A MASS ACCUMULATION OF SPONGES (HEXACTINELLIDA: DICTYOSPONGIIDAE) IN THE UPPER DEVONIAN CHAGRIN SHALE OF NORTHEASTERN OHIO CONTAINING EVIDENCE OF AXIAL SPONGE SEGMENTATION

A mass accumulation of sponges in the Upper Devonian (Famennian) Chagrin Shale of northeastern Ohio, composed of more-or-less flattened specimens, contains an assortment of rarely seen features and provides clear evidence of hexactinellid segmentation. The Chagrin Shale occurrence, at the base of an escarpment known as Old Baldy along Euclid Creek, is about coeval with the three-dimensional hexactinellid glass-sponge assemblages of southern New York State and northern Pennsylvania. The occurrence is mainly comprised of Hydnoceras cf. H. multinodosum, including tall, whole-body specimens, sponge fragments, sponge cross-sections, and root-tuft basal attachment areas. The height of the specimens is variable, the greatest being about 36 cm. The sponges are found in at least two main siltstone beds interpreted as tempestite and/or turbidite deposits. The sponges must have initially colonized a silt substrate, only to be smothered by further pulses of silt. Lycopods and inarticulate brachiopods are found with the sponges. Calamites and other lycopods must have been carried downslope with the silt pulses.

Upon death and subsequent compression, the annular segment boundaries of some of the Chagrin specimens of Hydnoceras separated, delineating a serially segmented skeleton. The laterally uniform, rimmed segment boundaries are evidence for an axially “diplosegmented” skeleton. Segmentation was indicated in the dictyospongiid Ceratodictya by Hall and Clarke (1898), but the evidence for such segmentation was much weaker than that seen in Chagrin Hydnoceras specimens. The newly recognized distinct segmentation of Hydnoceras is seen only in crushed specimens, in which the segments have separated. Such clear indications of segmentation are not seen, ironically, in the more three-dimensionally preserved New York Hydnoceras specimens. The clear evidence for segmentation implies the presence of some type of a hoxlike or other regulatory gene controlling segmentation in this early sponge lineage.