Arcticacanthus bicostatus, Valiukevičius, 2003

Valiukevičius, Juozas, 2003, Devonian acanthodians from Severnaya Zemlya Archipelago (Russia), Geodiversitas 25 (1), pp. 131-204 : 186-188

Arcticacanthus bicostatus n. sp. ( Figs 44A, B ; 45 )

HOLOTYPE. — LIG 35-2002. Scale ( Fig. 44B ). October Revolution Island , Pod ”emnaya River, outcrop 69, bed 26.

ETYMOLOGY. — From bicostatus (Latin) : two-ridged, referring to anteriorly bifurcated ridges on crowns.

MATERIAL EXAMINED. — 10 scales.

LOCALITY AND AGE. — October Revolution Island: Pod”emnaya River, outcrop 69, bed 26. Pioneer Island: sample P-12-7. Lower Devonian, Lochkovian, Pod”emnaya Formation to Emsian, Rusanov Formation. DIAGNOSIS. — Ischnacanthid having mid-sized subrhombic scales, with crowns ornamented by four to six sub-parallel high ridges, extending on two thirds of its length and bifurcating anteriorly. High neck with pore openings. Dentine-like tissue of crown perforated by a pore canal system, composed of radial canals high in the neck, longitudinal canals in crown along superficial grooves, connected by uneven ascending ones formed anew in each growth lamella.

DESCRIPTION

Species described from isolated scales. Their crowns measuring 0.4-0.85 mm long are subrhombic, flat, rarely overhanging base, and often wider than long. They are ornamented with four to six longitudinal sub-parallel, anteriorly higher ridges, which are of a moderately rounded quadrangular section, strongly widened and shortly bifurcated at the anterior margin. Ridges are separated by wide grooves. Posterior third or half of the crown length is smooth. The mid-high neck, shows each three round pore openings on postero-distal walls. The rhombic outlined base rarely protrudes the crown anteriorly.

The original histological structure is not yet sufficiently investigated since the available number of scales remains too small. The crown is composed of a dentine-like tissue and the base of highly cellular bone. 10 unevenly thick lamellae of superpositional growth are present in the crown ( Fig. 45B ). Over the base radial vascular canals are weakly developed. They are narrow, supposedly consisting of several branches. Growth lamellae ( Fig. 45A ) are evenly pierced by ascending canals with monotonous and slightly more broadened longer principal branches. Some characteristic short, weakly winding ascending tubules occur in each lamella of transitional dentine-mesodentine style. The crown is pierced by a system of wide pore canals. Their openings can be observed only in the scale neck, but supposedly they are also present in the grooves of crown plate as demonstrated in sections. Radial centripetal pore canals are situated high over the base ( Fig. 45B ). They are multibranched, as seen on Fig. 45A (left side). From them the arising ascending pore canals are extremely uneven ( Fig. 45A ), with variable shape, as wide as the radial canals. It seems they are formed anew in each new growth lamella, but continue within a principal direction during all growth (they seem to function with interruptions).

DISCUSSION

By their general crown ornamentation, these scales distantly resemble the representatives of Cheiracanthoides Wells, 1944 , but they differ in height and in the ridges bifurcation. The highly cellular base bone might not contradict with an assignment to nostolepids but the presence of a pore canal system in crowns allows to relate Arcticacanthus n. gen. to the Ischnacanthidae and among them, particularly to Poracanthodes scales which demonstrate two sorts of base tissues: cellular and acellular bone occurring together. A modified “ Poracanthodes ” - type histology is already noted both in Lietuvacanthus and Ectopacanthus ? Valiukevicius, 1998, two Early Devonian genera from the Baltic basin. The scales of Lietuvacanthus fossulatus have a deeply embayed anterior crown rim and a pore canal system with arcades and radials, but without upward directed canals opening on the surface ( Valiukevičius 1998: pl. 6, figs 10-14, pl. 12, figs 7, 8, pl. 13, figs 1-6). On the other hand Arcticacanthus n. gen. scales have distinct vertical connections of radial crown pore canals, producing uneven, differently shaped ascending branches, functioning during the all scale growth. Lietuvacanthus fossulatus shows radial pore canals displayed in chains one above the other in every growth lamella without vertical connections or occuring only in the primordial lamella. Ectopacanthus ? pusillus ( Valiukevičius 1998: pl. 7, figs 8-12, pl. 14, figs 2-4), apart from differences in its crown ornamentation (smooth narrow ridges and grooves placed in fans and extending over the all crown), has a pore canal system (with arcade, radial and ascending canals) without surface openings. The pore canals rise up only to the basal part of the latest growth lamella and open in the neck.

BIOSTRATIGRAPHIC SIGNIFICANCE Long-ranging species, occurring within the upper part of early Lochkovian Nostolepis minima Zone ( Diplacanthus poltnigi n. sp. Subzone,

A B

Pod”emnaya Formation) and disappearing in the early Emsian beds with Watsonacanthus costatus n. sp. (age analogues of Rusanov Formation on the Pioneer Island).

Acanthodii incertae sedis Order and family uncertain

Genus Acanthospina n. gen.

TYPE AND ONLY SPECIES. — acanthospina irregulare n. gen., n. sp.

ETYMOLOGY. — From acanthus (Latin): spine, thorn; and spina (Latin): needle, spine, referring to spiny scales.

AGE. — Early Devonian, early Lochkovian.

DIAGNOSIS. — As for the type and only species.

DISCUSSION

Acanthodian of uncertain systematic position, with chondrichthyan-type scales (hybodontiform according to its crown morphology),

rpc

cbo

composed of “ Nostolepis ” - type tissues, characteristic of acanthodian histological structure and ischnacanthid-like (particularly, Poracanthodestype) teeth.

It might possibly belong to a group of climatiiform acanthodians, which, by scale morphology may be distant from the ascertained Climatiidae species.

Figures

FIG. 44. — A, B, Arcticacanthus bicostatus n. gen., n. sp., scales, crowns, anterior upwards; A, LIG 35-2003; B, LIG 35-2002, holotype. Pod”emnaya River, outcrop 69, bed 26; C, Nostolepis sp. cf. N. robusta (Brotzen, 1934), scale, crown, anterior to right, LIG 35-1906. Matusevich River, outcrop 4, bed 3. Pod”emnaya Formation (A-C); D, Nostolepis sp. or Cheiracanthoides sp., scale, crown, anterior upwards, LIG 35-1696. Spokojnaya River, outcrop 41, bed 1; Krasnaya Bukhta Formation. Scale bar: 0.5 mm.

FIG. 45. — Arcticacanthus bicostatus n. gen., n. sp., scale histology, Pod”emnaya River, outcrop 69, bed 26, Pod”emnaya Formation; A, vertical transverse section; B, vertical longitudinal section, anterior to right, thin sections 3399 (A) and 3398 (B). Abbreviations: apc, ascending pore canal; avc, ascending vascular canal; cbo, cellular bone; dt, dentine tubule; gl, growth lamella; oc, osteocyte cavity; po?, pore opening?; rpc, radial pore canal; rvc, radial vascular canal. Scale bar: 0.1 mm.