Devonian alga mimicking Coleochaete
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Any fossil alga resembling Coleochaete in some way should deserve attention, for two reasons: The small freshwater alga Coleochaete is thought to be the closest relative to the earliest land plants, the ancestors of all extant "higher" plants, and it has never been found as a fossil.
A Lower Devonian alga of uncommon aspect as seen here shows several features also seen on an extant species of Coleochaete as described in detail by Oltmanns [1]:
(1) branching filaments emerging from a common base attached to a substrate,
(2) spherical capsules at the end of some branches which may become cup-like by splitting open,
(3) dark inclusions in the cells, probably the nucleus and a large elongated chloroplast.


sessile alga in Rhynie chertsessile filamentous alga in Rhynie chert, contours
Fig.1: Devonian alga in Rhynie chert, one specimen on the left branching near the base, seen attached to a substrate. Width of the picture 2.8mm,
scale bar 0.25mm.
Insert above right: Extant freshwater alga Coleochaete pulvinata for comparison, same scale. Photographs: H. Sahm.

In this sample, a bunch of filaments connected at their base (Fig.1 left) is attached to a largely decayed axis of a terrestrial plant of the Rhynie flora which apparently became submerged in flooding.
What looks like a partially coated capsule in Fig.1 above might be merely incidental but the cup-like forms in Fig.1 below right are most intriguing. Four of them are seen slightly higher magnified in Fig.2.
sessile alga in Rhynie chert with cup-like forms
Fig.2: Devonian alga with objects of cup-like aspect. Same sample as Fig.1.
cup with granular content on alga in Rhynie chert
Fig.3: Devonian alga: object of cup-like aspect with granular structure inside.

The peculiar objects of cup-like aspect are rather smooth on the outside but the cavity wall may be bulging inward, as seen on the empty cups. The polygonal structure in one cup suggests an interpretation as sporogenic tissue as it is known from the thick-walled "zygote fruits" or sporocarps of Coleochaete.

An interpretation of this fossil as being closely related to Coleochaete, driven by both wishful thinking and stunning similarities, should be questioned by weighing the arguments in favour and against.
The large difference in cell sizes is not relevant in this connection since sizes can greatly change for various reasons, as, for example, the widespread phenomenon of polyploidy. The time span of 400 million years between this fossil and Coleochaete would not necessarily wipe out old features, as we know from extant Nitella, which looks quite similar to the Lower Devonian Palaeonitella. (See Rhynie Chert News 10.) Hence, the sporocarp of Coleochaete could well be homologous to the round objects seen here.
A contrary argument is based on the similarity between Fig.1 and early growth stages of Palaeonitella, which do not yet grow the typical whorls of branches but more irregular branching near the base. Also it is known that fungus infection can cause Palaeonitella cells to become bloated into globular shape [2]. Hence, what is seen here could possibly be juvenile Palaeonitella with deformed cells. Anyway, the polygonal structure in one of the cups looks like some tissue, which is not compatible with the notion of a merely bloated cell. So it appears that this fossil remains enigmatic, and if it really would turn out to be closely related to Coleochaete, it could contribute to a better understanding of the origin of land plants.
Annotation: Among the numerous reconstructions depicting the various land plant species found from the Silurian up to the Early Devonian summarily called Cooksonia [3], a few show cup-like objects strikingly similar to those seen in the above images. Hence, an interpretation of this plant as a Cooksonia species would be another option. Considering that this name,
in a phylogenetic sense, is only a vaguely defined label, it would at least indicate that the present fossil may be neither close to Palaeonitella nor to Coleochaete. The absence of any cellular structure in Cooksonia reconstructions may be due to the fact that Cooksonia had always been found as compressions but never in chert. 

H.-J. Weiss      2012,     Annotation 2015

[1]  F. Oltmanns: Morphologie und Biologie der Algen, Gustav Fischer Verl. Jena 1922.
[2]  T.N. Taylor et al.: Paleobotany. Elsevier 2009. p.102.

[3]  H. Steur: steurh.home.xs4all.nl
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