At about the same time that wood rats were revealing the youngest fossil evidence of the region’s past, more conventional paleontologists were digging up some of the oldest in the wood rat–filled El Paso Mountains. Lying in the immediate rain shadow of the southern Sierra Nevada, the El Pasos are barren even for the Mojave. Their gray slopes look cadaverous when seen from colorful Red Rock Canyon, with just a faint fuzz of creosote bush among the rocks. So it is odd that they produced the earliest proof that southeast California not only wasn’t always desert but once supported lush subtropical forest.
In 1960, Malcolm McKenna, a UC Berkeley paleontology PhD who had recently joined the American Museum of Natural History, announced the discovery there of fossils from soon after the dinosaurs’ demise: “The oldest continental Cenozoic fossil mammals, fish, crocodilian, and chelonian [turtle] remains from California, or for that matter from anywhere in the United States west of Utah, have been collected near the town of Inyokern from a small number of localities.”
Collectors had found turtle and mammal fossils in the El Pasos in 1950, but those specimens had disappeared. McKenna had begun looking in 1954, and, after “extensive prospecting efforts,” had found a mammal tooth and jaw fragment in a place called Laudate Canyon. The tooth was that of a condylarth, a primitive group of hoofed herbivores only distantly related to living ungulates like horses or deer.
It was a promising start to learning what lived in today’s California desert at the dawn of the Age of Mammals, although it was not precisely the start. McKenna noted that Daniel Axelrod had explored Laudate Canyon in the 1940s, following up an 1896 report of two fossil plant species there. Somehow, he had excavated a number of identifiable fossil leaves “from the water well behind the cabin of C. E. French, veteran prospector of the region.” I can imagine the energetic Axelrod dangling from a rope as he dug them out.
Axelrod reported on the plant fossils at a Geological Society of America conference in 1949: “The Mohave [now Goler] formation . . . from El Paso Mountains was originally considered Eocene on the basis of two plants, but this assignment has not generally been accepted. An Eocene age is clearly demonstrated by a small flora (Anemia [a fern], Annona [pawpaw], Myrica [wax myrtle], Persea [avocado], Parathesis [a relative of burning bush]) whose species resemble those now in the warm temperate to subtropical forests of Mexico and Central America.” Axelrod’s inference that lush forest had grown where only creosote bush does now was reasonable, but his El Paso Mountains fossils didn’t cause much stir. His paper on them wasn’t published except as an abstract in the Geological Society’s bulletin. The scientific frictions G. Ledyard Stebbins recalled may have contributed to this. Geologists who said the Goler Formation was Miocene in age wouldn’t have liked hearing it was millions of years older on the basis of a few fossil plants.
Axelrod seems to have backed away from the hard-to-find El Paso Mountain fossils for a while after that. But McKenna persisted, grimly, as he reported in 1960: “Prospecting has been intermittently continued from 1955 to the present, but identifiable specimens are extremely rare. Approximately one man-week of prospecting per identifiable mammalian specimen has been required. The total available collection from the Goler Formation now consists of five identifiable mammal teeth and a scattering of other vertebrate remains.” Still, the fossils convinced McKenna that the Goler Formation was even older than Axelrod thought—not Eocene but Paleocene, the first epoch after the dinosaurs’ demise, lasting from about 65 to 55 million years ago.
Some of the fossils were less suggestive of subtropical forest than Axelrod’s leaves. The fish bones and some of the turtle bones may have been from marine species. Part of a leg bone “of some reptile adapted to powerful digging” suggested a land habitat, but that was about all. The teeth of multituberculates, small primitive mammals named for knobby cusps on their molars, also indicated little more than land origin. Multituberculates evolved long before marsupials or placental mammals and died out fifty million years ago. They may have eaten plants as rodents do, but nobody is sure. Like rodents, they probably lived in both wet and dry places.
On the other hand, the crocodilian was an alligator, so it could have inhabited swamp forest like living alligators. The condylarths, the primitive ungulates of which McKenna found fossils of two kinds, also may have lived in moist forest. Condylarth teeth lack the high enamel crowns and other chewing specializations of later ungulates, suggesting that they browsed soft vegetation instead of a dry habitat’s tough herbs and shrubs.
Primitive mammal fossils turned up again in 1965 from central Baja. William J. Morris, an Occidental College geology professor who excavated them, thought the fossils were as old as the Goler Formation condylarths because they included remains of barylambdids and tillodonts, beasts even more bizarre than the Goler ones. Multituberculates and condylarths may have vaguely resembled rodents and deer; barylambdids and tillodonts resembled nothing living. Barylambdids had small, flattened heads and long, muscular tails—reconstructions suggest miniature duck-billed dinosaurs. Tillodonts grew as large as bears but had teeth like rodents, and may have been burrowers.
The Baja site’s other fossil mammal was two-foot-tall, four-toed Hyracotherium, the earliest known precursor of horses. Since accounts of horse evolution usually feature a succession from early forest browsers to later grassland grazers, Hyracotherium seemed another sign that lush forest covered much of today’s California desert region for millions of years after the dinosaurs’ extinction.
A year later, in 1966, at a site in northern Baja called El Gallo, Morris found even better proof of long-ago forest: “an abundance of petrified wood.” He found the wood with fossils of dinosaurs that had lived a few million years before the late Cretaceous mass extinction ended the Age of Reptiles: “The fauna is the only one from the Pacific margin of the continent where dinosaur materials are abundant.” The fossils included many bones of a large duck-billed dinosaur, Hypacrosaurus, teeth and vertebra of a large predator related to Tyrannosaurus, and back plates of an armored dinosaur resembling Ankylosaurus. The trees evidently had matched them in size: “Logs 3.6 to 5 meters long were not uncommon.”
“The lithology of the El Gallo Formation is indicative of near shore lagoons and playas, a normal habitat for hadrosaurian [duck-billed] dinosaurs,” Morris concluded. “Vegetation was much more profuse than that which exists today, and the better drained areas were thickly wooded.”
Overall, the fossils substantiated Axelrod’s 1949 picture of early Age of Mammals California as supporting humid, warm temperate and subtropical forest. The only possible anomaly was that both McKenna and Morris thought some of their California mammal fossils might be of significantly different species than those of the Rocky Mountains, where more abundant plant fossils show that both dinosaurs and primitive mammals lived in humid places.
In the 1980s, work at the central Baja site by another American Museum of Natural History paleontologist, Michael Novacek, seemed to dispel that anomaly. Deciding that its fauna had lived in the early Eocene epoch, a few million years after the El Paso Mountains one, Novacek concluded that it “could be compared favorably with those of early Eocene age in New Mexico, Colorado, Wyoming, and Montana.” It even resembled a newly discovered Eocene fauna from the Arctic: “This showed an extraordinary level of similarity among different habitats. Imagine North America today with the same kinds of animals and plants stretching from the Pole to the equator . . . a hothouse that nurtured a Garden of Eden—with dense forests, lakes and rivers, and diverse mammals, birds, amphibians and reptiles—that has not reappeared over such a wide swath of the earth’s surface anytime since.”
Novacek and his team found salamander, alligator lizard, and boa fossils at the Baja site, further suggesting a moist forest habitat. The mammals also had included extinct carnivores called creodonts and primitive artiodactyls, early relatives of deer and cattle. The central Baja site remained more Sphinx-like botanically than Rocky Mountain ones, however. The only plant fossils cited from it were seeds of Celtis, hackberry, a tree genus that grows in warm temperate and subtropical forest throughout the world today, and also in California desert.
Dinosaur age fossil wood and ice age wood rat middens made convincing bookends for Axelrod’s Eocene to Pliocene Madro-Tertiary woodland theory. But other fossils could be more ambiguous. When petrified wood from the Miocene epoch turned up in southern Baja, for example, it suggested that the area had been forest then. But the wood probably belonged to a tropical tree named Tapirira that still lives in semidesert southern Baja, so the fossil didn’t prove that much. Other evidence could be even more ambiguous and Axelrod’s colleagues kept bringing it up, perhaps motivated in part by his prickly character.