DESCRIPTION OF THE SUPERCLASS OSTEOSTRACOMORPHI+ (NELSON 2006)
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SUPERCLASS OSTEOSTRACOMORPHI LINKS
| Osteostracomorphi (O-ste-o-strak-o-MORF-i) is formed from three Greek roots that mean bone [osteon (οστούν)] shell [ostraco (όστρακo)] forms [morphi (μορφή)]. The name was coined by Nelson (2006) and is a reference to the bony shell or exoskeleton in which the head is encased. |
| INTRODUCTION TO THE OSTEOSTRACOMORPHI The Osteostracomorphi appeared in the Ordovician period (443-488 MYA) and disappeared in the Devonian (360-416 MYA). They were jawless, dorsoventrally flattened, and heavily armored with bony scutes and head shields, which provide some spectacular fossils (Figure 1). The bony head shields resembled those of the Pteraspidomorphi. Such heavy armor and dorsoventral flattening of the fish suggest that the animals were bottom-dwellers. The respiratory system had characters that support a bottom-dwelling lifestyle. While lying on the bottom or feeding on the bottom, the mouth likely was occluded and they had an alternate nasal opening (much like those of the hagfishes and lampreys) that allowed water to enter and pass over the gills, whose exit pouches were on the underside of the head. Likely, the gills and the gill arches, which occupied the inside of the dome-like cephalic shield likely served as filtering devices. If so, the animal was a filter-feeder and used a velum-like device at the mouth to force water into the cavity, where it was filtered. |
| THE OSTEOSTRACI (CEPHALASPIDIFORMES) Of the three groups of the osteostracoderms, the Osteostraci ( cephalaspidomorphs) are the best known (Figures 1 and 2). Their tails were epicercal (rather than the primitive hypocercal condition). Furthermore, The head shields had curious regions of bony platelets, called cephalic fields (see the cephalic field on the margin of the head shield in Figure 2). The purpose of the cephalic field has been debated, but all seem to agree that it functioned in a sensory mode (reviewed in Janvier 1996). One of the most important features of the cephalaspids was the occurrence of paired pectoral fins that had a bony endoskeleton and, therefore, a presumed pectoral musculature. The pectoral fins of the Osteostraci in general bear important similarities to the paired pectoral fins of the gnathostomes. Thus, Wilson et al. (2007) consider the paired fins of the Osteostraci to be homologous to those of the gnathostomes. |
| THE PITURIASPIDS AND GALEASPIDS The other two groups are less well known. The pituriaspids (Figure 3), viewed as sisters to the Osteostraci, have been found in Devonian beds in Australia only. They had long cornuate processes and bony shields that covered more than half of their bodies but details of the main body are poorly known (Young 1991). The galeaspids (Figure 4) were sisters to the pituriaspid + cephalaspid clade (Figure 5). They had less bone in the shields of the head and little armor on the rest of the body, which suggests that they may have been more active swimmers. The galeaspids had no paired fins which suggests that they may have lost them, especially if the fin-like flaps of the Thelodontomorphi are homologous to the pectoral fins of the other osteostracomorphs and the gnathostomes. The alternative explanation would be that the fin-flaps of the thelodonts evolved separately are were not homologs, in which case the galeaspids evolved from a line that never had pectoral fins. |
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| FIGURE 1. A fossil of Cephalaspis, a cephalaspidomorph. Note the dorsoventrally flattened head shield and overlapping bony scutes behind the head. Image by: Ghedoghedo- Wikipedia | FIGURE 2. A reconstruction of Cephalaspis, a cephalaspidomorph. Note the cephalic field on the flanges of the head shield. Image by: Apokryltaros- Wikipedia | FIGURE 3. A reconstruction of several pituriaspids. Note that the head shield covers about half of the body. Image by: Apokryltaros- Wikipedia | FIGURE 4. A reconstruction of several several galeaspids. Note that the animals have no paired appendages. The small fish below with the appendages are Bothreolepis, Placoderms. Image by: Apokryltaros- Wikipedia |
FIGURE 5. A cladogram of the agnathan craniates showing the relationships between the three orders of the Osteostracomorphi according to Benton (2005). Galeaspida is the sister to the Pituriaspida + Osteostraci. Either the gnathostomes share a common ancestor with the osteostracomorphs (as indicated here), or the gnathostomes emerged from within the group (as is the view of Janvier 2008a) and Gai et al. (2011).
| SYSTEMATICS OF THE OSTEOSTRACOMORPHI The Osteostraci (Figures 1 and 2) and Pituriaspida (Figure 3) had a single “nasal” opening. This monorhine condition caused Nelson (1994) to unite the lampreys with the cephalaspidomorphs, a view which he changed in 2006. Janvier (1996) also objected to the association with the lampreys and noted later (Janvier 2008a and 2013) that the “nostrils” and their placement (2 or fused into one) was a plastic feature that should not be considered a synapomorphy. The osteostracomorphs are generally recognized as the agnathans that are closest to the gnathostomes (see both Benton 2005 and Janvier 2008a). The structure of the brain, semicircular canal, a bony endoskeleton, dermal bone in and the placements of the major vessels and nerves in the skull, all appear to be consistent with those of the Placodermi. Also, like placoderms, the osteostracomorphs had cellular bone, pectoral fins with an endoskeleton, and an epicercal tail. |
| LITERATURE CITED Benton, M. J. 2005. Vertebrate Paleontology. Third Edition. Blackwell Publishing, Malden, MA. Gai, Z., P. C. J. Donoghue, M. Zhu, P. Janvier, and M. Stampanoni. 2011. Fossil jawless fish from China foreshadows early jawed vertebrate anatomy. Nature. 476: 324-327. 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. 2013. Led by the nose. Nature. 493: 169-170. Nelson, J. S. 1994. Fishes of the World. 3rd edition. John Wiley and Sons, Inc. New York. Nelson, J. S. 2006. Fishes of the World. 4th edition. John Wiley and Sons, Inc. New York. 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. Young, G. C. 1991. The first armoured agnathan vertebrates from the Devonian of Australia. In: M. M. Chang, Y. H. Liu, and G. R. Zhang, eds. Early vertebrates and related problems of evolutionary biology. Science Press, Beijing. pp. 67-85. |
| By Jack R. Holt and Carlos A. Iudica. Last revised: 01/16/2014 |
