The Chondrichthyan Fossil Record

Two chondrichthyan groups are known only from fossilized remains. These are the stem chondrichthyes and the stem elasmobranchs.

Stem Chondrichthyes

Stem Elasmobranchs

Stem Chondrichthyes

The Stem Chondrichthyes represent a majority of the Palaeozoic chondrichthyan diversity.

Order Cladoselachidae

Cladoselache was a Devonian chondrichthyan with a shark-like body. It had paired pectoral and pelvic fins that were used primarily as hydrofoils (there is little evidence of paired-fin motility), as well as two dorsal fins with a dorsal spine in front of each. Its caudal fin was heterocercal internally, but symmetrical externally. Its endoskeleton was composed of calcified cartilage (like all other chondrichthyans), and it exhibited amphistylic jaw attachment (for an explanation of chondricthyan jaw attachment styles, see Characters and Anatomy). It had a large gape, and its teeth were pointed and three-cusped. This, along with its streamlined shape, suggests that it was a predator, who probably fed on other fishes or cephalopods. The skin of Cladoselache was only sparsely covered with dermal denticles.

Cladoselache - From Benton (2005)

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Order Symmoriiformes

The Symmoriiformes had bodies with the same general shape as the Cladoselachidae. The caudal fin was externally symmetrical, as in Cladoselache, but the dorsal fins lacked associated spines. Unique to Symmoriiforms was the presence of a long, pointed metapterygial axis (and extension from the pectoral fin) whose function is unknown. The Stethacanthids are a family of Symmoriiforms from the Carboniferous. They exhibit a unique "shoulder spine" that originates dorsally at the base of the head. This large brush-like spine is present in sexually mature males, and may have been used as a form of sexual display.

Three different Symmoriiformes:

Denaea (top), Falcatus (middle) and Stethacanthus (bottom) from Benton (2005)

Fossil Stethacanthus from the mid-Carboniferous of Scotland (from Benton, 2005)

Stethacanthus by Todd Marshall

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Order Eugeneodontiformes

The Eugeneodontiformes are a group of chondrichthyans known primarily from spiral-like whorls of fossilized teeth that formed a saw-like apparatus. Little else is known about the rest of the Eugeneodontiform skeleton, but it is assumed to be similar to other stem chondrichtyans.

Helicoprion tooth whorl (from Benton, 2005)

Helicoprion by Todd Marshall

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Order Petalodontiformes

The Petalodontiformes are a group of bizarre looking stem chondrichthyans from the Carboniferous and Permian. They had a rounded body that does not resemble those of the Cladoselachidae or the Symmoriiforms. Their jaws contained ridged teeth that were probably used for crushing, implying a diet of hard-shelled foods.

Belantsea from Benton, 2005

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Stem Elasmobranchs

Stem Elasmobranchs are the Mesozoic ancestors of extant sharks and rays. They share a number of characters with the Neoselachii, including the presence of an anal fin (a small ventral fin just anterior of the caudal fin), a tri-basal pectoral/pelvic fin attachment (pro-, meso- and metapterygium), and a number of derived braincase features

Order Xenacanthiformes

The Xenacanthiformes were freshwater sharks that existed from the Devonian to the Triasssic. The endoskeleton skeleton of their pectoral and pelvic fins was similar to that of modern sharks, but their tail was long and tapering (diphycercal) and not heterocercal. They also possessed a single dorsal fin, and a dorsal spine that originated directly behind the head. Xenocanthus had tri-cusped teeth, but unlike those of Cladoselache, the central cusp was the shortest. The long, narrow body shape of Xenocanthus implies a predatory lifestyle, probably in vegetation-thick waters.

Xenacanthus (from Benton, 2005)

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Order Ctenacanthiformes

The Ctenacanthiformes existed from the Devonian to the Triassic, and were similar in appearance to modern sharks. The endoskeleton of the pelvic fins was, again, comparable to that of modern sharks. Furthermore, their two dorsal fins were supported by anterior fin spines, as well as by basal cartilages with radials extending from them. The cadaul fin was heterocercal (externally asymmetrical), as it is in the Neoselachii.

Ctenacanthus (from Benton, 2005)

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Order Hybodontiformes

The hybodonts radiated in the Triassic, and lived alongside modern elasmobranchs until the late Cretaceous. The paired fins show signs of intrinsic musculature, and greater flexibility, implying that they were used in steering. They had a heterocercal tail, an anal fin and amphistylic jaw suspension. Hybodonts were also the first group to possess a heterodont dentition (= different shaped teeth along the length of the jaw). For instance, having sharp cutting teeth in the front part of their jaw and flat crushing teeth in the back part of the jaw allowed them to feed on a variety of food types. The Hybodontiforms also possessed a full set of hemal, as well as neural, arches around the notochord.

Hybodus (from Benton, 2005)

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Author: Andrew Gillis
Last updated: 15 November 2004
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Websites produced by students on the MSc Palaeobiology programme in the Department of Earth Sciences at the University of Bristol for academic year 2004-5.