Chordates: The Evolution and Diversity of Animals with Notochords
The phylum Chordata encompasses a vast array of animals, ranging from simple, translucent sea creatures to the most complex mammals on Earth. Defined by a unique set of anatomical features, chordates represent a significant branch of the animal kingdom, specifically within the deuterostomes—a group of animals where the first opening in the embryo becomes the anus rather than the mouth.
The term "chordate" is derived from the notochord, a flexible, rod-like structure that provides primary support for the body. While many adult chordates replace this structure with a bony spine, the notochord remains a fundamental blueprint for the group's body plan, enabling efficient movement and structural integrity.
Key Facts
- Temporal Range: Chordates have existed from the Cambrian period to the present (approximately 525 million years ago to 0 Ma).
- Defining Feature: All chordates possess a notochord at some stage of their development.
- Symmetry: They exhibit bilateral symmetry, meaning their left and right sides are mirror images.
- Biological Systems: They possess a coelom (body cavity), a closed circulatory system, and metameric segmentation.
- Diversity: The phylum includes over 66,100 species of vertebrates alone.
Anatomy and Biological Characteristics
Chordates are characterized by several synapomorphies (shared derived traits) that distinguish them from other phyla. These include an encephallic bulge (the beginning of a brain), a mouth opening, light sensors, and a post-anal tail.
In most familiar chordates, the notochord is replaced during embryonic development by a vertebral column. However, some species maintain a visible backbone without the complexity of a full skeletal system.
Classification of Chordata
The phylum is divided into three primary subphyla based on the complexity of their nervous systems and skeletal structures.
Cephalochordata (Lancelets)
Lancelets are small, fish-like marine animals that maintain a notochord throughout their entire lives. With only about 32 known species, they provide a glimpse into the ancestral chordate body plan.
Tunicata (Urochordata)
Tunicates, including sea squirts and salps, are unique because their chordate characteristics—such as the notochord and nerve cord—are often only present in their larval stage. There are approximately 3,000 species in this group.
Vertebrata (Craniata)
Vertebrates are the most diverse group, comprising over 66,100 species. They are defined by the presence of a cranium (skull) and usually a vertebral column. This group is further divided into:
- Agnatha: Jawless vertebrates, such as hagfish and lampreys.
- Gnathostomata: Jawed vertebrates, which include cartilaginous fish (sharks), bony fish, and tetrapods.
- Tetrapoda: Four-limbed vertebrates, encompassing amphibians, reptiles, birds, and mammals.
Evolutionary History and Phylogeny
The evolutionary lineage of chordates is ancient. Evidence suggests that the split between protostomes and deuterostomes occurred at least 558 million years ago. Some of the earliest evidence of chordate-like organisms comes from the Ediacaran period, with fossils such as Ausia fenestrata and Burykhia hunti appearing between 555 and 548 million years ago.
One of the earliest known fish, Haikouichthys ercaicunensis, dates back roughly 518 million years to the Cambrian period in China.
![Haikouichthys ercaicunensis, from around 518 million years ago in China, may be the earliest known fish[42]](/images/ae/5d/ae5d06aff8ca7754e900fc695967a732054f7ac5bc430f0e9ddda14711c984ce.png)
Over millions of years, chordates evolved from simple filter-feeders to the most extreme biological forms, from the fastest animals on land to the largest creatures to ever exist.
Related Non-Chordate Groups
While not chordates themselves, certain phyla are closely related. Hemichordates (such as acorn worms) and Echinoderms (such as sea stars) are the closest non-chordate relatives, sharing the deuterostome developmental path.
Chordate Summary Table
| Subphylum | Common Examples | Approx. Species Count | Key Characteristic |
|---|---|---|---|
| Cephalochordata | Lancelets | 32 | Notochord persists throughout life |
| Tunicata | Sea Squirts, Salps | 3,000 | Chordate traits primarily in larval stage |
| Vertebrata | Fish, Mammals, Birds | 66,100+ | Possess a cranium and usually a backbone |
Frequently Asked Questions
What is a notochord?
A notochord is a flexible, rod-shaped structure found in all chordates at some point in their development. It provides structural support and serves as the basis for the development of the vertebral column in vertebrates.
Are all animals with backbones chordates?
Yes, all vertebrates are chordates. However, not all chordates are vertebrates; for example, lancelets and tunicates are chordates that do not possess a bony backbone.
How do chordates differ from protostomes?
Chordates are deuterostomes. In deuterostome development, the first opening of the embryo (the blastopore) becomes the anus, whereas in protostomes, that first opening becomes the mouth.
Which is the oldest known chordate?
Fossils such as Ausia fenestrata and Burykhia hunti from the Ediacaran period (roughly 555 to 548 million years ago) are believed to be some of the oldest evidence of the chordate lineage.
What are the closest relatives to chordates?
The closest non-chordate relatives are the hemichordates (like acorn worms) and echinoderms (like sea stars), as they all belong to the superphylum Deuterostomia.