508
Journal of Paleontology 91(3):493–511
gen., which occur between the M and P elements, suggest that they may have had a similar function as the P elements. The apparatus of Iowagnathus n. gen. also differs in
several significant respects from the reconstructed apparatuses of the Silurian genera Pterospathodus (Barrick and Klapper, 1976), Astropentagnathus (Armstrong, 1990), Apsidognathus (Armstrong, 1990), and Aulacognathus (Armstrong, 1990), indicating no close evolutionary relationships to these genera. Thus, both Archeognathus and Iowagnathus n. gen. appear to be isolated taxonomically from other genera. This taxonomic iso- lation is consistent with the fact that hyaline conodonts are not recorded from the latest Ordovician, although they are well represented in Middle and early Late Ordovician faunas.
Discussion
Exceptionally preserved apparatuses of Archeognathus primus and Iowagnathus grandis n. gen. n. sp. from the Winneshiek Shale provide unusual material for the reconstruction of their apparatus architecture and the determination of their affinity. In reconstructing apparatus architecture, it is obviously important to investigate the degree of post-mortem movement of individual elements. Where no movement has occurred, paired elements occupy consistent positions and the arrangement of the elements follows a particular pattern. Unfortunately, such spe- cimens have not been available for many described apparatuses and their architectures have been inferred based on available evidence. Complete conodont apparatuses are very rare in the lower Paleozoic and virtually all are preserved in two dimen- sions on bedding planes, except the remarkable Upper Devonian Palmatolepis apparatus described by Lange (1968, pl. 1). Although element movement may occur during preservation, many elements in bedding-plane assemblages of both Archeognathus and Iowagnathus n. gen. from the Winneshiek Shale are arranged in morphologically similar pairs, some of which are located close together. This provides strong indication of little movement of these elements, especially as consistent patterns are evident in several apparatuses. As described before, the two pairs of shorter arche-
ognathiform elements in the Archeognathus primus apparatus are regarded as homologous with the P elements in Lange’s Palmatolepis apparatus and in numerous preserved apparatuses from the Pennsylvanian of Illinois (Aldridge et al., 1987), and the elongated coleodiform elements as S elements. The spatial arrangement of elements in the A. primus apparatus is best displayed in the specimens shown by Figures 2.1 and 2.2, in which the elements are consistently arranged, from left to right in the pictures, as paired P1,P2, and S elements. The distribution pattern of elements in the apparatus of I. grandis n. sp. is represented by the specimens shown in Figure 5, in which the location of the paired M elements always at one end of the apparatus and the P elements at the other is reminiscent of many other genera (Purnell et al., 2000). In addition to the illustrated specimens, similar elemental orientations of both taxa repeatedly occur in our collection, revealing consistent patterns that provide useful information for apparatus architecture reconstructions. The unusual morphology of the hyaline archeognathiform
(P) elements in Archeognathus has prompted several authors to discuss their classification, affinity, morphology, and function.
of A. primus Cullison (1938, pl. 29, figs. 16a, b) and argued that its structure “is entirely different from the structure we always find in conodonts, and there is no reason to believe that Archeognathus, fish or no fish, is closely related to conodonts” (Lindström, 1964, p. 122). Many other authors also studied such fossils, but concluded that there is no morphological or other evidence that eliminates Archeognathus from conodonts. Sweet (1988, p. 123) considered such fossils, especially Archeognathus,to “clearly exhibit the internal structure of conodonts” and remarked that “the basal structures have no counterpart in the Conodonta. No relationship to other groups is apparent”, thus they “might represent a separate class of the Conodonta.” The detailed micro- and macromorphology of Archeognathus were studied by Klapper and Bergström (1984), and the results were consistent with a conodont affinity, but they noted that “the fibrous conodonts apparently are a rather spe- cialized group” (Klapper and Bergström, 1984, p. 973). This study supports the identity of the Winneshiek specimens of Archeognathus and Iowagnathus n. gen. as true conodonts with particular element structures and arrangements in their appara- tuses. However, the morphology and arrangement of elements in apparatuses of both taxa are significantly different from most euconodonts. Other groups, such as hagfish, also have a laterally operating feeding apparatus. Thus, future discoveries and investigations may clarify the phylogenetic classification of these unusual Winneshiek taxa. Archeognathus and Iowagnathus n. gen. share several
unusual morphological features, perhaps the most prominent one being the large basal bodies with their ridge-and-groove surface. Although the basal bodies of both taxa differ from those in other conodonts, as discussed above, we do not regard this as sufficient to separate them from Conodonta. However,
Based on its “jaw like” appearance, the archeognathiform element, which was designated as the holotype of A. primus, was referred to as a toothed fish jaw or that of some other vertebrate (e.g., Miller et al., 1947; Barskov et al., 1982). Branson and Mehl (1944) grouped conodonts with a “jaw structure” and fibrous denticles into the family Coleodontidae, to which they referred the genera Coleodus, Erismodus, and Neocoleodus. Based on an investigation of ultrastructures, Barnes et al., (1973) divided their Ordovician conodonts into two major groups, hyaline conodonts and cancellate conodonts. In addition to other microscopic features, these two groups are mainly characterized by the presence or absence of white matter. Klapper and Bergström (1984) considered the archeognathiform elements to have functioned as teeth and assigned them to conodonts, but they found no evidence of a vertebrate affinity. Based on the chemical composition and unusual fibrous struc- ture of the elements, these fossils were regarded as possible vertebrates by Rhodes and Wingard (1957). Barskov et al., (1982) and Sansom et al., (1994) suggested the presence of vertebrate bone structures in such fossils, especially Coleodus, Neocoleodus, and Chirognathus. All these studies were based on isolated elements, some of them incomplete. Because of their uncertain affinity, Clark et al., (1981) listed these fossils as family Unknown (Neocoleodus) or Problematic Taxa (Archeognathus and Coleodus), and Sweet (1988) listed Archeognathus under order Unknown. Lindström (1964) studied the plastic-embedded holotype
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