Tuesday, January 19, 2016

Why Were Prehistoric Animals So Big?

The other day I stopped by the American Museum of Natural History to visit their newest dinosaur display: the Titanosaur!

Discovered just last year in Argentina, this new species hasn’t officially received a name yet, but it is one of the largest dinosaurs yet discovered – the museum reconstruction is 122 feet long – and a lot of the skeleton has been uncovered between several specimens, unlike most poorly-known large sauropods.

The new titanosaur mount at the AMNH,
Photo by me, standing directly under the tip of the tail.
The media-hype over this skeleton, and the recent scientific reports about massive ancient giraffes and giant marine crocodiles, has a lot of people revisiting an old question: why were prehistoric animals so much bigger than their modern-day relatives?

Let’s start with an important question:

Were prehistoric animals really that big?

Not all of them. In fact, most animals throughout the last few hundred million years were the same size or smaller than modern fauna. Even the average dinosaur was only the size of a cow, and many were much smaller. For every huge animal of the past, there were many, many little ones.

But there were plenty of ancient behemoths. Dinosaurs of course dominate the list of “largest land animals ever,” and prehistory seems full of over-sized versions of modern animals: giant snakes, giant insects, giant sharks, giant camels… What was so special about ancient times that fueled the growth of massive creatures?

The most general answer is simple: “the past” is a lot of time. Most major lineages of life have been around for tens of millions or hundreds of millions of years, and at some point most have had the opportunity to produce a titan. But they didn’t all happen at the same time. Titanoboa, the largest snake in history, lived 60 million years ago; the largest crocodile-relative appears to be Sarcosuchus from around 110 million years ago; the enormous griffinflies flew through 300 million year-old forests; Paraceratherium, the largest land mammal ever, lived around 30 million years ago; Megalodon the giant shark persisted for a while between 15 and 2 million years ago; and so on.

So there isn’t anything inherent about being prehistoric that causes an animal to grow to mega-sizes, and the few exceptional cases are scattered throughout the timeline. Many different periods of the past featured their own gigantic species.

But what about today?

Where have all the giants gone?

Not all enormous extinct species are from the distant past. As a matter of fact, many massive specimens were still around recently enough to co-exist with humans. These include history’s largest marsupials (Diprotodon), birds (the Giant Moa and Elephant Birds), primates (Gigantopithecus), bears (Arctotherium), and the largest terrestrial lizards (“megalania”), as well as some of the most massive members of the camels (the Syrian camel), elephants (mammoths), ruminants (long-horned bison), and big cats (including Smilodon and the American lion), not to mention the giant ground sloths, glyptodons, and chalicotheres. All of these animals – and many others! – were around within the last 100,000 years or so, and some as recently as the last few thousand years. Most of human history took place during an age of giants.

So what happened? Extinction.

Over the course of the Pleistocene Epoch, and particularly at the very end of the Ice Age, roughly 10,000 years ago, large species went extinct in great numbers. The underlying cause is debated, but the top two candidates are severe climatic changes, and the spread of the impressive predator Homo sapiens. Whatever the main reason, by the time humanity had begun crafting the beginnings of civilization, most of the largest species were gone.

We, modern-day humans, missed out on some of history's most impressive titans by only a few thousand years.

Are there still giants around today?

You bet there are!

Even though many behemoths are extinct, there are still quite a lot with us. Examples: modern-day elephants, giraffes, hippos, moose, and great apes are close to the largest animals their lineages have ever produced; living big cats and horses have few, if any, larger ancient relatives; the elephant seal is the largest pinniped (and carnivoran) of all time. There are many other examples of huge modern animals: Wikipedia has a rather informative list. They might not be dinosaurs, but to say all the giants lived in the past is just plain wrong.

And, of course, there are the whales. Huge by the standards of any time period. The blue whale is not just the largest whale, not just the largest mammal, but as far as we know it is the largest animal to have ever existed. Ever.

It can be easy to forget sometimes about the giants we have today.
"Whale watching Tadoussac 11" by Hans Bernhard, from Wikipedia.
Why do the biggest animals get so big?

There are many reasons why it may be good to be big (protection from predators being a great one), but there is no single answer to the question of why enormous animals were able get so enormous. The number of answers seems as diverse as the list of giant animals themselves:
  • Many of the world’s largest animals live in the ocean, where the water helps support more massive bodies.
  • Some researchers have suggested that particularly warm climates of the past have helped fuel large body size in ectothermic animals like Titanoboa.
  • On the other hand, Bergmann’s Rule describes the phenomenon that mammals tend to get bigger in colder climates, which may help explain the Ice Age megafauna.
  • For many animals, reaching enormous size is just a matter of waiting long enough; Cope’s Rule says lineages naturally develop larger sizes over evolutionary time.
Here are some often-cited factors that DON’T seem to have a big impact on giant animal evolution:
  • Contrary to what a lot of people blurt out, gravity in the past was NOT weaker.
  • High levels of oxygen (fueling large body size), or carbon dioxide (fueling widespread plant growth) have both been suggested as reasons for the massive size of dinosaurs, but evidence doesn't seem to support these ideas (high oxygen levels are, however, thought to explain the giant insects of the Carboniferous).
  • Interestingly, the large size of the dinosaurs also doesn't seem to correlate with high temperatures.
In many cases, large body size may be a result of certain animals’ unique physiology or lifestyle. To use dinosaurs as an example:
  • The largest of the dinosaurs, the sauropods, seem to have had bodies built for lots of food intake with little movement, helping to fuel big bodies while conserving energy.
  • Many dinosaurs, including the massive sauropods and the biggest carnivores, had pneumatized bones like birds: this means the bones were full of hollow spaces and reinforced areas, making them particularly sturdy without increasing weight.
  • Those hollow spaces in the bones were also full of air sacs, part of a very efficient respiratory system.
  • Finally, even the biggest mammals spend a whole lot of their precious energy developing and caring for young, a reproductive strategy that may not be able to support too large a body. Dinosaurs, on the other hand, laid relatively small eggs, and many aren’t thought to have cared for their young. 
This paper provides a great overview of different ideas on why dinosaurs got so big.

In sum, many of prehistory's largest specimens achieved their great size at different times, under different evolutionary pressures, and with the help of different environmental or physiological factors.

Wrapping up. 

Evolution has been producing giant animals for a long time. Many of them are long gone. Some are still with us today, even though we tend to overlook them while thinking of dinosaurs and other ancient titans. And there will be more in the future, though the questions of When, Why, and Which ones, are very difficult to answer.

And there are undoubtedly more prehistoric goliaths left to find. Surely before long some new discovery will over-shadow even the AMNH's proud new titanosaur.

It's so big!

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