Friday, August 29, 2014

An exceptional trilobite-trace fossil

Trilobites (see the link ) were Paleozoic arthropods that crawled along the shallow-sea floor. When a trilobite stopped to rest, it made a shallow burrow in the mud or silt. These burrows were commonly filled with sediment and later fossilized as "resting" trace fossils called by the Latin name, Rusophycus. Remember from one of my earlier blogs, a trace fossil shows behavior of a fossil organism.

The above picture is the bottom of a Cambrian Rusophycus from the Inyo Mountains, central California. The burrow is 10 cm in length. Scratch marks made by the trilobites legs are visible on the bottom of the burrow.

These two slabs (both about 11 cm, widest dimension) of slightly metamorphosed Cambrian siltstones from eastern California contain a cluster of Rusophycus. The burrows were probably aligned parallel to an ancient current that brought food to the trilobite. Rusophycus commonly has a bilateral symmetry that was caused by its two rows of legs that moved back and forth in its burrow. This leg action was necessary because its gills were attached to its legs, thus it had to move its legs (even when stationary in its burrows), in order to obtain oxygen from the shallow-sea water.

Ahh, finally we get to a truly exceptional specimen of Rusophycus. It is undoubtedly the best specimen I have ever seen. It is preserved three-dimensionally (8.5 cm long, 3.5 inches) and is of Cambrian age from the Salt Spring Hills, eastern California. The above picture is the bottom view, which shows the scratches made by the legs of the trilobite. The specimen is a plaster replica of the actual specimen, which is now in a museum collection. I painted the plaster replica so as to make it look more like the actual specimen.

This is a side view of the same specimen shown above. The bottom of the burrow is at the top of the picture. I painted the antennae red, so you can readily see them. The fact that the antennae are visible indicates that the remains of the trilobite that made this resting trace are within the burrow, thereby proving the trilobite made this burrow. In situ (in place) specimens of trilobites in their burrows are very rare.

Friday, August 15, 2014

Fossil whale ear bone

Fossil whale ear bones are among some of the most unusual fossils that a collector can find. Whales do not have external-ear structures. Instead, they use the lower jawbone for transmitting sound vibrations to their interiorly located ear structures. They have well developed middle and internal ears and a good sense of hearing.

Their middle ear is enclosed by a bowl-shaped bony structure called the tympanic bulla, which can be fossilized. A fossil bulla is illustrated below.

fossil whale ear bone (bulla), 7 cm wide; Neogene age (Miocene?), California?
Whale ear bones occur in pairs, and the following pictures show replicas of a pair of ear bones from a modern melon-head whale.

models of modern melon-head whale bullae; each one is 3 cm wide

For an interesting article on whale ear bones, including fossil ones, the link is–whale-ancestors-and-more-ear-bones/

This link provides information on how fossil whale-ear bones help support the evolutionary concept that whales have an ancestry with artiodactyls (modern ones are cows, deer, pigs, etc.).

Monday, August 4, 2014

Pholad boreholes

In the first pictures of my last blog, I showed boreholes made by pholad clams (bivalves) in relatively soft sediment. I decided to expand somewhat on the topic and show pholad boreholes made in harder substrate. Pholad boreholes are trace fossils made by highly specialized clams belonging to the family Pholadidae (pholad, for short) that bore into hard substrate consisting of mud, rock (including cement-covered pilings), coral, shell, and wood. 

The pholads shown below all bored into hard rock and are representative of pholad boreholes commonly found along rocky shorelines of ancient and modern oceans.

Boring is done by the rough file-like anterior of the shell, which is twisted back and forth by the clam, which, as a juvenile, enters a small hole in the rock. As the clam grows larger, it enlarges the borehole to accommodate its growing shell. The borehole is periodically cleaned of debris by jets of water emanating from inside the clam.

These pholad boreholes were made in solid, hard rock (8 cm in width). The middle borehole has both valves of the actual clam, which is in its living position.
A transported clast (eroded rock material) of pholad boreholes in an upper Pliocene, shoreline deposit, Simi Valley, southern California.  Other fossils are fragments of pectinid clams. The blue-colored part of the rock handle is 18 cm long.
Rocky shoreline large cobble containing several empty pholad boreholes. The large cobble has a maximum width of 15 cm.

Sand fillings (casts) of a cluster of pholad boreholes (the rock that once surrounded these boreholes is missing). The rock is 5 cm in width.