Description
Placenticeras placenta Ammonite Fossil – Bearpaw Shale, Maastrichtian Upper Cretaceous, Montana
This genuine Placenticeras placenta ammonite fossil is a highly collectable Upper Cretaceous marine cephalopod specimen from the Bearpaw Shale of Montana, USA. Dating from the Maastrichtian Stage, approximately 72 to 66 million years old, this fossil represents one of the last great ammonite lineages before ammonites disappeared at the end of the Cretaceous Period.
This fossil is a genuine specimen and includes a Certificate of Authenticity lifetime guarantee generic card. It has been carefully chosen as an individual fossil specimen, with a photo that shows the actual piece you will receive. Full sizing please see photo.
Geology and Geological Age
Placenticeras placenta comes from the Bearpaw Shale, a famous Upper Cretaceous marine formation known for beautifully preserved ammonites and other fossils from the Western Interior Seaway. During the Maastrichtian, large parts of North America were covered or influenced by this vast inland sea, which connected the Arctic Ocean with the Gulf region and created extensive marine habitats across what is now the central and western United States.
The Bearpaw Shale was deposited in a quiet marine setting where fine muds, clays and silts accumulated on the sea floor. These sediments later compacted into shale and claystone, preserving shells and other remains from a rich Late Cretaceous ecosystem. The fossil-bearing beds of Montana are especially valued by collectors because they can preserve ammonites with strong shell shape, fine surface detail and, in some related specimens, traces of original iridescent shell material.
Fossil Type and Species
This specimen belongs to Placenticeras placenta, a Late Cretaceous ammonite within the family Placenticeratidae and the order Ammonitida. Placenticeras is a well-known genus of compressed, elegant ammonites recognised from Cretaceous marine deposits across several parts of the world. The genus Placenticeras was named by Fielding Bradford Meek in the 19th century and is the type genus of the family Placenticeratidae.
Ammonites were extinct marine cephalopods related to modern squid, octopus and cuttlefish. Unlike their modern soft-bodied relatives, ammonites possessed an external chambered shell. The living animal occupied the outer body chamber, while the older internal chambers helped control buoyancy, allowing the ammonite to move through the water column.
Morphology and Collectable Features
Placenticeras ammonites are admired for their very distinctive shell form. They are typically highly involute, meaning the outer whorl covers much of the earlier growth, creating a smooth, compact and elegant outline. The shell often has slightly convex sides and a narrow outer edge known as the venter. Ornamentation may be subtle, with faint ribs, small tubercles or smooth adult whorls depending on age, preservation and growth stage.
Placenticeras placenta is especially attractive to collectors because of its classic flattened ammonite shape and strong association with the Late Cretaceous seas of North America. In well-preserved examples, the fossil may show shell contours, natural surface texture, whorl shape, chamber structure, sutures or mineral replacement. Suture lines, where visible, represent the complex boundaries between the internal chamber walls and the outer shell, and are an important feature in ammonite identification.
Bearpaw Shale and Western Interior Seaway
The Bearpaw Shale records the final marine advance of the Western Interior Seaway across parts of North America. This ancient sea supported a diverse ecosystem including ammonites, baculites, nautiloids, bivalves, gastropods, fish, sharks, rays, marine reptiles and other invertebrates. Fossils from this formation help show the richness of marine life during the closing stages of the Cretaceous Period.
Placenticeras ammonites from the Bearpaw Shale are closely associated with this ancient marine world. Some placenticeratid ammonites from related deposits are also known with bite marks, showing that they interacted with large marine predators such as mosasaurs. This makes the group particularly interesting for collectors who enjoy fossils with a strong palaeoecological story.
Depositional Environment
This Placenticeras placenta ammonite was preserved after its shell settled onto the sea floor of a warm Cretaceous marine environment. Fine sediment gradually covered the shell, protecting it from complete destruction. Over millions of years, the surrounding mud hardened into shale, while mineral-rich groundwater helped fossilise the shell and preserve its form.
The result is a natural fossil from a vanished seaway that once divided North America. It represents not only an individual ammonite, but also a record of the marine ecosystems that flourished shortly before the end-Cretaceous extinction event.
Authenticity and Display
This is a genuine Placenticeras placenta ammonite fossil from the Bearpaw Shale, Maastrichtian, Upper Cretaceous of Montana, USA. It includes a Certificate of Authenticity lifetime guarantee generic card and is suitable for fossil collectors, geology enthusiasts, natural history displays, educational collections, display cabinets or as a distinctive gift for anyone interested in prehistoric marine life.
The fossil has been carefully selected, and the photo shows the actual specimen you will receive. Full sizing please see photo.
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