Atopostroma distans ( Ripper, 1937 )

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 : 222-224

Atopostroma distans ( Ripper, 1937)

Fig. 3 A–F

Not Actinostroma frustulum Pocta, 1894:145 , pl. 18 bis, figs 1–4.–E. Flügel & Flügel-Kahler, 1968:168 (cum syn.).– May, 1999:176. – May, 2005:210, 212, 248 (table 26); pl. 25, figs 1a,b, 2a,b; pl. 34 fig. 1a,b, pl. 35, fig. 1; pl. 41, fig. 2a,b.

Actinostroma stellulatum var. distans Ripper, 1937:12 , pl. 2, figs 1–2.– Ripper, 1938:223, 236, 240.–Teichert & Talent, 1958:16.

? Actinostroma cf. distans Ripper. –E. Flügel, 1958:180.–H. Flügel, 1961:47.

Actinostroma (Actinostroma) distans Ripper. –E. Flügel, 1959:142.

Actinostroma stellulatum distans Ripper. –E.Flügel & Flügel- Kahler, 1968:403.

Trupetostroma cf. ideale Birkhead.–Stearn & Mehotra, 1970:16, pl. 5, figs 1,2.

Part Nexilamina dipcreekensis Mallett, 1971:244, pl. 14, fig. 4 only.

Atopostroma tuntouense Yang & Dong. – Stearn 1983:548, fig. 4E–H.

Not Atopostroma tuntouense Yang & Dong. – Stearn, 1990:496, figs 4.1, 4.2, 8.2.

Atopostroma distans (Ripper) .–Webby & Zhen, 1993:346, figs 11A–D, 12E.– Webby, Stearn & Zhen, 1993:171, figs 27F, 28A–D.–Prosh & Stearn, 1996:36, pl. 18 fig. 5.

Material. Four specimens, including SD170-210-1 ( AM F.134879, AM FT.15066, AM FT.15067), SD170-217-1 ( AM F.134880, AM FT.15068, AM FT.15069), SD170-270-3 ( AM F.134881, AM FT.15070, AM FT.15071) and SD170-274-3 ( AM F.134882, AM FT.15072, AM FT.15073), from 210 m, 217 m, 270 m and 274 m, respectively, above base of Lomandra Limestone (upper Emsian), SD170 section of Mawson et al. (1988) in Lomandra Creek type section; and other specimens from C.W. Mallett’s collection as follows: UQF.47741, from UQL. 2980, 500 m above base of section, Martins Well ( MW) traverse, 1 km SW of Martins Well ( Mallett, 1971) and UQF.47750, from UQL. 2981, 800 m above base of section of MW traverse of Mallett (1971), both from Dip Creek Limestone—note the former specimen was erroneously included in description of Nexilamina dipcreekensis by Mallett (1971, p. 244, pl. 14, fig. 4); UQF.47600, from UQL.2974—collected from loose rubble, probably derived from the Chinaman Creek Limestone at South Chinaman Creek, 5 km N of the abandoned Pandanus Creek homestead; two other specimens, respectively, UQF.48243 from UQL.2698, and UQF.48142 apparently comes from the same locality, 2.5 km SSE of Pandanus homestead, with one ( UQF.48142) a reworked sample (it came from “isolated rudites” ( Mallett, 1971, p. 243), now assigned to the Late Devonian Bulgeri Formation ( Lang, 1993); consequently, the other ( UQF 48243) is also a reworked specimen.

Description. Skeleton domical, with width and height up to 140 mm; growth surfaces smooth to slightly wavy, but only to extent of forming weak mamelons; growth interruptions define a few, 1 to 3 mm thick, latilaminae, though one specimen (UQF.47741) from the Dip Creek Limestone, SW of Martins Well, has conspicuous latilaminae, 4 to 5 mm thick ( Fig. 3B ). Astrorhizae moderately well developed, with centres near tops of mamelons or offset to sides, spaced much closer, 2.5 to 4.5 mm apart, in Broken River material ( Fig. 3E ), as compared with the holotype (NMV P141754-57, ex MUGD) from the Buchan Caves Limestone of eastern Victoria, with its centres spaced 7 to 9 mm apart; each localized centre has a stellate, outwardly radiating, cluster of up to 12 canals; usually canals are from 0.10 to 0.18 mm wide, and extend laterally within an interlaminar space for a millimetre or more; canals of each cluster are deflected sharply upwards into single, radially aligned, 0.2 to 0.3 mm wide, tabulated, astrorhizal passageway that leads towards growth surface ( Fig. 3A ).

In longitudinal section pachysteles (pillars where poorly preserved) superposed continuously through many laminae (up to 25 counted) though in a few places continuity much more limited, even locally absent or restricted to only one or two interlaminar spaces; commonly spreading upward, but sometimes more spool- or post-shaped, spaced from 9 to 12 in 2 mm laterally; usually range from 0.08 to 0.20 mm in diameter, but may become more expanded outwardly and upwardly towards top. Pachystromes (or laminae where microreticulation not clearly visible), continuous, of variable thickness (0.05 to 0.18 mm thick), and with spacing from 8 to 12 in 2 mm; sometimes clearly defined by microlaminae at top, from 0.020 to 0.025 mm thick (though in extremes to 0.05 mm thick), and one or two, discontinuous, slightly disordered, rows of microlaminae below, each pair intersected by set of short, pachystele-derived micropillars to form microgalleres, from 0.03 to 0.05 mm in width; in places tiny, darker, melanospheric specks, about 0.01 to 0.02 mm in diameter, visible along microlamina within more-or-less continuous dark line, or may be identified at intersections of microlaminae and micropillars; additionally, in well preserved specimen (SD170-210-1 – AM F.134879), in areas where spool-shaped pachysteles are intersected, rather disordered and disrupted row of microgalleries may develop above a microlamina ( Fig. 3D ); more commonly, however, skeletal material is less well preserved, with fine structures showing a more diffused compact microstructure, though ghosts of microreticulation may be preserved in form of scattered, vaguely aligned, rows of tiny melanospheric specks. Spar-filled gallery spaces prominent, commonly dome-shaped to vertically ovate or rounded; in some places subdivided by small, low convexity, dissepiments; rarely, longer, wavy, microlamina-like dissepiments occur, and these may act as local pachystromal floors; occasionally pores, about 0.05 mm wide, seen to disrupt continuity of bounding microlaminae. In pachystromal areas microstructure has characteristics of acosmoreticular, whereas in pachysteles microreticulation is irregular, somewhat disordered and slightly divergent, with larger microgalleries reaching 0.05 to 0.07 mm in width; hence these areas include a mix of acosmoreticular and clinoreticular patterns.

In tangential section skeletal framework of pachysteles and pachystromes shows broad differentiation of structural and microreticular elements between successive “interlaminar” layers, broadly divided into three parts ( Fig. 3C ): near the base the pachysteles appear as rounded to oval, occasionally more elongated to vermiform shapes within extensive, “open”, gallery spaces (and astrorhizae); pachysteles have microreticulate (?acosmoreticular) interiors of subspherical microgalleries, up to 0.04 mm in diameter, and bounded by rod-like microcolliculi and intersected by micropillars; where pachystele margins are incomplete, the ends of microcolliculi may be frayed; in middle part, pachysteles are more anastomosing to coalesced, with rounded to oval to irregular spar-filled gallery spaces, usually ranging from 0.06 to 0.15 mm in diameter; and inside pachystele walls is well developed acosmoreticular network of microgalleries, from 0.03 to 0.05 mm across; in upper part where pachystromal elements and microlaminae are intersected the microreticulate networks of microgalleries are darker and denser and on average about 0.03 mm in diameter; in small areas where beam-like microcolliculi are not intersected the microgalleries are incomplete or show only aligned rows of tiny rounded micropillars, 0.01 to 0.02 mm in diameter (or sometimes they appear to be preserved as similar-sized melanospheric specks); also in a few places microgalleries arranged in gently meandering patterns, appearing like walless microcanals, 0.02 to 0.03 mm wide, and traceable for distances of about 0.4 mm in length ( Fig. 3C ); the patterns of pachystromal elements with differentiated microlaminae maintain a finely porous, microreticulation, unless skeletons suffer significant diagenetic alteration that seems to cause production of more continuous sheet-like microlaminae.

Remarks. The preservation of one specimen (SD170-210-1 – AM F.134879, see Fig. 3A, C, D, F ) of Atopostroma distans ( Ripper, 1937) from the Lomandra Limestone of the Broken River area is exceptional in exhibiting so much microstructural detail. Described material from all other localities in Australia and elsewhere shows much less well preserved specimens. A close comparison of the holotype and some topotypes of A. distans from the Buchan Limestone at Heath’s Quarry in Victoria ( Ripper, 1937; Webby et al., 1993), has been made recently, in an attempt to reveal more detail about the microstructure. Unfortunately most of this material shows melanospheric, compact, and/or fibrous microstructures, all of which would appear to have developed from secondarily, diagenetic alternation of its original microreticular skeletal elements. Tangential sections of the types were first mentioned by Ripper (1937, p. 13) as having “an appearance which seems to anticipate the porous structure of Stromatopora and allied genera”, and Webby et al. (1993, p. 171) who reported “cellules” in structural elements of lower, less dense, parts of interlaminar spaces (see also Webby & Zhen, 1993, fig. 12E). The Victorian material in further detailed study shows little direct evidence in both longitudinal and tangential sections of having an original microreticulate skeleton. A number of specimens show discontinuous, single rows of microgalleries between pairs of microlaminae (including the topmost microlamina) and pachysteles (rather than pillars) are identified with disordered (acosmoreticular) and vaguely perpendicularto-growth microstructures in longitudinal sections of three specimens, including the holotype. Tangential sections of the holotype and another specimen show rounded, to elongate or irregular pachystele outlines with well-defined clusters of microgalleries in their interiors and frayed margins that represent the cut ends of rod-like microcolliculi. These microreticulate structures can all be closely matched with the A. distans material studied in the Broken River region. The Victorian types also show matching arrangements of melanospheric dots—patterns such as microlaminae exhibiting a single, discontinuous, parallel-to-growth, row of melanospheric specks (0.015 to 0.025 mm apart), and less common perpendicular-to-growth rows of dots within pachysteles (or pillars).

The New South Wales Jesse Limestone material of A. distans is morphologically similar to the Victorian type and other material, and apparently comes from limestones of more-or-less equivalent, early Emsian age (Webby & Zhen, 1993). Though the specimens come from widely separated localities in south-eastern Australia they exhibit surprisingly few skeletal or preservational differences. Just a few minor differences seemingly exist between them. For example, scattered, small, long-low to more upwardly convex cyst-like dissepiments occur close to bases of “interlaminar” spaces in specimens from both localities; only one significant difference is noted in some Jesse material, particularly one, specimen AM F. 85595 (formerly listed as Sydney University Palaeontology no. SUP97230), that has developed localized, wavy, microlamina-like dissepiments within wider than normal interlaminar spaces (see Webby & Zhen, 1993, fig. 11A). A few Jesse specimens are seemingly less well-preserved, showing a fibrosity that seems to moreor-less mimic traces of an original skeletal microreticulation, shown elsewhere in the same specimens as rows of lighter subspherical voids that possibly represent traces of the original microgalleries .

May (1999, 2005) in a revision of the Devonian stromatoporoids from Bohemia, Czech Republic, designated lectotypes of a number of species, including Actinostroma frustulum Pocta, 1894 from the Koněprusy Limestone (middle Pragian) near Koněprusy (“hill near Plesivec”). Study of the lectotype of A. frustulum and 12 new specimens from western and eastern quarries near Koněprusy (respectively, “Čertovy schody-Zapad” and “Čertovy schody-Vychod”; see May 2002, fig. 1) allowed May (1999, p. 168) to identify the species as having a microreticulate (clinoreticular) microstructure and consequently interpreted A. frustulum as a species of the genus Atopostroma . Furthermore, on the basis of inferred morphological similarities, May (1999, 2005) proposed A. distans ( Ripper, 1937) as a junior synonym of A. frustulum ( Pocta, 1894) . Stearn (2001, p. 14) initially questioned this interpretation on the basis that the Czech material had not been adequately illustrated but a much more complete documentation (and illustration) of the species has since been made by May (2005). The fuller description and illustrations allow the relationships between the two species to be clarified, and to determine that A. frustulum has a markedly different microreticulation. May (1999, 2005, p. 210) recorded a finer clinoreticular network, with microgalleries about 0.015 mm across in A. frustulum , whereas the dominant microreticulation in A. distans is acosmoreticular, though also clinoreticlar in areas where pachysteles expand upwards. In the type material from Victoria (Webby et al., 1993, p. 171) the microgalleries, though previously recorded as ranging from 0.01 to 0.03 mm in diameter, based on a recent reexamination of this material, are far more variable, from 0.01 to 0.05 mm (most commonly 0.03 to 0.04 mm) in diameter.

The second difference is that pachysteles based on the lectotype of A. frustulum (see May 2005, pl. 41 fig. 2b) are more slender than is characteristic of A distans . Another example of A. frustulum is illustrated by May (2005, pl. 25 fig. 1b) is more comparable but it is not a type specimen. Yet another specimen labelled as A. frustulum (illustrated in tangential section by May (2005, pl. 25 fig. 2b) shows pachysteles that are markedly broader than those exhibited by A. distans ; indeed this specimen seems to be more closely related to the second described species of Atopostroma from Bohemia, A. contextum ( Pocta, 1894) —see May 2005, p. 206, pl. 41 fig. 1b). Consequently, A. distans remains a valid and independent species—with its more coarsely acosmoreticular and locally clinoreticular microstructure than the exclusively finely clinoreticular patterns of A. frustulum and A. contextum , and with its pachysteles that are typically thicker than those of A. frustulum , and thinner than those of A. contextum .

The better preserved skeletal material of A. distans from the Broken River area of Queensland has a typical microreticulation within both pachysteles and pachystromes. The pachystromes are not easy to characterize but intergrade with adjacent pachysteles and may be associated also with sets of closely spaced microlaminae especially towards upper surfaces. It seems that no clear-cut determination of a specific type of microreticulate microstructure ( Stock, 1989) can be made for this well preserved species of A. distans . All the basic microreticulated structures of the skeleton seem to be acosmoreticular, except where pachysteles diverge upwards and in lower parts of spool-shaped pachysteles where the microreticulation locally converges upward, the condition is clinoreticular, ( Fig. 3C ).

Figures

Fig. 3. Atopostroma distans (Ripper, 1937):A, C, D, F, from Lomandra Limestone in SD170 section (210 m above the base); (A) longitudinal section,AM F.134879 (AM FT.15066), ×10; (B) longitudinal section, UQF.47741B from locality UQL.2980 of Mallett (1968), from Dip Creek Limestone, 1 km SW of Martins Well, ×5; (C) tangential section, AM F.134879 (AM FT.15067), showing enlarged small area (centre left in Fig. 3F), with irregularly shaped pachysteles in open, spar-filled, interlaminar, …[continued on facing page]