Wooster’s Fossils of the Week: Upper Ordovician brachiopods and bryozoans from paleontology class collections

January 6th, 2017

1-geopetal-tommyLast semester the Invertebrate Paleontology class at Wooster had its annual field trip into the Upper Ordovician of southern Ohio. We had a great, if a bit muddy, time collecting fossils for each student’s semester-long project preparing, identifying, and interpreting their specimens. Like all research, especially when it starts in the field, there were discoveries and surprises. I always highlight a particular specimen collecting by a student in this blog.

Above is a cross-section of a specimen found by Tommy Peterson (’19). It is the rhynchonellid brachiopod Hiscobeccus capax almost completely enveloped by an encrusting trepostome bryozoan. We’ve cut through the center of the brachiopod, revealing gray micritic sediment and clear calcite crystals. We can infer from this simple specimen that the brachiopod died and its shell remained articulated. Sediment drifted in, filling the bottom half of the shell. The bryozoan eventually sealed it all up as it used the brachiopod shell for a hard substrate on a muddy seafloor. The remaining void space was filled in by the precipitation of calcite crystals. You can see that the crystals nucleated from the outer margin of the cavity and grew inwards, a kind of calcareous geode. I’m intrigued by the irregular sediment surface and the manner in which calcite nucleated upwards from it. I suspect this sediment was itself cemented before the calcite crystals appeared.

This kind of structure is called a geopetal. It shows the “way up” at the time of crystal formation. Gravity held the pocket of sediment in the bottom of the shell, leaving the void at the top. Nice little specimen.

2-constellaria-alexisThis star-studded bryozoan found by Alexis Lanier (’20) was going to be the Fossil of the Week, but then I saw that last year I highlighted the very same species! I think the bryozoan Constellaria is cool. Read all about it and its history at the link.

3-table-of-traysHere are the completed specimen trays for half the class. (Grading this project took, as you might imagine, considerable time!). Every week in lab, after we had done the assigned work, we got out the trays and cleaned, prepared, and identified the specimens. Students learned how to use the rock saws and make acetate peels of the bryozoans and corals.

4-tray-insideInside a typical tray. We are very grateful for the many online sources to aid identification of these Cincinnatian fossils. Three in particular were most valuable: Alycia Stigall’s Digital Atlas of Ordovician Life, Steve Holland’s stratigraphic and paleontological guide to the Cincinnatian, and the spectacular Dry Dredgers website.

Ohio is a paleontological paradise!

Wooster’s Fossils of the Week: Geological Magic Lantern Slides from the 19th Century (Part III)

December 16th, 2016

18-devonion-period[Note: Wooster’s Fossil of the Week is on holiday until January 2017.]

This is the last post illustrating the 19th Century Magic Lantern Slides recently found in Scovel Hall of Wooster’s Geology Department. Please see the December 2 post and the week before for details. To review, these slides are 4×8 inches with the image fixed on glass bolted into a thin slab of wood with metal rings. They are chromolithograph slides, each stamped “T.H. McAllister, Optician, N.Y.”. McAllister was the most prominent of many American producers of lantern slides in the late 19th century.

This last set of slides in our collection was apparently used in our old “Historical Geology” courses to evoke the geological time periods. The top image is simply labeled “Devonian“. The trees on the right appear to be towering lycopods, a kind of seedless vascular plant. They were common in the Devonian and are still around today. I can’t tell what the other plants are in the image. The rapid rise of large plants in the Middle Devonian has been called the “Devonian Explosion”. These early forests had significant effects on atmospheric composition, soil formation, erosion, and sediment transport.

[UPDATE: Please see the excellent comments by Ben Creisler. He has given us much new information and numerous links explaining the history of these images. I’ve left my amateur text in place only to record the original post! MW]

19-carboniferous-periodCarboniferous” is the title of this slide. It is dramatic, seemingly showing a Carboniferous forest dominated by ferns being torn apart by a swelling tide. Could this be a comment on the interbedding of marine and terrestrial rock units so common in the Upper Carboniferous of North America?

20-permian-periodFerns are again in the foreground of this Permian scene. I have no explanation for the mountainous seashore landscape, except that the red color of the rocks may represent the New Red Sandstone of Great Britain.

21-transition-periodThis slide is enigmatically labeled “Transition Period”. I suspect it represents the Triassic, a period just after the Permian and thus part of the transition into the Mesozoic. The shrubby plants in the foreground appear to be cycads with massive yellow cones emerging from their tops.

22-eeocen-periodThis image of the “Eocene” is the first of these period slides to depict animals (the herd of ungulates across the river and the bird in the foreground). This may mean these slides were meant to show the progression of plant life over geological time. The forests here look dominated by conifers and angiosperms.

23-miocene-periodThis is a “Miocene” image. I don’t know how I’d distinguish it from the Eocene view above.

24-drift-periodOur final slide shows what the “Drift Period”, which is clearly the Pleistocene. Not only do we have cave bears in the foreground and a herd of bison in the river, there seems to be a massive pile of ice in the left rear!

I have not discovered the artist responsible for these illustrations. If you know, please tell me in the comments!

[UPDATE: Please see excellent information and links by Ben Creisler in the comments below. Thanks, Ben!]

 

Wooster’s Fossils of the Week: Ordovician bioerosion trace fossils

December 9th, 2016

screen-shot-2016-12-03-at-2-06-03-pmThis week’s post is a celebration of the appearance of a remarkable two-volume work on trace fossils and evolution. The editors and major authors are my friends Gabriela Mángano and Luis Buatois (University of Saskatchewan). They are extraordinary geologists, paleontologists and ichnologists (specialists on trace fossils). They led this massive effort of multiple authors and thousands of manuscript pages. Turns out they are inspiring scientific leaders as well as sharp-eyed editors.

My contribution is in the first volume within a chapter (co-authored with Gabriela, Luis, and Mary Droser of the University of California, Riverside) entitled “The Great Ordovician Biodiversification event”. We examine here the relationship between trace fossils and the critical evolution of marine communities through the Ordovician. My main responsibility was sorting out the changes in the bioeroders over the course of the period. Way back in 2001, Tim Palmer and I noticed a rise in bioerosion trace fossil diversity and abundance in the Middle and Late Ordovician. We grandly called it the “Ordovician Bioerosion Revolution”. The concept and name stuck.

The top image is Fig. 4.8 from the book. The caption: Upper Ordovician bioerosion structures. (a) Trypanites weisi (cross-sectional view) in a carbonate hardground. Katian, Grant Lake Limestone, near Washington, Kentucky, USA; (b) Trypanites weisi (bedding-plane view) in a carbonate hardground. Katian, Grant Lake Limestone, near Manchester, Ohio, USA; (c) Palaeosabella isp. in a trepostome bryozoan. Katian, Whitewater Formation, near Richmond, Indiana, USA; (d) Petroxestes pera. Katian, Whitewater Formation, Caesar Creek Lake emergency spillway, near Waynesville, Ohio, USA; (e) Ropalonaria venosa in a strophomenid brachiopod. Katian, Liberty Formation near Brookville, Indiana, USA.

screen-shot-2016-12-03-at-2-08-42-pmThe cover of the book, which is described here on the publisher’s website.

References:

Mángano, G., Buatois, L., Wilson, M.A. and Droser, M. 2016. The Great Ordovician Biodiversification event, p. 127-156. In: Mángano, G. and Buatois, L. (eds.), The trace-fossil record of major evolutionary events. Topics in Geobiology 39 (Springer).

Wilson, M..A. and Palmer, T.J. 2001. The Ordovician Bioerosion Revolution. Geological Society of America Annual Meeting, Boston, Paper No. 104-0. November 7, 2001.

Wilson, M.A. and Palmer, T.J. 2006. Patterns and processes in the Ordovician Bioerosion Revolution. Ichnos 13: 109-112.

Wooster’s Fossils of the Week: Geological Magic Lantern Slides from the 19th Century (Part II)

December 2nd, 2016

12-iguanodon-and-a-hyleosaurusThis is a continuation of last week’s post about a set of 19th century “Magic Lantern Slides” found in Scovel Hall at Wooster. These evocative scenes are taken from reconstructions of ancient life by Benjamin Waterhouse Hawkins (1807-1894). In 1855, Waterhouse Hawkins finished sculpting life-sized models of these extinct animals, along with many others, for the Crystal Palace gardens in London. Most of these extraordinary animal statues still exist.

Above is the Waterhouse Hawkins version of the Early Cretaceous dinosaurs Iguanodon (the critter on top) and Hylaeosaurus (the two on the lower level). These two genera, along with Megalosaurus, were used as the basis for the new Dinosauria erected by Sir Richard Owen in 1842, a mere dozen years before these models were created. Both of these dinosaurs were herbivorous, Iguanodon being an ornithopod and Hylaeosaurus a basal ankylosaur. They are said here to be from “the Secondary Epoch of the Earth’s history”.

13-an-iguanodon-and-a-hyleosaurus-by-benjamin-waterhouse-hawkins-1853A print version of the same scene. Modern reconstructions of these animals are dramatically different, of course. Waterhouse Hawkins was advised by Owen to make these versions as mammalian as possible. The stance and articulation of limbs is the largest change in our conception of these genera. The Iguanodon model is where a famous 1853 New Year’s Eve dinner party was held.

14-megatherium-glyptodonThis next slide is another Waterhouse Hawkins creation of a much later scene. These are reconstructions of the South American ground sloth Megatherium, which lived from the Pliocene through the Pleistocene. Aside from some unnecessary bulk, these reconstructions are not too far off from how we conceive the giant ground sloths today.

16-no-labelThis magic lantern slide from Wooster’s collection is unlabeled, and I’ve found no trace of the image online. The scene has a Mesozoic vibe, with a crinoid, ammonites (or nautiloids?), and a lurking reptile. Any identifying information would be appreciated!

17-anoplotherium-gracile-palaeotheriumAnother Waterhouse Hawkins theme, this time of Eocene ungulates. The label says they are Paleotherium (in the right foreground) and Anoplotherium gracile (on the left in the foreground). Both were originally described from the Paris region by the magnificent Georges Cuvier.

9-benjamin_waterhouse_hawkins-_photograph_by_maull__polyblankBenjamin Waterhouse Hawkins (1807-1894) was a Londoner skilled in natural history and art. His lifetime honors are a clue to his abilities: He was a Member of the Society of Arts, a Fellow of the Linnean Society, and a Fellow of the Geological Society of London. His Crystal Palace dinosaurs are his best know combination of art and science, but he produced much besides. For example, he drew figures for The Zoology of the Voyage of HMS Beagle. In 1868 he mounted a skeleton of Hadrosaurus in Philadelphia, the first dinosaur to be displayed in this way. Through his art and connections in the paleontological world, Waterhouse Hawkins brought fossils to life for millions of people in Victorian times.

 

Wooster’s Fossils of the Week: Geological Magic Lantern Slides from the 19th Century (Part I)

November 25th, 2016

1-teleosaurus-ichthyosaurus-pentacrinites-ammonites-gryphaea“Wooster’s Fossil of the Week” is not always about actual fossils, but our topics are each paleontological. Many years ago I discovered in an old box tucked away in the attic of Scovel Hall at Wooster a set of “Magic Lantern Slides” used in geology courses. I came across them again recently and thought I would share these ancient scenes. Lantern slides were the 19th Century equivalent of PowerPoint, so generations of Wooster geology students must have sat in rapture looking at these colorful images. (At least that’s how I imagine them now viewing my PowerPoint slides!) The above imagined seashore view includes the crocodylian Teleosaurus atop the layered rocks, Ichthyosaurus immediately below, four long-necked Plesiosaurus on the left, an orange cluster of the crinoid Pentacrinus rooted inexplicably in the beach sand, and a scattering of ammonite and oyster shells.  The caption on the image says these animals lived during “the Secondary Epoch of the Earth’s history”. We would now say this is a Jurassic scene. The ichthyosaur looks the most odd to us. Not only is it crawling on the land, it lacks a dorsal fin and the characteristic bi-lobed, shark-like tail. These were later discoveries about ichthyosaurs made only after specimens were found with skin impressions.

2-ammonite-lantern-detailThis close-up shows the detail in these images. Ammonites are on the left (“6”) and the oyster Gryphaea is on the right (“7”).

3a-magic-lantern-slide-geological-585The Magic Lantern Slides are 4×8 inches with the image on glass fixed in a thin slab of wood with metal rings. These are chromolithograph slides, each stamped “T.H. McAllister, Optician, N.Y.”. McAllister was the most prominent of many American producers of lantern slides in the late 19th century.

4-megalosaurus-pterodactyleThe quadrupedal beasts in the foreground are the of the Jurassic theropod dinosaur Megalosaurus, with pterodactyls in the background. We now know Megalosaurus was bipedal, like all theropod dinosaurs.

5-megalosaurus-headAnother detail showing the fine quality of these color images on glass.

6-gigantic-lizards-and-some-pterosauria-by-benjamin-waterhouse-hawkins-1853Most readers with any background in the history of paleontology recognize these reconstructions of ancient life from the work of Benjamin Waterhouse Hawkins (1807-1894). In 1855, Waterhouse Hawkins finished sculpting life-sized models of these extinct animals, along with many others, for the Crystal Palace gardens in London. He was advised for the anatomical details by Sir Richard Owen (1804-1892), a hero of paleontology but not a fan of Darwinian evolution. He is responsible for the dinosaurs of Waterhouse Hawkins looking rather mammalian. Most of these extraordinary animal statues still exist.

9-benjamin_waterhouse_hawkins-_photograph_by_maull__polyblankBenjamin Waterhouse Hawkins (1807-1894). More reconstructions from him, along with his brief biography, in the next installment.

 

Wooster’s Fossil of the Week: A juvenile conch from the Upper Pleistocene (Eemian) of The Bahamas

November 18th, 2016

inagua-lobatus-gigasI collected this beautiful shell from a seashore exposure of Pleistocene sediments on Great Inagua, the third largest island of The Bahamas. I was on an epic expedition to this bit of paradise with Al Curran and Brian White of Smith College in March 2006. We were pursuing evidence for a sea-level change event in the Eemian, about 125,000 years ago. This was some of the most exciting scientific work I’ve done, so this little shell brings back many memories. I found it loosely cemented into a small patch of carbonate sediments inside a hollow of an ancient coral reef. This shell and numerous other samples were basic data for a rapid rise and fall of sea level during the last interglacial interval. The project is summarized in the Thompson et al. (2011) reference below.

This is a juvenile of the common Queen Conch Lobatus gigas (Linnaeus, 1758). In its adult form with a flared aperture it is one of the most recognizable modern shells in the world. Some of you may be surprised by the generic name. I was. I knew this shell as Strombus gigas, the original name given to it by the sainted father of taxonomy Carolus Linnaeus in 1758. After several adventures in the literature, Landau et al. (2008) placed the species in the genus Lobatus Swainson 1837.

salvador-lobatus-gigas-1The species looks exactly the same today, at least in its shell. This is a similar modern Queen Conch juvenile collected from San Salvador Island in The Bahamas. Note the color patterns which are lost in the fossil.

salvador-lobatus-gigas-2This is the apertural view of the same modern shell. With time it would have grown a much thicker apertural margin to protect it from predators.

buonanni-strombus-gigas-figureThis is the earliest image known of the Queen Conch (Buonanni, 1684). For a long time the type specimen (the specimen of record defining the taxon) of Strombus gigas (the older Linnaeus name) was missing. In 1941 this figure — the figure itself — was designated a neotype (a replacement type) of the species. (First time I’ve heard of that move.) The original type specimen, though, was found in Sweden in 1953, so there is an actual shell in the collections and no need for this neotype.

bonanno-coverThat first figure of Lobatus gigas was drawn by Filippo Bonanni (1638-1723), a remarkable Italian Jesuit scholar. It is found in the book above, which is the first known guide to seashells for collectors. (Note the “SUPERIORUM PERMISSU”, meaning he published with the permission of his Jesuit superiors.) Bonanni was one of the first to suggest fossils had at least some organic origins, speculating that they were either organism remains or “products of natural powers.”

References:

Buonanni, F. 1684. Recreatio mentis, et oculi in observatione animalium testaceorum curiosis naturae inspectoribus italico sermone primum proposita. p. Philippo Bonanno . Nunc denuo ab eodem latine oblata, centum additis testaceorum iconibus, circaquae varia problemata proponuntur. Ex typographia Varesij, Romae, xvi + 270 + [10] pp., 139 pls.

Landau, B.M., Kronenberg G.C. and Herbert, G.S. 2008. A large new species of Lobatus (Gastropoda: Strombidae) from the Neogene of the Dominican Republic, with notes on the genus. The Veliger 50: 31–38.

Thompson, W.G., Curran, H.A., Wilson, M.A. and White, B. 2011. Sea-level oscillations during the Last Interglacial highstand recorded by Bahamas corals. Nature Geoscience 4: 684–687.

White, B.H., Curran, H.A. and Wilson, M.A. 2001. A sea-level lowstand (Devil’s Point Event) recorded in Bahamian reefs: comparison with other Last Interglacial climate proxies; In: Greenstein, B.J. and Carney, C., (editors), Proceedings of the 10th Symposium on the Geology of the Bahamas: Bahamian Field Station, San Salvador Island, p. 109-128.

Wilson, M.A., Curran, H.A. and White, B. 1998. Paleontological evidence of a brief global sea-level event during the last interglacial. Lethaia 31: 241-250.

Wooster’s Fossils of the Week: Modern vermetid snails, a slipper shell, and an oyster

November 11th, 2016

crepidula-vermetidaeNot actually fossils this week, but cool nonetheless. This complex specimen is in our Invertebrate Paleontology teaching collection with no label giving its original location. In the foreground is the underside of a slipper shell gastropod identified as Crepidula fornicata. The tangled mass of tubes encrusting it is a vermetid gastropod. A small round hole drilled by a predatory gastropod is visible in the slipper shell.

vermetidae-oysterTurning the specimen over we see a left valve of the oyster Ostrea encrusting the exterior of the slipper shell, along with another view of the vermetid tubes and gastropod boring.

rafinesque-constantine-1783-1840The twisty gastropod Family Vermetidae was named in 1815 by Constantine Samuel Rafinesque-Schmaltz (1783 – 1840).Rafinesque was a character. His name is immediately recognizable to paleontologists of the Ordovician because Hall and Clarke named the common brachiopod Rafinesquina after him in 1892. Rafinesque was born of a French merchant father (Rafinesque) and German mother (Schmaltz) near Constantinople in the Ottoman Empire. He was self-educated, learning classical languages before his teens and sorting through rocks, minerals, plants, animals and fossils at a prodigious rate. He began to write numerous articles and books on anthropology, botany, zoology, geology, paleontology, history, and linguistics. The naturalist and philosophical establishment rejected him, for the most part, so he was little praised in his life. Most scholars agree now that he was ahead of his time on many topics, including evolution.

In 1819, Rafinesque was appointed professor of Botany at Transylvania University in Lexington, Kentucky. He apparently attracted considerable trouble during his years in Kentucky. In 1826 he was either fired by the university president or he walked out in a huff. Legend is that he left an angry curse on the school! He died in Philadelphia in 1840 of stomach cancer, to which some attributed to his own homemade medications.

screen-shot-2016-11-07-at-9-49-57-amThe cover page of Rafinesque’s 1815 work in which he attempted to classify just about everything in the universe. Note the subheading: “Nature is my guide, and Linnaeus is my master.”

screen-shot-2016-11-07-at-9-49-32-amA suitably grand frontispiece for the 1815 book.

screen-shot-2016-11-07-at-10-37-25-amThis is the extent of establishing a new family in the early 19th century (Rafinesque, 1815, p. 144). No wonder Rafinesque could name, by his own count, over 6700 taxa.

References:

Hall, J. and Clarke, J.M. 1892. An introduction to the study of the genera of Palaeozoic Brachiopoda. Part I. Geological Survey of the State of New York, Paleontology 8, p. 1-367.

Rafinesque, C.S. 1815. Analyse de la nature: ou tableau de l’univers et des corps organisés. J. Barravecchia: Palermo. 224 pages.

Wooster’s Fossils of the Week: Demosponge borings in a muricid gastropod from Florida

November 4th, 2016

entobia-snail-2Technically these are “subfossils” since this appears to be an old shell still within the Holocene, although it is possibly eroded out of Pleistocene sediments and then redeposited on a Florida beach. It is a muricid snail eroded enough to erase any specific characters for further identification. It is cool because it is thoroughly bored by clionaid demosponges, producing a beautiful pattern of holes given the ichnological name Entobia Bronn 1838.

entobia-snail-1On the left side of the aperture of this snail shell is a fine reticulate pattern from an encrusting cheilostome bryozoan, also punctured by Entobia. That bryozoan is in a favored place for filter-feeding encrusters on snail shells, so it likely was there during the life of the snail.

As a trace fossil this structure would be known as Entobia. It is very common in the fossil record, especially in the Cretaceous and later.

Bronn 041616Entobia is common in the fossil record, especially in calcareous rocks and fossils from the Cretaceous on. The ichnotaxon was named (but apparently not described) in 1838 by Heinrich Georg Bronn (1800-1862), a German geologist and paleontologist we’ve met before in this blog. He had a doctoral degree from the University of Heidelberg, where he then taught as a professor of natural history until his death. He was a visionary scientist who had some interesting pre-Darwinian ideas about life’s history. He didn’t fully accept “Darwinism” at the end of his life, but he made the first translation of On The Origin of Species into German.

References:

Bromley, R.G. 1970. Borings as trace fossils and Entobia cretacea Portlock, as an example. Geological Journal, Special Issue 3: 49–90.

Bronn, H.G. 1838. Lethaea geognostica: oder, Abbildungen und Beschreibung der für die Gebirgs-Formationen bezeichnendsten. E. Schweizerbart’s Verlagshandlung, Stuttgart, 545 pages.

Buatois, L., Wisshak, M., Wilson, M.A. and Mángano, G. 2016. Categories of architectural designs in trace fossils: A measure of ichnodisparity. Earth-Science Reviews (DOI: 10.1016/j.earscirev.2016.08.009).

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. 2007. Macroborings and the evolution of bioerosion, p. 356-367. In: Miller, W. III (ed.), Trace Fossils: Concepts, Problems, Prospects. Elsevier, Amsterdam, 611 pages.

 

Wooster’s Fossil of the Week: A naticid gastropod from the Pliocene of southern California

October 28th, 2016

polinices-galianor-sd-pliocene-1-copyThis week’s fossil comes from our teaching collection. It’s label appears to be from the late 19th Century. It is a naticid gastropod (“moon snail“) listed as Polinices galianor. That name, which I can only find in two lists and never with an author, may be a corruption of Polinices (Euspira) galianoi Dall 1909. It was collected from the Pliocene of San Diego County, California. It is preserved as both an internal mold and thin sheets of remnant original shell.

polinices-galianor-sd-pliocene-2-copyThis is a view of the underside along the axis of coiling. The hole is known as the umbilicus and is distinctive for the naticids. These snails are predatory, moving through loose sand with a very large foot and capturing shelled prey, like clams and other gastropods. They then drill a beveled hold through the shell of the prey with specialized teeth in their radulae. We’ve discussed the trace fossils they leave (Oichnus) in a previous post.

The genus Polinices was named in 1810 by Pierre Dénys de Montfort (1766–1820), a French malacologist (one who studies mollusks).

screen-shot-2016-10-22-at-11-52-46-amThe title page of de Monfort (1810).

screen-shot-2016-10-22-at-11-49-26-amThis brief paragraph is all it took in the early 19th Century to name a new taxon. The system is much more elaborate now.screen-shot-2016-10-22-at-8-20-44-pmPierre Dénys de Montfort is a tragic figure in science. First, he had the misfortune of being a French intellectual during the chaos of the French Revolution and the resulting Napoleonic dictatorship. Scientists struggled then, but after service in the revolutionary army and an apprenticeship with a geologist, de Monfort gained a position in the Jardin des Plantes, a research botanical garden in Paris. He did a massive study of mollusks, producing systematic tomes. De Monfort was a whiz at languages, so he did well as a translator after Napoleon was  finally defeated in 1815 and the Allied armies occupied Paris. Then he went off the rails. He had since 1801 championed the reports of mariners that giant cephalopods occasionally rose from the sea and attacked shipping, as shown in his above print (de Monfort, 1801, p. 256). The modern roots of the kraken! De Monfort took the idea too far, was ridiculed in the scientific community, and eventually died of starvation and alcoholism in the streets of Paris in 1820. The later discovery of giant squid salvaged his reputation a bit, but no one has yet found evidence of “le poulpe colossal”.

References:

Dall, W.H. 1909. Contributions to the Tertiary paleontology of the Pacific coast. U.S. Geological Survey Professional Paper 59. U.S. Government Printing Office, 288 pages.

de Montfort, P.D. 1801. Histoire naturelle, générale et particuliere des Mollusques, animaux sans vertèbres et á sang blanc. Volume 2. Paris, 424 pages.

de Montfort, P.D. 1810. Conchyliologie systématique, et classification méthodique de coquilles. Volume 2. Paris, 692 pages.

 

Wooster’s Pseudofossils of the Week: Artifacts in thin-sections of Ordovician limestones from southeastern Minnesota

October 21st, 2016

1bubfirstIt is always exciting to a geologist when thin-sections of curious rocks are completed and ready for view. A thin-section is a wafer of rock (30 microns thick) glues to a glass slide and examined by transmitted light through a petrographic microscope. They provide fantastic views of the mineralogy, paleontology, and structure of a rock in exquisite detail. Thin-sections are also full of mysteries since we have essentially two-dimensional slices through three-dimensional materials.

Thin-sections from the Decorah Formation samples collecting by Team Minnesota this past summer were finally available this week for study. I took the first look at slides of limestones containing ferruginous (iron-rich) ooids we gathered as part of Etienne Fang’s (’17) Independent Study. The first structures I saw were the crazy dark outlines above. What sort of fossils are these, I wondered. Could they be sponges? Odd bryozoans? Borings through the rock fabric? I was ready to post them here as mystery fossils to solicit your opinions. Now, though, they instead make a cautionary tale.

2bub730There are many of these features in a single slide from the Decorah Formation exposed at the Golden Hill outcrop near Rochester, Minnesota. Some are astonishingly complex. It then began to occur to me that these structures were too convoluted and unpredictable to actually be fossils. It also bothered me that to focus on them required to put the rest of the field out of focus. That only made sense if these oddities were in the epoxy, not the rock itself.

3buboverlapEtienne showed me how to demonstrate that these funny loops were not in the rock with this view: You can just make out the greenish lines overlapping the cut surface of this ferruginous ooid. Turns out I was excited about air bubbles in the cementing epoxy. They have nothing to do with the rock. I nearly posted my own pseudofossils.

4trio7321I held out hope, though, that these odd white objects in another thin-section of ooid-rich limestone. They appear to be ghostly outlines of ooids with a finely-textured object inside. They look like seeds with embryos within. Several are scattered through the thin-section. Another mystery fossil!

5duo7321A closer view. Strange how each internal object seems connected to an ooid on the outside, making them asymmetrical in their placements.

6single7321Strange also how once again the details of the internal object can only be seen with the rest of the slide out of focus. Yes, another artifact in the epoxy. This time we may be looking at holes left by ferruginous ooids plucked from the rock through the grinding process, pulling a patch of epoxy with them. Somehow this happened when the now-missing ooid was wedged against another. Nothing to see here, folks.

7ooid7301fAt least I can take the opportunity to show how cool Etienne’s ferruginous ooids are. Note that this one has a greenish layer midway through the cortex. It looks like the mineral chamosite to me. Spectacular detail in the lamellae, but only visible if the image is over-exposed.

8bifoliate7301hThere are plenty of real fossils in these thin-sections, of course. My favorites are these bifoliate bryozoans (lower right half) with their zooecia filled with ferruginous material. Note that the ooid above has had some of its lamellae dissolved away, probably because of some mineral diversity. Also note in the upper right another one of those crazy air bubbles.

The lesson I learn over and over: think, but then think again.

 

 

 

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