A Brief History Of Dinosaurs

Dinosaurs first appeared in the latter half of the middle Triassic 230-240 million years ago; they were originally bipedal carnivores, and quite small relative to the colossal dinosaurs that proceeded them. These primitive dinosaurs rapidly evolved into the two major clades we know of today; Ornithischia and Saurischia, Saurischia being comprised of both Sauropoda and Therapoda. Saurischia makes up all carnivorous dinosaurs and birds, as well as the long-necked herbivorous sauropods while ornithischia is classified as any non-sauropoda herbivorous dinosaur such as the armoured dinosaurs (Ankylosauria). Dinosaurs truly “conquered” Earth, spanning across a giant 165-million-year period they dominated and spread to every inch of the land and skies; but how?

It’s believed that the success of dinosaurs was propelled by the misfortune of prior life, the Permian-Triassic extinction event, also known as the “great dying” resulted in the extinction of “90% of the sea creatures and 80% of those on land during this period about 250 million years ago” (Spears, 2004). The commonly agreed upon theory for the cause of the Great Dying is that over a period of thousands of years constant volcanic eruptions flooded the atmosphere with particulates, significantly altering the climate and depleting oxygen levels; essentially suffocating all life, however some scientists believe it was caused by a meteor that was travelling so quickly it vaporised upon impact, altering the planet’s climate to a point where the fallout was still being seen millions of years later (Spears, 2004).

Regardless of the exact cause, this huge loss of life meant that there were vast amounts of space and thus great ecological opportunity, giving rise to the dinosaurs. With all the big hunters extinct, dinosaurs burst out of South America and began occupying different habitats and climates; encouraging them to fill niches and speciate. As well as this, as the success and population of dinosaurs continued to exponentially climb, movement became more restricted and competition increased, this put further selection pressures onto dinosaurs to occupy small ecological niches and become more specialised to avoid competition with other species. The specialisation of dinosaurs was one of their greatest attributes, but it was also their greatest weakness, specialisation in dinosaurs was a double edged sword in that it allowed for every habitat to be potentially occupied by a species of dinosaur, but also meant that they were incredibly vulnerable to environmental change.

While in the recent past many scientists believed that after the Great Dying dinosaurs simply overwhelmed their competitors and spread throughout the corners of Earth, many believing “dinosaurs as very successful animals that outcompeted other species wherever they went” (Reisz, cited in How dinosaurs conquered the world! - ProQuest, 2010) a recent study suggests dinosaurs were in fact “opportunistic creatures” (Reisz, cited in How dinosaurs conquered the world! - ProQuest, 2010) that only moved into areas such as North America following the Triassic-Jurassic mass extinction event 201 million years ago; this was theorized from Sarahsaurus fossils which suggest that Sarahsaurus and fellow sauropods migrated “in three distinct waves and only after the extinction event, what habitats were relatively free to occupy” (Boswell, 2010). Overall, this evidence suggests with a moderately firm basis that early dinosaurs functioned at least partly on an opportunistic basis and chose to expand outward when mass extinction events made ecological gaps that they could fill.

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Additionally, another problematic competitor to the dinosaurs and their spread was the pseudosuchians. Prior to the Triassic-Jurassic extinction event they were significantly bigger and more successful than fellow dinosaurs of the time, however after the Triassic-Jurassic extinction occurred, they were almost completely wiped out, opening up a gap for dinosaurs to take on the role as the dominant reptiles. Although it isn’t completely known why dinosaurs survived the Triassic-Jurassic extinction, it’s theorized that their breathing system; the same as that of birds today made the saurischian dinosaurs far better equipped than mammals and “old” reptiles such as pseudosuchians to deal with intense environmental conditions in which the oxygen was half of modern day’s 21% (Ultra-low Oxygen could have triggered Die-offs, 2003). Based upon this information it’s somewhat clear that dinosaurs replaced pseudosuchians as the most common and widespread animal, and based on little evidence it’s inferable that a reason they may have survived the Triassic-Jurassic extinction is the efficiency of their bird-like lungs, but ultimately this theory is uncertain.

As the Jurassic period began and dinosaurs expanded due to few competitors, they began exploding in size, one reason which allowed this was the presence of pneumatized bones; these are bones which have hollow spaces within them, allowing them to be significantly lighter than usual whilst retaining a lot of strength; ultimately meaning they could be larger than typically possible whilst resisting the effects gravity would usually have on an animal of that size. Moreover, as touched on before many dinosaurs had a similar breathing system to modern day birds, besides lungs they also had air sacks that would protrude spaces in their bones and allow for highly effective breathing (How did dinosaurs get so big?, 2009), and more so than other competition of the time. Evidently dinosaurs being bigger reduced the number of animals that could compete and prey on them, which practically ensured the fact that they would conquer Earth.

Despite dinosaurs being so seemingly invincible, they were completely wiped out by the end-Cretaceous mass extinction that ended their reign 66 million years ago. An asteroid estimated 6 miles in diameter hit earth and left a crater over 180km across (Hartmann, n.d.). During the impact a colossal amount of kinetic energy would have been created and millions of tonnes of debris would have been thrust into the atmosphere; likely leading to an Earth “shrouded in a dust layer that blocked sunlight for several weeks or months” (Hartmann, n.d.). Additionally, debris would have eventually fallen back down to the troposphere, resulting in a bombardment of fireballs thousands of degrees hot. The energy released from these alone was probably enough to kill the majority of flora and fauna on the surface from heat stress. Organisms with no where to hide either and or due to their size and lifestyle like the dinosaurs “might have been decimated in hours or weeks” (Hartmann, n.d.), however organisms who lived underground had a far greater chance of surviving, which is likely why mammals proceeded dinosaurs after their extinction.

Following the primary effects of the asteroid, it’s likely an impact winter would have followed; this is the theorised result of an asteroid impact where millions of tonnes of debris being thrown into the atmosphere results in skies blanketed in ash, preventing the sun from reaching the surface and rapidly lowering temperatures. As touched on before, dinosaurs were a highly specialised species and were extremely vulnerable to change in an environment, therefore the adaptions required for them to survive the sudden change in climate the Chicxulub asteroid caused would have been all but impossible.

In conclusion, it’s clear the dinosaurs 180-million-year reign was both begun and ended by mass extinction events. Initially being on the side better adapted to survive in an environment wiped clean of competition following the Permian-Triassic extinction they thrived and spread throughout the world, taking advantage of the ability to fill various niches and specialise themselves to specific habitats. This effort was greatly supported by the advantages of their physiology such as pneumatized bones and an effective breathing system, which allowed them to grow to unimaginable sizes whilst also potentially being a factor in why they overcame the Triassic-Jurassic mass extinction and outlasted the majority of the pseudosuchians and other competitors. In the end the dinosaurs fell victim to their most powerful attribute of specialisation being unable to adapt in a post-asteroid world where the former ecological niches they belonged to no longer existed.