It is commonly thought this was because the dinosaurs were somehow superior to the mammals, but big does not mean superior. The opposite is the case, big tends to mean inferior. Declining clades, living fossils, tend to be big. For example the biggest trees: coastal redwoods and Sequoias, the biggest spiders: tarantulas, the biggest birds: ostriches, emus, cassowaries, the biggest fish: sharks, the biggest freshwater fish in South America: the arapaima, the biggest freshwater fish in North America: the alligator gar, and I could go on, and on.
And this is exactly what we observe, whales, seals, and sharks sit at or near the top of the oceanic food chain and none of them are bony fish. Today the sharks share the top of the ocean food chain with mammals that are only distantly related to most animals in the ocean because they are descended from land animals. In the Mesozoic it was the same story. Sharks largely shared the top of the food chain with ichthyosaurs, plesiosaurs, and mosasaurs in the Mesozoic. Ichthyosaurs, plesiosaurs, and mosasaurs were all descended from land animals.
Both today and in the Mesozoic many of the largest animals in the sea were descended from land animals. This is part of a larger trend. Organisms that are in environment separate from most of their relatives tend to be large. The reason is the same as it was for the living fossils. The animals in the new environment do not have a lot of close relatives that are smaller to give them communicable diseases.
Furthermore, smaller relatives are particularly dangerous because they are usually more numerous and have shorter life cycles. Because smaller relatives are more numerous they are more likely to have a favorable mutation that allows them to survive with the pathogen. This allows them to carry the disease and infect their larger relatives without dying, and perhaps without being greatly harmed. And because they have shorter life cycles they will go through more generations so they can more quickly evolve an immunity to the disease.
We can see this happening in the deer populations of the United States. The Virginia white tailed deer is the smallest deer in the United States. It carries several diseases, most famously a brain worm, that do not seem to harm the white tail, but are deadly to larger deer: mule deer, American elk (closely related to English red deer), and moose (which are called elk in England). As a result these larger deer are largely extinct within the range of the white tailed deer.
By understanding what is happening today we can better understand the fossils. The reason that paleontologists have misinterpreted the fossils of dinosaurs and Mesozoic mammals is that they do not understand what is happening today and the reason that they do not understand what is happening today is that the biologists do not understand what is happening today. The same principles that control the ecosystems today applied in the Mesozoic.
The therapsids were warm blooded animals that evolved before the Mesozoic in the middle or even early Permian. They dominated the large bodied terrestrial niches. This was the beginning of the dominance of the large bodied terestrial niches by warm blooded animals that has continued to this day. The largest therapsid preditors were very early. As therapsids diversified into smaller and smaller species, for example tiny insect eaters, the largest predators at the other end of the food chain became progressively smaller. It is my contention that the diseases that evolved among the small therapids went up the food chain and killed the large predators.
In the early Triassic the archosaurs seized the position of top predator. The secret of these archosur predators was size. Robert Bakker, Ph.D. claimed in his book The Dinosaur Heresies 1986 that the archosaurs, more specifically the crimson crocodiles claimed the position of top predator because they were superior. (The Crimson crocodiles were not acturally crocodiles and both Bakker and I believe these large, half ton prediors were warm blooded.) Given that the archosaurs were relatively new to the warm blooded competition and the therapsids had been at it for tens of millions of years it seems unlikely that the archosaurs were really superior. What was far more likely was that the huge archosaur predators dominated the smaller therapsid predators because they were immune to diseases that were specific to therapsids. Early on in the contest the prey were mostly therapsids. So the archosaur predators were not likely to be infected by the prey but the therapsid predators were. Furthermore, this immunity to therapsid specific diseases allowed the archosaurs to evolve into very large predators.
With time the archosaurs evolved and were able to compete for and win niches as middle sized and even small predators and large herbivores. But even so they were larger than than the therapsids. So it is reasonable to suspect that they were inferior to the therapsids who very were successfully defending the niches where the body size was very small.
The normal story is that the dinosaurs were superior and the mammals were forced into nocturnal niches with small body sizes. The mammals hid in burrows and up in trees and only came out at night. My view is that the mammals successfully took and defended those niches because they were superior to the dinosaurs. Because the environment had so many small mammals large mammals were not able to compete with the dinosaurs. Large mammals would have had to compete with small mammals to evolve immunities to the diseases they had in common and to compete with the dinosaurs that did not have the smaller relatives with the exception of pterosaurs and birds. Large mammals could not simultaneously win both of these competitions.
Dinosaurs on the other hand did not take the small niches of the mammals because the mammals were superior. Dinosaurs dominated the booby prize niches, large and predator. The mammals dominated the prize niches small and herbivore.
So the therapsid/mammal clad evolved first and was always superior, the archosaur/dinosaur/bird clad evolved later and was always inferior.
Warm blooded animals evolved twice and a superior clad and an inferior clad both survived in large part as a way to deal with communicable diseases. While I am saying the mammals are generally superior birds are clearly better at flying, particularly in the day time, than any mammal.
This theory is about the competion of roughly equivalent groups. I am not saying that insects are superior to vertibrates because they are generally smaller, or that bacteria or viruses are the most superior because they are the smallest. This is about more equal competions: sharks vs. bony fish, mammals vs. dinosaurs, insects vs spiders, etc.
The dinosaurs were big and impressive but under their feet there were far more numerous mammals. These mammals probably had far shorter life cycles and were therefore evolving far more rapidly. To these mammals belonged the future for when the Cretaceous ended the only dinosaurs left, the birds, were banished to the air.
I have many other pages that use the tendency of close relatives to share the same communcable diseases to explain biological phenomena. I also have other pages on biology, some like this have my speculation other have information. People seem to like how I organize information. At any rate you can check out my biology index page
There are several ways you can tell me just how wrong I am. Many are covered in this page on my contact information.
Last Updated July 29, 2020