DESCRIPTION OF THE SUPERCLASS ANASPIDOMORPHI+ (NELSON 2006)
| EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>BILATERIA>DEUTEROSTOMATA>VERTEBRATA>ANASPIDOMORPHI |
| Anaspidomorphi (an-as-pid-o-MORF-e) is formed of three Greek roots which mean “forms without a shield” [without- an (áv), a shield- aspis (áσπὶς), form [morphi (μορφή)]. The reference is to a jawless fish without a shield of bony armor that is common to many of the other agnathans. Anaspidomorphi was a name created by Nelson (2006). |
SUPERCLASS ANASPIDOMORPHI LINKS
| INTRODUCTION TO THE ANASPIDOMORPHI Anaspids were small (up to 15cm long) freshwater fishes that persisted from the Silurian (408-438 MYA) through the Devonian (360-408 MYA). As the name implies, these fishes did not have bony armor, particularly bony head shields. However, they did have bony scales, scutes, and plates. Like the thelodonts with which they are usually grouped, the tails of the anaspids were hypocercal, and they had terminal mouths, but the anaspids had a pair of pectoral spines (rather than pectoral fins) and their bodies were laterally compressed (Figure 1). The scaleless Euphanerops (in the Petromyzontomorphi) and its relatives have been associated with the anaspidomorphs because both had strongly recurved hypocercal tails and lacked bony headshields (Nelson 2006 and Janvier 1996a). Nelson (2006) and Benton (2005) suggest that the lack of bony plates is due to reduction in armor which allowed them to be more agile in their freshwater habitats. Janvier (2008a), however, indicates that the anaspidomorphs were more primitive than the other fishes with external structures of dermal bone in that they had not acquired a lateral line system enclosed in grooves and a canal. Because the anaspidomorphs likely did not have vertical semicircular canals or a differentiated cerebellum, Janvier (2008a) places the anasidomorphs at the base of the armored agnathans. |
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| FIGURE 1. Drawing of Pharyngolepis that illustrates the salient features of this group: a hypocercal tail, pectoral spines, and bony scales and scutes rather than body armor. Image taken from: www.tolweb.org/tree/ToLimages/anaspida.gif |
FIGURE 2. A cladogram of the agnatha higher taxa. The position of the Anaspidomorphi (Anaspida) within the vertebrates generally follows the system of Nelson (2006).
| Benton (2005) grouped the anaspidomorphs together with the thelodonts because both had terminal mouths, hypocercal tails, body scales, and pectoral fins. Neither the character of terminal mouth nor of hypocercal tail is a shared derived character, and the body scales that the two groups had were completely different. Thelodonts had placoid scales (dentine covered by enamel) while the scales of anaspidomorphs were of acellular dermal bone. Furthermore, the anaspidomorphs had a pair of pectoral spines while the thelodonts had true fin-like structures. All in all, anaspidomorphs and thelodonts may only bear a superficial resemblance to each other. Neither Nelson (2006) nor Janvier (2008a) group the thelodonts and anaspidomorphs together. Thus, we followed Nelson (2006; see Figure 2) and hold the Anaspidomorphi at a rank (superclass in this system) equal to that of the Thelodonti. |
| LITERATURE CITED Benton, M. J. 2005. Vertebrate Paleontology. Third Edition. Blackwell Publishing, Malden, MA. Janvier, P. 1996a. Early Vertebrates. Oxford Monographs in Geology and Geophysics, 33. Oxford University Press. Oxford. pp. 393. Janvier, P. 2008a. Early jawless vertebrates and cyclostome origins. Zoological Science. 25: 1045-1056. Janvier, P. and M. Arsenault. 2007. The anatomy of Euphanerops longaevus Woodward, 1900, an anaspid-like jawless vertebrate from the Upper Devonian of Miguasha, Quebec, Canada. Geodiversitas. 29(1): 143-216. Nelson, J. S. 2006. Fishes of the World. 4th edition. John Wiley and Sons, Inc. New York. |
| By Jack R. Holt and Carlos A. Iudica. Last revised: 02/03/2018 |
