Anomura

Some sea animals look like crabs.

They have ten legs. But two legs are very small. These tiny legs stay hidden. They help clean the animal.

This group has many types. Hermit crabs are one kind. King crabs are another kind. Some look very different from each other.

These animals have lived for a long time. Old fossils were found in a far land. They are a special group of sea life.

Anomura is a group of sea animals. They are part of the decapod group. This name means they have ten legs.

Most of these animals have ten legs. But the last pair is very small. These tiny legs stay hidden under a shell. They help clean the animal's gills. Because of this, many look like they have only eight legs.

This group includes many different kinds. You may know the hermit crab. King crabs and porcelain crabs are also in this group. Some look very different from each other. Some even look like true crabs. This change is called carcinisation. It is when an animal grows a crab-like body.

Anomura has lived for a very long time. Scientists found a fossil called Platykotta. It lived during the Late Triassic period. This fossil was found in the United Arab Emirates.

Anomura is a sister group to true crabs. This means they are close relatives. They belong to the larger group called Reptantia. This group includes lobsters and crabs too.

Anomura is a special group of sea animals. These animals are part of the decapod group. The name decapod means they have ten legs.

This group includes many different creatures. You might know the hermit crab. It also includes king crabs and porcelain crabs. Some members look very different from one another. This is why they are sometimes called Anomala. That name means they have many unusual shapes.

Most anomurans have ten legs to help them move. However, the last pair of legs is very small. These tiny legs are often hidden away. They stay inside the gill chamber under the carapace. The carapace is a hard shell. These small legs are used to clean the gills. Because they are hidden, many look like they have eight legs.

Some animals in this group look like true crabs. This happens through a process called carcinisation. Carcinisation is when an animal develops a crab-like body. This has happened several times in this group. King crabs have gone through this change. Porcelain crabs also show this crab-like shape. Even the hairy stone crab has done this. They all grew a body that looks like a crab.

These animals have a very long history. They are part of a group called Reptantia. This group includes lobsters and crabs. Anomura is a sister group to true crabs. This means they are close relatives. Scientists believe they split from true crabs long ago. This happened during the Late Triassic period.

We know about their history from old fossils. The oldest fossil found is called Platykotta. It belongs to the family Platykottidae. This fossil comes from the Ghalilah Formation. That place is in the United Arab Emirates. It dates back to the Norian–Rhaetian age. This helps us see how they lived long ago. It shows us how much they have changed.

Anomura is a diverse group of decapod crustaceans. Decapods are animals that possess ten legs. This group includes well-known creatures like hermit crabs. It also contains king crabs and porcelain crabs. Scientists sometimes use the name Anomala for this group. This name reflects the unusual variety of forms found within it. While many crabs look similar, anomurans can look very different from one another.

Anomurans are defined by a specific leg arrangement. They possess ten pereiopods, which are walking legs. However, the last pair of these legs is greatly reduced in size. These small legs are often tucked away inside the gill chamber. This chamber sits protected under the carapace, which is a hard shell. These tiny legs are used for cleaning the gills. Because these legs are often hidden, many anomurans appear to have only eight legs.

Some members of this group have undergone a process called carcinisation. Carcinisation is the development of a crab-like body plan. This process involves acquiring an armored body and a concealed tail. This evolutionary change has occurred independently in several different lineages. For example, king crabs (Lithodidae) have undergone carcinisation. Porcelain crabs (Porcellanidae) have also developed this form. The hairy stone crab (Lomisoidea) is another separate instance of this process.

Evolutionary data helps scientists understand the history of these animals. Anomura belongs to the group Reptantia. This group includes crawling or walking decapods like lobsters and crabs. Morphological and molecular data show that Anomura is a sister group to Brachyura. Brachyura are known as the "true crabs." Together, Anomura and Brachyura form a larger clade called Meiura. This means they share a common ancestor.

Anomurans have a very deep history on Earth. Scientists believe they diverged from true crabs during the Late Triassic period. We can learn about this era through the fossil record. The oldest known fossil attributed to Anomura is called Platykotta akaina. This fossil belongs to the family Platykottidae. It was discovered in the Ghalilah Formation. This formation is located in the United Arab Emirates. The fossil dates to the Norian–Rhaetian age.

The group contains many different superfamilies. One group is Aegloidea, which includes the Aeglidae family. Another group is Chirostyloidea, which contains squat lobsters. There is also the Galatheoidea superfamily. This group includes both squat lobsters and porcelain crabs. Another important group is Paguroidea. This superfamily includes hermit crabs, coconut crabs, and various other families. These groups show the massive biological variety within the Anomura.

Understanding Anomura helps scientists study how body plans change. The existence of carcinisation shows that similar shapes can evolve multiple times. It is fascinating that different families like Lithodidae and Porcellanidae reached similar forms. The distinction between Anomura and Brachyura is also vital. While they look similar, their different leg structures and evolutionary paths set them apart. Studying these crustaceans provides a window into the complex history of life in the sea.