Liu et al.—Exceptionally preserved Ordovician conodont apparatuses The denticles of the 14 paired elements are more or less
reclined and mostly rounded in cross section, and costae or keels may occur, especially on the cusp. However, the denticles of the relatively short process of the M element are essentially blade- or plate-like in shape (Figs. 7.8, 8.17). All the paired elements have robust basal bodies. The
surface of the bodies has distinct ridge-and-groove structures that radiate from the center of the element (Figs. 7.2, 7.6, 8.11–8.13) similar to those described in Archeognathus primus. There are some variations in the morphology of the
elements of I. grandis n. sp., and they show interrelated changes in element size and denticle length. It appears that as the element became larger during ontogeny, the denticles became thicker but comparatively shorter. This might reflect denticle wear, but detailed study of the denticle surfaces is necessary to confirm this.
Remarks.—The erection of a new genus and species for this type of apparatus is based on the absence of any previously named taxon with identical elements. The incompletely known Microcoelodus festivus Moskalenko from the Podkamennaya Tunguska and Moyero Rivers region in Siberia (Moskalenko, 1970), which was based on a single specimen and only illu- strated in posterior view, is similar to the P2 element of the present species, but no associated elements of its apparatus have been recorded. Coleodus mirabilis Moskalenko, 1970 has an unusual morphology similar to that of the alate S0 element in Iowagnathus grandis n. gen. n. sp., but Moskalenko’s (1970) illustrations suggest that this element differs in being albid and having a different type of process denticulation. In general appearance, and particularly denticulation, I. grandis n. sp. is similar to Erismodus? horridus Harris, 1964 from the lower Joins Formation of Oklahoma. However, the illustrations of E.? horridus (Harris, 1964, pl. 1.2a–1.2c) show two types of S elements that clearly differ from those of I. grandis n. sp. in that the third process of the S elements extends posteriorly from the base of the cusp rather than anteriorly.
Comparison with other conodont apparatuses
As described above, the elemental component, morphology and structure of Archeognathus primus and Iowagnathus grandis n. gen. n. sp. are significantly different. For comparison, the hypothesized architectures of A. primus and I. grandis n. sp., essentially based on elemental distributions in their apparatuses, are shown in Figure 9.1 and 9.2. Among the small number of natural conodont assemblages described from the Ordovician, none is closely similar to those from the Winneshiek Shale. As far as we are aware, the Winneshiek specimens are the first complete apparatuses with hyaline elements described. Another apparatus with hyaline elements, which was identified as Erismodus quadridactylus, was recorded from the Ordovician
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of North Dakota, USA (Dhanda, 2004). Judging from the unpublished Ph.D. dissertation by Dhanda (2007), it is far less well preserved compared to the Winneshiek apparatuses and may not be complete. In general architecture, the 6-element apparatus of A. primus is unique although it may be interpreted as a much-modified version of the prioniodinid apparatus. In contrast, the 15 element I. grandis n. sp. apparatus has a general architecture of prioniodinid type, although individual elements differ markedly in shape from those of, for example, post- Ordovician ozarkodinid and polygnathacean apparatuses (e.g., Aldridge et al., 1987). Especially striking is the difference in the morphology and arrangement of the S elements. A detailed comparison with post-Ordovician apparatuses is
beyond the scope of the present paper, so we restrict ourselves to a discussion of two important examples. The first is Promissum pulchrum Kovács-Endrödy from the Upper Ordovician Soom Shale of South Africa (Theron et al., 1990; Aldridge et al., 1995), which was originally thought to be a primitive land plant (Kovács-Endrödy, 1987). Aldridge et al., (1995) recorded 19 elements in its apparatus. The second example, also from the Soom Shale, is Notiodella keblon Aldridge et al., 2013, which has 17 elements in the apparatus. According to Bergström and Ferretti (2015), Notiodella is congeneric with Icriodella Rhodes, 1953, and we use that designation here. The archi- tecture of the Icriodella apparatus (Aldridge et al., 2013, fig. 12) is similar to that of Promissum, except in the case of theMand P elements, so we do not illustrate it here. The general elemental distribution of the Archeognathus,
Iowagnathus, ozarkodinid, and Promissum apparatuses is schematically compared in Figure 9. The Archeognathus apparatus is much simpler than the others, with a strikingly different template (Fig. 9.1), especially in the presence of only one pair of coleodiform (S) elements, suggesting no obvious relationship to the others. The multidenticulated morphology of the S elements suggests a similar function to those in Promissum. The apparatus of Iowagnathus n. gen. (Fig. 9.2), in contrast, exhibits a general similarity in organization to both Promissum and the ozarkodinid types of apparatuses. However, Iowagnathus n. gen. has a different basic architecture in that the element pairs appear to be lined up one after the other in two parallel rows rather than being positioned essentially side-by- side as is the case with most of the elements in the ozarkodinid (Fig. 9.3) and Promissum (Fig. 9.4) apparatuses. Also, the morphology of the M, S, and P elements, in particular, is too different to suggest any close relationship between these taxa. In general, the template of the Iowagnathus n. gen. apparatus shows a somewhat closer resemblance to the ozarkodinid type (cf., Mashkova, 1972; Purnell and Donoghue, 1998; Donoghue et al., 2008), which is best known from post-Ordovician strata, than to that of Promissum. As indicated by multiple apparatuses and specimens with paired elements from the Winneshiek Shale, the morphologically similar S1 to S4 elements in Iowagnathus n.
Figure 9. Comparison of the architecture of the 6-element apparatus of Archeognathus primus (1), the 15-element apparatus of Iowagnathus grandis n. gen. n. sp. (2), the 15-element apparatus of ozarkodinid type (3), and the 19-element apparatus of Promissum pulchrum (4): Figures (3) and (4) are after Aldridge et al., (1995), Donoghue et al., (2008), and Aldridge et al., (2013). All these figures are based on interpretation of bedding-plane assemblages. The figures of the I. grandis elements are mostly based on SUI 139888 (WS18-266), in which several individual element pairs are readily recognizable. The estimated length of the illustrated apparatuses of A. primus and I. grandis is approximately 20mm, but an incomplete I. grandis apparatus SUI 139895 (WS13-1) has a length more than 31mm with individual elements up to 16mm long, suggesting that much larger apparatuses were present. The occurrence of some element pairs in clusters, and the symmetrical arrangement of elements, indicate that they likely had a food-handling function.
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