After the Cretaceous–Paleogene mass-extinction it took 15 million years for the world to recover.  Mammals and birds then evolved to take over the role of the dinosaurs.  Towards the end whales arose too take over after the plesiosaurs.  Most mammals which arose then seem to have been forgotten.  The biggest exception I can come up with is the North American Eohippus (“dawn horse”).  This was an early ancestor of the horse the size of a lapdog.  I think it filled the same niche as a present-day mouse-deer.

Around 56 million years ago the climate was as warmest.  It even became warmer than it had been before the Cretaceous–Paleogene mass-extinction.  I think it was increased volcanism causing a small warming.  This triggered a release of methane from sea floors.  Methane is a potent greenhouse gas but not particularly long-lived.  It reacts with oxygen and increases the fraction of carbon dioxide in the air.  The Earth also has a mechanism to stop it from getting so far that it becomes uninhabitable.  When the average temperature rises erosion also rises.  Since erosion removes carbon dioxide this sinks the average temperature.  However, we are talking about a time scale of tens of thousands or hundreds of thousands of years.

It was primarily the movements of the continents which cooled down the climate in the long run.  First Australia splintered of from Antarctica.  An ocean current could then move freely around the entire southern hemisphere’s mid latitudes.  This isolated Antarctica from warmer water from the north.  Antarctica was cooled down and stated to be covered in ice.  Later South Asia collided with the already existing Eurasia.  The newly formed Himalayas additionally increased erosion.  Finally North America was connected to South America though an isthmus.  This prevented warm sea water from reaching the Pacific from the Atlantic.  Instead, the Gulf Stream arose which increased precipitation around the North Atlantic.  This made the ice sheet of Greenland possible.  In a colder and colder and dryer and dryer world grasses started spreading.  Their breakthrough made grasslands as we know them possible.

Most prehistoric mammals people tend to know about were contemporary with humanity.  There were additional animals which were at least as impressive.  For example, a mammoth was not the largest elephant humans have seen.  It was a Eurasian elephant which went extinct 25,000 years ago.  It most closely resembled a taller version of the present-day African savannah elephant.  However, I think it had smaller ears since its habitat was cooler.

Many prehistoric mammals are moreover misunderstood.  Glyptodonts I have explained here and described one genus here.  A few other examples:

Cave bear (Ursus spelaeus) was a relative of present-day brown and polar bears.  Although the species was the size of a polar bear it was largely herbivorous.  Skeletons of it have been found in Europe and West Asia.  It is called “cave bear” because so many have been found in caves.  It seems to have preferred to spend its hibernation there.

Dire wolf (Aenocyon dirus) was just a little larger than a human.  Previously it was placed in the same genus as today’s wolves.  Now it seems more likely it consisted its own genus.  It was found in open landscapes in the Americas.  The species managed to spread to what is now China.  However, it does not seem to have been long-lived there.

Giant deer (Megaloceros giganteus) was first misunderstood as an elk.  It was a relative of the fallow deer which could grow larger than living elks.  Otherwise it was primarily characterised by the males’ exceptionally large antlers.  To be able to support these antlers their backs between their shoulders were particularly high.  Remains have been found in Europe and parts of southern Siberia.

Ground sloths were large herbivores in the Americas.  Species varied in size from a little larger than human to almost like an elephant.  This then does not count the family Megalocnidae.  I think the rest descended from animals living in treas.  Otherwise, it became a bit hard to explain their foot anatomy.  They distributed their weight only on the front and hind feet’s outsides.  How could this have arisen if they lived on the ground all the time?

Mammoths (Mammuthus) were a genus of elephants specialised on eating grass.  (Present-day elephants prefer leaves.)  Most famous was the wholly which was the most cold-adopted.  I varied in size from as large as today’s largest elephant species and up to one third larger.  Remains of it have been found in Eurasia and North America.  In contrast it was not the largest.  The largest seen by humans was instead the Dixie mammoth.  Skeletons of it have been found in parts of North America.

Mastodonts (Mammut) were relatives of elephants with proportionally shorter legs.  On the other hand, their heads were longer.  They were larger than present-day elephants but smaller than the largest mammoths.  Skeletons from this genus have been found over large parts of North America.  All species lived in forests where they ate twigs, leaves and fruit.  This included from coniferous trees in colder areas.

Sable-toothed cats were felines which canine teeth protruded out of their mouths.  This they did to different degrees in different genera.  All were more closely related to each other than any present-day felines.  The ones surviving the longest were the size of present-day lions.  Homotherium represented a group with proportionally smaller canine teeth.  It first disappeared from Africa, and later from Eurasia and the Americas.  Smilodon was the most famous and had larger canine teeth.  It was only found in North and South America.

Short-faced bears (Arctodus) were a genus which has only been found in North America.  Two species have been identified.  The smaller one was the size of a brown bear and the larger one a little larger than a polar bear.  Above all both species had proportionally longer legs.  There is great uncertainty about these animals’ diet.  I think they were omnivores if they were not carnivores.

Wholly rhinos (Coelodonta) were simply rhinos which had adapted to a cold climate.  This genus arose on the Tibetan Plateau and later spread to northern Eurasia.  One species has also been found in North Africa.  No species seem to have ben larger than today’s largest rhinos.  In contrast they had shorter legs and proportionally longer heads.  Their horn most forward on their noses was much longer too.  Likely it was used to dig up grass under the snow.

All these died out before being described in writing.  This does not mean their world was so much different from ours.  To the opposite very few organisms have arisen since then.  Above all no new animals have evolved to fill their roles.  I think most of them – perhaps even all – were extinguished by humanity.  Not deliberately of cause but as an unwanted by-effect.

Sub-Saharan Africa has not been affected to such a great extent.  The current human species arose in this part of the world.  Organisms of Sub-Saharan Africa have evolved together with us.  The largest extinction took place after pre-humans learned to kill large animals.  Human-sized otters and viverrids were simply outcompeted.  Bears disappeared entirely from this part of the world.  Som carnivores gradually disappeared over a couple of hundred thousand years.  Certain large herbivores were extinguished trough hunting.  Nearly all which had then survived have managed until the modern day.

In Eurasia and North Africa pre-humans have long existed.  However, they primarily killed the young, and sick and weak animals.  In this way there was no difference to other large carnivores.  The current human species is more creative than them.  Our hunting became considerably more efficient.  Animals which were adult and healthy did not need bad luck to get killed.  Add the quick warming at the end of the last ice age.  I think it was this combination leading to several animals being extinguished.  These include cave bear, wholly rhino and the last sabre-toothed cats.  Hippos disappeared from Eurasia but survived in Africa.  What I find most tragical was the fate of the last wholly mammoths.  A singe heard of 10 individuals got stuck on Wrangel Island when the seas rose from shrinking ice sheets.  If it functioned as and elephant heard everyone was already related on their mother’s side.  This gave rise to a population of 300 animals.  The mammoths were dwarfed and badly inbreed.  In contrast the degree of inbreeding did not increase over the course of millennia.  Around 1700 BC the last mammoth died there.  It may have been a natural disaster affecting the entire island.  Or humans reached the island and soon ate them all up.

To a lesser degree Australia had its own giant animals.  Not only larger kangaroos but marsupial rhino (Diprotodon optatum), marsupial tapir (Palorchestes) and marsupial lion (Thylacoleo carnifex) too.  It is unclear if any pre-humans managed to get there.  Then they would have been able to imagine there to be land beyond a sea horizon.  Anyway, the current human species have existed there for tens of thousands of years.  I think many large animals there were extinguished by a combination of two different factors.  One was man-made fires.  The other one was climate change in the form of the peak of the last ice age.  This country is even today dry with for the most part lean soils.  At the peak of the last ice age, it must have been as driest.

In the Americas pre-humans have never been found.  The animals arising there were as such not familiar with people.  It has been suggested many such would have been quickly extinguished after the arrival of humanity.  Later it has turned out these continents were inhabited earlier.  People got there by boats while the land route was blocked by ice.  So I think this was also the result of human hunting and climate change.  The later means quick warming at the end of the last ice age.

Since then many animals on islands have been relatively quickly extinguished.  Either as a consequence of hunting or habitat destruction.  This affected not only mammals but also birds and reptiles.  The last of these extinction waves took place on New Zealand.  It was reached around 1200 AD by people coming sailing from the north.  Unfortunately, these had lost the knowledge of such large land masses existing.  So they did not realise the land’s resources were limited until it was too late.  Within the span of 200 years all larger animals had been extinguished.  But this affected birds and not mammals.  The only land mammals there were a few species of bats.  Which in themselves are misunderstood animals.

 

Uploaded on the 9th of September 2024.