A GIANT AMMONITE FROM CALIFORNIA AND BAJA CALIFORNIA, MEXICO

Ammonites are cephalopods (e.g., octopus, squid, and pearly Nautilus shells) that range in geologic age from the Devonian through Late Cretaceous periods, an interval of about 360 million years. For more information, please see my last posting. You can also search for this blog for previous posts concerning ammonites.

This post concerns a few specimens of a giant species of Late Cretaceous ammonite that lived in shallow-marine waters in south-central and southern California and northern Baja California, Mexico. Complete specimens like the ones shown are not common.

The next three images are of the holotype of this giant ammonite, which was first recognized by Anderson and Hanna, who, in 1928, named this species. The first image is a side view of the holotype (which is the "name bearer" of any species and theoretically the most complete of all the specimens initially used to define that species). There can be only one holotype. It has to be stored in an officially recognized museum. This specimen is 50.3 cm [503 mm] in diameter and is from Baja California, Mexico. 

The second image (shown below) is of a paratype (diameter 19 cm [190 mm]) of this ammonite. It too was picked and used by Anderson and Hanna, the original namers of this species, in 1928.  Paratypes are used primarily to show morphologic feature(s) not observable on the holotype.There is no fixed number as to how many paratypes there can be. They are of  auxiliary (or secondary importance) in the naming of a species. 

The third image (shown below) is of the preceding paratype (19 cm [190 mm]) shown immediately above. It shows an end view.

The next two images are additional (non-type specimens) of this large ammonite. Both are large and heavy! It takes at least two strong people to transport these specimens by hand. This large ammonite represents the last of the “giant” ammonites in the fossil record in this region. It can be nearly 20 inches (50.8 cm [508 mm]) in diameter (not including the “living” chamber which enclosed the actual animal), and the weight of a single specimen can be well over a hundred pounds. 

Parapachydiscus catarinae was the first officially recognized name of this giant ammonite, whose shells were first found in the 1920’s in northern Baja California, Mexico. There have been several changes in its genus name in the last 100 years. For example, Parapachydiscus is no longer used because it is a synonym of Pachydiscus, which was named earlier and therefore has priority in its usage.

Pachydiscus is differentiated by having noticeable ribs on its shell, but catarinae has a smooth shell. Some researchers, therefore, have chosen to identify this ammonite as Pachydiscus (Neodesmoceras) sp., aff. catarinae because Neodesmoceras resembles Pachydiscus but is smooth-shelled. Furthermore, in the above abbreviation “aff.” means “having affinity to,” implying that more study is needed in order to determine if the species really is catarinae or if it is a different species (even possibly a new species). 

Other paleontologic problems are 1) possibly the ribs might not be preserved, 2) the ribs change in strength from juveniles to adults, and 3) the rib strength might just be a variable feature. I should also add that some researchers believe catarinae also lived in Colorado. So, the “last word” on the actual identification of this large ammonite, as well as its geologic age and stratigraphic/geographic distribution, still need be figured out. Such matters are beyond the scope of this blog post. 

This geologic time range chart shows the geologic range of the family, genus, subgenus, and the questionable species: Pachydiscus (Neodesmoceras) catarinae?

The geologic age of this ammonite species is inexactly known, but it is either near the Campanian/Maastrichtian boundary (72 million years ago) of the Late Cretaceous or is early Maastrichtian age (72 to about 70 m.y. ago).