Propachastrella primaeva
Zittel 1878
Propachastrella primaeva is moderately common at Misburg. It can form bizarre, multiply folded, ear- or plate-like bodies as in the first two pictures. Smaller (younger) individuals, as in the picture below, tend to be of a simpler shape.
Propachastrella primaeva is one of the rare fossilized non-lithistid members of the class demospongia. Its skeleton is composed of a densely crowded mass of calthrops and simple triaene spicules which are not united via zygosis, or fused as in hexactinellid sponges. Instead, the endings of the spicules are often curved to form hooks which embrace neighbouring spicules, thus providing some stability to the skeleton.
Photomicrographs of spicules of Propachastrella primaeva
The first image shows a single dichotriaene.
The next plate shows a collection of calthrops and triaenes, some with bifurcated and/or curved arms.
The second plate shows several dichotriaenes as well as two amphioxes. The latter probably belong to the set of spicules of Propachastrella primaeva, but were not reported previously.
Spicules of Propachastrella primaeva (after Schrammen, 1910).
(a) Calthrops, some with hooks
(b) dto., with spherical arms
(c) dermal dichotriaene