Amonoids

Amonoids were a diverse group of extinct cephalopods that inhabited marine environments from the Early Devonian period (around 400 million years ago) until their mass extinction at the end of the Cretaceous period (about 66 million years ago). They are closely related to modern nautiloids and squids, sharing a similar body plan characterized by a coiled or straight external shell, a muscular foot, and a complex eye structure.

The most distinctive feature of amonoids was their coiled, chambered shell, which housed a series of compartments separated by septa. These chambers were filled with gas, allowing the animal to control buoyancy by adjusting the amount of liquid in the living chamber. The shell often exhibited intricate sutures, where the septa met the outer shell wall, which varied in complexity among different species. These sutures are a key feature used by paleontologists to classify and identify amonoids.

Amonoids thrived in a wide range of oceanic environments, from shallow seas to deep basins, and exhibited remarkable diversity in shell shape, size, and ornamentation. Some species had smooth, elongated shells, while others displayed elaborate patterns, spines, or ribbing. Their fossilized shells are among the most common marine fossils, providing valuable insights into ancient marine ecosystems and evolutionary history.

The extinction of amonoids coincided with the Cretaceous-Paleogene (K-Pg) extinction event, which also wiped out the dinosaurs and many other species. While the exact cause remains debated, evidence suggests a combination of factors, including asteroid impact, volcanic activity, and climate change. Despite their extinction, amonoids left a lasting legacy, influencing the evolution of modern cephalopods and contributing to our understanding of marine life through deep time.