Ammonites

Definition

Ammonites are extinct marine mollusks that belong to the subclass Ammonoidea, known for their distinctive coiled and chambered shells. These creatures lived during the Mesozoic era and are important index fossils, meaning they help geologists date and correlate the age of rock layers based on their presence.

5 Must Know Facts For Your Next Test

1. Ammonites first appeared around 400 million years ago and thrived until they became extinct approximately 66 million years ago during the mass extinction event that wiped out the dinosaurs.
2. The intricate patterns found in ammonite shells, including suture lines, are key features used to identify different species and determine their evolutionary relationships.
3. Ammonites inhabited a variety of marine environments, often living in open oceans and may have played a significant role in ancient ecosystems as both predators and prey.
4. These fossils are found worldwide, often in sedimentary rock formations, making them useful for geologists to establish relative ages of rock layers in different geographic locations.
5. Ammonites are known for their ability to regulate buoyancy through gas-filled chambers in their shells, allowing them to float at various depths in the ocean.

Review Questions

• How do ammonites serve as index fossils, and what significance does this have for stratigraphy?
  • Ammonites are effective index fossils because they had a wide geographical distribution and evolved rapidly over relatively short geological periods. This allows geologists to use their presence in rock layers to date those layers and correlate them across different regions. By identifying specific ammonite species within a stratigraphic column, geologists can establish a timeline for sediment deposition and understand the geological history of an area.
• Discuss the morphological features of ammonite shells and how they contribute to our understanding of their evolutionary relationships.
  • Ammonite shells exhibit unique morphological features such as coiling, chambering, and intricate suture patterns. These features vary among different species and provide valuable information for classification. The suture patterns help paleontologists distinguish between groups of ammonites and understand their evolutionary development over time. By analyzing these morphological traits, researchers can trace the lineage of ammonites and their adaptation to various marine environments.
• Evaluate the role of ammonites in ancient marine ecosystems and how their extinction impacted marine life during the Mesozoic era.
  • Ammonites played a crucial role in ancient marine ecosystems as both predators and prey. Their diverse feeding strategies allowed them to occupy various ecological niches, contributing to the complexity of marine food webs. The extinction of ammonites at the end of the Cretaceous period had significant consequences for marine life; it disrupted established food chains and allowed other cephalopods and marine organisms to evolve and fill ecological roles previously occupied by ammonites. This shift contributed to the restructuring of marine biodiversity during the Cenozoic era.

Related terms

index fossils:

Fossils of organisms that were widespread but existed for a relatively short geological time, used to date and correlate the age of rock layers.

cephalopods: A class of marine animals that includes ammonites, octopuses, squids, and nautiluses, characterized by their soft bodies and tentacles.

sutures: The lines or patterns formed on the shell of ammonites where the different chambers of the shell were joined together, important for classification and identification.