Nautiloid

Nautiloids are a fascinating group of sea creatures called cephalopods, which also include animals like squid and octopus. They belong to a larger family of animals known as mollusks. Nautiloids first appeared in the ocean a very long time ago, during the Late Cambrian period, and they have been an important part of marine life ever since.

Today, we still see nautiloids living in the ocean. The two main types we can find are called Nautilus and Allonautilus.

One of the most interesting features of many ancient nautiloids was their shells, which came in amazing shapes. Some shells were coiled like spirals, while others were long and straight. Though most of these ancient shapes disappeared long ago, a few coiled-shell types, the nautiluses, are still alive today. Nautiloids are one of the three main groups of cephalopods, along with the extinct ammonoids and the living coleoids, which include squid and octopus.

Taxonomic relationships

Nautiloids are part of the group called cephalopods. This group also includes ammonoids, belemnites, and modern coleoids like octopus and squid. Cephalopods are a class of mollusks. Mollusks also include gastropods, scaphopods, and bivalves.

Cephalopods are usually divided into four groups: orthoceratoids, nautiloids, ammonoids, and coleoids. Nautiloids share basic features and may have been the ancestors of the other groups.

Shell

All nautiloids have a large shell with two main parts: a narrowing area called the phragmocone and a broad, open body chamber where the animal lives. The shell’s outer wall, called the conch, gives the shell its shape. As the nautiloid grows, it adds new chambers and makes the body chamber bigger for the animal.

A special tube called the siphuncle runs through the shell’s chambers, helping the nautiloid stay afloat. The shell is made of aragonite, a type of calcium carbonate. Modern nautiluses have coiled shells, while ancient ones could be straight, curved, or coiled in different ways.

Modern nautiloids

Main article: Nautilus

We learn about ancient nautiloids by studying modern nautiluses, like the chambered nautilus. These creatures live deep in the Pacific Ocean from Samoa to the Philippines, and also in the Indian Ocean near Australia. They swim freely and have a head with simple lens-free eyes and many arms called tentacles.

Nautiluses have a smooth shell divided into sections filled with gas. This helps them stay balanced in the water. They hunt for food like crustaceans using strong, beak-like jaws. When they move, they push water out of a special funnel called the hyponome to jet around. Unlike some other sea creatures, nautiluses do not use ink for defense.

Fossil record

Nautiloids are often found as fossils in rocks from the early Palaeozoic time. Rocks from the Ordovician period in the Baltic coast and parts of the United States have many nautiloid fossils. You can find fossils like Discitoceras and Rayonnoceras in the limestones from the Carboniferous period in Ireland. Marine rocks from the Jurassic period in Britain often contain fossils of Cenoceras, and nautiloids such as Eutrephoceras are in the Pierre Shale formation from the Cretaceous period in the north-central United States.

Some Ordovician nautiloids, like Endoceras, grew very big, with shells up to 5.7 meters (19 feet) long. In places like Scandinavia and Morocco, many orthoconic nautiloid fossils were found, forming special kinds of limestone rocks.

Evolutionary history

Nautiloids first appeared in the late Cambrian period in what is now China. They lived in warm, shallow seas. Many early types disappeared, but one group survived and led to all later cephalopods.

During the Ordovician, nautiloids diversified greatly. They developed many different shell shapes and lived in various ways.

They stayed diverse through the Ordovician, Silurian, and Devonian periods. By the Devonian, they began to decline, possibly because of competition with other sea creatures. Only a few types survived into later periods. Today, just six species remain, living in the tropical Indo-Pacific Ocean.

Classification

See also: List of nautiloids

Nautiloids have been grouped in many ways by scientists, and these groupings have changed over time. A major book about them, called the Treatise on Invertebrate Paleontology, was published in 1964 but is now considered outdated.

More recent studies have tried new ways to group nautiloids. Some scientists divide them into five main groups based on how their muscles attached to their shells. These groups are:

• Subclass †Plectronoceratia
  • Order †Plectronoceratida
  • Order †Yanheceratida
  • Order †Protactinoceratida
• Subclass †Multiceratia
  • Order †Ellesmeroceratida
  • Order †Cyrtocerinida
  • Order †Bisonoceratida
  • Order †Oncoceratida
  • Order †Discosorida
• Subclass †Tarphyceratia
  • Order †Tarphyceratida
  • Order †Ascoceratida
• Subclass Nautilia
  • Order Nautilida
• Subclass †Orthoceratia
  • Order †Rioceratida
  • Order †Dissidoceratida
  • Order †Orthoceratida
  • Order †Pseudorthoceratida
  • Order †Actinoceratida
  • Order †Astroviida
  • Order †Endoceratida