The Middle Miocene: A time for diversity

The middle Miocene is roundly agreed to have been an apex in the evolution of new species. For some reason, that particular window, 16 to 12 million years ago (called the Barstovian NALMA – or North American Land Mammal Age in the U.S.) was the peak for faunal diversity. S. David Webb has called it the “Clarendonian Chronofauna”.

Here are names and dates for generally concurrent time periods around the world:

Langhian (15.97 – 13.82 Ma) Worldwide
Early Astaracian (15 – 13.5) Europe
MN 5 (15.97 – 13.65) Europe
Bairnsdalian (15 – 10.5 Ma) Australia
Barstovian (15.5 – 11.8 Ma) North American
Colloncurian (15.5 – 13.8 Ma) South America
Tozawan (15.97 – 13.5 Ma) Japan
Shanwangian (16.9 – 13.65) Asia
Clifdenian (15.9 – 13.65) New Zealand

Take a look at the following graphs. Each is from a separate study. Notice anything interesting?

In each case we see a mysterious pinnacle in biological diversity at around 15 ma. Now, I’m not here to try to establish what exactly caused this explosion in speciation, I’ll leave that to people more expert than I, but only that it happened. Having said that, hypotheses range from CO2 fertilization – carbon from the active volcanoes of the time brought back to earth and, coupled with an increase in rainfall, contributing to increased plant growth, which meant more food available which in turn fueled animal expansion and diversity – to continental drift causing reproductive isolation and thus an increase in speciation.

A few quotes:

“Clarendonian Chronofauna: Grassland Savanna Land mammal diversity in North America reached its zenith during the Barstovian mammal age (Webb, 1989). Savage and Russell (1983) recognized 16 families with 60 genera and 141 nominal species of land mammals in the Barstovian. The next highest numbers occurred in the Clarendonian (the next mammal age), with 55 genera and 117 species. These mammal ages are thought to indicate a savanna optimum in North America, with a rich mosaic of trees, shrubs, and grasses supporting an extraordinary variety of large and small, grazing and browsing, ungulates. It is not uncommon during this savanna acme to collect in a single site 20 genera of ungulates of which half are Equidae (Webb, 1983a; Voorhies, 1990).” ~ Effects of Past Global Change on Life, chapter: Global Climatic Influence on Cenozoic Land Mammal Faunas. S. David Webb and Neil D. Opdyke, p. 193

“During such robust chronofaunal intervals the continental mammal fauna stood near its ecological capacity…. The acme of land mammal diversity, dominated by horses and other savanna herbivores, is attained in the Barstovian…. the land mammal faunas experienced high diversity and long stable community development (chronofaunal evolution). This lends credence to the view that the ecosystem was near capacity during the Barstovian acme.” ~ Effects of Past Global Change on Life, chapter: Global Climatic Influence on Cenozoic Land Mammal Faunas. p. 203

“Changes in the productivity and species richness of terrestrial vegetation must have affected herbivore communities. Hoofed herbivorous large mammals on the North American continent show a maximum diversity during the middle Miocene climatic optimum… This maximum in both local and regional diversity greatly exceeds the diversity of ungulates in any present-day habitat, which implies a greater primary productivity than is seen today. Preliminary review data suggest that the pattern of elevated ungulate diversity is a global phenomenon, and, therefore, a global driving force is the most likely explanation. CO2 fertilization during the middle Miocene climatic optimum may have made possible the expansion of high-productivity terrestrial biomes that supported high-diversity browser communities.” ~ The impact of Miocene atmospheric carbon dioxide fluctuations on climate and the evolution of terrestrial ecosystems. Wolfram M. Ku ̈ rschner*†, Zlatko Kvacˇek‡, and David L. Dilcher§, p. 452

“The later Tertiary mammalian fauna of the World Continent was perhaps the richest that has ever existed on the face of the earth. It is as if mammalian life had been proliferating in ever increasing numbers, exploring every ecological nook and cranny that could be populated, testing how large or small you could get, how best to adorn yourself with tusks or horns, how to fly, swim, climb, run, dig, jump, hunt, eat, kill and defend yourself, better than ever before. All the manifestations have a single keyword:adaptation. Most of the evolutionary lines of the later Tertiary had a fairly long history behind them: they had got far enough to attain basic adaptation for a given way of life. What remained now was to perfect it. And so, in the late Tertiary, the mammals were increasing in efficiency, under the constant supervision of natural selection – a perfectionist potentate. And in general this would also involve an increase in beauty, in gracefulness, in elegance.” ~ The Age of Mammals, (chapter) The Miocene: Epoch of Revolutions by Björn Kurtén. Columbia University Press, 1971

“Horse diversity increased so dramatically that at some fossil sites from fifteen million years ago as many as a dozen species can be found. Today the world’s horses (and their relatives the zebras, asses and onagers) are reduced to the single genus Equus, whose wild members live only in parts of Asia and Africa…. the familiar horses, zebras, asses and onagers that share our modern world represent but a single surviving branch on a once luxuriant equid family tree that reached its full glory during the Miocene.” ~ Natural History 4/94. Article: The Heyday of Horses by Bruce J. Macfadden

“Canids experienced their second spurt of diversification in the middle Miocene… their maximum ecological breadth during this time.” ~ Dogs: Their Fossil Relatives and Evolutionary History by Wang and Tedford, p. 127

“If west coast marine tetrapod evolution in general had a peak, a time when it went as far as it could go, the mid-Miocene may have been it.” ~ Neptune’s Ark: From Ichthyosaurs to Orcas, p. 105. By David Rains Wallace

“Looking back the Pliocene [or the timeframe within the Pliocene now called the Miocene] is something of a paradise lost, a climax of the Age of Mammals before the coming of the cold; a time when life was richer, more exuberant than ever before or after.” ~ The Age of Mammals, (chapter) The Pliocene: Epoch of Climax by Björn Kurtén.

“Lasting for millions of years, the mid-Miocene must have seemed a kind of endless summer” ~ Neptune’s Ark: From Ichthyosaurs to Orcas, p. 113. By David Rains Wallace

Okay, so it happened. Now the question is, what would it have been like to live back then, during the Middle Miocene? A time when the earth was at it’s peak? Finding the answers took years of research. I bring it all together in Opalescence.

I hope you enjoy it!

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