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J. Valiukevičius. Silurian acanthodians from Lūžņi-4
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Remarks. Crown pores were not visible on examination of scales under a binocular
microscope, and they were considered possibly related to Gomphonchus boekschoteni
Vergoossen (1999 c, pl. 2, figs 24-25; pl.3, figs 26-31). As for Poracanthodes marssae,
most G. boekschoteni scales have large pores opening on all faces of the neck, pearshaped
crowns with strengthened and elongated posterior parts, and the similar neck
characters including a prominent posterior keel. The grade of the base convexity and
its position in advance of the crown in P. marssae were also within the range of the G.
boekschoteni morphovarieties. Discovery of pores on crowns of P. marssae precluded
assignment to G. boekschoteni and supported attribution of scales to the punctatiform
Poracanthodes.
Occurrence. See Table.
Biostratigraphical potential of acanthodians
A single studied locality is insufficient to determine exact acanthodian zonality, but
the Lužni-4 core fauna is interesting both in the subdivisional and correlational aspects
of Silurian acanthodians in Latvia and the wider region. The series of the Ludlow and
Pridoli is here acanthodian-based divided into eleven biostratigraphic intervals named
by the chosen index species (Fig. 9, right column), and defined by the first occurrence
of the nominative taxon. The intervals are neither interval zones nor partial range zones,
because they occupy only some parts of the partial ranges of indices, until the next
index appears in the sequence. This fact lowers the stratigraphic value of the interval
units. However, the entry levels of the index or other first appearing taxa are significant
to the regional record.
The most ancient acanthodian of the Lužni-4 borehole is a tiny indeterminable
nostolepid (of the putative N. striata-group?) at the depth of 458 m (see Table).
The first, Nostolepis gracilis interval starts at the depth of 388 m, about 50 m below
the top of the Ventspils Formation, that is coincident with the boundary of Ludlow/
Pridoli. The unit is based on only a few scales of N. gracilis at this single point, which
has yielded no other taxon. The finds are significant (I have no doubts concerning their
identification), because they mark the earliest occurrence of N. gracilis Gross (1947)
ever defined in the Baltic. All prior data (Märss 1986, 1992, 1997, 2000; Märss et al. 1995,
1996) indicated that this species did not exist in the Ludlow, and it was introduced as the
Pridolian index fossil of the Vertebrate Standard Scheme. Seemingly, its entry and the
biozonal boundary must be lowered into the late Ludlow.
The next, Nostolepis latvica sp. nov. interval (338.6-306.2 m) is approximately
attached to the Šilale Beds of the Minija Fm. The first occurrence of the index is about
1.3 m above the lower boundary of the Beds, and its range extends to the Venzava
Bedss of the Targale Fm. This basal Pridolian acanthodian association is based on
the entry of Gomphonchus? minutus sp. nov. and Nostolepis cf. amplifica as the
most definitive species (for full list of taxa of this and further intervals see Fig. 9). Its
correlational value is yet uncertain, as all three most significant taxa have not been
discovered in Lithuania. Of note, N. gracilis starts its ongoing occurrences in samples
in the upper part of the interval, above 319.9 m (see Table), about 20 m over the