that have ever existed roamed the Earth. In Part 11 of “Journey through the History of Earth,” we’ll be exploring the veritable Land of Giants, the time when animals the size of houses ruled over our world.

Part 1: Hadean Earth – The Violent Creation of Our World

Part 2: Archean Earth – Signs of Life

Part 3: Proterozoic Earth – The First Animals

Part 4: Cambrian Earth – An Explosion of Evolution

Part 5: Ordovician Earth – Colonising a Barren Land

Part 6: Silurian Earth – The First Breath of Air

Part 7: Devonian Earth – The Age of Fishes and Forests

Part 8: Carboniferous Earth – The Age Bugs

Part 9: Permian Earth – The Age of Amphibians

Part 10: Triassic Earth – The Rise of the Dinosaurs

Part 11: Jurassic Earth – The Land of Giants

Part 12: Cretaceous Earth – The Reign of Tyrants

Part 13: Paleogene Earth – The Rise of Mammals

Part 14: Neogene Earth – Human Ancestors

Part 15: Quaternary Earth – The Age of Man

For 31 million years, dinosaurs had been evolving at an unprecedented rate, radiating into an ever-diversifying range of families, filling the empty niches left behind after the catastrophic Permian-Triassic extinction. Rising to dominance throughout the Late Triassic, the dinosaurs were about to become larger and larger in a warming, high-oxygen world. Cowering in their shadows were our own mammalian ancestors, waiting their turn to rule over the ancient Earth.

The Jurassic Period presents the middle of the Mesozoic Era, the time when some of the most iconic dinosaurs of all time, such as stegosaurus, diplodocus and brachiosaurus evolved. The geological period is named after the Jura Mountains in the European Alps, owing to the limestone deposits dating from this time that were found there. It was first recognized and described by Prussian geographer Alexander von Humboldt in 1795.

Highlights of the Jurassic

  • Largest dinosaurs evolve
  • Major maritime radiation
  • First ceratopsians evolve
  • Pangaea supercontinent splits up
  • Stegosaurus evolves
  • Missing link between dinosaurs and birds appears

Mass Extinction Redefines the Path of Marine Evolution

Heinrich Harder

The dolphin-like ichthyosaurus lived among sea crocodiles and plesiosaurs throughout the Jurassic Period.

The Jurassic Period began following the Rhaetian mass extinction, one of the five biggest of such events in the history of the Earth. Among its many casualties were almost all families of ammonites, a group of marine molluscs that evolved 400 million years ago during the Devonian Period. The eel-like conodonts vanished entirely from the oceans while, on the land, many of the larger amphibians also disappeared as the dinosaurs and early crocodiles took over.

Although a third of all marine genera had disappeared by the beginning of the Jurassic, marine reptiles were about to go through one of the most important stages in their evolutionary history. Plesiosaurs (lizard-like) and ichthyosaurs (fish lizards), which had already been present for millions of years, would now move in to occupy the niches left behind by the casualties of the Rhaetian extinction and become the dominant animals of the Earth’s oceans.

One of the most iconic marine reptiles of the Early Jurassic was the ichthyosaurus, a dolphin-like creature that grew up to 6.6 feet (2 metres) long. In fact, one could be forgiven for thinking that the ichthyosaurus was a dolphin. However, ichthyosaurus lived some 150 million years before the first cetaceans evolved from terrestrial mammals. In other words, they are not related in any way. What this bizarre similarity represents is a perfect example of convergent evolution when two completely different species develop a similar anatomy due to living in similar environments. For the same reason, extraterrestrial life, provided it exists in a similar environment to that of Earth, might look alarmingly familiar.

Pelagosaurus, the lizard of the open sea, was a crocodyliform, a relative of ancient crocodiles. It grew up to an estimated 10 feet (3 metres) long.

Near the end of the Early Jurassic, joining the plesiosaurs and ichthyosaurs, was a group of crocodylomorphs named thalattosuchia, which literally means ‘sea crocodile’. Although they themselves are not direct ancestors to modern crocodiles, they formed a closely related and, for a time, a highly successful group. Among their many members were the long, slender and very fast swimmer teleosaurus and the 33-foot-long (10 metres) machimosaurus, the largest member of this family of marine predators.

Heavily Armed Herbivores and Lethal Predators Battle for Dominance

On display in London’s Natural History Museum, Sophie is the world’s most complete stegosaurus specimen, in spite of being 150 million years old. Despite the name, the animal’s real gender remains unknown.

While many groups had perished during the Triassic-Jurassic mass extinction, dinosaurs managed to escape relatively unscathed. Their dominance over terrestrial ecosystems strengthened yet still. Herbivorous families grew larger, developing protective spikes and plates to fend off predatory dinosaurs such as the coelophysoids, some of which could grow up to 20 feet (6 metres) in length.

Of course, the most iconic of all these well-protected beasts was the stegosaurus, which means ‘roof lizard’. The first stegosaurs appeared around 165 million years ago during the Mid Jurassic. The largest of the three species so far identified weighed over 2.4 tonnes. Adorned with a lavish array of spikes and plates across its back, tail and hips, this animal was more than capable of standing its ground against many of the predators of the time. Unfortunately, however, stegosaurus probably wasn’t the sharpest knife in the drawer, having a braincase the size of a sheep. It was for this reason that the popular misunderstanding that stegosaurus had two brains came about, one of the most common dinosaur myths of all.

Allosaurus was one of the dominant terrestrial predators of the Jurassic, with the largest reaching lengths of up to 40 feet (12 metres).

During the Late Jurassic, stegosaurus shared an environment in what is now North America with allosaurus, the apex terrestrial predator of the time. Three species of these theropod dinosaurs have so far been discovered, with the largest one reaching a length of 40 feet (12 metres). Fast and agile creatures with razor-sharp teeth up to four inches (10cm) long, allosaurus was one of the few animals that would have posed a serious threat to stegosaurus.

A common question in the world of dinosaurs is who would win a fight between stegosaurus and allosaurus. I would say probably the latter, with its undoubtedly greater wit and speed. Nonetheless, given the stegosaurus’s lethal thagomizer, allosaurus likely wouldn’t have won the fight unscathed.

The Late Jurassic also saw the appearance of the first ceratopsians, the group to which the Cretaceous triceratops belongs. However, the very first ceratopsians did not look much like the iconic triceratops, aside from their parrot-like beaks. These relatively small herbivorous ornithischians, such as the recently discovered chaoyangsaurus and yinlong, were all smaller than sheep.

Sauropod Dinosaurs Break All Records as Giants Roam the Land

Also known as seismosaurus (earthquake lizard), diplodocus hallorum was the largest of all diplodocids, reaching lengths of an estimated 108 feet (33 metres).

Another extremely successful group of herbivores that evolved during the Mid- to Late Jurassic were the ornithopods. Starting out as nothing more than small, bipedal grazers, these herbivorous dinosaurs grew larger as they adapted to the predatory environment of the Jurassic. The largest of these was shantungosaurus, which grew longer than a bus from head to tail. However, despite weighing up to an estimated 16 tonnes, these creatures were woefully overshadowed by the sauropods.

Dinosaur size is one of the most baffling things of all when it comes to this extremely diverse and successful clade of organisms. The smallest dinosaur, if we’re referring to the clade, rather than just the popular meaning of the word, is the hummingbird, weighing as little as a tenth of an ounce (3 grams). If we’re only referring to the extinct variety of dinosaurs, the non-avian ones, then the carnivorous microraptor was barely the size of a crow.

The largest dinosaurs may have weighed up to 100 tonnes, reaching lengths of 114 feet (35 metres). These enormous beasts were the sauropods, among the mightiest and most incredible of all the animals that have ever lived. Sauropods dominated terrestrial ecosystems throughout the Late Jurassic, while the closely related titanosaurs, some of which might have been even larger, took their place during the Cretaceous.

Diplodocids, of which diplodocus is the most famous genus, were by far the largest of the Jurassic sauropods. Although almost three times longer than a bus, these incredible creatures were not actually that heavy for their size, due to having extremely long, slender necks and tails. Diplodocids would almost certainly have used their whip-like tails in self-defence. Computerised simulations have even shown that these colossal beasts may have generated sonic booms of over 200 decibels by using their tails like bullwhips, leaving even the fiercest predators fleeing in terror.

Aside from their truly outrageous size, one of the most remarkable things about the diplodocids was the speed at which they grew. Their eggs were barely bigger than those of ostriches; their hatchlings smaller than domestic cats. Reaching sexual maturity by about 10 years of age, these ravenous herbivores would have put on so much weight so quickly during adolescence that you would literally have been able to see the difference in a single day.

Ancient Wings Take to the Skies

Archaeopteryx is quite literally part dinosaur and part modern bird, representing the missing evolutionary link between the two.

Since the Late Triassic, pterosaurs dominated avian niches in almost all areas of the world. Contrary to popular belief, the pterosaurs were not actually dinosaurs. Instead, they belonged new a unique and highly successful group of flying reptiles that lived right up until the cataclysmic extinction event that also claimed the dinosaurs. Throughout the Jurassic, pterosaurs continued to grow larger in size and number, paving the way for complete dominance of the skies in the Cretaceous Period that would follow.

Despite their iconic status in popular culture, pterosaurs leave no successors with us today. An order that is very much alive and well, however, is that of saurischia, one of the two orders to which all dinosaurs belong. One such saurischian dinosaur was archaeopteryx, which lived around 150 million years ago during Late Jurassic. Widely regarded as the missing link between feathered non-avian dinosaurs and modern birds, the discovery of archaeopteryx provides us with definitive evidence that birds are nothing other than modern dinosaurs. This remarkable revelation proves virtually beyond a doubt that our dinosauric friends remain with us to this day.

Archaeopteryx, which means ‘ancient wings’ in Greek, looked much like a bird because, in many respects, it was a bird. Like a modern bird, it had a beak and a furcula (wishbone) and was covered in plumage, though the structure of its feathers suggest that it had only evolved the ability to glide rather than powered flight. The pigeon-sized creature was just as much ancient dinosaur as it was a bird, however, not least because it still had teeth, claws on its wings and a bony tail.

Conclusion

By the end of the Jurassic, some of the largest and most magnificent dinosaurs that ever existed ruled over the Earth. Early crocodiles, lizards, frogs, turtles and the direct ancestors to birds all called the warm Jurassic world home. Vast redwood forests, home to some of the largest trees that have ever existed, spanned millions of square miles, their foliage fed upon by colossal beasts such as the 74-foot-long (22.5 metres) giraffatitan. In next week’s instalment, we’ll be exploring the Cretaceous, the final age of the dinosaurs, in which one of the most monstrous terrestrial carnivores that ever existed tyrannised the land.

Part 12: Cretaceous Earth – The Reign of Tyrants

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