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Unusual Flora Nodules


Rockaholic

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Here’s a couple of fern fossils that I found interesting.These are upper Carboniferous Mazon Creek type nodules collected from Indiana surface coal mine spoil piles.I question whether both of these specimens are single leaves or if two leaves have been preserved in close proximity giving the appearance of the two belonging to a single leaf.This Aphlebia like leaf appears to have two pinna that are fairly well defined and have discernible venation.

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Both pinna also have different shapes.One looks oblong the the other is more round in shape. This seems unusual for Aphlebia leaves.

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.The second nodule has a typical looking apex but the leaflets further down the rachis are oblong and seem out of place for this pinna.

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It could be that these are two separate specimens from two different fossil plants.Any thoughts?

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Nice specimens!

In the first specimen, what do you mean exactly with "two leaves"? Not sure I understood you correctly. Are you referring to the Aphlebia (indicated in yellow below) and the pinnule (indicated in blue), or are you suggesting that the Aphlebia (yellow) itself consists of two, overlapping structures? In case of the latter, could you annotate the figure to show what you mean? I don't see it...

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In the second specimen, I'd say you're dealing with one specimen. Main reason is the apparent traceability of the rachis (indicated in yellow below). Also, the pinnules don't look terribly out of place or different in shape to me.

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Searching for green in the dark grey.

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Regarding the second specimen, I'm not sure about the situation for your specific locality, but many Carboniferous plant fossils were transported (by water) for some distance before burial and fossilization. As such, you sometimes see bent, snapped or bruised plant remains. Below I show two examples of near-complete fronds that have displaced pinnae. Similar rearrangement happens regularly on the pinnule-scale also (no photographs at hand though).

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Searching for green in the dark grey.

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I'm questioning whether the pinnule you highlighted in blue is part of the Aphlebia or is it from a separate leaf.From the area I highlighted in green it appears to be part of the Aphlebia.Also if you believe that the second fossil is one specimen any thoughts as to its ID?

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I'd be highly surprised if the structure highlighted in blue (to my eye a pecopteroid pinnule) was connected to the structure in yellow (the Aphlebia) in the manner as suggested by the green highlighting. The two might be part of the same frond/plant, but even that is speculative. Regarding the second specimen, the pinnules look pecopteroid, and have entire margins, at least near the apex. Perhaps you could look at vein density and the number of times the veins fork, among other things, to ID it further. Sorry I can't be of more use. However, I'm quite sure that Jack Wittry (fiddlehead on TFF) will be able to help you further, if you shoot him a PM.

Searching for green in the dark grey.

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I'd be highly surprised if the structure highlighted in blue (to my eye a pecopteroid pinnule) was connected to the structure in yellow (the Aphlebia) in the manner as suggested by the green highlighting. The two might be part of the same frond/plant, but even that is speculative. Regarding the second specimen, the pinnules look pecopteroid, and have entire margins, at least near the apex. Perhaps you could look at vein density and the number of times the veins fork, among other things, to ID it further. Sorry I can't be of more use. However, I'm quite sure that Jack Wittry (fiddlehead on TFF) will be able to help you further, if you shoot him a PM.

Thanks. I always respect your insight. With the first nodule I agree the blue /green highlighted pinna does look Pecopteris like but it has an unusual curved appearance and even though it may be a fluke it does appear to be attached to the Aphlebia specimen.The second nodule has pinna at the apex that have bases that are broadly attached to the rachis while the linear pinna further along the rachis have rather long petioles.I would think that the presence of a petiole would rule out Pecopteris.

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Crazy thought.What if Aphlebia is an abnormal growth of a fern frond somewhat similar to a burl formation on a tree.And what if this nodule is a abnormal Pecopteris frond but the pinnule in question grew in a more normal form.

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[...] The second nodule has pinna at the apex that have bases that are broadly attached to the rachis while the linear pinna further along the rachis have rather long petioles.I would think that the presence of a petiole would rule out Pecopteris. [...]

Very good point... I didn't notice/appreciate that the most basal pinnule on the specimen show something petiole-like... If this is indeed a petiole (most likely, I guess), then it renders my assessment that the pinnules are pecopteroid rather dubious at best. Perhaps you're better off looking at neuropteroid species then. There are some that can show (a greater extent of) fusion to the rachis in pinnules close to the pinna apex (Neuropteris obliqua, for example). However, I'm not sure which form you'd be dealing with here. Alternatively (but less likely), the pinnule lamina may perhaps have been damaged somehow, close to the point of attachment. Not impossible, I guess, considering the overall disrupted state of the fossil, but there's no way to test such a thing, really... Anyway, it is a very interesting specimen. Hope you'll be able to find out more!

Searching for green in the dark grey.

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It is commonly understood that Rhacophyllum/Aphelibia is an abhorrent growth form, which grows from several types of true ferns, as it still does today. It is possible the two leaf forms you have grew from the same plant at one time, but that is far from a given, since as it was mentioned most Pennsylvanian aged plant fossils were transported to some degree. The second fossil is really a taxonomic question, it is an example of Neuropteris obtusa, which appears to be an Odontopteris near the apex of the pinna and transitions to Neuropteris lower down the pinna in the mature leaf form. Forms like this don't easily fit a mold, but the mature foliage is what the name is based on.

Hope this makes sense, if interested somewhere on the forum I wrote more on Rhacophyllum/Aphelibia

Jack

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Hi Jack,

Great information, I always learn something new from your posts. As I'd love to learn some more, could you perhaps explain what the differences are between odontopteroid and pecopteroid pinnules? It appears I have my definitions tangled. I thought that absence (or very poor visibility) of the midvein was the characteristic of odontopteroid pinnules. Yet, this specimen shows quite recognizable midveins. Is the distinction between odontopteroid and pecopteroid forms made on the basis of something else? Like the angle between the lateral veins and the pinnule margin, or something? Your feedback is much appreciated!

Thanks,

Tim

Searching for green in the dark grey.

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It is commonly understood that Rhacophyllum/Aphelibia is an abhorrent growth form, which grows from several types of true ferns, as it still does today. It is possible the two leaf forms you have grew from the same plant at one time, but that is far from a given, since as it was mentioned most Pennsylvanian aged plant fossils were transported to some degree. The second fossil is really a taxonomic question, it is an example of Neuropteris obtusa, which appears to be an Odontopteris near the apex of the pinna and transitions to Neuropteris lower down the pinna in the mature leaf form. Forms like this don't easily fit a mold, but the mature foliage is what the name is based on.

Hope this makes sense, if interested somewhere on the forum I wrote more on Rhacophyllum/Aphelibia

Jack

Thanks Jack.You’re a mind reader.You’re reply answered questions that I’ve been contemplating that I hadn’t even presented.You’ve further educated me on the complexity of fossil plant taxonomy.It’s seems like the more I know the less I understand.

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