Wooster Geologists

It doesn’t look like much, this long lump of gray stone. With a close view you might pick up a hint of a bryozoan or two, but mostly we see rather shabby shades of grey. One of the coolest perks of being a geologist, though, is that you get to use a saw to cut rocks in half to see what’s inside. So that’s what I did with this specimen from the Whitewater Formation (Upper Ordovician) of southeastern Indiana at a site we’ve visited often.

In this cross-section we see first a long, cone-shaped fossil made of white calcite. It is the rugose coral Grewingkia canadensis, one of the most common fossils in the upper part of the Upper Ordovician. This coral in life would have stood upright like an ice cream cone, spreading the tentacles of its polyp to catch very small swimming prey (and maybe to do a bit of symbiotic photosynthesis). The polyp sat in the cup-like cavity on the expanded end of the cone. The coral evidently died on the Ordovician seafloor and toppled over to be encrusted on one side, presumably the one that faced upwards.

This is a closer view of the cross-section showing the encrustations on the rugose coral skeleton. The image is annotated below.

The coral skeleton in the lower right was first encrusted by a trepostome bryozoan, which you can recognize by the tubes (zooecia) extending perpendicular from the substrate. This bryozoan is thickest on the upwards-facing surface of the coral, and it thins as it wraps around and then colonizes the cryptic space beneath (but not too far). This bryozoan is covered with a layer of sediment which appears to have rapidly cemented in place (a function of Calcite Sea geochemistry). The sediment then is encrusted by a another trepostome bryozoan with long zooecia and several layers.

In this closer view of the second bryozoan you can see that its base is irregular as it grew across the rough cemented sediment surface. In the middle of this view some of the bryozoan zooecia are occupied by dark spots known as brown bodies. These are likely the remains of bryozoan polypides (main parts of the individual zooids) that were sealed into their zooecia by some disturbance. In this case the whitish bit of sediment above the cluster may represent something that settled on the colony, stopping the growth of the zooecia below, and forcing those nearby to grow around it.

Moving down the coral skeleton away from its opening we come across borings drilled down through the coral skeleton (the white mass at the bottom of the image). The conical, large boring is filled with golden crystals of the mineral dolomite, which were formed long after burial. The shape of this boring is unusual. Typical borings in these corals have straight parallel sides, but this boring is cone-shaped. We’ll see if we can find more like it to get a better idea of its shape and distribution.

This week’s fossil, then, is a demonstration of the hidden wonders sometimes found in even the dullest of grey rocks!

Last week we looked at a dull gray rock found in a roadcut in southeastern Indiana near the town of Liberty. It is from the Saluda Formation (Upper Ordovician), a thin unit that was likely deposited in very shallow, lagoonal waters along the Cincinnati Arch. We know that it is primarily a platter formed by the mysterious fossil Tetradium, and that it is encrusted with a trepostome bryozoan that was infested by some sort of soft-bodied encruster on its surface, forming the trace fossil Catellocaula vallata. Now we’re examining the wonders revealed by cutting this rock in half. Above we see the surprising and spectacular geode that it is, with calcite crystals surrounding a dark cavity. Let’s see what the fossils look like when polished and magnified.

The orangish, irregular patch in the lower half of the section above is the crystalline calcite near the center of the rock. The sediment-filled tubes in the top half are of the Tetradium specimen. Note that the walls of the tubes are blurry and indistinct, and that they fade and disappear into the calcite crystals below. This is apparently because the skeleton of Tetradium was made of aragonite, an unstable form of calcium carbonate. It is likely that the aragonitc, tubular skeleton of Tetradium dissolved away in the center of this encrusted mass, forming the cavity that later filled with secondary calcite crystals. The remaining tubes were apparently preserved as ghostly molds by infillings of calcitic mud that didn’t dissolve.

In this section we are cutting the Tetradium tubes perpendicularly, rather than the longitudinal cuts we saw before. The cross-sections of the tubes show a four-part symmetry, which adds to the mystery of this group. (This is where the name “Tetradium” comes from.) It has been called a chaetetid sponge (as in Termier and Termier, 1980); a “calcareous filamentous florideophyte [red] alga” (Steele-Petrovich 2009a, 2009b, 2011; she renamed it Prismostylus), and most commonly a coral of some sort (as in Wendt, 1989). I now know enough about chaetetids to say that it is not in that group. Chaetetid tubes are not aragonitic, do not show tetrameral symmetry, and have diaphragms (horizontal floors). The corals of the Ordovician are decidedly calcitic, not aragonitic, and they too have internal features in their tubes not seen here. The four-part symmetry, though, is something you see in the coral’s phylum, Cnidaria, so there is that vague resemblance. The red algal affinity strongly urged by Steele-Petrovich may be our best diagnosis for the place of Tetradium.

On top of the tubes of Tetradium is the encrusting trepostome bryozoan. Its tubes (zooecia) are made of stable calcite, so they are well preserved compared to the aragonite tubes of Tetradium below it. Note that the bryozoan is made of two layers. One colony died or went into some sort of remission, and another of the same species grew across it. The second colony could have budded somewhere from the first colony.

This closer view of the bryozoan section shows details of the zooecia, including the horizontal diaphragms inside. The dark spots at the tops of the zooecia are brown bodies, the remains of polypides preserved here in clear calcite cement. (We’ve seen brown bodies before in this blog.) They likely represent some sort of traumatic event in the life of this bryozoan when this part of the colony essentially shut down and was covered with sediment.

Finally, there is a mineralogy story here too! Attached to the dog-tooth calcite spar in the center of this geode is this tiny gypsum flower. The gypsum crystals are white and very delicate. The dark needles among them are mysterious. Dr. Meagen Pollock and her students will subject them to x-ray diffraction in her lab later this semester. I’ll report the results here.

It is a simple tool, the rock saw. For geologists and paleontologists, it is one of our essential instruments for discovery.

References:

Hatfield, C.B. 1968. Stratigraphy and paleoecology of the Saluda Formation (Cincinnatian) in Indiana, Ohio, and Kentucky. Geological Society of America Special Papers 95: 1-30.

Li, Q., Li, Y. and Kiessling, W. 2015. The first sphinctozoan-bearing reef from an Ordovician back-arc basin. Facies 61: 1-9.

Palmer, T.J. and Wilson, M.A. 1988. Parasitism of Ordovician bryozoans and the origin of pseudoborings. Palaeontology 31: 939-949.

Steele‐Petrovich, H M. 2009a. The biological reconstruction of Tetradium Dana, 1846. Lethaia 42: 297-311.

Steele‐Petrovich, H M. 2009b. Biological affinity, phenotypic variation and palaeoecology of Tetradium Dana, 1846. Lethaia 42: 383-392.

Steele-Petrovich, H.M. 2011. Replacement name for Tetradium DANA, 1846. Journal of Paleontology 85: 802–803.

Termier, G. and Termier, H. 1980. Functional morphology and systematic position of tabulatomorphs. Acta Palaeontologica Polonica 25: 419-428.

Wendt, J. 1989. Tetradiidae — first evidence of aragonitic mineralogy in tabulate corals. Paläontologische Zeitschrift 63: 177–181.

I found this lump of a gray rock in southeastern Indiana along a highway near the town of Liberty. It is from the Saluda Formation (Upper Ordovician), a thin unit that was likely deposited in very shallow, lagoonal waters along the Cincinnati Arch. It is not especially notable in this view. I intend to show you the wonders that can be revealed in such dull rocks by simply sawing them in half. First, though, let’s have a look at the outside. Inn the view above you can see on the left side a large trepostome bryozoan with some irregular holes in it. We’ll come back to that.

Flipping the rock over we find that most of it is a fibrous fossil shaped like a dinner plate with limestone matrix and encrusting bryozoans covering most of the center.

A closer view of the fibrous part shows thousands of thin tubes radiating out from the center of the plate. This is the Ordovician fossil known as Tetradium. It is strange and mysterious enough that we will use the next Fossil of the Week blog post to describe it. It has been called a chaetetid sponge (as in Termier and Termier, 1980); a “calcareous filamentous florideophyte alga” (Steele-Petrovich 2009a, 2009b, 2011; she renamed it Prismostylus), and most commonly a coral of some sort (Wendt, 1989). Interesting range of options! We’ll explore later.

Now, back to the trepostome bryozoan visible on the top surface. There are three kinds of holes on this specimen. The smallest are the zooecia of the bryozoan itself, each of which would have hosted a zooid (a bryozoan individual). They are the background texture of the fossil. The large holes above are a bioclaustration structure that Time Palmer and I named in 1988 as Catellocaula vallata (little chain of walled  pits). It is explained thoroughly in one of the early Fossil of the Week posts. Basically they are pits formed when the bryozoan grew up and around some sort of soft-bodied colonial organism sitting on top of the surface, forming these embedment structures connected together by tunnels at their bases.

A third kind of hole in this bryozoan is a boring cut down into its skeleton. These are the trace fossil Trypanites, formed when some kind of filter-feeding worm bored straight into the calcite zoarium (colonial skeleton) to make a protective home, as many polychaete worms do today.

Now let’s cut this stone in half —

Inside we find a wonderful cavern of crystals — a geode! The crystals are mostly calcite, with dog-tooth spar lining the cavity and blocky spar replacing large parts of the Tetradium skeleton. There’s a story here, and it will be told in the next Fossil of the Week post!

References:

Hatfield, C.B. 1968. Stratigraphy and paleoecology of the Saluda Formation (Cincinnatian) in Indiana, Ohio, and Kentucky. Geological Society of America Special Papers 95: 1-30.

Li, Q., Li, Y. and Kiessling, W. 2015. The first sphinctozoan-bearing reef from an Ordovician back-arc basin. Facies 61: 1-9.

Palmer, T.J. and Wilson, M.A. 1988. Parasitism of Ordovician bryozoans and the origin of pseudoborings. Palaeontology 31: 939-949.

Steele‐Petrovich, H M. 2009a. The biological reconstruction of Tetradium Dana, 1846. Lethaia 42: 297-311.

Steele‐Petrovich, H M. 2009b. Biological affinity, phenotypic variation and palaeoecology of Tetradium Dana, 1846. Lethaia 42: 383-392.

Steele-Petrovich, H.M. 2011. Replacement name for Tetradium DANA, 1846. Journal of Paleontology 85: 802–803.

Termier, G. and Termier, H. 1980. Functional morphology and systematic position of tabulatomorphs. Acta Palaeontologica Polonica 25: 419-428.

Wendt, J. 1989. Tetradiidae — first evidence of aragonitic mineralogy in tabulate corals. Paläontologische Zeitschrift 63: 177–181.