Habrostroma alternum, 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 : 230-232

Habrostroma alternum n.sp.

Fig. 6 A–F

Material. Six specimens including holotype MW6-3 ( AM F.134908, AM FT.15128, AM FT.15129), and five paratypes MW1-2 ( AM F.134909, AM FT.15130, AM FT.15131— paratype A), MW1-6 ( AM F.134910, AM FT.15132, AM FT.15133— paratype B), MW1-12 ( AM F.134911, AM FT.15134, AM FT.15135— paratype C), MW6-5 ( AM F.134912, AM FT.15136, AM FT.15137— paratype D) and MW6-15 ( AM F.134913, AM FT.15138, AM FT.15139— paratype E), from Martins Well Limestone Member , Shield Creek Formation (late Lochkovian-early Pragian) of the MW section of Mawson et al. (1988) near Martins Well ; holotype as well as paratypes D and E are from 6 m above base of MW section, and the other paratypes, are from 1 m above base of section. Five other specimens MW1-3 ( AM F.134914, AM FT.15140, AM FT.15141), MW1-8 ( AM F.134915, AM FT.15142, AM FT.15143), MW1-19 ( AM F.134916, AM FT.15144, AM FT.15145), MW6-19 ( AM F.134917, AM FT.15146, AM FT.15147) and MW6-21 ( AM F.134918, AM FT.15148, AM FT.15149) from the same locality, with the first three listed specimens from 1 m above base, and the last two, from 6 m above base of section .

Derivation of name: Latin, alternum , other—relating to the transitional relationships that exist between this taxon and Atopostroma protentum n.sp.

Diagnosis. A species of Habrostroma with pachysteles relatively short, mainly irregular but sometimes tending to be somewhat upwardly expanded; not usually superposed (and continuous) through more than about five successive pachystromes though more extended local alignments exist, but the related pachysteles are usually incomplete structures, that do not cross entirely each successive pachystrome; pachystromes are spaced moderately widely, from 5 to 14 in 2 mm (usually 8 to 9 in 2 mm); each has a prominent, extensive, thin, gently undulating, microlamina at top, and has irregular microreticular meshwork, including complete and incomplete pachysteles immediately below; between successive pachystromes, laterally extensive patterns of thin, wavy, microlamina-like dissepiments are commonly developed and often act as pachystele floors; microstructure acosmoreticular.

Description. Skeleton is domical with growth surfaces smooth to broadly undulate; latilaminae rarely seen, but discontinuity surfaces occur ( Fig. 6A). Caunopore tubes of syringoporid tabulate coral affinity commonly seen in intergrowth relationships ( Fig. 6B,C,E); characteristically coral has deeply concave tabulae, and corallites usually have larger (0.7 to 0.85 mm) or smaller (0.3 to 0.55 mm) diameters; and in a few places lateral buds seen in smaller corallites; also rare solitary rugosans, and possibly a 0.3 to 0.4 mm diameter worm tube (possibly Helicosalpinx) occur. Astrorhizae only conspicuous in tangential section ( Fig. 6B,C) forming localized stellate clusters of irregularly radiating and branching astrorhizal canals, spaced from 4 to 9 mm apart; and individual astrorhizal canals from 0.01 to 0.02 mm wide; in a few places, one or more converge into a single, untabulated, vertical canal, 0.02 to 0.03 mm in diameter, and sometimes associated with small mamelons, 0.5 mm wide and 0.8 mm high.

Single, thin, dark, laterally continuous, flattened to weakly undulate microlaminae are most conspicuous skeletal elements in longitudinal section; where in contact with caunopore tubes microlaminae almost always gently downflexed; only in a few places are close-spaced, second or third microlaminae seen to split off (or coalesce with) a microlamina; widest spacing between successive microlaminae is 0.6 mm; spacing more usually about 0.25 mm; spacing of microlaminae on average between 8 to 9 in 2 mm; spacing extremes range from 5 to 14 in 2 mm; microlaminae typically 0.025 mm thick and where a close-spaced pair of microlaminae develop they may be accompanied by traces of short micropillars, defining a row of microgalleries, each about 0.03 to 0.05 mm across; also a few small pores (foramina) may interrupt lateral continuity of microlaminae, from 0.07 to 0.15 mm in diameter.

Overall pachysteles rather short, irregular and not commonly radially aligned through skeleton; may extend across “interlaminar” spaces, as irregularly post-like or tapering downward structures, but are sometimes shorter, confined to upper two-thirds of “interlaminar” space ( Fig. 6E,F); lower part is represented by poorly differentiated, spar-filled areas of galleries, astrorhizae and/or allotubes; in places where superposed, pachysteles occur they only extend for short distances to about 2 mm longitudinally; pachysteles are prominently microreticulate (largely acosmoreticular), and they are consistently spaced from 7 to 9 in 2 mm; individually pachystele dimensions are from 0.10 to 0.05 mm wide in lower part of interlaminar space to up to 0.25 mm wide in upper part; microgalleries are of irregular shape and size, with alignment of rows appearing to be orientated directly upward near axes of pachysteles but more splayed upward and outward towards pachystele margins.

“Interlaminar” spaces, especially in lower two-thirds, commonly exhibit small, thin, oblique to convex upward vesicle-like dissepments, and more extended, undulating, microlamina-like dissepiments ( Fig. 6E,F), that subdivide spar-filled gallery spaces and sometimes are also continuous through pachysteles; at tops the subdivided gallery spaces have dome-shaped outline, and usually 0.1 to 0.2 (rarely to 0.3) mm wide; they may extend up to overlying microlamina but are sometimes separated immediately beneath by a thin laterally spreading veneer of microreticulated skeletal material.

In tangential section towards the base, pachysteles composed of microreticulate skeletal material within rounded to irregular profiles, ranging from 0.05 to 0.20 mm in diameter ( Fig. 6C,D); pachysteles tend to show frayed margins where beam-like microcolliculi are intersected; astrorhizal canals seem to mainly occupy more open vermiform areas; pachysteles coalesce into more completely closed networks towards tops; microlaminae are represented by darker, dense, rather diffused layers that still seem to show many differentiated microgalleries.

In general microstructure is coarsely ascosmoreticular ( Fig. 6 C–F), with networks of irregular microgalleries, only crudely arranged in longitudinal and tangential rows, and ranging in most areas from 0.05 to 0.075 mm across, but may be smaller, down to 0.035 mm across, or exceptionally in places up to 0.1 or 0.15 mm across; micropillars rounded, about 0.01 mm in diameter, and may be from 0.02 to 0.03 mm apart in areas where more enclosed networks occur; in less well preserved areas appear to be replaced by darker melanospheric specks; areas of intersected microlaminae exhibit a finer microreticulation of microgalleries, usually about 0.025 mm in diameter.

Remarks: Habrostroma alternum shows features that are transitional to typical A. contentum but remains confidently referable to Habrostroma . It differs in exhibiting shorter and more irregular pachysteles and localized undulating microlamina-like dissepiments, though some characteristics of Atopostroma are also shown, such as the superposition (only partial) of pachysteles, and tendency for pachystele skeletal material to upwardly and outwardly veneer undersurfaces of successive microlaminae. Habrostroma alternum has little close resemblance to other known species of Habrostroma from Lower Devonian successions. In comparison with H. tyersense Webby, Stearn & Zhen, 1993 from the Pragian Cooper Creek Limestone and equivalents of Victoria, the Broken River species has pachystromes composed mainly of single microlaminae at tops of “interlaminar” spaces, rather than paired or multiple microlaminae with associated rows of microgalleries as in H. tyersense , and it also differs in having coarser and more widely spaced pachysteles as well as localized, microlamina-like dissepiments. Habrostroma centrotum ( Girty, 1895) from the Manlius and Coeymans formations (Lochkovian) of New York ( USA) as revised by Stock (1991, 1997) also exhibits some similarities, such as the presence of some localized, undulating, microlaminalike dissepiments, but pachysteles in H. centrotum are more continuously superposed and their laminae (or pachystromes) are more closely spaced. Another species, previously referred to as Habrostroma sp., from the Jesse Limestone (Emsian) of central-western New South Wales (Webby & Zhen, 1993), is probably closer to Parallelopora Bargatzky, 1881 , than to Habrostroma .