Chordate

A chordate is a special kind of animal in a group called Chordata. All chordates have five key features at some point in their lives. These features are a notochord, a hollow nerve cord on their back, a special structure called an endostyle or thyroid, pharyngeal slits, and a tail behind their anus.

Chordates are divided into three main groups. The first group is Vertebrata, which includes fish, amphibians, reptiles, birds, and mammals. These animals have a skull and a backbone that replaces their notochord. The second group is Tunicata, which includes sea squirts and salps. These animals only show the key chordate features when they are young. The third group is Cephalochordata, which includes lancelets. These animals look like jawless fish but do not have gills or a distinct head.

Chordates have been around since the Cambrian explosion, more than 539 million years ago.

Etymology

The name "chordate" comes from a special part of their bodies called the notochord. This helps give chordates their shape and lets them move. Chordates also have symmetry on both sides, a space inside their bodies for organs, a system that moves blood around, and a body plan that repeats in segments. The name Chordata was first used by a scientist named William Bateson in 1885, but others were already using it before that. Another scientist, Ernst Haeckel, described these animals in 1866.

Anatomy

Chordates are animals with special features at some point in their lives. These features include:

• A notochord, a stiff rod that runs down the middle of the body. In animals with backbones, this rod is replaced by parts of the spine.
• A hollow dorsal nerve cord, which becomes the spinal cord and helps control the body.
• Pharyngeal slits, which in fish become gills, but in other animals help with feeding.
• A post-anal tail, a muscular tail that sticks out behind the body.
• An endostyle, a groove in the throat that helps with feeding and storing certain materials.

These features help define what makes chordates special among animals.

Classification

This section shows how scientists group different kinds of animals called chordates. The groups are based on what features these animals share.

The main group is called Phylum Chordata, and it has three big subgroups:

1. Subphylum Cephalochordata – These are small sea animals called lancelets.
2. Subphylum Tunicata – These include sea squirts and other ocean animals.
3. Subphylum Vertebrata – These are animals with backbones, like fish, amphibians, reptiles, birds, and mammals.

The chart shows many smaller groups within these big ones, helping scientists understand how all these animals are connected.

Subphyla

See also: List of chordate orders

Cephalochordata: Lancelets

Main article: Lancelet

Cephalochordates are one of the three main groups of chordates. They are small, fish-like animals without brains or heads. These animals live buried in the sand and eat by filtering food from the water. They are the first group to split off from other chordates.

Tunicata (Urochordata)

Main article: Tunicate (Urochordata)

Tunicates have three different shapes when they are adults. All of their babies have the basic features of chordates, including long, tadpole-like tails. Their babies also have simple brains and sensors to see light and movement.

The smallest group of tunicates is the Appendicularia. They keep their tadpole-like shape and swim their whole lives. For a long time, they were thought to be babies of the other two groups.

The other two groups, sea squirts and salps, change shape when they become adults and lose some chordate features. Both groups are soft-bodied animals that eat by filtering food with mucus.

Sea squirts stay in one place on the sea floor their whole lives, eating tiny organisms in the water.

Salps float in the water, also eating tiny organisms. They have a special life cycle where one group lives alone, and the next group forms chains together.

Vertebrata (Craniata)

Main articles: Craniata and Vertebrata

All craniates have distinct skulls. This group includes hagfish, which do not have vertebrae. Most craniates have a backbone made of bone or cartilage that protects their spinal cord. Hagfish have incomplete skulls and no vertebrae, placing them in a special group. Recent studies suggest hagfish might actually be a type of vertebrate that lost their backbone.

Phylogeny

Overview

Scientists study the simplest chordates by looking at their DNA. This helps them learn how these animals are related. Some chordates may have lost certain features over time.

Researchers think chordates share a common ancestor. Evidence from proteins supports this. Early chordate fossils are from the Cambrian period, about 538.8 million years ago. These fossils include early fish and other chordates. Studying fossils and comparing DNA helps scientists make evolutionary family trees, though this can be hard.

Cladogram

Below is a family tree showing the likely evolutionary relationships between both extinct and living chordate groups.

Closest non-chordate relatives

The closest relatives of chordates are thought to be the hemichordates and Echinodermata. Together, they form the Ambulacraria. The Chordata and Ambulacraria make up the superphylum Deuterostomia.

Hemichordates

Main article: Hemichordate

Hemichordates share some features with chordates, such as openings in the pharynx that look like gill slits, and a dorsal nerve cord. There are two groups of hemichordates. The enteropneusts, called "acorn worms", have long, worm-like bodies and live in seafloor sediments. Pterobranchs are small, colonial animals that live in connected tubes.

Echinoderms

Main article: Echinoderm

Echinoderms look very different from chordates. As adults, they have a wheel-like body shape, supported by hard shells made of calcite. They also have special tube feet that help them move. Crinoids often look like flowers and stay in one place, while others like starfish and sea urchins can move around.