What may have actually happened: Any conclusions about what happened 64 million odd years ago have to be tentative, and the scenario of dinosaur extinction I outline here may soon be outdated by new discoveries, or even discoveries that are already being analyzed. 

With that in mind, here is a picture of what may have happened at the end of the era of the dinosaurs.  At the end of the Cretaceous, a very large asteroid hit what is now the Yucatan peninsula.  Much of the energy released from the impact came in the form of heat, and that heat, combined with superhot debris raining down across large parts of the planet probably ignited continent-wide forest fires in North America and Asia, incinerating the vast majority of plants and animals over much of the northern hemisphere.

The impact and the fires threw debris high in the atmosphere.  Some of that debris quickly rained out.  Enough debris stayed in the atmosphere that the next several growing seasons were extremely cold and harsh, with the sun’s light dimmed in a much nastier and long-lasting version of nuclear winter.  The debris raining out of the atmosphere created more problems, probably including acid rain that would make what we’ve experienced seem like bath water.  To make matters worse, a huge set of volcanoes erupted in India either shortly before or shortly after the asteroid strike, putting even more junk in the atmosphere.

The fact that the asteroid struck near the equator meant that it had a major impact on both hemispheres, though there is some evidence that the southern continents and especially Antarctica may not have been hit quite as hard as the northern ones.  North America, especially the Western part, was probably hit harder than Asia. Distance from the center of the blast helped a little, though that depended partly on how much superhot debris fell in an area. Areas to the west of the impact probably got the worst of the debris, along with an area roughly halfway around the planet.

The ecology didn’t just start returning to pre-Asteroid-strike equilibrium after the blast, or even after the worst of the asteroid winter was past.  Some key plant and animal species undoubtedly were already extinct or close to it.

What remained of the ecology would have been in shambles, with key seed dispersal and pollinating animals missing, making it harder for the remaining plants and the animal populations that depended on them to recover.  The few animals capable of coping with the rapidly fluctuating environment would experience population explosions, with few predators or competitors to keep them in check.

 

Adaptable ‘weed’ insects like cockroaches and flies would probably adapt and flourish to the point that they covered the landscape in the small pockets of surviving or recovering vegetation.  The small adaptable birds, lizards, and mammals that preyed on them would take a little longer to adapt to the harsh conditions, but then they would have population explosions.  Think of an ecology almost entirely made up weeds and vermin with wild population fluctuations among those weeds and vermin and you get the picture.

The ecology may have taken thousands or even tens of thousands of years to stitch itself back together into a reasonably stable and predictable entity.  It had to rebuild topsoil that had been eroded away or buried under ash. Then forests and grasslands could begin to reestablish themselves.

An equilibrium between predators and prey and between plants and animals would eventually be established. It wouldn’t be the same equilibrium as before. Too many plant and animal species were extinct.

During the thousands or tens of thousands of years of rapid fluctuations the ecology favored small adaptable animals that could eat almost anything and quickly rebuild populations after the latest catastrophe.

Some mammals and birds survived, though many of the more specialized species died out. Many lizards and some crocodile species survived, probably protected to some extent by their ability to go long periods without food.

Did the last of the dinosaurs die out within a few years of the impact, or did a few species struggle on for a few hundred or thousand years after it? The fossil record is probably not good enough to tell that. Life recovered, and eventually flourished, with the ancestors of animals who could adapt to chaotic conditions or at least survive them taking the lead.

Did the dinosaurs actually die out at the end of the Cretaceous? There is some question as to whether or not dinosaurs did actually die out in the Cretaceous. Some deposits in the early Paleocene of western North America do contain shed dinosaur teeth in considerable numbers. Those teeth may have somehow been washed out from earlier deposits, but that is by no means certain.

Western North America has probably the best explored fossil deposits that come close to the end of the Cretaceous and the best explored ones that appear to be from the early Paleocene, but dating on most of them is on very shaky grounds. In most cases the temporal relationships between groups of fossils is based on the presence or absence of certain common fossil species or group of species. That only works as a dating mechanism if the species lived at approximately the same time everywhere it occurred. Using key species like that can be useful, but it can also be deceptive.

If you used Great Apes as that kind of fossil marker, you would see them vanishing from Europe several million years ago, yet there are still several species of Great Apes living in Asia and Africa.

In situations where the fossil record is pretty fragmented and the dating isn’t very firm, presence of dinosaur fossils can be taken as evidence in and of itself that the deposits were formed in the Cretaceous.

You don’t really know that dinosaurs were gone from a given continent until (a) There is a good fossil record from several areas of that continent, and (b) Dinosaurs are entirely absent from that record. Using those criteria, we really don’t know one way or the other for Australia until about 25 million years ago—40 million years after most people figure dinosaurs died out worldwide.

Europe and North and South America have pretty good fossil records in the Paleocene, though the early part of it is a bit of a mess.

Parts of Asia have reasonably good fossil records for the Paleocene, but if Dinosaurs survived for thirty or forty million years past the end of the Cretaceous in some sort of refuge environment like Tapirs and Orang Utans have in Malaysia and in Indonesia, chances are we wouldn’t know about it given the current state of the fossil record. I’m not saying that’s likely, I’m just saying that we don’t know one way or the other.

What might have happened: Let’s assume for now that dinosaurs died out worldwide in the aftermath of the asteroid strike and volcano activity around sixty-four million years ago.

Let’s say that the asteroid impact was a few tens or hundreds of miles north, or northeast of where it actually happened. North America and Asia feel the immediate impact to an even greater extent than they did historically.  A somewhat larger percentage of the debris goes into the atmosphere of the northern hemisphere. The southern hemisphere still gets hit hard, but not as hard as it did historically.

A few small, relatively adaptable omnivore or predator dinosaur species survive somewhere in the still-joined continents of Australia, Antarctica, and South America. For good measure let’s say that other species of similar dinosaurs survive in Africa.

What impact would they have? Probably not as much as most people would visualize. As the ecologies knit themselves together and reach reasonable stability, these dinosaur species become part of the new ecologies developing on those continents.  However those dinosaurs don’t automatically take over the same niches that dinosaurs held prior to the meteor strike. The ecologies aren’t recovering in the sense of going back toward the old balance. They are establishing a new balance based on the interactions of the surviving species, and on how well those species can handle the new environments that are developing.

Ecologists have been talking about the web of life, and how every species interacts with other species in a delicate balance for years. That has a major impact on what would happen next.

The huge dinosaur plant-eating machines have been removed from the ecology. Nothing that big or that efficient at devouring plants has developed yet to replace them. That alone changes the balance between forests and more open areas. Historically in modern Africa savannahs sometimes turn into closed woodlands when elephants are all poached out of an area. The extinction of the big dinosaur herbivores like triceratops could easily have the same kind of impact.

A species doesn’t even necessarily have to be big to change the character of an entire habitat. In the modern southwest arid areas where kangaroo rats are kept out develop a much richer and diverse group of plant species.

The point is that the new ecology after asteroid is likely to be very different than the one before the asteroids, and dinosaurs may not be able to play the same roles that they did before.

No matter how well adapted an animal is for a role in an ecology, if that ecology disappears the animal has to adapt or disappear. A fast running small dinosaur that is superb at running down prey in the open country may not be able survive in thickly wooded areas where tracking and patient stalking are important and running fast in a straight line usually isn’t.

The surviving dinosaurs also find that their competitors have been improving over the eons of dinosaur dominance. Birds have probably been around for over a hundred million years by the time of the asteroid strike. They keep improving though. They keep getting more efficient and faster at flying. They keep getting smarter too. As they improve, they are able to move into niches that had been mainly filled by dinosaurs.

A flock of birds can in some cases be the ecological equivalent of a dinosaur, and as they become stronger and more efficient flyers they can usually get to emerging food sources more quickly than dinosaurs.

As bird flight becomes more efficient, it may become nearly impossible for any kind of land animals to compete in some ecological niches. If a food source is widely scattered and accessible to birds they’ll get to it first most of the time.

Much of the speculation I’ve read on dinosaurs assumes that mammals replaced them. In some cases that’s true. In others cases, they may very well have been replaced by birds. Take a small, fast, sight-oriented hunting dinosaur that preyed on animals less than thirty pounds for example. The nearest current-day analog to that dinosaur may well be a hawk or an eagle, not a small mammal predator.

More efficient predator birds might also change the balance of advantages between egg-laying and non-egg-laying animals.

Anything that hatches from an egg has to pass through a period where they weigh less than thirty pounds, which means that they are small enough to be killed and carried away by predatory birds. Animals that give birth to live young have the option of letting the young develop inside the womb until they are too large to be carried away by hawks and eagles. Most large open country mammals either have burrows or have young to large to be attacked by hawks and eagles. That factor would not by itself keep dinosaurs from occupying niches, but it would tilt the balance against them by increasing the cost of reproduction.

Snakes also emerged as formidable predators of small animals at some point. As they developed advanced predatory techniques like venoms and constricting that undoubtedly would cut into the niches available for our surviving dinosaurs.

Mammals had been refining themselves during the age of dinosaurs too. They developed better hearing, probably a better sense of smell, more efficient locomotion, and more efficient means of reproducing.

Mammals generally weren’t very big until after the end of the Cretaceous. However, if the distribution of niches was anything like it is today there were probably far more mammal species than there were dinosaur species in the last part of the Cretaceous.

The ancestors of today’s placental and marsupial mammals controlled many of the niches for insectivores and small carnivores—up to cat-sized. A group called the Multiberculates controlled most of the niches that are now held by rats, mice, and squirrels. Bats and primates may already have been pushing into the niches that they currently hold.

My guess is that the surviving dinosaurs would hold some but not all of the medium to large carnivore niches in South America, Antarctica and Australia. They might also develop fast-running herbivore forms that resembled ostriches and rheas. Would they develop large quadruped herbivore forms too? Not necessarily. They would be at least as far away from that niche as mammal herbivores were, maybe further. I’m guessing that they would end up filling about the same roles in South America, Australia, and Antarctica that flightless birds did in reality in South America.

Would very large predators like Tyrannosaurus develop? I doubt it. No carnivorous mammals or flightless birds ever got anywhere close to that large. Part of the reason for that might have been that continents were more isolated from one another through most of the period after the Cretaceous. That would have reduced the land area available to large animals, and thus restricted the maximum size somewhat.

Also, heavily forested areas tend to support far less animal biomass than more open areas, so the forests that covered much of the world in the early part of the age of mammals would further reduce the potential for very large dinosaur predators. I could see predators in the half ton to ton class developing, but probably nothing much bigger. That would still be impressive enough.

As Australia and South America became more isolated from Antarctica the maximum size of dinosaur predators would probably drop even further.

Would the dinosaurs spread to North America and Asia from South America? There was some kind of transient partial connection between the two continents somewhere around fifty-five to sixty million years ago. In our reality a few herbivore species made it across, but apparently no carnivores.

What about the African dinosaurs? I’m guessing that they would fill the same niches as the ones in South America/Australia, but that they would play a less dominant role in those niches. They would be competing against placental predators rather than marsupial ones, and placental mammals find it easier to produce fast-running forms because they don’t have to maintain the grasping ability of the front paws in order to crawl to the mother’s pouch.

Also, Africa started out the age of mammals as an island continent. When the connection with Asia was reestablished, African animals would be at a disadvantage due to the fact that Asia is a larger continent, with more room for species to develop. On the other hand, medium-sized carnivorous dinosaurs were superbly adapted for running fast. I’m guessing that they would hold on to some of the medium to large predator niches in Africa for an extended period of time, but not be able to expand into Europe or Asia to any great extent.

If they didn’t expand out of Africa, the African dinosaurs would gradually become less and less of a factor in that continent too. They might end up fading out completely in Africa and only surviving in South America and Australia. Of course they could disperse to Madagascar before they die out in Africa, which might set up a dinosaur/lemur fauna there, which would be fun to explore.

If dinosaurs eventually fade out in Africa, they could and probably would still survive in South America until the Central American land bridge developed a couple of million years ago. At that point they would have to compete with lions and wolves and bears, assuming that those animals developed, which isn’t at all certain in this scenario. In any case they would be competing with formidable mammal predators from North America and ultimately Asia.

If the South American dinosaurs survived long enough and a few of them moved north, they could turn a lot of shlock fiction into reality by putting dinosaurs, mammoths, saber-tooth tigers, and even early man (PaleoIndians) in the same fauna for a while.

Of course that assumes that recognizable humans developed in this scenario, which is by no means assured given how early and how large the changes would have been. Certainly it would be very difficult to imagine Spain existing as such and sending Columbus to find the New World in this scenario.

So, what do we have here? The most likely dinosaurs to survive would probably have surprisingly little impact on the world, essentially replacing South America’s large predator birds, replacing some of Australia’s larger predator marsupials and monitor lizards and taking over the rhea/emu/ostrich niche. In other words they maintain themselves as bit players in a predominately mammal and bird dominated world.