Here are some photos of the shale outcrop:
Clam Fossil; you are seeing the cross section of a clam. Photo by Jeff Mitton
Bell's Twinpod, Physaria bellii, is limited to shale outcrops between Fort Collins and Denver. Photo by Jeff Mitton
Bell's Twinpod and Clam Fossil; the plant, by virtue of its tiny distribution, will probably not last
very long. In contrast, the clam Genus Inoceramus ( now extinct) lasted about 135 million years. Photo by Jeff Mitton
Habitat generalists, like pigeons and dandelions, thrive in a wide variety of environments. In stark contrast, some habitat specialists are adapted to rare or ephemeral environments, limiting them to tiny distributions and predisposing them to extinction. Bell’s twinpod is a pretty flower that is limited to shale outcrops along the Front Range between Denver and Fort Collins.
Bell’s twinpod, Physaria bellii, is relatively new to science, for it was only described in the 1960s. I suspect that it was overlooked by botanists because it is so rare. It is limited to shale outcrops of the Niobrara and Pierre formations between Fort Collins and the Ken-Caryl Ranch Natural Area in Denver. Approximately twenty small populations of Bell’s twinpod are known; some are tiny, just dots on the map of Boulder County.
One of the most convenient local spots to see Bell’s twinpod is at the intersection of Neva Road and Route 36, just north Boulder. The road cuts at that intersection dissect a shale outcrop. Twinpods are small rosettes of blue green, flat, spatulate leaves that persist year round. In late April and May they produce a canopy of bright yellow flowers that is conspicuous against the dark shale.
Other species of Physaria are widespread in the west. Why is it that Bell’s twinpod has such a limited geographic range? It is likely that a closely related species, perhaps roundtip twinpod, P. vitulifera, was growing close to the shale outcrops, and occasionally casting seeds onto the shale and its derived soils. Shale is not a good substrate for plants; it usually supports a depauperate plant community. In the Allegheny Mountains, shale barrens are known for sparse plant cover and endemic species, or species that grow nowhere else. During a period of high rainfall and salubrious weather, Physaria became established on the shale. Natural selection then modified the physiology and morphology of these plants so they could persist in the unusual environment. This process produced a new species, Bell’s twinpod.
Plants that tolerate shale soils have the advantage of very few competitors. But its adaptation to shale prevents Bell’s twinpod from colonizing surrounding environments. Although it is common on the shale, it stops at the margins of the shale outcrops, trapped by changing soil conditions.
In an evolutionary sense, specialists may have a tenuous grip on life. If a species, such as a clam, is narrowly adapted to the environment in which waves pound against sandy beaches, the species is guaranteed a safe haven. Since our planet has had oceans, waves have been pounding against beaches, and that will probably never change. If, on the other hand, a specialist depends on other species, or on environments that appear and then disappear, the specialist will probably not last very long, on a geological or evolutionary scale. Bell’s twinpod grows on shale, but shale is not permanent.
Erosion brings shale outcrops to the surface, where they weather, erode, and disappear. If another outcrop does not appear as a shale outcrop disappears, the stranded population is doomed. Furthermore, miniscule populations are more likely to suffer calamities that wipe them out.
The shale outcrop at the west end of Neva Road is novel not only because it supports Bell’s twinpod, but also because it presents a stark contrast of persistence in evolutionary time. Bell’s twinpod grows among fossils of large, robust marine clams. The clams, in the genus Inoceramus, persisted for 135 million years. It is a delightful juxtaposition: a small, fragile plant, probably not destined for a long play in this world, grows among the stony impressions of a species that endured for eons.