Remember This Mystery Fossil From 2010?

[Harry Pristis]

Do you remember this mystery fossil from 2010?

(Here's the thread: http://www.thefossilforum.com/index.php?/topic/11836-quiz-what-is-this-invertebrate-from-the-peace-river/?hl=002mystery)

We had identified this fossil as (probably) the silicified cast of an invertebrate burrow, an ophiomorpha made by a shrimp.

I finally got around to searching for more of these trace fossils from the Peace River of Florida, and here's what I found:

The construction of this fossil is identical to the first one - cemented sand grains with no micro-structure apparent under magnification. Only the gross morphology indicates association with a life form.

I'd really appreciate some opinions on this more elaborate example. Do you believe we got the correct identification in 2010 and that if might apply to this second example?

[piranha]

That appears to be the logical conclusion. Here is an excellent paper that details the morphology and preservation characteristics of some common ophiomorphids.

LINK

Bromley, R.G., & Frey, R.W. (1974)

Redescription of the trace fossil Gyrolithes and taxonomic evaluation of Thalassinoides, Ophiomorpha and Spongeliomorpha.

Bulletin of the Geological Society of Denmark 23(3-4):311-335

[Harry Pristis]

Thank you for that PDF link . . . that is an informative paper.

I ask for reconsideration of these fossils on the basis of an opinion from a professional invertebrate paleontologist. When I submitted these images to him, he thought that the object might be a hydrozoan colony. He says that he has encountered the same preservation in other inverts - starfish. It is difficult for me to imagine this is a cast of a hydrozoan colony.

Does this fossil (specifically, my images of this fossil) create something of an optical illusion? That is, lobes up, colonial animal . . . lobes down, a feeding burrow?

And, that's what I think this may be: a feeding burrow ('fodichnia'). It's difficult to compare these puny efforts at burrowing to the elaborate and extensive burrows of say Thalassinoides as illustrated in the Bromley & Frey paper.

Any other thoughts?

[Missourian]

Yeah, Ophiomorpha can occasionally throw a curveball.

[piranha]

I found this image of a crustacean burrow from the Eocene of Texas that looks very similar to your specimen.

[Harry Pristis]

Okay! Two votes for burrow rather than colonial hydrozoan. Anyone else have an opinion?

[Al Dente]

I would have guessed bryozoan or hydrozoan colony for these fossils. There appears to be pores covering the fossil (unless the dark specks I'm seeing are sand grains). Here is what I'm talking about:

One of your specimens appears to have formed in layers the way a bryozoan or hydrozoan would grow:

If this were a cast of a burrow, I don't understand why it would be covered with pores or why it would be layered.

[Harry Pristis]

I would have guessed bryozoan or hydrozoan colony for these fossils. There appears to be pores covering the fossil (unless the dark specks I'm seeing are sand grains). Here is what I'm talking about:

colony1-pores.jpg

One of your specimens appears to have formed in layers the way a bryozoan or hydrozoan would grow:

colony2-layers.jpg

If this were a cast of a burrow, I don't understand why it would be covered with pores or why it would be layered.

Those are reasonable deductions, Al . . . Except "The construction of this fossil is identical to the first one - cemented sand grains with no micro-structure apparent under magnification. Only the gross morphology indicates association with a life form."

I was hoping that someone would take note of the layering of the fine sand material. That layering appears to be either the sandy base upon which the colonial animal grew, OR, it may represent the opening to the burrow, infilled with low-energy waves of sediment.

Okay, two votes for burrow, one vote for colonial animal. Anyone else?

[Herb]

One more for the bryozoas

[Auspex]

I cannot quite reconcile, in my mind, either the "slightly concave base" or the "layering" with its being a burrow; either or both could be inventively explained, but it just seems odd. Other than my slight misgivings here, it does seem to me most similar to a complex, in-filled crustacean burrow in form, shape, and preservation.

[Fossil Diver Steve]

I'm leaning toward some type of soft coral whose polyps were closed and did not leave details in the cast. My reasons are the layering / encrusting area at the base, the random, yet same general growth direction of the branches including the secondary offshoots, and the tips all appear rounded instead of random points. Another thought is that if it were a soft coral, it could have grown on top of a layered, dead, encrusting hard coral and we are seeing both. Just something to think about.

Thanks,

Steve

[Harry Pristis]

I cannot quite reconcile, in my mind, either the "slightly concave base" or the "layering" with its being a burrow; either or both could be inventively explained, but it just seems odd. Other than my slight misgivings here, it does seem to me most similar to a complex, in-filled crustacean burrow in form, shape, and preservation.

My thoughts on layering, Chas, run like this: The layering is either the lowermost portion of the cast (colonial animal) OR it is the uppermost portion of the cast (a burrow).

If it is the lowermost portion, the colonial animal had a holdfast that is not distinguishable from the rest of the colony except for the layering. There is no impression of a hard surface (like a mollusk shell or a bit of coral) to which the holdfast was affixed.

If the layering is the uppermost portion of the cast (a burrow opening), it is reasonable to assume that the burrow mouth was expanded by water currents. Imagine a recently abandoned burrow in dense phosphatic clay. Water currents and eddies remove clay particles from the rim of the burrow opening even as larger-grain beach sand is being funneled into the aperture. Once the burrow was full of sand, the cratered mouth of the burrow would accumulate some sand as well . . . hence, the layering.

It is this scenario which leads me to think that this cast is not from the basement Tampa Limestone, but more likely from the Early Pliocene.

But, that is just my speculation. Any other ideas?

[piranha]

Attached are photos of recent crustacean burrows that may bolster the suggestion of a top-side-up (entrance) orientation.

[Harry Pristis]

Attached are photos of recent crustacean burrows that may bolster the suggestion of a top-side-up (entrance) orientation.

burrow 1.jpg

burrow 2.jpg

Thank you, 'piranha.' Those images are instructive.