A chordate is a type of animal in the phylum Chordata. All chordates have five special traits at some point in their life, whether as a baby or adult. These traits include a notochord, a hollow nerve cord along the back, an endostyle or thyroid, openings in the throat called pharyngeal slits, and a tail that extends beyond the anus.

Scientists have also found two unique features in the proteins of chordates: cyclophilin-like proteins and a type of mitochondrial enzyme called ATP23. These features are only found in chordates, including vertebrates, tunicates, and cephalochordates. These traits help scientists identify chordates accurately.

Chordates are grouped into three main categories: Vertebrata, which includes animals with backbones like fish, amphibians, reptiles, birds, and mammals; Tunicata, which includes sea squirts and similar creatures that only show the special traits as larvae; and Cephalochordata, which includes lancelets that look like jawless fish but lack gills or a clear head. Vertebrates and tunicates are part of a group called Olfactores, which is closely related to Cephalochordata. Some extinct chordates, like conodonts, are known, but their exact placement is unclear. Hemichordata, which includes acorn worms, was once thought to be a chordate group but is now classified separately. Hemichordata and echinoderms (like starfish) are grouped together as Ambulacraria, which is closely related to chordates. Chordates, Ambulacraria, and possibly another group called Xenacoelomorpha are thought to belong to a larger group called Deuterostomia, though some recent research questions this classification.

Chordata is the third-largest animal group, after arthropods and mollusks. Fossils of chordates date back to the Cambrian period, over 539 million years ago. Among the more than 81,000 living chordate species, about half are ray-finned fish, and most of the rest are tetrapods, which are land-dwelling animals that evolved lungs to breathe air.

Etymology

The name "chordate" comes from the notochord, a shared trait among chordates. The notochord helps shape the body and supports movement in chordates. Chordates have a body that is symmetrical on both sides, a coelom (a body cavity), a closed circulatory system (blood flows through closed tubes), and repeating body segments. The term "Chordata" was named by William Bateson in 1885, but people were already using it by 1880. In 1866, Ernst Haeckel grouped tunicates, cephalochordates, and vertebrates into a taxon. He used the German term for the name, which is allowed by the ICZN code because it was later changed to Latin.

Anatomy

Chordates are a phylum of animals that must have certain body parts at some point in their life. There are certain characteristics that help distinguish chordates from other animal groups, but these characteristics are not part of the official definition.

Classification

The diagram comes from the 2015 edition of Vertebrate Palaeontology. The groups of invertebrate chordates are from Fishes of the World. It is usually organized to show how species are related through evolution, similar to a cladogram. However, it also keeps the traditional classification system from Linnaean taxonomy.

Subphyla

Cephalochordates are one of three groups of chordates. They are small, fish-like animals that do not have brains, clear heads, or special organs for sensing the environment. These animals live in burrows and filter food from water. They are the oldest group of chordates to branch off from the main line.

Tunicates have three different adult forms. Each form belongs to a group that shares a common ancestor. All tunicate larvae have features typical of chordates, such as long, tail-like bodies. Their larvae also have simple brains, light-sensing structures, and balance-sensing structures.

The smallest group of tunicates is called Appendicularia. These animals keep their tail-like shape and swim actively throughout their lives. For many years, they were thought to be the larval stage of other tunicate groups.

The other two tunicate groups are sea squirts and salps. Both change into adult forms that lose the notochord, nerve cord, and tail. They are soft-bodied animals with many gill slits that filter plankton from water using mucus.

Sea squirts stay in one place for their entire lives. They attach to the ocean floor and use structures to pump water and filter food.

Salps float in the ocean and also filter plankton. They have a life cycle with two generations: one that lives alone and another that forms groups like chains.

The name "Urochordata" comes from Greek and Latin words meaning "tail" and "cord," because the notochord is only found in the tail. The name "Tunicata" is now more commonly used and was first introduced by Lamarck in 1816.

Craniates are animals with distinct skulls. This group includes hagfish, which do not have backbones. Scientists note that craniates are defined by their skulls, similar to how chordates are defined by their tails.

In most craniates, the notochord is replaced by a backbone made of bony or cartilaginous vertebrae. These vertebrae protect the spinal cord and connect to each other. Hagfish lack vertebrae and have incomplete skulls, leading to debate about whether they are vertebrates. Some scientists argue they may have lost their backbones over time.

Studies using DNA analysis suggest that hagfish and lampreys (which have backbones) are closely related in a group called Cyclostomata. This supports the idea that hagfish are vertebrates that lost their backbones. Fossil evidence also supports this view. As a result, the groups Craniata and Vertebrata are now considered the same for living animals.

Phylogeny

Scientists are still studying chordates using DNA comparisons to understand their simplest forms. Some species without backbones or notochords may have lost these structures over time, making it harder to classify them. DNA analysis is sometimes needed to find chordate lineages when no physical evidence of chordate-like features remains.

Research into how chordates evolved has led to several theories. Today, most scientists agree that chordates are monophyletic, meaning all chordates share a single common ancestor. Tunicates are considered the closest relatives of vertebrates. In 2016, scientists found two unique features in proteins found only in vertebrates, tunicates, and cephalochordates. These features strongly support the idea that all chordates share a common ancestor.

The oldest known chordate fossils were found in the Early Cambrian Chengjiang fauna and include species classified as fish, which are vertebrates. However, the fossil record for early chordates is incomplete, so scientists use molecular phylogenetics to estimate their origins. This method is controversial because it is difficult to determine exact dates for evolutionary changes. Traditional classifications of living chordates are also unclear, as many groups are paraphyletic, meaning they do not include all descendants of a common ancestor.

Although scientists have known since the 19th century that vertebrates are not monophyletic, modern classifications are still changing. Most complex animals, except for jellyfish and other cnidarians, are divided into protostomes and deuterostomes, with chordates belonging to the latter group. Fossils from the Ediacaran period, such as Kimberella, suggest that protostomes and deuterostomes split at least 558 million years ago, before the Cambrian period began. Three Ediacaran fossils—Aisia fenestrata, Yarnemia ascidiformis, and Burykhia hunti—may be early tunicates, but their classification is debated because their fossils are poorly preserved.

Fossils of echinoderms, like starfish and sea urchins, are common from the start of the Cambrian period. Other Cambrian fossils, such as Rhabdotubus johanssoni, Yunnanozoon, and Haikouella lanceolata, show features of early chordates or related groups. Pikaia, found in the Burgess Shale, is also considered a primitive chordate. However, early chordate fossils are rare because they lack bones or teeth. The best-known early tunicate is Shankouclava shankouense, found in the Lower Cambrian Maotianshan Shale.

Scientists have debated the relationships between chordates and their deuterostome relatives since 1890. Studies using anatomy, embryology, and fossils have produced conflicting "family trees." Some older ideas, like linking chordates and hemichordates, are now rejected. Recent research combining genetic data with fossil evidence suggests tunicates may be "basal deuterostomes," a group from which other deuterostomes evolved. Some scientists think craniates are closely related to cephalochordates, while others believe tunicates are their closest relatives.

Because early chordates leave few fossils, scientists use molecular phylogenetics to estimate key evolutionary dates. One study suggests deuterostomes appeared over 900 million years ago and chordates around 896 million years ago. However, these estimates often conflict with each other and with fossil evidence, as they rely on the assumption that molecular changes occur at a constant rate.

Traditionally, Cephalochordata and Craniata were grouped into "Euchordata," considered the sister group to Tunicata. More recently, Cephalochordata has been linked to "Olfactores," which includes craniates and tunicates. This relationship is still debated.

Two unique features in proteins found only in vertebrates and tunicates support the idea that vertebrates are more closely related to tunicates than to cephalochordates.

A phylogenetic tree of the chordate phylum shows evolutionary relationships between extinct (marked with a dagger) and living species. Key groups include:

• Cephalochordata (lancelets)
• Appendicularia (larvaceans)
• Hyperoartia (lampreys)
• "Placodermi" (paraphyletic)
• "Acanthodii" (paraphyletic)
• Selachimorpha (sharks)
• Actinistia (coelacanths)

Closest non-chordate relatives

The closest relatives of chordates are hemichordates and Echinodermata, which together form the group called Ambulacraria. Chordates and Ambulacraria are part of a larger group called Deuterostomia.

Hemichordates, sometimes called "half-chordates," share some traits with chordates. They have branchial openings that lead into the pharynx and resemble gill slits. They also have stomochords, which are similar to notochords but form a circle around a "collar" near the mouth. Additionally, they have a dorsal nerve cord and a smaller ventral nerve cord.

There are two living groups of hemichordates. Enteropneusts, also known as "acorn worms," are solitary animals with long proboscises and worm-like bodies. They have up to 200 branchial slits and can grow up to 2.5 meters (8.2 feet) long. They dig through seafloor sediments. Pterobranchs are colonial animals, usually less than 1 millimeter (0.039 inch) long. They live in connected homes and use branched tentacles to filter food. Each has a short, shield-shaped proboscis. Graptolites, now extinct, were colonial animals with fossil shapes like tiny hacksaw blades. They lived in tubes similar to those of pterobranchs.

Echinoderms differ from chordates and their relatives in three key ways. As larvae, they have bilateral symmetry, but as adults, they have radial symmetry, meaning their bodies look like wheels. They also have tube feet and are supported by dermal skeletons made of calcite, a material not used by chordates. Their hard, calcified shells protect their bodies, which are covered by thin skins. The tube feet are powered by a unique system of water-filled canals called the water vascular system, which also acts as a "lung" and is surrounded by muscles that function as pumps. Crinoids are usually stationary and resemble flowers, with feather-like arms that filter food from water. Most live attached to rocks, though some can move slowly. Other echinoderms, like starfish, brittle stars, sea urchins, and sea cucumbers, are mobile and have varied body shapes.