Chordata: Vertebrates and Their Relatives
In a Nutshell: What is Chordata?
The phylum Chordata is one of the most important groups in the animal kingdom, encompassing vertebrates (mammals, birds, reptiles, amphibians, and fish) as well as a few invertebrate groups such as tunicates and lancelets.
Chordates share several defining characteristics at some stage in their development, including a notochord (a flexible rod that supports the body), a dorsal nerve cord, pharyngeal slits, and a post-anal tail. These features have allowed chordates to evolve into some of the most complex and diverse organisms on Earth.
Chordates are found in virtually every ecosystem, from deep-sea trenches to dense rainforests and high mountain peaks. The group includes everything from tiny fish to massive whales, demonstrating an incredible range of adaptations.
Keep reading to learn more about Chordata!
Key Characteristics of Chordata
Chordates are defined by a set of anatomical features that distinguish them from other animal phyla.
Evolutionary Traits
- Ancient origins – Chordates first appeared over 500 million years ago during the Cambrian Explosion.
- Diverse adaptations – The phylum includes both invertebrates (tunicates and lancelets) and vertebrates.
- Complex nervous systems – The dorsal nerve cord evolved into the spinal cord and brain in vertebrates, supporting advanced behaviors.
Anatomical and Physiological Features
- Notochord – A flexible, rod-like structure providing support, which in vertebrates is replaced by the vertebral column.
- Dorsal hollow nerve cord – Develops into the central nervous system (brain and spinal cord) in vertebrates.
- Pharyngeal slits – Openings near the throat, which in aquatic species become gill structures and in land vertebrates contribute to throat and ear structures.
- Post-anal tail – Present in embryos of all chordates, but may be reduced in some adult forms.
Reproductive and Developmental Aspects
- Most chordates reproduce sexually, though some tunicates and fish exhibit asexual reproduction.
- The majority develop from eggs, but mammals and some fish give birth to live young.
- Vertebrate embryos show clear chordate features, even in groups where some traits disappear later in development.
Major Groups of Chordata
Chordata is divided into three major subphyla, each with distinct evolutionary traits.
Vertebrata (Vertebrates)
The most diverse and well-known chordates, vertebrates possess a bony or cartilaginous backbone. This subphylum includes:
- Mammals – Warm-blooded vertebrates with fur and milk production.
- Birds – Feathered, warm-blooded vertebrates capable of flight.
- Reptiles – Cold-blooded vertebrates, including lizards, snakes, and turtles.
- Amphibians – Animals with aquatic larvae and terrestrial adults, such as frogs and salamanders.
- Fish – The most ancient vertebrates, including cartilaginous and bony species.
Tunicata (Urochordata)
Also called tunicates, these marine animals include sea squirts and salps. They are sessile (attached to surfaces) as adults but have free-swimming larvae that display key chordate traits.
- Filter-feeders – Tunicates strain plankton from seawater using their pharyngeal slits.
- Protective outer covering – They are named for their tunic, a cellulose-like outer layer.
- Close relatives of vertebrates – Despite their simple and very different appearance, tunicates are more closely related to vertebrates than lancelets.
Cephalochordata (Lancelets)
Lancelets are small, fish-like invertebrates that retain all chordate features throughout life.
- Burrowing lifestyle – They live partially buried in sand, filtering food from water.
- Segmented muscles – Similar to those found in vertebrates, allowing for simple swimming movements.
- Evolutionary link – Lancelets provide key insights into the early evolution of vertebrates.
Chordata vs. Other Animal Phyla
Chordates are often compared to other major animal groups, such as arthropods and echinoderms.
| Feature | Chordata (Chordates) | Arthropoda (Arthropods) | Echinodermata (Echinoderms) |
|---|---|---|---|
| Body Plan | Bilateral symmetry | Bilateral symmetry | Radial symmetry (adults) |
| Skeleton | Internal (endoskeleton) | External (exoskeleton) | Internal (calcium-based) |
| Nervous System | Dorsal nerve cord | Ventral nerve cord | Nerve ring |
| Circulatory System | Closed system (vertebrates) | Open system (most) | Water vascular system |
| Key Examples | Mammals, fish, birds | Insects, spiders, crabs | Sea stars, sea urchins |
Key Takeway: Chordates are unique for their internal skeleton, dorsal nerve cord, and complex organ systems, distinguishing them from other major animal phyla.
Summary
- Chordata is the phylum that includes vertebrates and two small invertebrate groups, tunicates and lancelets.
- The defining chordate features include the notochord, dorsal nerve cord, pharyngeal slits, and post-anal tail.
- Chordates range from primitive, filter-feeding tunicates to intelligent mammals, with adaptations for nearly every environment.
- This phylum is one of the most successful and diverse groups in the animal kingdom, shaping the evolution of complex life forms on Earth.
Chordates dominate land, sea, and air, making them one of the most evolutionarily significant groups in the history of life.
Animals in the phylum Chordata
