Biology Index
By Richard Bruce BA, MA, and PhC in Economics

Mysteries of the Mesozoic Explained

The Mesozoic has some very big mysteries. For example, why were dinosaurs so big compared to the mammals that lived in the last 65 million years since the dinosaurs, except birds, went extinct? Another mystery is, given that the proto-mammals, also called therapsids, became warm blooded millions of years before the proto-dinosaurs, also called archosaurs, why were the proto-dinosaurs able to evolve in the first place? One would think that newly evolved warm bloods, the archosaurs, would be poorly adapted compared to the old warm bloods, the therapsids, and therefore the archosaurs would be quickly eliminated. Still another mystery, is why was Quetzalcoatlus the last of the pterosaurs also quite possibly the largest? Why were so many of the largest animals roaming the oceans descendants of land animals? Finally, why did the dinosaurs dominate the mammals?

Why Were Dinosaurs So Big?

Dinosaurs were very large compared to today's land mammals, for example elephants. One reason for this is the Mesozoic equivalent of mice, rats, and shrews were mammals. The mammals held the small warm blooded niches. So to the degree that the diseases these small mammals were specific to mammals the dinosaurs would be immune to them. This allowed the dinosaurs to get much bigger than today's land mammals which have to contend with communicable diseases carried by small rodents.

We see a similar pattern today with whales. The smallest mammals in the sea are more than ten thousand times heavier than the smallest land mammals. So whales do not have to deal with the diseases carried by really small mammals the size of a mouse. Therefore, the whales get much larger than elephants, and other large land animals which do have to avoid or survive the diseases carried by those small mammals.

Returning to the Mesozoic, communicable disease is a key factor for understanding why dinosaurs were so big and as we will see many of the other mysteries of the Mesozoic.

This disturbs many scientists. They fret about our inability to measure disease in long extinct animals. Good point, but I am simply assuming Mesozoic biology works the way today's biology does. Closely related plants and animals share many of the the same pathogens, or diseases. Our closest relative the chimpanzee shares all of our communicable diseases. So, this is a very conservative assumption. While we may not be able to observe disease we can easily determine which animals are closely related, paleontologists do that all the time. Making this simple conservative assumption gives us a new tool to answer many of the mysteries of the Mesozoic and other periods of natural history.

Why Was Quetzalcoatlus the Last of the pterosaurs the Biggest?

As long as there were small pterosaurs flying around the diseases carried by those small pterosaurs put a limit on the maximum size of the pterosaurs. As the small pterosaurs were driven in to extinction by the birds the problem of communicable disease was reduced and pterosaurs could and did become larger. The large pterosaurs could compete with the birds using their larger size, and the birds could not grow as large because they were limited by the diseases carried by the smaller birds. So it is quite natural that the last of the pterosaurs, Quetzalcoatlus, is also the largest, or at least one of the largest.

Once again we can see this phenomena in our world. In South America the largest purely freshwater fish is the arapaima. It diverged from its closest relative, the African arowana, in the Mesozoic, more specifically in the late Triassic, the earliest of the three major divisions of the Mesozoic. This happened 220 million years ago. Of course African arowanas do not infect South American arapaimas because they live on a different continent. The closest relative that an arapaima ever meets or eats in the wild is the South American arowana whose lineage diverged from the arapaima's even earlier. The South American arowana is also a very large fish, so we have to go back even farther to find the very small relatives of the arapaima. The arapaima is part of the order Osteoglossiformes which diverged from the family tree of all the other fish in the arapaima's environment, and menu, in the middle Permian, before the Mesozoic began, very roughly 275 million years ago. So as the arapaima has no close relatives it can not become infected by a close relative. This allows the arapaima, like the Quetzalcoatlus, to grow very large.

It might help to compare the arapaima to man. The last common ancestor between an arapaima and a South American arowana lived before the first mammals. So if we measure how related two species are by when their most recent common ancestor lived then all mammals are more closely related to man than the South American arowana is to the arapaima. Science has identified five to six thousand species of mammals. With our modern transportation we are in contact with, and can get diseases from all of them. The palm civet gave us SARS, which killed half its victims. Scientific evidence seems to be saying that the one humped camel, dromedary, is giving us the new disease related to SARS which is coming out of Saudi Arabia. This is the type of challenge that the arapaima does not have, and Quetzalcoatlus did not have.

In North America the largest purely freshwater fish is the alligator gar. Like the arapaima and all but one of the arowanas the gars are large predatory fish. Also like arapaima and the arowanas the gars are generally considered "primitive" because they diverged from all presently existing fish except the bowfin hundreds of millions of years ago, even earlier than the arapaima. The gar and bowfin group, holostei, diverged 320 to 330 million years ago. Once again we have a group of large predatory living fossils that are probably largely immune to many, perhaps most diseases in their environment because they do not have many smaller close relatives in their environment. This immunity to disease allows them to grow larger than fish that have close relatives.

In Europe there is still another similar story. In most of Europe there is only one catfish, and it is giant, the wels catfish. The wels catfish is the second largest catfish in the world and the largest predatory catfish. This is a third example of the same mechanism just among freshwater fish.

The same mechanism works with many other organisms other than freshwater fish. For example the giant redwood and the giant sequoia, the two largest single stem trees. They are living fossils, their only relatives are one another and the dawn redwood in China. Their ranges do not overlap, so they are genetically isolated. So once again, there are many examples of this phenomena today, all of which follow the same pattern as Quetzalcoatlus.

Why Were Almost All of the Largest Animals in the Sea from the Land

In the Mesozoic animals from the land invaded the sea and were usually the largest animals. There are three major groups the ichthyosaurs, plesiosaurs, and mosasaurs. All three groups independently left the land and became very large in the ocean. The ichthyosaurs looked like fish or modern dolphins. The plesiosaurs came in both short and the famous long necked varieties. The mosasaurs had short necks and were awesome predators. None were dinosaurs or descended from dinosaurs. The mosasaurs were descended from lizards.

In the modern world we see the same pattern, the largest animals in the sea are descended from land animals. The whales are the largest animals. They are descended from land animals and their closest relatives are the hippopotamuses. The largest seals are smaller than the filter feeding whale and basking sharks but the male southern elephant seal may average larger than the great white shark, the largest predatory fish in the ocean. Male southern elephant seals are generally several times larger than the largest bony fish, the ocean sun fish.

So we see the same strange pattern in both the Mesozoic and modern ocean, I suspect for the same reason. Coming from the land these animals have a physiology that is radically different from the fish and other animals in the ocean. For example all five groups, ichthyosaurs, plesiosaurs, mosasaurs, whales, and seals breathed air. All had lungs, none had gills. Furthermore none were closely related to the fish or invertebrates which made up most of the ocean fauna. This no doubt protected the Mesozoic marine reptiles and presently protects the marine mammals from many diseases that are specific to fish and invertebrates. Put simply, if you are at the top of a food chain of bony fish, it is best not to be a bony fish. This is why the bony fish, in both the Mesozoic and today are generally not the largest animals in the sea. The sharks and the descendants of land animals are.

Is this disease factor important. There are several reasons to think it is, beyond what I have already pointed out. One both whales and seals generally do not eat warm blooded animals, birds or mammals. Humans follow a similar policy we eat sushi, including raw fish and vertebrates from salt water. But we cook the flesh of mammals and birds before we eat it. The whales and seals can not cook, so they limit themselves to fish and invertebrates.

There are two notable exceptions, the killer whale and the leopard seal. Both animals eat mammals and birds. Apparently part of their specialization is having immune systems that can deal with the diseases they get from their warm blooded prey. I think it is significant that these animals that do eat warm blooded pray, eat both birds and mammals. This suggests to me that the fact that the animals are warm blooded is significant, and I suspect that is because eating warm blooded prey is particularly dangerous for warm blooded animals.

A second point that illustrates the importance of disease is its importance in the world today. The most common wild mammal that is as large or larger than a person is the Virginia white tailed deer. The white tailed deer carries several diseases and parasites, most famously a brain worm, that do not seem to harm the white tail much, but kill its larger relatives, black tailed deer, mule deer, elk, and moose. It has been said these other species can not live in the white tailed deer's range because of the diseases. At very least they make living in the white tailed deer's range difficult.

Smaller animals are frequently more numerous at any one moment in time. They also frequently have a shorter life cycle so they are even more numerous than their larger relatives over time. This means they have more opportunities to develop mutations that can protect them from a parasite or disease. Thus like the white tailed deer they may carry the disease without it killing them. This is why it is so important for larger animals to avoid the range of smaller closely related species.

Note that I did not have to go far for this example. Writers try to strengthen their points by searching far and wide for a relevant example. Once again, I am simply pointing to the most common wild mammal in the world as large or larger than a human perfectly illustrates my point.

We can also see the same phenomena with our closest relatives, the great apes. They are suffering because their close relative, that is the human race has reached seven billion and is infecting them with our many diseases. I have read that this is a major factor in the decline of the orangutan. Once again we do not have to search hard for examples of this phenomena.

How did the Archosaurs Survive and Thrive against Therapsid Completion

Robert Bakker argued in his book Dinosaur Heresies that the ancestors of the mammals, the therapsids, evolved a warm blooded, endothermic physiology in the middle Permian. This was tens of millions of years before the ancestors of the dinosaurs, the archosaurs did the same thing in the early Triassic, once again according to Bakker. As Bakker tells it the archosaurs entered the competition as very large carnivores, larger than any of the therapsid carnivores that they were competing against. This very much impressed Bakker because as he sees it the fight for the position of top carnivore is "particularly unforgiving." The best will win.

My position is the opposite. Because of disease the position of top carnivore is actually one of the easiest positions for a new group to take from an old group. The fittest group will take lower positions on the food chain and send their pathogens up the food chain wiping out their close relatives. But the very distantly related carnivores of the less fit group will be unaffected by many of those diseases and thereby take over the top carnivore position. If you are at the top of a food chain of bony fish it is a massive advantage not to be a bony fish because diseases will accumulate like mercury, but it is even worse this diseases not only accumulate the viruses, bacteria, and other parasites multiply. So whales, seals, and sharks do not have to be the equal of the bony fish to be the top carnivores of the sea and archosaurs did not need a superior physiology as Bakker imagines to take the top carnivore position away from the therapsids, they just needed a different physiology that was relatively immune to therapsid diseases not because their archosaur physiology was better but because it was different.

So if you ever wondered, as I did, why there were two major groups of warmblooded animals that evolved independently, now you have a plausible explanation. The fact that there are two groups is a barrier to the spread of communicable diseases.

Were the Dinosaurs the Winners in the Age of Dinosaurs?

Eventually, at the end of the Triassic and in the Jurassic the Dinosaurs took over as the large terrestrial animals. The mammals were small. They hid in burrows, and trees, and only came out at night. Mammals survived but only at the edge of the triumphant dinosaur's kingdom.

Once again I offer a different view. The therapsids and their descendants the mammals successfully evolved into small forms. This is very difficult for a warm blooded animal to do because the smaller warm blooded animal has a lot of surface area to lose heat and little mass per unit of surface area to generate heat. Furthermore, it was difficult for the large therapsids and any large mammals that might have evolved to survive because all those small mammals were carrying diseases that could kill them.

Today small warm blooded animals like mice, moles, and shrews, massively outnumber large warm blooded animals, I suspect it was the same in the Mesozoic. In terms of numbers of individuals the Jurassic and the Cretaceous were probably an age of mammals, just as in terms of numbers of individuals the Triassic and the last half of the Permian had been an age of therapsids. Any large mammal would have had to compete with the dinosaurs, but also the small mammals that could infect them with diseases. It was the combination of these factors that prevented them from evolving.

Communicable Disease Explains Many of the Mesozoic Mysteries

The more closely related two species are the more communicable diseases they have in common. Knowing this helps us understand why the dinosaurs were so big, why the last of the pterosaurs was quite possibly the largest, why the land dwelling reptiles because the largest animals in the sea, why the archosaurs were able to survive against the more highly evolved therapsids, and why the mammals were relegated to being small, nocturnal, burrowers and tree dwellers.

The biologists and paleontologists may not like this because we have no direct measurements of disease in the Mesozoic, but if we are willing to assume that biology worked then as it does today then the puzzling patterns of the Mesozoic are a lot less puzzling.

Paleontologists look for answers in fossils. More recently we study genomes to determine when one group diverged from another. As they are scientists, paleontologists look at the evidence. In this essay I take a different approach. By better understanding what is happening today, I better understand what was happening then.

You can find more of my speculation on my biology index page.

Once your found the fatal errors in my reasoning on this page or one of the other pages you can pass on your insights to me several different ways. Here is my contact information

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