Ammonites are extinct marine cephalopods belonging to the subclass Ammonoidea. They are characterized by their spirally coiled shells, often exhibiting complex suture patterns and a variety of external ornamentation.
Here's a breakdown of the scientific definition:
* Phylum: Mollusca
* Class: Cephalopoda
* Subclass: Ammonoidea
* Key characteristics:
* Spirally coiled shell: The most distinctive feature, providing protection and buoyancy control.
* Septa: Internal partitions dividing the shell into chambers.
* Suture patterns: Complex, intricate lines on the shell's inner surface formed by the intersection of the septa with the outer shell wall.
* Siphuncle: A tube connecting the chambers, allowing the ammonite to regulate buoyancy.
* External ornamentation: Various ridges, ribs, and spines that may have served defensive or hydrodynamic purposes.
Evolutionary Significance:
Ammonites appeared during the Devonian period (~419 to 359 million years ago) and flourished throughout the Mesozoic Era, becoming extinct at the end of the Cretaceous period (~66 million years ago). Their fossil record is extensive and highly diverse, making them valuable tools for biostratigraphy (using fossils to date rock layers).
Note: The term "ammonite" is often used interchangeably with "ammonoid" in a broader sense. However, "ammonite" specifically refers to the Ammonoidea subclass, while "ammonoid" encompasses all extinct members of the Ammonoidea subclass, including its various orders.
