The phylum Chordata represents a major division of the animal kingdom, containing a vast and diverse array of species that share a specific set of physical characteristics. This group includes familiar animals like fish, amphibians, reptiles, birds, and mammals, as well as less-known marine invertebrates like sea squirts. The phylum encompasses tremendous biological variety, from tiny sessile organisms to the blue whale. Understanding Chordata is central to comprehending the evolution of complex animal life, as it traces the lineage that ultimately gave rise to the vertebrates.
The Four Unifying Features
The definition of a chordate rests on four distinct anatomical features that must appear at some stage of the animal’s life cycle, even if only briefly during embryonic development. The notochord is a flexible, rod-shaped structure composed of large, fluid-filled cells encased in a fibrous sheath. This structure provides skeletal support along the animal’s length, giving the phylum its name and acting as a precursor to the backbone in many species.
Running along the dorsal side, superior to the notochord, is the dorsal hollow nerve cord. This cord originates from the ectoderm, the outermost layer of the embryo, and rolls into a hollow tube during development. In contrast to the solid, ventral nerve cords found in most other animal phyla, this structure develops into the brain and spinal cord, forming the central nervous system.
The pharyngeal slits are a series of openings in the pharynx, the region immediately posterior to the mouth, that extend to the outside environment. In aquatic chordates, these slits function in filter-feeding by allowing water to exit while trapping food particles, or they are modified into gill supports for respiration. In terrestrial species, these structures are transiently present during the embryonic stage and later develop into components of the inner ear, tonsils, and other throat structures.
The post-anal tail is a posterior elongation of the body that extends past the anus. This tail contains both skeletal elements and muscle tissue, serving as a primary source of locomotion for aquatic species, such as fish and larval forms. While often reduced or vestigial in many adult terrestrial species, like the coccyx in humans, its presence during embryonic development confirms the animal’s classification as a chordate.
The Non-Vertebrate Subphyla
The phylum Chordata includes two subphyla of invertebrates: Urochordata and Cephalochordata. Urochordata, commonly known as tunicates or sea squirts, are primarily sessile marine filter feeders as adults. The adult form is a sack-like organism covered by a tough, cellulose-like tunic, which often obscures their chordate identity.
It is the tunicate larva, a free-swimming, tadpole-like form, that clearly exhibits all four chordate features, including a notochord, dorsal hollow nerve cord, and post-anal tail. During metamorphosis, the larva settles, and its notochord, nerve cord, and tail are reabsorbed, leaving a simplified adult body focused on filter-feeding through numerous pharyngeal slits. Cephalochordata, or lancelets (like Branchiostoma), are small, fish-like marine animals that retain all four chordate characteristics throughout their entire life cycle.
In lancelets, the notochord extends the full length of the body, even into the head, providing continuous axial support. They are filter feeders that use their numerous pharyngeal slits to strain plankton from the water.
Vertebrates The Dominant Group
The subphylum Vertebrata contains the most familiar and ecologically diverse members of Chordata, all of which are distinguished by the presence of a vertebral column, or backbone. This structure is a series of segmented bony or cartilaginous elements called vertebrae, which surround and largely replace the embryonic notochord. The vertebral column provides a strong yet flexible axis for the body, allowing for sophisticated movement and larger body size.
Vertebrates possess a well-developed internal skeleton, or endoskeleton, composed of bone and cartilage, which includes a cranium, or skull, that encases and protects the brain. The dorsal hollow nerve cord is elaborated in vertebrates, with the anterior end expanding into a complex, multi-region brain. This brain, coupled with advanced sensory organs for sight, hearing, and smell, enabled the sophisticated behaviors and cognitive abilities that characterize the group.
The pharyngeal slits, while present in the embryo, undergo modifications in adult vertebrates. In aquatic forms like fish, they are supported by skeletal arches and develop into gills used for extracting oxygen from the water. In terrestrial vertebrates, these structures are reorganized during development to form parts of the jaw, the inner ear bones, and glandular tissue like the tonsils and thymus.
The post-anal tail persists in many adult vertebrates, such as fish and reptiles, where it aids in propulsion or balance, but it is often greatly reduced in mammals and birds.
The Evolutionary History of Chordates
The ancestry of the phylum Chordata is rooted in a lineage of organisms called deuterostomes, which also includes the phyla Echinodermata (sea stars and sea urchins) and Hemichordata (acorn worms). The earliest chordate fossils date back to the Cambrian period, approximately 508 million years ago, with the discovery of Pikaia gracilens in the Burgess Shale. This small, eel-like organism possessed a notochord and segmented muscle blocks, establishing it as one of the first known primitive chordates.
The evolution of the chordate body plan involved the integration of the dorsal nerve cord and the segmented axial musculature, which allowed for improved swimming ability. Following these soft-bodied forms, the transition to the first true vertebrates involved the development of hard tissues, specifically bone and cartilage. The earliest vertebrates, which appeared around 400 million years ago, were jawless fish that began to replace the flexible notochord with a vertebral column.
The acquisition of a mineralized skeleton, initially seen as bone armor in the skin of early fish, provided structural advantages and protection. This development of internal bony or cartilaginous elements, including the skull and the vertebral column, facilitated the diversification of vertebrates into the many forms that populate aquatic and terrestrial environments today. This shift from a simple notochord to a complex, articulated skeleton was a key evolutionary step.