reconstructions of three shark relatives Cladoselache (top left, Middle Devonian), Ischyodus (top right, Upper Jurassic)
and Hybodus (bottom, Lower Jurassic) ©

More about Chondrichthyes
(sharks and their kin)

Cartilage and teeth; many many teeth

Sharks, skates, rays and holocephalans belong to a group of fishes called the Chondrichthyes (=cartilaginous fishes). The most distinctive characteristic of chondrichthyans is their skeleton. Except for mineralized dorsal fin spines, their skeletons are made up entirely of cartilage, the stiff material that gives our outer ears their distinctive shape. However, chondrichthyan cartilage differs from ours because it contains thousands of minute prisms containing the same mineral that strengthens our own bones (hydroxyapatite). Upon death, a chondrichthyans cartilaginous skeleton can decompose rapidly and the reinforcing prisms are soon scattered. Consequently, intact fossils are exceedingly rare.

On the other hand, sharks and other chondrichthyans produce distinctive teeth that are highly resistant to decay. As a general rule, tooth replacement and total tooth production is relatively modest in ancient sharks and other chondrichthyans, but it can be prodigious in neoselachians (modern sharks). An individual shark may replace teeth within a matter of days and produce several hundred or even thousands of teeth during a lifetime. One consequence of this incredible tooth production is that neoselachian teeth are typically the most abundant vertebrate fossils in Cretaceous and Cenozoic marine deposits.

Chondrichthyans may be the oldest known vertebrates. Shark-like denticles (minute teeth-like structures that cover much of the body) have been recovered from Late Ordovician and Lower Silurian deposits. The first unambiguous shark teeth, however, don’t appear until the Lower Devonian. The frequency and diversity of fossils increase substantially in the Upper Devonian and Carboniferous, decline somewhat thereafter, then again increase in the Cretaceous and Tertiary. Sharks and their relatives constitute the second most abundant and diverse group of fishes alive today.

shark diversity over geologic time

Thanks to a number of exceptionally preserved fossils from the Cleveland Shale of Ohio, Cladoselache is the best known of the Devonian sharks. With its streamlined body, large pectoral fins, two prominent dorsal fins and a heterocercal (upturned) tail, Cladoselache superficially resembles modern sharks. However, Cladoselache and many other Paleozoic sharks differed from modern sharks by having a long mouth that extended from the gill arches to the front of the snout and by having a simple and relatively weak jaw joint. They also lacked the solid vertebral column found in modern sharks.

Chondrichthyans experienced a major radiation during the Latest Devonian and Carboniferous. Several unusual groups emerged that were either much reduced or extinct by the end of the Permian. The Eugeneodontida (e.g., Helicoprion) had shark-like bodies but also had large symphseal (front lower jaw) teeth arrayed in a complex teeth whorl. Symmoriida (e.g., Stethacanthus and Falcatus) also had shark-like bodies, but possessed elaborate structures situated where one would expect to find the anterior dorsal fin. Petalodonts (e.g., Belantsea) and iniopterygians (e.g., Iniopteryx) were bizarre fishes with conspicuously unshark-like bodies. Many of these and other Paleozoic chondrichthians are represented in the exceptionally preserved Lower Carboniferous marine deposits at Bear Gulch, Montana.

The Holocephali (e.g., Ischyodus) is one group that first appeared during the Carboniferous radiation but has survived to the present. This group, which includes chimaerids and rabbitfishes, are represented today by fewer than 40 known species. They are characterized by an operculum covering the gill arches, an upper jaw fused to the braincase and crushing plate dentition.

The Xenacanthida (e.g., Orthacanthus), Ctenacanthida (e.g., Ctenacanthus) and Hybodontids (e.g., Hybodus) are three groups of sharks that appeared during this Latest Devonian/Carboniferous radiation and persisted into the Mesozoic. Unlike most chondrichthians, xenocanths were predominantly freshwater fishes. Ctenacanths and hybodonts share several features with modern sharks including greatly strengthened jaw articulations and an arrangement of cartilage in the pectoral and pelvic fins that allowed for greater freedom of movement. Xenacanths and ctenocanths become extinct in the Triassic, but hybodonts were abundant and diverse throughout most of the Mesozoic.

Neoselachians (modern sharks, skates and rays) first appear in the Triassic and are believed to have evolved from within the Ctenacanthida. They are distinguished from the ctenacanths and hybodonts by the presence of solid vertebral centra. The possession of a series of non-compressible vertebrae probably facilitated greater speed and maneuverability by allowing for the use of stronger trunk muscles. Neoselachians and hybodonts shared several features concerning the mouth such as the migration of the mouth from the tip of the snout to behind and underneath it, and the extensibility of the upper jaw. However, both of these characteristics are more developed in the Neoselachii. Examples of neoselachian sharks include Palaeospinax, Carcharocles and Carcharinus.

Rays and skates first appear during the Jurassic. They are characterized by the expansion of the pectoral fins into a disk that in some of the more advanced forms extends well in front of the snout. Examples include Heliobatis, Myliobatis and Raja. Neoselachian sharks and skates and rays continued to diversify throughout the Cretaceous and Cenozoic and apparently didn’t suffer any major extinctions.

Unlike most fishes, cartilaginous fishes reproduce by internal fertilization. The males have paired intromittent organs called claspers attached to their pelvic fins. Once the fertilized eggs develop they're encased in a sturdy "shell" and released into the water (ovipary) or they're encased in a thin "shell" and kept inside the body until they hatch (ovovivipary).

Two sharks, Ctenacanthus sp. and Ageleodus pectinatus, were found at Red Hill.

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Web:
Discovery Online's videos of prehistoric sharks:
dsc.discovery.com/convergence/sharkweek/
video/prehistoric/prehistoric.html
Fiona's Shark Mania:
www.oceanstar.com/shark/
Palaeos.com's overview of the Chondrichthyes:
www.palaeos.com/Vertebrates/Units/070Chondrichthyes/070.000.html
ReefQuest's web pages on shark evolution:
www.elasmo-research.org/education/evolution/evol_s_predator.htm
Richard Lund & Eileen Grogan's website on the Bear Gulch fauna:
www.sju.edu/research/bear_gulch/
Shark Friends' web pages on prehistoric sharks:
www.sharkfriends.com/paleo.html
U.C. Museum of Paleontology's introduction to the Chondrichthyes:
www.ucmp.berkeley.edu/vertebrates/basalfish/chondrintro.html
Books:
Carroll, R.L. 1988. Vertebrate Paleontology and Evolution. New York: W. H. Freeman & Co.
Janvier, P. 1996. Early Vertebrates. Oxford: Claredon Press.
Long, J.A. 1995. The Rise of Fishes: 500 Million Years of Evolution. Baltimore and London: John Hopkins Univ. Press.
Maisey, J.G. 1996. Discovering Fossil Fishes. New York: Henry Holt & Co.
Scientific Papers:
Maisy, J.G. 1982. "The anatomy and interrelationships of mesozoic hybodont sharks." Amer. Mus Novitates 2724: 1-48.
Patterson, C. 1965. The phylogeny of chimaeroids. Phil. Trans. Roy. Soc. London, B 249: 101-219.
Zangerl, R. 1981. "Chondrichthys I. Paleozoic elasmobranchs." in: H.P. Schultze (ed.) Handbook of Paleoichthyology, Vol. 3, Elasmobranchi. Stuttgart: Gustav Fishcher Verlag.
Image Credits:
All images are copyrighted © 2002, Dennis C. Murphy. (See Terms of Use.) The reconstructions of Cladoselache, Hybodus and Ischydodus were based on Denison (1979) and Moy-Thomas and Miles (1971).Patterson (1965), and Zangerl (1981).

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