THREE REPRESENTATIVE AMMONITE FOSSILS FROM WESTERN USA

Ammonites are extinct cephalopods (mollusks) that had soft parts that resembled squids but also had a hard, external shell (used for protection).

1) MIDDLE TRIASSIC NEVADITES AMMONITE FROM WESTERN NEVADA

 

In 1975, I collected specimens of the ceratitid ammonite Nevadites hyatti? (Smith) from anoxic black shale/limestone = the Middle Triassic Prida Formation, which was deposited along the ancient continental coastline that existed at that time.

Fig. 1. Back side of a specimen of the Middle Triassic ammonite Nevadites hyatti from Nevada.

2) UPPER CRETACEOUS SCAPHITES AMMONITE FROM       NORTHWESTERN NW NEW MEXICO

In 1968, when I was a summer field assistant working for Shell Oil Company in northwestern New Mexico, I collected specimens of this ammonite. They occurred only in heavy concretions that had weathered out from nearby cliffs. I had to bust up a representative concretion with a sledge hammer in order to recover any fossils, which were predominantly the ammonite Scaphites sp. Not much time was spent by the crew I was with, when it came to collecting fossils. We were there to measure a stratigraphic section, but the fossiliferous concretions were a wonderful “bonus” for me!

 

These ammonites are of Late Cretaceous age (about 92 million years old, from the Gallup Sandstone, which correlates with Upper Cretaceous Turonian Stage). The ammonites are heteromorphs, meaning that their shell became uncoiled with growth (the uncoiled parts were broken off on the specimens I collected). I identified these ammonite specimens as Scaphites.

Fig. 2. Back side of the Late Cretaceous ammonite Scaphites sp. from northwestern New Mexico.

3) UPPER CRETACEOUS HOPLOSCAPHITES AMMONITE FROM MONTANA

This widespread genus of shallow-marine ammonite was previously reported as confined to Late Cretaceous age rocks. In recent years, however, shallow-marine Hoploscaphites ammonites have been reported in Early Paleocene (Danian Stage) rocks in Denmark, the Netherlands and in the USA. This new information does not support the previous viewpoint held by most earth scientists that all ammonites died out at the end of the Cretaceous Period. 

In North America, Hoploscaphites lived primarily in the Late Cretaceous warm waters of the Western Interior Seaway---from eastern Alaska to northern Mexico. The early part of the shell of this animal was tightly coiled, but with growth the shell became somewhat uncoiled (i.e., J-shaped). This genus has complicated septa (= curved partitions that divide the shell into chambers). 

Overriding the septa are complicated suture lines (white on these images). The suture lines of Holploscaphites are especially complicated  which indicates that this genus was an advanced form of ammonite that lived during Cretaceous time.

Figure 3. Back side of the Late Cretaceous ammonite Hoplocsaphites sp. from Montana (this specimen was a gift given to me).