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J. Valiukevičius. Silurian acanthodians from Lūžņi-4
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of the biozonal unit and wider correlations, as in the type locality (Linley Brook near
Much Wenlock, UK) it is only known from the Downton Castle Sandstone Fm., the
lower part of Pridoli (Miles and Märss 1999).
Poracanthodes punctatus accompanied by Gomphonchus sp. cf. G. hoppei first
appear at the same point, and distinguish the next interval (280.3-260 m). P. punctatus
is the second acanthodian species of the Silurian Vertebrate Standard Scheme (SVSS),
having its interval zone attached to the upper part of the Pridoli (Märss 1986, 1992,
1997, 2000; Märss et al. 1995, 1996). In the Lužni-4 core, the interval correlates to the
Minija/Targale Fm. boundary beds, perhaps supporting the prior opinion on its first
occurrence in Estonian localities. However, in Lithuania, particularly in the west area,
which is characterized by the deepest marine sediments and most complete geological
profile, it ranges for the entire Pridoli, in the Minija and Jura formations.
The Arenaceacanthus arcuatacanalis interval (260-223 m) comprises almost full
ranges of the index species (Valiukevicius 2004) and “Pruemolepis wellsi” Vieth (1983)
(the last being only a specialized morphological variety of nostolepids), and Nostolepis
musca Valiukevicius (2003 b) appearing high at the top of the interval. Both most
valuable taxa demonstrate distinctly later stratigraphical entry as compared to Lithuania,
where A. arcuatacanalis appears in Wenlock? (with some doubt according to conodont
dating of rocks) and N. musca in the Ludlow.
The next, Fecundosquama basiglobosa interval (223-210.8 m) is attached to the
upper part of the Venzava Beds (Targale Fm.) and probably the previous showing a
shorter range in comparison with Lithuania, where the nominative species makes the
first occurrence in the Minija Fm. (Valiukevicius 2004). This species is of great interest
in acanthodian biozonality as it is easy identifiable, with a restricted stratigraphic range
and wide regional distribution. It is recognized in all Baltic republics and the Kaliningrad
district of Russia. Of note, in this interval nostolepid squamae margaritatae (Gross
1971) first appear, possibly identifying a stratigraphic level useful for correlation over
a wide area.
The Garzde and Lužni Beds of the Targale Fm. are connected with the flourishing
time for acanthodians in the late Pridoli. The acanthodian associations (likely as in the
whole Baltic region) have shorter ranges, and the new indices and characteristic taxa
reach maximum numbers here and appear with the highest frequency.
The short-ranging Poracanthodes sulcatus sp. nov. interval (210.8-202 m) is
distinguished at the base of Garzde Beds and marks a sequential dominance of
ischnacanthid acanthodians in the taxa-rich associations. Alongside the index species,
it is marked by the entry of two more Poracanthodes, P. cf. stonehousensis (sensu
Vergoossen 2000) and P. marssae sp. nov., and also two representatives of
Gomphonchus, G. cf. mediocostatus Vergoossen (1999 c) and G. hoppei (Gross 1947).
Poracanthodids are not yet defined in Lithuania, but the stratigraphical level may be
approximated by the representatives of Gomphonchus occurring in the Kelme Beds of
the Jura Fm.
The Monospina erecta interval (202-196.5 m) marks an essential renewal of
acanthodian associations with the entering of most valuable Monospina erecta
Valiukevicius (2003 d), Nostolepis elegans (Brotzen 1934), N. alifera sp. nov., N.
amplifica Valiukevicius (2003 b) and ?Diplacanthus sp., whereas the overlying
Nostolepis alta interval (196.5-186 m) yields only the index species of significance