Silurian Plant, Cooksonia ?, Wenlock Limestones, Shropshire

[araucaria1959]

Hello,

due to my interest in early land plants (in my area, I collected lots of plants in pragian and emsian strata), I bought two specimens of purported Cooksonia from the Wenlock Limestones, Wenlock Edge, Shropshire, UK. For the first of the depicted specimens, I got also the information about the exact locality (Shadwell Quarry). Since the matrix looks exactly the same, the second specimen is probably also from that quarry.

If so, these specimens would be among the oldest macrofossils of land plants; though spores that are attributed to tracheophytes go back into the second half of the ordovician, the oldest uncontroversial tracheophyte macrofossils are from Wenlock times and assigned to Cooksonia. That's what makes this unimpressive stuff so interesting.

However, I have my doubts; this time not because of its size (like the one from the Bertie Group), but because of the fully marine setting. Wenlock Limestones in general, and Shadwell Quarry also, are famous for corals and brachiopods - typical for a marine setting (possibly near the shore). Since early Cooksonia were very tiny plants, it seems quite implausible to find them together with corals and brachiopods. That's an unusual taphonomic situation for land (or shoreline) plant fossils.

On the other hand, the second specimen (and that's why I bought it though I already had the first one) shows a circular structure that may well be a Cooksonia sporangium. Its diameter is about 3 - 4 mm (scale: match = 45 mm) and it shows some structures on the "cup", though not good enough preserved for further identification. It may well be that one looks at the upper side of a sporangium. If so, it is detached from the tiny stem, since the stem continues. Since Cooksonia sporangia (and those of other very early land plants) were situated terminally, the position of the purported sporangium in my second specimen can not be its original position.

I hoped to find lots of information about the earliest occurrences of Cooksonia in that area in the internet, but that's not true (there are, however, reports of Cooksonia from Shropshire from younger strata, e.g. Pridoli and Lochkovian).

There is a website about Shadwell Quarry, but it doesn't even mention Cooksonia:

http://fossil-world.lefora.com/2010/06/07/wenlock-edge-stuff/

This stuff remembers me a lot of the coral reefs from the middle devonian of the Eifel area where I never found any fragments of land plants - though the land vegetation was already much more resistant in the middle devonian than at Wenlock times.

However, there is a single hint for Cooksonia in Wenlock Limestones of Shropshire on another website:

http://fossil-world.lefora.com/2010/06/07/wenlock-edge-stuff/

That's all I got so far - and so my doubts remain.

Since this is an international forum, perhaps someone knows more about that?

araucaria1959

[Wrangellian]

I don't know anything about that locality but I know that plants can and often do occur in marine sediments - I find them all the time in my local Cretaceous below-tide sediments. (not sure how deep, but it's fine shale). I suppose Cret. leaves are more robust than the Silurian ones, so yours would of course be more rare, I'm just pointing out that it's possible. From what I know of theory, Cooksonia and other early land plants were all pretty much near-shore flora because they needed water. Later plants evolved better resistance to drought/etc. So I would think being near shore would increase the likelihood of being washed out to sea during storms.

Well that ended up more wordy than I intended..

I wonder if your 2nd specimen above (17:33) could be prepped out more?

[pleecan]

Yes they (photo 1) look like Cooksonia sp. morphology similar to the ones found in the Williamville Formation Bertie Group...

[pleecan]

photo 2 is an interesting early plant... beautiful little specimen. You might want to ask Dr Joe Botting his thoughts.... as he is well versed on UK fossils.

Edited October 27, 2012 by pleecan

[Malcolmt]

Picture #1 is definately cooksonia

[araucaria1959]

Thank you - it is good to know that I really have some stuff of this famous old plant. I'll also try to ask Dr Joe Botting, but before that, I try to make pictures of the sporangium-like structure under magnification.

Since I like reconstructions (as drawings or dioramas) of old landscapes, here is a link to a pic from a diorama in a museum in Den Haag with these unfamiliar tiny plants:

http://franziskaloeding.files.wordpress.com/2012/09/museoncooksonia1.jpg

araucaria1959

[Malcolmt]

Great picture, never seen that one before.....

[araucaria1959]

I got the answer from Dr. Joe Botting, confirming my doubts from the first post. I showed only the second specimen, since this one remained under discussion. In fact, he missed any signs of carbonisation, the environment is wrong, and the morphology doesn't fit enough with regard to Cooksonia. So he suggests two alternative explanations, (i) remnants of a pyritisized burrow or (ii) staining as a consequence of modern plant rootlets that grew into a crack, and doesn't think that this is a fossil plant (of any genus) at all.

araucaria1959

[Wrangellian]

Disappointing! I hope you didn't pay a lot.

Oh well, it's good to know.

[araucaria1959]

Beautiful specimens - and no doubts that they are tracheophytes because of the diagnostic water-conducting filaments.

Meanwhile, I started a general discussion about purported land plant material from the Wenlock Limestones and Shadwell Quarry in the other forum (Dr. Joe Botting) and showed my first (y-shaped) specimen there, still waiting for answers. Since the arguments that Dr Botting mentioned apply to all of the purported land plant material from the Wenlock Limestones/Shadwell Quarry that is "around in the net and/or ebay", that's a question of general interested. If so, a lot of stuff is mislabeled and misunderstood.

On the other hand, here is a link to a picture of a "non-commercial" specimen from Cooksonia specimen from the lower devonian (and another locality) which seems to have similarities with regard to its preservational style. Sadly, the picture is not large enough for direct comparison:

http://www.google.de/imgres?q=Cooksonia&num=10&hl=de&tbm=isch&tbnid=ISkpLwC4c5J6LM:&imgrefurl=http://www.sciencephoto.com/media/473225/enlarge&docid=FZm8PH3YCMLdvM&imgurl=http://www.sciencephoto.com/image/473225/large/C0136542-Cooksonia_plant_fossil-SPL.jpg&w=530&h=421&ei=JWehUO_HCM3EtAbc2oCoCA&zoom=1&iact=hc&vpx=770&vpy=170&dur=2406&hovh=200&hovw=252&tx=128&ty=108&sig=116452918458064347845&page=3&tbnh=149&tbnw=186&start=60&ndsp=31&ved=1t:429,r:4,s:60,i:277&biw=1246&bih=857

So, even if my second specimen is no land plant at all, the case of land plant fossils in Wenlock Limestones/Shadwell Quarry is still open for discussion.

araucaria1959

[pleecan]

Some examples of Cooksonia from the Williamsville member of the Bertie Dolostone.

These closeups show the diagnostic water filament.

Very nice collections John.... Cooksonia are one of my favorites from the Bertie.

PL