Animal Phyla/Chordata
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- Chordata
- Name Meaning: Cord
- English Common Name: Chordates
- Major distinguishing characteristics: Hollow dorsal nervous chord
- Approximate number of species described: about 100,000+
Natural History
[edit | edit source]Chordates occur in most habitats on Earth.
Their natural history is as varied as they are diverse. They range from sedentary tunicates, to birds with near global migratory routes. They eat everything from organic particles suspended in water, to blue whales (the largest animal living). Most are ectothermic (relying on the environment to regulate their body temperature), but many are endothermic (with internal temperature regulation).
They are such a diverse group, it's not really possible to put together a simple expression of their natural history.
Taxonomy
[edit | edit source]Partly because of their ready availability to study, the chordates have a rather complicated taxonomy and is frequently revised. The list below is one version that should give an overview of the diversity within the Phylum.
Phylum Chordata
- Group Cephalochordata
- Family Pikaiidae (an extinct group considered to be among the earliest chordates, it isn't readily placed within the hierarchy)
- Class Leptocardii
- Order Amphioxiformes (lancets)
- Group Olfactores
- Subphylum Tunicata (tunicates, filter-feeding chordates)
- Class Ascidiacea
- Class Thaliacea
- Class Larvacea
- Subphylum Tunicata (tunicates, filter-feeding chordates)
- Clade Craniata
- Class Myxini (hagfishes)
- Class Hyperoartia (lampreys)
- Class Vertebrata (vertebrates, including humans)
Anatomy
[edit | edit source]Chordates have a notochord, a rod of cartilage inside of the body. They also have dorsal neural tube, which develops into a spinal cord in most subgroups. Additionally, chordates have pharyngeal slits along part of the throat. The slits may only be present in the embryonic animal, and develop to form gills in fish.
All chordates have an anus, and a post-anal tail, which is a body part that extends past the tail at the posterior end of the animal.
Though all have bilateral symmetry, the form of some chordates is not alike at first glance. In general form, the tunicates, for instance, appear somewhat blob-like, especially compared to many tetrapods (amphibians, reptiles, birds, and mammals).
The Fossil Record
[edit | edit source]Chordates have a long and diverse fossil history. The earliest known chordate fossil is 525 million years old, from the Cambrian from China.
Fossil chordates have been found on all continents. Many have bony skeletons and fossilize well.
Chordate fossils include representatives of very well known groups, including dinosaurs, fish, reptiles, birds, and mammals.