Saturday, August 19, 2017

Two Late Cretaceous species of the bivalve Pterotrigonia

In some cases, recognition of two species of the same genus can be problematic. In the examples shown here, however, the recognition  is straightforward. The species are of a shallow-marine bivalve (clam) of Late Cretaceous age from the west coast of North America. These species lived as burrowers in subtidal shelfal depths.

Pterotrigonia klamathonia, 4.5 cm length, Santa Ana Mountains,
Turonian age, Orange County, southern California.

Pterotrigonia klamathonia (Anderson, 1958), which is of Turonian age, is characterized by its closely spaced radial ribs, 20 to 25 in number.

Pterotrigonia evansana, 4.5 cm length, Campanian age,
Simi Hills, Ventura County, southern California.
Pterotrigonia evansana (Meek, 1858) is geologically younger and is of Coniacian through Campanian age (see time table below). This species is characterized by its widely spaced radial ribs, commonly 10 or so in number.

Genus Pterotrigonia, which is the state fossil of Tennessee, is extinct. It belongs to the family Megatrigoniidae, whose geologic time range is latest Jurassic through the end of the Cretaceous. The family was very widespread in the world during the Cretaceous.

Saturday, August 5, 2017

"Desert Roses"

“Desert roses” are clusters of crystals of baryte or gypsum containing inclusions of sand. “Baryte” is the new official spelling of the mineral barite. “Desert roses” are also called “sand roses,” “rose rock,” “gypsum rose,” and “gypsum rosettes.” The clusters can be spherical shaped, irregular, or columnar shaped. They are commonly red or reddish brown.

These three pictures shown baryte "desert roses," and the largest one shown above is 2 inches in diameter. They feel heavy to the touch because of the element barium.

"Desert roses" form as evaporites when shallow waters (marine lagoons or lakes) rich in sulfates containing the elements barium or calcite precipitate out of solution. 

“Desert roses” are not that common although it seems that every collector I know has one or two. They are found in Kansas, Oklahoma, Arizona, California, Egypt, and a few other places in the world where lake beds have dried up and become evaporites. They can be of any geologic age (e.g., Permian [250 million years old] or Plio-Pleistocene [less than about 3 million years old]).

Some of the most famous and largest (up to 39 inches tall and weighing as much as 1,000 pounds) specimens of “desert roses” are from the Permian-age (about 275 million years old) Garber Sandstone in Oklahoma. In fact, in 1968, “desert roses” became known as the official state rock of Oklahoma.

Like the one shown above (1.5 inches long), not all clusters of baryte contain inclusions of sand nor do they necessarily have to form as evaporites.