Crustacean and crystal
deutsche Version

Castracollis moult Crustacean and crystal have got something in common here: Either has grown in water. This may be surprising to those who subconciously adhere to the old-fashioned terms of "animal kingdom" and "mineral kingdom", and relate the latter to the idea of old age and high temperature of formation. This does not apply here since the well-shaped quartz crystals had formed much later than silicification had turned the crustacean moult into a "fairy bathtub" (Fig.1). The illusion of a seashell as a bathtub, a favourite motiv of painters, is brought about here by the level boundary suggesting the idea of water and steam. As mentioned in connection with similar phenomena, levels of such kind do not indicate a former water surface. They indicate a former interface between water and a watery suspension. (See Rhynie Chert News 142, 143, 144, 145.)

Fig.1: Crustacean moult with fancy fill, cavity with quartz crystals. Frame width 4.3mm.

The moult with the silica-rich swamp water inside provided a nearly closed compartment where silica clusters formed and settled into a suspension. Continuing influx of silica by diffusion finally turned the shell with content and surroundings, except the big cave below right, into solid chalzedony.
Most probably the moult belongs to the crustacean Castracollis named after the farm Castlehill near Rhynie and first published in 2003 [1]. Another moult part (Fig.2), found on the same cut face of the sample, supports the assumption.
Castracollis head shield
Fig.2: Castracollis head shield, cut across its rather involved 3D-shape. Frame width 2mm, same scale as Fig.1.

Apparently the 3D-shape of the shield on the front part known as head shield is not readily imaginable as a whole.
In Fig.2, clear chalzedony offers a spatial aspect of the strongly incurved edge above.
As expected, the contours of the few random cuts available differ among themselves, and they do not seem compatible with the reconstruction of the creature in [1]. (See Rhynie Chert News 24.)  
A tiny detail in Fig.2 confirms that the object is really of organic origin: The yellow crack on the right is deflected around the strongly curved part of the moult (above), which means that the moult offers an easy crack path.
The cavity with quartz crystals in Fig.1 requires an explanation. In a swamp, gas bubbles ascend or become trapped. If the swamp water is rich in silica, beginning silicification of the surrounding water and swamp matter can stabilize the bubbles. Later on, the gas escapes and water saturated with silica enters by diffusion through the silica gel. Constant conditions with slight supersaturation gave rise to slow growth of regular quartz crystals over a long time. Later on the water disappeared and left the cavity as we see it now.
Sample: Rh6/53, found by S. Weiss in 2003
, this Part 8 (end) given to Barron.

H.-J. Weiss     2019

[1]  S.R. Fayers, N.H. Trewin: A new crustacean from the Early Devonian Rhynie Chert ...
     Trans.Roy. Soc. Edinburgh, Earth Sci. 93(2003), 355-382.

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