Atopostroma stearni, 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 : 224-226

Atopostroma stearni n.sp.

Fig. 4 A–F

Not Atopostroma tuntouense Yang & Dong, 1979:74 pl. 41, figs 7, 8.

? Atopostroma tuntouense Yang & Dong. – Stearn, 1990:496, figs 4.1, 4.2, 8.2.

Material. Five specimens, including holotype MW6-8 ( AM F.134883, AM FT.15074, AM FT. 15075, AM FT.15076) and four paratypes MW1-5 ( AM F.134884, AM FT.15077, AM FT.15078 – paratype A), MW20-3 ( AM F.134885, AM FT.15079, AM FT.15080 – paratype B), MW20-4 ( AM F.134886, AM FT.15081, AM FT.15082, AM FT.15083— paratype C), and MW35-8 ( AM F.134887, AM FT.15084, AM FT.15085, AM FT.15086— paratype D), are from the Martins Well Limestone Member of the Shield Creek Formation (late Lochkovian-early Pragian) in the MW section of Mawson et al. (1988) near Martins Well ; holotype from 6 m above base of MW section, and paratypes, respectively, from 1, 20 and 35 m above base of section, and two additional paratypes, JAL-T/4 ( AM F.134888, AM FT.15087, AM FT.15088— paratype F) and JAL-T/6 ( AM F.134889, AM FT.15089, AM FT.15090— paratype E), from the upper limestone member of the Jack Formation (earliest Lochkovian) of the Broken River Gorge section are referred to the species. Two less well preserved specimens, JAL-T/3 ( AM F.134890, AM FT.15091, AM FT.15092) and JAL-T/17 ( AM F.134891, AM FT.15093, AM FT.15094), are from the same horizon and locality .

Derivation of name. After Colin W. Stearn, who has contributed so much to contemporary understanding of Palaeozoic stromatoporoids in general, and to orders Stromatoporoida and Syringostromatida , including the genus Atopostroma , in particular.

Diagnosis. Species of Atopostroma with pachysteles typically exhibiting upwardly expanding to irregularly post-shaped, and commonly superposed through up to 20 successive pachystromal layers; each pachystrome has a capping of an extended, rather even thin microlamina at top and unevenly suspended microreticular network of pachystrome and associated pachystele elements below; pachystromes variably spaced, about 12 to 17 in 2 mm but usually exhibits two distinctly different successional spacing arrangements, with closely clustered microlaminae, and more widely spaced pachystromal elements, and between these there are elongated, wavy, microlamina-like dissepiments that in places act as raised pachystele floors; microstructure largely acosmoreticular.

Description. Skeleton domical to laminar with dimensions up to 160 mm in height and 260 mm diameter. Growth surfaces smooth, flattened to gently undulating except for a few small, locally upraised, mamelons up to 1 mm high, at least 0.6 mm wide ( Fig. 4A); astrohizae most conspicuous in tangential sections, with centres spaced from 3 to 7 mm apart, and including 5 to 8 main, non-tabulated, radiating branches, from 0.10 to 0.20 mm wide ( Fig. 4B); most commonly astrorhizal canals located in lower interlaminar spaces but characteristically upturn in places into single, tabulated, longitudinally-orientated, canals that interconnect with astrorhizal centres; sometimes canals are with mamelons. Latilaminae defined by growth interruptions and phase changes, from 1 to 6 mm thick; in one part of holotype an extensive layer of disordered microreticular, dissepiment and canal-like skeletal material, up to 1 mm thick, has developed between successive latilaminae ( Fig. 4A, 4E); this layer may represent a kind of basal phase to growth of overlying latilamina.

In longitudinal section skeletal structures dominated by long, thin, even, laterally persistent, microlaminae at tops of pachystromes, and irregularly upward-spreading pachysteles below, these latter commonly superposed but not usually through more than about 12 to maximum of 20 successive “interlaminar” spaces ( Fig. 4A, 4D); the single, mainly continuous, microlaminae usually appear as dark, rather dense, plates, about 0.02 mm thick; only rare breaks in continuity occur, that suggest presence of occasional, scattered, small pores, with diameters of 0.02 mm (and perhaps related to original microreticulation). Skeletons show spacing of pachystromes varying widely; on average measurements range from 12 to 17 in 2 mm, though successively sets of microlaminae are either wide spaced or narrowly spaced; where microlaminae are widely spaced (e.g., 0.15 to 0.35 mm apart), individual “interlaminar” spaces are divisible broadly into two parts, a lower, occupied by gallery spaces, astrorhizae and narrower, ends of the pachysteles, and an upper part represented substantially by spread of pachystele and pachystrome-derived microreticular material that even extends around tops of associated gallery spaces; wavy microlamina-like dissepiments may locally intervene between lower and upper parts, and where present may provide raised floors for development of pachysteles, and may subdivide larger spar-filled galleries into two ( Fig. 4C,D). Pachysteles typically expand upwards, from 0.03 to 0.10 mm wide in lower part, to 0.15 mm, even to 0.2 mm wide towards top; and a spacing from 9 to 10 in 2 mm laterally; skeletal material is predominantly acosmoreticular, though at margins of some pachysteles, patterns may be vaguely clinoreticular. Where narrowly spaced, the laterally persistent subparallel to parallel microlaminae occur in sets of two, three or more ( Fig. 4D); spaces between microlaminae may be partially occupied by microgalleries containing microcolliculated microlaminae and short micropillars (from 0.02 to 0.04 across), or micropillars are absent or incomplete, appearing as a row of?melanospheric dots along a microlamina, about 0.01 mm wide; or these structures may develop a vaguely radial fibrosity possibly where poorly preserved. In addition to undulating microlamina-like dissepiments some small, blister-like dissepiments may be scattered in places within skeleton; they mainly subdivide galleries, especially in lower parts of “interlaminar” spaces; spar-filled galleries characteristically dome-shaped, from 0.10 to 0.20 mm across.

In tangential section, pachysteles rounded to elongated, or irregular, from 0.05 to 0.15 (rarely to 0.20) mm in diameter in “open” gallery areas of lower part of “interlaminar” space; usually pachysteles have frayed margins as result of rod-like, microcolliculli being intersected; pachysteles spread outwards to develop more completely anastomosing network across middle part of “interlaminar” space, including completely enclosed, spar-filled galleries, typically 0.10 to 0.15 mm in diameter ( Fig. 4F); microreticular elements acosmoreticular, with microgalleries from 0.025 to 0.050 mm across, and orderly rounded micropillars (0.010 to 0.015 mm across) where microcolliculi intersected. In upper part of “interlaminar” space more uniformly, denser and darker band-like areas ( Fig. 4B), that mainly represent parts of obliquely intersected microlaminae near tops of “interlaminar” spaces; finely porous, microreticular network of microgalleries, which are 0.02 mm across (on average); rounded micropillars where they can be differentiated are about 0.01 mm in diameter.

Remarks. The type species, A. tuntouense Yang & Dong, 1979 , from the Yujiang Formation (Emsian) of South China bears a resemblance to A. stearni but its pachysteles are thicker, consequently more widely spaced (6 to 8 in 2 mm), and more regularly superposed, and it lacks any significant development of dissepiments. Material from the Lower Devonian (upper Lochkovian) Stuart Bay Formation of Arctic Canada was first described as A. tuntouense by Stearn (1990, p. 496), but has since been recognized as belonging to a different species from the type species (see Webby et al., 1993) having, for example, a much closer spacing of laminae (16 in 2 mm). This Stuart Bay species, now assigned by Stearn (in press), and illustrated in the Treatise volume as “ Atopostroma n.sp. = A. tuntouense of Stearn, 1990, p. 496”, bears close relationships to the late Lochkovian/ early Pragian A. stearni of the Martins Well Limestone Member in northern Queensland. However though it has a similarity in the spacing of laminae (or pachystromes), the Stuart Bay form shows certain differences that may be of taxonomic importance. Firstly it is described as exhibiting a clinoreticular microstructure, whereas A. stearni is dominantly acosmoreticular. Secondly, though it is recorded as having “broadly undulate” microlaminae where they are locally close spaced, the Stuart Bay form does not apparently exhibit the association of more widely spaced microlaminae and intervening wavy microlamina-like dissepiments. Consequently, the Canadian form is only doubtfully regarded as conspecific with A. stearni .

Atopostroma stearni also bears close resemblances to A. distans ( Ripper, 1937) from the Early Devonian (Emsian) successions of Victoria (Webby et al., 1993), central New South Wales (Webby & Zhen, 1993), and North Queensland (herein), but differs mainly in having on average more closely spaced pachystromes (or laminae), from 12 to 17 in 2 mm, compared with spacing of pachystromes (or laminae), from 7 to 11 in 2 mm in A. distans , and A. stearni more commonly exhibits dissepiments both long, wavy microlamina-like and small cyst-like types. Also, in A. distans , pachysteles may be spool-shaped with the microreticulation, not only typically diverging upwards, but also, locally, converging in the lower parts of pachysteles, a feature not seen in A. stearni , and the pachysteles are more markedly superposed in A. distans than in A. stearni .

Atopostroma sp. 1 from the Dayville Member of the Coeymans Formation (Lochkovian) in New York ( Stock, 1997) is another possibly related form, though its pachysteles are slightly thicker than in A. stearni , and no long, wavy, dissepiments occur; consequently Stock’s Atopostroma sp. 1 is not conspecific with A. stearni . A second species recorded by Stock (1997) as Atopostroma sp. 2 from Deansboro Member of the Coeymans Formation is likely to belong to another syringostromatid genus, possibly Coenostroma Winchell, 1867 .

Somewhat transitional relationships exist between species of Atopostroma such as A. stearni , and species of Habrostroma , not only in the Martins Well Limestone Member. For example, the microreticulation is similar, that is, mainly acosmoreticular, in both genera Atopostroma (herein), and Habrostroma ( Stock, 1991, p. 903) . Secondly, the pachysteles of A. stearni are less continuous and regularly superposed than in some other species of Atopostroma , whereas though typically species of Habrostroma have short pachysteles, some do have superposed pachysteles not unlike A. stearni . For example, the Lochkovian species Habrostroma microporum and H. centrotum (see Stock, 1991, 1997, figs 4–7) exhibit superposed pachysteles, and like A. stearni commonly exhibit long, wavy, microlaminalike dissepiments.