Chondrichthyes is a monophyletic group of fishes that belongs to subphylum Vertebrata. Within the Vertebrata, chondrichthyans are thought to lie between the Placodermi (a group of extinct vertebrates whose body was covered with plates of dermal armour, and who were the first group of fishes with jaws) and the Acanthodii.
Chondrichthyan phylogeny (= the interrelationships of groups within chondrichthyes) is still a matter of much contention among systematic biologists and palaeontologists. Particularly problematic are the interrelationships of early (i.e. "stem") chondrichthyans. The phylogenetic tree below illustrates one evolutionary hypothesis, and includes both extinct and extant members (each of these groups will be discussed in detail on fossil forms and modern forms pages).
This is a group that comprises the earliest known chondrichthyans. They appear in the Silurian, and diversified through the Devonian. They are known only from fossil remains. These remains are often isolated scales and teeth (as complete cartilaginous skeletons are rarely fossilized). Some fully articulated fossil stem chondrichthyans do exist, however. One such specimen is Cladoselache.
- Order Cladoselachida
- Order Symmoriiformes
- Order Eugeneodontiformes
- Order Petalodontiformes
The Holocephali is the group that includes the bizarre looking, deep water-dwelling Chimaeras or ratfishes. This group appeared near the Silurian-Devonian boundary, with modern forms appearing in the Jurassic. There is one extant order in the Holocephali (Order Chimaeriformes).
Elasmobranchs, the group that
includes modern sharks, skates and rays and their relatives, are
divided into two subgroups: 1) the stem elasmobranchs
and 2) the Neoselachii.
Stem elasmobranches are the extinct relatives of modern sharks, skates and rays. They were shark-like in appearance, and existed from the Devonian to the Triassic.
- Order Xenacanthiformes
- Order Ctenacanthiformes
- Order Hybodontiformes
The neoselachii comprises all
living sharks, skates and rays. The first neoselachii appear in
the early Mesozoic,
and exhibit a number of modern "shark-like" characters.
The neoselachii are subdivided as follows:
- Order Heterodontiformes (bullhead sharks)
- Order Orectolobiformes (carpet sharks)
- Order Lamniformes (mackerel sharks)
- Order Carcharhiniformes (ground sharks)
- Order Hexanchiformes (six- and sevengill sharks)
- Order Echinorhiniformes (bramble sharks)
- Order Squaliformes (dogfishes)
- Order Squatiniformes (angelsharks)
- Order Pristiophoriformes (sawsharks)
- Batoidea (rays)
Author: Andrew Gillis
Last updated: 15 November 2004
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