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How They Formed
According to The Rock Shed, septarian fossils were formed during the Cretaceous period, approximately 50 to 70 million years ago. This was a period of time when the Gulf of Mexico reached Utah. This period was characterized by volcanic eruptions, which killed sea life. Decomposing sea life chemically reacted with the sediment around them to form mud balls. As the ocean receded, the mud balls that were formed hardened and cracked, trapping sea life inside of them.
Composition
According to Best Crystals, as decomposing sea life seeped down into the cracks in the mud balls that were formed, calcite crystals formed. The site states that the outer walls of calcite transformed into aragonite. The balls cracked in seven points in every direction, thereby accounting for its unique patterns. Aside from calcite and aragonite, which account for the yellow fossil centers and brown cracked lines, respectively, the outer shell of the fossils are composed of limestone.
Meaning
The word "septarian" is derived from the Latin name "septem." According to The Rock Shed, this means "seven," a reference to the seven points at which the mud balls According to The Crystal Cache, the fossils or parts of the fossils that decomposed into the rock can still be seen inside the rock.
Fossils
Septarian fossils are commonly found in the northern most areas of the United States where the Gulf of Mexico used to exist. Specifically, this area is classified as southern parts of Utah, Wyoming and Colorado. This location is part of an area where specimens of fossilized fish are found in septarian fossils. According to The Rock Shed, the fish most commonly found in the area is the Knightia, which was a herring-like fish. Some fossils found in this area are up to 60-million years old. Septarian fossils are also found in parts of New Zealand.