DESCRIPTION OF THE SUPERCLASS THELODONTOMORPHI+ (JAEKEL 1911)
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SUPERCLASS THELODONTOMORPHI LINKS
| Thelodontomorphi (THE-lo-don-ta-MOR-fe) is derived from three Greek roots that mean “nipple-toothed forms” [nipple- theli (θηλή), tooth- donti (δόντι), form- morphi (μορφή)]. The reference is to jawless fishes whose pharynx was covered with small tooth-like structures. The name was coined by Jaekel (1911) but used in the current sense by Nelson (2006). |
| INTRODUCTION TO THE THELODONTOMORPHI The thelodonts were similar to the Anaspidomorphi in that they had hypocercal tails and had terminal mouths (Figures 1 and 2). They differed in being marine animals that were generally dorso-ventrally compressed (though the fork-tailed taxa tended to be laterally-compressed). Their bodies were covered with placoid scales, and they had pectoral fins. Most seem to have been adapted as bottom-dwellers; however, the fork-tailed thelodonts (the Furcacaudiformes) likely were adapted to life in the open water with laterally-compressed bodies, large tails, and large eyes. The most striking feature of the thelodonts is the lack of body armor or obvious imbricate scales. Instead, their bodies were covered with placoid scales, the same types of scales that cover sharks. Each placoid scale had the same general structure as a tooth with an inner pulp covered by dentine, a type of bone (Figure 3). The outermost layer was enamel. The scales were quite numerous during the periods and in the environments where thelodonts lived. So, although articulated thelodont remains are rare, the scales are common enough to serve as stratigraphic markers from the Silurian to lower Devonian. The pectoral fins were paired structures that had no internal fin rays, structures that did occur in the in-line fins, especially the caudal fins. Thus, they might not be homologous with the pectoral fins of gnathostomes. The positions of the pectoral fins varied also. Longiella had pectoral flaps that were on top of the gill slits (Figure 1) while the furiciforms (Figure 2) had elongate fin-like structures that inserted below the gill slits. Thelodonts were strong swimmers and most had large eyes. The furciforms were flattened laterally suggesting that they were fish of the open water. Given that these animals were jawless, what did they eat? Because they were clearly active, the assumption is that they must have been predatory. Perhaps they fed on soft-bodied worms. Their feeding habits might be inferred from the occurrence of tooth-like structures in the mouth and pharynx. Furthermore, Wilson and Cladwell (1993) reported that the thelodonts, at least the furciforms, had stomachs, which means that they were ingesting more than body fluids or bottom ooze. |
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| FIGURE 1. An illustration of Loganiella, a dorsoventrally flattened fish. Note large pectoral fin over the gill openings. Image from: http://www.palaeos.com/Vertebrates/Units/050Thelodonti/050.800.html | FIGURE 2. A furciform thelodont. Note the deep body, almost homocercal tail, and pectoral fin. Image from: http://www2.biology.ualberta.ca/wilson.hp/mvhw/thelodont.html | FIGURE 3. A section through a thelodont scale which shows the pulp chamber (P), dentine (D), and enamel. Image from: Karatajute-Talimaa (2002) |
FIGURE 4. A cladogram of the agnathan craniates showing the relationship of the Thelodontomorphi relative to the other taxa. This topology is derived from Benton (2005), who considers the thelodonts to be a sister group to the Anaspidomorphi in a clade. Neither Nelson (2006) nor Janvier (2008a) consider these taxa to be grouped together this way.
| Benton (2005) also grouped the anaspidomorphs together with the thelodonts because both had terminal mouths, hypocercal tails, body scales, and pectoral fins (see Figure 4). Neither the character of terminal mouth nor of hypocercal tail is a shared derived character. Also, 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. The anaspidomorphs had a pair of pectoral spines while the thelodonts had true fin-like structures. All in all, anaspidomorphs and thelodonts superficially resembled each other. Neither Nelson (2006) nor Janvier (2008a) grouped the thelodonts and anaspidomorphs together. |
| LITERATURE CITED Benton, M. J. 2005. Vertebrate Paleontology. Third Edition. Blackwell Publishing, Malden, MA. Karatajute-Talimaa, V. N. 2002. Lower Devonian (Lochkovian) thelodonts from October Revolution Island (Severnaya Zemlya Archipelago, Russia). Geodiversitas. 24(4): 791-804. JaeKel o. 1911. — Janvier P. 1977. — Contribution à la connaissance d Jaekel, O. 1911. Die Wirbeltiere. Eine Übersicht über die fossilen und lebenden Formen. Borntraeger. Berlin. 252 pp. 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. Wilson, M. V. H. and M. W. Caldwell. 1993. New Silurian and Devonian fork-tailed ‘thelodonts’ are jawless vertebrates with stomachs and deep bodies. Nature. 361: 442-444. |
| By Jack R. Holt and Carlos A. Iudica. Last revised: 02/04/2018 |
