Definition

1. Noun:
   • An ammonoid is the coiled, chambered, fossilized shell of an extinct marine mollusk. These fossils are characterized by their typically spiral shape, similar to a coiled snake or a ram's horn, and complex internal chambers separated by walls called septa. They are important index fossils for dating rock layers.

Examples of Usage

• Noun:
  • The paleontologist carefully extracted the ammonoid from the limestone.
  • This particular ammonoid species is used to identify the Jurassic period.
  • The museum's collection features an exceptionally large and well-preserved ammonoid.

Advanced Usage

• "Ammonoid suture": Refers to the intricate, wavy line formed where the internal chamber walls (septa) meet the outer shell wall. The complexity of this suture pattern is a key feature for classifying different ammonoid groups.
  • The complex ammonoid suture indicated it was a more evolutionarily advanced species.

Variants and Related Words

• Ammonite (n): A specific subgroup of ammonoids, often with more complex suture patterns. In common usage, "ammonite" is frequently used interchangeably with "ammonoid."
• Nautiloid (n): A related but distinct group of cephalopods with chambered shells, including the modern nautilus, which often has a simpler, curved suture pattern compared to ammonoids.

Synonyms

• Fossil shell: A general term for the preserved hard part of an ancient organism.
• (In specific contexts) Ammonite: See "Variants and Related Words."

Related Phrases

• Chambered shell: Describes the internal structure of the ammonoid, divided into compartments.
• Index fossil: A fossil used to define and identify geologic periods. Ammonoids are classic index fossils.

Noun

1. one of the coiled chambered fossil shells of extinct mollusks