The Time of Great Weirdening

The Miocene (~23-5 Ma) was a tremendously eventful period in Earth's history. With non-avian dinosaurs long gone, the Age of Mammals was coming to its climax, with mammalian diversity reaching unprecedented levels which remain unmatched to this day. It seemed as though evolution had let go of the reigns of normality, and species, regardless of class, reached sizes unheard of or filling impossible niches. Each continent, and every sea, had its share of brand new weirdo denizens.

These bizarre new creatures didn't appear out of nowhere. Global cooling was causing a drastic change in the world's ecosystems. The land was becoming drier, the seas becoming cooler and more fertile. The Miocene marked the end of the era of primeval forests and swamps the size of continents, and slowly the post-dinosaurian world was fading into the past. Mammals were reaching the peak of their diversity. Many modern families of birds came into existence. The tremendous boas, crocodiles, and turtles were becoming more and more rare, their lineage surviving in a few sole species in the wetlands of South America.

Just as the puzzling world of the Paleogene was slipping into the past, the modern world was coming into view. Recognizable horses, rhinoceroses, elephants, antelope, whales, and many other families made their debut. The great apes were coming into fruition, and in a few tens of millions of years their descendants would becoming mankind.  However, the variety of species, even of recognizable ones, was far greater than any period of the Cenozoic, and has never been matched. In this post, I hope to highlight for you some of the most fascinating and, of course, the weirdest of the great Miocene menagerie.

Rise of the Planet of the Artiodactyls
With the disappearance of the rainforests in North America, a whole new environment was created. It may come as a surprise that, up until approximately 20Ma, grasses were rare if not completely absent from major environments. Now that large swathes of land were clear of overgrowth, vast prairies and savannahs began to form. It was on these savannahs that herbivorous mammalian diversity began to skyrocket, and artiodactyls, the even-hoofed mammals, had their first moment of glory.

For tens of millions of years, from the end-Cretaceous extinction until the rise of the grasslands, the majority of herbivorous mammals were perissodactyls, the family of mammals which includes horses, rhinoceroses, and tapirs. Their success was due, in part, to the fact that their simpler stomachs were more suited to digest nutritious plants of the understory. With rainforests so widespread, they found their niche as ground-level browsers. Slowly, artiodactyls were replacing even the largest perissodactyls; in China, the last of the indricotheres, the largest mammals to ever walk the earth, lived alongside some of the first giraffes. While perissodactyls were still commonplace on most plains, they became far outnumbered by strange artiodactyls.

A glimpse at life from the late Miocene of North America, by Jay Matternes. While rhinoceroses and horses were still common, they were outnumbered by camels, pigs, and other artiodactyls.

Grass is low in nutrients and requires a complex multi-chambered stomach to fully reap its benefits. Artiodactyls were already well-equipped - with three to four stomachs, depending on the family, they were quick to radiate onto the prairies and begin grazing. All over the world, brand new families of mammals were springing up - giraffes, true antelope, cattle, and hippopotami were widespread from mainland Europe and Africa all the way to eastern China. Camels, pigs, and pronghorns filled virtually every herbivorous niche in North America.

Some artiodactyls even became some of the most fearsome predators to roam the American plains. The entelodont Daeodon was as tall as a man at the shoulders and twice as long as a man's height, and was the veritable tyrannosaur of the Miocene. It was far larger than any other mammalian carnivore of its environment, and its power and jaws would have scared any other predator from its kill in an instant.

The terrible pig Daeodon approaching the carcass of the rhinoceros Teleoceras on the plains of North America. The artist Chavez describes Daeodon was "the T. rex of the Miocene."

In a few million years, grasslands the world over were populated by a myriad of never-before-seen grazers and browsers. Stephen Jay Gould's The Book of Life has an awesome two-page spread comparing Miocene North American and modern African mammals, and the similarity between the two faunas is incredible. However, the land wasn't the only place which was experiencing a faunal sea change.

The Endless Coast: Life in the oceans
The fertile oceans of the Miocene created an explosion in the abundance of large marine life. Not since the late Cretaceous, when seas were full of serpents, had the oceans seen such diversity and size. Cetaceans, pinnipeds, seabirds, and sharks were all at the peak of their diversity, most inhabiting the tremendous proto-Pacific Ocean. In those days, the Bering Sea had not yet opened up, and an expansive land bridge still connected Siberia to Alaska.

The physeteroid Zygophyseter, a whale not as large as sperm whales
today but just as intimidating.

All along this endless coastline, marine life flourished. The first recognizable dolphins, baleen whales, belugas, and porpoises abounded. Physeteroid whales, represented today by the sperm whale, were the first cetaceans evolved to prey upon other cetaceans. With their huge, pointed teeth, they were the orcas of their day. In recent years, the discovery of the enormous physeteroid Livyatan revealed just how large these predators could be - the size of a modern sperm whale, Livyatan (formerly Leviathan) sported both top and bottom teeth which were so large they have previously been mistaken for the tusks of mastodons. These carnivores likely specialized in feeding on smaller baleen whales, as well as smaller cetaceans and sharks. Strange rhabdosteid whales, like giant river dolphins, also patrolled the waters, feeding on small fish and cephalopods. Whales would never again be so diverse, though their prominent role in marine ecology would remain.

The four-tusked walrus Gomphotaria pugnax (top)
alongside other Miocene pinnipeds and flightless
auks on the ancient Californian coast. Reconstruction
by avancna on deviantArt.

Pinnipeds, including seals, sea lions, and walruses, were also incredibly diverse. A variety of predatory pinnipeds evolved to feed on the abundance of flesh in the water. Many evolved stranger dentition than any pinniped seen today; the walrus-relative Pelagiarctos (literally, "sea bear") sported sharp canines ideal for hunting smaller seals. Another relative, Gomphotaria, sported two pairs of tusks, blunter and shorter than the modern walrus's, ideal for rooting up and smashing shellfish. All along the proto-Pacific coast, pinnipeds of all sizes and relations were common. Some reached sizes comparable to those of modern elephant seals, making them the largest seals to ever swim.


A few coastal mammals during the Miocene were some of the strangest to ever live. From Baja California to Japan, one group of tubby mammals stood out from all the rest: the bizarre desmostylians. These mammals are closely related to both sirenians, including dugongs and manatees, as well as proboscideans such as elephants. However, their affinity with these groups, beyond the fact that there is some affinity, is largely unknown. The desmostylians were a chimera of oddities, sort of like a hippopotamus with a strunken head and giant, paddle-like feet. They were herbivorous, filling niches that sirenians would have in more tropical waters; the coldness of their environment allowed them to exploit such a niche. The desmostylians were a relatively short-lived group, evolving in the late Oligocene, just before the Miocene, and lasting to the end of the next period. They have no living relations beyond questionably related elephants and manatees.

Skeletal reconstruction and life restoration of the bizarre aquatic
mammal Paleoparadoxia. Life restoration by Roman Uchytel.

While marine life was strange throughout the world, the shores of one continent held perhaps the most bizarre and unlikely of all coastal creatures. This island continent was a land seemingly lost in time, still ruled by tremendous reptiles, towering rodents, birds that could run down and kill horses, and vultures the size of airplanes.

The Real Lost World
South America has always been a land of the strange, largely due to the fact that, up until a couple million years ago, it was still an island continent. Since the Mesozoic, it had been separated from all other continents, and thus South American fauna evolved independently of all other life on earth. The huge equatorial continent was a labyrinth of forests and swamps, and had any man set foot on such a land, they would have surely though they had stumbled into a nightmare.

A bit south of the endless coastline from southern California to southeastern Asia, the diversity and weirdness of the proto-Pacific did not stop in South America. The Peruvian coast has provided exquisite fossils representing a Galapagos-like environment chock full of seabirds, including several species of giant penguin, boobies (teehee), gannets, and saw-toothed pelicans. But beyond the myriad of bird species, one mammal ventured where none of its kind had been before: for a brief moment in time, sloths became ocean explorers.

The sea-sloth Thalassocnus. Reconstruction by Guillermo Navalon Fernandez. Alright, I'll admit, this represents the Pliocene Pisco Formation, a few million years later, but it's the best reconstruction I could find.

That's right, there were once marine ground sloths. Thalassocnus was a large sloth which evolved in a very different direction of its giant cousins of the savannahs, for instead of browsing treetops, it was an able swimmer that grazed on sea grasses. The sea-sloths lived alongside the bizarre whales of the coastal proto-Pacific, and if they had ventured far enough out at sea, they would have likely fallen prey to physeteroids and sharks, including the mega-shark Carcharocles (/Carcharodon) megalodon, which swam just beyond the South American coastline.

The giant caiman Purussaurus, which shared its environment
with several other giant crocodilians.

Northern South America at the time was largely covered in wetlands, and these huge steamy areas of water and vegetation were the ideal environment for giant reptiles. Enormous crocodilians, including the giant caiman Purussaurus, the huge gharial Gryposuchus, and the huge filter-feeding croc Mourasuchus all lived alongside one another, each reaching lengths upwards of 10m. The combination of heat, humidity, and area all contributed to the continued reign of giant reptiles. These tremendous creatures shared their environment with a variety of other strange reptiles including the stupendous turtle Stupendemys, as well as the largest rodents to ever live. Some, like Phoberomys, the "fearsome mouse," reached lengths of 3m, and would have made ideal prey for the crocodilians of the area.

Perhaps even more bizarre, and a bit frightening, is the fact that South American crocodilians at the time were not restricted to the water. Like taking a glance back to the Mesozoic, big land-crocodiles were still terrorizing the land, preying upon large mammals both in the wetlands and on dry land. Sebecosuchians, like rauisuchians from the Triassic, searched for prey on long legs which held their bodies high above the ground for easy movement on land. The fact that such dinosaurian wildlife continued to exist in South America, long after the reign of reptiles had ended across the rest of the globe (except for Australia, where enormous monitor lizards were predators par none), is an eye-opening discovery, and holds testament to just how isolated the continent was from the rest of the planet.

"Return to the Triassic," an aptly-named reconstruction of Miocene Venezuela. The sebecid Langstonia attacks the giant tapir-like mammal Granastrapotherium. Reconstruction by Zimices, from deviantArt.

The Miocene was a time of strangeness in every aspect of life. The shift in climate left behind a primordial world of unrecognizable mammals and created the brave new world of modern life on Earth. Many species of the Miocene would have been fairly recognizable to those living today, as many of the families of animals found today had their origins during this time. Yet, in the first period of "modernity," life was still extremely foreign - life had not been this big, this diverse, and this weird since the time of the dinosaurs.