Crusts are old. Maybe sometime soon I’ll write about the idea that the first terrestrial communities were cyanobacterial biocrusts, rather than true land plants…or I’ll ask someone else to do it who knows a lot more. Recently in a geology journal that few biologists are likely to wander to, Simpson et al. (2010) report on a fossilized crust from the Cretaceous. Now, the Cretaceous isn’t that old, it’s noted as the last stand of the dinosaurs and the coming out party of the angiosperms…but the amazing thing is that their fossil looks very much like a living modern day crust.
It was found in my old stomping grounds of Grand Staircase-Escalante National Monument near the contact between the Wahweap formation and the Kaiparowits formation. The fossil was located near Cottonwood Canyon where I would often camp in the shade after long hot days of hiking to remote sampling sites, and attempt to remove the multi-day sunscreen & soil crust that had accumulated on me in the nearby creek. The sandstone portions of the Wahweap formation can support some very nice biocrust today if you work harder than a cow and walk far away from water sources. So needless to say this stirs some nostalgia in me. Little did I know there were fossil crusts in the rocks, it really makes you wonder what other formations have them.
As you can see from the photos, they have a pinnacled structure like most of the Colorado Plateau crusts of today. This puzzles me a bit, we have long thought that frost heaving plays a large part in generating uplift in these pinnacled crusts. Naturally, erosion sculpts things that stick up, but eventually some equilibrium is found where the sculpted pinnacled form is well-preserved by a late successional biocrust. But the Cretaceous was a much warmer time than today, the areas where the rocks were formed would not likely have experienced frost, correct me if I’m wrong. I was similarly puzzled a few years ago when I saw a fantastic crust on the Oregon Dunes while cycle touring. It had the exact same pinnacled structure as Colorado Plateau crusts, despite the maritime climate and lack of frost. Maybe we need to rethink the origin of this structure.
Read more about the fossil crust here. All photos are from Simpson et al. 2010.
Simpson, W.S., Simpson, E.L., Wizevich, M.C., Malenda, H.F., Hillbert-Wolf, A.L., Tindall, S.E. 2010. A preserved Late Cretaceous biological soil crust in the capping sandstone member,Wahweap Formation, Grand Staircase-Escalante National Monument, Utah: Paleoclimatic implications. Sedimentary Geology 230: 139-145.