66 Million Years Ago: Dinos on the Decline

Surprisingly, two very similar-seeming things can possess vast differences. The Cretaceous period, 145-66 million years ago, was the last time dinosaurs truly roamed our Earth. The remnants of which evolved to become what we know today as birds. 

Unlike the dinosaurs, birds are avian creatures. Avian dinosaurs are also known as Pterosaurs, such as the Pterodactyl. Despite their deceptively similar appearance, Pterosaurs are actually a separate species from dinosaurs. Our knowledge of prehistoric fauna is still very limited and unequal; we know much more about the northern hemisphere than the southern. This creates a great divide in our understanding of the two hemispheres, painting an incomplete picture of Cretaceous biodiversity worldwide. Historically, the north has yielded more fossils, overall providing better insight into our old world. Yet recently, things are changing.

It’s not that the southern hemisphere does not contain any fossils, it is just that they are few and far between. The fossilization process is biased; favoring creatures with a skeletal structure. Resulting in soft-bodied organisms being cast aside. When was the last time you heard about a fossil of a jellyfish or a worm? 

The importance of fossils found in the Southern Hemisphere cannot be understated. In 2020, a new dig site in South America opened called Cañadón Tomás. In March 2023, many groundbreaking discoveries were made at this site. The first of which being fossils hailing from a herd of a duck-billed dinosaur species. Hadrosaurus is the term referring to dinosaurs that shared a duck-bill feature, encompassing many subspecies. Dinosaurs with this feature before the Cañadón Tomás excavations were not found within the southern hemisphere. This is the first discovery of its kind.

Another fossil was found among this herd of Hadrosaurs: a snake vertebrate. All hailing from the Cretaceous. Nothing like this was previously seen in the southern hemisphere. Nothing like it will be seen from a later period, given the Cretaceous was the final period before the Cretaceous-Paleogene Boundary. This was the mass extinction event when the Earth was hit by an asteroid and all non–avian dinosaurs ceased to exist. Pterosaurs and Plesiosaurs continued living after this event, this extinction was exclusive to the dinosaurs.

Biodiversity and evolution are immensely related to one  other, however there is much to be discovered surrounding periods such as the Cretaceous and Paleogenic eras. These points in time, identified closely with geologic knowledge, carry information that can be incredibly difficult to come across. This is why it was a huge breakthrough when an excavating site finally discovered the fossilized remains of a certain dinosaur species. Such a species was previously never found within the area, or within the southern hemisphere at all.

Throughout Earth's history, biodiversity has shifted and evolved with the ever changing environment. Many evolutionary processes considerably altered our knowledge on dinosaurs and geological periods. The Cretaceous era specifically can be difficult to understand without enough resources, we currently have a partially incomplete picture. However, based on the fossils recently uncovered, we’ve learned more about how survival was towards the end of the Cretaceous period, just before the non-avian dinosaurs were hit with the mass extinction known as the Cretaceous-Paleogene event. 

Avian animals are recognized as birds or bird relatives, such as certain dinosaurs. Around the time just approaching the mass extinction event, survival became difficult for the dinosaurs previous to their evolution. Results shown from a study on this idea show that carnivores may have experienced a decline in numbers nearly 10 million years before the K/Pg event (mass extinction). The major decline in herbivore availability/variety is a large component in the delayed decline of carnivorous dinosaurs. From our data, we can conclude that the asteroid was not the only element that had caused certain species of dinosaurs to go extinct.

Although they began adapting even before the dinosaur’s extinction, birds have evolved mostly after the fact. Nowadays, there are around 11,000 different species of birds. Birds quickly evolved to survive in their changing habitats and environment, and were able to survive millions of years onwards.

This holds great significance, birds can technically be classified as avian dinosaurs. They are descended from raptor-like small dinosaurs, from which they evolved. Birds even had teeth for  a while! Now, toothed birds have since gone long extinct, but  dinosaur DNA is still extremely prominent within them. Like many species of dinosaurs, birds are feathered. Feathers greatly help  birds, providing camouflage, warmth, dryness, mobility and much more. Paleontologists believe that part of the reason birds kept these feathers was so they could fly better. Improving their mobility, survival and overall livelihood.

However, evidence suggests that dinosaurs may have already been in decline. Roughly 76 million years ago, right in the middle of the Cretaceous period, on a significant scale, diversity of herbivores plummeted as did the temperatures due to climate change. 

Biodiversity is essential to ensure an ecosystem successfully functions. Now we can see that even before the asteroid made impact, the dinos were on the decline.