Habrostroma Fagerstrom 1982

Habrostroma Fagerstrom, 1982

Type species. Stromatopora proxilaminata Fagerstrom, 1961.

Discussion. In the type species of Habrostroma, H. proxilaminatum (Fagerstrom, 1961), Prosh & Stearn (1996, p. 36) described the pachystele microstructure as “orthoreticular to clinoreticular” in longitudinal section, and as having a “cellular (=irregular microreticular)” microstructure, presumably the “acosmoreticular” type of Stock (1989) in tangential section. However, other writers have recognized the genus Habrostroma as exhibiting only acosmoreticular microstructure (see Stock, 1991, 1997; Webby et al., 1993). For comparative purposes, the genus Atopostroma, based on A. tuntouense Yang & Dong, 1979, has been described as exhibiting “orthoreticulate to clinoreticulate” microstructure (see Stearn, 1993, Stearn et al., 1999), but also in Stearn (1990, p. 498) and Stock (1997, p. 549), as only exhibiting a clinoreticular type of microstructure, as in Columnostroma and Coenostroma, respectively. In the present study, the species of Atopostroma show examples of both acosmoreticular and clinoreticular microstructural types; no orthoreticular types have been found.

The diagnostic features of the two genera are as follows: Habrostroma typically has pachysteles that are shorter, more irregular, confined largely within “interlaminar” spaces (rarely superposed) and internally composed of an irregular network of acosmoreticulate skeletal material; and the pachystromes prominent but not thick, with one or more microlaminae, and sometimes microlaminalike dissepiments. Atopostroma, on the other hand, has pachysteles that are typically superposed, formed by upward and outward accretion to produce irregularly spread network of acosmoreticular to clinoreticulate skeletal material beneath successive, laterally extensive, microlaminae. The two described species H. alternum and A. protentum occur together in the Martins Well Limestone Member and are truly transitional forms, showing some features characteristic of each genus. Some consideration has been given to combining them, even treating them as different species of the one genus. However, between the mid-range and extremes of variation of the two species they are clearly representatives of different genera.