Acritolepis urvantsevi, Valiukevičius, 2003

Valiukevičius, Juozas, 2003, Devonian acanthodians from Severnaya Zemlya Archipelago (Russia), Geodiversitas 25 (1), pp. 131-204 : 146-151

Acritolepis urvantsevi n. sp. ( Figs 13-16 )

HOLOTYPE. — LIG 35-A-382. Anterior part of the trunk with paired pectoral and prepectoral fin spines, fragmentary scapula, tesserae-like plates and squamation preserved laterally ( Fig. 13A, B ). Pod”emnaya River, outcrop 67, bed 12.

ETYMOLOGY. — In honour of one of the first discoverer and researcher of Severnaya Zemlya Archipelago: N. N. Urvantsev ( Russia).

OTHER MATERIAL EXAMINED. — LIG 35-A-381 and LIG 35-A-402: parts of unidentified fin spines in ventral preservation and squamation. Matusevich River (both), outcrop 1, bed 21. Another specimen LIG 35-

cbo A-383: poorly exposing squamation alone. Spokojnaya River, outcrop 41, bed 12.

LOCALITY AND AGE. — October Revolution Island: Matusevich River, outcrop 1, bed 21; Spokojnaya River, outcrop 41, bed 12 and Pod”emnaya River, outcrop 67, bed 12. Top of the Severnaya Zemlya Formation, Lower Devonian, lower Lochkovian.

DIAGNOSIS. — Climatiid with long, deeply inserted, uniformly eight-ribbed pectoral fin-spines, wide basal tuberculated prepectoral spines and rounded slender scapula. Outer mesodentine of spine with numerous osteocytes. Middle and basal layers made of vascularised cellular bone with fragmentary osteons. Scales with elongated crowns, down sloping anteriorly; with two to three short anterior ridges. Composed of Stranggewebe and simple mesodentine. It shows a diversified system of principal vascular canals at the junction of base and crown.

DESCRIPTION

Poorly articulated specimens exhibiting a shoul- der girdle with fragment of scapula, pectoral spines and mostly distorted squamation. Body measurements and proportions are indistinct.

Fin spines

Pectoral spines ( Figs 13A, B ; 15A ) thick-walled, massive, slightly caudally bent with eight uniform, sharply distinct smooth ribs separated by deep grooves. Ribs and grooves are approximately of the same width, quadrangular, with a slightly rounded profile through the whole spine length. Preserved parts (22.6 mm for spine fossilized laterally just downward the prepectoral, and 20.7 mm for other, oriented ventrally) are hollow, with single fully closed central cavity. In transverse sections the spine ( Fig. 15A ) shows a slightly rounded triangular form with parabasally widened sides (about 3 mm). The inserted portion (about 6.3 mm) demonstrates a deep insertion in the body.

Spine histology contains cellular bone and mesodentine structures. The thin superficial layer (ridged part) is formed of mesodentine ( Fig. 14A, B ) with a dense network of short, winding dentine tubules; it contains lacunae and osteocytes even at the surface. Middle spine part is composed of cellular bone, densely penetrated by osteocyte cavities and linear fibers. Large longitudinal vascular canals and elongated cell structure ( Fig. 14D ) are constricted in mature spines by concentrically thin-laminated osteons, better seen in transverse sections ( Fig. 14B ). Large pore openings, distributed in grooves are visible on the spine surface. They continue into deeper levels of the middle layer as wide irregular vascular canals ( Fig. 14A ). The basal layer, lining the spine cavity, is approximately of the same volume as the middle one, formed of bone with increased cell number and less numerous enlarged vascular canals.

Prepectoral spine ( Fig. 15D ) shows a sharp tip and a wide basal triangular form in lateral view. It is slightly asymmetrical. Its total length is 5.0 mm, base length, 4.1 mm and the ratio of base length to spine length is about 0.82. 10 longitudinal rows of rounded and oval tubercles – beaded ridges – are present ( Fig. 15D ). Tubercle size decreases both toward the sides and the tip. Basal parts of tubercles are mostly extensively overlapped and thus to the tip, their turned margin has a higher angle in profile.

The histological structure of mesodentine and cellular bone composing this spine is similar to that of pectoral ones, except for osteon formation ( Fig. 14C ).

Shoulder girdle

The unarticulated scapula is present over the pectoral spine. Its visible part is round, 3.3 mm high and shows a base diameter of 3.2 mm. Just above the base, its diameter is reduced to half. On the walls pointed openings of large vascular canals ( Fig. 15C ) are seen. They penetrate the perichondral bone and are regularly distributed in lines.

Squamation

In the specimens, only small areas show an intact scale cover. This squamation is mainly distorted and represented by isolated scales or small articulated scale groups ( Fig. 15B ). Two morphological scale varieties can be recognized. First: enlarged examples with multiangular isometric, rhombic, often elongated or widened scales, slightly convex base and aside to other ones with twice or more times reduced crown ( Fig. 13E, F ). Maximum diameter of the base reaches 1.5 mm. The crown is anteriorly low and the neck is either absent or very low. The wide medial, slightly concave area gives to this crown a sort of tongue shape. Crowns are devoid of ornamentation or with two or three very short blunt ridges. This scale variety is mainly observed to the left (anteriorward) of pectoral fins in the holotype and treated as head scales. They represent a morphological transition to tesserae-like scales or plates, like those known in climatiids. The second variant ( Fig. 13C, D, G, H ) recognized as trunk scales shows the following features: crown size varies from 0.2 mm to 1.2 mm and crown is flat or slightly concave in the central part ( Fig. 13C ). Rhombic, elongated or leafformed with rounded and widened anterior margin, they are ornamented (up to one third of the crown length) with three to four short parallel ridges, and possess a sharpened posterior margin, sloping slightly upward and overhanging the base ( Figs 13H ; 15B ). Their base is moderately convex ( Fig. 13D ), centrally or anteriorly vaulted. A low and stout neck is separated from the base by a well developed rim of rhombic or sub-circular outline.

The scale histology of both morphological varieties is practically uniform. Head scales or tesserae-like scales ( Fig. 16A, E ) as well as trunk scales ( Fig. 16 B-D, F) are composed of oriented (Stranggewebe after Gross 1971) and simple mesodentine, characteristic of the “ Nostolepis ”- type histology. Both varieties contain widened main circular and radial vascular canals, located at the junction between the base and crown ( Fig. 16E, F ). Ascending canals, rising from them, develop wide principal branches in every growth lamella, of which the number does not exceed three. Growth is superpositional. Oriented mesodentine composes the large posterior scales area primordial lamella included. Horizontally elongated lacunae are displaced densely to each other without leaving any space for dentine tubule connections. The outer strips of lamellae are composed of a simple mesoden-

lvc

tine with a network of winding, narrow dentine tubules and numerous osteocytes with short processes. An analogous type of tissue composes entirely the anterior crown area. The crown mesodentine gradually turns into the cellular bone of base. The base tissue is extremely thin-layered with a maximum number of multiangular or slightly rounded osteocyte cavities.

DISCUSSION

Acritolepis urvantsevi n. gen., n. sp. is comparable to Nostolepis striata Pander, 1856 and Canadalepis linguiformis Vieth, 1980 only by several selective features. Less sculptured “Kronenplättchen” (tesserae coronate) ( Gross 1971: pl. 5, figs 5-7, 10) of N. striata are distantly similar by their large inconstantly shaped base, their anteriorly lowered crown with small numbers of short and blunt ridges. Scales grown together or distinct tesserae of the new species do not meet. Unfortunately, the histological structure of “Kronenplättchen” observed by the author is very fragmentary, with the supposition they could be close to N. striata body scales. The vascular canals piercing the base, are not found in A. urvantsevi n. gen., n. sp. Fin spines of the compared species are rather different. Short and stout, tuberculated spines of N. striata ( Gross 1971: pl. 8, figs 2-26) are distantly close only to prepectorals of A. urvantsevi n. gen., n. sp. Longitudinally smooth-ribbed spines like pectorals of A. urvantsevi n. gen., n. sp. have not been described in N. striata . The main spine histological structure is comparatively similar. Superficial mesodentine of the new species contains more numerous osteocytes and a more complicated network of dentine tubules. Middle and basal bony layers are characterised by a magnified tissue vascularisation; osteon structures are only fragmentarily observable (compare with Gross 1971: text-fig. 14A-E), whereas they show a great density in the cellular bone of mature spines in N. striata .

Morphological similarity of A. urvantsevi n. gen., n. sp. and Canadalepis linguiformis is only restricted to the general shape and crown ornamentation of tesserae-like scales with developed basal plates ( Vieth 1980: pl. 7, figs 9-18). Essential differences occur in the histological structure. In C. linguiformis the Stranggewebe is absent ( Vieth 1980: text-fig. 23A-D), the simple mesodentine contains rare osteocytes and lacuna-like widen- ings of dentine tubules. The main system of diversified vascular canals, characteristic for A. urvantsevi n. gen., n. sp., does not exist in C. linguiformis . The cellular base bone of new species contains a larger numbers of osteocytes.

BIOSTRATIGRAPHIC SIGNIFICANCE

Identical to Acritolepis ushakovi n. gen., n. sp.

Family CLIMATIIDAE Berg, 1940 Genus Nostolepis Pander, 1856

Figures

FIG. 13. — Acritolepis urvantsevi n. gen., n. sp.; A, B, holotype (LIG 35-A-382), Pod”emnaya River, outcrop 67, bed 12, Severnaya Zemlya Formation; A, paired pectoral and prepectoral fin spines, tessera-like plates and squamation, preserved in lateral view, anterior to left; B, magnified area with pectoral and prepectoral fin spines; C-H, scales and tesserae; C, LIG 35-2049, normal trunk scale, area behind the pectoral spine, crown; D, LIG 35-2053, scale from the same area, lateral view; E, LIG 35-2048, tessera-like plate, area of prepectoral spine, crown; F, LIG 35-2051, tessera-scale from the same area, crown; G, LIG 35-2097, scale from the mostly rightward distanced area; H, LIG 35-2052, slightly asymmetrical trunk scale. All scales from the holotype specimen. Abbreviations: pfs, pectoral fin spine; pps, prepectoral spine; sc, scapula; tlp, tessera-like plate. Scale bars: A, B, 10 mm; C-H, 0.5 mm.

FIG. 14. — Acritolepis urvantsevi n. gen., n. sp., histological structure of fin spines, Severnaya Zemlya Formation; A, transverse section through the basal part of pectoral spine containing pore openings in the grooves, thin section 3475; B, transverse section through the two-ribbed fragment of pectoral spine (supposedly, middle part), thin section 3481; C, transverse section through the fragment of tuberculated prepectoral spine, thin section 3473; D, longitudinal section through the single-ribbed fragment of pectoral spine, thin section 3472; A, C, D, holotype (LIG 35-A-382). Pod”emnaya River, outcrop 67, bed 12; B, specimen LIG 35-A-402. Matusevich River, outcrop 1, bed 21. Abbreviations: cbo, cellular bone; dt, dentine tubule; lvc, longitudinal vascular canal; md, mesodentine; oc, osteocyte cavity; os, osteon; po, pore opening. Scale bar: 0.1 mm.

FIG. 15. — Acritolepis urvantsevi n. gen., n. sp., drawings of selected elements, Severnaya Zemlya Formation; A, pectoral spine in lateral view with transverse section through median part; B, trunk scales, arrangement in lines; C, basal portion of scapula with linear openings of vascular canals; D, magnified prepectoral spine; A, C, D, holotype (LIG 35-A-382). Pod”emnaya River, outcrop 67, bed 12; B, specimen LIG 35-A-402, Matusevich River, outcrop 1, bed 21. Abbreviations: bas, basal (inserted) spine part; po, pore opening; sr, spine rib; tlp, tessera-like plate; tub, tubercle. Scale bars: A, 2 mm; B-D,1 mm.

FIG. 16. — Acritolepis urvantsevi n. gen., n. sp., histology of scales, all scales from the holotype (LIG 35-A-382), Pod”emnaya River, outcrop 67, bed 12, Severnaya Zemlya Formation; A, vertical longitudinal section through the tessera-scale with multiangular base, thin section 3351; B, vertical longitudinal section through the elongated trunk scale, thin section 3348; C, vertical transverse section through the central part of normal trunk scale, thin section 3347; D, magnified area of the primordial growth lamella of the same scale; E, vertical transverse section through the anterior part of tessera-like scale with quadrangular base, thin section 3349; F, vertical longitudinal section through the normal trunk scale, thin section 3350. Abbreviations: avc, ascending vascular canal; cbo, cellular bone; dt, dentine tubule; gl, growth lamella; oc, osteocyte cavity; rvc, radial vascular canal; smd, simple mesodentine; stg, Stranggewebe (oriented mesodentine). Scale bars: 0.1 mm.