Acritolepis ushakovi, Valiukevičius, 2003

Valiukevičius, Juozas, 2003, Devonian acanthodians from Severnaya Zemlya Archipelago (Russia), Geodiversitas 25 (1), pp. 131-204 : 136-146

Acritolepis ushakovi n. sp. ( Figs 1-12)

HOLOTYPE. — LIG 35-A-380. Adult specimen missing head and tail, in lateral view. Articulated anterior and posterior dorsal, pectoral, pelvic and anal fin spines, two prepectorals, scapula, head covering plates, tesserae-scales, trunk scales ( Figs 1; 2 ). Matusevich River, outcrop 1, bed 21.

PARATYPES. — LIG 35-A-384: articulated dorsal fin spines in lateral view and squamation in dorsal view ( Fig. 4 A-C, E); LIG 35-A-385: spine fragments, tesserae-scales and trunk scales; LIG 35-A-390: jawbone with teeth in lateral view ( Fig. 3A ), disarticulated scales and denticulate tectal plates; LIG 35-A-404: articulated specimen in lateral preservation exposing squamation (trunk scales, tesserae-scales, tesserae) and fragments of spines, fully dissolved during preparation; LIG 35-A-348: specimen in lateral view with dorsal fin spines, fragment of anal spine and squamation ( Fig. 2B ). All specimens from the Spokojnaya River, outcrop 41, bed 12 ; LIG 35-A-371: articulated specimen with pectoral and anal(?) fin spines in inner view, scapula, tesserae and tectal plates of prepectoral area and intact scales (small and enlarged, tesseraelike); LIG 35-A-386: specimen exposing jawbone, articulated teeth (main cusps, intercusps and palatine), denticles, branchial(?) cones, tectal plates, denticulate tesserae-scales and tesserae of polyodontode type ( Fig. 3B, C ). Both specimens from the Matusevich River, outcrop 1, bed 21 .

ETYMOLOGY. — In honour of one of the first discoverer and researcher of Severnaya Zemlya Archipelago: G. A. Ushakov ( Russia).

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

DIAGNOSIS. — Moderately deep-bodied climatiid with two lowly erected dorsal fin spines, long pectorals, two pairs of prepectorals; no intermediates; anal spine opposite to posterior dorsal, and pelvic midway between pectorals and anal; spines longitudinally rib- bed, composed of acellular bone and highly vascularised mesodentine (outer part); slender, cylindrical scapula with wide procoracoid? blade; jaw bones with ankylosed principal cusps and intercusps of trabecular dentine; tuberculate or denticulate tectal plates, polyodontode tesserae and denticulate tesserae-scales composed of mainly acellular bone (basal parts), vascularised osteon-bearing cellular bone (middle) and mesodentine (superficial layer); uniform body scales with short-ridged and flattened crowns, composed of durodentine (superficial strip), simple mesodentine and Stranggewebe restricted to each growth lamella, and bases of variable cellular bone.

DESCRIPTION

Body shape and proportions

Described from specimens missing head and tail, preserved laterally flattened, Acritolepis ushakovi n. gen., n. sp. is a moderately deep-bodied (maximum depth to supposed length ratio of 0.22-0.24), small-sized (up to 80-83 mm long, based on proportions with the longest anterior dorsal spine in LIG 35-A-384) climatiid with two poorly erected, deeply inserted dorsal fin spines, the posterior longer and wider. The pectoral spines are longer than dorsals and inserted in a low lateral position. It possesses two pairs of prepectoral spines, and short anal and pelvic spines. The anal spine lays opposite or just in front of posterior dorsal, and pelvic one at midway between pectorals and anal. Measurements on Table 1.

Fin spines

Pectoral spines are thick-walled, with a single central cavity, open at parabasal part. They are slightly curved and ornamented with six to eight longitudinal ribs on each side ( Figs 1C; 2A ). The last three distal ribs near the inserted portion may be interrupted (tuberculated) or short, converging ( Fig. 6A ). Deep grooves separate them. The holotype demonstrates a shallow insertion (inserted part represents 2.2 mm of a whole 14.6 mm spine length). Spines are sharpened and show in section at the base a flattened horseshoe form, a rounded isosceles triangle in the middle part and blunt triangle form at the tip. Prepectoral spines ( Fig. 2A ) are different both in size and proportions; it is why they are treated as two pairs. The anterior is 1.6 mm in length with

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A B C D E

A-D, H-O

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a base length of 2.4 mm and ratio of base length/spine length about 1.59. The second, situated closer to pectoral spines, show a length of 2.1 mm and a base length of 3.4 mm, with a ratio base length/spine length of about 0.6. Randomly distributed and extensively overlapping rounded tubercles compose their ornamentation. An arrangement in lines is partly seen on the second one. The most complete dorsal spines preserved on paratype LIG 35-A-348 have a length of 14.9 mm (for the anterior spine) and about 17 mm (for the posterior). Their basal width is 1.5 mm and 2.3 mm respectively. The biggest anterior dorsals might reach 23 mm in length (on paratype LIG 35-A-384) and posterior dorsals 26-27 mm. These spines are slightly curved posteriorly, moderately erected, hollow, of a flattened rounded triangular form in section ( Fig. 6B ). Their sculpture is made of six longitudinal ribs. Specialized enlarged scales ( Fig. 4A, C ) surround the deeply inserted spine part. Pelvic and anal spines are shorter than pectorals ( Fig. 1C), with six longitudinal ribs on each side. In morphological features they look identical to pectorals. The anal spine is inserted in the body musculature at a higher angle than dorsals. Large pore openings of vascular canals lay within the grooves on dorsal and pectoral spines.

The histological structure of spines is uniform. Two thirds of their walls are composed of dense, thin-lamellar acellular bone, lining the central cavity, which may be of a complicated configuration at the basal part ( Fig. 8A). The outer third (ribbed spine part) is composed of highly vascularised mesodentine containing large longitudinal canals, contracted in mature spines by concentric denteons ( Fig. 8A, B). Superficial canal openings connect with denteons of deeper layers. A network of short winding mesodentine tubules of “ Nostolepis ” - type includes a small number of lacunae.

Shoulder girdle

A slender scapula of cylindrical form slightly narrowing upwards represents the dermal shoulder girdle; it measures 4 mm in height in the holotype ( Fig. 1C) and 4.5 mm in LIG 35- A-371 ( Fig. 6C ). A wide basal blade (procoracoid?) with a deep groove for the pectoral spine(?) articulation makes a low cone with a parabasal diameter of about 4.1 mm ( Fig. 6C ). Its perichondral bone is highly vascularised; openings of large vascular canals on scapula walls are distributed randomly or in lines.

Dentition

The dentition is represented only by fragments of dentigerous jawbones with teeth of the main lateral row (32 mm long in paratype LIG 35-A-390 and 13.1 mm long in LIG 35-A-386); it also exposes ornamented denticles and palatine teeth. Firmly ankylosed principal cusps (five seen in LIG 35-A-390) of lateral biting edge have a triangular

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longitudinal and stretched rhombic transverse basal form with a rounded outer line. It bears an emerged inner keel, and shows sharp side flanges ( Fig. 3A ). The maximum cusp height is 2.1 mm in the first specimen and about 3.5 mm (with broken tip) in the second. It decreases in size in a row frontward. Main cusps may bear smaller lateral cusps. Three to four ( Fig. 3A ) intercusps are about one third as high as principals, essentially blunter and ornamented by vertical wavy interrupted groovelets (striated). Mesial side of the lateral ridge is covered by blunt striated denticles of irregular shape and height. Their parabasal form in transverse section changes from circular to oval. Several denticles are displaced between the lateral and mesial ridges ( Fig. 3C ). This area also bears randomly situated palatine teeth ( Fig. 5A ) of flattened and elongated sharp-flanged lingual form. They do not exceed 0.4 mm in length.

The teeth are composed of trabecular dentine ( Fig. 9C ) showing principal ascending canals and their obliquely upstream sideward branches. For principal cusps the material is supposed to be of specialized flange dentine (no section have been done).

Branchial? cones

The jawbone interridge area in LIG 35-A-386 bears separate random, hollow and thin-walled cones ( Figs 3C ; 4D ; 7J, K ); they are ornamented by linear perpendicularly or obliquely-directed mainly tricuspid spinelets.

Tectal plates and tesserae

The rostral area (anterior to shoulder girdle) of specimens (holotype, also LIG 35-A-371, LIG 35-A-386, LIG 35-A-390) is covered with three types of tesserae and plates. Polygonal irregularly shaped plates with extensively overlapping, rounded, flattened, unornamented tubercles ( Fig. 7I ) and with ridged stellate tubercles distributed without overlap ( Fig. 7H ) dominate on the shoulder girdle of the holotype ( Fig. 1C). A first sort of plates resembles prepectorals in their ornament, whereas the second bears blunt denticles resembling those from the mesial side of lateral jawbone ridge. All plates are composed of three tissue layers ( Fig. 9A, B ): 1) basal: laminar, mainly acellular bone; 2) middle: highly vascularised cellular bone with distinct osteon structures filling gradually the principal vascular canals during growth; and 3) superficial (ornamented): certain “ Nostolepis ” - type mesodentine with dense network of winding, mainly up streamed dentine canals with lacunae-like widenings.

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Polyodontode tesserae ( Fig. 5 E-G), the second plate type, are short and wide (1-1.4 mm). They are composed of numerous small odontodes growing in areal style. Like ordinary scales they have moderately convex bases and low necks. Some of these tesserae are distinguished by an anterior row of higher, separate odontodes, the remaining crown part consisting of poorly distinct odontodes: they are densely grown together into one compact flattened plate, concentrically striated by wavy grooves, lining the distal edge ( Fig. 5F ). This tesserae type is supposedly mainly connected with rostral head areas, represented in specimens exposing jawbones and dentition (LIG 35-A-386, LIG 35-A-390). Both varieties of the second-type tesserae have basal plates made of acellular thin-lamellar bone ( Fig. 12A, B ) and crowns with a slightly different composition. Crowns of the first variety show typical “ Nostolepis ” mesodentine with a network of winding dentine tubules, including lacunae ( Fig. 12A ). The junction strip between base bone and crown mesodentine contains princi-

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pal circular vascular canals constricted by concentric denteons. The same mesodentine is only present in anterior odontodes in tesserae of the second type ( Fig. 12B ). These are secondary odontodes (added later). The main part, including primordial lamella, is composed of thin-lamellar dentine without vascular canals, except for the principal circular ones, located higher in the neck.

The third type is represented by denticulate tesserae-scales with one-side (posterior) areal growth ( Fig. 5 B-D). They are small (0.3-0.4 mm long) and represent transitional forms passing to typical trunk scales. Several (dominant 3-4) show unornamented or radially ridged crowns with extensive overlaps (up to half of length) joined by a common base. Base’s bone contains rare osteocyte cavities ( Fig. 9D ); the neck area includes major circular vascular canals surrounded by wide strip of concentric denteons. The external mesodentine does not differ from that of other plates and tesserae.

Squamation

Except for the head and shoulder area, squamation is surprisingly uniform. Trunk scales small (0.17-0.32 mm long), with two to six parallel, short, rounded ridges, prolonging up to one third of crowns length, which is down sloping anteriorly. Low neck and convex, centrally or slightly anteriorly vaulted base ( Fig. 5 H-O) separated from the neck by a rim of sub-rhombic outline. Scales, recognized as tail scales, are narrower and longer ( Fig. 5J, N ). At the posterior dorsal spine in LIG 35-A-384 ( Fig. 4C ) we can see small area of magnified, slightly asymmetric scales with increased number of ridges (eight) like figured on Fig. 5K . Scales displaced densely in oblique lines without overlap ( Fig. 4E ). Trunk and tail scales composed of cellular bone (base), durodentine and mesodentine (crown) ( Fig. 10 ). Superpositional growth type. Cellular bone extremely thin-lamellar, with well defined long Sharpey’s fibres of longitudinal and perpendicular orientation ( Fig. 10D, F ) and inconstant number of osteocyte cavities. Simple mesodentine composes anterior, whereas Stranggewebe the posterior parts of every growth lamella, including primordial. Their number does not exceed four. Stranggewebe surrounded by outer strips of simple networked mesodentine. Osteocyte cavities and lacunae meet high, even in the superficial layer of primordial lamella, but never in the last formed, which are partly composed of durodentine. Scales only rarely show presence of large circular vascular canals ( Fig. 10A ) displaced over the base.

The scale-like crowned tesserae ( Fig. 7 A-F) consist of ridged rhombic, leaf-shaped elongated or widened asymmetrical crown, on a variable base exceeding crowns measurements. Tesserae may be two-crowned ( Fig. 7C, E, F ) side grown or grown antero-posteriorly, with smaller secondary proximal crown ( Fig. 7C ). These are found on the head area. One-crown tesserae are similar to trunk scales in histological structure ( Fig. 11B ). Differently from them, base plate composed of highly cellular bone and crown contained better developed enlarged radial and ascending vascular canals. Two-crowned (without overlap) tesserae ( Fig. 11A ) show histological structure

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of two trunk scales with separate convex bases, later joined by additional common basal plate. Both basal parts composed of thin-lamellar acellular bone resembling tectal tesserae of polyodontode type. Superposed lamellae (up to five) in the crown are composed of a simple mesodentine with enlarged principal circular vascular canals and a dentine tubule network with osteocytes.

DISCUSSION

Acritolepis ushakovi n. gen., n. sp. is closest to A. urvantsevi n. gen., n. sp. Despite the less complete preservation of the latter, several morphological elements and their histological structure can be compared. Uniting affinities are as follows: massive, thick-walled pectoral spines with a single central cavity and distinct longitudinal ribs, do not exceed eight on each side; deep grooves in-between containing in pectoral and dorsal spines numerous openings of vascular canals; rounded triangular flattened or isosceles triangular section; presence of prepectorals ornamented by extensively overlapping tubercles; slender cylindrical scapula with wide procoracoid? blade, containing openings of vascular canals; shape and proportions of body scales with poorly ridged lingual crowns, arranged in lines without overlap; Stranggewebe in crowns composing posterior parts of every growth lamella including the primordial one and surrounded by simple mesodentine strip; presence of large longitudinal vascular canals in spines.

However, the differences between species compared seem more essential. These are: deep insertion of pectoral spines in A. urvantsevi n. gen., n. sp. versus the shallow insertion in A. ushakovi n. gen., n. sp.; one pair of prepectorals in the first versus two in the second; spine histological structure composed of three distinct layers in A. urvantsevi n. gen., n. sp.: 1) basal bone with increased cell number and decreased number of vascular canals; 2) middle, also highly cellular bone with principal vascular canals restricted in mature spines by concentric osteons; and 3) superficial mesodentine with network of tubules and numerous osteocytes even at the outer strip. In A. ushakovi n. gen., n. sp. two thirds of spines are composed of a thin-lamellar acellular bone, and only the ribbed part is made of highly vascularised mesodentine containing few osteocytes; a higher density of denteons occur in the outer strip. Scales of A. urvantsevi n. gen., n. sp. bear less ridges (up to four), and are more elongated; the number of growth lamellae in crowns does not exceed three; there is a simple networked mesodentine with a diversified system of principal vascular canals and osteocytes even at the outer strips of the last growth lamella; the crown mesodentine gradually merges into cellular bone of scale base. A. ushakovi n. gen., n. sp. scales bear more ridges (up to eight) and crowns are more flattened; up to four to five growth lamellae; extremely thin-lamellar base bone with irregular (mainly small) number of osteocytes; simple mesodentine without system of principal vascular canals; the osteocytes never reach the outer strips of at least the two latest growth lamellae.

BIOSTRATIGRAPHIC SIGNIFICANCE

A. ushakovi n. gen., n. sp. is a key species of the Poracanthodes menneri Subzone , based on association of articulated acanthodians from the topmost part of Severnaya Zemlya Formation. It characterises the lower part of the Nostolepis minima Zone , traced throughout the regions worldwide. Age is early Lochkovian. Tentatively correlates to the woschmidti Zone of the standard conodont scale.

Figures

FIG. 2. — Acritolepis ushakovi n. gen., n. sp., Spokojnaya River, outcrop 41, bed 12, Severnaya Zemlya Formation; A, detailed rostral part of the holotype (LIG 35-A-380) with tuberculated prepectorals, head tesserae and tectal plates, lateral view; B, paratype (LIG 35-A-348), lateral preservation, anterior to right, anterior and posterior dorsal fin spines, fragment of anal spine, squamation. Abbreviations: adfs, anterior dorsal fin spine; pdfs, posterior dorsal fin spine; pfs, pectoral fin spine; pps1, pps2, first and second prepectoral spines; sc, scapula; sq, squamation; tp, tectal plate. Scale bars: 10 mm.

FIG. 4. — Acritolepis ushakovi n. gen., n. sp., Severnaya Zemlya Formation; A, paratype (LIG 35-A-384), lateral view, dorsal fin spines and squamation, anterior to right; B, magnified anterior dorsal fin spine of the same specimen; C, posterior dorsal fin spine with enlarged asymmetrical scales of the same specimen, Spokojnaya River, outcrop 41, bed 12; D, branchial? cone (LIG 35-2099) from the paratype LIG 35-A-386, Matusevich River, outcrop 1, bed 21; E, enlarged area of squamation with myomeric structures in the central part of paratype LIG 35-A-384, dorsal view, Spokojnaya River, outcrop 41, bed 12. Abbreviations: adfs, anterior dorsal fin spine; ascl, asymmetric magnified scales; con, branchial? cone; pdfs, posterior dorsal fin spine; sq, squamation. Scale bars: A-C, 10 mm; D, E, 1 mm.

FIG. 3. — Acritolepis ushakovi n. gen., n. sp.; A, jawbone with the main and intercusps, lateral view, paratype (LIG 35-A-390), Spokojnaya River, outcrop 41, bed 12; B, C, jawbone with teeth of the lateral row, denticles, palatine teeth and branchial? cones in lateral view (B) and medial-occlusal view (C), paratype (LIG 35-A-386), Matusevich River, outcrop 1, bed 21, Severnaya Zemlya Formation. Abbreviations: con, branchial? cone; dnt, denticle; ic, intercusp; jbo, jawbone; lr, lateral tooth row; mc, main tooth cusp; mr, medial ridge; pt, palatine tooth. Scale bars: 10 mm.

FIG. 6. — Acritolepis ushakovi n. gen., n. sp., drawings of selected elements of paratypes; A, part of pectoral? spine with longitudinally ribbed anterior and interruptedly ribbed (tuberculated) distal part (LIG 35-2138), paratype LIG 35-A-385, Spokojnaya River, outcrop 41, bed 12; B, middle? part of dorsal spine (LIG 35-2133); C, scapulocoracoid with openings of vascular canals (LIG 35-2124); B, C, from paratype LIG 35-A-371, Matusevich River, outcrop 1, bed 21, Severnaya Zemlya Formation. Abbreviations: po, pore opening; proco, procoracoid; sc, scapula; sr, spine rib; tubr, tuberculated rib. Scale bars: 1 mm.

FIG. 5. — Acritolepis ushakovi n. gen., n. sp.; A, palatine tooth with bony base (LIG 35-2103); B-D, denticulate tesserae-scales of one-sided areal growth, crowns; B, LIG 35-2106; C, LIG 35-2105; D, LIG 35-2100; E-G, polyodontode-type tesserae of areal growth from the head area, crowns; E, LIG 35-2104; F, LIG 35-2102; G, LIG 35-2101, paratype LIG 35-A-386, Matusevich River, outcrop 1, bed 21; H-O, scales; A-G, scales from the paratype LIG 35-A-404, Spokojnaya River, outcrop 41, bed 12, Severnaya Zemlya Formation; H, LIG 35-2047, normal trunk scale, crown; I, LIG 35-2045, side view; J, LIG 35-2041, scale from the tail area, crown; K, LIG 35-2044, specialized asymmetrical tessera-like scale, crown; L, LIG 35-2042, trunk scale, side view; M, LIG 35-2046, asymmetrical scale, crown; N, LIG 35-2040, scale from the tail area, crown; O, LIG 35-2039, trunk scale, crown. Scale bars: 1 mm.

FIG. 9. — Acritolepis ushakovi n. gen., n. sp., histologic structure of separated elements, Severnaya Zemlya Formation; A, vertical section through the tectal plate with starlet-like stellate tubercles (LIG 35-2136), (see Fig 7H), thin section 3499; B, magnified area of the same plate, microstructure of the central tubercle; A, B, paratype LIG 35-A-390, Spokojnaya River, outcrop 41, bed 12; C, trabecular dentine of the palatine tooth (see Fig. 5A), thin section 3501; D, vertical longitudinal section through the denticled tessera-scale of one-sided areal growth (see Fig. 5C), anterior to right, thin section 3500; C, D, paratype LIG 35-A-386, Matusevich River, outcrop 1, bed 21. Abbreviations: abo, acellular bone; cbo, cellular bone; cvc, circular vascular canal; dt, dentine tubule; dto, denteon; gl1, gl2, gl3, lamella of areal growth; lac, lacuna; md, mesodentine; oc, osteocyte cavity; os, osteon; trd, trabecular dentine; vc, vascular canal. Scale bars: 0.1 mm.

FIG. 7. — Acritolepis ushakovi n. gen., n. sp., drawings of selected elements of paratypes; A-F, head tesserae, crowns, anterior upwards; A, asymmetrical, two-crowned (LIG 35-2126); B, LIG 35-2125; C, rhombic, two-crowned, small anterior as a secondary scale (LIG 35-2137); D, LIG 35-2129; E, F, two-crowned, sidegrown; E, LIG 35-2127; F, LIG 35-2130; G, trunk scales, arrangement in lines without overlap (LIG 35-2134); H, I, tectal plates, with separated starlet-like striated denticles (H, LIG 35-2136) and with extensively overlaped unornamented tubercles (I, LIG 35-2131); J, K, branchial? cones with tricuspidate denticles, LIG 35-2122 (J) and LIG 35-2123 (K); A, B, D-G, I, from paratype LIG 35-A-371; C, from paratype LIG 35-A-385; J, K, from paratype LIG 35-A-386, Matusevich River, outcrop 1, bed 21, Severnaya Zemlya Formation; H, from paratype LIG 35-A-390, Spokojnaya River, outcrop 41, bed 12, Severnaya Zemlya Formation.Abbreviations:stub, stellate tubercle; tsp, tricuspidate spinelet; tub, tubercle. Scale bar: 1 mm.

FIG. 12. — Acritolepis ushakovi n. gen., n. sp., histology of polyodontode-type tesserae, from paratype LIG 35-A-386, Matusevich River, outcrop 1, bed 21, Severnaya Zemlya Formation; A, vertical transverse section through the tessera like figured on Fig. 5G, thin section 3502; B, vertical longitudinal section through the tessera with the secondary growth area on the crown (see Fig. 5F), thin section 3503. Abbreviations: abo, acellular bone; cvc, circular vascular canal; dto, denteon; gl1, gl2, gl3, gl4, lamellae of areal growth; seco, secondary odontode; smd, simple mesodentine; thd, thinlamellar dentine. Scale bar: 0.1 mm.

FIG. 10. — Acritolepis ushakovi n. gen., n. sp., scale histology, from paratype LIG 35-A-404, Spokojnaya River, outcrop 41, bed 12, Severnaya Zemlya Formation; A, vertical longitudinal section through the asymmetrical trunk scale (see Fig. 5K), thin section 3359; B, vertical transverse section through the central part of small elongated tail scale (see Fig. 5J), thin section 3363; C, D, vertical longitudinal sections through the central part of trunk (C, thin section 3355) and tail scale (D, thin section 3358); E, vertical transverse section through the head scale (see Fig. 5H), thin section 3360; F, vertical longitudinal section through the less ornamented (with two short ridgelets) trunk scale, thin section 3357. Abbreviations: cbo, cellular bone; cvc, circular vascular canal; dd, durodentine; gl, growth lamella; lsh, longitudinal Sharpey’s fibres; md, mesodentine; oc, osteocyte cavity; sqsh, transverse Sharpey’s fibers; stg, Stranggewebe (oriented mesodentine). Scale bar: 0.1 mm.

FIG. 11. — Acritolepis ushakovi n. gen., n. sp., histology of tesserascales, Severnaya Zemlya Formation;A, vertical longitudinal section through the central part of two-crowned tessera, thin section 3491, paratype LIG 35-A-404, Spokojnaya River, outcrop 41, bed 12; B, vertical longitudinal section through the slightly asymmetrical tessera-scale (see Fig. 7A), thin section 3492, paratype LIG 35-A- 371, Matusevich River, outcrop 1, bed 21. Abbreviations: abo, acellular bone; avc, ascending vascular canal; cbo, cellular bone; cvc, circular vascular canal; gl, growth lamella; md, mesodentine; rvc, radial vascular canal; smd, simple mesodentine; stg, Stranggewebe (oriented mesodentine). Scale bar:0.1 mm.