Liu et al.—Exceptionally preserved Ordovician conodont apparatuses
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Figure 3. Camera-lucida drawings of the three types of elements in the Archeognathus primus apparatus: (1)P1 element, SUI 139884 (WS14-522); (2)P2 element, SUI 139885 (WS12-382); (3) S element, SUI 139886 (WL13). Scale bar = 2mm.
Material.—Complete or nearly complete apparatuses: SUI 102853,WL143,WL238,WL240,WL281,WS16-1, WS17-34; incomplete apparatuses: WL70, WL213, WL227, WL229, WL256, WL277, WS10-48, WS10-61, WS12-199, WS13-589, WS15-342, WS16-540; individual elements: WL13, WL58, WL211,WL225,WS12-251,WS12-382,WS14-522,WS16-386.
Diagnosis.—The same as for the genus above.
Occurrence.—Darriwilian to Sandbian (Middle to Upper Ordovician) ofNorth America and possibly the Siberian Platform.
Description.—Representatives of Archeognathus primus are some of the most common conodonts in the Winneshiek fauna and they are preserved both as individual elements and as apparatuses. A complete apparatus of this taxon (Figs. 2.1–2.3, 4, 9.1) includes six elements in three pairs, one of coleodiform and two of archeognathiform elements, all of them with robust basal bodies. The elements in each pair appear to be mirror images of each other, reflecting a bilateral symmetry. Locations of the three paired elements are consistent in its apparatus as shown by Figure 2.1 and 2.2 and illustrated by Figure 4. The complete apparatuses of A. primus from theWinneshiek
purposes, we use conventional terminology. Based on their position at one end of the apparatus, as well as their multidenticulated morphology, it is likely that the single pair of coleodiform elements in the A. primus apparatus represents a highly modified S element. On this basis, the two pairs of archeognathiform elements can be designated P elements and are here referred to as P1 (Pa) and P2 (Pb) elements (Figs. 3, 4) occupying a caudal position according to the scheme of anatomical notation by Purnell et al., (2000). The coleodiform S element of A. primus resembles
Shale are the first of this taxon discovered anywhere in theworld. Because the element morphology and architecture of the apparatus differ greatly from those previously described from theOrdovician, applying the location systemof Sweet (1988; also see Purnell et al., 2000), which is now commonly used in conodont multielement classification, is not straightforward. In the collections at hand, we have failed to find any typical M elements in this type of apparatus. The absence of such an element is not a unique feature and has been reported in a variety of Ordovician multielement taxa, as well as in taxa of other ages. Although several complete apparatuses of A. primus occur
in our collection, it is still a challenge to recognize the orientation of the apparatus in the animal without evidence of the soft parts. However, the general location of these pairs of elements is consistent in these apparatuses. For descriptive
a slightly curved and elongated saw blade and can reach a length of >10mm in the Winneshiek specimens. The degree of curvature of this element varies (Figs. 2.1, 2.2, 3.3, 8.4). Numerous, short, slightly reclined in the same direction, and densely packed (8–10 per mm) denticles form a uniform row along the convex margin. No cusp is present in either the left or right representatives of this pair of elements. The basal groove has a robust and thick basal body. In some specimens, an ear- like projection (indicated by the red arrow in Fig. 8.4) occurs near the posterior end of the concave side of the basal body of the element. This type of element is always the largest one in its apparatus (the example illustrated in Fig. 3.3 is 12.5mm long). The archeognathiform P1 and P2 elements in the apparatus
of A. primus are smaller than the S element and are typically 2–3mm long. Klapper and Bergström (1984), however, recorded specimens of this element 5–7mm long and noted that they had seen specimens twice as large. Because they described the morphology of these elements in considerable detail, we restrict our description here to a few details. The P1 and P2 elements usually have 10–16 prominent, somewhat cone-shaped, denticles. Costae may occur on some denticle surfaces. These denticles are well separated and arranged in a row along the margin of the crown of the element. They are much longer and thicker than those in the S element with a length of ~0.5mm that may vary along the denticle row. In many specimens, slightly shorter denticles occur between the larger ones, but none is developed as a distinct cusp. In several apparatus specimens, the denticles of the P elements point in the
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