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Journal of Paleontology 90(1):78–91
In more recent global zonation schemes (Cramer et al., 2011; Melchin et al., 2012), there is a degree of uncertainty concerning the correlation of the expansa Zone to the graptolite zonation, although it is still found to span the Rhuddanian-Aeronian boundary. Pseudolonchodina expansa has also been reported fromCanada (Zhang and Barnes, 2002),Alaska (Dumoulin et al., 1997), Estonia (for references, see Männik and Malkowski, 1998), Poland (Männik and Malkowski, 1998), Russia (Männik et al., 2009), Australia (Simpson and Talent, 1995), and Spain (Sarmiento et al., 1994). The Distomodus kentuckyensis Zone is commonly assigned
to the strata below the first occurrence of Pseudolonchodina expansa, making D. kentuckyensis the lowermost Rhuddanian biozone in the global zonation scheme compiled by Cramer et al. (2011). Distomodus kentuckyensis has been reported from North America (for references, see Zhang and Barnes, 2002), the United Kingdom (Aldridge, 1972, 2002), Sweden (Dahlqvist and Bergström, 2005), Norway (Aldridge and Mohamed, 1982), and Russia (Zhang and Barnes, 2007b). It frequently ranges from the lowest Rhuddanian into the Aeronian. Distomodus kentuckyensis has been recorded across the Ordovician-Silurian boundary in Sweden (Bergström et al., 2012), and terminates before the first appearance of D. staurognathoides (Walliser, 1964) (see Stott and von Bitter, 1999; see synonymy for additional references). Ozarkodina hassi is a characteristic species of the
Distomodus kentuckyensis Zone (Aldridge, 1972; McCracken and Barnes, 1981). The first occurrence of O. hassi [and/or O. oldhamensis (Rexroad, 1967)] was considered to be indicative of the Ordovician-Silurian boundary by McCracken and Barnes (1981), and O. hassi is the nominate species of the lowest Llandovery biozone from Anticosti Island (Zhang and Barnes, 2002), as well as from Sweden (Bergström et al., 2012). Bergström et al. (2012) reported occurrences of O. hassi in the latest Ordovician, across the Ordovician-Silurian boundary, and into the Telychian. Ozarkodina hassi also occurs in North America (for references, see Zhang and Barnes, 2002), the United Kingdom (Aldridge, 1972, 2002), and Norway (Aldridge and Mohamed, 1982). Pranognathus siluricus has only been reported from
Aeronian strata (Pollock et al. 1970; Aldridge, 1972; Uyeno and Barnes, 1983; Dahlqvist and Bergström, 2005). It was first recognized by Pollock et al. (1970) as Aphelognathus siluricus from northern Michigan and Ontario. It has since been documented in the United Kingdom (Aldridge, 1972), Canada (Uyeno and Barnes, 1983; Stott and von Bitter,1999), Sweden (Dahlqvist and Bergström, 2005), and Russia (for references, see Dahlqvist and Bergström, 2005). The first appearance of Pr. siluricus in the Welsh Borderland (Aldridge, 1972) occurs within the same sample as the first appearance of Pranognathus tenuis (Aldridge, 1972), at the base of the Pr. tenuis Zone (Cramer et al., 2011). In central Sweden (Dahlqvist and Bergström, 2005) and Ontario (Stott and von Bitter, 1999), Pr. siluricus and Pr. tenuis also coincide, and both have been shown to terminate below the Distomodus staurognathoides datum (Stott and von Bitter, 1999). Panderodus and Pseudooneotodus are long-ranging, simple
coniform genera that are frequently found in Silurian strata. Panderodus serratus ranges from the Ordovician to the
before the first appearance of Pseudolonchodina expansa,and range to the middle Aeronian or higher. The range of Pranognathus siluricus has been shown to be mid-Aeronian B3 through C1 zones (Stott and von Bitter, 1999). This corresponds to the Pr. tenuis Zone of Cramer et al. (2011), which marks the upper boundary of the Ps. expansa Zone. Pseudolonchodina expansa has been found to occur above the D. staurognathoides datum (Männik, 2007), which marks the upper boundary of the Pr. tenuis Zone (Cramer et al., 2011), therefore the range of Ps. expansa can overlap with that of Pr. siluricus. Pranognathus siluricus has not been reported to occur below the first appearance of Pr. tenuis, therefore it is most likely that the range of Pr. siluricus begins at or within the Pr. tenuis Zone. At Yamaan Us the lowermost sample within the upper
Ludlow (Mellgren, 2010), and Panderodus unicostatus and Pseudooneotodus beckmanni range from the Ordovician to the LowerDevonian (Zhang andBarnes, 2002; Corradini, 2007; Zhang, 2011, 2013); as such, these taxa are of little use for age constraint. Distomodus kentuckyensis and Ozarkodina hassi occur
Zalaa Formation to contain diagnostic conodonts (YU-D) contains Pranognathus siluricus. This is indicative of the Pr. tenuis Zone at that level. No diagnostic conodonts were recovered from below YU-D to further constrain the lower part of the section. Taxa with a first appearance above the Pr. tenuis Zone (i.e., Distomodus staurognathoides) were not found in the studied section. The uppermost samples in the lower Sharchuluut (YU-Q andYU-R) contain elements of Distomodus kentuckyensis, therefore the lower Sharchuluut is restricted to a zone below the D. staurognathoides first appearance datum. The highest zone below the D. staurognathoides Zone is the Pr. tenuis Zone, recognized in the lowest diagnostic sample of the upper Zalaa. This restricts the uppermost Zalaa and lower Sharchuluut formations to the Pr. tenuis Zone, indicative of a late-middle to early-late Aeronian age for the onset of the Silurian carbonate platform within the Gobi-Altai Zone.
Paleoenvironment.—Marine faunas such as Late Ordovician conodonts can be used to interpret paleolatitude (Bergström, 1990). Llandovery conodonts frequently occur in areas located at tropical paleolatitudes (i.e., Laurentia and Baltica) although this does not display provincialism because little is known about high-latitude faunas. The Silurian conodont faunas that have been recorded in areas of higher paleolatitudes are similar to typical Llandovery fauna (Lehnert et al., 1999). Llandovery conodonts have been found to be more useful for interpreting depth rather than latitude (Sarmiento et al., 1994; Lehnert et al., 1999; Dahlqvist and Bergström, 2005), and display environ- mental preference corresponding to facies depth. Because the sample size of the Yamaan Us fauna is small, true relative abundance of taxa is difficult to determine and the paleoecological interpretation must be general. Ozarkodina hassi is generally considered to prefer
shallower water (Idris, 1984; Armstrong, 1990; Aldridge, 2002; Dahlqvist and Bergström, 2005). In contrast, Pseudolonchodina is most abundant in deeper water slope facies (Armstrong, 1990; Aldridge, 2002; Dahlqvist and Bergström, 2005). Species of Distomodus have also been considered to prefer shallower water (Aldridge, 2002), although D. kentuckyensis was found
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