More About Acanthodians (spiny fins)

reconstructions of three acanthodiansClimatius (top left; Lower Devonian), Diplacanthus (top right; Middle Devonian)
and Acanthodes (center; Lower Permian) ©

Enigmatic Early Fishes

Acanthodians are a poorly understood group of extinct jawed fishes that are distinguished by the bony spines projecting in front of their fins and by minute, diamond-shaped scales. Most of them also have relatively large eyes set near the front of their short blunt heads. Like most early fishes, acanthodians had heterocercal caudal fins (tail fins with the top longer that the bottom).

Acanthodians are among the earliest jawed vertebrates known. Fragmentary remains have been recovered from as early as the Upper Ordovician of North America and Lower Silurian of China. Their record extends about 160 million years to the Lower Permian. Diversity is greatest from the Upper Silurian through the Devonian.

acanthodian diversity over geologic time

Although they lived for nearly 160 million years, acanthodians exhibited relatively little diversity. Most authorities recognize from four to nine families belonging to three main groups. Climatiiforms account for most of the earlier acanthodians, but become uncommon after the Middle Devonian. One family, the Gyracanthidae, however, extends through the Carboniferous. Climatiiforms are characterized by stout spines, dermal bones covering much of the shoulder girdle and/or head, and one or more sets of intermediate ventral spines; The variety of intermediate spines, dermal bones and dentition has led some authories to question whether climatiiforms are monophyletic. Examples include Climatius, Diplacanthus and Gyracanthus.

Ischnacanthiforms are distinguished by robust dermal jaw bones with rows of large triangular teeth. They are represented by a single family that extended from the Middle Silurian through the Carboniferous. The last surviving acanthodians belong to the Acanthodiformes. This group, which extends from the Lower Devonian through the Lower Permian, is characterized by a single dorsal spine and fin, absence of dermal bones covering it head and shoulder girdle, lack of teeth and the presence of well-developed gill rackers. Acanthodes is the best known member of this group.

The earliest acanthodians were exclusively marine, but freshwater forms predominated during and after the Middle Devonian. Most were small, but some gyracanthids and ischnacanthids could exceed 1 meter in length. Most also had streamlined bodies and were probably good swimmers. The teeth of ischnacanthiforms and many climatiiforms suggest that they were predators, whereas the presence of gill rackers and the absence of teeth suggest that the acanthodiforms were zooplankton feeders. Indeed, the stomach contents of some Late Devonian acanthodids included numerous small crustaceans.

Acanthodian have been found in many of the more productive fossil fish localities throughout rhe world. However, their remains tend to be poorly fossilized. Most species are known only from isolated spines and/or pectoral bones. Some fossils exhibit exquisite external details of the spines, fins and scales, but contain essentially no information about the weakly ossified internal skeleton. Anatomical details of the braincase and gill arches are known only from Acanthodes, one of the last and most specialized members of the group.

The paucity of information about the internal skeletons has hindered study of acanthodian relationships both within the group and with other vertebrates. Similarities in body shape, scales and spine morphology suggest that they were an evolutionarily conservative group. However, since so little is known about crucial elements of the braincase and gill arches, acanthodian diversity might be greater than is generally recognized.

The relationships of acanthodians to other vertebrates has been the subject of considerable debate. Early researchers considered them to be most closely related to the ray-finned fishes, but most scientists during the mid-20th Century considered them to have a closer affinity to the sharks. Opinion has now generally swung back in favor of a closer relationship with ray-fins, but this is not universally accepted.

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Web:
Palaeos.com's web section on Acanthodii:
www.palaeos.com/Vertebrates/Units/090Teleostomi/
090.100.html#Acanthodii
Books:
Carroll, R. L. 1988. Vertebrate Paleontology and Evolution. New York: W. H. Freeman & Co.
Denison, R.H. 1979. Acanthodii. Handbook of Paleoichthyology, Vol 5. Stugggart: Gustav Fisher Verlag.
Janvier, P. 1996. Early Vertebrates. Oxford: Claredon Press.
Long, J.A. 1995. The Rise of Fishes: 500 Million Years of Evolution. Baltimore & London: John Hopkins Univ. Press.
Maisey, J.G. 1996. Discovering Fossil Fishes. New York: Henry Holt & Co.
Moy-Thomas, J. A. and R. S. Miles, 1971. Paleozoic Fishes. London: Chapman and Hall.
Image Credits:
The acanthodian reconstruction at the top and the spindle diagram are copyrighted, © Dennis C. Murphy, 2002. (See Terms of Use.) The reconstructions of Acanthodes, Climatius and Diplacanthus were based on Denison (1979) and Moy-Thomas and Miles (1971).

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