Tuesday, September 30, 2014

"Blind" trilobite Itagnostus from Utah

Trilobites like the one shown above are well known to most collectors of fossil invertebrates (animals without backbones). This trilobite, which is 4 cm long, has a cephalon or head (with eyes), a thorax (body with many segments), and a short pygidium ("tail"). This particular specimen is an Elrathia kingi of Middle Cambrian age (see time diagram below) from the Wheeler Shale near Delta, Utah.

Another type of trilobite found alongside E. kingi is Itagnostus interstictus, until recently known as Peronopsis interstrictus, which belongs to one group of so-called "blind" trilobites. Technically speaking, "blind trilobites" were not blind because they never had eyes. Itagnostus is an agnostid trilobite, which are characterized by having a thorax consisting of only two segments and a cephalon and pygidium of approximately the same size.

This is a "cluster" of two specimens of Itagnostus interstices. The largest specimen is nearly 1 cm  long. 

Sunday, September 21, 2014


This post concerns scaphopods, which are mollusks that belong to the same phylum as snails, squids, octopi, etc. Scaphopods are tusk-shaped shells that are partially infaunal, that is to say they burrow into the ocean floor but do not burrow deep enough to cover the top part of their shell.

The drawing shows how the shell projects into the sediment. The mouth is surrounded by feeding tentacles which bring microscopic-sized food to it. The gut is straight. Water is brought down into the shell and also is excreted through a hole at the top of the shell.
This is a fossil scaphopod of the Eocene Dentalium stramineum The specimen is 7 cm long and almost complete. It is from Simi Valley, southern California. Fossil scaphopods are not common. 
This is a modern specimen of Dentalium. If it were not for modern specimens, scientists would probably never have determined that these simple tubes were made by mollusks.
This is another modern specimen of Dentalium.

Sunday, September 14, 2014

Molds and casts

This picture shows a specimen of the Eocene Turritella andersoni (height 7.5 cm) from southern California (please see one of my earlier posts if you want more information about this species). If the specimen is removed (i.e., is weathered away or simply fell out), it leaves behind an external mold = an impression of the external surface. This external mold is a negative surface, that is to say, it could literally "hold water." 
This next picture shows the external mold of the original specimen and, to the right, an external cast of this mold. Most collectors commonly do not bother to collect external molds. At some localities, however, that is all you can find, and if you need to identify the genus and species, it will help if you create a latex external cast of the external mold. That way, you can make a "positive" out of a "negative." All you need is some liquid latex, like the kind you can put on the back of a rug to keep it from sliding around on a floor. Carefully pour the liquid latex into the external mold (try not to create any bubbles), and let the latex dry. Removal of the latex cast is easy; just pull it off. A latex external cast is shown above just to the right of the external mold. In some, cases nature creates external casts by filling in the external mold with some foreign substance. In the above, because I used latex to make the external cast, therefore, it is called an artificial-external cast.
The above picture shows an internal cast, which shows the interior of a high-spired gastropod shell (most likely, a Turritella). The 6-cm high shell was hollow, and silt and mud filled the shell after the death of the gastropod. Later, the sediment converted into solid rock, the shell was destroyed, and all that is left is the internal cast. Equivalent terms for an internal cast are endocast, internal "mold," or, my personal favorite, steinkern (= a German word meaning a "rock center or core"). Steinkerns are not very useful for determining genus or species. Some early workers, unfortunately, used them for naming new species. Doing so has caused serious taxonomic (classification) problems for subsequent workers, who commonly refer to such a species as a nomen dubium = a name representing a species that is not identifiable from the original specimen (type) used to describe it.  
This final picture shows the external mold (6 cm wide) of the bivalve Laevicardium californiense. The fossil is of Plio-Pleistocene age and from the Santa Barbara Formation, Santa Barbara, southern California.

Sunday, September 7, 2014

fossil pine cones

a fossil pine cone, height 10.5 cm

I have always been intrigued by fossil pine cones, especially the ones that were transported by river currents to a ocean shoreline and ended up being deposited in the shallow-marine environment alongside Turritella shells (see some of my earlier posts) and shark teeth. 

The rare specimen shown above is from float material (weathered out and laying loose on the surface) from the Pico Formation near Newhall, southern California. The formation in this area was deposited in a marine-delta environment. There had to have been pine trees growing in the adjacent, ancient San Gabriel Mountains east of the delta. This pine cone floated down a braided river (full of coarse debris consisting of pebbles and cobbles) and was deposited in fine-grained sandstone near the ancient shoreline. 
another fossil pine cone, height 8.5 cm

The rare specimen shown above is from float material found near Topanga State Beach, southern California. Much less is known about its provenance (origin) than the Pico Formation specimen.

Identification as to the family or genus of pine tree for both of the illustrated specimens is needed. The identification process is not an easy task. Just the presence of pine cones in a sedimentary rock deposit is most helpful, nevertheless, because they indicate that a mountainous area was adjacent to the burial site.