Palaeostachya Dircei N. Sp., an Authigenically Cemented Equisetalean Strobilus From the Middle Pennsylvanian of Southern Illinois

Abstract

Palaeostachya dircei n. sp. is described from an authigenically cemented specimen collected from the Anna Shale Member occurring above the Herrin (No. 6) Coal Member and below the Brereton Limestone in the Carbondale Formation, Kewanee Group (Middle Pennsylvanian). The strobilus is three‐dimensionally disposed within the matrix, allowing the preparation of ground thin sections, as well as selected maceration of the specimen. The imbricate strobilus is preserved for at least 7 cm of its original length and is composed of alternating whorls of sterile bracts and fertile sporangiophores. An articulated axis extends the length of the strobilus and attains a maximum width of 3 mm at the nodal areas. A whorl of 24 sterile bracts arises at each node, with each bract emerging at a 90° angle from the axis. Bracts are free except for a slight adaxial fusion at their point of origin. A slight downward‐projecting keel develops at the point where the bract begins ascending at least past the second supra‐adjacent node, where it is appressed into an abaxial furrow of the superposed bract. A whorl of sporangiophores originates above the bracts and is equal in number to the bracts. The sporangiophores are obliquely inserted on the axis and possess four superposed and thin‐walled sporangia inserted upon a (?) cruciate head. Spores assigned to Calamospora have been recovered and range in diameter from 68–115 μm (x̄ = 89 μm). The cone appears to be homosporous. Palaeostachya dircei n. sp. is compared to the reported permineralized and coalified compression species and appears similar to P. vera Seward and P. gracilis Renault.

Taxonomic Considerations for Carboniferous Coalified Compression Equisetalean Strobili

“…Species delimitation of coalified compression species within Palaeostachya, and hence other strobilar genera, should be approached cautiously. Presently, species appear to be based upon cone size, internodal distance, bract length, bract number, and sporangiophore number, even though taxa have been erected with considerably less than this amount of data (Gastaldo, 1981). Strobilus size does not appear to be a valid criterion except when distinguishing a very large cone from a small cone.…”

“…The majority of known equisetalean fructifications have been assigned to either Palaeostachya (21 species) or Cala-mostachys (62 species). In order to delimit Palaeostachya dircei Gastaldo (1981) from the recognized forms of equisetalean strobili, species characteristics in all the genera were compiled for comparison. A number of taxa within Palaeostachya and Calamostachys were encountered which either did not fit the accepted generic diagnosis or were established on insufficient data.…”

“…Palaeostachya paucibracteata von Sandberger (1866) is also described from a fructification exposed in longitudinal view, and also represents a limited aspect of exposure. If a longitudinal exposure of P. dircei (Gastaldo, 1981, fig. 6) was the only aspect of the strobilus considered, a bisporangiate configuration may have been incorrectly interpreted.…”

“…Therefore, structural characters utilized to separate anatomically preserved specimens cannot be used when attempting to delimit coalified compressions. It has been demonstrated that most species erected within Palaeostachya have been created with less than sufficient data to justify their specific status (Gastaldo, 1981). The same condition of overspeciation is true in Calamostachys.…”

“…The same condition of overspeciation is true in Calamostachys. A suite of 16 morphological characters which may be utilized in order to separate coalified compression species has been published (Gastaldo, 1981) as a guide for future referral. DISCUSSION-Good-(1975) concludes that Calamostachys and Palaeostachya, which are based solely upon sporangiopore insertion and sporangium number, are badly defined and probably represent artificial genera.…”