Transcription
During the Mesozoic, there were three major groups of animals that ruled the sky, land, and waters: those being the pterosaurs, non-avian dinosaurs, and marine reptiles, respectively. Pterosaurs, in particular, were quite successful. Through their 160 million year reign, they experienced great strides in size and expansion, spreading throughout every single continent on Earth.
Although they weren't alone, as there was a myriad of flying insects that traversed the skies as well. Plus, there was also the avian dinosaurs, of course, which included birds. This class of animals originated during the late Cretaceous, about 72 million years ago, but their earlier, primitive ancestors had been around for far longer, first appearing in the middle Jurassic Period, approximately 165 million years ago.
From the get-go, they were rather successful on prehistoric Earth, yet they did not reach the same kind of dominance that the pterosaurs enjoyed, mainly because pterosaurs had evolved earlier, thus giving them a head start in a largely untouched niche. With the oldest specimens of pterosaurs dating to the late Triassic, or around 228 million years ago, and through the years of coexisting, birds largely remained much smaller in stature and worse at flying longer distances. This, in many ways, made them inferior to pterosaurs. And yet, as you may know, it was birds, not pterosaurs, that would go on to survive the K-T Extinction event 66 million years ago, securing their place as the new rulers of the sky. The question is, though, how?
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And now, back to the question on why birds ended up surviving the K-T Extinction event when pterosaurs, despite their many advantages, did not. And before answering this, it's important to realize that most birds did, in fact, die during this event. The Mount Everest-sized asteroid, which struck Earth at speeds 12 times greater than a tank shell, caused many catastrophic effects, including a thermal pulse, shock waves, mega tsunamis, earthquakes, and wildfires. On top of all this, the material that was thrown up by the impact eventually had to come back down, creating immense friction and heat in the atmosphere, essentially turning the Earth into a giant oven that killed anything exposed. As a result, many birds did, of course, perish. Yet, a small handful did survive and eventually recovered. On the flip side, while pterosaurs may have survived initially, like birds, they clearly never recovered. To this day, scientists are not 100% certain on the reasons behind this, but believe there could have been a few factors, with a potentially major one being the difference in the two's sizes.
As pterosaurs were first on the scene, they were able to establish a foothold in many niches without challenge. They became both large and small in form, although as time passed, they specialized more and more to fill the niche of being an apex predator, leading to a tendency of the species being rather big. This growth in size may have also been exacerbated by birds in the later stages of the Cretaceous, as it was then that birds began to truly thrive and take over the smaller carnivore and omnivore niches within their ecosystems, which led to perhaps the putting evolutionary pressure on pterosaurs to get increasingly large. This discrepancy was, for a long time, very beneficial for pterosaurs, as it meant that birds were no match head-to-head. However, when the asteroid came, the tables were turned, as the impending effects that followed were harder on larger animals, which had trouble both from a food source perspective and from being able to shelter from the elements, leading to nothing bigger than a full-grown raccoon surviving the K-T Extinction event.
On the other hand, birds, with their smaller bodies, could venture deeper into more protected areas, such as marshlands and thick brush, to avoid some of the lethal conditions that developed. Their size would also come into play in the post-apocalyptic world that emerged, as they require less energy than most animals, which was huge when considering that resources were extremely scarce. On the other hand, surviving pterosaurs would have had a very difficult time, as during the final days of the Cretaceous, the giant pterosaur family known as azhdarchids were the most abundant and widespread pterosaurs around. A significant issue, as many members, like Quetzalcoatlus, were giants who required large amounts of calories, which simply didn't exist after the impact. And that isn't to say that all pterosaurs were giants, but it should be understood that even the small pterosaurs of the late Cretaceous were still relatively big, with wingspans being between 1.5 and 2 meters, or 4.9 to 6.6 feet, making them just as big as the largest birds of the time.
The pterosaurs also likely struggled after the initial devastation due to their relative lack in diversity, with birds again holding an edge here. Surprisingly, there are actually more known types of pterosaurs than birds from the Mesozoic, with 268 known pterosaurs and 263 known birds. However, this changes when focusing solely on the Cretaceous, where bird genera outnumbered pterosaurs roughly 2:1. As a result of their higher diversity, Cretaceous birds varied a lot in terms of preferred diet and habitat, whereas pterosaurs did not have such diversification. This made recovery easier for birds, as they had a higher likelihood of being accustomed to any given ecosystem left standing, while remaining pterosaurs had less of a chance, as there were fewer ecosystems that they specialized in.
This habitat resilience is actually demonstrated by the pollen differences above and below the K-T boundary, which suggests that only two types of ferns dominated the world after the impact. This supported the idea that a great deforestation of trees had occurred, thus wiping out many tree-dwelling animals, including birds that lived in trees. This could have spelled doom for birds as a whole. However, thanks to their diversity, there were ground-dwelling birds as well, allowing them to more easily adjust to a world overrun by ferns. Additionally, the diversity advantage didn't just help them outperform pterosaurs when it came to habitats, but also with what food remained. It's currently thought that insects, seeds, and nuts were the most abundant source of food after the event, which proved to be a huge win for birds that had small, precise, yet tough beaks that could pluck up these items with accuracy and crush them.
However, this ability wasn't always unique to birds, at least when it came to insects, as in the beginning, both pterosaurs and birds were toothed and liked to snack on small invertebrates. Then, as millions of years passed, the pterosaurs, in addition to getting bigger, began to slowly shift diets, with them ultimately ending up eating food that was almost exclusively composed of fish and meat. While birds, on the other hand, slowly lost their toothy nature in favor of relatively small beaks. This being said, there are some birds from the late Cretaceous that still had teeth, exemplified by Ichthyornis, but most were toothless. This, as mentioned, allowed them to eat smaller, harder items, giving them one foot up over pterosaurs, who at the time were also mostly toothless but had large beaks not designed to pick and pluck smaller food items.
And birds didn't stop here, as while the vast majority of pterosaurs could no longer eat such varied foods, birds were still able to eat meat and fish on top of the smaller food items mentioned. This was made possible by their tough beaks, yet again, which could shear flesh off of prey or grip fish, which could then be swallowed whole. This is also the part where their gizzards would come into play. This is an organ that functions as a second part of their stomach and is extremely muscular, allowing them to grind up food that has been swallowed whole, like pieces of meat, fish, and small invertebrates. All these tools gave birds an array of food options that pterosaurs simply could not compete with, which ultimately granted birds yet another win.
And to add fuel to the fire, birds still had other tools up their wings that made them the favored animals to endure the K-T Extinction event. An overlooked one being their intelligence. Birds are one of the smartest animals that we know of and have large brains relative to their bodies. Their intelligence is well-documented, and some are even known to have occasionally create and used tools, as demonstrated by crows. This general level of heightened intelligence may have allowed birds to make smarter decisions and adapt their behavior to the harsh conditions that followed the impact, whereas pterosaurs may have struggled. Unfortunately, pterosaur intelligence is not well understood, and we still have no concrete idea of how smart they really were, and this makes it hard to compare them to birds. Though, from what anecdotal evidence we have and preliminary studies that have been carried out, it does seem that they were not as sharp as birds, possessing smaller brains when compared to their bodies. It's been proposed that this hypothetical difference in brains was due to birds inheriting their gray matter from big-brained theropods, while pterosaurs got theirs from relatively small-brained archosaurs. However, further research needs to take place before we can confirm this.
Ultimately, intelligence, along with diet diversity and size, may have been the deciding factors in the fate of birds and pterosaurs. Yet, it should be remembered that it could have come down to luck as well, with birds simply being in the right places at the right time. And that about wraps it up for this video. But if you found this interesting, you should check out our video on the largest pterosaur to ever fly, as well as the rise of the most terrifying family of birds to exist, the terror birds. And like always, thanks for watching, and until next time on Extinct Zoo.