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Lake Tahoe Trip #1

Poleta Limestone In The White Mountains Of California

Archaeocyathids were sessile, marine organisms of shallow, tropical and subtropical waters that lived during the early Cambrian period about 550 million years ago. They were widespread in Cambrian seas throughout the world and were the first major reef-builders before the true corals (phylum Coelenterata). [Ancient colonies of cyanobacteria called stromatolites formed massive "algal reefs" over one billion years ago.] The extinction of archaeocyathids by the middle Cambrian coincides with a rapid diversification of sponges. There has been considerable disagreement about their taxonomy. They are sometimes placed in the phylum Porifera (sponges), although some authorities have placed them in the extinct phylum Archeocyathida (Archaeocyatha). Cladistical analysis indicates that the Archaeocyathida is a clade within the phylum Porifera.

An individual archeocyathid has a conical or vase-shaped skeleton of calcite similar to a pair of perforated, nested ice cream cones. The concentric inner and outer walls are separated by a space (intervallum) containing numerous partitions (septa). The concentric walls and septa are commonly observed in fossilized limestone. The abundance of archaeocyathids in limestone outcrops indicates that they grew over one another in a tangled profusion as sponges do.

Poleta limestone from the White Mountains of Inyo County, California. Abundant fossils of archeocyathids occur in outcrops of limestone in the pinyon-juniper woodland at about 7,000 to 8,000 feet elevation. An individual archeocyathid had a conical or vase-shaped skeleton of porous calcite similar to that of a sponge (or an ice cream cone). The red arrows show remnants of conelike archeocyathids embedded in the limestone. Massive colonies of archeocyathids were major reef-builders in ancient Cambrian seas approximately 550 million years ago.

Archaeocythids in Poleta limestone of the White Mountains (Inyo County, California).

Archaeocyathids in Poleta limestone of the White Mountains of California. Note
the radiating septa between the concentric outer and inner walls of the skeleton.
The bodies of several rust-colored individuals can be seen in longitudinal view.

Archaeocyathid in Poleta limestone of the White Mountains, California. Note
radiating septa between the concentric outer and inner walls of the skeleton.

Minute Pits In Poleta Limestone: Mystery Finally Solved

Small pits in limestone are often caused by the calcium-loving crustose lichen Verrucaria calciseda. At the base of each pit is a conical spore-bearing, reproductive body called a perithecium. The small perithecia are only 0.1 to 0.3 mm in diameter. Since the lichen thallus is endolithic (living within the calcite), the black, conical perithecia reveal its presence. Lichens produce phenolic acids that slowly eat away at calcareus rock surfaces. For non-calcareous, granitic rock surfaces, the etching process is probably mechanical. Crustose rock lichens are able to grow on bare rock, sinking their spreading thallus into every minute nook and cranny. Microscopic rock fragments intermeshed with the lichen thallus become loosened by expansion and contraction, as the thallus is alternately moistened and dried. These pits are apparently not caused by a lichen.

After 10 years I may have found the cause of minute pits in the Poleta limestone in the White Mountains. The origin does not appear to be caused by lichens because I cannot see any of the lichen structures. Lichenologist and curator of the University of California Lichen Herbarium, Kerry Knudsen, just sent me the following e-mail message (23 Nov 2016):

"I am actually working on saxicolous microfungi on limestone in the White Mountains which we will be sequencing next year in Italy. We are finishing working on the ones on non-calcareous substrates from Italian Alps, San Bernardino and White Mountains right now for the description of a new order. The current probable genus name for your fungus is a Lichenothelia."

The origin of these etch pits on the limestone surface was unknown until my recent e-mail from lichenologist Kerry Knudsen (see above message).

Microscopic view of black body within an etch pit of limestone. It appears to be composed of fungal cells. Magnification 400 x.

Microscopic view of black body within an etch pit of limestone. It appears to be composed of fungal cells. Magnification 400 x.

A Pit-Causing Marine Lichen

Close-up view of small acorn barnacles on the coast of Santa Barbara, California. The minute pits in the calcareous skeleton of the barnacles are caused by the endolithic marine lichen Pyrenocollema halodytes. The head of an ordinary straight pin is shown as a size comparison. The diameter of the pin head is 1.5 mm. Magnification 30 x.
According to Lichen Flora of the Greater Sonoran Desert Region (Vol 1) by T. H. Nash et al. (2002), Pyrenocollema halodytes is now listed as Collemopsidium sublitoralis. This species has been verified from Cabrillo National Monument by K. Knudsen (personal communication, 2008).

  See Rock Lichens and Desert Varnish  
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See Dimensions Of Above Pin Head

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