Today, we unearth dinosaur fossils of unfathomable size.
And we live to tell stories about how extraordinary they were.
How did they get so big? And what was their demise?
What is the lineage of dinosaurs?
If you turn back the clock 252 million years back, dinosaurs didn’t exist. But ideal conditions were shaping up for dinosaurs. Earth’s climate was much hotter and drier. Eventually, rainforests collapsed and deserts swept the land.
Amphibians already partially evolved to land conditions. But reptiles were different because they could lay their hard-shelled eggs on land and travel long distances. Overall, reptiles cut ties with the ocean and adapted to the drier land conditions.
Reptiles gained an ecological advantage because there was less competition for food in dryland conditions. This triggered the start of the age of reptiles. And dinosaurs are just evolved reptiles.
The era of dinosaurs
Welcome to the era of dinosaurs. As desert-like conditions began to spread, dinosaurs (terrible lizards in Greek) began to evolve on the supercontinent of Pangaea. Similar to reptiles, they had scaly skin and hatched eggs like reptiles.
- ARCHOSAURS: Dinosaurs rose from archosaurs. For example, the first fossil record is from the “Nyasasaurus”. This proto-dinosaur first appeared in Tanzania about 231.4 million years ago.
- THEROPODS: About 230 million years ago, dinosaurs branched off into two-legged “Theropods” and long-necked “Sauropods”. At the same time, a different group of reptiles “Pterosaurs” took flight in the sky.
- ICHTHYOSAURS: Finally, a group of marine reptiles “Ichthyosaurs” dominated the sea. Next, the “Plesiosaurus” had four fins, a stubby tail and a snake-like neck.
In the Triassic period, a variety of dinosaurs ruled the land, sea, and sky. Next came the Jurassic Period and is when the reign of dinosaurs reached its peak level of dominance.
For the next 160 million years, dinosaurs were the reigning land vertebrates on Earth. This existed until the Cretaceous-Paleogene extinction.
Why did gigantism exist in dinosaurs?
If you want to understand gigantism in dinosaurs, you have to understand the environment they lived in. Conifers filled the landscape. Oxygen levels increased to 35% compared to 21% today.
A previous theory is that the enormous frame of dinosaurs correlated with oxygen content in the atmosphere. But we mostly attribute the biology and evolutionary pressure for which these reptile-like mammals grew extraordinarily in size.
- DINOSAUR EGGS: For example, dinosaurs laid small eggs on land. This allowed for baby dinosaurs to grow outside their mother’s body. Unlike humans, babies are constrained to the size of their mother.
- AIR SACS: Also, dinosaurs had sacs of air in their frame which helped expand their size and shape. But mammals have dense, heavy bones that limit our size.
- ECOLOGICAL ADVANTAGE: Lastly, dinosaurs that were larger gained an ecological advantage. For example, dinosaurs with longer necks could reach higher for foods. Like an evolutionary arms war, the larger, carnivorous dinosaurs at the top of the food chain gained a significant energy advantage.
But throughout history, bigger is not always better. The only survivor is feathered dinosaurs which are the birds we see flying today.
What happened during the Cretaceous–Paleogene (K–Pg) extinction event?
About 66 million years ago, 75% of species became extinct during the Cretaceous–Paleogene Extinction. Rates of extinction broadly swept the land, sea, and air. In the oceans, ammonites disappeared.
All non-avian dinosaurs became extinct. But avian dinosaurs survived because it was birds that descended from theropod dinosaurs. Eventually, mammals emerged as dominant large land animals.
The cause of this extinction event was from an asteroid impact which left an impact called the Chicxulub Crater. Also, giant floor basalts aggravated called Deccan Traps.