“In the bones of the rock the fossils are living, crinoid and ammonite; … The fossils are moving… aching for the sea.” — Norman Nicholson

https://www.youtube.com/watch?v=-qcZdtwpemA

Characteristics

Ammonites were marine cephalopod mollusks that existed from the Devonian to the Cretaceous periods. Here are their key physical characteristics:

• Shell: Characterized by their distinctive coiled shells (called "phragmocones") with complex suture patterns where the chamber walls meet the outer shell wall

• Size: Ranged dramatically from less than 1 cm to 2 meters in diameter, with most species between 5-30 cm

• Structure: Shell divided into chambers separated by walls called "septa," with the animal living in the outermost chamber

• Buoyancy system: Chambers filled with gas or fluid allowed for buoyancy control, similar to modern Nautilus

• Tentacles: Possessed numerous tentacles extending from the aperture (shell opening) for capturing prey
• Body: Soft-bodied creature within the shell with a mantle, gills, jet propulsion system, and complex eyes

Time Period

Ammonites had a lengthy and significant presence in Earth's oceans:

• First appearance: Devonian period (approximately 409 million years ago)

• Peak diversity: Jurassic and Cretaceous periods (201-66 million years ago)

• Extinction: End of the Cretaceous period (66 million years ago) during the K-Pg mass extinction event that also eliminated the dinosaurs

• Fossil record: Incredibly abundant and well-preserved, making them important index fossils for dating rock layers

• Diversification: Experienced several adaptive radiations throughout their existence, evolving into over 10,000 species

  

Habitat

Ammonites were exclusively marine organisms with specific habitat preferences: