Epibionts And 'xenobionts'

[Missourian]

Does anyone know of any instances -- or suspected instances -- of organisms that are known only from associated fossils, i.e. organisms that left absolutely no trace (i.e. 'xenobiont'), but are detectable from various epibionts that were once attached?

 

In the Pennsylvanian of Kansas City, there are thin limestone beds in the lower Wea Shale that contain an abundance of Crurithyris brachiopods and ammovertellid forams that range persistently at least for several tens of miles.

 

From south Kansas City, MO:

 

 

From Excelsior Springs, MO, approximately 30 miles to the north:

 

 

The ammovertellids are the little white things, and the Crurithyris are the bluish bb-like shells.

 

Normally, in other strata, these fossils are sparse or only locally present. I didn't think much of these beds until I read that both fossil types have been found as epibionts attached to calcareous algae (both) and echinoids (Crurithyris). (See http://palaios.geoscienceworld.org/content/18/4-5/435.abstract and PDF at http://www.kgs.ku.edu/Current/2005/sawin/sawinandwest.pdf (page 9) ). This made me wonder if there were some soft bodied organism (perhaps sea-grass-like algae) that were present in abundance but left no trace -- carbon films, impressions, root/holdfast impressions, etc -- but did leave behind a mess of formerly attached shells.

 

I can imagine vast 'gardens' stretching over wide expanses of the shelf.

[Auspex]

Your hypothesis has merit. Are there taphonomic clues as to the depositional environment; ie: current sorting or such?

[Missourian]

Nothing in particular stands out. The lower Wea is basically a continuous bed of limestone or a tight bundle of limestone flags (seen here as the overhanging bed above the sunglasses):

 

 

This thin bed appears to be a 'condensed' regressive limestone, which could resulted from a quickened sea level drop or an increase in siliciclastic sedimentation. The lack of oxidation or weathering in the limestone or overlying shale tells me there was an influx of siliciclastic mud that terminated limestone deposition while still submerged.

 

The homogeneity and even distribution of the fossil hash seems to indicate some kind of energy (currents or wave action), but there isn't any sorting, cross-bedding, etc. apparent. I'm not really sure, but with this evenness, as well as with the fossil types present, I would guess the hash was deposited well offshore in fairly shallow water.

 

Fossil Forum members eagerly collect shark teeth and other goodies from the lower Wea:

 

 

The transgressive limestone (Block Limestone) and darkish maximum-depth core shale can be seen just below the ledge surface.

[Missourian]

I should mention that if this is a regressive phase, the hash may not necessarily have been deposited at the same time across its range. As the sea level dropped, the area of deposition would have shifted to the south ahead of the receding shoreline.

[Missourian]

Some modern examples of epibionts on seagrass (not my photos):

 

 

 

In a typical carbonate environment, the seagrass would leave no trace in the fossil record. Once the plants disintegrated, the epibionts would scatter and settle on the sea floor.