DESCRIPTION OF THE SUPERCLASS PTERASPIDOMORPHI+ (NELSON 2006)
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SUPERCLASS PTERASPIDOMORPHI
| Pteraspidomorphi (ter-a-spid-o-MORF-i) is derived from three Greek roots that mean winged [pterya (πτέρυγα)] shield [apsida (ασπίδα)] forms [morphi (μορφή)]. The name is a reference to the large bony head shield on the animal. Pteraspidomorphi was coined by Goodrich (1909) but was redefined in the narrow sense in which we use it here by Nelson (2006). |
| INTRODUCTION TO THE PTERASPIDOMORPHI The pteraspidomorphs were entirely extinct jawless fish with very large head shields made of acellular dermal bone (Figures 1 and 2). The rest of the body was covered by overlapping scale-like bony plates. They had no paired appendages, and their dorsoventrally flattened bodies indicate that they likely were bottom-dwellers rather than active swimmers in the water column. They appeared in the Ordovician and persisted through the mid Devonian. Benton (2005) suggests that the pteraspids were more derived than the conodonts by the acquisition of dermal bone on the outside of the body, which formed the head shield plates and bony scales. Janvier (2008a), while accepting the pteraspid clade of Arandaspids+Heterostracans+Astraspids (as in this system), proposes that the anaspids, rather than the pteraspids, are the most basal of the fishes with dermal bone on the outside of the body. Janvier’s pteraspid clade is the most basal group with a lateral line system, vertical semicircular canals, and a cerebellum. |
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| FIGURE 1. Arandaspis resembled Astraspis but it had had a large bony head shield. Image by: Apokryltaros- Wikipedia | FIGURE 2. Astraspis had a flattened body covered with small bony plates. The eyes were set wide apart on either side of the mouth. The tail was little more than a laterally flattened end of the body. Image by: Apokryltaros- Wikipedia |
FIGURE 3. A cladogram showing the relationships between the subclasses of the Pteraspidomorphi (Arandaspida, Astraspida, and Heterostraci) according to Janvier (2008a). They are the most basal group to have armor made of dermal bone.
| THE ASTRASPIDA AND ARANDASPIDA The pteraspidomorphs are represented by three unequal higher taxa that Nelson (2006) regards as subclasses: Astraspida, Arandaspida, and Heterostraci. Janvier (2008a; see Figure 3) considers taxa in the Subclass Arandaspida to be basal to the rest of the taxa in the class. Arandaspis (Figure 1) is typical of the group with large bony head shields and eyes placed far forward on the head, on either side of the mouth. They were notable in that they had paired pineal and parapineal openings (Nelson 2006). They had ten gill openings, each with a separate cover. The Arandaspida were restricted to the Ordovician Period. The Subclass Astraspida has a sole genus (assuming that the second genus, Pycnaspis, is a synonym of Astraspis). The head of Astraspis (Figure 2) was covered in small bony plates like the rest of the body. The eyes were small and placed forward on the broad head. They had eight gill slits that were not covered. Donoghue et al. (2000) suggest that Astraspis might be more derived and sisters to the gnathostomes. |
| THE HETEROSTRACI The Subclass Heterostraci was by far the most diverse of the three subclasses and persisted from the lower Silurian through the middle of the Devonian. According to Janvier (2008a), they were sisters to the Astraspida. Unlike the astraspids, however, the bones of the head shields were heavy and fused as a carapace in some like those of the arandaspids. The rest of the body was covered in an armor of overlapping bony scales. The tail was lobed and generally hypocercal, as opposed to the tadpole-like tails of the other two subclasses. The body plans tended to be dorsoventrally flattened with a head shield that covered the front third to half of the body. Pteraspis (Figure 4) had a large head shield made of multiple plates. Notable features are the large bony rostrum, dorsal spine, a branchial plate, and a bone of the shield that covered the gill openings. Another heterostracan was Drepanaspis (Figure 5), a flattened animal with a very broad head shield that had lateral “wings” which must have functioned as stabilizers when swimming. The overall shape is curiously reminiscent of the Osteostracomorphi. Drepanaspis was large, more than a meter wide, and had a mouth that opened upward. |
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| FIGURE 4. Pteraspis had a distinctive rostrum, dorsal spine, brachial plate, and hypocercal tail. Image from: Biodidac | FIGURE 5. Drepanapsis had a broad head shield with “wings” and an upturned mouth. Image by: Apokryltaros- Wikipedia |
| LITERATURE CITED Benton, M. J. 2005. Vertebrate Paleontology. Third Edition. Blackwell Publishing, Malden, MA. Goodrich, E. S. 1909. Vertebrata Craniata. First fascicle: Cyclostomes and fishes. In: Lankester, E. R., ed. A Treatise on Zoology. Vol. 9. Black. London. 518pp. Janvier, P. 2008a. Early jawless vertebrates and cyclostome origins. Zoological Science. 25: 1045-1056. Nelson, J. S. 2006. Fishes of the World. 4th edition. John Wiley and Sons, Inc. New York. Sweet, W. C. and P. C. J. Donoghue. 2001. Conodonts: past, present, future. Journal of Paleontology. 75:1174-1184. |
| By Jack R. Holt and Carlos A. Iudica. Last revised: 02/03/2018 |
