Why are modern land mammals so much smaller than animals in the dinosaur 🦕 age?

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During the dinosaur age, some were just massive. Why are modern land mammals, and others so tiny. Comparing a modern tiger to a sabertooth is no contest.

In: Biology

Mammals evolved from the rodent-like animals.

Also there are many ages of time (eras) on earth when dinosaurs roamed in different shapes and sizes.

For examples birds were the dominant species for the long period of time

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Structural and ecological differences. Oxygen wasn’t actually relevant for dinosaurs, just for insects that lived long before dinosaurs.

Comparing dinosaurs to mammals, they had some really important differences. For example, their life history was totally different.

Mammals have one or a few young at a time (big mammals usually have only one) which they care for for several years and which, when independent from its mother, is big enough to fill the same ecological niche. Dinosaurs produced large clutches of eggs which hatched out into tiny babies. In some cases the parents provided some care, in some cases they didn’t, but in either case the care was less extensive than in mammals. And the babies occupied a _totally_ different environmental niche than the adults.

Mammals grow to adult size and then start reproducing and stop growing, all at about the same time. Dinosaurs start reproducing when about half grown and just kept reproducing as they get larger.

So what does this mean for size? Well, egg layers can lay more eggs as they get bigger. Mammals can’t really scale up their reproduction the same way, a big elephant still has only one baby. And bigger animals tend to also have longer gestation times. So dinosaurs benefited from being larger in a way mammals didn’t.

This population structure also means that dinosaurs could have much smaller populations of really big adults, because there was always a larger population of subadults and a ton of juveniles around. Really big mammals could just have a relatively small population of really big adults, which puts them at higher risk of extinction. Even if times got bad for dinosaurs though, and all the really big adults died, the species could be saved because some of the zillions of babies would probably survive.

Dinosaurs also had some structural adaptations that mammals don’t. Mammal cartilage can only get so thick, because it doesn’t have blood vessels. This puts a bit of a limit on how much weight it can support. Dinosaurs on the other hand seem to have had vascularized cartilage and it could definitely get thicker. This let them get bigger. It’s probably no coincidence that whales, which don’t need to support their weight on land, are enormous even compared to most dinosaurs. They don’t have that problem.

Aside from cartilage, sauropods and therapods at least had air spaces in their bones, just like modern birds do. And like modern birds, they probably had air sacs elsewhere in their bodies. This meant they were lighter for their size than an equivalent mammal, which again helps them be bigger.

Sauropods, which again are the real standouts in size (a few big land mammals do match the hadrosaurs and ceratopsians in size) also had an extremely efficient method of eating, with heads on long necks adapted allowing them to efficiently reach a whole lot of plant matter, and food processing that occurred mostly in the stomach rather than the head, freeing them up to consume more food. That let them intake huge amounts of food to support huge sizes. Once you are big enough (and medium sized and up sauropods are big enough) you can just eat full sized leaves and they will take so long to pass through your digestive tract that they will break down entirely.

So there’s some reasons for ya, here’s an interesting article on the topic (be sure to read the comments at the bottom for most of the discussion)

https://svpow.com/2018/01/12/what-allowed-sauropods-to-get-big-and-what-kept-them-from-getting-any-bigger/

First of all, you mention sabertooth, but those did not exist during the time of the dinosaurs. You are conflating two very distant time periods, I assume you’re in part thinking of the last ice age.

At the end of the last ice age about 15,000 years ago, there was a large extinction event, where a lot of large mammal species went extinct. Large creatures take the most food to support themselves, so they’re usually the first to start suffering during a planetary crisis. There’s a lot of theorizing about exactly why this extinction happened, the common consensus is that both climbing temperatures, and overhunting of the land by early humans, were involved.

Many reasons, and not all of them are known, or certain. One possible reason is the size of the continents. Large animals need lots of room to roam. When the dinosaurs first appeared, all of the continents had come together in one super continent call Pangaea. Even when Pangaea started to break up into the continents we have today, it was slow and there were multiple intermediate continents along the way. By the time the continents were fully broken up, the dinosaurs were on their way out. Less land means less room to roam to find food. It also meant major changes to the climate, possibly less friendly to large animals. The asteroid that killed them off may actually have just been the final knock-out punch for an already declining population.

Another thing to note, the saber tooth cats did not coexist with the dinosaurs; they came much later after the dinosaur extinction event. In fact, there were many large mammals (megafauna) that came after the dinosaurs all died. Not as big as the biggest dinosaurs, but still big. Woolly mammoths, woolly rhinos, giant sloths, giant deer, cave bears, and all sorts of massive animals bigger than their modern cousins. These went extinct for different reasons than the big dinosaurs, most notably from human hunting. Not the only reason (probably), but the dinosaurs never had to deal with humans, so it’s definitely different.