Stigmaria – fascinating fossil from Arran

The field trip to Arran was disappointingly devoid of interesting fossils. But, on the last day, the paleos (plus a group of honorary geologists) went (on an invigorating trip over a mountain) to work on an Arthropleura trackway. Arthropleura was the largest land vertebrate we know, a colossal millipede (or maybe centipede… the actual relationships are not clear) that could grow to over two metres in length! It lived in near-coast areas of the Carboniferous period, some 300 million years ago, and was a heavily protected herbivore that would have fed mostly on plant litter or detritus. 

The group was too large for us all to work on the short preserved trackway, so I was delegated to examine the other fossil on the site: Stigmaria, a plant root belonging to one of the colossal Carboniferous club mosses (which are not mosses at all: they are closer to true trees, but distinguished by having leaves attached directly to the stem, i.e. no branches in between).

Stigmaria is a morphotaxon, a group of body parts that is too poorly known to know its evolutionary origin, but is still given a taxonomic name so that it can be easily referred to. Morphotaxa are common practice in paleobotany, the study of extinct plants, since you rarely find fossils of entire plants, but usually only leaves, pollen, trunks or branches and nothing more. It is then the challenge of paleobotanists to puzzle certain morphotaxa together into a whole organism, if possible (which rarely is the case). Stigmaria is considered to be the roots of Lepidodendron, a 40 metres tall club moss, or lycopod. (So, Stigmaria is part of Lepidodendron, but only Lepidodendron is an actual, classified organism; Stigmaria is just the roots.)

Reconstruction of Lepidodendron with its Stigmaria roots.

 Image from http://commons.wikimedia.org/wiki/File:Stigmaria_Heimans.jpg

Some of the best specimens are quite large and look like this:

Excellent specimen of Stigmaria from http://the-haskellok.com/fossils/id113.html

But the ones on Arran are not as spectacular.

These are much smaller and only have the outline preserved. They look nothing like the example specimens above, and I would never have guessed they were the same unless the professor had told us. (I am still a bit sceptical…)

There was a whole bed full of these tiny root pieces.

The root pieces are short (5-10 cm, mostly) and abundant. They are tapering (i.e. narrow toward one end) and mostly gently curved, as one can expect of plant roots, which need to be hard enough to penetrate the soil, but soft enough to wriggle around and form a net to cover a large volume from which to absorb nutrients and water. Some are branching, some are not. The branching is simple. (I did not see any that branched more than once.)

This makes a bit of sense considering that the club mosses are quite primitive vascular plants, so I do not expect their roots to be amazingly complex. However, those pits in the surface of the better specimens from above make me curious. Could they have been simple adaptations to increase the surface area for absorption of the roots, or might they have been points where small rootlets or root hairs came out? I found this reconstruction on the internet:

Interpretation of the purpose of the depressions on the surface of Stigmaria seen in better-preserved specimens.

Image from http://commons.wikimedia.org/wiki/File:PSM_V18_D633_Stigmaria_ficoides.jpg

I interpret it as suggesting that smaller projections came out of a main root to enhance the area of soil the roots were in contact with and could take up water and nutrients from. Actually, I am inclined to speculate that the small, featureless parts I was looking at could be those protrusions only. The only thing that makes me want to think twice before jumping to conclusions is that a great number of the fossils I was examining were branching, which may or may not make sense with that idea. If those mini-roots were branching, it was probably to the advantage of the plant, but since I have not heard of anything like it before, I am reserved not to make a judgement just yet. I prefer to be cautious and merely put forward the idea rather than take a stand here and now.

Regardless, Stigmaria ought to have been an efficient root form to be able to anchor and support a giant such as Lepidodendron. Taller trees need stronger roots to keep them from falling over from strong winds. The roots must also absorb enough water and nutrients for the large plant to survive. Without roots, we would not have many plants managing to grow more than a few decimetres over the ground, and Stigmaria was one among many pioneering roots in the early forests of the world.