Wooster’s Fossils of the Week: The mysterious Paleozoic encrusters Ascodictyon and Allonema

September 12th, 2014

 

1 Slide01The above pair of fossils are small sclerobionts commonly found on hard substrates in shallow marine sediments through much of the Paleozoic, especially the Silurian and Devonian. Paul Taylor and I have been studying them for a few years now and our first paper on them was published this summer (Wilson and Taylor, 2014). Ascodictyon (Silurian-Carboniferous) is on the left and Allonema (Silurian-Permian) is on the right. Both are calcitic encrusters and look, at least in this view, very different from each other. We present evidence in our paper, though, that strongly suggests Ascodictyon and Allonema are actually manifestations of the same organism. What that organism is, exactly, still eludes us. We are persuaded at the very least that they are not bryozoans as originally described by Nicholson, Ulrich and Bassler. Since they are so common their identity is important for studies of fossil diversity and paleoecology.
2 Slide07The above view through a light microscope of Ascodictyon and Allonema shows the perspective paleontologists have had of these encrusters until recently. The clear calcite skeletons sitting on a calcitic brachiopod shell (this is from the Devonian of Michigan) makes for little contrast and poor resolution, and the microscope-camera combination has a very limited depth of field. The rest of the images in this post were made with a Scanning Electron Microscope (SEM) expertly operated by Paul. The difference in morphological detail is not just astonishing, it is a revolution in the study of tiny fossils like this.
3 Slide16 siluriense UKThis is a typical view of Ascodictyon. It consists of stellate clusters of inflated vesicles (like little calcite balloons) connected by thin calcitic tubes called stolons. (Ascodictyon siluriense from the Silurian of the England.)

4 Slide24 waldronense S GotlandThis is a typical Allonema. The primary form is a series of porous vesicles attached in chains like sausages. (Allonema waldronense from the Silurian of Gotland, Sweden.)

5 Slide29 Silica MIHere is where these obscure little encrusters get interesting. This is a specimen from the Silica Shale (Middle Devonian) exposed in Michigan. It was collected in a beautiful suite of fossils by that intrepid citizen scientist, Brian Bade. It consists of Allonema sausages connected to Ascodictyon stolons which are themselves connected to Ascodictyon stellate vesicle clusters. Clear evidence that Allonema and Ascodictyon are end members of a morphological continuum produced by the same organism.

7 Slide33 Silica MIA critical feature we see in this Ascodictyon/Allonema complex is the occurrence of “sockets” at the bases of vesicles like the above from the Silica Shale. These are almost certainly places where some erect portion of the organism extended above the substrate. Maybe these were feeding devices? Reproductive parts? We’ve found no trace of them.

8 Slide39 S GotlandOur hypothesis is that Allonema (left) and Ascodictyon (right, both from the Silurian of Gotland, Sweden) are the basal parts of some as yet unknown erect organism. They may have stored nutrients for the creature. We are convinced they were not bryozoans, foraminiferans, corals or sponges. Unfortunately we can only classify them as incertae sedis or Microproblematica. At some point we’ll have to figure out how to name this complex with two genera and over a dozen species.

It was fun work, and the project continues. For more detail, see Wilson and Taylor (2014).

References:

Nicholson H.A. and Etheridge R. 1877. On Ascodictyon, a new provisional and anomalous genus of Palæozoic fossils. J. Nat. Hist., Series 4, 19: 463-468.

Ulrich E.O. and Bassler R.S. 1904. A revision of the Paleozoic Bryozoa. Smith. Misc. Coll. (Quart.) 45: 256-294.

Wilson M.A. and Taylor P.D. 2001. “Pseudobryozoans” and the problem of encruster diversity in the Paleozoic. PaleoBios 21 (Supplement to No. 2): 134-135.

Wilson, M.A. and Taylor, P.D. 2014. The morphology and affinities of Allonema and Ascodictyon, two abundant Palaeozoic encrusters commonly misattributed to the ctenostome bryozoans. In: Rosso, A., Wyse Jackson, P.N. and Porter, J. (eds.), Bryozoan Studies 2013. Studi trentini di scienze naturali 94: 259-266.

Wooster’s Fossils of the Week: A hardground with rugose corals from the Upper Ordovician of southern Ohio

September 5th, 2014

Hdgd small 090114The above slab is a carbonate hardground from the Liberty Formation (Upper Ordovician) of southern Ohio. Carbonate hardgrounds are cemented seafloors, so we’re actually looking at the hard rocky bottom of an Ordovician sea. I’ve long found the idea of a hardground fascinating — it is like a bit of ancient time frozen before us. This hardground is especially interesting because of the fossils associated with it. The knobby nature of the surface is probably due to a burrow system that was preferentially cemented and then exhumed by currents that washed away the loose sediment. The intersecting tunnels, now ridges, provided numerous crannies for encrusting, boring and nestling organisms to inhabit. The high points hosted encrusting bryozoans that needed currents for their filter-feeding.

brach coral 090114There are several shelly fossils found in the low points of this hardground surface. The brachiopod in the upper left is the orthid Plaesiomys subquadrata (Hall, 1847), and the conical rugose coral in the lower right is Grewingkia canadensis (Billings, 1862)

two corals 090114Here is another detailed view of the hardground showing a second rugose coral on the left. I suspect that the corals and maybe even the brachiopod are actually in place (or “in situ” to use the fancy words). I’ve seen such occurrences before and passed them off as just examples of loose fossils rolling into holes. Here, though, we can see that both corals have the calyx (the cup in which the coral polyp was located) facing upwards. These G. canadensis corals did not attach to hard substrates like some of their cousins, but lay recumbent and curved upwards on the seafloor. What better place to do so than in the cozy hollows of a hardground?

This slab is certainly a nice vignette of a marine community nearly 450 million years old.

References:

Billings, E. 1862. New species of fossils from different parts of the Lower, Middle, and Upper Silurian rocks of Canada. Paleozoic Fossils, Volume 1, Canadian Geological Survey, p. 96-168.

Hall, J. 1847. Paleontology of New York, v. 1: Albany, State of New York, 338 p.

Palmer, T.J. 1982. Cambrian to Cretaceous changes in hardground communities. Lethaia 15: 309–323.

Wilson, M.A. and Palmer, T.J. 1992. Hardgrounds and hardground faunas. University of Wales, Aberystwyth, Institute of Earth Studies Publications 9: 1–131.

First Wooster paleontology field trip of the year: the glorious Ordovician of Ohio

August 31st, 2014

1CaesarCreek083114Today the Invertebrate Paleontology class at The College of Wooster drove south to one of our favorite outcrops: the Waynesville, Liberty and Whitewater Formations (= Bull Fork Formation) at the emergency spillway in Caesar Creek State Park. I enjoy taking students to this extensive exposure because it has diverse fossils, is easy for beginners, and it is hard to get lost here! The rain was unrelenting on our drive down, and it continued well past our arrival at the Visitor Center. The park manager very helpfully showed us a new park movie and gave us a talk about the Army Corps of Engineers (which runs the dam and lake). This occupied us as the rain slowed and finally ended soon after we approached the rocks. You’ll see surface water as a theme in these photos because the spillway turned into meandering streams and wetlands.

2Brachs083114Above is an example of why we visited Caesar Creek. The fossils are fantastically abundant and well preserved. The students had a simple charge: collect a diverse array of fossils, enough to fill two large bags each. They will prepare and identify their fossils throughout the semester as a field/lab exercise. Since I’ve shown these fossils many times in this blog, I’ll use the rest of the post to show off my students.

3TrevorBrianKevin083114Heading up the north end crew is (from the left) Trevor Shoemaker, Brian Merritt, and Kevin Komara. (You’ll see that I get better with the people photos as I moved south.)

4Chloe083114Chloe Wallace is here in the foreground braving the mud on her knees.

5KelliSpencer083114Kelli Baxstrom (upper left) has a nearly full bag, and the flash of purpleness in the lower right is Spencer Zeigler.

6Mary083114Mary Reinthal shows her enthusiasm for her first fossil expedition.

7Andrew083114Andrew Conaway was always easy to find on the outcrop.

8Annette083114Annette Hilton is here examining a slab full of small strophomenid brachiopods.

9Cassidy083114Cassidy Jester has a slab almost too large for her bag. Note the standing water behind her.

CurtisGalen083114Curtis Davies (back) and Galen Schwartzberg show the happiness that comes with fossil collecting (especially when the rain stops).

DanJeffKrysden083114Dan Peraza-Rudesill and Jeff Gunderson are joyfully receiving instruction from the class Teaching Assistant, Krysden Schantz.

GalenSharron083114Galen Schwartzberg (left) demonstrates that he can even find fossils here with his eyes closed as a cynical Sharron Osterman looks on. Both of these students are from Seattle, so rain is no discomfort for them. (Nor mud, in Galen’s case.)

MaeKaitlin083114Mae Kemsley and Kaitlin Starr proudly carry their first bags of fossils.

Meredith083114Meredith Mann reaches with mud-stained fingers for more fossil treasures.

William083114William Harrison is a senior who wanted to come along for the fun and to possibly add to his senior independent study materials. He was a great help with his advanced paleontological knowledge.

zPaleoGroup083114And here is the class at the end of the session with their collections. I was very pleased to see how dry everyone was. We had a window of respite for collecting because soon after lunch it began to rain again. We were only missing Julia Franceschi, who had a scheduling conflict. I’m looking forward to seeing all these fossils once they are cleaned and prepared in our paleontology lab. At the end of the semester each student will have a full report on the fossil fauna at Caesar Creek, including identifications and paleoecology.

Wooster’s Fossils of the Week: Orthid brachiopods from the Middle Devonian of New York

August 29th, 2014

Tropidoleptus carinatus 585On the first day of the Invertebrate Paleontology course at Wooster, I give all the students a fossil to identify as best they can. Everyone gets the same kind of specimen, and they can use any means to put as specific a name on it as possible. Most students struggle with the exercise, of course — I just want them to spend some time looking at fossils online and getting a feel for distinguishing characteristics and preservation. This week, though, one student nailed it. Meredith Mann (’16) identified the target fossil above as Tropidoleptus carinatus (Conrad, 1839) from the Middle Devonian of  New York. I suppose if I asked she could have told me it was from the Kashong Shale Member of the Moscow Formation, and that it was collected by my friend Brian Bade. Nicely done, Meredith!

Tropidoleptus carinatus (Conrad, 1839) is a member of the Orthida, an order of brachiopods that lived from the Early Cambrian up to the Permian extinction. Orthids are a difficult group to characterize because they were so variable in shell shape and form. T. carinatus, for example, is one of the few orthids to have a concavo-convex shell, meaning that one side is concave (on the right in the image above) and the other convex (left). Most orthids are biconvex, meaning that both sides are convex. (A lima bean would also be biconvex by this definition.)

I like these little brachiopods because their shells are often encrusted by wonderful little creatures like bryozoans, Allonema, Ascodictyon, and microconchids. Each shell had the potential of hosting its own little community of encrusters.

Wooster’s Fossils of the Week: Remanié fossils in the Lower Cretaceous of south-central England

August 22nd, 2014

Faringdon ammonite smThe last two editions were about a bryozoan and borings from the Faringdon Sponge Gravels (Lower Cretaceous, Upper Aptian) of south-central England. This week we have some Jurassic fossils from the same unit. That sounds a bit daft at first — Jurassic fossils in a Cretaceous unit? — until it becomes obvious that these are older fossils reworked into a younger deposit. In this case underlying Jurassic ammonites have been unearthed and tossed around with sediment in Cretaceous high-energy tidal channels. These older fossils in a younger context are called remanié, meaning they have been “rehandled” in a fancy French way.

The above image is an example of remanié in the Faringdon Sponge Gravels. It is a partial internal mold of a Jurassic ammonite. Drilled into it are several holes attributed to Early Cretaceous bivalves and called by the trace fossil name Gastrochaenolites. The ammonite fossil was eroded out of an outcrop of Jurassic rock and then bored while rolling around in what would become the Faringdon Sponge Gravels.
Ammonite frag 2 072014This is another Jurassic ammonite internal mold. The jagged lines are the sutures of the ammonite (remnants of the septal walls). This mold was phosphatized (partially replaced with phosphate) before it was reworked into the Cretaceous gravels. Many remanié fossils are phosphatized because of long exposure on the seafloor.
Ammonite frag 1 072014Finally, this is a fragment of another Jurassic ammonite internal mold in the Faringdon Sponge Gravels. It has an odd shape because it has disarticulated along the sutures. We are looking at the face of one of the septa, or at least where this septum would have been if it hadn’t dissolved. You can see some tiny borings that were made by Cretaceous polychaete worms.

In one of the cobbles in the Faringdon Sponge Gravels I found an identifiable ammonite. It was Prorasenia bowerbanki, which indicated that the cobble was derived from the Lower Kimmeridge Clay or Upper Oxfordian clays. The above ammonites are likely from the same Jurassic sequence. This means these fossils were roughly 45 million years old when they were reworked into the sponge gravels. Today it would be as if Eocene fossils were eroding out of a cliff and being incorporated within a modern sediment. When you think about it, this is a relatively common occurrence.

References:

Murray-Wallace, C V. and Belperio, A.P. 1994. Identification of remanié fossils using amino acid racemisation. Alcheringa 18: 219-227.

Pitt L.J. and Taylor P.D. 1990. Cretaceous Bryozoa from the Faringdon Sponge Gravel (Aptian) of Oxfordshire. Bulletin of the British Museum (Natural History), Geology Series, 46: 61–152.

Wells, M.R., Allison, P.A., Piggott, M.D., Hampson, G.J., Pain, C.C. and Gorman, G.J. 2010. Tidal modeling of an ancient tide-dominated seaway, part 2: the Aptian Lower Greensand Seaway of Northwest Europe. Journal of Sedimentary Research 80: 411-439.

Wilson, M.A. 1986. Coelobites and spatial refuges in a Lower Cretaceous cobble-dwelling hardground fauna. Palaeontology 29: 691-703.

Wooster’s Fossils of the Week: Abundant borings in Early Cretaceous cobbles from south-central England

August 15th, 2014

Faringdon cobble in matrix 071714Last week I described a cyclostome bryozoan on the outside of a quartz cobble from the Faringdon Sponge Gravels (Lower Cretaceous, Upper Aptian) of south-central England near the town of Faringdon. This week I’m featuring a variety of heavily-bored calcareous cobbles from the same unit. One is shown above in its matrix of coarse gravel. The holes are bivalve borings known as Gastrochaenolites. As a reminder, these gravels are very fossiliferous and were deposited in deep channels under considerable tidal current influence (see Wells et al., 2010).

Faringdon cobble 1 071714The large and medium-sized flask-shaped borings are all Gastrochaenolites. In the suite of cobbles described in Wilson (1986), there are three ichnospecies of bivalve borings: G. lapidicus, G. cluniformis and G. turbinatus. It is thus likely, although not necessarily, an indication that at least three bivalve species were boring the soft calcareous claystone to make secure homes for their filter-feeding. The thin, worm-like borings are Maeandropolydora, which were probably made by polychaete “worms”.

Faringdon cobble 3 071714Some of the Gastrochaenolites lapidicus borings have remarkably spherical chambers, a testament to the uniform lithological character of the rock.

Faringdon cobble 5 071714Occasionally bivalve shells are found still preserved in their crypts, along with nestling brachiopods. Some shell bits are visible in the borings above.

FaringdonCobble 585 071714Some of the cobbles are so heavily bored that they fall apart quickly on removal from the matrix. On the Cretaceous seafloor this intensity of boring must have reduced many cobbles to bits before burial — a classic example of bioerosion.

Diagram 071714What is very cool about these Faringdon cobbles is that the borings often overlapped inside, creating a network of tunnels and small cavities that hosted dozens of bryozoan, foraminiferan, sponge, annelid worm, and brachiopod species. This is a diagram from Wilson (1986) showing the combination of external encrusters in a high energy, abrasive world, and coelobites (cavity dwellers) in the protected enclosures. A diverse community can be found on each cobble, inside and out. In a future post I will describe some of these coelobite fossils.

References:

Pitt L.J. and Taylor P.D. 1990. Cretaceous Bryozoa from the Faringdon Sponge Gravel (Aptian) of Oxfordshire. Bulletin of the British Museum (Natural History), Geology Series, 46: 61–152.

Wells, M.R., Allison, P.A., Piggott, M.D., Hampson, G.J., Pain, C.C. and Gorman, G.J. 2010. Tidal modeling of an ancient tide-dominated seaway, part 2: the Aptian Lower Greensand Seaway of Northwest Europe. Journal of Sedimentary Research 80: 411-439.

Wilson, M.A. 1986. Coelobites and spatial refuges in a Lower Cretaceous cobble-dwelling hardground fauna. Palaeontology 29: 691-703.

Wooster’s Fossil of the Week: An Early Cretaceous cobble-dwelling bryozoan

August 8th, 2014

Faringdon quartz 071714One of my formative experiences as a young paleontologist was working in the Faringdon Sponge Gravels (Lower Cretaceous, Upper Aptian) of south-central England while on my first research leave in 1985. (I was just a kid!) These gravels are extraordinarily fossiliferous with sponges, brachiopods, corals, vertebrate bones, and a variety of cobbles, both calcareous and siliceous. These coarse sediments were deposited in narrow channels dominated by tidal currents with significant energy reworking and sorting the fossil and rock debris. Above is a cobble of very hard vein quartz from the Sponge Gravels. On the left end you see an encrusting bryozoan with an unusual morphology.
LhwydThe fossils of the Faringdon Sponge Gravels have been studied for a very long time. The first formal notice of them is a museum catalogue compiled by Edward Lhwyd (image above) and published in 1699. Lhwyd (1660-1709) was a Welsh natural philosopher better known by his Latinized name Eduardus Luidus. He had an unfortunate childhood being the illegitimate son of what has been reported as a “dissolute and impractical” (and poor) father. Still, he was better off than most and had schooling all the way up to Oxford (but he could not afford to graduate). In 1684 he became an assistant to Robert Plot, the Keeper of the Ashmolean Museum in Oxford. He became a great scientific traveler and collector, specializing in plants and fossils and (eventually) ancient languages of Britain. In 1691 he was appointed Keeper at the Ashmolean. His book detailing fossils of Britain (Lithophylacii Britannici Ichnographia) was published with financial assistant from his good friend Isaac Newton.
Corynella in Lhwyd plate 18This is plate 18 from Lhwyd (1699). The fossil in the upper right is the sponge Corynella from the Faringdon Sponge Gravels.

Lhwyd’s views on the origin of fossils are with describing. This is a summary from Edmonds (1973, p. 307-308):

He suggested a sequence in which mists and vapours over the sea were impregnated with the ‘seed’ of marine animals. These were raised and carried for considerable distances before they descended over land in rain and fog. The ‘invisible animacula’ then penetrated deep into the earth and there germinated; and in this way complete replicas of sea organisms, or sometimes only parts of individuals, were reproduced in stone. Lhwyd also suggests that fossil plants known to him only as resembling leaves of ferns and mosses which have minute ‘seed’, were formed in the same manner. He claimed that this theory explained a number of features about fossils in a satisfactory manner: the presence in England of nautiluses and exotic shells which were no longer found in neighbouring seas; the absence of birds and viviparous animals not found by Lhwyd as fossils; the varying and often quite large size of the forms, not usual in present oceans; and the variation in preservation from perfect replica to vague representation, which was thought to represent degeneration with time.

What is most interesting about these ideas is that they have no reference to Noah’s Flood or other divine interventions.

In 1708, Lhwyd was elected a Fellow of the Royal Society in 1708. He didn’t enjoy this privilege long for he died of pleurisy the next year at age 49.
Reptoclausa hagenowi Cretaceous England 071714Now back to the bryozoan on the Faringdon cobble. It is the cyclostome Reptoclausa hagenowi (Sharpe, 1854). It has an odd form of irregularly radiating ridges of feeding zooids (autozooids) separated from each other by structural zooids (kenozooids). I like to think (although I have no evidence) that this morphology was resistant to abrasion in the rough-and-tumble life of living on a cobble in a high-energy channel. There are few other encrusters on the outer surfaces of the Faringdon cobbles.

The next two Fossils of the Week will also be from the fascinating Faringdon Sponge Gravels.

References:

Edmonds, J.M. 1973. Lhwyd, Edward, p. 307-308. In: Gillespie, C.C. (ed.). Dictionary of Scientific Biography, 8. Charles Scribner’s Sons, New York, 620 pp.

Lhwyd, E. 1699. Lithophylacii Britannici Ichnographia. London, 139 pages.

Meyer, C.J.A. 1864. I. Notes on Brachiopoda from the Pebble-bed of the Lower Greensand of Surrey; with Descriptions of the New Species, and Remarks on the Correlation of the Greensand Beds of Kent, Surrey, and Berks, and of the Farringdon Sponge-gravel and the Tourtia of Belgium. Geological Magazine 1(06): 249-257.

Pitt L.J. and Taylor P.D. 1990. Cretaceous Bryozoa from the Faringdon Sponge Gravel (Aptian) of Oxfordshire. Bulletin of the British Museum (Natural History), Geology Series, 46: 61–152.

Wells, M.R., Allison, P.A., Piggott, M.D., Hampson, G.J., Pain, C.C. and Gorman, G.J. 2010. Tidal modeling of an ancient tide-dominated seaway, part 2: the Aptian Lower Greensand Seaway of Northwest Europe. Journal of Sedimentary Research 80: 411-439.

Wilson, M.A. 1986. Coelobites and spatial refuges in a Lower Cretaceous cobble-dwelling hardground fauna. Palaeontology 29: 691-703.

Wooster’s Fossils of the Week: An Ordovician hardground with a bryozoan and borings — and an unexpected twist

August 1st, 2014

1 Hardground Bryo Large 071514aThe view above, one quite familiar to me, is of a carbonate hardground from the Upper Ordovician Grant Lake Formation exposed near Washington, Mason County, Kentucky. We are looking directly at the bedding plane of this limestone. The lumpy, spotted fossil covering about half the surface is a trepostome bryozoan. It looks like a dollop of thick pudding plopped on the rock. In the upper left are round holes that are openings of the trace fossil Trypanites, a common boring in carbonate hard substrates.
2 Closer hdgd bryo 071514bThis closer view shows the bryozoan details in the right half. You can barely pick out the tiny pin holes of the zooecia (the tubes that contained the individual zooids) and see the raised areas called maculae, which may have assisted in directing water currents for these colonial filter-feeders. Without a thin-section or peel I can’t identify the bryozoan beyond trepostome, but I suspect it is Amplexopora. The Trypanites borings in the hardground surface are also visible.
3 Hardground oblique Ordovician sm 071514cThis oblique view brings all the elements together. The bryozoan has closely encrusted the microtopography of the hardground surface. The Trypanites borings are shown cutting directly through the limestone of the hardground. Both of these observations confirm that the hardground was cemented seafloor sediment when the encrusters and borers occupied it.
4 Cross section hdgd 071514dHere is a full cross-section view showing the borings and the draping nature of the bryozoan. Now for the twist — I’m showing the specimen upside-down! It was actually found in place with the bryozoan down, not up. This is the roof of a small cave on the Ordovician seafloor. The bryozoan was hanging down from the ceiling, and the boring organisms were drilling upwards. The true orientation of this specimen is thus —
5 Cross section hdgd right side up 071514dThe cave was apparently formed after the carbonate hardground was cemented on the seafloor. Currents may have washed away unconsolidated muds underneath the hardground, forming a small cavity then occupied by the borers and the bryozoan: an ancient cave fauna. Brett & Liddell (1978) showed similar cavity encrustation in the Middle Ordovician, and I recorded a nearly identical situation in the Middle Jurassic of Utah (Wilson, 1998). Other detailed fossil marine caves are described from the Jurassic by Palmer & Fürsich (1974) and Taylor & Palmer (1994).

I should write up this Ordovician story someday!

References:

Brett, C.E. and Liddell, W.D. 1978. Preservation and paleoecology of a Middle Ordovician hardground community. Paleobiology 4: 329– 348.

Bromley, R.G. 1972. On some ichnotaxa in hard substrates, with a redefinition of Trypanites Mägdefrau. Paläontologische Zeitschrift 46: 93–98.

Palmer, T.J. 1982. Cambrian to Cretaceous changes in hardground communities. Lethaia 15: 309–323.

Palmer, T.J. and Fürsich, F.T. 1974. The ecology of a Middle Jurassic hardground and crevice fauna. Palaeontology 17: 507–524.

Taylor, P.D. and Palmer, T.J. 1994. Submarine caves in a Jurassic reef (La Rochelle, France) and the evolution of cave biotas. Naturwissenschaften 81: 357-360.

Taylor, P.D. and Wilson. M.A. 2003. Palaeoecology and evolution of marine hard substrate communities. Earth-Science Reviews 62: 1–103.

Wilson, M.A. 1998. Succession in a Jurassic marine cavity community and the evolution of cryptic marine faunas. Geology 26: 379-381.

Wilson, M.A. and Palmer, T.J. 1992. Hardgrounds and hardground faunas. University of Wales, Aberystwyth, Institute of Earth Studies Publications 9: 1–131.

Wooster’s Fossil of the Week: A faulted oyster ball from the Middle Jurassic of Utah

July 25th, 2014

Split oyster ball 062914I’m returning this week to one of my favorite fossil types: the ostreolith, popularly known as the “oyster ball”. These were lovingly described in a previous blog entry, so please click there to see how they were formed and some additional images. They are found almost exclusively in the Carmel Formation (Middle Jurassic) of southwestern Utah.They are circumrotatory (a fancy word for “rolling around while forming”) accumulations of small cup-like oysters along with minor numbers of plicatulid bivalves, disciniscid brachiopods, cyclostome bryozoans (see Taylor & Wilson, 1999), and mytilid bivalves that drilled borings known as Gastrochaenolites. They are nice little hard-substrate communities originally nucleated on bivalve shells (Wilson et al., 1998).

oyster ball close 062914Here is a close view of the oyster valves on the outside of the ostreolith. They are attached to similar valves below them, and it is oysters all the way to the center.

What is special about our specimen here is that it managed to obtain a fault right through its center! The chances of this happening are slim, given that they are relatively rare in the rock matrix. The faulting was probably during the Miocene related to a “left-lateral transfer zone that displaces north-south–trending crustal blocks of the eastern Basin and Range Province to the west” (Petronis et al., 2014, p. 534). This is an interesting tectonic region between the Basin and Range Province and the Colorado Plateau.

Slickenfibers 062914A close view of the fault surface shows it is a striated slickenside. The striations (called slickenlines) are parallel to the direction of movement, not that we have to guess when we look at the ostreolith itself. There are also calcitic deposits here formed during faulting called slickenfibres. These elongated crystals have tiny step-like breaks in them that show the actual direction of movement.

Another nice specimen combining paleontology and structural geology.

References:

Petronis, M.S., Holm, D.K., Geissman, J.W., Hacker, D.B. and Arnold, B.J. 2014. Paleomagnetic results from the eastern Caliente-Enterprise zone, southwestern Utah: Implications for initiation of a major Miocene transfer zone. Geosphere 10: 534-563.

Taylor, P.D. and Wilson, M.A. 1999. Middle Jurassic bryozoans from the Carmel Formation of southwestern Utah. Journal of Paleontology 73: 816-830.

Wilson, M.A., Ozanne, C.R. and Palmer, T.J. 1998. Origin and paleoecology of free-rolling oyster accumulations (ostreoliths) in the Middle Jurassic of southwestern Utah, USA. Palaios 13: 70-78.

Wooster’s Fossils of the Week: Silicified productid brachiopods from the Permian of West Texas

July 18th, 2014

Productids ventral valves 052514The three beauties above are productid brachiopods from the Road Canyon Formation (Middle Permian, Roadian, approximately 270 million years old) in the Glass Mountains of southwestern Texas. They are part of a series we’ve done on the silicified fauna of a block of limestone we dissolved in the lab many years ago. The calcitic shells have been replaced with silica during the process of fossilization, so they can be extracted from the carbonate matrix with hydrochloric acid. This is a primary way we can see delicate parts of a fossil, like the long hollow spines above. Ordinarily these would have been lost under the usual processes of taphonomy.

The specimens are highly convex ventral valves, which are characteristic of the productid brachiopods. The long hollow spines helped distribute the weight of these brachiopods on soft and unstable substrata, like a sandy or muddy sediment. This is often called “the snowshoe effect”. Below is a diagram reconstructing productid brachiopods on a sandy substrate with their spines keeping them from sinking below the sediment-water interface.

productid diagramProductid Permian Texas 585Here is a closer view of the ventral valve exterior of one of these productid brachiopods. You can see how delicate the hollow spines are.

Productid interior ventral Permian Texas 585This is the interior of the same valve. Each spine has a hole connecting it to the inside of the shell. The mantle, which secretes the shell and has other physiological functions, extended out into each spine to build its length and possibly carry some sort of sensory abilities.

I have been unable to identify these brachiopods because of the bewilderingly large number of them described by Cooper and Grant in the 1960s and 1970s. Maybe one of our readers can give it a shot!

References:

Brunton, C.H.C., Lazarev, S.S. and Grant, R.E. 1995. A review and new classification of the brachiopod order Productida. Palaeontology 38: 915-936.

Cooper, G.A., and Grant, R.E., 1964, New Permian stratigraphic units in Glass Mountains, West Texas. American Association of Petroleum Geologists Bulletin 48: 1581-1588.

Cooper, G.A., and Grant, R.E. 1966. Permian rock units in the Glass Mountains, West Texas, In: Contributions to stratigraphy, 1966: U.S. Geological Survey Bulletin 1244-E: E1-E9.

Cooper, G.A. and Grant, R.E. 1972. Permian brachiopods of West Texas, I. Smithsonian Contributions to Paleobiology 14: 1–228. [and five other volumes]

Shiino, Y. and Suzuki, Y. 2007. Articulatory and musculatory systems in a Permian concavo-convex brachiopod Waagenoconcha imperfecta Prendergast, 1935 (Productida, Brachiopoda). Paleontological research 11: 265-275.

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