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

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ADVANCES IN PALAEOICHTHYOLOGY
Order ISCHNACANTHIFORMES Berg, 1940
Family ISCHNACANTHIDAE Woodward, 1891
Genus Gomphonchus Gross, 1971
Type species. Gomphonchus (pro Gomphodus Pander, 1856) sandelensis (Pander).
Upper Silurian, Pridoli; Saaremaa, Estonia.
Etymology. Minutus (Latin) - small.
Gomphonchus? minutus sp. nov.
Fig. 6 A-K
Holotype. LIGG 25-A-2581, flank scale (Fig. 6 B).
Type horizon. Šilale Beds of the Minija Formation, Pridoli, Upper Silurian.
Range. Minija Formation, Pridoli, Upper Silurian.
Material. Total about 4500 scales.
Diagnosis. Ischnacanthid having tiny rhomboidal scales with deep bases which protrude
slightly beyond the crowns on all sides. Crown ornament varies from almost smooth
with only shallow anterior incisions, to short, low, flattened parallel ridges, that are
rarely sub-radial and widened anteriorly. Scale crowns are composed of modified
acellular networked mesodentine with neither lacunae nor outer durodentine present.
Thin-lamellar acellular bone in bases contains long traces of Sharpey’s fibres.
Description. Morphology. Scales are tiny; the length of crown rarely reaches 0.5 mm.
Crowns are flat, without an anterior downslope, isometrically rhomboidal or slightly
longitudinally stretched. A minority have crowns wider than long (Fig. 6 E). Crowns do
not overhang bases. The crown sculpture varies from almost smooth, without ridges
but with short, shallow incisions along the anterior edges (Fig. 6 A, C), to short, uneven,
rounded parallel ridgelets fading out in the anterior part or a little longer (Fig. 6 B, D).
Some scales have stout, rounded, sub-radially placed ridges that widen anteriorly and
fade out at one-third of crown length (Fig. 6 E). The rest of the crown surface is smooth
except on rare scales which have shallow slits on the posteriormost part (Fig. 6 B)
reflecting the growth zones. Scale necks are well developed but short, porose on the
anterior or all faces (Fig. 6 E and B-C), with from two to four pores per side. Bases are
rhomboidal, extending slightly beyond the crown on all sides, deeply convex, with the
deepest part centrally.
Histology. Six lamellae in crowns of the superpositional growth are composed of
simple acellular ?mesodentine. Durodentine is absent or possibly present superficially
in only the two outer lamellae. A fine mesodentinal network of canaliculi (Fig. 6 I) is
developed among the ascending vascular canals, of which the main branches are
distinguished well only in the lower (neck) crown part. Horizontal longitudinal dentine
canals are also interwoven with many small canaliculi. No lacunae are visible in crowns.
Radial vascular canals, of comparable width to the ascending canals, are positioned
over the flat-pyramidal base that is composed of fine-lamellar acellular bone pierced
by traces of long Sharpey’s fibres.