Zemes un vides zinātnes Earth and Environment Sciences - Latvijas ...

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ADVANCES IN PALAEOICHTHYOLOGY
rest of the crown. The dentine has clear principal branches of ascending canals (Gross
1971, figs 20 A-E, 21 A-D) which branch differently compared to those in G.? minutus,
and do not form a complicated network of canals of mesodentine-type. Horizontal
canals in crowns of G. sandelensis form separate branches which are not interwoven
with dentine canaliculi.
Scales of Gomphonchus turnerae (Burrow and Simpson 1995) from the Ludlow of
northeastern Australia resemble those of G? minutus in several features - they are small
scales, also with slits following the growth lines on the posterior crown surface. The
preservation of the histological structure of G? turnerae scales is poor, but they also
show an acellular base, and a crown tissue intermediate between non-lacunal mesodentine
and orthodentine of the usual Gomphonchus-type.
Two further species of Gomphonchus, G. mediocostatus Vergoossen (1999 c, pl. 2,
figs 18-23; see also Valiukevicius 1998, pl. 7, figs 23-24; pl. 8, figs 1-7) and G. nordicus
Valiukevicius (2003 a, figs 23 N-P, 35 A-D) show “Acanthodes”-type histology in
crowns concerning even small details, but their bases are formed of cellular bone.
Sometimes the latter tissue does not differ from that in scales of the “Nostolepis”-type
(Valiukevicius 2003 a, fig. 35 A-B). Based on sections of scales from the Lužni-4, G.
mediocostatus also has cellular bone in scale bases. The outstanding dcharacteristic of
G. mediocostatus is a pair of median crown ridges fading out half way along the crown,
and shorter, often radial ridgelets that converge with the central ridges or one with
another. Sometimes short ridges are uniform, sub-radial.
A variant of G.? minutus scales having more robust, rounded and anteriorly widened
sub-radial anterior ridges (like in Fig. 6 E) resembles G. volborthi (Rohon 1893; also
Lehman 1937, pl. 3, fig. 53; Vergoossen 1999 a, figs 10-12) in the general scale shape
(flat crowns smaller than deep bases) and form of ridges (widened anteriormost edges),
but differs in that they are not as marked and separated by wide, deep grooves forming
deep incisions along the anterior crown edge as in G. volborthi.
Gomphonchus? sp. (Burrow et al. 1999, fig. 8 A-G) from the Pridoli of Arctic
Canada, though different in scale shape (in my opinion, more Acanthoides-like), is
comparable to the Lužni species in having short anterior ribs (though these are different
in form) or being smooth, and in posterior slits on the crown surface which zig-zag are
parallel to the growth zones. The histologic structure can not be compared as only a
horizontal section of the upper crown of the Canadian Gomphonchus? sp. was figured
giving little information.
Gomphonchus minicostatus and G. abruptus (Valiukevicius, 2003 d) provide
controversial data in the problem of categorizing histologic structure and systematic
position of this genus. G. minicostatus has scales ornamented with few parallel very flat
anterior ridges and composed of orthodentine-like tissue in crowns and cellular bone in
bases, whereas scales of G. abruptus are smooth, with vertically or obliquely cut posterior
crown edges and have typical”Acanthodes”-type histology, containing acellular bone
in bases and the crown orthodentine similar to that of G. sandelensis.
Some of the differences noted for Gomphonchus and Gomphonchus? possibly
indicate that, based on the histological similarities, G? minutus and G? turnerae should
be assigned to a new genus. The morphological and histological differences shown by
“Gomphonchus” hoppei seem to support Vergoossen’s (1999 c) creation of a new genus
Gomphonchoporus for G. hoppei Gross.