Thursday, October 30, 2014

Rudistid bivalve

          Coralliochama orcutti White, 1885, height (incomplete) 12.7 cm, width 7.7 cm

Rudistid bivalves (also known as rudists) are among some of the more unusual (aberrant) bivalves (clams). They are of Late Jurassic to Late Cretaceous in age (see time diagram at bottom of this post) and had widespread distribution in equatorial (tropical) areas. They were gregarious, and their shells were closely packed together. At many locales, their abundant shells commonly formed thick beds (biostromal accumulations).

As shown above, the lower valve (right valve) of a rudistid belonging to genus Coralliochama has the form of an elongate thick-walled cone. The upper valve (left valve) is reduced to a domed structure. The commissure is the line of junction of the two valves.  

The following picture is of the same specimen as above but shows the front of the bivalve. This specimen is of Late Cretaceous age (approximately 72 million years old) and is from northwestern Baja California.

Tuesday, October 21, 2014

Ammonite sutures in interior of shell much simpler than you would expect

      Late Mesozoic ammonite whose maximum diameter is 10.5 cm (4 in.);

       the specimen is from Madagascar
This blog is a follow-up on my previous blog, which also deals with ammonite sutures. What I want to show you now is something that I noticed years ago about suture lines. I have looked through many textbooks for a similar depiction, but I have never found any illustration of what I am showing here. The photograph above is of the exterior (partly worn) surface. Along the upper part of the photograph, you can see complex ammonitic sutures.

This second photograph shows one half of the interior of the same specimen that is illustrated above (i.e., the specimen was sliced in half). The bright yellow material on the right side is secondary foreign mineral material that replaced some of the chambers. Notice that in the upper part of the photograph, the partitions that form the chambers are very simple curves. They are not like the complex sutures lines show in the first photograph. The suture lines are complex ONLY at the junction between the chamber partitions and the outer shell wall.

The presence of complexity of the sutures at the junction with the wall, but not in the main parts of the chambers, begs the question "why." The answer is not clear because ammonites are extinct organisms. Some experts believe that the complex suture lines are related to the strengthening of the outer shell wall, so as to resist being crushed by hydrostatic pressure as the ammonite slowly swan (descended) into relatively deep depths in the ocean environment. The main parts of the chambers must have not needed the extra reinforcement.

Tuesday, October 14, 2014

Ammonite sutures

Ammonite shells are some of the most popular fossils for collectors. For a link that shows representative pictures of ammonites, click HEREAmmonites were cephalopods (e.g., squids and octopus) that resemble the modern-day Nautilus. Like Nautilusammonites have curved partitions (septa), which divide the shell into chambers. The lines (sutures), which formed where the septa made contact with the inside wall of the shell, consist of distinctive curves that characterize different groups of ammonites. Sutures are only visible when the outer wall is removed or has been nearly stripped away.

Ammonite sutures gradually changed (see diagram above) during the 250 million years through which they ranged; namely, from the Middle Paleozoic (Devonian Period) to the end of the Mesozoic (Cretaceous Period) (see diagram below). 

The ammonite sutures gradually became increasingly more complex, and these changes enable the paleontologist to use ammonite shells as "geologic clocks" for helping to determine geologic time.

The gonitatic shells were the earliest (oldest) and the most simple; the ceratitic shells were intermediate in age and complexity; the ammonitic shells were the latest (youngest) and the most complex. The following pictures show a representative specimen for each type of sutures.

This is an example of deeply undulating goniatitic sutures. It is a partial specimen of Gonioloboceras coniolobum of Late Paleozoic (Pennsylvanian) age from Texas. The maximum dimension of this specimen is 6 cm.

This is questionably a specimen of Uddenoceras of Late Paleozoic age. It is an example of ceratitic sutures. The maximum dimension is 9 cm. 
This is Hoploscaphites brevis? of Late Cretaceous age from Montana and is an example of ammonitic sutures. The maximum dimension is 3.25 cm. The shell shows impressions of its straight ribs, which must not be confused with the sutures. This specimen displays "mother-of-pearl" luster, which indicates that the preservation of the shell material is very good. 

Tuesday, October 7, 2014

Oldest fossil shells in California

The oldest known fossil shells in California are poorly preserved hollow tubes of Wyattia reedensis Taylor, 1966 They are found in uppermost Precambrian to possibly lowermost Cambrian (see the time diagram in my last post) carbonate rocks in the Inyo Mountains of eastern California. A representative tube, as indicated in the above image by the short black arrow, is approximately 10 mm in length and 3 mm in diameter. The rock surrounding this tube contains many other, but less distinct, tubes of W. reedensis. 

Wyattia is a probable molluscan fossil and is pre-trilobite in age. Wyattia and other small-sized shells are important because they represent the oldest skeletonized faunas on Earth. These fossils occurred during an interval of geologic time between when only impressions of soft-bodied organisms have been found versus when the first shells of larger animals (e.g., trilobites) have been found.

Late Precambrian to early Cambrian Wyattia-like fossils have also been found in Esmeralda County Nevada and near Caborca in northern Sonora, Mexico.

Wyattia was named for J. Wyatt Durham (deceased), who was a professor of paleontology (study of fossils) for many years at the University of California, Berkeley.