fOrder ANASPIDIFORMES (Birkeniae). Six to 15 or more pairs of external lateral gill openings; branchial region posteriorly placed with first gill pouch well behind eye (as in lampreys); eyes large and lateral; tail hypocercal with large epichordal lobe (perhaps in part or entirely corresponding to the posterior dorsal fin); anterior dorsal fin absent, but a series of dorsomedian scutes present; unique pectoral spines or rods present; anal fin reduced or absent; body usually covered with dorsoventrally elongated ornamented scales (which are virtually absent in Lasanius); body fusiform and somewhat compressed; mouth terminal; complex dermal head armor present in some; bone cells absent. Maximum length about 15 cm. Silurian (primarily Upper Silurian, although some Late Devonian taxa, e.g., Endeiolepis of Quebec, may be anaspidiform), predominantly freshwater.
Genera include Birkenia, Lasanius, Pharyngolepis, and Rhyncholepis (e.g., Arsenault and Janvier, 1991; Janvier, 1996). Jamoytius and Euphanerops are regarded as sister taxa to the Anaspida (Donoghue et al., 2000).
This group is known primarily from isolated micromeric scales, important for stratigraphic correlations (e.g., Soehn et al., 2001; Turner, 2004; and discussion above under "VERTEBRATES"), although many near complete body fossils are known. Most thelodonts are depressed, with horizontal mouth, asymmetrical tails, one dorsal fin and paired pectoral fin flaps, but species of Furcacaudiformes are compressed, have near tubular mouths, and have a nearly symmetrical tail. Upper Ordovician to Upper Devonian (Turner, 1992). Ordovician genera include Sandivia (Karatajute-Talimaa,1997) and Stroinolepis (Marss and Karatajute-Talimaa, 2002).
There are questions on the monophyly of thelodonts and their interrelationships. For example, Wilson and Caldwell (1998) placed thelodonts less the Furcacaudiformes in a polytomy with gnathostomes, and the Furcacaudiformes were regarded as a sister group to all. Turner (1991) and others suggested that thelodonts and gnathostomes are closely related. Both taxa share features such as lateral line continuing on body, lining of buccal cavity, pharynx and branchial skeleton with denticles and complex platelets; and Marss and Ritchie (1998) noted that Shielia taiti and Lanarkia species have pelvic fin flaps and epicercal tails, respectively. Donoghue and Smith (2001) found Turinia pagei and the Galeaspida to be sister taxa, and the sister group to the Osteostraci plus jawed vertebrates. In their phylogenetic analysis, Donoghue and Smith (2001) also regarded the thelodonts with a depressed body as a monophyletic group, of which T. pagei was the least derived member. The furcacaudiforms were resolved as an unnatural group, one taxon
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