An area that I found as unexpectedly enchanting as Red Rock Canyon during early desert trips was Algodones Dunes between Anza-Borrego and the Colorado River. Dunes hadn’t interested me particularly as natural places; I’d associated them mainly with movies about Rommel’s desert war. But another writing project sent me to the Algodones in 1985. When I got there one late afternoon in March, I might as well have arrived at a desert war. One side of the road was solid with parked pickups and trailers, and the dunes beyond swarmed with ORVs from which came occasional gunfire and a steady screeching, grinding roar.
Luckily for me, the Bureau of Land Management had recently closed the dunes on the road’s other side to ORVs. So the roar faded as I fled across the sands, apparently the only pedestrian within miles. Although this must have been an illusion, I saw little sign that the machines had been grinding over the dunes for decades before the closure. I guess sand is a good ecological cosmetic. They seemed improbably pristine, and very alive.
First I crossed an orchardlike expanse of big creosote bushes and ephedras—the biggest I’d seen, almost tree size—then increasingly bare waves of rippled sand, pale orange in the late afternoon light, leading up to sharp dune crests. But calling the sand bare is a misnomer. It was written over with hundreds of animal tracks—uppercase swirls of reptiles, lowercase jottings of mammals and insects—and illuminated with wildflowers, their colors enhanced by the orange background—white of big evening primroses and desert lilies, pink of sand verbenas, bluish pale gray-green of their foliage.
The creosote bushes were blossoming and were full of native bees the same pale yellow as the flowers. I noticed little pits in the dunes and saw a bee fly into one. In a moment it emerged and pushed a pinch of sand out, evidently digging a nest. Seeing yet more pits as the wind blew clouds of creosote bush petals overhead, I realized the dunes were full of the nests, full of the creosote bush pollen that the bees were storing.
I saw a movement in a small depression, and a tarantula slowly crawled out and made a peculiar pulsing movement with its abdomen, then dropped out of sight so quickly it might have dematerialized. I began to feel that I’d wandered into the metaphysical essence of desert.
Then I saw something that bolstered the impression. The sand had blown conical mounds around the shrubs, and as I approached one of these, a big sand-colored lizard suddenly materialized under my feet, dashed toward the mound, and vanished into it. As I walked through a group of mounds, this happened enough times that I surmised that I was not seeing vagrant whiptails, desert iguanas, or other generalists. I had found that paragon of California sand dune adaptation—the fringetoed lizard. A number of fringe-toed lizard species of the genus Uma live in North American deserts—three in California—and they are famous for adaptations that allow them virtually to swim in sand as they flee enemies or pursue insect prey. They rate a lengthy description in a global compendium, Living Reptiles of the World:
In the first place, the body is flattened, but unlike that of the ungainly horned toad, still elongate, enabling the lizard to knife its way through the sand yet leaving it enough length to employ “swimming” undulations. The tip of the snout is sharpened to an edge by having the lower jaw shorter than the upper and actually countersunk into the upper. The push given by the hind legs in sand swimming is multiplied by the development of a fringe of scales on the toes. As the feet are drawn forward the fringing scales collapse; as the feet are kicked back the scales stick out, thus increasing the surface area brought to bear against the sand. The fringes act like snowshoes when Uma runs on the loose surface of the dunes. Finally, the nostrils and ear openings are protected from sand grains by valves and the eyes by fringed eyelids.
Fringe-toed lizards belong to a group that includes some of the desert’s commonest species, such as zebra-tailed lizards. Called sand lizards, the group generally has adaptations to sandy soils, such as protected ear openings and burrowing ability, although they live in many habitats. California fringe-toed lizards, however, inhabit only the relatively small parts of the desert where dunes prevail. The only others I’ve seen were in Death Valley dunes. This might seem to simplify the origin of at least one California lizard genus. If they live only in desert dunes, it would seem that fringe-toed lizards probably evolved in them. And there is circumstantial evidence that they evolved on dunes here relatively recently. They are less adapted to dune life than many Old World dune species. According to three herpetologists: “The specializations that allow Uma to inhabit Aeolian sand deposits are behavioral and morphological; physiologically Uma does not differ from other iguanid lizards.” This seems another evidence of Daniel Axelrod’s “recent regional desert.”
The Uma origin question is not quite that simple, however. When Edward Cope first noticed the fringe scales on its toes in 1866, the genus was known only from California and Arizona, a situation that prevailed until the 1940s. This seemed to support its recent evolution here. Then an Uma species turned up in Mexico’s Chihuahuan Desert, isolated from the Arizona and California species and different from them.
“The Chihuahuan Desert cradles at its center a huge, hot dry lowland system referred to as the Bolson de Mapimi,” wrote a herpetologist, David J. Morafka, in 1977.
Climatic conditions in the Mapimian region may have been highly insulated and continuous since middle Neogene [late Miocene] time . . . virtually all Chihuahuan endemic herpetofaunal groups are either centered in, or restricted to, the Mapimian subprovince by their distributions. Moreover, a striking majority of these endemics are extremely primitive species, often with mixed character states indicating a nearly transitional condition between a mesic ancestral genus and a more arid adapted genus or species group. . . . The example of Uma is particularly favorable to the model of Mexican plateau origins, since the Chihuahuan species is more primitive than Mohavian and Peninsular congenitors.
One primitive aspect of the Mexican Uma, Morafka noted, is that it is less specialized for dunes than its congeners—even though today’s Chihuahuan Desert has dunes. It doesn’t burrow in sand as readily or often as the California species. It doesn’t just live in dunes: “The eastern or Chihuahuan desert is virtually without a highly adapted dune herpetofauna. . . . Even when sister species are represented in both western and eastern deserts, only the western member is clearly committed to sand dune life. This is true for the sand lizards.” The Chihuahuan fringe-toed lizard might provide evidence that Uma is older than the highly specialized California species; that the genus first evolved in an ancient Mexican desert and later spread northwest.
But lacertilian ambiguities, like Chinese boxes, seem always to open into further ambiguities. Morafka did not interpret the Chihuahuan fringe-toed lizards’ primitiveness as evidence of ancient Mexican desert origin. He saw it as evidence of ancient Madro-Tertiary flora origin: “Vegetation in the [Mapimian] region is strikingly similar in both organization and, to a lesser extent, content, to the Mojavia Madro-Tertiary Geoflora from which AXELROD (1958) suggested that a modern desert vegetation evolved. . . . Thus, the Mexican Plateau would be favored as one center of origin for Madro-Tertiary vegetation and Young Northern faunas, but certainly not the center. The lizard genus Uma is a good example where the primitive member is Chihuahuan.”
Morafka saw the California fringe-toed lizards’ dune specializations as an aspect of Axelrod’s recent regional desert:
The underlying cause is linked with the reverse ice age histories of the two dune systems. Refugial western desert dunes actually expanded as glacial sea levels dropped by as much as 130 meters along the coastlines of the Gulf of California. The isolated eastern desert faunas were trapped in isolated valleys or bolsons of the Mexican plateau. During glacial times, pluvial lakes filled the valleys, inundating most exposed dunes, and leaving as refugial habitats only the outcrops, talus, and alluvium that became lake shores. The same climatic events severed the two desert systems as cooling climates and downward-placed vegetation intruded across the Continental Divide, expanded the coastal dune systems of western desert refugia, and simultaneously contracted or obliterated those of the east.
So, according to Morafka, the dune-specialist Uma species evolved in recent regional desert, but the genus Uma itself evolved in the Madro-Tertiary flora’s older semideserts. California fringe-toed lizards—like desert bushes and wood rats—thus seemed to fit the neo-Darwinian scenario on which Axelrod based his ideas. They evidently evolved rapidly in response to regional desert’s recent expansion because rising mountains and increasing dryness isolated them and because they were preadapted to spreading dunes.