Danielsen and Over—Lower Silurian conodonts, Shine Jinst region, Mongolia
2005 Distomodus kentuckyensis; Dahlqvist and Bergström, p. 16, fig. 12(l).
2007b Distomodus kentuckyensis; Zhang and Barnes, p. 507, fig. 9.17–9.21.
2012 Distomodus kentuckyensis; Bergström et al., fig. 7G.
Holotype.—Distomodus kentuckyensis (M element), University of Missouri C675-1, locality 1970, Brassfield Formation, Madison County, Kentucky, USA (Branson and Branson, 1947, pl. 81, figs. 40, 41).
Occurrence.—Occurs globally in the Distomodus kentuckyensis assemblage Zone that is typically correlated with early Rhuddanian to late, although not latest, Rhuddanian strata (Cramer et al., 2011). Distomodus kentuckyensis typically ranges from the lowest Rhuddanian through the Aeronian (Dahlqvist and Bergström, 2005), although it has been documented in the latest Ordovician across the Ordovician- Silurian boundary (Bergström et al., 2012), and terminates at the D. staurognathoides datum (Stott and von Bitter, 1999).
Description.—P1 element: Platform outline is generally cruciform, with a slight curve in the transverse axis concave
toward the posterior. Longitudinal processes typically more elongate than lateral processes. All processes develop irregular, coalesced nodes and ridges. Entire aboral surface is deeply excavated (Rexroad, 1967). P2 element: Cusp is straight or slightly recurved, with a
sharp anterior margin and a rounded or angular posterior margin. A single process extends from the anterior base of the cusp, directed downward, can be twisted laterally, and bears subround to laterally compressed, partially fused denticles. Posterior-lateral base of the cusp flares broadly, creating an expanded shallow basal cavity that is deepest under the cusp and extends under the anterior process (Rexroad, 1967). M element: Cusp is straight or curved, increasing in
convexity toward the inner side of the base, with sharp or blunt anterior and posterior margins; specimens from Yamaan Us typically show a sharp anterior margin on the inner lateral side; straight anterior process extends downward from the cusp to a sharp point; anterior and posterior processes can bear one or two small, compressed denticles; basal cavity is wide and deep beneath the cusp, and extends as grooves beneath both processes (Branson and Branson, 1947; Rexroad, 1967;
85
Zhang and Barnes, 2002). Some specimens from Yamaan Us have an outer lateral costa (Fig. 3.6); anterior and posterior costae were described previously by Rexroad (1967). S0 element: Symmetrical element nearly triangular in cross
section. Curved cusp can be slightly twisted, and bears two lateral and one posterior costae that give rise to downward- directed, denticulate lateral and posterior processes. Cusp is deeply excavated; basal cavity extends under the processes (Rexroad, 1967; Aldridge, 1972).
Remarks.—P1 elements recovered are fragmented, yet display the anterior, posterior, and lateral processes. Specimens illu-
strated in Figure 3.1 and 3.2 might be pieces of a single element. Cooper (1977) discussed the difference betweenDistomodus
kentuckyensis and D. staurognathoides that can be seen in the P elements. The P1 elements are easily distinguished because D. kentuckyensis will only have four processes whereas D. staurognathoides will have five or more. The recovered elements are fragmented, making classification based on the number of processes difficult, yet ornamentation of the processes show distinct differences between the two taxa. Distomodus staurognathoides typically displays an irregular network of ridges, and some processes develop a carina with discrete nodes (see Bischoff, 1986); specimens figured byAldridge (1972) show rows of well-developed denticles on reduced platforms. Distomodus kentuckyensis develops more robust ornamentation of irregular nodes that can coalesce to formridges (see Zhang and Barnes, 2002 for Pa element synonymy). The S and M elements of Distomodus kentuckyensis and
D. staurognathoides from Anticosti Island were distinguished by patterns in microstriations by Zhang and Barnes (2002). The striations in D. staurognathoides are more regular and pronounced, whereas those in D. kentuckyensis are discontin- uous and of low relief. The microstriations of D. kentuckyensis specimens from Yamaan Us are comparable to those recovered from Anticosti Island (Fig. 3.7c).
Distomodus sp. Figure 3.8
Remarks.—Several fragmented S elements of Distomodus sp. were recovered. Some specimens (e.g., Fig. 3.8) might be unique Distomodus elements. They have clear microstriations but are not comparable to either D. kentuckyensis or D. staurognathoides striations from Anticosti Island (Zhang and
Figure 3. Scanning electron microscope images of Llandovery conodonts from the upper Zalaa and lower Sharchuluut formations at Yamaan Us. Collection stored at the Research Center for Stratigraphy and Paleontology of the Mongolian University of Science and Technology, with call numbers CND/09-01–CND/
(a), oblique inner lateral view (b), outline indicating view in Fig. 3.7c, magnification of Fig. 3.7b to show striations (c), CND/09-07, YU-G. (8) Distomodus sp., S element, inner lateral view (a), outline indicating view in Fig. 3.8b, magnification of Fig. 3.8a to show striations (b), CND/09-08, YU-D. (9) Pseudooneotodus beckmanni, upper view (a), oblique lower view (b), CND/09-09, YU-I. (10) Panderodus serratus, lateral view, CND/09-10, YU-D. (11) Panderodus unicostatus, lateral view, CND/09-11, YU-N. (12, 13) Panderodus sp., lateral view; (12) CND/09-12, YU-D; (13) CND/09-13, YU-G. (14) Icriodella sp., P1 element, upper view (a), lower view (b), CND/09-14, YU-G. (15–18) Pseudolonchodina expansa; (15) S0 element, oblique lower view, CND/09-15, YU-G; (16) S0 element, posterior view, CND/09-16, YU-G; (17) P2 element, posterior view, CND/09-17, YU-D; (18) M element, inner lateral view, CND/09-18, YU-G. (19) Pseudolonchodina sp., S element, lateral view, CND/09-19, YU-D. (20–24) Pranognathus siluricus; (20) P2 element, outer lateral view (a), inner lateral view (b), CND/09-20, YU-G; (21) P2 element, upper view, CND/09-21, YU-G; (22) P1 element, lower view, CND/09-22, YU-G; (23, 24) P1 elements, lateral views (a), upper views (b); (23) CND/09-23, YU-G; (24) CND/09-24, YU-G. (25) Ozarkodina sp., P2/S element, lateral view, CND/09-25, YU-D. (26–28) Ozarkodina hassi; (26, 27) P2 elements, lateral views; (26) CND/09-26, YU-Q; (27) CND/09-27, YU-G; (28) P1 element, lateral view (a), upper view (b), CND/ 09-28, YU-D. Scale bars=100 µm.
09-28. Call numbers are followed by sample name: (1–7) Distomodus kentuckyensis; (1, 2) P1 elements, upper views (a), lower views (b); (1) CND/09-01, YU-Q; (2) CND/09-02, YU-Q; (3) P1 element, upper view, CND/09-03, YU-Q; (4) P2 element, posterior view (a), anterior view (b), CND/09-04, YU-G; (5) S0 element, posterior view (a), lateral view (b), CND/09-05, YU-G; (6) M element, outer lateral view, CND/09-06, YU-F; (7) M element, outer lateral view
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