Nostolepis timanica, Valiukevičius, 2003

Valiukevičius, Juozas, 2003, Devonian acanthodians from Severnaya Zemlya Archipelago (Russia), Geodiversitas 25 (1), pp. 131-204 : 162-167

Nostolepis timanica n. sp. ( Figs 25 ; 26 )

HOLOTYPE. — LIG 35-2023. Scale ( Fig. 25B ). Pioneer Island , sample P-10-1. Lower Devonian, Member 8.

ETYMOLOGY. — After Timan-Pechora region, where the species has been identified first.

MATERIAL EXAMINED. — 60 scales.

LOCALITY AND AGE. — October Revolution Island: Matusevich River, outcrop 4, bed 3; Spokojnaya River, outcrop 40, bed 27. Pioneer Island: samples P-9-1 and 10-1. Lower Devonian, Lochkovian to Emsian, Pod”emnaya to Rusanov formations.

DIAGNOSIS. — A Nostolepis with minute scales with elongated, horizontally flattened crowns with three to

four short, converging anterior ridges of both inconstant form and length. Simple mesodentine in crown with typical canal network or with long, smooth ascending and branchy radial vasculars resembling those of dentine-type tissue. Bone of base highly cellular.

DESCRIPTION

Species described from isolated scales. Their size varies in length from 0.2 to 0.45 mm. Crowns have an elongated triangular form with rounded and widened anterior edge and tapered posterior, overhanging base. Three to four stout, rounded sub-parallel anterior ridges of inconstant form and length converge but do not reach medial crown part. Two thirds of the crown surface are flat, unornamented, only with a shallow longitudinal medial concavity. Crown plate is horizontal. Scale bases are rhombic; differently convex anterior and side margins are protruding crown. Most convex ones are centrally vaulted.

We observe up to four growth lamellae in the crown composed of uniform simple mesodentine. Several specimens demonstrate comparatively long, smooth main branches of ascending vascular canals ( Fig. 26C, D ), less characteristic for the “ Nostolepis ” - type histology. Mesodentine canal network includes medium numbers of osteocyte spaces, increasing in primordial lamella ( Fig. 26B, E ). Enlarged and branching radial dentine canals occur at the base-crown junction. Base bone is highly cellular, uniformly thin lamellar, with long Sharpey’s fibres.

DISCUSSION

Nostolepis timanica n. sp. follows N. striata Pander, 1856 - N. minima Valiukevicius, 1994 phylogenetic lineage of nostolepids. It differs from the close N. minima by its weaker ridges both in number and strength, including loss of oblique neck ridges. N. timanica n. sp. has no lateral crown ridges, like those pointing distally in N. minima ( Valiukevičius 1994: pl. 21, figs 10-12; 1998: pl. 1, figs 5-9). Furthermore differences occur in histological structure. N. minima scale crowns are composed of a simple mesodentine with a more bushy canal network and a Stranggewebe (scales from the

A

B

Tareya River section, Tajmyr; Valiukevičius 1994: fig. 75.4) only fragmentary developed: it can completely fill all posterior area of crowns (scales from the type Baltic region; Valiukevičius 1998: pl. 12, figs 2-4), embracing also the primordial growth lamella. N. timanica n. sp. shows no Stranggewebe, and ascending or radial vascular canals are sometimes more characteristic of true dentine than mesodentine style.

BIOSTRATIGRAPHIC SIGNIFICANCE Long-ranging species, occurring into the Nostolepis minima (upper part) Zone assemblage (Diplacanthy poltnigi Subzone ), upper part of Lochkovian (Pod”emnaya Formation). The latest representatives are found on Severnaya Zemlya Archipelago within the age analogues of the Rusanov Formation on Pioneer Island, in beds with Watsonacanthus costatus n. sp. (Emsian).

A B C D

A B C A B C D

B, C D K L J M O J-P P

E F G G-I I H N

A-C E F

D-G G J I K H H-K N-P, S N M L O R S P

Genus Acanthacanthus n. gen.

TYPE AND ONLY SPECIES. — Acanthacanthus ornatus n. gen., n. sp.

ETYMOLOGY. — Twice repeated acantha (Greek): thorn.

AGE. — Early Devonian, early Lochkovian.

DIAGNOSIS. — As for the type and only known species.

Figures

FIG. 25. — Nostolepis timanica n. sp., scales, crowns, anterior upwards, Pioneer Island, sample P-10-1, Lower Devonian, Member 8; A, LIG 35-2022; B, holotype, LIG 35-2023. Scale bar: 0.5 mm.

FIG. 26. — Nostolepis timanica n. sp., scale histology, Pioneer Island, sample P-10-1, Lower Devonian, Member 8; A, vertical longitudinal section through the central part of scale, anterior to right, thin section 3375; B, vertical transverse section through the anterior part of scale, thin section 3376; C, vertical transverse section through the posterior scale part, thin section 3377; D, E, vertical longitudinal sections through the scale with maximum vaulted, convex base (D) and maximum flattened scale (E), anterior to right (D) and to left (E), thin sections 3378 and 3379. Abbreviations: avc, ascending vascular canal; cbo, cellular bone; dt, dentine tubule; gl, growth lamella; lsh, longitudinal Sharpey’s fibres; rvc, radial vascular canal; smd, simple mesodentine. Scale bar: 0.1 mm.