Dinosaurs were incredible organisms, some of the largest to ever walk the earth. They reigned over the Mesozoic era of Earth’s geological history which dates from 251 to 66 million years ago. When an asteroid hit the Earth 66 million years ago, it wiped out the non-avian dinosaurs, meaning all dinosaurs that walked and swam. The avian dinosaurs that survived evolved into modern day birds.
Let’s track the geological events and life history to see where dinosaurs fit in.
3.5 billion years ago – Early life on Earth
Life would begin around 3 and a half billion years ago in the primordial seas as single celled organisms. Early organisms evolved in a greenhouse gas heavy atmosphere. Photosynthesising organisms would evolve and flood the atmosphere with oxygen. This would trigger the evolution of organisms that would utilise oxygen and at the same time, poisoning and killing the then organisms that were adapted to the greenhouse heavy atmosphere. Poor things.
550 million years ago – Cambrian explosion
Around 550 million years was the ‘Cambrian Explosion’. Anything before this is commonly referred to as ‘Pre Cambrian’. This technically covers about 3 billion years of evolution of organisms. A lot happened in this time that would lay the foundation of the organisms that thrived in the Cambrian explosion and beyond. The Cambrian Explosion saw a major shift in diversification of species with newly evolved skeletons and body plans.
520 million years ago – Vertebrates
Vertebrates evolved near the beginning of the Cambrian explosion around 520 million years ago, and have diversified massively since. Vertebrates have a backbone, limbs and nervous system which all connect at the head. Vertebrates would first evolve to become jawless and then bony fish, then arthropods on land developing into semi-aquatic amphibians. Then would come the reptiles of which dinosaurs are a part of, and from them birds would evolve. The mammals would evolve at the same time as reptiles though from an earlier evolutionary line giving rise to a completely different organism in behaviour and appearance, though mammals would dominate much later on in Earth’s history.
‘Invertebrate’ isn’t actually a term in taxonomy, it is too broad to include all the organisms that don’t have a backbone. The majority of species are invertebrates, as much as 95%.
320 million years ago – Vertebrates Split
The vertebrates would split into two main groups, the sauropsids and synapsids. The sauropsids would evolve into reptiles including dinosaurs and modern day birds. The synapsids would evolve into mammals.
251 to 66 million years ago – Mesozoic Era
This is the era of geological history where dinosaurs would first evolve and then die out by the end of it. This era is bracketed with extinction events marking the geological changes that give rise to its dating. It is sometimes dubbed ‘The Age of Reptiles’.
251 to 201 million years ago – Triassic Period
The Triassic period was marked either end with extinction events. It begins with The Great Dying extinction event which is the third extinction event of the big five, and the worst extinction event ever recorded. It would take several million years for life to bounce back. At the end of the Triassic comes another extinction event which marks the fourth event of the big five. This extinction event happened in several stages and badly affected the oceans. It did affect land animals with early species of dinosaurs dying out but they would survive into the Jurassic. During this time, oxygen levels stayed quite low so the dinosaurs in the Jurassic evolved more efficient breathing systems which they passed onto modern day birds.
In the Triassic, the landmasses were connected as the supercontinent Pangaea. Pangaea started to come apart at the end of the Triassic around 200 million years ago and this breaking up of the landmass lasted well into the Jurassic. As the landmass of Pangaea was so large, coastline and water areas were naturally less abundant. The land would have been quite arid in the middle however the seasons were quite drastic with very cold winters and hot summers. There were heavy monsoons as well.
What we know collectively as ‘dinosaurs’ would begin to evolve from earlier reptiles in the late Triassic. They would survive the fourth extinction event and come to dominate the Jurassic. Early mammals were around in the Mid-Triassic, they would stay relatively small and out of the way while the reptiles dominated.
201 – 145 million years ago – Jurassic Period
At the beginning of this period, Pangaea the supercontinent would start to drift apart. It would break into two main landmasses, Laurasia to the north and Gondwana to the south. Any dinosaurs that survived the fourth extinction event would have begun to evolve different traits as these landmasses started to drift apart. Generally the Jurassic period was quite warm with atmospheric co2 maybe four times higher than it is today.
It was in this period of time that the first birds would begin to appear. The Pterosaurs were the earliest vertebrates with powered flight, and they evolved in this period and would continue into the next. Also crabs, lizards and the diversification of mammals. Though mammals in this period were small and likely underground and nocturnal due to larger predators roaming on land during the day. The oceans were teaming at this time too with new species after the extinction event including new species of sharks.
As well as dinosaurs, other reptiles to evolve here were turtles and crocodiles. There were many smaller reptile creatures living in the waters as well as on land, each diversifying into many different species.
145 to 66 million years ago – Cretaceous Period
The warm temperature would continue into the Early Cretaceous period. This was partly due to the extra co2 that would have been released into atmosphere due to the breaking up of Pangaea. During this time, new species of dinosaurs would evolve as well as some species surviving from the Jurassic.
It was in this period around 130 million years ago that flowering plants would evolve. This will come to include all plants with seeds so not just flowers but crops as well. This would have widely increased the available foods for herbivores and potentially prompting new eating and digesting methods in order to process and extract different elements from these foods.
It was at the end of this period the asteroid hit, and wiping out all non-avian dinosaur species. The ones that survived evolved into modern day birds. Other species today with common ancestors of dinosaurs would be modern day lizards and crocodiles.
Dinosaur Classification
Classification of organisms is an organisation system called taxonomy that groups organisms together according to similar characteristics, usually in body plan. Different body plans evolved to adapt to different environments and create different behaviour patterns in order to successfully survive and thrive in that environment.
The classification system begins with the three kingdoms (sometimes four) of plants, fungi and animals. These are then split into various classification levels of phylums, classes, orders, families, genus and species, with many levels in between such as superfamilies and clades.
The dinosaurs come under the kingdom of Animalia and the phylum of Chordates (vertebrates). There is a superclass within Chordates called Tetrapoda (four limbed verterbrate) and within that, a clade of Amniota. Within Amniota resides the vast majority of land and semi-aquatic vertebrates. Within Amniota are the clades of sauropsids and synapsids. There were other clades but they are now extinct. The sauropsids would evolve to be reptiles, dinosaurs and modern day birds. Mammals are the only group of synapsids that have survived to today.
Dinosaurs come under the clade of Dinosauria. Dinosaurs are typically split into two orders, Saurischia (lizard hipped) and Ornithischia (bird-hipped). All the dinosaurs fall under one of these two orders. They are then further divided into various families, genus and species with a few more layers in between of classification.
Dinosaur genus and species
While there are many species of dinosaurs, let’s explore a few in timeline order of evolution.
Brachiosaurus is a genus of dinosaurs from the family Brachiosauridae that lived in modern day North America. They evolved in the late Jurassic around 150 million years ago. They were on average 20 metres long with a very long neck and small skull, munching on tree tops. These were famously in the Jurassic Park film and so gained familiarity with the public.
Stegosaurus is a genus of herbivore dinosaur from the family Stegosauridae that evolved in the late Jurassic, approximately 150 million years ago. It had plates along its back like armour and spikes on their tails. There are a few recognised species of this genus and they have been found in modern day Western US and in Portugal. They were large and heavy with the largest ones measuring 7.5 metres long.
T. rex, officially known as Tyrannosaurus rex is a species from the family Tyrannosauridae. They have been found in modern day Asia and North America and evolved in the Late Cretaceous around 80 million years ago. The species from this family would grow to be some of the largest and fearsome of dinosaurs, giving it worldwide fame in the modern age. T. rex stands on two hind legs with short arms, with an average body of 13 metres and a head of 5 feet.
Velociraptor is a genus of dinosaur that originated in today’s Asia in the late Cretaceous, around 75 million years ago. There are only a couple of species that have been recognised in this genus, but it has still made it a popular one for the Jurassic Park films as the small but vicious dinosaur. Its body was around 1.5 – 2 metres long, has feathers, with a long tale and enlarged claws.
Triceratops is the name of a genus of dinosaur from North America, from the family Ceratopsidae. Some species of this genus included prorsus and horridus. These were plant eating dinosaurs and evolved in the late Cretaceous period around 68 million years ago. Species would have a parrot-like beak and a huge frill on the back of its head.
Dinosaurs will long fascinate mankind far into the future, and who knows whether genetic engineering will truly recreate extinct species.
