Wooster’s Fossil of the Week: A bored rhynchonellid brachiopod from the Middle Jurassic of France

July 22nd, 2016

1 Kutchi dorsal 585Another beautiful brachiopod this week from our friend Mr. Clive Champion in England. His donations to our collections have considerably enriched our teaching program, especially for brachiopods! This specimen is the rhynchonellid Kutchirhynchia morieri (Davidson, 1852) from the Middle Jurassic (Upper Bathonian) of Luc-sur-Mer, France. This is a view of the dorsal side with the dorsal valve on top with the ventral valve (containing the round opening from which the stalk-like pedicle extended) seen below it. Like most rhynchonellids, the valves have distinct plicae (thick ridges) where the shell is tightly folded.
2 Kutchi ventral 585This is the ventral view showing only the exterior of the ventral valve. Note the curved serpulid worm tube attached near the center, and the squiggly borings. These were likely sclerobionts (hard substrate dwellers) that occupied the brachiopod shell when the animal was still alive, since the dorsal and ventral valves are still articulated. The borings are probably of the ichnogenus Talpina, but I would have to grind down the shell to know for certain.
SSBuckmanThe genus Kutchirhynchia was named by Sydney Savory Buckman (1860-1929) in 1917. We met Buckman earlier in this blog when looking at another of his Jurassic rhynchonellid genera, Burmirhynchia. We learned a lot more about Buckman this summer during our expedition to the Jurassic of Dorset, where he did much of his work. He is best known there as an ammonite worker and stratigrapher (and massive taxonomic splitter).
3 Thomas DavidsonThe species Kutchirhynchia morieri was named by the Scottish paleontologist Thomas Davidson (1817-1885), who originally placed it in the large genus Rhynchonella. Buckman acknowledges Davidson in an ammonite monographs as one of his “earliest geological friends”. (Davidson was 43 years older than Buckman.) Davidson was born in Edinburgh to wealthy parents. He studied at the University of Edinburgh and then in France, Italy and Switzerland, where he made many long geological tours. He was convinced by the German paleontologist Christian Leopold von Buch (1774-1853) to work on fossil brachiopods. (Von Buch was 43 years older than Davidson. Nice to see the older generation having an effect on those kids!) Davidson stayed with brachiopods his entire career, producing massive monographs on both fossil and recent forms. He engraved his own plates on stone, and there are more than 200 of them. Davidson was elected a fellow of the Geological Society of London in 1852, awarded the Wollaston medal in 1865. In 1857 he was elected a Fellow of the Royal Society, receiving their Royal medal in 1870. Upon his death in Brighton, England, in 1885, his entire collection of fossil and recent brachiopods went to the British Museum.
4 Elizabeth GrayThis is a good place to mention Elizabeth Anderson Gray (1831-1924), an important fossil collector in Scotland who supplied Thomas Davidson and many other paleontologists with critical specimens for their work. She is one of the many unnoticed heroes of paleontology, being rarely acknowledged publicly and then overshadowed by her husband. She worked primarily in the Ordovician and Silurian and so did not give Davidson Jurassic rhynchonellids, but she provided hundreds of brachiopods from the early Paleozoic. I love this image of her knocking out fossils with a hammer, just like we do today. Trowelblazers has an excellent biographical page on Elizabeth Anderson Gray.

References:

Buckman, S.S. 1917. The Brachiopoda of the Namyau Beds, Northern Shan States, Burma. Palaeontologia lndica 3(2): 1-254.

Gilman, D.C., Thurston, H.T. and Colby, F.M., eds. 1905. Davidson, Thomas (paleontologist). New International Encyclopedia (1st ed.). New York: Dodd, Mead.

Shi, X. and Grant, R.E. 1993. Jurassic rhynchonellids: internal structures and taxonomic revisions. Smithsonian Contributions to Paleobiology, Number 73, 190 pages.

Wooster’s Fossils of the Week: A molluscan assemblage from the Miocene of Maryland

July 15th, 2016

1 Calvert Zone 10 Calvert Co MD 585Earlier this month a gentleman stopped by The Department of Geology and donated the above beautiful slab of fossils to our program. Dale Chadwick of Lancaster, Pennsylvania, is an avid amateur fossil collector with a very useful website and considerable generosity. His gift to the department makes an excellent Fossils of the Week entry. Later I’ll show you the equally-impressive other side of this specimen!

We have here a fine sandstone from the famous Calvert Formation (lower to middle Miocene) exposed at the Calvert Cliffs, Plum Point, Calvert County, Maryland, in the stratigraphic Shattuck Zone 10. As you can see, some horizons are densely fossiliferous with large numbers of gastropods and bivalves. This is what we refer to us a death assemblage, meaning these shells are not preserved in their life positions but how they accumulated just before final burial. These rocks and their fossils were the initial basis of Susan Kidwell’s important work on taphonomic feedback, or how shell accumulations affect the succeeding living communities.

So what are the prominent fossils in this slab? Dale has the answers on his website. I’ve annotated the image and made a list below:

2 Calvert Zone 10 Calvert Co MD 585 labeledA Turretilla variabilis (a turritellid gastropod)
B Stewartia sp. (a lucinid bivalve)
C Turritella plebia (a turritellid gastropod)
D Cardium laqueatum (a carditid bivalve)
E Siphonalia devexa (a buccinid gastropod)

So how did several of these animals die on that seafloor long ago? You’ve probably guessed predation by looking at that round hole in specimen B, a lucinid bivalve.

3 Naticid borehole Calvert 585The beveled nature of this round drillhole tells us it was made by a predatory naticid gastropod, which used its radula (a tongue-like device with sharp teeth) to penetrate the calcareous shell and damage the muscles holding it tight against the attack. About half the specimens in this slab show similar predatory penetrations. Wooster alumna Tricia Kelley did critical work on predation styles, intensities and evolutionary patterns with Calvert specimens like these.

Thank you again to Dale Chadwick for his gift!

References:

Kelley, P.H., 1983, Evolutionary patterns of eight Chesapeake Group molluscs: Evidence for the model of punctuated equilibria: Journal of Paleontology 57: 581–598.

Kelley, P.H. 1988. Predation by Miocene gastropods of the Chesapeake Group: stereotyped and predictable. Palaios 3: 436-448.

Kidwell, S.M. 1986. Taphonomic feedback in Miocene assemblages: Testing the role of dead hardparts in benthic communities: Palaios 1: 239–255.

Kidwell, S.M., Powars, D.S., Edwards, L.E. and Vogt, P.R. 2015. Miocene stratigraphy and paleoenvironments of the Calvert Cliffs, Maryland, in Brezinski, D.K., Halka, J.P. and Ortt, R.A., Jr., eds., Tripping from the Fall Line: Field Excursions for the GSA Annual Meeting, Baltimore, 2015: Geological Society of America Field Guide 40, p. 231–279.

Wooster’s Fossil of the Week: An ammonite from the Middle Jurassic of southern England

July 8th, 2016

Leptosphinctes microconch Jurassic Dorset 585We’re featuring just a workaday fossil this week because of other summer activities. This is the ammonite Leptosphinctes Buckman 1929 from the Inferior Oolite (Middle Jurassic) at Coombe Quarry, Mapperton, Dorset, southern England. Cassidy Jester (’17) and I collected it last month during our 2016 England research expedition. Our friend Bob Chandler generously identified it. It popped out of a rock we were pounding into submission, providing a direct application of ammonite biostratigraphy to our work. As with many ammonites, the group is well known but the names are still a bit dodgy.

This specimen is a microconch, meaning it is the smaller version of a species pair, the larger being the macroconch. It is presumed that this is sexual dimorphism and that the microconch is the male because it didn’t need to carry resources for egg-laying. This is one reason why the taxonomy of these ammonites is in perpetual revision.

References:

Buckman, S.S. 1909–1930. Yorkshire Type Ammonites & Type Ammonites. Wesley & Son, Wheldon & Wesley, London, 790 pl.

Chandler, R B., Whicher, J., Dodge, M. and Dietze, V. 2014. Revision of the stratigraphy of the Inferior Oolite at Frogden Quarry, Oborne, Dorset, UK. Neues Jahrbuch für Geologie und Paläontologie-Abhandlungen 274: 133-148.

Galácz, A. 2012. Early perisphinctid ammonites from the early/late Bajocian boundary interval (Middle Jurassic) from Lókút, Hungary. Geobios 45: 285-295.

Pavia, G. and Zunino, M. 2012. Ammonite assemblages and biostratigraphy at the Lower to Upper Bajocian boundary in the Digne area (SE France). Implications for the definition of the Late Bajocian GSSP. Revue de Paléobiologie, Vol. spéc, 11: 205-227.

Wooster’s Fossils of the Week: Iron-oxide oncoids (“snuff-boxes”) from the Middle Jurassic of southern England

July 1st, 2016

1 Snuffbox colection BBThese fossils (in the broad sense!) are inevitable for our weekly feature considering how much time we spent studying and collecting them during last month’s fieldwork in Dorset, southern England. “Snuff-boxes” are the subject of Cassidy Jester’s (’17) Senior Independent Study project, so here we’ll just broadly cover what we think we know about them.

These discoidal objects are called “snuff-boxes” because their carbonate centers (usually a bit of limestone or shell) often erode faster than their iron-oxide exteriors, making them weather a bit like boxes with lids.
2 Quote from Buckman 1910 67This quote from Buckman (1910, p. 67) is the earliest reference I can find to the snuff-box term. Snuff-boxes were sometimes works of art in the 18th and 19th centuries, although quarrymen probably had more homespun varieties in mind.
1 Snuffbox serpulidssWe’re counting these snuff-boxes as fossils here because they formed through biotic and physical processes. The cortex of a snuff-box has layers of serpulid worm tubes, as shown above.
4 Palmer Wilson Fig 3There are also cyclostome bryozoans embedded within the iron-oxide layers, as shown in this image from Palmer and Wilson (1990, fig. 3).
3 Snuff-box horn 061716We believe the snuff-boxes grew by accretion of microbially-induced layers of iron-oxide formed on their undersides, which would have been gloomy caverns on the seafloor. They then would have occasionally turned over and grew layers on the other side. Many snuff-boxes have extensions on their peripheries that look in cross-sections like horns, as seen above. The layers are separate from those that formed around the nucleus. They may have grown after the snuff-box became too big to be overturned by currents or animals.
6 Platy minerals pdt19573Paul Taylor and I looked at the cortex of a snuff-box with Scanning Electron Microscopy (SEM) and had the above surprising view. The odd platy materials may be limonite, an iron-oxide that is amorphous (non-crystalline).
7 Hebrew letters pdt19572Sometimes the plates look like they’ve partially evaporated, leaving remnants that resemble Hebrew letters!
8 iron ooid pdt19576Associated with the snuff-boxes are small “iron ooids” that are about sand-size. They too have the platy materials, and so their origin may be similar to that of the snuff-boxes.

Cassidy has an interesting project ahead of her testing various origin hypotheses and sorting out the paleontology, mineralogy and geochemistry.

References:

Buckman, S.S. 1910. Certain Jurassic (Lias-Oolite) strata of south Dorset and their correlation. Quarterly Journal of the Geological Society 66: 52-89.

Burkhalter, R.M. 1995. Ooidal ironstones and ferruginous microbialites: origin and relation to sequence stratigraphy (Aalenian and Bajocian, Swiss Jura mountains). Sedimentology 42: 57-74.

Gatrall, M., Jenkyns, H.C. and Parsons, C.F. 1972. Limonitic concretions from the European Jurassic, with particular reference to the “snuff-boxes” of southern England. Sedimentology 18: 79-103.

Palmer, T.J. and Wilson, M.A. 1990. Growth of ferruginous oncoliths in the Bajocian (Middle Jurassic) of Europe. Terra Nova 2: 142-147.

Wooster’s Fossils of the Week: Encrusting cyanobacteria from the Upper Ordovician of the Cincinnati region

June 24th, 2016

1 pdt19598 D1253Deep in the basement of the Natural History Museum in London, Paul Taylor and I were examining cyclostome bryozoans encrusting an Upper Ordovician brachiopod with a Scanning Electron Microscope (SEM). This is one of our favorite activities, as the SEM always reveals tiny surprises about our specimens. That day the surprises were the smallest yet – fossils we had never seen before.

2 Infected brachWe were studying the dorsal exterior surface of this beat-up brachiopod from a 19th Century collection labelled “Cincinnati Group”. (Image by Harry Taylor.) We knew it was the strophomenid Rafinesquina ponderosa, and that the tiny chains of bryozoans encrusting it were of the species Corynotrypa inflata. We’ve seen this scene a thousand times. But when we positioned the SEM beam near the center of the shell where there was a brown film …

3 pdt16920 D1253… we saw that the bryozoans were themselves encrusted with little pyritic squiggles. These were new to us.

4 pdt19580 D7139In some places there were thick, intertwining mats of these squiggles. We later found these fossils on two other brachiopod specimens, both also Rafinesquina ponderosa and from 19th Century collections with no further locality or stratigraphic information.

5 pdt19578 D7139Last week Paul and I scanned these specimens again and began to put together an analysis. We believe these are fossil cyanobacteria. They are uniserial, unbranching strands of cells that range from 5 to 9 microns in length and width. Some of individual strands are up to 700 microns long and many are sinuous. The cells are uniform in size and shape along the strands; there are no apparent heterocysts. They appear very similar in form to members of the Order Oscillatoriales.

6 CyanobacteriaCyanobacteria are among the oldest forms of life, dating back at least 2.1 billion years, and they are still abundant today. The fossils are nearly identical to extant forms, as seen above (image from: http://www.hfmagazineonline.com/cyanobacteria-worlds-smallest-oldest-eyeball/).

7 pdt19599 D1253Paul made this remarkable image, at 9000x his personal record for high magnification, showing the reticulate structure preserved on some of the fossil surfaces. Note that the scale bar is just 2 microns long. These are beautiful fossils in their tiny, tiny ways.

We have not seen these cyanobacteria fossils before on shell surfaces, so we submitted an abstract describing them for the Geological Society of America annual meeting in Denver this September. We are, of course, not experts on bacteria, so we are offering our observations to the scientific community for further discussion. Here is the conclusion of our abstract —

“We suggest the cyanobacterial mats developed shortly before final burial of the brachiopod shells. Since the cyanobacteria were photosynthetic, the shells are inferred to have rested with their dorsal valve exteriors upwards in the photic zone. That Cincinnatian brachiopod shells were occupied by cyanobacteria has been previously well demonstrated by their microborings but this is the first direct evidence of surface microbial mats on the shells. The mats no doubt played a role in the paleoecology of the sclerobiont communities on the brachiopods, and they may have influenced preservation of the shell surfaces by the “death mask” effect. The pyritized cyanobacteria can be detected with a handlens by dark squiggles on the brachiopod shells, but must be confirmed with SEM. We encourage researchers to examine the surfaces of shells from the Cincinnatian and elsewhere to find additional evidence of fossilized bacterial mats.”

References:

Noffke, N., Decho, A.W. and Stoodle, P. 2013. Slime through time: the fossil record of prokaryote evolution. Palaios 28: 1-5.

Tomescu, A. M., Klymiuk, A.A., Matsunaga, K.K., Bippus, A.C. and Shelton, G.W. 2016. Microbes and the Fossil Record: Selected Topics in Paleomicrobiology. In: Their World: A Diversity of Microbial Environments (pp. 69-169). Springer International Publishing.

Vogel, K. and Brett, C.E. 2009. Record of microendoliths in different facies of the Upper Ordovician in the Cincinnati Arch region USA: the early history of light-related microendolithic zonation. Palaeogeography, Palaeoclimatology, Palaeoecology 281: 1-24.

Wooster’s Fossils of the Week: Symbiotic interactions in the Silurian of Baltica

June 17th, 2016

EcclimadictyonThis week’s fossils are from work Olev Vinn (University of Tartu, Estonia) and I did last summer that is soon to appear in the journal Lethaia. (An early electronic version of the manuscript has been available since November.) After numerous smaller studies describing symbiotic relationships recorded in Silurian fossils in the paleocontinent Baltica, we wrote a summary paper under Olev’s leadership. All the images are take by Olev and in the paper itself. I love this kind of study because it is about fossils as living, interacting organisms, not just static sets of characteristics.

For example, the top image is of the stromatoporoid Ecclimadictyon astrolaxum (a kind of hard sponge) with embedded rugosan corals (Palaeophyllum, with arrows) from the Jaagarahu Formation (Sheinwoodian) exposed at Abula cliff, Saaremaa Island, Estonia. The stromatoporoid and corals were growing together, each having their particular needs met and maybe even enhanced by the other.
syringoporidThe network of holes in this stromatoporoid from the Paadla Formation (Ludfordian) of Katri cliff, Saaremaa, represent the corallites of a syringoporid coral. Again, the coral and sponge formed an intergrown association.
ChaetosalpinxThis is a thin-section view of what was likely a soft-bodied worm (represented by Chaetosalpinx sibiriensis, noted by a white arrow) embedded in the tabulate coral Paleofavosites cf. collatatus from the Muksha Subformation (Homerian), Bagovitsa A, Podolia, Ukraine. Again, the worm was embedded in the living tissues of the host.

We found 13 such symbiotic associations in the Silurian of Baltica. Most of these interactions involved large skeletal organisms like stromatoporoids and corals, which provided stable hosts for smaller sessile filter-feeders and micro-predators. This work is part of a larger study looking at evolutionary trends in symbiotic associations during the Paleozoic.

References:

Tapanila, L. 2005. Palaeoecology and diversity of endosymbionts in Palaeozoic marine invertebrates: trace fossil evidence. Lethaia 38: 89–99.

Vinn, O. and Wilson, M.A. 2016. Symbiotic interactions in the Silurian of Baltica. Lethaia 49: 413–420.

Vinn, O., Wilson, M.A. and Motus, M.-A. 2014. Symbiotic endobiont biofacies in the Silurian of Baltica. Palaeogeography, Palaeoclimatology, Palaeoecology 404: 24–29.

Wooster’s Fossil of the Week: A fracture-shaped bioerosion trace from the Pliocene of Cyprus

June 10th, 2016

Caedichnus_01_scale_Mark 500This past semester I worked with three colleagues on a massive trace fossil review paper, which we hope meets success in the next month or so. My primary job on the team was to sort out bioerosion traces, especially those that are macroscopic. As always with such studies, I learned a great deal when forced to do a systematic literature review. One of the ichnogenera new to me was Caedichnus, a wedge-shaped excision found primarily in gastropod shells. It was only described last year by Stafford et al. (2015). Above is an example we happened to have in our collections. Note the fractured margins in this Fusinus shell aperture from the Pliocene of Cyprus. It was likely made by a predatory crustacean (such as a crab or lobster) bashing away at the shell to get to the living snail inside. The predator may have been successful in this case since there is no sign of healing in the snail shell.
Fusinus Cyprus Pliocene 500Above is an undamaged Fusinus showing a complete aperture. This snail also had its travails, though. Note the round, incomplete borehole just above the aperture. This was made by some kind of drilling predator, likely a naticid snail.

These shells come from the 1996 Wooster-Keck expedition to Cyprus with Steve Dornbos (’97) and me. Like the rest of the Cypriot specimens on this blog, it is from the Nicosia Formation (Pliocene) exposed on the Mesaoria Plain in the center of the island. This specimen comes from the “Exploration” locality described in Dornbos and Wilson (1999).

References:

Dornbos, S.Q. and Wilson, M.A. 1999. Paleoecology of a Pliocene coral reef in Cyprus: Recovery of a marine community from the Messinian Salinity Crisis. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen 213: 103-118.

Molinaro, D.J., Stafford, E.S., Collins, B.M., Barclay, K.M., Tyler, C.L. and Leighton, L.R. 2014. Peeling out predation intensity in the fossil record: A test of repair scar frequency as a suitable proxy for predation pressure along a modern predation gradient. Palaeogeography, Palaeoclimatology, Palaeoecology 412: 141-147.

Stafford, E.S., Dietl, G.P., Gingras, M.P. and Leighton, L.R. 2015. Caedichnus, a new ichnogenus representing predatory attack on the gastropod shell aperture. Ichnos 22: 87-102.

Stafford, E.S., Tyler, C.L. and Leighton, L.R. 2015. Gastropod shell repair tracks predator abundance. Marine Ecology 36: 1176-1184.

Wooster’s Fossils of the Week: A bored Ordovician hardground from Ohio, and an introduction to a new paper on trace fossils and evolution

June 3rd, 2016

Bull Fork hdgdAbove is an image of a carbonate hardground (cemented seafloor) from the Upper Ordovician of Adams County, Ohio. It comes from the Bull Fork Formation and was recovered along State Route 136 north of Manchester, Ohio (Locality C/W-20). It is distinctive for two reasons: (1) the many external molds (impressions, more or less) of mollusk shells, including bivalves and long, narrow, straight nautiloids, and (2) its many small borings called Trypanites, a type of trace fossil we’ve seen on this blog before.
Bull Fork boringsIn this closer view we can see the shallow external molds of small bivalve shells, especially on the left side, and the many round perforations of the Trypanites borings.

The dissolved mollusk shells (from bivalves and nautiloids) were originally composed of the calcium carbonate mineral aragonite. This aragonite dissolved early on the seafloor, liberating calcium carbonate that quickly precipitated as the mineral calcite in the sediment, cementing it into a rocky seafloor (hardground) that was then bored by the animal that made Trypanites. This all happened because of the distinctive geochemistry of the ocean water at that time. High levels of carbon dioxide and a decreased Mg/Ca ratio dissolved aragonite yet enabled calcite (the more stable polymorph of calcium carbonate) to rapidly precipitate. This geochemical condition is known as a Calcite Sea, which was common in the early to middle Paleozoic, especially in the Ordovician. This is not the case in today’s marine waters in which aragonite is the primary calcium carbonate precipitate (“Aragonite Sea“). See Palmer et al. (1988) for more details on this process and the evidence for it.

I’m using this Ordovician carbonate hardground to introduce a new paper that just appeared this week in the Proceedings of the National Academy of Sciences (PNAS): “Decoupled evolution of soft and hard substrate communities during the Cambrian Explosion and Ordovician Biodiversification Event“. The authors are the renowned trace fossil experts Luis Buatois and Gabriela Mángano, the ace geostatistician Ricardo Olea, and me. I’m excited about this paper because it adds to the literature new information and ideas about two major evolutionary radiations: the “explosion” of diversity in the Cambrian (which established basic body plans for most animals) and the diversification in the Ordovician (which filled in those body plans with abundant lower taxa). This is one of the few studies to look in detail at the trace fossil record of these events. Trace fossils (records of organism behavior in and on the sediment substrate) give us information about soft-bodied taxa otherwise rare in a fossil record dominated by shells, teeth and skeletons. It is also the first systematic attempt to compare the diversification of trace fossils in soft sediments and on hard substrates (like the hardground pictured above).

As for the paper itself, I hope you can read it. Here is the abstract —

Contrasts between the Cambrian Explosion (CE) and the Great Ordovician Biodiversification Event (GOBE) have long been recognized. Whereas the vast majority of body plans were established as a result of the CE, taxonomic increases during the GOBE were manifested at lower taxonomic levels. Assessing changes of ichnodiversity and ichnodisparity as a result of these two evolutionary events may shed light on the dynamics of both radiations. The early Cambrian (Series 1 and 2) displayed a dramatic increase in ichnodiversity and ichnodisparity in softground communities. In contrast to this evolutionary explosion in bioturbation structures, only a few Cambrian bioerosion structures are known. After the middle to late Cambrian diversity plateau, ichnodiversity in softground communities shows a continuous increase during the Ordovician in both shallow- and deep-marine environments. This Ordovician increase in bioturbation diversity was not paralleled by an equally significant increase in ichnodisparity as it was during the CE. However, hard substrate communities were significantly different during the GOBE, with an increase in ichnodiversity and ichnodisparity. Innovations in macrobioerosion clearly lagged behind animal–substrate interactions in unconsolidated sediment. The underlying causes of this evolutionary decoupling are unclear but may have involved three interrelated factors: (i) a Middle to Late Ordovician increase in available hard substrates for bioerosion, (ii) increased predation, and (iii) higher energetic requirements for bioerosion compared with bioturbation.

Thank you to Luis Buatois for his leadership on this challenging project. I very much appreciate the way this work has placed the study of trace fossils into a critical evolutionary context.
Fig1_PNASFigure 1 from Buatois et al. (2016): “Ichnodiversity changes during the Ediacaran-Ordovician. Ichnogenera were plotted as range-through data (i.e., recording for each ichnogenus its lower and upper appearances and then extrapolating the ichnogenus presence through any intervening gap in the continuity of its record).”

References:

Buatois, L.A., Mángano, M.G., Olea, R.A. and Wilson, M.A. 2016. Decoupled evolution of soft and hard substrate communities during the Cambrian Explosion and Ordovician Biodiversification Event. Proceedings of the National Academy of Sciences (in press).

Palmer, T.J., Hudson, J.D. and Wilson, M.A. 1988. Palaeoecological evidence for early aragonite dissolution in ancient calcite seas. Nature 335: 809-810.

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: Echinoderm holdfasts from the Upper Cambrian of Montana

May 27th, 2016

Pelmatozoans051216The white buttons above are echinoderm holdfasts from the Snowy Range Formation (Upper Cambrian) of Carbon County, southern Montana. They and their hardground substrate were well described back in the day by Brett et al. (1983). We have these specimens as part of Wooster’s hardground collection. (The largest collection of carbonate hardgrounds anywhere! A rather esoteric distinction.)

These holdfasts are the cementing end of stemmed echinoderms, conveniently called pelmatozoans when we don’t know if they were crinoids, blastoids, cystoids, or a variety of other stemmed forms. I suspect these are eocrinoid attachments, but we have no evidence of the rest of the organism to test this.
Snowy bedThe hard substrate for the echinoderms is a flat-pebble conglomerate, a distinctive kind of limestone found mostly in the Lower Paleozoic. They are in some places associated with limited bioturbation (sediment stirring by organisms) and early cementation, but there are other origins for these distinctive sediments (see Myrow et al., 2004).
Snowy crossThis particular flat-pebble conglomerate was itself cemented into a carbonate hardground, as seem in this cross section. The pelmatozoan holdfasts are just visible on the upper surface.

These pelmatozoans are among the earliest encrusters on carbonate hardgroounds and thus have an important position in the evolution of hard substrate communities.

References:

Brett, C.E., Liddell, W.D. and Derstler, K.L. 1983. Late Cambrian hard substrate communities from Montana/Wyoming: the oldest known hardground encrusters: Lethaia 16: 281-289.

Myrow, P. M., Tice, L., Archuleta, B., Clark, B., Taylor, J.F. and Ripperdan, R.L. 2004. Fat‐pebble conglomerate: its multiple origins and relationship to metre‐scale depositional cycles. Sedimentology 51: 973-996.

Sepkoski Jr, J.J. 1982. Flat-pebble conglomerates, storm deposits, and the Cambrian bottom fauna. In: Cyclic and event stratification (p. 371-385). Springer, Berlin Heidelberg.

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

Wooster’s Fossil of the Week: A phyllocarid crustacean from the Middle Cambrian Burgess Shale of British Columbia, Canada

May 20th, 2016

Canadaspis perfecta Burgess Shale 585We are fortunate at Wooster to have a few fossils from the Burgess Shale (Middle Cambrian) collected near Burgess Pass, British Columbia, Canada, including this delicate phyllocarid Canadaspis perfecta (Walcott, 1912). This species is one of the oldest crustaceans, a group that includes barnacles, crabs, lobsters and shrimp. Please note from the start that I did NOT collect it. The Burgess Shale is a UNESCO World Heritage Site, so collecting there is restricted to a very small group of paleontologists who have gone through probably the most strict permitting system anywhere. I had a wonderful visit to the Burgess Shale with my friend Matthew James in 2009, and we followed all the rules. (The photo below is of the Walcott Quarry outcrop.) Our Wooster specimen was in our teaching collection when I arrived. I suspect it was collected in the 1920s or 1930s and probably purchased from a scientific supply house.

walcottquarryMarrellaSuch a dramatic setting, which is perfect for the incredible fossils that have come from this site.

Canadaspis perfecta drawing

Canadaspis perfecta has been thoroughly studied by Derek Briggs, the most prominent of the paleontologists who have studied the Burgess Shale fauna. The above reconstruction of C. perfecta is from his classic 1978 monograph on the species. He had spectacular material to work with, including specimens with limbs and antennae well represented. Our specimen is a bit shabby in comparison! Nevertheless, we can still make out abdominal segments and a bit of the head shield.

Briggs (1978, p. 440) concluded that C. perfecta likely “fed on coarse particles, possibly with the aid of currents set up by the biramous appendages”. This is a similar feeding mode to many of the trilobites who lived alongside.

References:

Briggs, D.E. 1978. The morphology, mode of life, and affinities of Canadaspis perfecta (Crustacea: Phyllocarida), Middle Cambrian, Burgess Shale, British Columbia. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 281: 439-487.

Briggs, D.E. 1992. Phylogenetic significance of the Burgess Shale crustacean Canadaspis. Acta Zoologica 73: 293-300.

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