Why Don't We Have Animals As Large as Dinosaurs Today?

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Have you ever wondered why dinosaurs got so gigantic but today most animals are relatively small by comparison? This video from PBS Eons explains why.

Why haven't other very large animals, other than a few whales, evolved? There even used to be a giant sloth. Given, the blue whale is actually the largest animal of all time reaching up to 198 tons and a length of almost 100 feet. But there were lots of large dinosaurs, some as big as a five-story building.

When large dinosaurs existed, from the Jurassic to the Cretaceous Periods, our ancestors were tiny little shrew-like creatures. The size of mice. While these little mammals scraped out a living in the dirt, dinosaurs like the supersaurus, sauroposeidon, and argentinosaurus were literally shaking the earth with their footfalls. Among the different groups of dinosaurs, such as sauropods (sauropodomorphs), theropods, and ornithopods, the sauropods were the largest and the heaviest. Even the smallest among them were large than anything else around. And the largest, well, no terrestrial animal has ever come close to their size.

Here are some more of the heaviest and longest sauropods which existed:

Heaviest Sauropod Dinosaurs

  • Argentinosaurus huinculensis: 55.1–106.3 tons (110,200 to 212,600);
  • Antarctosaurus giganteus: 43.5–88.2 tons
  • Notocolossus gonzalezparejasi: 49.5–83.7 tons
  • Mamenchisaurus sinocanadorum: 83 short tons
  • Apatosaurus ajax: 36.0–80.0 tons
  • Patagotitan mayorum: 76 tons
  • Sauroposeidon proteles: 44–66 tons
  • Dreadnoughtus schrani: 24.4–65.4 tons
  • Paralititan stromeri: 22–65 tons

Longest Sauropod Dinosaurs

  • Argentinosaurus huinculensis: 82–130 ft (25–39.7 m)
  • Turiasaurus riodevensis: 98–128 ft (30–39 m)
  • Patagotitan mayorum: 110–121 ft (33.5–37 m)
  • Supersaurus vivianae: 107–115 ft (32.5–35 m)
  • Diplodocus hallorum: 98–115 ft (30–35 m)
  • Mamenchisaurus sinocanadorum: 85–115 ft (26–35 m)
  • Sauroposeidon proteles: 89–112 ft (27–34 m)
  • "Antarctosaurus" giganteus: 75–108 ft (23–33 m)
  • Xinjiangtitan shanshanesis: 98–105 ft (30–32 m)
  • Ruyangosaurus giganteus: 98 ft (30 m)

Once we inherited the Earth after their demise, why did we never grow as large as they did?

As mentioned, we mammals can take credit for the largest animal to ever walk the…swim. The blue whale is even larger than those colossal dinosaurs. But really, its easier to be so large in the water. All that extra blubber actually makes whales more buoyant. On land, such weight would be, shall we say, problematic. Growing large on land is much more difficult.

Largest Land Mammal to Ever Live?

The biggest mammal to ever walk on land was the hornless rhinoceros (Paraceratherium). They lived during the Oligocine epoch, around 33.9 million to 23 million years ago. It is estimated to have weight from 5 to 20 tons (33,000 to 44,000 lbs) and to have stood around 15.7 feet at the shoulder. It was still quite a bit smaller than the smallest sauropod.

Laying Eggs Instead of Carrying Young

Unlike the hornless rhino, which was a placental mammal like we humans, the dinosaurs did not carry their young. They laid eggs. In fact, even largest dinosaurs came from eggs no larger than a soccer ball. Compare that to a large mammal like the elephant, which carries one offspring at a time which grows very large, and gestates for two years. This requires enormous resources. The larger an animal like the elephant or the hornless rhinoceros becomes, the larger their young become and the more inefficient this process becomes. This places a size limit on placental mammals. Growing too large would mean more energy would be required to gestate, and this gestation period would become longer. Being pregnant for years at a time takes its toll! Growing giant babies in their wombs would not have been very efficient for dinosaurs. But by laying eggs, they sidestepped this problem.

Skull of the hornless rhinoceros, Paraceratherium transouralicum, largest land mammal that ever lived
Skull of the hornless rhinoceros, Paraceratherium transouralicum, largest land mammal that ever lived

Light Bones with Internal Air Sacs

Another advantage that allowed dinosaurs to grow so large is that they were remarkably light for their size. This may not make sense, given an animal like Argentinosaurus which could weigh over 100 tons and stretch to 130 feet. But their bones, like their bird relatives, were light. Not having massive heavy bones allowed them to grow so very large without being weighed down by an excessively heavy and hard to move skeleton. One way it is believed they achieved this is through systems of air sacs in their bodies, much like birds have today. These air sacs were positioned in channels throughout their skeleton, and not only do we have evidence from birds today but from fossils, where you can see where these channels would have been. Some of the sacs may have run inside the bones themselves. Although we usually only have bits and pieces of fossilized dinosaurs bones to study, when scientists scan some of these bones, they find hollow spaces where, it is surmised, these air sacs fit. You can think of these as pneumatic skeletons.

This air sac system is theorized to have helped strengthen the relatively light bones. Remember that muscles make us move by moving bones. A large heavy dinosaur-sized bone would take a lot of power to move. Lighter bones buttressed by air sacs would have been much easier to move. Not only does this keep down the total body weight, but less energy required to move means less heat is produced.

Dinosaur internal air sacs compared to bird air sacs
Dinosaur internal air sacs compared to bird air sacs

Hot Blooded!

Dinosaurs were not cold-blooded lizards like we once thought. The were warm-blooded. In fact, the largest were hot-blooded. Their massive bodies make them like giant radiators. Producing less heat, and thus have less excess heat to dissipate, would have been another advantage that made a larger size feasible. This despite the fact that they lived during a time when the average global temperature was higher. The air sac may also have been a part of their respiratory system, allowing more efficient air flow. Like the sauropods, the theropod had air sacs, but not all dinosaurs had them. Then again, they were not so large!

But, Why Did They Get So Large?

So, we can understand why today's land mammals do not get so large. And, we can understand how some dinosaurs became giants. But, that doesn't really tell us why. Just because you can grow very large, doesn't mean it makes sense to do so. In terms of evolution, we do not really understand why this happened. What was the advantage?

One possibility, especially given the largest dinosaurs were herbivores, is that this larger size made them safer from predators. They could also cover more ground. As well, this large size, especially matched with a super-long neck, meant they could forage food more efficiently without moving around. They could reach all the leafy branches in a large area without traveling. Or, perhaps, becoming so large wasn't an advantage at all. In the video, it is brought up that, after all, these dinosaurs are gone. I'd point out this is spurious argument given that the dinosaurs existed for millennia and were, basically, the most successful animals to ever roam the Earth. However, it was the smallest animals that survived the events which killed off the dinosaurs. Like those tiny shrew-like creatures from which we evolved. The smaller lizards are still around as well, while the only thing left of the dinosaurs are their tiny bird ancestors, the largest of which is the ostrich.

Posted on 04 Apr 2018 18:04

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