Phylum Chordata

Tree of Life

Phylum Chordata

From the first dawn of life, all organic beings are found to resemble each other in descending degrees, so they can be classed in groups under groups.

Charles Darwin, Origin of Species, Chapter 13

 Haikouella lanceolata, Early Primitive Chordate from ChengjiangPhylum Chordata comprises those animals most often occupying the top of the food chain, including the fishes, reptiles (and thus the extinct dinosaurs), reptiles, birds and mammals (including humans). The Chordates primary common feature is a notochord, a rod extending most of the length of the organism. Lying dorsal to the gut but ventral to the central nervous system, it stiffens the body and acts as support during locomotion. More specifically, the notochordat should be present at least during some part of the organisms development. Other common morphologies include bilateral symmetry, segmented body, a well-developed coelom, a single, dorsal, hollow nerve cord, usually with an enlarged anterior end (i.e., brain), a tail projecting beyond (posterior to) the anus at some stage of development, ventral heart, with dorsal and ventral blood vessels and a closed blood system, a complete digestive system, and usually, a bony or cartilaginous endoskeleton.

Note: Classification for vertebrates is in a state of change. As genomes are sequenced and the phylogeny of genes studied for homology across species, changes are sure to result, and in turn the implied evolutionary decendency in the table below.

This table ends at Tetrapoda, with its obvious importance in the tree-of-life and evolution. Please link to the next table when it is available.

Phylum Chordata
Subphylum Urochordata Lancelots
Subphylum Cephlachordata
Class Myxini Hagfish (often grouped with jawless fish)
Subphylum Vertebrata
Superclass Agnatha (jawless fish) Myxini (hagfish)
Petromyzontidae (lampreys)
Infraphylum Gnathostomata
(have jaws)
Class Placodermi (note 2) Armored fish
Class Chondrichthyes Cartilaginous fish
Class Acanthodii (note 3) Spiney sharks (extinct)
Osteichthyes (note 4)
Class Actinopterygii (note 5 - ray fished fishes) Bony Ray-finned fish
Class Sarcopterygii (lobe-finned fish) Order Actinistia Coelacanths
Subclass Dipnoi Lungfishes
Superclass Tetrapoda Four-legged Vertebrates
  1. Cephalaspidomorphi: Lamprey is a jawless fish with a toothed, funnel-like sucking mouth that most species use to bore into other fishes to suck blood. Lampreys have enormously different morphology and physiology than other fishes.
  2. Placodermi, Antiarcha, Bothriolepidae, Bothriolepis canadensisPlacodermi are armored fishes only known from fossils from the Devonian . Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked. Placoderms were the first of the jawed fishes, their jaws having evolved from the first of their gill arches.
  3. Acanthodii: The Acanthodians make up a class of extinct fishes, having features of both bony fish ( Osteichthyes) and cartilaginous fish (Chondrichthyes). They are sometimes called spiny sharks, appeared in the early Silurian and persisted into the late Permian. Importantly, they are the earliest known jawed vertebrates and they had stout, fixed spines supporting their fins like a shark's dorsal fin.
  4. Class Osteichthyes are the bony fish, a paraphyletic group with land vertebrates, and hence, four-legged vertebrates are shown in the table above. Most Osteichthyes belong to the Actinopterygii. The other seven living species are called lobe-finned fish, and include lungfish and coelacanths. Some of species of lobe-finned fish have jointed bones. They are traditionally treated as a class of vertebrates, with subclasses Actinopterygii and Sarcopterygii, but newer schemes may divide them into several separate classes. The vast majority of fishes are bony fish, and therefore belong to Osteichthyes. Osteichthyes are the most successful group of vertebrates based on the number of species.
  5. The Actinopterygii are the ray-finned fish. They are the largest group of vertebrates, with over 27,000 species across fresh water and marine environments.