ASTYLOSPONGIIDAE Zittel, 1877

Family ASTYLOSPONGIIDAE Zittel, 1877 ASTYLOSPONGIIDAE indet.

( Figure 5 View FIGURE 5 E-F)

Material. A single thin section from the San Juan reef mound block at Niquivil locality Middle Ordovician (Dapingian), CEGH-UNC 27597.

Description. A single thin section from a reef mound slab shows a fragmented skeleton of a sponge composed of fused sphaeroclones that form an isodictyal net with triangular interspaces in all directions. The sphaeroclones are six-armed anapodal desmas (dichotriders) with terminal expansions (zygosis).

The spicule net shows an open structure, at least as far as is visible in the small area preserved, with sphaeroclones being anaxial desmas with five to six ray-like arms extended from one side of a globular centrum and showing knobby or spiny terminations. The globular centrum of the sphaeroclones in the specimen is variable in size ( 0.4 to 0.8 mm) and formed by the fusion of the zygome endings of the rays (arms), which are generally slender and long ( 0.7 to 1.1 mm), forming an open triangular structure. In well-preserved sphaeroclones, or incompletely fused centra, the arms are composed of a cladome with a simple bifurcated zygome; the zygomes of multiple spicules interlock to form the globular center.

Remarks. The characteristics exhibit by the spicular net seen in the thin section are not fully comparable with, or are difficult to relate to, described astylospongiids. Similar characteristics are seen in the Upper Ordovician and Silurian genera Astylostroma ( Rigby and Webby, 1988) and Astylospongiella ( Rigby and Lenz, 1978) . In particular, Astylostroma has similar zygome junctions; however, the lack of fine detail in the spicule preservation, and the limited material, prevent a more comprehensive comparison.

Most of the described genera of the family have a wide distribution but are restricted to the Upper Ordovician and Silurian, with only a few occurrences having been recorded from Middle Ordovician rocks (see Finks and Rigby, 2004c). It therefore appears that the early Middle Ordovician (Dapingian) age of our material may represent the oldest record of the family.