DIVERSITY OF LIFE
CLASS CHONDRICHTHYES (HUXLEY 1880)

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Chondrichthyes (kon-DRIK-thes) is derived from two Greek roots meaning "cartilaginous fishes" [cartilage- kondros (χόνδρος) and fishes- ichthyos (ιχθύος)].  The name refers to the skeleton of prismatic calcified cartilage that is diagnostic for this group.  The name was coined by T. H. Huxley (1880)  

INTRODUCTION TO THE CHONDRICHTHYES

Grogan and Lund (2004) define the two synapomorphies of the Chondrichthyes as: having skeletons of cartilage and having pelvic fins in the males modified as claspers.  In addition, most sharks, rays, and chimeras have a heterocercal tail, large pectoral fins, relatively small pelvic fins, and an obvious first dorsal fin.  Another characteristic is the tooth-like placoid scale (Figure 1), which is made of dentine with a pulp chamber on the inside and a layer of enamel on the outside.  Note that the structure of the placoid scale is similar to the scales of the thelodonts.

The cartilaginous fishes date from the Devonian to the present, but typical shark teeth and scales occur earlier in the Silurian.  Although the Chondrichthyes has a long fossil history with at least two major periods of evolutionary radiation, today the diversity has been reduced to two groups: the elasmobranchs (the sharks, skates, and rays) and holocephalians (the chimeras or ratfish, Figures 2-A and 2-B).  The 964 different species of living Chondrichthyes are unevenly divided between the open water sharks (45%) and the skates and rays (55%).  There are only about 30 extant species of chimeras.  Most shark species are on decline from over fishing and habitat destruction.

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FIGURE 1

 

FIGURE 2A. A cladogram of the gnathostome fishes according to Benton (2005).  The two major clades of the Chondrichthyes: Euchondrocephali and Elasmobranchii both have extant taxa.

MAJOR CLADES OF THE CHONDRICHTHYES

1.  Euchondrocephalian Clade

2.  Elasmobranch Clade

3.  Neoselachian Clade

4.  Squalean Clade

FIGURE 2B. A cladogram of the Chondrichthyes using the Acanthodii+Osteichthyes as the outgroup.  We have used Benton (2005) and Nelson (2006) as the basis for its structure.  Use this cladogram as the framework to understand the descriptive text below.  We have indicated numbers for the various clades that are discussed below.  Note that clades 2-4 are nested.  Taxa that are entirely extinct are in red.

 

 

The Euchondrocephalian Clade (1)

The Euchondrocephali ( a name from 3 Greek roots that mean "True Cartilaginous Heads") are commonly called chimeras, named for a monster of Greek mythology that had the head of a lion, the body of a goat, and the tail of a dragon.  These odd fish appear to be chimeroid in that they have characteristics of sharks and bony fishes (also, many have a rat-like tail; see Figure 3).  The name is a reference to the fusion of the jaws with the chondrocranium.  In addition, they have no placoid scales, no spiracle, no stomach, no ribs, and no cloaca.  However, they do have teeth that have been reduced to crushing plates and gill slits that are covered by an operculum (a characteristic of all bony fishes).  Males also have structures on their heads that are used in copulation in addition to claspers.   All living taxa are in the Holocephali.  The Paraselacimorpha, a sister group to the Holocephali, were restricted to the Paleozoic and had teeth that were similar to those of the Elasmobranchii.

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FIGURE 3

The Elasmobranch Clade (2)

The elasmobranchs [from two Greek roots: elasmo- plate-like and branch (pronounced brank, which means gill) are named for the multiple (usually 5) separate gill slits, usually with spiracles. These fish vary in form from the familiar cylindrical sharks of open water to extremely-flattened skates and rays.  All have skin that is covered with tooth-like placoid scales.  Their jaws and braincases (chondrocrania) are not fused, and they have a cloaca (a common opening for the urogenital and digestive systems).  Also, their teeth are not fused to their jaws and are readily replaced as they are lost or worn out.  

The sharks went through two major radiations.  The first was during the later half of the Paleozoic (Devonian-Permian) followed by a second radiation through the Mesozoic (Triassic-Cretaceous).  One of the most successful early taxa was the Devonian (360-416 MYA) taxon, Cladoselache (Figure 4).  Much is known about this genus due to preservation of fine details, including internal organs.  It was recognizable as a shark with a cylindrical body, a skeleton of hyaline cartilage, two dorsal fins, teeth of dentine and enamel, and placoid scales, though the scales were restricted to the areas of the eyes and the mouth.  Cladoselache and other sharks of the Paleozoic and Mesozoic Eras had teeth with three cusps (the center one being the largest).  In addition, their mouths were terminal, rather than the characteristic subterminal  position.   Many of the early sharks showed strong sexual dimorphism; however, Cladoselache and perhaps other taxa that are not as well known, do not have any evidence of having claspers, which calls into question that character as a synapomorphy for the whole class.

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FIGURE 4

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FIGURE 5

The Neoselachian Clade (3)

The Neoselachii are the more recent Elasmobranchs, within which there are three groups of taxa.  In this system, the living taxa all occur in three groups defined as superorders: Galeomorphi (280 species), Squalomorphi (124 species), and Batidiomorphi (534 species, Clade 9).  The galeomorphs are characterized by having an anal fin and are typically cylindrical in form, making them good open water swimmers.  Among them are the whale shark, the thresher shark, the hammerhead shark, and the great white shark (Figure 5).  Though most are predators, the whale shark and the basking shark (Figure 6), the largest fish alive, feed only on plankton.


FIGURE 6

The Squalean Clade (4)

The Squalea are sisters to the Galeomorphi, but they have no anal fin.  The squalomorphs range in form from cylindrical (e.g. spiny dogfish, Figure 7) to flattened (e.g. guitarfish).  The main distinctions of the squalomorphs include no anal fin and locomotion mainly by the tail.  Pough et al. (2009) do not consider the squalomorphs to be monophyletic.  Indeed, if the Paleozoic sharks are considered, the squalomorphs become paraphyletic or part of the batidomorph lineage (Janvier 1996). 

Anal fins are completely absent in both the squalomorphs and batidomorphs.  Though some are open water swimmers, most are dorsoventrally-flattened and adapted to life in association with the bottom (Figure 8).  More than half of all of the living chondrichthyes species are members of the batidomorphs, the skates and rays.   They are bat-shaped with large pectoral fins which flap like bat wings and serve as the primary means of locomotion.  Their gill slits and mouths are located ventrally, and the spiracles, which serve as the primary water intake are located dorsally, an arrangement that works well for the animals as they lie on the bottom.

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FIGURE 7

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FIGURE 8

LITERATURE CITED

Benton, M. J. 2005. Vertebrate Paleontology. Third Edition. Blackwell Publishing, Malden, MA. [L]

Grogan, E. D. and R. Lund. 2004. The origins and relationships of early Chondrichthyes. In: J. C. Carrier, J. A. Musick, and M. R. Heithaus, eds. Biology of Sharks and Their Relatives. CRC Press. Boca Raton, Florida. pp. 596.

Huxley, T. H. 1880. On the application of the laws of evolution to the arrangement of the Vertebrata and more particularly of the Mammalia. Proceedings of the Zoological Society of London. 1880: 649662.

Janvier, P. 1996a. Early Vertebrates. Oxford Monographs in Geology and Geophysics, 33. Oxford University Press. Oxford. pp. 393.

Nelson, J. S. 2006. Fishes of the World. 4th edition. John Wiley and Sons, Inc. New York.

Pough, F. H., C. M. Janis, and J. B. Heiser. 2009. Vertebrate Life. 8th ed. Benjamin Cummings. New York. pp. 688.

 

By Jack R. Holt and Carlos A. Iudica.  Last revised: 02/03/2018