Sharks, with lateral gill openings, anterior edge of the pectoral fin not attached to the side of the head, and pectoral girdle halves not joined dorsal-ly, are regarded here as forming a separate taxon from the rays (including skates), which have ventral gill openings, anterior edge of the enlarged pectoral fin attached to the side of the head (forming the disc in most species), and pectoral girdle halves joined dorsally.
There are currently two conflicting hypotheses regarding the phylogenetic relationships of euselachians, one based on morphological evidence and one based on molecular evidence (interestingly, older non-cladistic morphological studies showed the same conflict, some placing rays within sharks and others not). This conflict is common in fish systematics. In the present case, following Compagno's (1973, 1977) work, there was growing acceptance that while sharks and rays form a monophyletic group, sharks were a paraphyletic group without the inclusion of rays. Compagno (2001), Shirai (1992a, 1996), and de Carvalho (1996) agreed that rays (batoids or rajiforms) and pristio-phoriforms are sister taxa and that both belong in the squalomorph clade.
A comparison of the revised higher classification of the elasmobranch division Neoselachii herein of de Carvalho (1996) (his infraclass Neoselachii) and of Shirai (1996) is given on the next page (for Shirai's Squalea, unless otherwise stated, the orders have one family). The higher classification by Compagno (2001) for his cohort Neoselachii is based on a consensus of his earlier works and of de Carvalho (1996) and Shirai (1996) in giving superorder Squalomorphi with the orders Hexanchiformes (including the Chlamydoselachiformes), Squaliformes, Squatiniformes, Pristiophoriformes, and Rajiformes and the superorder Galeomorphi (as recognized herein) (note that Compagno's sequencing of the two extant euselachian superorders is reversed from the others). The equivalency of some taxa recognized herein is given in parentheses.
However, while the above studies of de Carvalho (1996) and Shirai (1996) present sound morphological studies and analyses, it may be premature to follow their cladistic implications. The cytogenetic data reviewed by Schwartz and Maddock (2002) and the molecular studies of Arnason et al. (2001) and Douady et al. (2003) presented preliminary evidence supporting the mono-phyly of sharks (without rays), and strong evidence for the same conclusion de Carvalho (1996)
Division Galeomorphii (= superorder Galeomorphi)
Division Squalea (= superorder Squalomorphi + subdivision Batoidea) Superorder Notidanoidea Order Hexanchiformes Superorder Echinorhinoidea Order Echinorhiniformes Superorder Squaloidea Order Squaliformes Superorder Hypnosqualea Order Squatiniformes Order Pristiophoriformes Order Rajiformes
Superorder Galea (= superorder Galeomorphi)
Superorder Squalea (= superorder Squalomorphi + subdivision Batoidea) Order Chlamydoselachiformes Order Hexanchiformes (two families) Order Echinorhiniformes Order Dalatiiformes (four families) Order Centrophoriformes Order Squaliformes
Order Squatiniformes Order Pristiophoriformes Order Rajiformes (four suborders and 12 families)
was given by Maisey et al. (2004) and Naylor et al. (2005). This conclusion was accepted in McEachran and Aschliman (2004), Musick et al. (2004), and Musick and Ellis (2005). With both modern sharks and rays going back to at least the Early Jurassic, I accept that the paleontological evidence can be best interpreted to support the hypothesis of Maisey et al. (2004) and that proposal is accepted here. Maisey et al. (2004) discussed the conflict between the molecular- and morphological-based trees, and they regarded the strata-graphic data as highly congruent with the molecular data; in addition, they regard the batoids as basal to the modern sharks. There are thus two current hypotheses expressing the relationships of sharks and rays:
i) the hypnosqualean hypothesis—the batoids (rays) are sister to the Pristiophoriformes and that clade (coined the Pristiorajea by de Carvalho, 1996) is sister to the squatiniformes (the resulting clade being the Hypnosqualea), and all share a common ancestry with the Squaliformes.
ii) the selachian/batoid (shark/ray) hypothesis—the sharks, as conventionally defined, are monophyletic without the inclusion of the rays.
The latter hypothesis is accepted here. Although I normally feel it best to accept the implications of morphological data for classifications when there is a conflict with molecular evidence, in this case with such apparently strong conflicting evidence, I prefer to recognize sharks and rays in separate taxa (as done, for example, in Berg, 1940, and Nelson, 1976, 1984), pending further work. However, the cladistic results of de Carvalho (1996) and Shirai (1996) may yet warrant changing our classification. If the molecular evidence is correct, then the morphological features used to argue for a monophyletic Hypnosqualea would be the result of convergence (i.e., there would be much homoplasy in the morphological data).
Regardless of which system is employed and whether a different taxonomic rearrangement is used, for common names, the terms shark or selachian (non-batoids) and rays or batoids still seem appropriate to use, as opposed to using the term sharks to include rays as given in Compagno (2001). This follows the principle of keeping common names as stable as possible; thus scientific names are intended to apply to monophyletic taxa while common names are intended only to refer to classical recognized groups, monophylet-ic or not. The rankings assigned to various taxa and the terms applied, unfortunately and especially so for the non-taxonomist, vary in the literature, more so with the euselachians than with most fish groups. In addition, the content of some taxa varies as well with authors. Some of this is the result of the uncertainty of the current state of our knowledge of the phylogeny of the group. In order to better express relationships, the classification presented here recognizes more categories than some users may wish to use. Users wishing to recognize fewer categories may do so by recognizing only the classical and more familiar taxonomic names and, in reducing the number of categories, changing the category names of the retained taxa (e.g., to subclass Euselachii and superorder Selachii).
Teeth are especially important in the fossil record of sharks and exhibit much variation between taxa. Many publications describe the teeth of living and fossil taxa, for example, works by S. P. Applegate. R. Lund, and J. G. Maisey have worked on the taxonomy and systematics of fossil elasmobranchs. A general review of some aspects of shark behavior and acoustical biology may be found in Myrberg and Nelson (1990) and Myrberg (2001). Schwartz and Maddock (2002) review the cytogenetic data of euselachians. See "Chondrichthyes" above for references giving major revisions to our knowledge of chondrichthyans and explanations to the many terms describing the reproductive phenomena oviparity and viviparity with their many variations as differing means of supplying nutrients to the embryo. Many websites give information on sharks and rays (e.g., http://www.flmnh.ufl.edu/fish/).
Thirteen orders, 51 families, 178 genera, and about 937 species of extant elasmobranchs or euselachians. Some 403 species are sharks and 534 are skates and rays. At least 28 species of sharks and rays are known primarily from freshwater. There are many species of sharks and rays yet to be described, with the rays still outnumbering the sharks. The current classification of euselachi-ans is very split compared to that of other fishes. The mean number of species per family is 18 and the median number is 5.3. About 50% of the species of sharks and rays are in four of the 51 families, Rajidae, Scyliorhinidae, Dasyatidae, and Carcharhinidae, and about one-fifth of the families (11) are monotypic, having only one species in each.
An overview of the higher categories of living (extant) euselachians adopted here is as follows:
Division Neoselachii Subdivision Selachii (sharks) Superorder Galeomorphi
Order Heterodontiformes (one family) Order Orectolobiformes (seven families)
Order Lamniformes (seven families) Order Carcharhiniformes (eight families) Superorder Squalomorphi
Order Hexanchiformes (two families) Order Echinorhiniformes (one family) Order Squaliformes (six families) Order Squatiniformes (one family) Order Pristiophoriformes (one family) Subdivision Batoidea (rays)
Order Torpediniformes (two families) Order Pristiformes (one family) Order Rajiformes (four families) Order Myliobatiformes (ten families)
fOrder CTENACANTHIFORMES. Two dorsal fins, each with a spine; anal fin near caudal fin; cladodont-type tooth (as with many fossil taxa, there are no synapomorphic characters to show monophyly). Maximum length about 2.5 m.
Three families (Zangerl, 1981; Cappetta, 1987; Cappetta et al., 1993). Middle Devonian to Triassic. Unassigned genera include Acronemus and Carinacanthus.
Family CTENACANTHIDAE. Upper Devonian and Mississippian. e.g., Ctenacanthus and Goodrichthys. Family BANDRINGIDAE. Pennsylvanian.
Primarily freshwater. Snout elongated; caudal fin externally heterocercal. e.g., Bandringa.
Family PHOEBODONTIDAE. Middle Devonian to Upper Triassic. e.g., Phoebodus.
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