Ammonites

By | September 9, 2016

Ammonite fossils are unmistakeable, thanks to their beautiful coiled shapes. These sea creatures were close relatives of today’s octopuses and squids but lived inside a shell, which they enlarged with new chambers as they grew, forming a spiral. They lived throughout the seas and swam by squirting water, the hollow inner chambers of their shells acting as air tanks to help them float.

FAMILY FACT FILE

ammonite-description

Because the shell’s air filled inner chambers floated upward, ammonites swam with their heads beneath their bodies

Key features
■ Coiled shell divided into chambers
■ Soft body inside the outermost chamber
■ Large head and well-developed eyes
■ Long tentacles for capturing prey

When
Ammonites appeared 425 million years ago and were very common in the oceans throughout the age of the dinosaurs. They perished at the same time as the dinosaurs, 65 million years ago.

Scaphites (scaff-EYE-tees)

scaphites

Scaphites fossil


When: 144–65 million years ago (Late Cretaceous)

Fossil location: Europe, Africa, India, N. America, S. America

Habitat: Shallow seas

Size: Up to 8 in (20 cm) across

Scaphites was an unusual ammonite. Instead of forming a neat spiral, its shell grew in a crooked shape. As a result, the opening for its head would have gotten tighter and tighter as Scaphites grew, eventually starving the animal to death. Perhaps Scaphites only lived long enough to lay its eggs, dying soon after, as happens in octopuses.

Promicroceras (pro-my-CROSS-e-ras)

marston-marble


When: 200 million years ago (Early Jurassic)

Fossil location: Worldwiammonite-mainde

Habitat: Seas

Size: Up to 3/4 in (2 cm) across

Vast numbers of Promicroceras died at the same time, carpeting the seafloor with shells. Over time, these turned into fossils, forming an amazing type of rock called Marston marble, which consists of almost nothing but ammonites. The cause of the mass death is a mystery, but one possibility is poisoning of seawater by algae (microscopic plants).

Echioceras (ECK-ee-oh-se-ras)

echioceras


When: 200 million years ago (Early Jurassic)

Fossil location: Worldwide

Habitat: Seas

Size: Up to 2.1/2 in (6 cm) across

Echioceras had a tightly coiled shell, which may have made it difficult to move rapidly. It preyed on other slow-moving creatures in the Jurassic seas.

Bifericeras (BYE-fuh-ih-suh-ras)

microconch

Microconch (female) fossils formed of the mineral iron pyrite (“fool’s gold”)


When: 200 million years ago (Early Jurassic)

Fossil location: Europe

Habitat: Open seas

Size: 1.1/4 in (3 cm) across

Bifericeras fed on small invertebrates that lived in the seas. The larger shells (“macroconches”) belonged to the females and the smaller ones (“microconches”) to the males. Females needed larger body sizes for producing and protecting their eggs.

Aturia (ay-TOO-ree-a)

aturia


When: 65–23 million years ago (Paleogene to Early Neogene)

Fossil location: Worldwide

Habitat: Open waters

Size: Up to 6 in (15 cm) across

Although the ammonites died out at the same time as the dinosaurs, closely related animals called nautiloids survived.

Aturia was a fast-swimming nautiloid that probably preyed on fish and shrimp. Its shell was smooth and streamlined for speed, without the ribs seen in many ammonites.

LIVING RELATIVE

pearly-nautilus

The pearly nautilus is a living nautiloid and a relative of the ammonites. Like its prehistoric cousins, it lives in a spiral shell divided into chambers, and it swims by squirting water. It has up to 90 tentacles, which it uses to capture small fish and crustaceans.

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