Danielsen and Over—Lower Silurian conodonts, Shine Jinst region, Mongolia


to occur in both shallow and deep water facies by Idris (1984), and Distomodus was considered characteristic ofmoderate depth by Dahlqvist and Bergström(2005). Because the present fauna is typical of both deep and shallow water facies, deposition most likely occurred atmoderate depth in a dynamic environment.This is supported by the lithology and associated fauna of the upper Zalaa where the shale and thin carbonates mark the transition from a deep shelf-basin to shallower shelf setting. The cosmopolitan nature of the fauna also indicates connection and circulation with the open ocean, thus, in light of the present conodont fauna, the Zalaa and Sharchuluut formations were deposited in an open basin-shelf environment.


Systematic paleontology


Apparatus notation is based on that of Purnell et al. (2000), although terms to describe orientation (e.g., anterior, posterior, inner, outer, upper, and lower) are used conventionally (Sweet, 1981, 1988) and do not refer to true biological orientation. Descriptions are based on both Yamaan Us specimens and works cited herein. The collection is stored at the Research Center for Stratigraphy and Paleontology of the Mongolian University of Science and Technology. Figured specimens are identified with call numbers CND/09-01–CND/09-28.


Family Balognathidae Hass, 1959


Genus Pranognathus Männik and Aldridge, 1989 Type Species.—Amorphognathus tenuis Aldridge, 1972.


Pranognathus siluricus (Pollock, Rexroad, and Nicoll, 1970) Figure 3.20–3.24


1970 Aphelognathus siluricus Pollock, Rexroad, and Nicoll, p. 749, pl. 114, figs. 1–4.


1972 Aphelognathus siluricus; Aldridge, p. 165, pl. 3, fig. 3. 1977 Llandoverygnathus siluricus; Cooper, p. 1064, pl. 1, figs. 2–4, 8–10, pl. 2, figs. 4, 7 (see for synonymy).


1981 Pterospathodus siluricus; Uyeno in Uyeno and Barnes, pl. 1, fig. 1.


1983 Pterospathodus siluricus; Uyeno in Uyeno and Barnes, p. 25, pl. 1, figs. 7–13.


1989 Pranognathus siluricus;Männik and Aldridge, p. 904. 2005 Pranognathus siluricus; Dahlqvist and Bergström, p. 16, fig. 12c–e.


Holotype.—Aphelognathus siluricus (P1 element), Indiana University/Indiana Geological Survey 12511, Neahga Shale,


locality 16, sample 6, Niagara Falls, Ontario, Canada (Pollock et al. 1970, pl. 114, fig. 2).


Occurrence.—Pranognathus siluricus has only been described from Aeronian strata (Aldridge, 1972; Cooper, 1977; Uyeno and Barnes, 1983; Dahlqvist and Bergström, 2005) corresponding to the Pr. tenuis Zone of Cramer et al. (2011). Pranognathus siluricus terminates below the Distomodus staurognathoides datum (Stott and von Bitter, 1999).


Description.—P1 element:Blade is slightly arched, widens at the base, and consists of a slightly larger cusp and typically 11–14


83


subequal, subrounded denticles that form an arch in lateral view. In oral view, the posterior process is offset, and small, unorna- mented, offset lateral processes are visible. Entire aboral surface is deeply excavated (Pollock et al., 1970; Aldridge, 1972). P2 element: Blade consists of a short, stout central cusp with


denticulate anterior and posterior processes, equal or unequal in length, which are downwardly directed and flare to create a wide base. Denticles are either discrete or almost entirely fused to the apices. A low ridge directed anteriorly on the outer lateral face of the cusp can be considered a third or rudimentary process that is usually unornamented, as in the Yamaan Us specimens, but can be serrated. Occasionally a small, serrated ridge forms on the posterior inner lateral face. Entire aboral surface is wide and deeply excavated (Aldridge, 1972).


Remarks.—Pranognathus siluricus was first described by Pollock et al. (1970) as a species of Aphelognathus Branson, Mehl, and Branson, 1951, which is an Ordovician taxon. Cooper (1977) placed A. siluricus in Llandoverygnathus Walliser, 1972, which was considered a junior synonym to Pterospathodus Walliser, 1964 by Klapper and Murphy (1975) and Barrick and Klapper (1976). Uyeno in Uyeno and Barnes (1981) therefore assigned the taxon to Pterospathodus. Männik and Aldridge (1989) then described Pranognathus, a newly recognized genus encompassing three previously described taxa with potential ancestry to Pterospathodus, including


Aphelognathus siluricus,


Amorphognathus tenuis Aldridge, 1972, and Pterospathodus posteritenius Uyeno and Barnes, 1983. Uyeno and Barnes (1983) described the P1 element of


Pranognathus siluricus as having a basal margin flare only on the outer side, and compared this with Pterospathodus celloni (Walliser, 1964) that exhibits lateral flares on both sides. This description of Pr. siluricus differs from the description of


the holotype by Pollock et al. (1970) that specifically describes flares on both lateral faces. Cooper (1977) and Männik (1998) discussed that P elements of Pranognathus differ from Pterospathodus in having a wide, deep basal cavity. When compared with the holotype of Pt. celloni, Pr. siluricus has a wider, deeper basal cavity. Pranognathus siluricus can have less prominent lateral processes, although there is variation in shape and extension. This can be seen in the Yamaan Us specimens (Fig. 3.23, 3.24).


Family Distomodontidae Klapper, 1981 Genus Distomodus Branson and Branson, 1947


Type species.—Distomodus kentuckyensis Branson and Branson, 1947.


Distomodus kentuckyensis Branson and Branson, 1947 Figure 3.1–3.7


1947 Distomodus kentuckyensis Branson and Branson, p. 553, pl. 81, figs. 21–23, 27, 29–33, 36–41.


1996 Distomodus kentuckyensis; Bergström and Bergström, fig. 9D, 9E, 9H?.


2002 Distomodus kentuckyensis; Zhang and Barnes, p. 13, fig. 14.18–14.26 (see for synonymy).


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