Chordates

The phylum Chordata belongs to the animal (or deuterostome) kingdom. Collectively called chordates, the phylum consists of animals with a flexible rod (called a notochord) that supports their dorsal, or backsides. As you might expect, etymology can help describe the animal group. The word “chordata” derives from the Latin root chord, meaning string, and the Latin word “chordatus”, meaning “having a spine.”

Characteristics of Chordates

As with any animal group, taxonomists look for a set of defining characteristics to create parameters to help understand animal evolution. Since science, in general, constantly develops, these characteristics just distinguish chordates from, say, earthworms and sea jellies. Currently, taxonomists support four or five characteristics that delineate chordates from other phyla: the notochord, the nerve tube, the postanal tail, the pharyngeal slits, and the endostyle. The first three are relevant to the movement, support, and connectivity of the body structure, while the latter two are important for how chordates eat and access nutrients. While all chordates have these characteristics, they might not be consistent structures throughout the organism’s life cycle. Many only share them as embryos, only to have the organs develop into something more specified or complex as the animal develops.

Notochord

The namesake of the phylum, the notochord is perhaps the most significant structure of chordates because it gives the animal internal structural support. Consisting of a rod of vacuolated cells and cartilaginous tissue, the notochord lies along the dorsal side of the animal. A cross-section of the notochord reveals concentric rings, which provide strength. Additionally, the cells of the notochord have a large vacuole, which, when pressurized, gives rigidity to the structure. Extending along the length of the animal, the muscles and connective tissue branch off from the notochord, allowing the animal to have more powerful movement.

For the subphylum Vertebrata, the notochord precurses the backbone or spine. Only present in the embryo stage of fish, amphibians, reptiles, birds, and mammals, the notochord is replaced by a bony segmented vertebral column later in the organism’s development. Not all chordates develop a bony spine. In fact, the two other subphyla, Cepholochordates and Urochordates, are examples of invertebrate chordates. For these animals, the notochord is present throughout their life.

Nerve Cord

Going hand in hand with the notochord is the second defining characteristic of chordates. The nerve cord is a hollow neural tube that lies on top of (dorsal to) the notochord. The nerve tube branches to other areas of the body, developing into the central nervous system. Most notably, the nerve tube develops on the dorsal side of the body, as opposed to ventral or lateral placement as found in other animal phyla. As the nerve tube grows, pairs of nerves split off to connect with muscles and other organs. For vertebrates, the nerve tube will develop into the spinal cord and brain. Additionally, the developing spine will encase the nerve cord, giving it extra protection. For invertebrate chordates, their dorsal hollow nerve cords lie directly between the notochord and the skin, leaving this precious organ more vulnerable.

Postanal Tail

For many chordates, the notochord and nerve cord extends past the anus, becoming a third defining characteristic of chordates: the postanal tail. While all chordates have this trait, it again isn’t consistent throughout an organism’s lifecycle. Take frogs and humans as the first examples.  In humans, our embryonic tail is reduced to a small nub of bone (the tailbone) that does not protrude from the body. Frogs have tails as tadpoles but lose them slowly as they metamorphose into adults. But for so many other creatures, the tail developed to be an incredibly beneficial adaptation. From monkeys swinging from trees to snow leopards balancing on rocky cliffs to salmon powerfully swimming upstream, the tail allows chordates to access forms of movement unique to the phylum.

Endostyle

Also called the hypopharyngeal groove, the endostyle is a strip of mucus-producing tissue on the pharynx. The groove is ciliated, meaning it’s covered in tiny hairs, called cilia, that move water more thoroughly across the surface of a cell. Between a mucus coating and the cilia, the endostyle aids organisms in gathering food and transporting it to the esophagus. For the fully aquatic urochordates and cephalochordates, the endostyle is present throughout their lifecycles. In addition to helping transport food, the endostyle produces hormones similar to those from vertebrate thyroid glands. Therefore, many consider the endostyle and the thyroid to be homologous structures. This theory is supported by the lifecycle of lampreys. In their larval form, lampreys have a clear endostyle that metamorphoses into the adult’s thyroid gland.

Pharyngeal Slits

Also called the pharyngeal cleft or pharyngeal gills in some animals, these openings are found in the pharynx region in the back of the mouth. For aquatic animals, these gill slits allow for water to exit the mouth that was taken in during feeding, helping to filter food particles. Similar to other chordate characteristics, pharyngeal slits can be found clearly in the embryonic stage of all the animals in the Chordata phylum. But vertebrate evolution has modified pharyngeal slits for a wide range of purposes. They develop into gill supports for vertebrate fish and jaw supports for jawed fish. For terrestrial vertebrates, the slits become specialized ear components, tonsils, and jaw supports.

Evolution & Phylogeny of Chordates

To better understand how chordates evolved, we can take a closer look at the fossil record. However, before diving into the milestones of chordate discoveries, it’s important to note that due to the nature of two subphyla not having the endoskeletons of vertebrates, they can’t be as easily preserved. While the fossil record can be informative, it can’t give us the full picture, especially for cephalochordates and urochordates which are hard to distinguish from other soft-bodied animals.

In fact, some of the first chordate fossils discovered were initially thought to be worms! The first was discovered in the early 20th century, only to be reexamined in the 1970s. These “worms” turned out to be Pikaia gracilens, an extinct animal from the mid-Cambrian, possibly a member of the cephalochordates. More significantly however was the 1995 find of the Yunnanozoon lividium fossil in China. This cephalochordate was much older than the Pikaia spp., supporting theories that chordates first evolved with the Cambrian Explosion over 530 million years ago. Other fossils from around this time presented more of an enigma. Animals appeared to have features from both Chordata and Echinodermata. Called hemichordates, these animals are the sister phylum to echinoderms and the likely ancestor to modern chordates.

The earliest fossil evidence for vertebrates also dates back over 500 million years, when jawless fish first appeared. From there, chordate evolution tracks more clearly.  Jawed fish evolved during the Silurian between 445-415 million years ago. Soon after, the first tetrapod fossil, evidence of early amphibians was discovered from roughly 360 million years ago. Reptiles appeared on the scene during the Carboniferous period, with mammals following during the Triassic, around 250 million years ago. Finally, the discovery of Archaeopteryx from 150 million years ago indicated the evolution of birds and feathered flight.

Chordate Subphylum

Chordates share morphological characteristics that distinguish them from other members of the animal kingdom. This group includes many animals that are very familiar, including humans and other critters with a backbone. Chordates are further divided into three subphyla: Cephalochordata, Tunicata, and Vertebrata. What follows is an overview of each of these three subphyla.

Cephalochordates

Upon first glance, the Cephalochordata subphylum doesn’t draw much attention. With only about 30 species that spend most of their time buried in sand, they don’t have either the impressive number of insects or the allure of large predators. However, they encompass the earliest chordate species to appear. So what they lack in quantity or charisma, they make up for by helping elucidate the morphology and evolution of all chordates, including humans.

Also called lancelets or amphioxus, these spear-shaped filter-feeders that live in shallow marine habitats, often partially buried in the sediment. Adults are usually less than three inches long, and they spend most of their lives filtering food particles from the water into a rudimentary digestive system. They fertilize eggs externally, and the young hatch into a free-swimming larval stage.

Fun Fact: The name ‘amphioxus’ is derived from the Latin roots: amphi- meaning ‘on both sides’ and -oxus meaning ‘sharp.’ The name refers to the shape being sharp on both sides!

Urochordates

The subphylum Urochordata (also called Tunicates) goes by the more common name of sea squirts. With around 3,000 species and more diverse than the lancelets, sea squirts lead a marine life. Interestingly, the larval form of the tunicate is more complex than the adult form. The larvae are free-swimming with a notochord, a dorsal nerve cord, pharyngeal slits, and a postanal tail. However, once it attaches to a hard surface, it loses its tail and most of its nervous system. As an adult, the body plan is relatively simple. Tunicates have sac-like bodies with two siphons where water enters and exits. Most tunicate species lead a sessile existence, and they can live in both in groups or solitary. Some species, known as salps, float freely as plankton with jelly-like bodies in large colonies.

Vertebrates

Vertebrates, also called craniates, are the largest group of chordates, with more than 62,000 living species giving way to impressive diversity. This group includes some of the most familiar and most studied animal species, namely, of course, human beings. But we didn’t spontaneously appear from a sea squirt. The evolution of vertebrates roughly follows the organization of the seven vertebrate classes, all of which share a few common characteristics including a skull and brain, the extensive endoskeleton, and a closed circulatory system.

Jawless Fish

The class Agnatha, which includes hagfishes and lampreys, represents the first true vertebrate clade. They have gills and a two-chambered heart like most fish, though they lack paired appendages or fins. Therefore, they have a tubular body structure and a suction-like mouth.

Jawed Fish

The radiation of jawed fish (gnathostomes) gave way to the two other fish classes and a world of diversity. The development of the jaw is one of the most significant advances in Craniata evolution. The jaw allows an organism to grasp and tear its food, opening up food chains and diversifying diets. Early jawed fish also developed two sets of paired fins, allowing them to make more acute and powerful locomotion. Essentially, the evolution of the jaw gave way to the evolution of predators.

Defined by their cartilaginous skeleton, class Chondrichthyes is diverse, consisting of sharks, rays, and skates, together with sawfishes and a few dozen species of fishes called chimaeras (ghost sharks). Most cartilaginous fish live in marine habitats, with a few species living in freshwater for a part or all of their lives. Most are also carnivores that hunt down live prey, but some are suspension feeders that feed on plankton.

The majority of present-day fishes belong (about 30,000 species) to the class Osteichthyes, making it the largest group of vertebrates worldwide. Aside from their ossified skeletons, bony fish also have skin covered in overlapping scales and mucus glands that reduce drag in the water.

Chordates Move Out of the Water: Amphibians

Frogs, toads, and salamanders are animals that represent the class Amphibia. Because these animals divide their lifecycles between water and land, they can give us a clue to how chordates evolved to become terrestrial. Notably, amphibians have legs (after metamorphosis) and lungs, along with a three-chambered heart. While some species need aquatic environments periodically to keep moist skin, they all have water-bound reproduction, otherwise, their eggs would dry out. Therefore, the larva start life with gills and a body structure similar to a fish.

Land & Sky: The Amniotes

Once animals started to travel on land, the game-changing development came in the form of the amniotic egg. A fluid-filled membrane (amniotic sac) surrounds the egg to that supports embryonic development. This evolution meant eggs wouldn’t dry out and therefore weren’t completely reliant on aquatic environments. Now the land and sky could be explored! Reptiles, birds, and mammals all fall under the amniote category, though they each occupy a distinct class.

Reptiles

The class Reptilia were the first amniotes to evolve. All reptiles have lungs to breathe and skin that doesn’t need to be moist. They have keratin scales, and some (crocodiles and likely dinosaurs) have a four-chambered heart. In addition to the amniotic sac, their eggs usually have a leathery or hard shell that helps keep the embryo from drying out.

Birds

The class Aves includes all the birds.  They also have amniote eggs with a calcium carbonate shell. Instead of scales, birds developed feathers of keratin, along with a muscular-skeletal system allowing them to fly. Birds also have air sacs integrated into their respiratory system to aid with flight. Additionally, birds are endothermic (warm-blooded), and have a four-chambered heart, allowing for more efficient circulation.

Mammals

Dogs, cats, mice, humans, and most other large animals are members of the vertebrate class Mammalia. Apart from having hair or fur, all mammals conceive their young within the reproductive tract of the mother and nourish them with milk produced by their mammary glands. Significantly, mammals have differentiation in their teeth – incisors, canines, premolars, and molars.  This allows them to chew their food into small pieces before swallowing it. An appealing attribute, especially if you’ve ever benefited from the idiom “How to eat an elephant… bite by bite.”

Despite our familiarity with members of the Chordata phylum, they encompass less than 4% of all animal species. But even in that small number, we find incredible diversity and adaptations. Chordates have developed some incredible morphologies that allow for unique body movements, nervous systems, and digestive systems that have allowed them to be some of the largest and most impactful animals on Earth.

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