In the footsteps of Charles Darwin: Geological excursion into the Central Andes

October 1st, 2014

01ViewOppositeAconcagua100114MENDOZA, ARGENTINA–Today I had one of the finest geological field trips in my life. The scenery was stunning, the geology extraordinary, and the history deeply moving. Being able to share the experience with so many of my geologist friends, old and new, was a bonus. I also thought how much my Wooster Geology colleagues would have enjoyed this excursion. There were volcanics for Meagen, glacial deposits for Greg, and the most extraordinary structural geology for Shelley. This was a mid-meeting field trip of the Fourth International Palaeontological Congress entitled “Darwin and the Highest Andes”. It was led by Victor Ramos and Laura Giambiagi.

The purpose of the trip was to explore the Central Andes from Mendoza almost to the border with Chile, cutting a cross-section through the ranges. We followed the route Charles Darwin took between March 29 and April 5, 1835. This experience was heaven for me: history and geology combined.02TectonicContext100114The above image is from Jordan et al. (1983, issue 3 of Episodes) reproduced in our guidebook. It shows the flat-slab subduction beneath the Andes as well as the three ranges we visited (Precordillera, Frontal Cordillera, and Principal Cordillera). We traveled from right to left on this diagram perpendicular to the mountains.

03Precordillera101014The Precordillera was impressive enough by itself. Here is the dirt road we followed up the eastern slope. It is an enlarged mule path used for centuries as part of the road between Chile and Argentina. Most famously this is the route General José de San Martín used in his epic 1817 Crossing of the Andes to defeat the Spanish as part of the Wars of Independence. Charles Darwin, of course, took the same route in 1835.

04Guanico100114We saw many guanacos (Lama guanicoe) on the way up into the mountains. These camelids native to the area have delightful black faces and skittish ways.

05PrecordilleraNearTop100114Looking west across the Precordillera near the top. The clouds we see stayed around the Mendoza area, but we were above them and had a wonderful sunny day.

06DarwinTree100114Our first major stop was at a petrified forest in Triassic rift valley sediments. Darwin described these tree trunks, which are coarsely replaced by calcite, as part of a buried conifer forest that had been tilted to the west. The sediments are mostly fine volcanic grains. He counted 52 trees, but I found only a few during our stop.

07GuidebookPillar100114Darwin wrote that one of the fossil trees reminded him of “Lot’s wife turned into a pillar of salt.” Who could guess that one of my photographs from Israel would make it into the guidebook as a consequence!

08DarwinForestViewWest100114The view west from Darwin’s Forest shows the beginning of the Frontal Cordillera. The terrain here is very much like that of the Mojave Desert in California, even including a species of Larrea (creosote bush).

MZ and me from Michal Rakocinski1My Polish colleague Michał Zatoń and me with the magnificent Frontal Cordillera. Thank you Michal Rakocinski for the image.

09MarkDarwinForest100114Here I am at a plaque honoring Darwin at the fossil forest. An earlier marker had been destroyed by a small group of local Creationists. Evolution is not very controversial in Argentina, but some of their Creationists are apparently vandals. This plaque has been made especially thick and heavy.

10LunchStop100114We ate lunch while admiring this view of Triassic volcanics near the start of the Frontal Cordillera. The stone work is the Picheuta Bridge.

11PuentedelInca100114We are now in the Principal Cordillera of the Andes. I’m looking along old railroad tracks at Puente del Inca into the magnificent High Andes.

12SouthviewPuentedelInca100114This is the opposite view. Note the fine, fine weather. We needed only light jackets or sweaters.

13BridgePuentedelInca100114Puente del Inca itself is a natural bridge of travertine-cemented landslide debris. Thermal springs like these are scattered along the axes of the Principal Cordillera of the Andes. This bridge, sadly, had nothing to do with the Incas, although they did extend this far south.

14GuidebookCrossSection100114Darwin drew a cross-section of the Principal Cordillera, including this site at Puente del Inca (on the middle right). This is an image from the guidebook of our trip.

15Aconcagua100114The most impressive part of our trip (I’m avoiding “high point”!) was this view we had of Aconcagua, the highest mountain in the southern and western hemispheres. Essentially it is Miocene volcanic rocks unconformably atop folded Cretaceous sedimentary deposits thrust on top of Paleogene rocks. This iconic mountain often has snow blowing off its crest, producing white clouds. Darwin mistook these for ash emissions and thought this was an active volcano.

16DarwinShelter100114Our last stop was at Casuchas del Virrey, a brick structure built in 1765 for Spanish officials making the hazardous trip between what would become Chile and Argentina. (They would have rather communicated by sea through the Magellan Strait, but the British were making that difficult.) Darwin himself stayed here, so this little building is often called “Darwin’s Shelter”. The doorway is so high because of the accumulation of snow here in the winter.

17VictorRamos100114Thank you to Victor Ramos for being such an effective, knowledgeable, humorous and patient guide for this trip. He is a structural geologist, but also quite good with sedimentology, paleontology and history. He made this excursion one we will long treasure in our memories of the Andes. Thank you also to my Wooster colleagues for making this trip possible for me.

Darwin and the High Andes copy(Late update, October 4th: Field trip group photo courtesy of Beatriz Aguirre-Urreta.)

Nothing quite like the feeling of completing your presentation: Day 2 of the International Palaeontological Congress

September 30th, 2014

DSC_4540MENDOZA, ARGENTINA–I promise, the images will be much more interesting in the next post! Today we concentrated on talks. I finally was able to deliver mine in the same session as Leif Tapanila above. It was a crowded little room, but the presentations kept us well entertained and informed.

I learned a lesson: without any outward indication of the time in a talk, I do tend to prattle on. There were no clocks or green-yellow-red lights for the speakers, and I was reluctant to try to read my watch in the darkness. To my surprise one of the session chairs told me mid-talk that I had two minutes left. Aghast and in a panic as the rule-follower I am, I sprinted to my last slides and finished. The organizers were laughing afterwards, though, because when they say “two minutes” it is for Latin American speakers who will typically take at least five minutes to end. I had a full five minutes, but as time-structured Norte Americano, I used only one!

My talk was titled “Changing Diversity and Intensity of Marine Macroboring Through the Phanerozoic”. Two critical slides, for the record, are below. They were surrounded by caveats!

Slide48_093014Slide49_093014I even earned a certificate! (As did everyone else.)

DSC_4551I very much enjoyed the presentations by Luis Buatois and Gabriela Mangano on Ediacaran and Early Cambrian bioturbation and trace fossils, as well as a talk by Al Curran and colleagues on the trace fossil Ophiomorpha. Leif Tapanila’s lecture on Triassic wood borings was fascinating, as was a spectacular presentation on Early Jurassic dinosaur nests and eggs in Patagonia.

IPC4 talk from Michal RakocinskiMichal Rakocinski managed to get a photo of me as I started my talk. I’ve never seen myself before in talking mode.

Tomorrow is field trip day, so this blog will return to images of geological scenery!

The Fourth International Palaeontological Congress starts well

September 29th, 2014

Zaton092914MENDOZA, ARGENTINA–After an excellent opening lecture last night by Dr. Beatriz Aguirre-Urreta (“Palaeontology in the Southern Hemisphere: Benchmarks in the History of Discovery and Research”), we got down to the technical talks today in the Mendoza Sheraton for the 4th International Paleontological Congress. There were many presentations to choose from, as usual, so I had an eclectic mix today. Above is my colleague Michał Zatoń delivering an interesting talk on Devonian faunas in Laurussia. I also enjoyed a presentation by Linda Frey on Devonian paleoecology in Morocco (full disclosure: Paul Taylor and I were in the long list of co-authors), Andrei Dronov on Ordovician trace fossils in Siberia, and Bruce Runnegar on the classic Ediacaran fossil Dickinsonia. It is a joy to have so many of my people in the same place at the same time.

Posters092914Well, usually. As you can quickly deduce from this scene, the almost 1000 attendees at this meeting have stretched the capacity of the venue. This is one of the lanes for the poster session. I got in about six feet. Nevertheless, I had a good time speaking with Marcelo Carrera about his Lower Ordovician tabulate corals, and Alycia Stigall (whose poster is visible on the right) on her fantastic online Ordovician Atlas program.

Tomorrow I have a Paleontological Society meeting in the morning, and then I give my talk in the afternoon. As always, I’ve divided this meeting mentally into two parts: pre-talk and post-talk!

Wooster’s Fossil of the Week: A crinoid calyx from the Upper Ordovician of southern Ohio

September 26th, 2014

Xenocrinus baeri (Meek, 1872)_585This week’s contribution from the Wooster collections will be short. If all is going well, as this is posted I’m on my way to the Fourth International Palaeontological Congress in Mendoza, Argentina. I hope to have a few posts from that exotic place!

The fossil above is the crown of a monobathrid crinoid called Xenocrinus baeri (Meek, 1872). It was found by Bianca Hand (Wooster ’14) in the Bull Fork Formation (Upper Ordovician, Richmondian) on an Invertebrate Paleontology field trip to the emergency spillway at Caesar Creek State Park in southern Ohio (seen below). Thank you to my friend Bill Ausich of The Ohio State University for identifying this fossil. It is an unprepared specimen of a common species, and it is not nearly so flashy as in other collections. Still, it is one of the best finds from our class field trips, and it is cool. The calyx is on the right and mostly buried in matrix. Four filter-feeding arms extend to the left. Where the matrix is broken away on the far right you can see tiny ossicles from the pinnules on the arms. Someone using a needle very carefully under a microscope could expose more details of this crinoid, but I like leaving something to the imagination!
CaesarCreek2011References:

Schumacher, G.A. and Ausich, W.I. 198). New Upper Ordovician echinoderm site: Bull Fork Formation, Caesar Creek Reservoir (Warren County, Ohio). The Ohio Journal of Science 83: 60-64.

Wooster’s Fossils of the Week: A nest of cornulitid tubeworms and friends from the Upper Ordovician of northern Kentucky

September 19th, 2014

Cornulitids and bryozoan Bellevue 585This fascinating and complicated little cluster of cornulitid wormtubes was found by my current Independent Study student William Harrison while we were doing fieldwork near Petersburg, Kentucky. (Just down the road from the infamous Creation Museum, ironically.) It was collected from a roadcut in the Bellevue Member of the Grant Lake Formation (Upper Ordovician, locality C/W-152). We’ve seen all the elements before (cornulitids, bryozoans and stromatoporoids), but not in such a tight set of relationships. I find this aspect of paleontology to be one of the most delightful: who lived with whom and how?
reconstr1The tubes are of the common Paleozoic genus Cornulites Schlotheim 1820, and the species is Cornulites flexuosus (Hall 1847). These long-extinct little marine animals had calcitic shells and likely bore a filter-feeding lophophore, as shown in the reconstruction above by my friend Olev Vinn. They appear to be related to brachiopods, bryozoans, phoronids, and some other tubeworms that shared this feeding device and certain features of the shell. Their life goal was to keep their lophophore or equivalent apparatus free of obstructions so they could collect nutrients from the surrounding seawater.
cornulitid whole specimen 091214The bryozoan, which makes up the primary substrate of the specimen (seen above) is a trepostome. Its skeleton contains hundreds of tiny tubes (zooecia) that held individuals (zooids) in the colony (zoarium — these terms are for my paleo students this week!). Each zooid in this type of bryozoan had a lophophore for filter-feeding.
cornulitid, dermatostroma, bryozoanAbove we see a thin, light-colored, bumpy sheet in the center of the image covering three of the cornulitid tubes and some of the bryozoan. This is the stromatoproid Dermatostroma papillatum (James, 1878). Stromatoporoids were a kind of sponge with a skeletal base, so this organism was also a filter-feeder. (It was originally known as Stromatopora papillata James, 1878.) Here we see the interesting symbioses (living together) aspects of this tiny assemblage. In the top right you see a cornulitid tube growing over the bryozoan, but the bryozoan in turn is overgrowing its proximal parts. The bryozoan and the cornulitid were thus alive at the same time. The stromatoporoid is growing over the bryozoan and the three cornulitids, but it is overgrown by cornulitids on the left. In addition, the stromatoporoid did not obstruct the cornulitid apertures, an indication that they were occupying living tubeworms. My hypothesis, then, is that all three of these characters were alive at the same time growing in response to each other.

It could be that this represents a tiny hard substrate tiered assemblage, meaning that the organisms were selecting food resources at slightly different heights and particle sizes (see Ausich and Bottjer, 1982, for a start on the tiering literature). The cornulitids may have taken the largest bits, the bryozoans the next size, and then the stromatoporoids, as minuscule sponges, got the finest particles. This is another paleontological hypothesis that can be tested with further specimens.

It is also an example of the value of getting sharp-eyed students on the outcrops as often as possible. Good work, William!

References:

Ausich, W.I. and Bottjer, D.J. 1982. Tiering in suspension feeding communities on soft substrata throughout the Phanerozoic. Science 216: 173-174.

Galloway, J.J. and St. Jean, J., Jr. 1961. Ordovician Stromatoporoidea of North America. Bulletins of American Paleontology 43: 1-102.

Morris, W. R. and H. B. Rollins. 1971. The distribution and paleoecological interpretation of Cornulites in the Waynesville Formation (Upper Ordovician) of southern Ohio. The Ohio Journal of Science 71: 159-170.

Parks, W.A. 1910. Ordovician stromatoporoids of America. University of Toronto Studies, Geology Series 7, 52 pp.

Schlotheim, E.F. von. 1820. Die Petrefakten-Kunde auf ihrem jetzigen Standpunkte durch die Beshreibung seiner Sammlung versteinerter und fossiler Ueberreste des their-und Planzenreichs der Vorwelt erlaeutert. Gotha, 437 p.

Taylor, P.D., Vinn, O. and Wilson, M.A. 2010. Evolution of biomineralization in ‘lophophorates’. Special Papers in Palaeontology 84: 317-333.

Vinn, O. and Mutvei, H. 2005. Observations on the morphology and affinities of cornulitids from the Ordovician of Anticosti Island and the Silurian of Gotland. Journal of Paleontology 79: 726-737.

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.

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