Mastadons in Our Midst

The East Bay's Miocene Menagerie

by on July 01, 2001

 

Illustration by Carl Buell

 

 

Illustrations by Carl Buell

The old Gomphotherium, out in front of her group of cows and calves, scans the floodplain and the rolling hills beyond. There’s a hint of ash on the wind from a volcano west of the bay, and a line of summer rain clouds is moving in. She notes the herds of horses drifting through the grasslands and the camels browsing at the edge of the streamside woods. She’s alert for predators, too; they’re no threat to her, but dangerous to youngsters who stray too far from the protective matriarchs.

The Gomphotherium’s world looks much like the modern African savanna, with similar players in the major roles. But the trees in the gallery forest are elms and poplars, not acacias. The horses have three toes; the treetop browsers are camels, not giraffes; some of the big cats are not quite cats. And the “elephants”—the Gomphotherium and her herd—have four tusks, double the complement of their modern relatives.

This isn’t Africa, of course. This is Contra Costa County in the Clarendonian age of the Miocene epoch, 9 to 10 million years ago, future site of the Blackhawk Ranch fossil quarry—the Bay Area’s richest lode of vertebrate fossils. The nearby body of open water is an arm of the Pacific, not the San Francisco Bay we know today. There are active volcanoes in the rising Berkeley hills, as well as to the north and east. Mount Diablo has not yet been uplifted by faulting. The land is like a broad valley in the modern Coast Range: imagine Napa Valley without vineyards or tourists. Trees related to the coast live oak and the Engelmann oak of Southern California clothe the hills, along with mountain mahogany, sumac, and—most likely—bunchgrasses. The climate is mild, without seasonal extremes. Rain, totaling 25 inches a year, falls in summer as well as winter.

It’s 55 million years after the dinosaurs’ demise, and mammals have reached the peak of their diversity. The rise of the Central American landbridge and the southward advance of the glaciers are far into the future. Somewhere in the forests of Africa, the common ancestor of chimpanzees and humans still lives. And the ancestors of the Gomphotherium have wandered out of Africa, across Asia, and into a new world.

A Mammalian Cornucopia

The Gomphotherium and her kind are the largest plant-eaters in a North America where broadleaf forest has given way to savanna. In this mosaic of grassland dotted with trees and veined with riparian forest, new niches have opened up for grass-eaters and those mammals that can eat either leaves or grasses (mixed feeders). The continent is now home to more than 50 genera of hoofed mammals (ungulates) and large carnivores, a richness unmatched anywhere in our modern world; the plains of Africa come closest. (By contrast, 21st century North America has only 10 ungulate and four large carnivore genera.)

In the arcane language of paleontology, the Gomphotherium is a bunodont, trilophodont, tetrabelodont, longirostrine masto­don. All those polysyllables concern her jaws and teeth. Long upper tusks curve downward and outward, and there’s also a stubby, flattened pair at the end of her elongated lower jaw. Softball-sized molars with cusped grinding surfaces process her diet of leaves.

Some ungulates, like the 12-foot-tall long-necked camel Aepycamelus and the tiny pronghorn Merycodus, also browse the tender foliage of trees and bushes. Groups of archaic leaf-eaters called oreodonts (relatives of the camels) are among the last of their kind; these ungainly beasts have long tails, and toes instead of hooves. Peccaries and other generalists feed on whatever is available.

But the grass-eaters outnumber the browsers and mixed feeders. The horse family has diversified into 12 North American genera, most of them grazers. One, the three-toed Hipparion, is among the most abundant Miocene hoofed mammals. Like modern zebras, its herds make long migrations for fresh forage.

At least eight genera of ungulates, including a second horse and two smaller camels, share the Gomphotherium’s habitat. And where there are herds, there are hunters. Borophagus, a dog with massive, bone-crushing teeth, is capable of making its own kills but not above scavenging. Paleontologist Bjorn Kurten described Borophagus as “one of the few animals to be unpopular several million years after its death. Paleontologists do not like to have their bones broken.” (Nor, presumably, their subjects’.) Like the modern spotted hyena, it may have had a pack-hunting lifestyle.

 

Barbourofelis
“Where there are herds, there are hunters.” Here, the powerfulpaleosaber Barbourofelis ambushes a three-toed Hipparion, relative ofthe modern zebra. Illustration by Carl Buell

There are solitary killers as well, such as leopard-sized Nimravides, a prototype of the sabertooth cats. The largest predator on the scene is a sabertooth, but not a cat: the powerfully-muscled Barbourofelis, who has huge upper canines that slot into flanges in its lower jaw, is a surviving member of the ancient carnivore paleosabers. Miocene carnivores, as paleontologist Christine Janis points out, aren’t built for speed; there are no cheetah-like sprinters, or pursuit predators like the wolf or African wild dog. Stocky Barbourofelis is an ambush hunter, delivering a killing bite with its fangs.

These Blackhawk mammals are part of what the University of Florida’s S. David Webb has named the “Clarendonian chronofauna.” This suite of species, adapted to a savanna that spans the continent, has dominated North America for more than 10 million years. In that long stretch of stability, complex relationships have evolved among the browsers, grazers, and predators.

Smaller, more familiar creatures share the grasslands and riparian woods: rabbits, beavers, ground squirrels, foxes, relatives of the raccoon. Cranes soar overhead. Down at the stream, in a placid backwater, a western pond turtle—virtually indistinguishable from its descendants—basks on a log.

Bones and the Stories They Tell

It’s that stream that provides a window into this ancient world. Leaves, bones, teeth, Gomphotherium skulls, and turtle shells all found their way into the sediments that filled its channel. Horses and other grassland creatures may have been caught near the water by predators, or had their remains transported here by flooding. There’s no evidence of mass deaths or tar pit entrapment, only of gradual, piecemeal accumulation in alluvial silt and volcanic ash. As the Diablo range rose, the ancient, buried streambed was lifted and tilted by faulting, and eventually exposed by erosion.

At the Blackhawk Museum in Danville, you can see a reconstructed Gomphotherium skeleton downstairs from the collection of classic autos. It’s a composite, with casts of bones from some of the 50 individuals found at the fossil quarry. The actual fossils repose at the University of California Museum of Pale­ontology (UCMP) on the Berkeley campus.

There, in a warehouse-sized room of tall metal cabinets, UCMP Curator of Vertebrates Pat Holroyd pulls out a drawer labeled “Blackhawk.” It contains the teeth and jaws of the doglike Borophagus. “This was the most common carnivore at Blackhawk,” she says. “The length of the skull is like a coyote, but it’s much more robust, with a massive face and deep jaws.” And the teeth are impressive, well-suited for crunching bone.

Holroyd presides over a vast collection of skulls and bones, a sort of paleomorgue. Somewhere here is a Pleistocene ground sloth discovered at the site of the Berkeley BART station. The Bay Area is surprisingly rich in vertebrate fossils, many salvaged from construction sites in the 1950s. But most locales yielded only isolated fragments; none have been as productive as Blackhawk.

She opens another drawer. “Here’s Nimravides. It’s not a sabertooth, but it’s getting there.” Smaller than the modern lion, this cat was heavily built, with huge chewing muscles and teeth with serrated edges. Its descendants may have included Smilo­don, our state fossil. The paleosaber Barbourofelis is represented in the collection by a single formidable fang.

Gomphotherium has two cabinets all to itself; the Blackhawk specimens range from calf to older adult. I recognize the elongate lower jaw with its peglike tusks, and the molars with their three cusps, or “lophs.”

Behavior doesn’t fossilize. We can’t be sure that gomphothere herds were led by experienced matriarchs, or that Barbourofelis attacked from ambush. But some things can be inferred from anatomy. A confusing array of horse teeth Holroyd shows me is a case in point.

Grass blades, deceptively tender-looking, are full of silica, which wears down teeth. Mammals, such as the horses, with high-crowned cheek teeth—taller than they are wide—had an edge in the Miocene savanna, where grasses and grazers co-evolved. Although no grass fossils have been found at the Blackhawk site, the presence of grazers like the horse Hipparion implies the presence of grasses. In addition to shape, the teeth of grazers show distinctive wear patterns. There are also chemical clues to diet: the enamel of grazers’ teeth contains an isotope of carbon unique to grasses.

Holroyd shows me another drawer filled with the delicate jaws of the pronghorn Merycodus, which was roughly the size of small African antelopes today—more evidence of the diversity of Miocene hoofed mammals. “See, there are no first or second premolars,” she says. “It nibbled with its incisors.” Other drawers hold a supporting cast of rabbits, rodents, and small carnivores.

Blackhawk birds, reptiles, and fish have their own drawer. Some bones are too fragmentary for precise identification; we can only call it some kind of duck or owl. But an extinct species of crane, more primitive than the modern sandhill, has been described from the site. I’m a bit disappointed that Smilodon-ichthys, the sabertooth salmon, is represented only by a cryptic bit of backbone. In life, this anadromous fish reached a length of eight feet, with two-inch-long fangs

One specimen I’d recognize anywhere, though: the intact, if squashed, shell of a pond turtle. Except for the patina of age, its owner could have been swimming in Jewel Lake yesterday. While that whole exotic bestiary flourished around it and then died out, the turtle persisted in its low-key way and is still with us. There’s something to be said for conservative design.

Down in the Quarry

Some weeks later, I get to see where these specimens came from. Daniel Dunn, Director of the Blackhawk Museum, has taken time off from setting up an exhibit on the extinct Tucker automobile to show me the site of the Blackhawk Ranch quarry. We’ve jounced up an overgrown track in a Ford Explorer and walked down the ten-foot-deep trench to the quarry’s bone bed. The summit of Mount Diablo shows over the next ridge to the north. The excavated area, a broad notch at the crest of a hill, would fit comfortably in a basketball court, or in one of the homes in the gated community below. It’s hard to believe this place has yielded so many specimens.

Dunn picks up something small, shiny-brown, and slightly concave and hands it to me: “Here’s a piece of gomphothere tusk. This must have weathered out since the last time we were up here.” I would have missed it. Dunn spots another tusk fragment, then a shard of mammal bone. He shows me a sizable rock on the north side of the trench, finer-grained than most, where I can make out the knife-blade shape of a willow leaf, small oval leaves that could be mountain mahogany, and bits of other things. I’m impressed again by the productivity of this place and how broad a spectrum of ancient life is preserved here, from huge browsers like Gomphotherium to the plants they ate. It’s likely that even more remains to be discovered.

Although there is no active excavation project, UC classes still visit the site. “Because of continuing urbanization, Blackhawk is now one of the few places we can take students and show them how vertebrate fossils are preserved and how they are properly collected,” paleontologist William Clemens tells me later. “There used to be a wealth of other localities we could visit, but most of them have disappeared under parking lots or in the landscaped yards of housing developments.”

Paleontologists began excavating the quarry in the 1920s after a ranch hand noticed some protruding bones. The owner of Blackhawk Ranch at the time, R. C. Force, lent employees and equipment to the first digs, and subsequently had his name bestowed on a three-toed horse. During the Depression, crews from the Works Progress Administration took over.

Dunn’s involvement began in 1991, when the Blackhawk Museum and UCMP launched a community dig. Supervised by graduate students, small groups of museum members sorted through backfill from the excavations of the ’20s and ’30s. “It was the most popular project we’ve ever done,” he remembers. “The public had a great deal of interest in the quarry, virtually in their backyards. Other museums at the time were creating vicarious fossil sites, but this was the real thing.” Whole families came to the site, and some regulars showed up every day. All finds became UC property; volunteers took home the excitement of discovery.

The project ended three years ago when the museum turned toward a more exclusive focus on antique cars, but Dunn says he still gets inquiries and wouldn’t mind seeing it revived. He’s something of a partisan of Miocene mammals, and it bothers him that dinosaurs get all the paleontology publicity. “The fossil record isn’t just about dinosaurs,” he says. “Just in helping to dispel that notion, the quarry has earned its keep.”

Farewell to the Miocene Animals

Extinction was the common fate of both the dinosaurs and the Miocene megafauna. By the Ice Age, about 2 million years ago, the gomphotheres, Borophagus, and paleosabers had been replaced by mammoths, dire wolves, and true sabertooth cats. How did it happen?

Species, like individuals, are mortal; even those like the pond turtle that enjoy long runs eventually die out. Paleontologists say there is a steady background rate of such extinctions. But there are peaks of mass extinction in the fossil record, catastrophic die-offs like that at the end of the Cretaceous era (65 million years ago), when the dinosaurs and a host of sea creatures were wiped out, or the Permian event (250 million years ago), when the planet lost nine-tenths of its species. Impacts from extraterrestrial objects have been blamed for some of these mass extinctions. What took place beginning nine million years ago seems to have been somewhere between the extremes of mass and background extinction.

Not long after the Blackhawk deposits formed, two dozen genera of large mammals drop out of the North American fossil record, including five genera of horses and the last of the oreodonts. Three million years later, 21 more genera vanished, and another 35 disappeared five million years ago, at the boundary between the Miocene and Pliocene epochs. A few Clarendonian mammals, such as Borophagus, survived into the Pliocene, but most were gone.

There’s no evidence of a smoking asteroid, and human hunters can’t be blamed. Webb and other paleontologists believe the Blackhawk creatures may have been victims of less dramatic, but nonetheless crucial, trends in climate and plant communities. Toward the end of the Miocene, the Antarctic ice sheets expanded, sea level dropped, and the ocean depths became colder. By the late Miocene, permanent glaciers were in place in West Antarctica and the Alaskan mountains. The growth of the glaciers reduced rainfall worldwide and ended summer rains in the West. In our region, the elevation of the Coast Ranges created a rain shadow, intensifying the drier climate and seasonal temperature extremes.

This late Miocene drying trend transformed the vegetation across North America. In the interior, pure grassland replaced the mixed forest and grass habitat of the savanna. What woodland remained was discontinuous, which may have affected migration routes of the plant-eaters. According to David Webb, “The rich array of browsers and mixed feeding ungulates was devastated.” Surviving species were more likely to have dental equipment that could cope with a diet of grasses. A less varied ecosystem meant there were fewer ways of making a living; a complex web of co-adaptation was disrupted. The demise of keystone species set off a chain of other extinctions.

“Large mammals—ungulates and carnivores—tend to extend over very large parts of a continent, and changes in their adaptations, or waves of extinction, tend to carry across broad parts of their range,” says Webb. So changes in the heart of the continent could have affected peripheral areas like California, even if the change from savanna to grassland was less complete here.

Whatever the causes, the long summer of the Miocene came to an end some five million years ago. The great North American savannas, with their multitudes of hoofed mammals and attendant predators, gave way to a world more like our own. Modern plant communities, such as live oak woodland and chaparral, took shape during that transition. Grasses continued to diversify, and other plant families—composites, legumes, mints, mustards—produced an array of new species. The advent of dry summers favored the evolution of annuals—plants whose seeds can wait out unfavorable conditions. Many of the plants we think of as typically Californian made their appearance as the mammalian megafauna was fading out.

There are visible geologic remnants of the Miocene locally, like the 9.5 million-year-old basalt lava at Sibley Volcanic Regional Preserve in the Berkeley Hills and volcanic ash deposits underlying Little Grizzly Peak in Tilden Regional Park. But tectonic forces have altered the ancient topography beyond recognition, and—with exceptions like that persistent turtle—the Miocene menagerie is long gone.

Gone, but not irretrievable. Paleontology is a kind of time travel; working with an imperfect fossil record, scientists try to put flesh on the old bones and reconstruct vanished ecosystems. Sites like the Blackhawk Ranch quarry, with its wealth of plant and animal remains, are rare portals into the depths of time. Viewed through the prism of the past, our landscape is reinhabited by the ghosts of gomphotheres.

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