The Clarendonian Land Mammal Age

Many science writers often describe the Pleistocene of North America as resembling the modern day African Serengeti.  I debunked that notion 7 years ago in an article I wrote for this blog (See: https://markgelbart.wordpress.com/2013/12/09/the-faunal-diversity-of-pleistocene-north-america-was-less-than-that-of-modern-day-africa/ ) In terms of biomass Pleistocene North American might have been as impressive but not when it comes to biodiversity.  Africa has almost twice as many species of mammals as Pleistocene North America. However, there was a time period during North America’s natural history when it was biologically more diverse than modern day Africa.  The Clarendonian Land Mammal Age during the middle Miocene lasted from ~13 million years ago to ~9 million years ago.  The age is named after the Clarendon local fauna based on fossils found from 24 sites in Donley County, Texas.  Scientists are aware of 34 mammal families that lived in North America during this age.  This includes 8 genera of artiodactyls such as camels and llamas, peccaries, deer, and pronghorns.  There were 15 genera of horses plus tapirs and 2 species of rhinoceros.  1 species of primitive oreodont still clung on, though they were formerly more diverse.  Bear-dogs (Amphycyon sp.) also still survived but were headed for extinction.  4-tusked gompotheres, kin to elephants, entered North America by crossing the Bering Land Bridge and colonized the continent.  Predators included 8 genera of canids and 11 genera of weasels, and there were 9 genera of rodents.  River dolphins and dugongs swam in the waters.  The bone-eating dogs (Borophagine), ancestors of saber-tooth cats (Nimravides), and false saber-toothed cats (Barbourofelis) were the dominant large predators.  Fanged cats and cat-like animals came in all sizes.

Among the amazing diversity of mammals were some remarkable morphological convergences with modern day species of African fauna.  There were giraffe-like camels that evolved long necks to feed on the tops of trees, aquatic hippo-like rhinos, and fast running gazelle-like horses.

Teleoceras | Animal of the world Wiki | Fandom

The hippo-like rhino Teleoceras.

Aepycamelus | Extinct animals, Ancient animals, Prehistoric animals

The giraffe-like camel Aegypcamelus.

Nannippus sp. by Dinogod.deviantart.com on @DeviantArt

The gazelle-like nannihippus.

Climate over most of North America during the middle Miocene was warm and mostly non-seasonal.  Before the Clarendonian Land Mammal Age tropical and sub-tropical forest covered most of North America, but the uplift of the Sierra Nevada and Cascade Mountain Ranges caused increased aridity.  Warm savanna grassland and open woodland replaced the thick forest, and this resulted in a greater diversity of mammals, taking advantage of this more productive habitat.  Grazing herds of ungulates and burrowing populations of rodents in deep grassland soils thrived in this environment.  Climate change brought an end to the Clarendonian Land Mammal Age.  Conditions became even more arid and seasons became more pronounced.  Warm savannahs and open woodlands were replaced with steppe grasslands where  winters started to trend toward sub-freezing temperatures.  Many species of mammals could not adapt to harsher winters and simply went extinct. By the end of the Miocene and beginning of the Pliocene large mammal diversity was much reduced, but new cold-adapted species from Eurasia (crossing the Bering Land Bridge) and new immigrants from South America (crossing the newly emerged Isthmus of Panama) helped replenish biodiversity in North America. Though large mammal diversity never again approached that of the Clarendonian, it was an healthy cavalcade until the end of the Pleistocene when man wiped most of them out.

2 Responses to “The Clarendonian Land Mammal Age”

  1. Zach Matthews Says:

    Very solid post, Mark. I was unaware of most of this; fascinating time period. What was the primary reason for the sub-tropical/tropical forest blanketing North America prior to this period? Was the continent significantly more southerly/equatorial or was it just a geological factor like lower mountain ranges allowing more uninterrupted warm air mass from the tropics?

  2. markgelbart Says:

    I think the continents were pretty close to where they are today. Worldwide mountain ranges were lower. I think that was the bigger factor.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s


%d bloggers like this: