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. However, the ancient past was filled with many more kinds of nautiloids. Scientists have found fossils of over 2,500 different species. During a time called the early Paleozoic era, nautiloids were among the top hunters in the sea.

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, sometimes as big as a person's body. 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 known as cephalopods, which also includes ammonoids, belemnites, and modern coleoids like octopus and squid. Cephalopods are a class of mollusks, which also includes gastropods, scaphopods, and bivalves.

Traditionally, cephalopods have been divided into four groups: orthoceratoids, nautiloids, ammonoids, and coleoids. Nautiloids are considered a group that shares basic features and may have been the ancestors of the other groups.

Shell

All nautiloids have a large external shell with two main parts: a narrowing chambered region called the phragmocone and a broad, open body chamber where the animal lives. The shell’s outer wall, known as the conch, gives the shell its shape and texture. As the nautiloid grows, it adds new chambers and enlarges the body chamber to make space for the growing animal.

A special tube called the siphuncle runs through the shell’s chambers, helping the nautiloid control its buoyancy. The shell is made of aragonite, a type of calcium carbonate, and often has a smooth surface, though some fossil shells have spines or ribs. 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, which 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 early Palaeozoic rocks. The rocks from the Ordovician period in the Baltic coast and parts of the United States contain many nautiloid fossils. Specimens such as Discitoceras and Rayonnoceras can be found in the limestones of the Carboniferous period in Ireland. Marine rocks from the Jurassic period in Britain often have fossils of Cenoceras, and nautiloids like Eutrephoceras are found in the Pierre Shale formation from the Cretaceous period in the north-central United States.

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

Evolutionary history

Nautiloids first appeared in the late Cambrian period in what is now China, living in a warm, shallow sea. Though many early types disappeared, one group survived and led to all later cephalopods. During the Ordovician, nautiloids diversified greatly, developing many different shell shapes and living in various ways.

They stayed diverse through the Ordovician, Silurian, and Devonian periods. By the Devonian, however, 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. Scientists think that the spread of certain sea animals, like seals, helped reduce their numbers over time.

Classification

See also: List of nautiloids

Nautiloids have had many different ways of being grouped by scientists, and these groupings have changed over time. One 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