Scolecopteris spores
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Scolecopteris sporangia
Pinnules and sporangia of the "maggot fern" Scolecopteris are common fossils in the Permian cherts of the Döhlen basin but the related mature spores are seldom seen. More often one may find immature spores within sporangia, possibly not yet full size.
Same as with any other fern, the spores grow in globular groups of 4, called tetrads, where they are symmetrically arranged such that every one borders with 3 faces to neighbouring ones. The 3 plane faces of one spore meet at 3 edges which meet at a common point where they make a blunt tip. This can be seen after disintegration of the tetrad into separate spores. The symmetry of the arrangement makes it resemble a Mercedes "star". Hence, every spore has got a 3-fold symmetry axis.
"Stars" with 3-fold symmetry are also seen at the 4 locations on the surface of the tetrads where 3 spores meet. (Hence, one can expect to see "stars" on randomy oriented tetrads 4 times oftener than on randomly oriented spores.)
After the disintegration of the tetrads, the quarter spheres
become slightly inflated and more globular so that their blunt tip becomes even blunter. With limited magnification it may be not quite easy to distinguish between globular tetrads and globular spores. For simplicity, all globular grains inside and outside sporangia are called spores here. 

Fig.1 (above): Cross-sections of two neighbouring pinnules of the maggot fern Scolecopteris elegans from Döhlen basin (Lower Permian), sporangia stuffed with yellow spores, debris without relevant structure below. Width of the picture 4.3mm.
Scolecopteris sporangia

Fig.2 (right):
Cross-sections of maggot fern pinnule, detail of Fig.1. Width of the picture 2.15mm.

The tissue structure of the pinnules and sporangia has largely vanished so that the tissue is transparent now. On the right in Fig.2 one can see the fills of two sporangia through their walls.
The big sporangium in Fig.1 on the right is cut open along a narrow strip. On either side the yellow spores are seen through the wall.
Remains of the pinnule tissue are faintly seen in Fig.2.

As a conspicuous detail, the upper boundary of the pinnules is clearly seen. This is due to the combined effect of biology, chemistry, mechanics, and optics:
This fern, like all terrestrial plants, protected itself against exsiccation by exuding substances to make a thin cuticle on top of its epidermis. The cuticle is made of highly rot-resistant organic polymers which persist through ages. The interface between cuticle and chalzedony remains weakly bound. Under mechanical stress, as by shrinkage, the interface provides an easy crack path where a crack can start or an oncoming crack can be guided along. If the crack is not much narrower than the wavelength of light, it reflects the incident light, which makes sthe sharp contrast along the upper boundary of the pinnule.

Fig.3 (below): Longitudinal section of two sporangia of the maggot fern, detail of Fig.1,2.
Width of the picture 0.8mm.
Scolecopteris sporangia
On Fig.3 one can see an opening in the sporangium on the right, as it is known from Scolecopteris, with some spores fallen out.

These sporangia are shown here in detail because the maggot fern from Döhlen basin has been repeatedly reported in palaeobotany literature with erroneous size data:
The sporangia in Fig.3 are shown in [1]
, Bild 191, too large by a factor 1.7.  (Fig.3 shows the present state after polishing, hence there are small differences between the pictures.)
The sporangium in Fig.2 on the right is shown in [2], Abb.209, too large by a factor 2.7. 
The sporangia in [3], S.70, are too large by a factor 2.

The erroneous size data should have aroused suspicion even without original samples
available for comparison. After [2], Abb.209, the spore size would have been 70µm, which is bigger than the big spores of  Scolecopteris macrospora [4] and hence highly questionable. Checking against reality would have provided their real size of hardly 30µm.
Contrary to the too big size data of sporangia and spores in [1-3], other parts of this plant are shown with size data too small by factors 3 to 10:
 [2], Abb.210 und 130.
Erroneous size data are compiled under errors
and mistakes.
Sample: own collection, B/51.2 .

H.-J. Weiss     2016

[1]  R. Rößler: Der versteinerte Wald von Chemnitz. Museum f. Naturkunde Chemnitz, 2001.
[2]  M. Barthel: Die Rotliegendflora der Döhlen-Formation. Geologica Saxonica  61 (2) 2015, 105-238.
[3]  M. Barthel: Die Madensteine vom Windberg, in: U. Dernbach, W.D. Tidwell (eds.):  Geheimnisse versteinerter Pflanzen, D'ORO 2002, 64-77.     
[4]  J.R..Jennings, M.A. Millay: A new permineralized marattialean fern from the Pennsylvanian. Palaeontology 21(1978), 709-716,
  Scolecopteris pinnule cross-section, Sardinia Permian Chert News15
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