THE PALEOZOIC ENCRUSTING SCLEROBIONTS ALLONEMA AND ASCODICTYON: COMPONENT PARTS OF ORGANISMS BELONGING TO THE SAME PROBLEMATIC GROUP

Allonema (Silurian-Carboniferous) and Ascodictyon (Silurian-Permian) are common encrusting fossils, especially on Devonian skeletal substrates. Both were originally described as ctenostome bryozoans despite having calcitic skeletons and lacking zooecial apertures. Many paleontologists still include them in faunal lists as bryozoans, which distorts the evidence for bryozoan evolution and paleoecology, and obscures our record of other encrusting taxa.

Allonema, named in 1904 by Ulrich and Bassler, consists of sausage-shaped calcareous vesicles in chains adhering to hard substrates. The vesicles are porous but have no aperture. Ulrich and Bassler cited a “porelike depression near the end of the vesicle”, but we can find no such structure in the types or other specimens examined using SEM. Ascodictyon, described in 1877 by Nicholson and Etheridge, consists of clusters of vesicles connected by radiating stolons. Well-preserved vesicles and stolons exhibit a median row of pores. The centers of the vesicle clusters often have what appears to be a socket for the articulation of an unknown erect structure, presumably including the feeding organs. As with Allonema, the lack of apertures indicates that Ascodictyon was not a bryozoan, unless all of the feeding zooids were located in the hypothetical erect parts.

New findings of well-preserved specimens of Allonema and Ascodictyon from the Silica Shale (Middle Devonian) of Michigan include Ascodictyon vesicles having pores identical to those of Allonema, and Allonema vesicles having stolons like those of Ascodictyon with distinctive median pores. It appears that Allonema and Ascodictyon are end-members of a continuum of forms characterized by variably porous calcitic vesicles, sometimes in radial clusters, that may be connected to each other by stolons. Assessing the systematic position of these genera therefore requires looking at both as manifestations of the same group of organisms. The lack of apertures in Allonema and Ascodictyon not only excludes them from the bryozoans, it also virtually eliminates the possibility that they are foraminiferans, the next most likely affinity group. We recommend that Allonema and Ascodictyon be considered incertae sedis pending additional evidence for their systematic position.