DESCRIPTION OF THE CLASS ACANTHODII+ (OWEN 1846)
| EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>BILATERIA>DEUTEROSTOMATA>VERTEBRATA>GNATHOSTOMATA>ACANTHODII |
CLASS ACANTHODII LINKS
| Acanthodii (a-kan-THO-de-i) is derived from the Greek root acantha (Ακανθα), which refers to a spine. The reference is to the spines at the anterior of each of the body fins. The name was created by Richard Owen (1846) from a series of lectures for the Royal Society of Surgeons of England in 1844 and 1846. |
| INTRODUCTION TO THE ACANTHODII Commonly called spiny sharks, acanthodians had numerous fins (both in-line and paired), most of which were supported at the anterior end by a large spine (Figure 1). They were basal gnathostomes that shared a suite of characters with the Osteichthyes and the Chondrichthyes (e.g. gills covered by an operculum, placoid-like scales, etc.). They were small fish but could be as long as 2 meters. That most of them had large eyes suggests that they lived at great depth. By and large, the acanthodians were lightly armored with scales of bone and dentine. The scales covered the body of the animal and showed evidence of concentric growth rings. Though a few had teeth, most were toothless and may have been able to secure prey by back-pointed gill rakers. Those with teeth were variable in that some had teeth fused to the jaw (e.g. Ishnacanthiformes) and others did not (e.g. Climatiiformes). Although acanthodians have long been considered sisters or precursors to the Chondrichthyes, Janvier (1996), Wilson et al. (2007) and Benton (2005) consider the acanthodians to be sisters to the Osteichthyes (see Figure 2). Indeed, they suggest that the distinctions between the two groups might be arbitrary and that the acanthodians are paraphyletic with regard to the bony fishes. That is, the Acanthodians may be a basal group of Osteichthyes. |
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| FIGURE 1. Illustrations of two acanthodians showing the large eyes, gills covered by an operculum, and the spine at the leading edge of the fins. Images from: http://vivaldi.zool.gu.se/Fiskfysiologi_2001/Course_material/Introduction_fish_evolution/Images/Acanthodians.gif |
FIGURE 2. A cladogram of the gnathostome fishes after Wilson et al. (2007) and Benton (2005). In this topology, the acanthodians are basal to the Osteichthyes.
| LITERATURE CITED Benton, M. J. 2005. Vertebrate Paleontology. Third Edition. Blackwell Publishing, Malden, MA. Janvier, P. 1996. Early Vertebrates. Oxford University Press, Oxford. Nelson, J. S. 2006. Fishes of the World. 4th edition. John Wiley and Sons, Inc. New York. Owen, R. 1846. Lectures on the comparative anatomy and physiology of the vertebrate animals. Delivered at the Royal College of Surgeons 1844 and 1846. (no publisher given). London. Pt. I. Fishes. 304 pp Wilson, M. V. H., G. F. Hanke, and T. Märss. 2007. Paired fins of jawless vertebrates and their homologies across the “agnathan”-gnathostome transition. In: J. S. Anderson and H.-D. Suess, eds. Major Transitions in Vertebrate Evolution. Indiana University Press. Bloomington. pp. 122-149. |
| By Jack R. Holt and Carlos A. Iudica. Last revised: 12/02/2015 |
