Liu et al.—Exceptionally preserved Ordovician conodont apparatuses
503
Figure 6. Schematic illustration of the apparatus of the holotype SUI 139888 (WS18-266) of Iowagnathus grandis n. gen. n. sp. showing the element locational notations used in the present study. The apparatus is complete, but the left P1, the right P2, and both M elements are partly embedded in the shale matrix. Although not shown in this figure, the S3 elements have an anterior process, as seen in the paratype illustrated in Figure 5.1.
and a schematic illustration of the element notation is provided in Figure 6. Representative elements are illustrated in Figures 7 and 8.7–8.17. A schematic model of the apparatus elemental distribution is shown in Figure 9.2. The ramiform, asymmetrical to more or less symmetrical
paired M, S, and P elements of this species are not conspicuously different in general morphology. The unpaired, essentially symmetrical, and comparatively large S0 element, however, has a highly distinctive alate shape that is reminiscent of the corresponding element in the Devonian genus Apatognathus Branson and Mehl, 1934. All the elements of Iowagnathus grandis n. gen. n. sp. are
multidenticulated, and one of the denticles of each element is usually prominent in size and/or shape. For descriptive purpose, we call this denticle a cusp to distinguish it from others, although a cavity is absent. The S0 element (Figs. 5, 6, 7.1, 8.7–8.9) has a small but identifiable cusp and two symmetrically placed lateral processes that form an angle, which varies from acute to obtuse, or form a wide curve (Fig. 8.7–8.9). There is no anterior or posterior process. The denticles are straight or slightly reclined, of nearly the same length (~0.5mm), and sharply pointed and rounded in cross section. However, the two denticles on each lateral process adjacent to the cusp are significantly shorter than the others, resulting in a small gap on each side of the cusp (see Fig. 8.7, 8.9). The length of the lateral
processes of the S0 element is somewhat variable: the widely arched elements usually have somewhat shorter processes
(commonly <5mm long) and fewer denticles (usually <30). However, a total of 48 denticles were counted in the specimen shown in Figure 8.7. Another S0 element (lower specimen in Fig. 8.8) has a lateral width more than 16mm, making it one of the largest conodont elements ever recorded. The 14 paired elements in the I. grandis n. sp. apparatus are
roughly similar in morphology, being ramiform with a central cusp and two lateral processes. However, whereas the S1,P2, and M elements have only two lateral processes, the S2,S3,S4, and P1 elements have a well-developed denticulated anterior process extended from the cusp (Fig. 7). The anterior process is hidden in the sediment in some specimens, but its presence is revealed in posterior view by a more or less rectangular extension of the basal body (Figs. 6, 7.5). The S2 through S4 elements, which usually have anterior processes, can be identified based on minor differences in shape and size. For example, the S3 element usually is smaller and has fewer denticles than other S elements in the apparatus. The paired P1 and P2 elements are essentially similar to the S elements with multidenticulate processes, but the P1 element is nearly symmetrical and with well-developed denticulated anterior process, and the P2 element is slightly asymmetrical with only two processes. Morphologically the P2 element is close to the S3
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