Milesacanthus sp.

Hairapetian, Vachik, Valiukevičius, Juozas & Burrow, Carole J., 2006, Early Frasnian acanthodians from central Iran, Acta Palaeontologica Polonica 51 (3), pp. 499-520 : 508-513

Milesacanthus sp. aff. M. antarctica Young and Burrow, 2004

Figs. 8 , 9 , and 10.

Material.—Ten scales from sample H1, 121 scales from sample GI4, both middle Mesotaxis falsiovalis to Palmatolepis hassi conodont zones of Chahriseh section, Kaftari Mt., central Iran.

Description

Morphology.—All specimens examined are identified as flank scales. They are of a medium size; the largest reach 1.15 mm in length and width, but most are 0.6–0.8 mm. Crowns are flat, isometrically rhomboid, from slightly to distinctly rounded, or rarely with a subcircular outline. Crowns protrude beyond the base on all sides or at least posteriorly. Eight (only rarely), or most often twelve to fourteen subparallel ridges form a fan−like pattern over the crown. Ridge crests are flattened, decreasing gradually in height and narrowing towards the posterior, and extend the entire crown length ( Fig. 8A 1 ; AEU 311), or else fade out leaving the posteriormost crown part unornamented ( Fig. 8B 1, C 1 ; AEU 312–313). One or two median ridges may show short bifurcations at the anterior crown edge. The flatness of ridges often gives the impression of deep, linear and narrow grooves on the crowns (especially in lateral view). Scales have well outlined, medium to high concave necks carrying large pores and two symmetrically positioned “warts” on the posterior faces. “Warts” may be circular ( Fig. 8B 3 ) or transversely elongated ( Fig. 8C 3 ) in outline. Very large pores (five−seven per side), arranged in three lines ( Fig. 8A 3 ), open out from a complicated system (or systems) of canals. Some canals rise vertically and open out in the grooves of the anteriormost crown ( Fig. 8B 1 ), while others extend diagonally through the posterior crown ( Fig. 8A 2 ).

Scale bases are isometric or wide rhomboid, of medium convexity, with the deepest part centrally or, more frequently, slightly anteriorly.

Histology.—Scales have Diplacanthus − type histological structure. Up to ten thin growth lamellae in the crowns are composed of syncitial (non−lacunal) mesodentine and (particularly in the anterior crown) multiple−branching, long and winding ascending dentine canals ( Figs. 9A, B 1 , 10B 2 ) which have many narrow branchings of canaliculi. The posterior part of the crown is characterised by less branched, straighter ascending canals. The horizontal parts of growth zones in the crown plate have many narrow, short dentinal canaliculi directed perpendicularly outward and emanating from large ascending hyphae vascular canal fungi horizontal canals which run under the surface grooves ( Figs. 9A , 10A 2, C 2 ). The primordial zone (first growth lamella) contains a knot of very densely interwoven canals ( Fig. 9B 1 ). Pore canals are rarely observed in thin sections of crowns (possibly due to the preservation mode) and are distinguishable as rounded black spots ( Figs. 9B 1 , 10C 1 ) placed randomly at different heights in the neck area.

Scale bases are composed of acellular bone arranged in thin layers and pierced by long and straight Sharpey’s fibres which would have given a strong anchorage in the corium. As well as short and winding canaliculi (canals of Williamson sensu Ørvig 1951), bases contain longer and wider canals ( Fig. 10C 1 ) clearly distinguishable from the canaliculi.

ascending growth lamella vascular canal dentine tubule

Discussion.— Milesacanthus antarctica from the Givetian Aztec Siltstone of Antarctica was diagnosed on the skeletal structure of articulated fish and other isolated remains (scales and fin spines) discovered in the same horizons ( Young and Burrow 2004). According to the diagnosis of the genus, scales “... are ornamented with 14–24 subparallel ridges or grooves extending from the anterior edge to at least midcrown; scales have pore canal system in the anterior part of the crown with pore openings in the grooves and the lower neck ...” ( Young and Burrow 2004: 26). Iranian specimens fit closely to M. antarctica according to scale size and crown/ neck/base proportions and pore openings in the crown and neck. However, the crown ornamentation of the Iranian specimens (8–14 linear grooves rather than low flattened ridges) shows fewer grooves as compared to the Antarctic ones, and is more fan−like rather than the almost parallel ridges and grooves in M. antarctica . The bifurcation of ridges occasionally observed in the median crown of M. sp. aff. M. antarctica is not referred to in M. antarctica . Histological structure of the Antarctic scales is poorly preserved, but the illustrations of Young and Burrow (2004: fig. 3A–F) demonstrate a better developed system of branching pore canals and horizontal dentinal canals in the lower crowns, and a comparable appearance of ascending canals in the vertical growth zones. The possibly vascular canals in bases of the Iranian scales are considerably more pronounced in length and width than for the Antarctica scales, based on the small canaliculi fragments exemplified by Young and Burrow (2004: fig. 3D).

Morphologically, the Iranian scales resemble most closely an Emsian (Early Devonian) diplacanthid from the Jawf Formation of Saudi Arabia ( Forey et al. 1992) which was recently studied in detail and described as Milesacanthus ancestralis Burrow, Lelievre, and Janjou ( Burrow et al. 2006: figs. 7, 8). These scales were diagnosed by having six to eighteen subparallel longitudinal deep linear grooves rather than ridges on a flat crown, pores opening out in crown grooves and extending from a pore canal system in the anterior crown part, deep neck with lower and upper rows of large pores, and “warts” on the posterior neck. The Saudi scales differ from the Iranian ones in having a complicated syncitial mesodentinal network in the lower crown (neck area) filling the growth zones between the main branches of the ascending vascular canals ( Burrow et al. 2006: fig. 8.1), and also lacunal widenings of dentine canals, particularly dense in the lower neck; and the horizontal canals running under the grooves of the crown plate are the longest and widest of the species compared, and supplied by numerous winding canaliculi ( Burrow et al. 2006: figs. 7, 8).

Ectopacanthus ? pusillus Valiukevičius, 1998 from the upper Lochkovian (Lower Devonian) of the Baltic region also shows some similarities with Milesacanthus sp. aff. M. antarctica . Its scales have nine to eighteen narrow ridges and linear fan−like grooves extending over the crown; some ridges bifurcate; scale neck is high and concave. However, the scales are tiny, Ṥ 0.5 mm. Histologically, their crowns differ to those of the Iranian taxon in being composed of dentine with a thick durodentine superficially, in up to twelve growth lamellae; their modified Poracanthodes− type structure was defined on the presence of radial, arcuate and ascending pore canals forming a system with openings only in the neck and not on the crown surface ( Valiukevičius 1998: pl. 14: 2–4). The taxon was classified as an ischnacanthiform based on the pore canal system in the scales. Perhaps it might rather belong with Milesacanthus in a group of diplacanthids distinguished from the genus Diplacanthus by having scales with a crown sculpture mainly consisting of grooves, and an internal pore canal system.

Scales previously attributed to the genus Diplacanthus which resemble Milesacanthus sp. aff. M. antarctica in the crown sculpture are: (1) Diplacanthus poltnigi Valiukevičius, 2003b , which has 10 to 23 sub−parallel grooves extending the whole crown or fading out almost at the posterior corner; also attributed to this taxon are scales from the uppermost Emsian of Graz, Austria which Poltnig (1984: pl. 2: 1–7) assigned to D. longispinus Agassiz, 1845 ; and others from the upper? Lochkovian ( Vieth 1980) or possibly Pragian ( Langenstrassen and Schultze 1996) of Arctic Canada which Vieth (1980: pl. 5: 14a, b) assigned to Nostolepis gracilis ; and (2) Diplacanthus pechorensis Valiukevičius (2003a : figs. 23G, H, 24D, E) from the Lochkovian to Pragian of the Timan−Pechora region ( Russia) which have crowns ornamented with symmetric bipartite ridgelets and fan−like grooves, up to twenty on each side. The scales of both taxa have the Diplacanthus− type microstructure: crowns of mesodentine with upwardly directed bushy dentine tubules emanating from wide ascending and radial vascular canals (some of which arise high in the neck); no lacunae in dentinal canals nor pore canal systems present in crowns; acellular bone of scale bases pierced by long apically−directed vascular canals.

Diplacanthid spine fragments

Fig. 11 .

Material.—Three spine fragments from sample GI4, middle Mesotaxis falsiovalis to Palmatolepis hassi conodont zones of Chahriseh section, Kaftari Mt., central Iran.

Description

Morphology.—The longest fragment of fin spine is about 2.8 mm long and 0.5 mm wide ( Fig. 11A ; AEU 320), and (subcircular) crescent−like in cross−section. The hard tissue forming the spine is thickest along the leading edge, thinning slightly laterally, with a large circular central cavity which is wide open along the trailing edge of all fragments. Remnants of 5–6 smooth longitudinal parallel ribs extend almost the whole fragment length. The ribs are rounded−quadrangular with steep lateral faces; all are of equal width or with a slightly wider rib along the leading edge; ribs are separated by deep narrow uniform grooves. No pore openings are observed in the grooves; some large vascular canals are exposed in areas damaged during preparation or fossilisation; these canals become the grooves between the narrow ridges of the insertion area as exposed at the extreme left end of the fragment shown in Fig. 11A 1 .

Histology.—The outer ribbed layer was lost during sectioning. The preserved part of the spine is composed of cellular bone ( Fig. 11B ; LIGG 3807) filled by long and wide longitudinal (linear in outline) and radial (subcircular in outline) vascular canals, with no surrounding osteons and without an inner lamellar layer around the central cavity. Polygonal osteocyte lacunae are disposed randomly throughout ( Fig. 11B 2 ).

Discussion.—The few, small spine fragments collected through this study as isolated remains preclude a stricter taxonomical attribution because of absence of most diagnostic characters. However, straight spines with a rib pattern of 5–6 uniform ribs per side, separated by narrow grooves, suggest a diplacanthid rather than ischnacanthiform (or other) affinity. By these features, the Iranian diplacanthid spines resemble those of Milesacanthus antarctica ( Young and Burrow 2004: figs. 4B–D, 5A–D) which have from three ribs (per side) plus a wider leading edge rib on the paired pectoral and pelvic spines, and up to five longitudinal ribs on the dorsal and anal spines. Tissues composing spines in M. antarctica articulated fish and isolated, better preserved spines ( Young and Burrow 2004: fig. 5A, D) “...are quite vascular between the central cavity and the leading edge, with a radiating structure more evident anteriorly” ( Young and Burrow 2004: 34). The principal histological structures of spines were listed as: vascular canals surrounded by narrow denteons; the spines lack a subcostal canal; the thin inner spine layer presumed osseous. The Iranian spines also lack a subcostal canal, and similarly have regularly−spaced longitudinal and radial canals, but are mainly composed of a cellular bone with abundant osteocytes from below the ribs to the central cavity. However, in Milesacanthus antarctica spines, the thin inner lamellar layer is only developed in the exserted part, increasing in thickness distally. The proximal exserted part of the Saudi Arabian spines also lacks an inner lamellar layer ( Burrow et al. 2006: fig. 9.11). Fine histological detail was not preserved in the spines from Antarctica and Saudi Arabia, so it is unclear if the lack of osteocytes in their thick osteodentine/ cellular bone layer is an original absence or a result of preservation.

Figures

Fig. 8. Diplacanthid acanthodian Milesacanthus sp. aff. M. antarctica Young and Burrow, 2004 from early Frasnian, horizon GI4 of an unnamed formation in the Chahriseh section, Kaftari Mt., central Iran. A. AEU 311. B AEU 312. C. AEU 313. SEM micrographs of flank scales in three positions respectively, in crown (A1, B1, C1), basal (A2, B2, C2), and postero−lateral (A3, B3, C3) views. In A1, A2, B1, B2, C1, and C2, anterior is upward.

Fig. 9. Diplacanthid acanthodian Milesacanthus sp. aff. M. antarctica Young and Burrow, 2004 from early Frasnian, horizon GI4 of an unnamed formation in the Chahriseh section, Kaftari Mt., central Iran. Photomicrographs of thin sections of flank scales in vertical longitudinal (A) and vertical transverse (B) sections.A. Thin section LIGG 3800. Structure of syncitial mesodentine with mainly upwardly directed dentine tubules in the upper crown part, unbranched ascending vascular canals in the lower crown part, and numerous elongated?vascular canals and Sharpey’s fibres piercing an acellular bone in scale base. B. Thin section LIGG 3801. B1, ascending, radial and?pore canals of the neck area, knot of canals in primordial zone, and small?vascular canals in an acellular bone of base, all exposed in a section made through the anterior, most ridged crown part; B2, positioning of fungal hyphae of a magnified area in the upper right corner of crown of the same scale. Scale bars 0.1 mm.

Fig.1. Map of Iran (A) and close−up of the Chahriseh area showing location of the Kaftari Mountain section (B).

Fig. 3. Climatiid acanthodian Iranolepis ginteri gen. et sp. nov. from early Frasnian, horizon GI4 of an unnamed formation in the Chahriseh section, Kaftari Mt., central Iran. SEM micrographs of flank scales. A. Holotype, AEU 301 in crown (A1, anterior upward), basal (A2, anterior upward), and lateral (A3, anterior to right) views. B. AEU 302 in crown (B1) and basal (B2) views. C. AEU 303 in crown view. In B1, B2, and C anterior is upward.

Fig. 2. Simplified stratigraphic column of the Chahriseh area with a magnification of the basal Frasnian deposits and position of acanthodian−bearing horizons H1 and GI4.

Fig. 11. Diplacanthid spine morphology and histology. Early Frasnian, horizon GI4 of an unnamed formation in the Chahriseh section, Kaftari Mt., central Iran. A. AEU 320, fin spine in lateral (A1) and leading edge (A2) views. B. Histological microstructure of a diplacanthid fin spine. Vertical transverse section LIGG 3807 of spine fragment which had two roundedquadrangular sculptural ribs (lost during sectioning, but were positioned at top). B1, general structure of an osseous vascularised tissue embracing all of the preserved spine; B2, abundant osteocyte cells densely displaced among the large vascular canals appeared in two perpendicular directions, longitudinal and transverse.