Sunday, October 18, 2015

Miocene fish scales

Macrofossils are not common in deep-marine mudstones of the Miocene Modelo Formation in southern California. A lucky find would be a partial or complete bony fish skeleton, but these are  rare. Fish scales, however, can be locally common, but a collector needs to know what they look like. Before enrolling in my paleontology class, most of my students would find a fish scale and not even know what they were looking at.

The fish scales shown below are of late Miocene age and from the Modelo Formation (commonly referred to as the Monterey Formation). Each scale is about 10- to 15-mm-wide and represents a paper-thin imprint on bedding planes of mudstone. Identification as to the family or genus of fish is not a trivial process and requires very specialized knowledge. At the very least, however, their presence indicates an aquatic environment.

Two late Miocene fish scales belonging to the same genus. There might be a third fish scale in
 the lower right-hand corner.

Another late Miocene fish scale, genus different from the two shown in the photo above. 
There have been only a few paleontologists that have published on Miocene fish scales from southern California. The principal workers were: D. S. Jordan (published between 1900 and 1920), Lore Rose David (published during the 1940s), and Richard Pierce (published during the 1950s). 

Sunday, October 4, 2015

Fossil dolphin ear bones

Fossil-dolphin ear bones, which can somewhat resemble small pebbles, consist of very hard, dense bone that can be readily fossilized. Most examples of these fossils that are illustrated online these days are from rocks of Miocene to Pleistocene age from the Atlantic coast of the United States (e.g., Miocene rocks in Calvert Cliffs, Maryland; and Pliocene rocks at the Lee Creek Mine, North Carolina.

During life, the earbones were located in a cavity in the middle-ear area, located near the back of the dolphin skull. This cavity is filled with a dense foam that, along with ligaments, to support the ear bones. The bones consist of the tympanic and the periotic bones, and the entire structure is referred to as the “tympano-periotic complex.” The two bones are partially fused together with flanges, grooves, small holes, and a large bulbous (i.e., bowl-shaped area) region, called the bulla. The inner ear is located within the tympano-periotic complex.

Examples of fossil tympano-periotic complexes are shown below. They are from the shallow-marine Miocene Temblor Formation found at Sharktooth Hill in southern-central California.
Miocene tympano-perdiotic complex, greatest dimension 42 mm (approximately 1.75 inches).

Opposite side of same specimen shown above.
A different specimen, greatest dimension 43.5 mm.

Note: In one of my previous posts (8/15/2014) entitled “Fossil whale ear bone,” I illustrated a whale tympanic bulla.