DESCRIPTION OF THE CLASS PLACODERMI+ (MCCOY 1848)
| EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>BILATERIA>DEUTEROSTOMATA>VERTEBRATA>GNATHOSTOMATA>PLACODERMI |
CLASS PLACODERMI LINKS
| The Placodermi (PLAK-o-DER-me) is derived from two Greek roots which means “plated skin” [plate- plaka (πλάκα), and skin- derma (δέρμα)]. The name, which was coined by McCoy (1848), refers to the plated armor bone that covered the head and much of the thorax. |
| INTRODUCTION TO THE PLACODERMI The placoderms were the earliest vertebrates to have true jaws, and, therefore, were the first true gnathostomes. They flourished from the Silurian to the end of the Devonian (359-443 MYA). They were heavily-armored with the head and thorax covered by fused bony plates made of dentine. Usually, the head shield articulated with the thoracic armor to allow for movement of the jaws. The rest of the body generally was naked or covered by small scales, usually hexagonal and tuberculated. The gill chamber was shielded by a movable operculum, much like that of the bony fish. The endoskeleton was mineralized as perichondrial bone, as in the osteostracans. Also, like the osteostracans, the placoderms had pectoral fins with well-developed endoskeletons. Usually, the pectoral girdles were robust and anchored to the thoracic shield. Some taxa had armored plates which articulated directly with the thoracic dermal armor. Bothriolepis (Figure 1) had pectoral fins that were so heavily armored that it looked like a crab. Some of the groups also had pelvic fins, but they were not armored, and the pelvic girdle was anchored only in the body wall of the animal. The in-line fins of the placoderms included an epicercal tail and a dorsal fin. The placoderms seem to have evolved in two directions. Most of them were bottom dwellers with dorso-ventrally flattened bodies and subterminal mouths (Figures 1-2). Indeed, many of the groups resembled taxa of the Chondrichthyes like the skates, rays, and chimeras. However, most of the arthrodires (Figures 3 and 4) were adapted to open water with laterally-compressed bodies, large eyes, and terminal mouths of shearing bony plates. Despite the diversity in form within the placoderms, Goujet and Young (2005) argue that they are monophyletic and share five synapomorphies: – a joint between the head shields and thoracic shields, and a thoracic shield that also serves as the pectoral girdle – the same pattern of dermal bones in the skull roof and cheek covers – simple jaws with bony tooth plates – the same type of opercular structure – the same type of derived dentine (called semidentine) in the bone of the exoskeleton. According to Goujet and Young (2004) and Janvier (1996a), the order Acanthothoraciformes (literally meaning spined thorax forms) was the stem group from which the diversity of the other orders sprang. Thus, it is more properly understood to be paraphyletic. Nelson (2006) also notes that the primitive nature of the group implies that the first jawed vertebrate was contained within this group, most of which were flattened bottom-dwellers with very short thoracic armor. Janvier (1996a) notes that although they show the range of diversity that one might expect from a paraphyletic group, they all have a brain case (neurocranium) that is very similar to that of the osteostracoderms. Both Benton (2005) and Janvier (2008a) illustrate the pivotal position of the placoderms between the most derived of the agnathans and the other gnathostomes (see Figure 5). Likely, they evolved from an osteostracoderm with which they share the structure of the neurocranium, the armored head shield, pectoral fins, and epicercal tail. The main distinction is the advent of the jaw. |
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| FIGURE 1. Bothriolepis, a dorso-ventrally flattened bottom dwelling placoderm. Note the armored head and thoracic shields. Also note the armored pectoral fins. Image from: http://bill.srnr.arizona.edu/classes/182/Placoderm.htm | FIGURE 2. Phyllolepis, a dorso-ventrally flattened bottom dwelling placoderm like Bothriolepis but it had pelvic fins. In addition, it was a blind predator. Note the armored head and thoracic shields. Image by: Dimitry Bogdanov | FIGURE 3. Dunkelosteus was an arthrodire that was a top predator in the Devonian seas. The tooth-like structures were shearing plates. Note the animal had both pectoral and pelvic fins. Image by: Nobu Tamura | FIGURE 4. Coccosteus was a small arthrodire that occupied freshwater and near-shore marine environments during the middle to late Devonian. Image by: Nobu Tamura |
FIGURE 5. A cladogram of the gnathostome fishes that illustrates the pivotal position of the Placodermi as a sister group to all other gnathostomes. It was taken from Benton (2005) and Janvier (2008a).
| ORIGIN OF THE GNATHOSTOMES According to Benton (2005), the placoderms were the first vertebrates to possess jaws, but where did jaws come from? The state of answers to this question can be summed up well by Kimmel et al. (2001) as “a wonderful problem, long debated in the literature”. The classical answer is that a jaw is a modified gill arch that lost its function as gill support and assumed the role as an articulated lower jaw. That was the solution presented by Mallatt (1996; Figure 6) who argued that the first gill arch moved forward in the mouth. These gill arches that occurred in the more derived agnathans, and still occur in the gnathostomes, are jointed. Mallat (1996) said that if such a jointed rod were joined by musculature, it would become an efficient method for ventilating the gills (and filtering food through the gills). A formal joint would make the structure more effective as a ventilator, which then could be co-opted as jaws in a protognathostome. Janvier (2006 a; 2007c; Figure 7), proposed a different solution. In short, the main problem with the classical theory is that there are no ostracoderms with adequate structure to suggest the transition of a gill to a jaw. The homology between the visceral arches of living lampreys and bony fishes also is in question (Janvier 2007c; and Kimmel et al. 2001). Clearly, the function of the structure(s) that evolved into jaws must have had something to do with the oral opening like the velum of the lamprey ammocoetes larva. The velum forms a fleshy wall that covers the esophagus while the animal is ventilating water through its gill pouches. Janvier (2007c) suggests that such a structure, if hardened and then covered with denticles (such as the scales of Chondrichthyes), would become a functional jaw. Still, the mystery remains. What is known is that jaws appeared relatively suddenly with the appearance of the placoderms and acanthodians in the Silurian. Furthermore, they coexisted with the Osteostracomorphi until the end of the Devonian. |
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| Figure 6. A sequence of events as presented by Mallatt (1996) which shows the more classical theory of the development of the first gill arch to become the jaws (both upper and lower. The internal gill arches typical of the more derived agnathans and the gnathostomes are segmented. Mallatt suggests that the first gill arch moved forward in the mouth with muscles attached to bend the gill arch in order to serve as a more efficient gill ventilator than the velum. The red is Meckel’s cartilage. |
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| Figure 7. A sequence of events as presented by Janvier 1996a and 2007c. The cartilaginous support for the velar skeleton as seen in the lamprey (and likely also occurring in the extinct armored agnathans) expanded forward in the mouth. Muscles attached to the cartilaginous bars served to ventilate the gills and, perhaps to force more water over the gills to be filtered. That structure then became co-opted as moveable jaws. |
| LITERATURE CITED Benton, M. J. 2005. Vertebrate Paleontology. Third Edition. Blackwell Publishing, Malden, MA. [L] Goujet, D. and G. C. Young. 2004. Placoderm anatomy and phylogeny: new insights. In: G. Arratia, M. V. H. Wilson, and R. Cloutier, eds. Recent Advances in the Origin and Early Radiation of Vertebrates. Verlag D. Friedrich Pfeil. München, Germany. pp. 109-126. Kimmel, C. B., C. T. Miller, and R. J. Keynes. 2001. Neural crest patterning and the evolution of the jaw. Journal of Anatomy. 199: 105-119. Janvier, P. 1996a. Early Vertebrates. Oxford Monographs in Geology and Geophysics, 33. Oxford University Press. Oxford. pp. 393. Janvier, P. 2007c. Homologies and evolutionary transitions in early vertebrate history. In: J. S. Anderson and H.-D. Sues, eds. Major Transitions in Vertebrate Evolution. Indiana University Press. Bloomington. pp 57–121. Janvier, P. 2008a. Early jawless vertebrates and cyclostome origins. Zoological Science. 25: 1045-1056. Mallatt, J. 1996. Ventilation and the origin of jawed vertebrates: a new mouth. Zoological Journal of the Linnean Society. 117:329-404. McCoy, F. 1848. On some new fossil fish of the Carboniferous period. Annals of the Magazine of Natural History. 2: 1-10. Nelson, J. S. 2006. Fishes of the World. 4th edition. John Wiley and Sons, Inc. New York. [L] |
| By Jack R. Holt and Carlos A. Iudica. Last revised: 01/16/2014 |
