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
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/
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