Atopostroma protentum, Webby & Zhen, 2008

Webby, Barry D. & Zhen, Yong Yi, 2008, Devonian Syringostromatid Stromatoporoids from the Broken River Region, North Queensland, Records of the Australian Museum 60 (3), pp. 215-236 : 227-229

Atopostroma protentum n.sp.

Fig. 5 A–G

Material. Eleven specimens, including holotype MW1-1 ( AM F.134892, AM FT.15095, AM FT.15096), and ten paratypes MW1-20 ( AM F.134893, AM FT.15097, AM FT.15098— paratype A), MW6-4 ( AM F.134894, AM FT.15099, AM FT.15100— paratype B), MW6-6 ( AM F.134895, AM FT.15101, AM FT.15102— paratype C), MW6-7 ( AM F.134896, AM FT.15103, AM FT.15104— paratype D), MW6-9 ( AM F.134897, AM FT.15105, AM FT.15106— paratype E), MW6-10 ( AM F.134898, AM FT.15107, AM FT.15108, AM FT.15109— paratype F), MW6-11 ( AM F.134899, AM FT.15110, AM FT.15111— paratype G), MW6–12 ( AM F.134900, AM FT.15112, AM FT.15113— paratype H), MW6-13 ( AM F.134901, AM FT.15114, AM FT.15115— paratype I) and MW6-20 ( AM F.134902, AM FT.15116, AM FT.15117— paratype J), from the Martins Well Limestone Member , Shield Creek Formation of the MW section of Mawson et al. (1988) near Martins Well; holotype from 1 m above base of MW section, and paratypes, respectively, from 1, 6, and 20 m above base of section. Two other specimens, MW1-18 ( AM F.134903, AM FT.15118, AM FT.15119), and MW35-9 ( AM F.134904, AM FT.15120, AM FT.15121) are included in the species, and doubtfully, MW1-9 ( AM F.134905, AM FT.15122, AM FT.15123), MW1-17 ( AM F.134906, AM FT.15124, AM FT.15125) and MW6-18 ( AM F.134907, AM FT.15126, AM FT.15127); these all come from the same horizon and locality as the type material, respectively, from 1 m, 6 m and 35 m above base of section. Three of the specimens [ MW6-6 and MW6-7 ( AM F.134896— paratype C, and AM F.124896— paratype D), and specimen MW1-18 ( AM F.134903)] are intergrown with an unnamed cyanobacterium .

Derivation of name: Latin, protentum , stretched out, lengthened—alluding to the dominantly wide-spaced microlaminae.

Diagnosis. Species of Atopostroma with pachysteles typically exhibiting upwardly expanding to irregular, even locally, spool to post-like, shapes, and may be superposed through up to 12 successive pachystromal layers; each pachystrome has capping of laterally continuous, gently undulating, thin microlamina at top, and unevenly suspended microreticular network beneath, composed of pachystromal and associated pachystele elements; spacing of pachystromes varies from 5 to 11 (usually 7 to 8) in 2 mm, that is, they are relatively widely spaced, and commonly they have intervening, thin, wavy, microlamina-like dissepiments that act as raised pachystele floors; microstructure dominantly acosmoreticular.

Description. Skeleton laminar to domical, with maximum height of 110 mm and maximum diameter of 220 mm. Growth surfaces smooth to slightly wavy, rarely showing small mamelons. Latilaminae apparently absent, but a few discontinuity surfaces preserved. Astrorhizae conspicuous in skeletons, especially well shown in tangential sections ( Fig. 5B ), with individual stellate clusters centred between 4 and 10 mm apart; centres connect to vertical astrorhizal canal, 0.2 mm across; each centre has up to 10 outwardly radiating and branching astrorhizal canals, from 0.1 to 0.2 mm wide; no tabulae associated. Small, updomed, mamelonate structures, possibly related to astrorhizal system, are 1mm high and 0.5 mm wide.

Caunopore tubes commonly associated in stromatoporoid skeletons, as intergrown, phaceloid (longitudinally aligned) and tabulated corallites of coral Syringopora; corallites vary from 0.3 to 1 mm (usually 0.7 to 0.8 mm) in diameter, and typically show tabulae with infundibuliform floors and/or axial syrinxes ( Fig. 5A,B ); also in a few places tabulate coral exhibits lateral branching. Microlaminae of stromatoporoid characteristically downflexed slightly at intersections with caunopore tubes. Small, solitary rugosan, about 1.0 to 1.7 mm in diameter, also intergrown in places, and helicoid spiralling “worm tubes” (Helicosalpinx) with diameter of 0.9 to 1.0 mm. In addition, in localized areas of some stromatoporoid skeletons, especially close to, or along, discontinuity surfaces (perhaps associated with intervals of slowing, or cessation, of stromatoporoid growth) an undescribed, problematical, cyanobacterium occurs as upright dendroid growths of dark, thread-like tubules ( Fig. 5F ) that may be partitioned into “cellules”, 0.02 to 0.035 mm across, to maximum width of 0.05 mm; as well as various parallel-to-growth aspectswhere threads may appear like strings of beads and as clusters on surfaces adjacent to, or within individual microlamina; at points of lateral budding a noticeable tendency occurs for chainlike row of “cellules” to increase rapidly in size; colonization by this microorganism is typically within the microreticulate networks of pachysteles and within some porous microlaminae.

Stromatoporoid skeleton exhibits prominent, evenly spaced, laterally continuous, flattened to gently undulating, and relatively thin, microlaminae; most commonly a single, microlamina occurs, but locally in a few places may divide into two, but rarely continues in a closely paired relationship, even laterally may merge into a single microlamina again; in addition localized, thin, compact, undulating, microlaminalike dissepiments occur, but typically these occur towards middle of more widely spaced “interlaminar” spaces (where successive microlaminae are spaced between 0.25 and 0.95 mm apart); these microlamina-like dissepiments have comparatively limited lateral continuity; also smaller, irregular, low convexity and obliquely aligned, dissepiments (like normal cyst plates) may occur, usually in lower part of “interlaminar” space. Microlaminae usually have a variable thickness (0.01 to 0.03 mm thick) and appear to have compact microstructure but some show a mid-line row of tiny, darker (?melanospheric) dots, about 0.035 mm apart, or in other places are vaguely microreticulated; also breaks in continuity of microlaminae occur in places suggesting small pores, 0.02 to 0.025 mm across; a few other pores are larger, to 0.05 mm across; spacing of microlaminae ranges from 5 to 11 in 2 mm (most commonly 7 to 8 in 2 mm).

Pachysteles are from 0.06 to 0.20 mm in diameter near base, and characteristically spread upward and outward, and enclose irregular microreticular network, coalescing against undersurfaces of overlying microlaminae; however, much irregularity exists in pachystele shapes ( Fig. 5G ), with some more spool- or post-like, or may be less well developed in certain areas, for example, where microlaminae are close spaced, less than 0.1 or 0.2 mm apart; or where they are more widely spaced, pachysteles may be incompletely developed below middle parts of “interlaminar” space, being replaced by spar-filled areas (galleries, astrorhizal structures) or may be separated by floors formed by dissepiments; most typically, however, pachysteles are regularly superposed (rarely branching) through up to 15 successive microlaminae, i.e., for distances of up to 3.5 mm, and have spacing ranging from 6 to 9 in 2 mm laterally. Microreticulation of pachysteles shown by slightly inclined upward and outward rows of micropillars and microcolliculi that define microgalleries; variation in sizes and shapes of microgalleries suggest irregular, acosmoreticular microstructure; usually microgalleries range from about 0.03 to 0.04 mm, rarely to 0.05 mm across; sometimes aligned rows of tiny melanospheric dots occur, possibly representing points where micropillars and microcolliculi intersected in their original state prior to diagenetic alteration. Between pachysteles, spar-filled gallery spaces tend to be upwardly elongated and slightly narrowing, about 0.1 to 0.2 mm across, with domelike tops in contact with upper microlaminae, and typically divided in two by a thin, wavy microlamina-like dissepiment; in these areas microlaminae do not exhibit larger pores but microreticular-sized pores may have existed but no longer clearly preserved in longitudinal sections.

In tangential section, lower parts of “interlaminar” spaces show isolated, rounded to irregular pachysteles from 0.15 to 0.2 mm in diameter within open spar-filled gallery spaces ( Fig. 5C,E ), and these grade into more elongate to vermicular pachystele shapes a little higher up within more sinuous gallery spaces, some of which probably represent traces of astrorhizal canals ( Fig. 5D ); pachystele margins in these lower-middle parts of “interlaminar” space are typically frayed ( Fig. 5E ), due to cut ends of beam-like microcolliculi; microgalleries vary from 0.03 to 0.05 mm across, and from shapes of microgalleries are evidence of acosmoreticular microstructure. In upper parts of “interlaminar” spaces, pachysteles become more coalesced though small, rounded spar-filled gallery spaces remain in places, from 0.10 to 0.15 mm in diameter. Closer to upper microlamina, skeletal material is almost entirely microreticulate, composed of rounded micropillars, 0.02 mm in diameter, and very fine rod-like microcolliculi, 0.007 mm thick, that define microgallery margins, typically 0.03 to 0.04 mm across. A gradational change occurs into darker, and more densely, microreticulated areas of upper microlamina, though microgallery dimensions remain much the same ( Fig. 5D ); also in a few places microgalleries seem to have become aligned and interconnected in arrangements that appear like slightly sinuous, very fine, tubule-like, pathways; however, in some, rather more dense and diffused areas the primary microreticulation of a microlamina has become rather obscure ( Fig. 5C ), possibly owing to its diagenetic alteration into a more continuous, solid sheet with scattered dark?melanospheric specks.

Remarks. Atopostroma protentum , though it bears close similarities to the type species A. tuntouense Yang & Dong, 1979 from the early Emsian of South China, especially in the spacing of tangentially directed structural elements (called “laminae” in the type species but appearing more like microlaminae), but exhibits differences in the longitudinally orientated elements (called “pillars” in the type species) with thicker, slightly wider spaced, and more continuously superposed structures than in the equivalent pachysteles of A. protentum n.sp. The present material is also comparable with the type material of A. distans ( Ripper, 1937) from the Buchan Caves Limestone of Victoria (Webby et al., 1993), material from the Jesse Limestone of New South Wales (Webby & Zhen, 1993), and also, as described from the Broken River area, herein, but A. protentum n.sp. as described here has slightly coarser, though less continuously superposed longitudinal structural elements; the pachystele spacing is from 6 to 8 in 2 mm in A. protentum , whereas in the Victorian type material of A. distans it is 8 to 9 in 2 mm; also dissepiments are comparatively uncommon in A. distans , and certainly none of the longer, microlaminalike dissepiments present in A. protentum . In addition the tangential structural elements of A. protentum are comparatively thinner (composed mainly of microlaminae) and more widely spaced, whereas the “laminae” of the Victorian type specimens of A. distans are relatively thicker and more closely spaced.

Comparisons with A. stearni n.sp. are also warranted especially as both A. stearni and A protentum come from the same locality and horizon in the Martins Well section. The two species have many structural microstructural similarities, but A. stearni exhibits markedly different successional patterns of spacing of microlaminae; first, a closely spaced phase that includes a very limited development of pachystele structural elements, only some very short, rather scattered, micropillars; and secondly, a wide-spaced phase that includes a full range of longitudinally orientated features, such as upward-flaring pachysteles with acosmoreticular microstructure, and wavy, microlamina-like dissepiments. Examples of the close-spaced phase with sets of up to four microlaminae occur at irregular intervals through the skeleton. In contrast, A. protentum is dominantly composed of the wide-spaced phase of development—it mainly lacks developments of clustered close-spaced microlaminae. Also overall, A. stearni exhibits a closer spacing of pachystromes, on average between 12 and 17 in 2 mm, compared with A. protentum that shows a spacing of pachystromes, averaging between 7 and 8 in 2 mm.

Figures

Fig.5. Atopostroma protentum n.sp., A, B, D, G, holotypeAM F.134892 from MartinsWell Limestone Member (Shield Creek Formation), 1 m above base of MW section; (A, B), longitudinal (AM FT.15095) and tangential (AM FT.15096) sections, showing astrorhizae and associated caunopore tubes, ×10; (C, E), tangential sections of paratype E, AM F.134897 (AM FT.15100), from Martins Well Limestone Member, 6 m above base of MW section; (C), showing characteristic morphological … [continued on facing page]