Acanthopora, Valiukevičius, 2003

Valiukevičius, Juozas, 2003, Devonian acanthodians from Severnaya Zemlya Archipelago (Russia), Geodiversitas 25 (1), pp. 131-204 : 179-186

Poracanthodes sp. cf. P. subporosus Valiukevicius, 1998 ( Figs 28 E-I; 39)

MATERIAL EXAMINED. — Single paratype LIG 35-A-376 ( Fig. 28E): exposing poorly articulated unidentified fin spines and squamation. One more sample yields four scales.

LOCALITY AND AGE. — October Revolution Island: Spokojnaya River, outcrop 51a, beds a-c and outcrop 41, bed 12. Upper Silurian, Pridoli, Krasnaya Bukhta Formation and Lower Devonian, lower Lochkovian, Severnaya Zemlya Formation.

REMARKS

Crown pore openings are mainly organised in four radial lines, starting at midlength and extending to postero-lateral margins. Smaller numbers of scales have one to two weakly developed latero-distal groovelets, but pores are also arranged in radial lines. Referring to this, these scales are distantly comparable with the smoother ornamented and porose scales of Poracanthodes menneri ( Valiukevičius 1992: pl. 10, fig. 2). Typical P. subporosus , described from the Baltic ( Valiukevičius 1998: pl. 7, figs 13-22), demonstrates radial lines of pores or, rarely, pores grouped into two longitudinal areas without a strong linear arrangement. The Severnaya Zemlya specimens are dominated by large crown parts (up to the complete anterior half) devoid of pores. Their histological structure is composed of dentine and durodentine tissue ( Fig. 39A, B ), of a superpositional growth type; the style of radial pore canals and upstreamed superficial openings, as well as reduced cellular or acellular base bone, are without essential differences in comparison with P. subporosus ( Valiukevičius 1998: pl. 15, fig. 9, pl. 16, fig. 1).

Fin spines, preserved articulated in paratype LIG 35-A-376, are slightly recurved, with eight longitudinal ribs on each side. The proximal rib is wider than others. Spines have a widened tri-

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angular cross section with a slightly concave base. Sectioned spine fragments show a tripartite lamination: a basal layer composed of compact acellular thin-lamellar bone; a middle layer made of bone? with increased numbers of enlarged vascular canals, restricted during the growth by well developed concentric osteons; and an outer layer made of simple mesodentine network with plenty of osteocytes ( Fig. 39C, D ).

lvc

Spine histology is rather different from that of the well known P. menneri ( Valiukevičius 1992: fig. 11A, B), of which complicated tissue types depending on growth stages distinguish it and, inter alia, the structure of the middle and, particularly, the outer mesodentine layer.

BIOSTRATIGRAPHIC SIGNIFICANCE

Species appearing with the Poracanthodes punctatus Zone assemblage (Pridoli, Krasnaya Bukhta Formation) and spanning the Silurian/ Devonian boundary associates with the Poracanthodes menneri Subzone (Severnaya Zemlya Formation) mainly defined by an original composition of articulated acanthodians early Lochkovian in age.

Genus Acanthopora n. gen.

TYPE AND ONLY SPECIES. — Acanthopora transitans n. gen., n. sp.

ETYMOLOGY. — From Acantha (Greek): spine, thorn; and poros (Greek): pore.

AGE. — Early Devonian, early Lochkovian.

DIAGNOSIS. — As for the type and only species.

DISCUSSION

Through the presence of some randomly displaced pore openings in crowns of its scales, this taxon may be related to Poracanthodes , but it is different in its modified “ Poracanthodes ” - type histology, differing both by its structure and the function duration of its pore canal sys-

tem. Several affinities are also marked with the served holotype specimen is a small-sized fish, scales identified by Gross as Gomphonchus or which might have reached about 70-75 mm in Poracanthodes ( Gross 1971: pl. 9, figs 30, 31) length. Body slender: its height, both at the secfrom the Beyrichia Limestone (Upper Silurian). ond dorsal and the pectoral spines amounts

about 13 mm. Supposedly a long area behind the

anal fin spine preserved.

Acanthopora transitans n. sp.

( Figs 40-43 )

Fin spines

HOLOTYPE. — LIG 35-A-403: a specimen missing From the holotype specimen and paratype LIG head and tail, preserved laterally, with a small fragment 35-A-369, it is supposed to be a long-spine fish of pectoral spine, longer posterior dorsal and anal fin with mid-erected spines. The anal spine of holospines, and squamation, exposing myomeric structures type is slightly recurved and might be 13 mm long ( Fig. 40 A-C). Matusevich River, outcrop 1, bed 21.

(preserved 10.4 mm). The posterior dorsal (pre-

PARATYPES. — LIG 35-A-387: specimen 33 mm long, served on 6 mm) was inserted in a point vertically exposing only the squamation and a small fragment of unidentified fin spine; LIG 35-A-401: specimen about above and just behind the anal spine. Both are 51 mm long, exposing only squamation ( Figs 40D, E ; deeply inserted. Pectoral spine is in a low posi- 41D-G); LIG 35-A-395: small fragment of specimen tion. Its preserved part in holotype is about 2 mm. with intact scales. All from the Matusevich River, The ventral orientation does not allow to compare outcrop 1, bed 21. LIG 35-A-369: specimen about 58mm long, missing head, shoulder and tail areas the, ornamentation of pectoral spine with that of preserved laterally, with parts of posterior dorsal, anal others. The best preserved anal spine of holotype and unidentified spines and squamation ( Fig. 41 A-C). is strongly longitudinally ribbed with enlarged, Ushakov River, outcrop 21, bed 9. stout, sub-quadrangular proximal rib and four ETYMOLOGY. — From Transitans (Latin): variable, narrower ones on each side. Spine section is triantransitional, referring to a systematic position close to gular, basally wide, and narrow towards the tip. Poracanthodes .

Paratype LIG 35-A-369 exposes the ventrally

LOCALITY AND AGE. — October Revolution Island: preserved spines, supposedly belonging to a larger Matusevich River, outcrop 1, bed 21; Ushakov River,

specimen than the holotype. Its spines are wider,

outcrop 21, bed 9. Lower Lochkovian, top of the Severnaya Zemlya Formation. one small inarticulate displaced fragment bear a

sharp tip. The single central cavity is mainly open

DIAGNOSIS. — Small-sized ischnacanthid with slender body and long, deeply inserted, longitudinally ribbed and closed only at the tip. fin spines; two dorsal spines; pectoral spines in a low The spine histology shows a tripartite composiposition. Three tissue layers in spines: basal acellular tion. A basal lamellar acellular bone, lines the bone; porose, highly vascularised bone with osteons

central cavity; it is thin at the parabasal part

and small osteocyte numbers in the middle layer; superficial networked mesodentine with plenty of ( Fig. 42A ) and thick at the tip ( Fig. 42D ). The osteocytes. Scales small with rhombic crowns contai- middle layer is composed of very porose, highly ning some random posterior pores. Histology of vascularised bone with a small number of osteomodified “ Poracanthodes ”- type: crown dentine and durodentine without radial vascular, and pore system cytes ( Fig. 42A, B ). Large longitudinal vascular – without radial pore canals. Pore canals function canals ( Fig. 42E ) of different length and form only in two latest growth lamellae, replaced by vascu- in mature spines are restricted by concentric lar canals in the earlier ones. osteons. The superficial mesodentine layer

includes dense osteocytes and a network of

DESCRIPTION winding dentine tubules ( Fig. 42D, E ). Species described from articulated specimens missing head and tail areas or exposing only Squamation squamation, or part of the body behind the Dense lines of small body scales are displayed anterior dorsal spine. The body proportions are without overlap or with extremely small overlap weakly determinable. The most completely pre- areas ( Figs 40E ; 41C ). Sometimes wavy

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pdfs

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pdfs C

myomeric reflections of the body musculature can be seen ( Fig. 40B ). Scale crowns are of 0.18- 0.40 mm long, with elongated rhombic to rounded triangular forms, flat and with small proximal depressed medial area for the pointed edge of the overlapping crown. Rare examples have shallow linear groovelets along the laterodistal margins ( Fig. 41F ). The crown plate con- tains some pore openings on its distal part. They are displayed randomly or rarely grouped in two irregular strips ( Fig. 41E ). Scale bases are rhombic in outline, moderately convex, with anteriorly protruding crowns ( Fig. 41G ). Their mid-high neck contains small pore openings on the latero-distal walls ( Fig. 41D ). No obvious differences or marked varieties in scale shape can be observed along the preserved trunk of fishes.

The scale histology is certainly of a modified “ Poracanthodes ”- type. Up to eight growth lamellae in crowns composed of dentine and durodentine can be identified. Ascending vascular canals are almost without principal branches. All lamellae spaces are pierced by monotonous upstreamed, interwoven dentine canals, only rarely enlarged near the base ( Fig. 43A, E ). No radial vascular canals are encountered in crowns. Slightly complicated mesodentine-like network of canals is identified in primordial lamella ( Fig. 43C ). Neither osteocytes nor lacunae or lacunae-like widenings occur in crowns. The pore canal system, including neck and superfi- cial openings in crown, functions only within the two last growth lamellae ( Fig. 43 B-D), whereas in the earlier ones they are replaced by extremely widened strips of ascending vascular canals ( Fig. 43B, E ). This, and also the lack of radial and arcade pore canals, allows to suppose a formation of pore canal system anew in each next growth lamella. The scale bases are composed of thin-lamellar acellular bone, and exceptionally with Sharpey’s fibres demonstrating no more structural elements.

DISCUSSION

The presence of pore canals and particularly of superficial openings allows to place A. transitans n. gen., n. sp. close to the representatives of Poracanthodes Brotzen, 1934 . According to the pore openings, which do not cover all the crown plate, but its posterior part, A. transitans n. gen., n. sp. is the nearest to P. subporosus ( Valiukevičius 1998: pl. 7, figs 13-22), but with a different pattern of pore distribution. A. transitans n. gen., n. sp. does not demonstrate pore openings in radial lines. Furthermore differences occur within their histological structure. The originality in tissue composition (both of crown and base) and pore canal system discriminates the taxa under comparison. The principal differences are the lack of radial pore and vascular canals, the presence of thick superficial durodentine strip in crowns and the exclusively acellular bone in scale bases of A. transitans n. gen., n. sp. Pore canal system in all known representatives of Poracanthodes is functioning with no interruption during the growth, whereas in A. transitans n. gen., n. sp. it functions only within the last lamellae, and is replaced in the earlier ones by vascular canals.

BIOSTRATIGRAPHIC SIGNIFICANCE

Key species of the Poracanthodes menneri Subzone , based on association of articulated acanthodians from the topmost part of Severnaya Zemlya Formation. It corresponds to the lower part of the Nostolepis minima Zone , widely traced through the regions. Age is early Lochkovian. Tentatively correlates to the woschmidti Zone of conodont standard.

Genus Arcticacanthus n. gen.

TYPE SPECIES. — Arcticacanthus bicostatus n. gen., n. sp.

ETYMOLOGY. — From Arctic: referring to the arctic origin of the fish; and acantha (Greek): thorn, spine.

AGE. — Early Devonian.

DIAGNOSIS. — As for type and only species.

DISCUSSION

According to highly cellular bone in bases of scales, this genus might be classified among Climatiidae or Ischnacanthidae , but the presence of an original pore canal system in crowns defines a modified “ Poracanthodes ” - type histology, thus they will be placed within Ischnacanthidae .

Figures

FIG. 39. — Poracanthodes sp. cf. P. subporosus Valiukevicius, 1998, scale and spine histology, from paratype LIG 35-A-376, Spokojnaya River, outcrop 41, bed 12, Severnaya Zemlya Formation; A, vertical longitudinal section through the central part of scale with pore canal openings on the posterior crown half, anterior to left, thin section 3465; B, vertical transverse section through the central part of scale with pore canal openings on the extreme posterior part only, thin section 3467; C, D, transverse sections through the unidentified fin spines, thin sections 3471 (C) and 3470 (D). Abbreviations: abo, acellular bone; arcpc, arcade pore canal; avc, ascending vascular canal; dd, durodentine; dt, dentine tubule; lvc, longitudinal vascular canal; oc, osteocyte cavity; os, osteon; po, pore opening; rpc, radial pore canal; smd, simple mesodentine. Scale bar: 0.1 mm.

FIG. 40. — Acanthopora transitans n. gen., n. sp., Matusevich River, outcrop 1, bed 21, Severnaya Zemlya Formation; A, specimen exposing pectoral, posterior dorsal and anal fin spines and squamation with myomeric structures, lateral view, anterior to left, holotype, LIG 35-A-403; B, enlarged area of squamation with myomeric structures in the central part of the holotype; C, enlarged fin spines of the same specimen; D, paratype LIG 35-A-401 in lateral preservation, squamation; E, enlarged area of squamation of the same paratype. Abbreviations: afs, anal fin spine; pdfs, posterior dorsal fin spine. Scale bars: A-D, 10 mm; E, 1 mm.

FIG. 41. — Acanthopora transitans n. gen., n. sp., Severnaya Zemlya Formation; A, posterior dorsal, anal and undeterminate fin spines and squamation, lateral view, anterior to left, paratype LIG 35-A-369; B, enlarged area of the same specimen with anal and undeterminated spines; C, intact scales from the same specimen displaced behind the anal spine; Ushakov River, outcrop 21, bed 9; D-G, scales; D, LIG 35-2058, side; E, LIG 35-2056, crown; F, LIG 35-2057, crown; G, LIG 35-2059, side. All scales from the paratype LIG 35-A-401; Matusevich River, outcrop 1, bed 21. Abbreviations: afs, anal fin spine; pdfs, posterior dorsal fin spine. Scale bars: A, B, 10 mm; C-G, 0.5 mm.

FIG. 42. — Acanthopora transitans n. gen., n. sp., spine histology, paratype LIG 35-A-369, Ushakov River, outcrop 21, bed 9, Severnaya Zemlya Formation; A, transverse section through the parabasal part, thin section 3455; B, magnified left corner of the same section; C, transverse section through the spine tip, thin section 3456; D, magnified proximal area of the same section; E, longitudinal section of spine, thin section 3458. Abbreviations: abo, acellular bone; cbo, cellular bone; cc, central cavity; lvc, longitudinal vascular canal; oc, osteocyte cavity; os, osteon; smd, simple mesodentine. Scale bars: 0.1 mm.

FIG. 43. — Acanthopora transitans n. gen., n. sp., scale histology, Severnaya Zemlya Formation; A, vertical transverse section through the central part of scale, thin section 3342; B-E, vertical longitudinal sections of scales, anterior to right (B-D) and to left (E); B, scale like figured on Fig. 41E, thin section 3454; C-E, scales like figured on Fig. 41F, thin sections 3341 (C), 3343 (D) and 3344 (E). Paratype LIG 35-A-369 (A, C-E); Ushakov River, outcrop 21, bed 9; and paratype LIG 35-A-395 (B), Matusevich River, outcrop 1, bed 21. Abbreviations: abo, acellular bone; avc, ascending vascular canal; dd, durodentine; gl, growth lamella; pc, pore canal; po, pore opening; prgl, primordial growth lamella. Scale bar: 0.1 mm.