Living fossil - Lingula

Lingula is a Brachiopod that has been found in the fossil record from the Cambrian all the way up to today's ocean floors. It's a very primitive genera and that is probably a reason for it's longevity. Many animals specialize in a particular niche within the ecosystem. This makes them vulnerable to extinction when the environment changes or a food source disappears. Lingula have survived every major extinction event througout history because they are not picky about where they live or what they eat.

Lingula gets it's name from the similarity of it's shell to that of a small tounge (Lingulata, from the Latin word for "tongue"). They are inarticulate brachiopods which means their shells do not open and close along a hinge line. Rather they are pushed apart by a muscle within the shell thus allowing them to feed.

Here is a picture of a modern day Lingula from the Colorado State University website.



For comparison, here is a Lingula from the Mahantango formation.

Favosites turbinatus

Not every post will be about the Mahantango, today I want to show off a coral from the middle Devonian called Favosites turbinatus. This is an interesting coral that is known to form a smooth base in the sediment that often has a curve to it. I've been told two different stories from two different sources about how it received it's species name: turbinatus.

A few years ago I took a trip to Lousiville, KY and had the chance to collect from the Devonian aged Jeffersonville limestone (Eifelian). I found a couple of examples of this coral and they all looked like a helmet with a round surface on the bottom and eroded corallites on top. The locals call them Kneecap corals and I was told they were named turbinatus because they often flipped over during storms (turbulent storms). The specimen below came from my good friend Mike who runs the great blog Lousiville Fossils







The second story came from a collecting companion while visiting Hungry Hollow near Arkona, Ontario in Canada. He told me that it was named due to the similarity of the appearance of the coral mound to a Turban. Here is an example from the Centerfield formation (Givetian) of New York that shows a much more mounded form:





Now, both stories make sense when you look at them from the local perspective. In the Louisville area the fossils are often found as silicified fossils that have eroded from the limestone. Most times the corallites have a very gentle mound or have eroded some and so look like bowls. However, at Arkona the most commonly found specimens have the coral mound intact, are formed of Calcite and do look like turbans. I have been assured that the latter story comes straight from the researcher who named the species so I think I will go with that one as the official reasoning.

Here are some more pictures of other specimens:

From the Friends of UMMP Picture Archive: http://strata.geology.wisc.edu/mibasin/showfull.php?id=10

From the Indiana Memory Collection website:
http://cdm15078.contentdm.oclc.org/cdm4/item_viewer.php?CISOROOT=/p181901coll35&CISOPTR=510

Dalmatia

This is one of many exposures of the Mahantango formation in Pennsylvania. The site is a small borrow pit on the south side of route 147 north of the town of Dalmatia.



It's hard to tell from the photo but the rocks are steeply tilted and the layer surface is exposed along the length of the cut. As a result you only get one or two thin layers that are easily studied. This can be good and bad since it allows you a narrow view of a particular point in time. In this case a hash layer is the primary fossil producer here. A hash layer is one where a number of shells have collected into a small thin area and are very crowded together. This is usually indicative of a large violent storm like a hurricane or maybe a tsunami. The shell debris is washed together and often broken. You can find lots of different species but there are not many good fossils to be had since there was so much jumbling and compression. Below is a picture of a large pelecypod (possibly of the genus Leiopteria) that was exposed on the surface. Since it was parallel with the bedding plane the shell had been scoured out of it's buried position and deposited with other debris.





You can see other small fossils that surround and compose the matrix for the larger shell. There are lots of individual crinoid column segments and one of them was even compressed into the surface of the shell.

Goniatites

Here are some Goniatites from the Mahantango formation of Pennsylvania. These were all found in the Sherman Creek member, which used to be called the Upper Frame Shale member, at a site near the village of Seven Stars in Pennsylvania. I find what appears to be two different genus:

A 2-3 cm variety:



And a 6-7cm variety:



The smaller I think is Tornoceras but it's hard to see the suture lines to confirm. The larger seems to be Agoniatites. It's hard to get a firm ID on these as there is very little information published on the Goniatite genera from the Mahantango. Agoniatietes is the only genus that is listed in the "Fossil Collecting in Pennsylvania" book which is the best reference for anyone looking for fossils sites in PA. The next best reference is "Stratigraphy and Paleontology of the Mahantango Formation in South Central Pennsylvania" and that book does not discuss them at all. This could be because they are rare fossils to find in the Mahantango and, truth be told, I've only found them at one site. In any case, Tornoceras and Agoniatites are the two most common genera in the Middle Devonian so it might be safe to use them to ID what I have found.

More Pleurodictyum

My previous post concerning Pleurodictyum was composed primarily of text I'd written a few months ago. Since then I've found another example of the genus that looks like the beginnings of a paper wasp nest.





This was found in the Centerfield formation near East Bethany, NY during a recent trip. The fact that the genus colonizes shells or has a calcareous foundation seems to be indicative. Also, colonies are typically small and thus are probably not reef forming. Another important bit of info is that this specimen was found in limestone indicating a much cleaner water column in terms of sediment load. Since the specimens I've found previously have been in shale or mudstone it is obvious that the genus is wide ranging in it's habitat tolerance.

Also I found a few more pictures of other Pleurodictyum fossils

From the Falls of the Ohio web site: http://www.fallsoftheohio.org/PleurodictyumPix.html

From the Friends of the UMMP picture archive: http://strata.geology.wisc.edu/mibasin/showfull.php?id=1068

Pleurodictyum

I thought I'd start my blog with a post on a favorite fossil coral: Pleurodictyum found in the Mahantango formation (Middle Devonian/Givetian, 385 Ma). This is a tabulate coral of the Favositida Order whose members can be found in rocks from the late Silurian (422.9 Ma) to the middle Mississippian (342.8 Ma).(*1) I've been collecting a lot of fossils of this genus from the Sherman Ridge (Upper Frame Shale) member of the Mahantango Fm. (*2, *3) and have formulated some observations. For the purposes of this post I've named the species of the corals based on illustrations and pictures of similar specimens. It's impossible to truly nail down the species name without examining the fossil much closer.

The specimens I find occur in two distinct types:

1. Colonies that colonized clam shells or other solid foundations (Pleurodictyum styloporum)








2. Colonies that formed on the floor of the ocean without any obvious external foundation (Pleurodictyum lenticulare (or possibly americanum))










What I find interesting is that the two species are found in different rocks within the formation. I've found P. lenticulare only in a fine grained, dark grey shale that weathers light gray and often breaks into needle like fragments during weathering while P. styloporum has only been found in the thick bedded light grey siltstones that weathers brown and lay above the dark grey shale.

The P. styloporum fossils are found as casts where the mud has filled the hollow spaces of the animal after it died. This is the result of the animal being buried and fossilized as per normal but at some point the calcite based skeleton was dissolved away leaving only the siltstone cast. One of the benefits of this process is that you are able to see the underside of the colony where it was attached to the shell and the internal details of the animals living space. In the two examples above you can see one colony grew on a worm tube and the other looks like it grew on a Modiomorpha Pelecypod. The latter mode is much more common in the thick mudstones found at Seven Stars, Pennsylvania where I've found many pelecypod shells encrusted with Pleurodictyum colonies. This is a common growth pattern seen in other corals of the period such as the Aulopora colonies that are common on the Paraspirifer et. al. brachiopods found in the Silica Shale.

The P. lenticulare fossils are found as calcite casts with the hollow areas filled with shale. There is little to no evidence of the deeper weathering associated with the siltstones found above them so the overall structure and growth habit of the colony is preserved. For some reason these colonies had no trouble growing on the muddy bottom where they established colonies without a shell or other obvious foundation to start on. I've most often found these in the dark grey shale where there is also very little evidence of large species of Pelecypod.

I have never found Pleurodictyum fossils in association with other Tabulate corals or Rugose coral species.

So what have I learned so far?

- I know that when looking at the stratigraphy there are two slightly different paleoenvironments that the fossil colonies lived in.
- P. lenticulare formed without the benefit of a solid foundation and that tells me that the substrate was firmer and thus they could establish colonies on the surface of the mud. If the substrate was firm then there must not have been much water movement and could indicate a sheltered lagoon or deeper water.
- P. styloporum formed on shells of large exposed Pelecypods or worm tubes which may indicate that the shells were washed in rather than formed in-situ. This in turn could indicate a more turbulent environment which may mean it was closer to the shore line or a breakwater.

As I've been reading about the Mahantango formation I've learned that it formed as a series of fining sediments that graded upwards until the cycle would repeat itself again.(*2) This means that the base of a series would be sandstone and the sediment particles would get smaller and smaller as time wore one so that it would grade into a finer sandstone, then shale, then silt and mudstone before going back into a sandstone. This represents an actively changing environment with rapid deposition which is the result of the erosion of the mountains formed during the Allegheny Orogeny (mountain building event) to the southeast. It also records episodes of transgression and regression of the ocean which covered much of Pennsylvania at the time. As the sea rose, it would drown river deltas and thus the sediments that were deposited would become finer and finer until the sea level dropped again. The mechanism for the sea rising and falling could be due to ice ages or regional deformation as the Allegheny Orogeny continued.

It's hard to come to any firm conclusions as I'm not an expert in understanding how the geology and stratigraphy of the rocks relates to the paleoenvironment. Also, my sample size is somewhat small with only two specimens of P. lenticulare and a handful of P. styloporum specimens in my collection. What I think I'm observing is a limited window in time that illustrates how species growth habits change as the environment does.

I'm still unsure as to what drove the coral to decide where to grow. This site, ( http://www.fossilguy.com/sites/18mile/18_col.htm#coral )which has pictures of P. americanum, clearly shows both colonial habits occuring in the same formation (in this case the Moscow Formation which is contemporary with the Mahantango Fm.). Is it environmental or just happenstance? I can see why a polyp would want to colonize an existing shell as, to me, it seems more stable; but does this give it more of an advantage over those who colonize the substrate?


Here are some links to other photos of similar Pleurodictyum specimens around the world:

From Yale: http://www.yale.edu/ypmip/taxon/anthozoa/tabulata/7066.html

From the Kentucky Geological Survey: http://www.uky.edu/KGS/fossils/tabulatemound2.htm

From Lousiville Fossils: http://louisvillefossils.blogspot.com/2009/09/recent-kentucky-coral-finds.html

From Germany: http://www.panoramio.com/photo/1698308

From the website of SAMPUZ in Spain: http://www.sampuz.com/?q=node/116

From the Museum Victoria in Australia: http://museumvictoria.com.au/prehistoric/time/corals.html


Bibliography

*1 - "Paleobiology Database" Available at: http://paleodb.org/cgi-bin/bridge.pl?action=basicTaxonInfo&taxon_no=4996

*2 - Ellison, Robert L. Stratigraphy and Paleontology of the Mahantango Formation in South Central Pennsylvania, General Geology report G 48. Harrisburg: Pennsylvania Geological Survey, 1965

*3 - Faill, Robert T.; Hoskins, Donald M.; Wells, Richard B. Middle Devonian Stratigraphy in Central Pennsylvania - A Revision, General Geology report G 70. Harrisburg: Pennsylvania Geological Survey, 1978