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4.05" Highly Iridescent Fossil Ammonite (Sphenodiscus) - South Dakota
This is a 4.05" Sphenodiscus sp. ammonite specimen that was collected from the Fox Hills Formation of South Dakota. The shell is still intact and exhibits phenomenal iridescence. It's been wonderfully prepared free from the hard concretion it was found in.
There is a repaired crack with some minor gap fill and the ammonite has been coated in a thin layer of sealant/epoxy. It comes with an acrylic display stand.
There is a repaired crack with some minor gap fill and the ammonite has been coated in a thin layer of sealant/epoxy. It comes with an acrylic display stand.
About These Ammonites
These 70-million-year-old Cretaceous ammonite fossils come from a time when South Dakota was submerged beneath the Western Interior Seaway, a warm, shallow inland ocean that once stretched from the Arctic to the Gulf of Mexico. Ammonites were extinct marine cephalopods related to modern squid, octopus, and cuttlefish, but unlike their soft-bodied relatives, they lived inside coiled, chambered shells. As the animal grew, it added new chambers, using them to regulate buoyancy and move efficiently through the ancient seas. Their abundance and rapid evolution make ammonites some of the most important index fossils for dating marine rocks.
After death, these ammonites settled into seafloor sediments where mineral-rich waters slowly formed concretions around the shells, protecting them from crushing and decay for tens of millions of years. When these stone nodules are split open today, the fossils are revealed locked inside and must be carefully hand-prepared to remove the surrounding rock. This delicate, time-consuming process—often done with air scribes and fine tools—can take many hours per specimen, gradually exposing the shell’s ribs, sutures, and natural curvature. The finished fossils are striking remnants of South Dakota’s ancient ocean, preserving a moment from a world that vanished long before dinosaurs walked on land.
These 70-million-year-old Cretaceous ammonite fossils come from a time when South Dakota was submerged beneath the Western Interior Seaway, a warm, shallow inland ocean that once stretched from the Arctic to the Gulf of Mexico. Ammonites were extinct marine cephalopods related to modern squid, octopus, and cuttlefish, but unlike their soft-bodied relatives, they lived inside coiled, chambered shells. As the animal grew, it added new chambers, using them to regulate buoyancy and move efficiently through the ancient seas. Their abundance and rapid evolution make ammonites some of the most important index fossils for dating marine rocks.
After death, these ammonites settled into seafloor sediments where mineral-rich waters slowly formed concretions around the shells, protecting them from crushing and decay for tens of millions of years. When these stone nodules are split open today, the fossils are revealed locked inside and must be carefully hand-prepared to remove the surrounding rock. This delicate, time-consuming process—often done with air scribes and fine tools—can take many hours per specimen, gradually exposing the shell’s ribs, sutures, and natural curvature. The finished fossils are striking remnants of South Dakota’s ancient ocean, preserving a moment from a world that vanished long before dinosaurs walked on land.
SPECIES
Sphenodiscus sp.
LOCATION
South Dakota
FORMATION
Fox Hills Formation - Trail City Member
SIZE
Ammonite 4.05" Wide
CATEGORY
SUB CATEGORY
ITEM
#144816
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