Large animals over 99 pounds or 45 kilos are defined as Megafuana. Now that we know what exact weight threshold we are looking for, we can look at what time period this first evolved. We are starting the investigation in the Cambrian period, as before then, most organisms were far too small to be Mega fauna. Looking at the Cambrian period, 550-500 Mya, we find a lot of big life, but none reaching sizes nearly big enough to be considered Megafauna. Additionally, even if we saw organisms close to the size of humans in this time period, they would not weigh enough to be considered Megafauna as they lacked the rigid heavy structures that allowed animals to become so large. So going further down the timeline to the Ordovician starting around 500 Mya, we do actually find many organisms that are considered Megafauna. The earliest known Megafauna is an arthropod called Aegirocassis Benmoulae, which lived 480 Mya. The key reason that this animal was so large, 6’6” and around 99 pounds, was that they were filter feeders. Which segues perfectly into why being so big is so beneficial.
In the Ordovician it was supremely beneficial to be larger, as not only could animals better filter feed, they also had very few natural predators. As time went on and more and more mutations occurred and animals became larger and larger, for those two exact reasons. However, as animals got larger, and structures got more complex, jaws evolved, as well as other sorts of weapons. Generally at this point the larger an organism is, the better it is at being an apex predator. These more complex structures helped benefit apex predator survival. So not only were there basically no animals that predated upon the apex predator, it had many prey to choose from. Furthermore, there are even more benefits to being large. Such as sexual selection, and thermal regulation against colder weather. Which is why we find more large organisms closer to the poles.
However, animals didn’t simply stop growing once they became Megafauna, they kept growing. They were further naturally selected to be larger until we get to organisms like placoderms. Nearly 8 meter long heavily plated monsters, that well exceeded the weight of Megafauna, some reaching over 4 tons. This is why I would like to propose a new threshold of giant animals, as the difference between 100 pounds and 1000 is very vast. These would be organisms that reach a weight of one ton. I will be calling them Tortafauna. These organisms first started to emerge in the post cambrian period and are present in the modern day. The only reason that this worked so well is due to filter feeding animals, as well as many other species evolving to be bigger over time as well. Predatory Tortafauna organisms would not be able to survive if most other species remained small as they need high amounts of calories in order to retain their lifestyle. So in some ways, growing bigger became an evolutionary arms race. At this point life found its way on land, and land Tortafauna began to thrive. However, we are focusing on marine Tortafauna.
Looking even a million years ago from today, there were many Tortafauna that we no longer see. There is no known ratio of Tortafauna that existed a few million years ago as compared to now, but looking at the fossil record it is more than clear that we have lost much of Tortafauna diversity. This is due to a multitude of factors, but to sum up why we see so much less of them, is that the environment just doesn’t support their niche as much anymore. Think of how recently the Megalodon was alive, it only went extinct 3.4 million years ago. Which in the time frame of the world is an incredibly short time. The food supply simply wasn’t around anymore, and they died out. There’s also the human element to consider. Humans took out many Tortafauna, such as whales or the wooly mammoth if you are looking at land animals, and most likely more. This resulted in a lessened food pool for other predatory Tortafauna. The environment of the modern day is far better suited for smaller, faster animals, rather than dueling behemoths. However, none of this is to take away from the giant predatory organisms we still do have today. Such as the sperm whale, giant and colossal squid, elephant seals, crocodiles, and a handful of sharks. The other marine Tortafauna we have today are filter feeders, such as baleen whales, and large filter feeding sharks. It is a little disheartening to see how few we have remaining on this earth. Which is why it is integral that we ensure their survival, otherwise we could face an issue of Tortafauna extinction. The vast majority of these species are keystone species, as they occupy the niche of filter feeders and apex predators. So losing them could result in catastrophic results for the oceanic ecosystem.
Featured image of whale shark provided by wikimedia user MarineMegafuana
