tClass THELODONTI
being the sister taxon to the "conventional" thelodont clade, and the other, the sister taxon to this clade plus galeaspids, osteostracans, and jawed vertebrates. However, Donoghue and Smith (2001) had few relevant characters and few species in their analysis, and Wilson and Marss (2004) in their phylo-genetic study found evidence that the Thelodonti and the Furcacaudiformes are monophyletic.
I have chosen to follow Donoghue et al. (2000) on the higher classification of craniates and thus place the thelodonts here (with thelodonts as sister to Osteostracomorphi + Gnathostomata), but their position is uncertain, as these authors only used one thelodont taxon, Loganellia, in their analysis. The classification below is provisional and follows, as does the orthography, Marss et al. (2002); further changes are expected when cladistic studies such as Wilson and Marss (2004) are conducted with better material. Genera not placed in the following classification include Apalolepis, Stroinolepis, and Turinia (of worldwide distribution [Jiang, 1992]; the internal anatomy of T. pagei, with eight pairs of gills and a stomach, is especially well known but remains controversial [Donoghue and Smith, 2001]).
Order LOGANELLIIFORMES. Loganelliidae (= Loganiidae) (Illoganellia and Loganellia; Karatajute-Talimaa, 1997, revised this family and also placed in it Angaralepis, Paralogania, and Sandivia) and Nunavutiidae (Nunavutia).
Order SHIELIIFORMES. Shieliidae (Paralogania, Praetrilogania, and Shielia). Shielia taiti has paired fin flaps that are interpreted as pelvic fins (Marss and Ritchie, 1998).
Order PHLEBOLEPIDIFORMES. Phlebolepididae (Erepsilepis, Helenolepis, and Phlebolepis) and Katoporodidae (Goniporus, Katoporodus, Overia, and Zuegelepis).
Order THELODONTIFORMES (= Coelolepidiformes). Coelolepididae (Thelodus), Lanarkiidae (Lanarkia and Phillipsilepis), Archipelepididae (Archipelepis), Nikoliviidae (Chattertonodus and Nikolivia), Talivaliidae (Glacialepis and Talivalia), and provisionally Eestilepididae (Eestilepis). Marss and Ritchie (1998) suggested that Lanarkia horrida have epicercal, heterocercal tails, and tail fins have scale-covered ray-like supports (as with forktail thelodonts).
Order FURCACAUDIFORMES (forktail thelodonts). Body compressed, eyes lateral and large, branchial openings in an oblique row; stomach present (barrel-shaped); dorsal and ventrolateral fin flaps present in some; caudal fin with large dorsal and ventral lobes and scale covered ray-like fin supports. The lateral line branches to both lobes of tail. Wilson and Caldwell (1993) were the first to interpret a group of thelodonts as having compressed bodies, rather than depressed bodies as in other thelodonts. Furcacaudidae (Canonia, Cometicercus, Drepanolepis, Furcacauda, and Sphenonectris) and Pezopallichthyidae (Pezopallichthys) and provisionally Barlowodidae (Barlowodus and Sophialepis) (Wilson and Caldwell, 1998; Wilson and Marss, 2004; Marss et al., 2002).
+SUPERCLASS OSTEOSTRACOMORPHI
The osteostracomorphs (comprising the cephalaspidiforms or osteostracans, galeaspidiforms, and provisionally the poorly known pituriaspidiforms) are now considered to be the sister group to the jawed vertebrates (gnathostomes) by many researchers. Janvier (2001), in assuming that cephalaspidiforms and galeaspidiforms are the closest well-known outgroups to the gnathostomes and that ostracoderms as a group are more closely related to gnathostomes than to either hagfishes or lampreys, reconstructed the characters of various hypothetical ancestors of certain clades. More systematic work is required to present convincing arguments on possible gill-arch homologies with jaws to have a strong hypothesis on which agnathan group shared a common ancestry with the first jawed vertebrates. There is no evidence of gnathostome-like gill arches in cephalaspidiforms, and the sensory line system is restricted to the head.
Two semicircular canals; some bony regions in cephalaspidiforms may have true bone cells; single dorsomedian nostril (nasohypophyseal) opening between eyes with pineal eye behind except in the galeaspidiforms.
fOrder CEPHALASPIDIFORMES (Osteostraci). Dorsal and lateral areas of cephalic shield with depressed areas in exoskeleton and associated canals present (this may have been an electric or sensory organ); usually 10 pairs of gill chambers and 10 pairs of external ventral gill openings; branchial region anteriorly placed (first gill opening at least level with eye); eyes dorsal; sclerotic ring present; endolymphatic duct present; tail, assumed to be epicercal, heterocercal, with a pair of horizontal caudal flaps in ventral position; head with complex, ornamented, polygonal interlocking plates; body with dorsoventrally elongated ornamented scales; head depressed anteriorly, triangular posteriorly; body triangular in cross section; mouth ventral; pectoral fins, possibly homologous to gnathostome pectoral fins, present in some (e.g., the basal Ateleaspis) but absent in the derived tremataspids; long rostral process present in species of Boreaspis. Maximum length about 60 cm, but most are much smaller. Upper Silurian to Upper Devonian, predominantly freshwater. These are the best known of the fossil agnathans. This group is almost always known as the Osteostraci by paleontologists.
In a study of granular labyrinth infillings in such osteostracans as Waengsjoeaspis nahanniensis and Superciliaspis gabrielsei, Sahney and Wilson (2001) suggested that one function of the endolymphatic pore openings in osteostracans is similar to that in living chondrichthyans, namely that exogenous material gets into the labyrinth of the inner ear by entering through the endolymphatic pores.
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