Nostolepis decora, Valiukevičius, 2003

Valiukevičius, Juozas, 2003, Devonian acanthodians from Severnaya Zemlya Archipelago (Russia), Geodiversitas 25 (1), pp. 131-204 : 151-159

Nostolepis decora n. sp. ( Figs 17 ; 18 )

HOLOTYPE. — LIG 35-A-346. A specimen preserved laterally, exposing only squamation, with the head and tail areas, fin spines also missing ( Fig. 17A ).

ETYMOLOGY. — From Decorus (Latin): beautiful, lovely.

LOCALITY AND AGE. — October Revolution Island: Pod”emnaya River, outcrop 67, bed 12. Lower Devonian, lower Lochkovian, Severnaya Zemlya Formation.

DIAGNOSIS. — Small-sized nostolepid with leaf-like scales, having anteriorly down sloping crowns with

two long, posteriorly pointed ridges, forming an elevated medial area. Lowered lateral notched areas present. Two enlarged pores open latero-distally on the neck. Crown composed of Stranggewebe with large lacunae and, in only a small proximal area, of simple networked mesodentine.

DESCRIPTION

This description is based on the holotype which is the only representative of this species.

Squamation

Scales are distributed densely in oblique lines with small extensive overlap. Trunk scales have oval or leaf-like crowns with a rounded, widened anterior and tapered, pointed posterior edge. The largest crowns reach in length 0.65 mm. They slope down anteriorly with a prevailing low angle, sometimes to 45°. We always distinguish a medial area of elongated triangular form ( Fig. 17C, E ) to stretched strip ( Fig. 17F ) with two blunt, magnified and higher ridges pointing and protruding posteriorly beyond the base. Slightly concave plates between them may be ornamented with one or two shorter ones. Somewhat lower notched lateral crown areas join upward uneven, rarely “denticulate”, sides ( Fig. 17F, G ). The base is moderately convex, anteriorly vaulted, with a prevailingly low neck. Rare examples (mostly with higher neck) contain two enlarged pore openings on the latero-distal neck walls. Two longitudinal lines of enlarged scales (especially widened, to 0.75 mm) with asymmetrically ridged crowns may be defined as lateral line scales ( Fig. 17B ) or tesserae-scales with often wavy or interrupted ridges of both inconstant number and shape ( Fig. 17H ). Their crowns do not show a division into medial and lateral areas. Their neck may also contain two pore openings.

Trunk scales, normal ( Fig. 18A ) and extremely elongated ( Fig. 18C ), as well as asymmetrical lateral line scales ( Fig. 18F ), are composed of classical Stranggewebe in crowns with horizontally stretched long lacunae, connected with principal circular, radial and ascending vascular canals. They present no surrounding strips of simple mesodentine. Medial area shows two lamellae of superpositional growth ( Fig. 18D ), whereas laterals join sideward (areal). Crown contains simple mesodentine with plenty of osteocytes and a small number of dentine tubules composes the minor proximal crown area only. It turns gradually into the highly cellular base bone. Scales from the lateral line ( Fig. 18F ) or the special tesserae-scales ( Fig. 18B ) have a complicated system of enlarged vascular canals, leaving almost no space for dentine tubules or lacunae.

Branchial? plates

Rare pointed fragments of plates with slightly convex or flat thin base and randomly displaced oblique or vertical sharply-tipped spinelets demonstrate a dentine structure ( Fig. 18E ) with a main (pulpar) ascending canal in the centre and small branching tubules. Their base is supposedly composed of mesodentine. By these features they differ from branchial? cones of Acritolepis n. gen.

DISCUSSION

By its original crown ornamentation, including the two-ridged medial area and lateral notched “wings”, but also by the latero-distal neck pore openings (no pores on crowns surface), Nostolepis decora n. sp. differs from all known nostolepids. A distant similarity arises only with one morphological variety of normal trunk scales of N. striata Pander, 1856 , figured by Gross (1947: pl. 26 (7), figs 10, 11; 1971: pl. 6, figs 1, 2). The latter have parallel and numerous converging anterior ridges on the flat and wide medial crown area, and also regularly shaped narrow notched lateral areas, which may contain a double line of notches.

BIOSTRATIGRAPHIC SIGNIFICANCE

Identical to the representatives of Acritolepis n. gen.

Nostolepis fragilis n. sp.

( Figs 19 ; 20 A-G; 21; 22)

Nostolepis sp. no. 7 – Valiukevičius 1988a: 604, 605.

D A-C, E, F

HOLOTYPE. — LIG 35-A-353. Fragmentary specimen preserved in lateral view with two fragments of pectoral spines, two ventral fin spines and squamation ( Fig. 19A, B ). Matusevich River, outcrop 1, bed 21.

ETYMOLOGY. — From Fragilis (Latin): fragile, referring to squamation.

LOCALITY AND AGE. — October Revolution Island: Matusevich River, outcrop 1, bed 21. Lower Devonian, lower Lochkovian, Severnaya Zemlya Formation.

DIAGNOSIS. — Nostolepid with two stout ventral (intermediate?) fin spines with nodose ridges and longitudinally ribbed pectoral ones, composed of cellular bone and mesodentine (the ribbed part only). Scales with crowns anteriorly down sloping at a high angle, ornamented with two ridges, forming a medial area, and two symmetrical side ridges; base concave with a deep posterior fossa. One or two growth lamellae in crown composed of simple mesodentine with vascular tubules, distantly resembling the lacunae of Stranggewebe.

llsc

A, C, D, F B, E

DESCRIPTION

Species described from the single holotype specimen.

Fin spines

The best preserved spines are the two ventral fin spines (intermediate?) following one another with a distance of about 10 mm. The first one measures 7.3 mm in length, the second c. 8.0 mm. These spines are stout, anteriorly with a narrowed oval cross section, almost without curvature, hollow, only slightly inserted or missing an insertion base. They bear four robust nodose ridges on each side ( Fig. 19B ). The two anterior ones, which do not reach the top end, meet on the anterior margin in a typical V-shape pattern. Nodes increase in length both anteriorly and basally. Grooves between ridges are deep and wide, with numerous pore openings.

In front and above the first ventral are displaced two fragments of the supposed pectoral spines. Their length are respectively 4.0 mm and 1.3 mm. The latter, closer to the ventral spine, bears three longitudinal ribs, the proximal one

A-G

H-O

being enlarged, quadrangular in cross section, and separated from its neighbour by a very deep, wide groove containing some pore openings.

All spines are composed of highly vascularised cellular bone ( Fig. 22A, B) and, only in their ribbed part, of mesodentine with characteristically up streamed dentine tubules.

Squamation

Scales show small crown overlaps. Scales with elongated sub-rhombic crowns (0.4-0.6 mm long) dominate. They possess sharp and posterior tapering edges sloping at an angle of 30-50° toward the horizontal base ( Figs 19C ; 20B ). Proximal crown part slopes downward to the

cbo A, vertical longitudinal section through elongated scale, thin section 692; B, vertical transverse section through elongated scale, thin section 695; C, magnified area of crown of the same scale; D, vertical longitudinal section through the widened scale, thin section 691. Dotted canals better seen with slightly changed focus. For locality and stratigraphic arrangement see Fig. 19 . Abbreviations: avc, ascending vascular canal; cbo, cellular bone; dt, dentine tubule; oc, osteocyte cavity; op, osteocyte process; smd, simple mesodentine. Scale bars: 0.1 mm.

base outline. The distal pointed edge may overhang the base on twice their length ( Fig. 20C ). Short, widened crowns ( Fig. 20D ) and enlarged bases met rarely. Two long ridges delimit a large medial crown area, surrounded by a pair of lateral areas, somewhat lowered ( Fig. 20D, G ) or connected to the base sides at right angle ( Fig. 20B ). The rhombic base is small, anteriorly vaulted ( Fig. 20E ) and posteriorly depressed, with a deep fossa ( Figs 19D ; 20C ).

Scales are made of a moderately cellular bone (base) and mesodentine showing only one or two growth lamellae (crown). Enlarged radial and ascending vascular canals ( Fig. 21A, C ) are present with arising and mainly superficially directed winding dentine tubules. They contain widenings similar to the lacunae in Stranggewebe. The crown mesodentine looks similar to bone tissue of the base due to the presence of osteocytes even high in the crown and in well preserved bases.

DISCUSSION

By their crown downsloping anteriorly at a high angle, overhanging the base and by their medial area, outlined by two long ridges, the scales of Nostolepis fragilis n. sp. are similar to N. arctica Vieth, 1980 ( Vieth 1980: pl. 5, figs 1-9). But at the difference of the new species, N. arctica scales are larger, more massive and bear more ridges. Additional ridges may also be present on lateral areas. Except for medials and one pair of laterals, N. fragilis n. sp. has no extra ornamental details. Its concave and fossulated base strongly differs from the convex and centrally vaulted ones of N. arctica . Scale crowns with their increased number of growth lamellae (five), their more diversified system of principal vascular canals and the presence of typical Stranggewebe are characteristic for N. arctica ( Vieth 1980: fig. 20A-E).

BIOSTRATIGRAPHIC SIGNIFICANCE

Identical with representatives of Acritolepis n. gen.

Figures

FIG. 17. — Nostolepis decora n. sp., all scales from the holotype specimen, Pod”emnaya River, outcrop 67, bed 12, Severnaya Zemlya Formation; A, holotype (LIG 35-A-346) in lateral preservation, squamation, anterior to right; B, enlarged area of the same specimen, trunk scales and magnified asymmetric scales along the lateral line; C-G, trunk scales; C, LIG 35-2030, crown; D, LIG 35-2025, side; E, LIG 35-2031, crown; F, LIG 35-2032, crown; G, LIG 35-2033, crown (anterior upwards for all crowns); H, tessera-scale from the lateral line, crown, LIG 35-2026. Abbreviations: ll, lateral line; llsc, scale from lateral line; sq, squamation. Scale bars: A, 10 mm; B, 2 mm; C-H, 0.5 mm.

FIG. 18. — Nostolepis decora n. sp., histologic structure of trunk scales, tesserae-scales and denticled branchial? plates; A, normal trunk scale (like figured on Fig. 17G) in anise oil, crown, LIG 35-2077; B, tessera-scale (see Fig. 17H) in anise oil, crown, LIG 35-2076; C, vertical longitudinal section through the central part of scale with posteriorly pointed elongated crown (see Fig. 17C), thin section 3447; D, vertical transverse section through the anterior part of stretched scale with linked upward, notched lateral areas (see Fig. 17F), thin section 3449; E, slightly recurved denticled plate in anise oil, LIG 35-2075; F, vertical longitudinal section through the asymmetrical tessera-scale from the lateral line, thin section 3448. For locality and stratigraphic assignement see Fig. 17. Abbreviations: avc, ascending vascular canal; cbo, cellular bone; cvc, circular vascular canal; oc, osteocyte cavity; rvc, radial vascular canal; smd, simple mesodentine; stg, Stranggewebe (oriented mesodentine). Scale bars: 0.1 mm.

FIG. 19. — Nostolepis fragilis n. sp., Matusevich River, outcrop 1, bed 21, Severnaya Zemlya Formation; A, B, holotype (LIG 35-A-353); A, in lateral preservation, anterior to left, supposed fragments of pectoral fin spines, two ventral spines and squamation; B, magnified first ventral spine and squamation; C, trunk scale, side, anterior to right, LIG 35-2061; D, trunk scale, base, anterior to page bottom, LIG 35-2063. Abbreviations: pfs, pectoral fin spine; sq, squamation; vfs, ventral fin spine. Scale bars: A, 10 mm; B, 3 mm; C, D, 1 mm.

FIG. 20. — Drawings of scales; A-G, Nostolepis fragilis n. sp., holotype; A-C, LIG 35-1275, crown (A), side (B) and base (C), anterior upwards (A, C) and to left (B); D, E, LIG 35-1276,crown (D, anterior upwards) and side (E, anterior to left); F, G, LIG 35-1277 and 35-1279, crowns, anterior upwards. For locality and stratigraphic arrangement see Fig. 19; H-J, Diplacanthus solidus n. sp.; H, I, holotype, LIG 35-1283 crown (H, anterior upwards) and side (I, anterior to left); J, LIG 35-1285, crown, anterior upwards, Matusevich River, outcrop 5, bed 151 (top), Vstrechnaya Formation; K, L, Diplacanthus carinatus Gross, 1973, crowns, anterior upwards; K, LIG 35-1254; L, LIG 35-1282, Matusevich River, outcrop 5, bed 149, Vstrechnaya Formation; M-O, Cheiracanthus sp. cf. Ch. longicostatus Gross, 1973; M, LIG 35-1286, crown, anterior upwards; N, O, LIG 35-1284 crown, anterior upwards (N) and side, anterior to left (O). Matusevich River, outcrop 5, bed 153 (M) and top of bed 151 (N, O), Vstrechnaya Formation. Abbreviation: fos, fossa. Scale bars: 0.5 mm.

FIG. 21. — Nostolepis fragilis n. sp., scale histology, holotype;