Maggot stone black-red-yellow
deutsche Version
MadensteinChert with abundant enclosed fern pinnules from the Lower Permian of the Döhlen Basin is known as "maggot stone". A specimen of this type, distinguished by conspicuous colours, is shown here.
The black aspect in Fig.1 at the bottom is the primary one with carbon from the decayed organic substance of gyttja. Above, the carbon had vanished by oxidation so that the substance had become light-coloured. Iron compounds entering into some strata by diffusion became oxidized into red hematite. This occurred after the siliceous substance had become solid chalzedony, which can be inferred from cracks acting as diffusion barriers. Tiny amounts of hematite in chalzedony can make a conspicuous effect
The left edge of the sample is obviously part of a nearly plane crack face across the stiff chert layer. Probably, displacements within the sediment strata caused the brittle layer to fragment already in the Permian. Dissolved iron came along the crack, entered up to 2cm into the chalzedony, and became oxidized there into yellow Goethite. A thin layer (up to 2mm) at the very edge became bleached later.
The chert layer fragments got pried loose from the Permian strata by the proceeding Cretaceous sea and became part of the boulders on the beach. Together with the sand they turned into Cretaceous sandstone. Later, weathering broke down the sandstone, leaving the heavy chert boulders on the spot. Glacial rivers moved and deposited them across the then softly undulating area. This sample has still kept its Cretaceous surface, as seen from tiny remains of sandstone sticking to it.

Fig.1: Old fragment of a chert layer: silicified mud, coloured as a result of diffusion and oxidation, with pinnules of the tree fern Scolecopteris.
Sample (2001): Bu7/184, Part 2, Height 18cm; Freital, Kohlenstraße.

The fern pinnules as the smallest parts of the big fronds are well seen as a row of cross-sections, as in Fig.1 below the middle on the right, and in Fig.2.
Less clear are the aggregations of randomly oriented pinnules, as in Fig.3 with sporangia, here in groups of mainly 4 on a common stalk.
The pinnules lacking sporangia in Fig.2 have got hairs below the midrib.
In the middle of Fig.2, the bulging end of an apparently shorter pinnule is tangentially cut so that lateral veins are dimly seen.
fern pinnules
Fig.2,3 (left): Pinnules without and with sporangia.
Image widths 17mm.fertile fern pinnules

The aspect of the silicified pinnules in other samples had been misinterpreted as insects more than once, hence the name "maggot stones". Accordingly, this tree fern got the popular name "maggot fern", which became translated into the scientific name Scolecopteris
pinnules and rods

Fig.4 (right): Pinnules cut lengthwise; enigmatic cylindrical fragments. 
Image width 8.5mm.

Width (2mm in Fig.2) and length (4.5mm in Fig.4) of the pinnules fit to the most abundant variety of the maggot fern in the Döhlen Basin. Incidentally, the extant fern Athyrium growing in the present Döhlen Basin has got pinnules of similar size but is not even distantly related to Scolecopteris.
Enigmatic are cylindrical fragments, 0.05-0.3mm wide, in this sample abundant in some places, partially not stained for unknown reasons and thus clearly seen in Fig.4. They are to be the subject of another contribution.

H.-J. Weiss       2018 

[1] E. Zenker: Scolecopteris elegans, ein neues fossiles Farrngewächs mit Fructification. Linnaea 11(1837), 509-12.
Scolecopteris pinnule cross-section, Sardinia Permian Chert News 22

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