It seems perversely counterintuitive that an Andean-size massif dominating the far west at the Age of Reptiles’ end should have persisted to become the present Sierra and Great Basin ranges but have left only esoteric traces of its existence throughout the Age of Mammals. The West Coast’s jagged peaks and steep canyons look so young compared to the hulking Appalachians, which are supposed to be truly old mountains. Yet Science and Nature published no letters in opposition to the articles proposing this reversal of long-accepted notions. Subsequent issues carried arguments about climate change, animal rights, drug abuse, human evolution, science funding—not about Miocene rain shadows.
One 2004 Geology article did offer some evidence of newly rising farwest mountains: “Recent geologic data have polarized the debate about whether the Sierra Nevada underwent late Cenozoic (ca 10 Ma to the present) uplift. The debate has suffered from a lack of landscape erosion rates particularly from the rugged southern Sierra Nevada. We report new erosion rates that link many of the previous data sets and inspire new conceptual models of the late Cenozoic topographic evolution of the range.” The three authors determined the “new erosion rates” by dating the sediments that have accumulated in caves of southern Sierra Nevada marble deposits, and thus estimating how long the Sierra’s rivers had been eroding the landscape. They thought that the southern Sierra had begun to rise between ten and three million years ago “in a pattern that steepened the gradients of westward flowing rivers. These rivers responded in a wave of incision that propagated upriver from the edge of the Central Valley, deepening preexisting canyons. Incision in the marble belt reached its maximum rate as the wave of rapid incision passed from 5 to 2 Ma.”
A Sierra Nevada that rose rapidly between five and two million years ago might reaffirm the conventional view that the West Coast was low lying before then. The Geology authors didn’t think so, however. They agreed with the Science and Nature articles:
Low temperature geochronology studies suggest that the southern Sierra Nevada had high elevations and relief as early as the late Cretaceous, when the range was an active volcanic arc. The . . . minerals east of the Sierra Nevada crest suggest a persistent rain shadow throughout the Miocene (Poage and Chamberlain, 2002) indicating that elevation was higher then as well. These data have been used to argue for a monotonic decline in mean elevation and local relief through the Cenozoic, implying no recent uplift....
We note that these data are not necessarily at odds. Tectonically driven rock uplift ... would rejuvenate incision in pre-existing canyons, resulting in further flexural isostatic uplifts.
Just because the western mountains started getting higher quickly some five million years ago, in other words, it doesn’t mean that they haven’t been high since the dinosaur age.
But if Daniel Axelrod was askew, he was not overturned. The lack of published disagreement with the Science and Nature articles didn’t necessarily signal agreement. As Jeffrey Schaffer complained in 1999, biologists continued to espouse the Madro-Tertiary paradigm. In an excellent 2008 book on California desert ecology, botanist Bruce Pavlik summarized the paradigm better than Axelrod:
Prior to the rise of the Cascade-Sierra-Peninsular chain, western North America was a low undulating plain dominated by forests of conifers and broad-leaved hardwoods. Twenty-five million years ago, the coast ranges did not exist, and the ancient Pacific Ocean lapped a continental edge along the east side of what is now the Central Valley. The Nevadan hills ran north to south along this coast, eroded remnants of a once high range of sedimentary mountains. With an estimated elevation of only 3,500 feet, the hills were too low to block storm fronts, and rainfall was abundant, reliable, and evenly distributed across the continental interior. To the south, the influence of subtropical climates created oak and palm forests, rich pine and juniper woodlands, and thorny shrublands with exposure to seasonal drought and high temperatures, but there were no desert ecosystems at this time. The ancestors of modern desert species were, however, already establishing lineages under these semiarid conditions.
Axelrod’s paradigm is not, after all, imaginary, but a genuine theory based on a great deal of informed observation and evidence, including a fossil collection that is a major scientific contribution in itself. The circumstance that his intellectual ingenuity and powerful, sometimes abrasive personality contributed to its ascendancy was not an unusual one in the real practice of science. If he was an occasional obstacle to further inquiry, he was an incitement to it as well. Whether or not they all prove accurate, Axelrod’s ideas have value as the product of resourceful, perceptive exploration, just as do Miguel del Barco’s, Ivan Johnston’s, and Jerzy Rzedowski’s.
In any case, the new western geology did not have much to say about coastal mountains south of the Sierra Nevada. Even if the Sierra and Great Basin were much higher much earlier than once thought, that doesn’t necessarily mean the Transverse and Peninsular ranges now rain-shadowing California’s southern Mojave and Sonoran were. Lowland Madro-Tertiary vegetation might have covered what is now southeast California even if high mountains loomed to the north.
Early paleontologists’ vision of Age of Mammals southern California as a megafauna-rich plain covered with grass, savanna, or woodland remained compelling, and there was even some new fossil evidence for it. In 2006, the paleontologists who described the Pliocene-epoch Anza-Borrego region as “a lowland of meandering rivers and riparian streams” announced a discovery that questioned an earlier challenge to Axelrod’s paradigm, although it challenged some of Axelrod’s own fossil identifications in the process. They reported the finding of “permineralized monocot wood, morphologically and anatomically identical to modern fan palm, Washingtonia filifera” in Pliocene deposits at least three million years old there:
This may represent the first documented fossil Washingtonia sp. within California. Previously (Axelrod, 1939, 1950 b) specimens of this taxon have been assigned erroneously to the genus Sabal based on palm frond impressions. This leads to paleobotanic confusion, since it is difficult to identify fossil specimens accurately from their leaf stem impressions alone. The leaf and associated fossil wood material from Anza Borrego, however, is indistinguishable from the genus Washingtonia sp. (based on anatomical pore structure morphology) and does not resemble Sabal palm. It is unlikely that Sabal (usually referred to the species Sabal miocenica) and Washingtonia sp. co-existed in Anza Borrego during the Pliocene. Washingtonia sp. favored temperate conditions and had a much wider distribution.
The Anza-Borrego paleontologists didn’t mention James Cornett’s 1989 theory that Washingtonia is a desert genus that only spread from Baja to Alta California after the ice age, but they seemed to negate it. They said in effect that Washingtonia was originally a palm of low-lying Madro-Tertiary riparian woodland and that the late Miocene and Pliocene palms that Axelrod and others had identified as Sabal were actually Washingtonia.
Other fossils besides plants and land mammals might suggest a low-lying southern California coast before three million years ago. A chain of marine deposits full of whale, shark, and pinniped bones that runs from the southern Sierra foothills down through Baja shows that much of today’s coastward land was undersea during the late Oligocene and Miocene epochs. There might have been a wide coastal plain inland.
On the other hand, neither riparian tree fossils nor marine fossils rule out the possibility of high coastal mountains. Riparian forest grows along high-altitude streams if the climate is warm enough. I once rode a train across Mexico’s Sierra Madre just north of the Tropic of Cancer from Los Mochis to Chihuahua. After climbing the steep subtropical canyons that open onto the narrow coastal plain, we emerged into an oak, madrone, and pine forest with cottonwood-bordered streams like coastal California’s today, but thousands of feet higher. If California’s mountains stood above nine thousand feet in the mid-Miocene, when climate was tropical, such a forest might have grown high on them as well.
If Washingtonia’s ancestors were as widely distributed in California Miocene and Pliocene woodland as the Anza-Borrego paleontologists suggest, they also might have grown at higher elevations. Most fan palm groves occur in steep, rocky terrain today. The only apparently natural old grove outside California, in the Kofa National Wildlife Refuge in western Arizona, grows not in a canyon bottom as might be expected but along spring channels near the top of overhanging cliffs. I searched most of a morning before I finally looked up and saw the wind-tossed main grove, which somehow reminded me of Marcel Duchamp’s vertiginous painting, Nude Descending a Staircase. If Washingtonia’s Pliocene ancestors did grow in a lowland of meandering rivers and riparian streams, which sounds a lot like modern coastal valleys, why don’t they grow naturally in modern coastal valleys?
Even if southern California lacked high coastal mountains, rainfall wasn’t necessarily ample. A low-lying coast might have been desert if warmer climate pushed the midlatitude arid belt northward. In the 1920s, a geologist likened the Miocene environment of the southern Sierra foothills’ richest marine deposit, Sharktooth Hill, to that of today’s Topolobampo, a town on the Gulf of California just west of Los Mochis. Sonoran desert grades into tropical thorn scrub there, but the road to Topolobampo seemed venerably desertlike to me while driving it one night, with cardons looming out of the dark and yellow eyes streaking across—a landscape of marble and moon dust.
If California was a tropical land of towering mountains and rain-shadowed basins for most of the Age of Mammals and before, it raises another possibility that seems ironic. Perhaps, instead of evolving in Mexico and later spreading north, as botanists from Asa Gray to Jerzy Rzedowski have surmised, an “earth-old” desert evolved in California and only spread south into Mexico as global climate cooled in the Pliocene. Anza-Borrego’s Washingtonia palm fossil wood might be a relic of ancient California desert.
All this is pretty thin speculation without more analyses of southern California geohistory. Meanwhile, the new geology’s counterintuitive quality makes it hard to even speculate on desert life’s past. After all, fossils show that many more kinds of animals lived in southeast Alta California and Baja before the ice age than afterward. It seems that the region should have been more verdant, more arcadian, to support them. Death Valley has yielded thirty-million-year-old fossils of titanotheres, ancient rhinoceros relatives almost as big as elephants. I can’t see giant rhinos thriving there today.
Yet rhinos live in southern African deserts today and may have lived in them long enough to shape the vegetation, as Daniel Janzen suggests. Many deserts without much wildlife now supported more even during historical times. A little over two millennia ago, the Greek adventurer Xenophon described what is now western Iraq: “In this region the ground was unbroken plain, as level as the sea, and full of wormwood [Artemisia]; and whatever else there was on the plain by way of shrub or reed was always fragrant, like spices; trees there were none, but wild animals of all sorts, vast numbers of wild asses and many ostriches, besides bustards and gazelles.”
California never had gazelles, ostriches, or bustards—turkey-size plovers. But it had large herds of equids and pronghorns as well as camels and mastodons for at least twenty million years. The fact that artists conventionally show them living among oaks and cottonwoods instead of creosote bushes and Joshua trees doesn’t prove that they always did. It seems that we still just don’t know enough about what was happening in North America before two million years ago to be sure when or how the desert evolved. And, given the twenty-first century’s information explosion and the consequent fragmentation of scientific disciplines, new paradigms as coherent as Axelrod’s may be indefinitely postponed.
The diversity of bushes and lizards that struck me at Mitchell Caverns three decades ago remains a riddle, especially in the case of the big bush-eating lizards. Two lizard experts observed in 2003 that plant eating has evolved several times in the group to which chuckwallas and desert iguanas belong, concluding that “exactly what drove this remains a mystery, but herbivory has one clear benefit: in many environments where temperatures are warm, plants appear to be a virtually unlimited resource. Even in deserts with low productivity, plants are unlimited for short-term periods, and lizards are capable of long-term fasting. So the impetus may simply have been the superabundance of plants relative to lizard abundance.”
That is not much of a conclusion in terms of exactly when and how California desert originated. Did lizards start eating desert bushes when megafauna that had preempted lusher flora dwindled? If so, did they start many millions of years ago, maybe so long ago that the dwindling megafauna was dinosaurian? Or did they start a few million years ago when the dwindling megafauna was mammalian? Or did bushes and lizards coevolve in a different way? The Keeper of the Seals still says nothing.