Tag Archives: taphonomy

Teaching on an Old Friend, Sapelo Island

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(This post is the fourth in a series about a spring-break field trip taken last week with my Barrier Islands class, which I teach in the Department of Environmental Studies at Emory University. The first three posts, in chronological order, tell about our visits to Cumberland Island, Jekyll Island, and Little St. Simons and St. Simons Islands. For the sake of conveying a sense of being in the field with the students, these posts mostly follow the format of a little bit of prose – mostly captions – and a lot of photos.)

When planning a week-long trip to the Georgia barrier islands with my students, I knew that one island – Sapelo – had to be included in our itinerary. Part of my determination for us to visit it was emotionally motivated, as Sapelo was my first barrier island, and you always remember your first. But Sapelo has much else to offer, and because of these many opportunities, it is my favorite as an destination for teaching students about the Georgia coast and its place in the history of science.

Getting to Sapelo Island requires a 15-minute ferry ride, all for the low-low price of $2.50. (It used to cost $1.00 and took 30 minutes. My, how times have changed.) For my students, their enthusiasm about visiting their fourth Georgia barrier island was clearly evident (with perhaps a few visible exceptions), although photobombing may count as a form of enthusiasm, too.

I first left my own traces on Sapelo in 1988 on a class field trip, when I was a graduate student in geology at the University of Georgia. My strongest memory from that trip was witnessing alligator predation of a cocker spaniel in one of the freshwater ponds there. (I suppose that’s another story for another day.) Yet I also recall Sapelo as a fine place to see geology and ecology intertwining, blending, and otherwise becoming indistinguishable from one another. This impression will likely last for the rest of my life, reinforced by subsequent visits to the island. This learning has always been enhanced whenever I’ve brought my own students there, which I have done nearly every year since 1997.

As a result of both teaching and research forays, I’ve spent more time on Sapelo than all of the other Georgia barrier islands combined. Moreover, it is not just my personal history that is pertinent, but also how Sapelo is the unofficial “birthplace” of modern ecology and neoichnology in North America. Lastly, Sapelo inspired most of the field stories I tell at the start of each chapter in my book, Life Traces of the Georgia Coast. In short, Sapelo Island has been very, very good to me, and continues to give back something new every time I return to it.

So with all of that said, here’s to another learning experience on Sapelo with a new batch of students, even though it was only for a day, before moving on to the next island, St. Catherines.

(All photographs by Anthony Martin and taken on Sapelo Island.)

Next to the University of Georgia Marine Institute is a freshwater wetland, a remnant of an artificial pond created by original landowner R.J. Reynolds, Jr. More importantly, this habitat has been used and modified by alligators for at least as long as the pond has been around. For example, this trail winding through the wetland is almost assuredly made through habitual use by alligators, and not mammals like raccoons and deer, because, you know, alligators.

Photographic evidence that alligators, much like humans prone to wearing clown shoes, will use dens that are far too big for them. This den was along the edge of the ponded area of the wetland, and has been used by generations of alligators, which I have been seeing use it on-and-off since 1988.

An idealized diagram of ecological zones on Sapelo Island, from maritime forest to the subtidal. This sign provided a good field test for my students, as they had already (supposedly) learned about these zones in class, but now could experience the real things for themselves. And yes, this will be on the exam.

When it’s high tide in the salt marsh, marsh periwinkles (Littoraria irrorata) seek higher ground, er, leaves, to avoid predation by crabs, fish, and diamondback terrapins lurking in the water. Here they are on smooth cordgrass (Spartina alterniflora), and while there are getting in a meal by grazing on algae on the leaves.

Erosion of a tidal creek bank caused salt cedars (which are actually junipers, Juniperus virginiana) to go for their first and last swim. I have watched this tidal creek migrate through the years, another reminder that even the interiors of barrier islands are always undergoing dynamic change.

OK, I know what you’re thinking: where’s the ichnology? OK, how about these wide, shallow holes exposed in the sandflat at low tide? However tempted you might be to say “sauropod tracks,” these are more likely fish feeding traces, specifically of southern stingrays. Stingrays make these holes by shooting jets of water into the sand, which loosens it and reveals all of the yummy invertebrates that were hiding there, followed by the stingray chowing down. Notice that some wave ripples formed in the bottom of this structure, showing how this stingray fed here at high tide, before waves started affecting the bottom in a significant way.

Here’s more ichnology for you, and even better, traces of shorebirds! I am fairly sure these are the double-probe beak marks of a least sandpiper, which may be backed up by the tracks associated with these (traveling from bottom to top of the photo). But I could be wrong, which has happened once or twice before. If so, an alternative tracemaker would be a sanderling, which also makes tracks similar in size and shape to a sandpiper, although they tend to probe a lot more in one place.

Just in case you can’t get enough ichnology, here’s the lower, eroded shaft of a ghost-shrimp burrow. Check out that burrow wall, reinforced by pellets. Nice fossilization potential, eh? This was a great example to show my students how trace fossils of these can be used as tools for showing where a shoreline was located in the geologic past. And sure enough, these trace fossils were used to identify ancient barrier islands on the Georgia coastal plain.

Understandably, my students got tired of living vicariously through various invertebrate and vertebrate tracemakers of Sapelo, and instead became their own tracemakers. Here they decided to more directly experience the intertidal sands and muds of Cabretta Beach at low tide by ambulating through them. Will their tracks make it into the fossil record? Hard to say, but I’ll bet the memories of their making them will last longer than any given class we’ve had indoors and on the Emory campus. (No offense to those other classes, but I mean, you’re competing with a beach.)

The north end of Cabretta Beach on Sapelo is eroding while other parts of the shoreline are building, and nothing screams “erosion!” as loudly as dead trees from a former maritime forest with their roots exposed on a beach. Also, from an ichnological perspective, the complex horizontal and vertical components of the roots on this dead pine tree could be compared to trace fossils from 40,000 year-old (Pleistocene) deposits on the island. Also note that at this point in the trip, my students had not yet tired of being “scale” in my photographs, which was a good thing for all.

Another student eager about being scale in this view of a live-oak tree root system. See how this tree is dominated by horizontal roots? Now think about how trace fossils made by its roots will differ from those of a pine tree. But don’t think about it too long, because there are a few more photos for you to check out.

Told you so! Here’s a beautifully exposed, 500-year-old relict marsh, formerly buried but now eroding out of the beach. I’ve written about this marsh deposit and its educational value before, so will refrain from covering that ground again. Just go to this link to learn about that.

OK geologists, here’s a puzzler for you. The surface of this 500-year-old relict marsh, with its stubs of long-dead smooth cordgrass and in-place ribbed mussels (Guekensia demissa), also has very-much-live smooth cordgrass living in it and sending its roots down into that old mud. So if you found a mudstone with mussel shells and root traces in it, would you be able to tell the plants were from two generations and separated by 500 years? Yes, I know, arriving at an answer may require more beer.

Although a little tough to see in this photo, my students and I, for the first time since I have gone to this relict marsh, were able to discern the division between the low marsh (right) and high marsh (left). Look for the white dots, which are old ribbed mussels, which live mostly in the high marsh, and not in the low marsh. Grain sizes and burrows were different on each part, too: the high marsh was sandier and had what looked like sand-fiddler crab burrows, whereas the low marsh was muddier and had mud-fiddler burrows. SCIENCE!

At the end of a great day in the field on Sapelo, it was time to do whatever was necessary to get back to our field vehicle, including (gasp!) getting wet. The back-dune meadows, which had been inundated by unusually high tides, presented a high risk that we might experience a temporary non-dry state for our phalanges, tarsals, and metatarsals. Fortunately, my students bravely waded through the water anyway, and sure enough, their feet eventually dried. I was so proud.

So what was our next-to-last stop on this grand ichnologically tainted tour of the Georgia barrier islands? St. Catherines Island, which is just to the north of Sapelo. Would it reveal some secrets to students and educators alike? Would it have some previously unknown traces, awaiting our discovery and description? Would any of our time there also involve close encounters with large reptilian tracemakers? Signs point to yes. Thanks for reading, and look for that next post soon.

 

 

Alien Invaders of the Georgia Coast

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(This is the first in a series of posts about invasive species on the Georgia barrier islands, their traces, the ecological impacts of these traces, and why people should be aware of both their traces and impacts.)

Paleontologists like me face a challenge whenever we study modern environments while trying to learn how parts of these environments might translate into the geologic record. Sure, we always have to take into account taphonomy (fossil preservation), through which we acknowledge that nearly none of the living and dead bodies we see in a given environment will become fossilized; relatively few of their tracks, trails, burrows, or other traces are likely to become trace fossils, either.

Because of this pessimistic (but realistic) outlook, paleontologists often rub a big eraser onto whatever we draw from a modern ecosystem, telling ourselves what will not be there millions of years from now. We then retroactively apply this concept – a part of actualism or, more polysyllabically, uniformitarianism – to what happened thousands or millions of years ago. When paleontologists do this, they assume that today’s processes are a small window through which we can peer, giving insights into processes of the pre-human past.

Feral horse (Equus caballus) tracks crossing coastal dunes on Cumberland Island, Georgia. During their evolutionary history, horses originated in North America and populations migrated to Asia, but populations in North America went extinct during the Pleistocene Epoch about 10,000 years ago. Using the perspective of geologic time, then, could someone argue that horses are actually “native,” and these feral populations are restoring a key part of a pre-human Pleistocene landscape? (Photograph by Anthony Martin.)

However, a huge complication in our quest for actualism is this reality: nearly every ecosystem we can visit on this planet is a hybrid of native and alien species, the latter introduced – intentionally or not – by us. Thus when we watch modern species behaving in the context of their environments, we always need to always ask ourselves how non-native species have cracked the window through which we squint, through the past darkly.

This theme is considered in Charles C. Mann’s most recent book, 1493: Uncovering the New World Columbus Created, in which he argues how nearly all terrestrial ecosystems occupied by people were permanently altered by the rapid introduction of exotic species worldwide following Columbus’s landfall in the Western Hemisphere. Going even further back, though, the introduction of wild dogs (dingoes) into mainland Australia by humans about 5,000 years ago irrevocably changed the environments of an entire continent. Examples like these show that European colonization and its aftermath in human history during the last 500 years was not the sole factor in the spread of non-native species, and hints at how species invasions have been an integral part of humanity and its movement throughout the world.

Something tells me we’re not in Georgia any more. A male-female pair of dingoes (Canis lupus dingo) pose for a picture in Kakadu National Park, Northern Territory, Australia. Although now considered “native,” dingoes are an example of an invasive species that had a huge impact once brought over by people from southeast Asia about 5,000 years ago. For one, its arrival is linked to the extinction of native carnivorous mammals in the mainland Australia, such as thylacines (Thylacinus cynocephalus) and Tasmanian devils (Sarcophilus harrisii). (Photograph by Anthony Martin.)

Well-meaning (but deluded) designations of “pristine,” “untouched,”and “unspoilt” aside, the Georgia barrier islands are no exception to alien invaders. Moreover, like many barrier-islands systems worldwide, they differ greatly from island to island in: which species of invaders are there; numbers of individuals of each species; and the degree of how these organisms impact island ecosystems and even their geological processes.

Feral cat tracks in back-dune meadows of Jekyll Island, Georgia. Jekyll is one of the few Georgia barrier islands with a significant human presence year-round, hence these cats are descended from domestic cats that were either purposefully or accidentally let loose by residents. What impact do these cats have on native species of animals and ecosystems, and are these effects comparable to those of other invasive species on other islands? Scale = 15 cm (6 in). (Photograph by Anthony Martin.)

This is one of the reasons why I devoted several pages of my upcoming book, Life Traces of the Georgia Coast, to the traces of invasive species – tracks, trails, burrows, and so on – despite their failing an “ecological purity test” for anyone who might prefer to focus on native species and their traces. With regard to invasive species, the genie is out of the bottle, so we might as well study what is there, rather than apply yet another metaphorical eraser to species that are drastically shaping modern ecosystems and affecting the behavior of native species, thus likewise altering their traces.

A large pit of disturbed sand in a back-dune meadow caused by feral hogs (Sus crofa) on St. Catherines Island, Georgia. Because feral hogs are wide-ranging omnivores with voracious appetites, they cause considerable alterations to island habitats, from maritime forests to intertidal beaches. How do these traces affect the behavior and ecology of other species, especially native ones, in such a broad range of environments on the Georgia barrier islands? Can their traces actually alter the geological character of the islands? (Photograph by Anthony Martin.)

What are some of these invasive species? What makes for an “invasive species” versus a mere “exotic species”? How do the traces of invasive species affect native species on the Georgia barrier islands, and the ecology and geology of the islands themselves? And how do paleontologists and geologists figure into the study of invasive species?

These are all questions that I hope to explore in upcoming weeks here, and for the sake of simplicity, I will showcase an invasive species of mammal and its traces each week. Some of the photos shown here serve as a visual teaser of the invasive species and their traces that will be covered: feral horses (Equus caballus), cattle (Bos taurus), hogs (Sus crofa), and cats (Felis domestica). Yes, I know, there are many others, but these four are among the most ecologically significant species, they consist of animals that nearly everyone knows, and – best of all – they make easily identifiable traces. So these fours species will provide a starting point in our learning how the Georgia barrier islands can be used as case studies in the traces and ecological effects of traces made by invasive species.

Trail made by feral cattle (Bos taurus) cutting through a salt marsh and extending to the horizon, providing a clue of how this forest-dwelling animal can travel deeply into and affect marginal-marine environments. How might such traces show up in the geologic record, and was there a species that might have made similar traces on the islands in the recent past? (Photograph by Anthony Martin.)

Fossils in Progress

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Despite whatever lamentations are made about the “incompleteness” of the fossil record, fossils are actually quite common. This truism is brought home even more so whenever trace fossils – tracks, burrows, and other evidence of organismal behavior – are included in a fossil checklist (as well they should be) when examining any given outcrop of sedimentary rocks formed in the past 550 million years or so.

For example, many a time I have visited an outcrop described previously as “lacking fossils,” and instead found it filled with trace fossils; hence what people meant was “lacking fossils” equals “no body fossils.” Normally these trace fossils are invertebrate burrows, which might be glibly identified as “worm burrows,” but tracks or other trace fossils may also reveal themselves to those who are looking for them. Indeed, this expectation of finding fossils is such that on occasions when geologists find a sedimentary rock layer devoid of either body or trace fossils, this is odd enough to cause geologists to scratch their heads and ask why.

But how do the former bodily remains of plants or animals, or traces of their behaviors, become preserved as fossils in the first place? This question other related ones are answered by the science of taphonomy. Coined by Russian paleontologist Ivan Yefremov, the etymology of this term stems from Greek, in which taphos ( = burial) and nomos (= law). In such a term, he was thus alluding to an expectation that natural processes that result in fossils becoming preserved are orderly and predictable.

An overview of taphonomy as a field of study would be far too lengthy to explore here, so instead I will use one example from the Georgia coast to show how it is supposed to work. This superb case in point is a relict marsh. It is what’s left of a salt marsh from about 500 years ago, and it has been revealing its nature to paleontologists, geologists, and students for the past few decades.

Overall view of relict marsh exposed on Cabretta Beach, Sapelo Island, Georgia. Me for scale, but photo taken 7 years ago, so the scale might now be slightly wider now. (Photograph taken by Ruth Schowalter.)

Just a little more than a week ago, my colleague Steve Henderson and I took a group of students from Emory University to Sapelo Island for a weekend field trip (detailed last week). One of our goals on this trip was to take them to a relict marsh on Cabretta Beach so that they could better appreciate how a sedimentary deposit makes a transition from living ecosystem to inert rock, yet filled with evidence of its formerly teeming life. Similar relict marshes are on St. Catherines Island and other Georgia-coast islands, but when it comes to teaching about taphonomy in the field, I prefer using the one on Sapelo.

Closer view of relict marsh on Sapelo Island, showing 500-year-old remains of smooth cordgrass (Spartina alterniflora), cross section of its muddy sediments, and quartz sand deposited on top by tides, waves, and wind. (Photograph by Anthony Martin.)

As mentioned in a previous entry, modern salt marsh on the Georgia coast have a few key components that make them among the most productive of all ecosystems: smooth cordgrass (Spartina alterniflora), marsh periwinkles (Littoraria irrorata), mud fiddler crabs (Uca pugnax), and ribbed mussels (Geukensia demissa). So if a Georgia salt marsh were to be buried quickly – say, by a storm that dumps a thick layer of sand on it – what would be preserved? The Cabretta relict marsh partially answers that question, showing us incipient trace and body fossils of these biota. They are not quite fossils, but on their way there, giving us a glimpse of the fossilization process well before it is completed.

For example, the tall, green or golden stalks of smooth cordgrass that we see today, adorned my millions of marsh periwinkles (Littoraria irrorata), are absent from the relict marsh. Only the lowermost ochre-colored stubs and extensive root systems remain, and traces made by the roots below what was the marsh surface.

Modern smooth cordgrass (Spartina alterniflora) and its constant companions, marsh periwinkles (Littoraria irrorata) on Sapelo Island, Georgia.

Cross-sectional view of relict marsh, what is left from a formerly magnificent marsh: stubs, roots, root traces, and not many periwinkles. (Both photographs by Anthony Martin.)

Once in a while, I also find old marsh periwinkle shells scattered on the surface of the relict marsh. These are made of calcium carbonate and will dissolve in slightly acidic waters, so these might not last for long once exposed. The real reason for why these tend to disappear quickly, though, is modern hermit crabs. Hermit crabs encounter these periwinkle shells on the relict marsh surface, say “Hey, free shells!”, then happily trot away with these, not caring that their “new” homes are actually 500 years old.

No mud-fiddler crab remains were apparent on the surface, nor have I seen them in 20-30 visits to this relict marsh. This is not surprising, as their exoskeletons are made of chitin and dissolve more quickly than molluscan shells. Nonetheless, their burrows are always abundantly evident on the surface as perfectly round holes, which are sometimes accompanied by new burrows made by modern fiddler crabs, as well as bivalves that will bore into this firmground.

Modern salt marsh surface on Sapelo Island with mud fiddler crabs (Uca pugnax) showing off a few of the behavioral traits they do best: eating, fighting, mating, and burrowing. Note that burrows, surface scrapings, and pellets are a few of the traces they make. Which of these traces get preserved?

Close-up of eroded relict marsh surface, showing cross-sections of old fiddler-crab burrows now being filled with modern beach sand. Think of how this will look in the fossil record. (Scale in centimeters).

Longitudinal view of former fiddler-crab burrows associated with smooth-cordgrass root traces. Fill the deeper parts of these burrows with sand, and they’re more likely to get preserved as trace fossils. Scale to right is 15 cm (6 in) long. (All photographs by Anthony Martin.)

Modern ribbed mussels are harder for us to see in the field because we would have to wade into soft, deep, sulfurous mud to get close to them, and however amusing that might be, we don’t have time to do our laundry before getting back into our rental vans for the ride home. So the students take our word for it that those mussels are indeed in the marsh, then we point to the old ones clumped on the relict-marsh surface that are still in life position.

Cluster of ribbed mussels (Guekensia demissa) directly associated with stubs of smooth cordgrass on relict marsh surface. Now that they’re exposed, how long will these shells last on the surface? (Photograph by Anthony Martin.)

Oysters (Crassostrea virginica) are less common in the relict marsh, but given the right exposure, these can be observed on some visits too. These clumps of oyster shells mark the edges of tidal creeks that wound through the marsh.

(Top) Modern salt marsh with tidal creek cutting through it and oyster bank exposed at low tide, Sapelo Island.

Former oyster bank peeking out of relict marsh, formerly buried for about 500 years, now revealed by erosion of the modern shoreline. (Both photographs by Anthony Martin.)

Because it was all too easy to spot the similarities between this relict marsh and a modern one less than 100 meters (330 feet) from where we stood, I then asked about other differences. For instance, take the fact that we were standing on the relict marsh while discussing its traits: could we do the same in the modern marsh nearby? No, was the universal answer, and I affirmed that they would likely be up to their waists in ribbed-mussel-produced mud. (I asked for volunteers to test this hypothesis, and they very smartly declined.)

This led to a discussion of why the relict marsh could be so firm, which introduced them to the concept of diagenesis: how a sedimentary deposit can change over time, an important consideration in taphonomy. Such alterations are especially apparent in muds, which lose considerable volume as these lose their water content, causing a “softground” to become a “firmground,” then eventually a “hardground.” The students were surprised when I told them that the relict marsh acting as the floor of our “classroom” was likely 2-3 times as thick as what was there now.

Would these students so blithely walk around on a modern salt marsh? I don’t think so, and please don’t experiment with this yourself. Nevertheless, a relict marsh, thanks to dehydration of its muds and compaction, is just fine for exploring on foot. (Photograph by Anthony Martin.)

We spent only about an hour at the relict marsh before regretfully walking back to our field vehicle, followed by a ferry ride to the mainland part of Georgia and a long drive home to Atlanta. Yet I felt assured that the lessons about taphonomy, ancient environments, ichnology, and diagenesis imparted by this relict marsh encompassed enough material to fill 4-5 class sessions in an indoor classroom. Moreover, if we had been all enclosed by four walls and a ceiling, and without a former marsh underfoot, there was no guarantee that these concepts would be understood or retained.

This is why we geoscientist-educators take our students outside, enriching our collective awareness of how environments change through time and how we piece together the clues left behind from ancient environments. It’s memorable, it’s fun, and it works. But don’t take my word for it. Whether you’re an educator or student, try it yourself sometime, whether on the Georgia coast or elsewhere, and see what happens.

Further Reading

Basan, P.B., and Frey, R.W. 1977. Actual-palaeontology and neoichnology of salt marshes near Sapelo Island, Georgia. In Crimes, T.P., and Harper, J.C. (editors), Trace Fossils 2. Liverpool, Seel House Press: 41-70.

Edwards, J.M. and Frey, R.W. 1977. Substrate characteristics within a Holocene salt marsh, Sapelo Island, Georgia. Senckenbergiana Maritima, 9: 215-259.

Frey, R.W. and P.B. Basan. 1981. Taphonomy of relict Holocene salt marsh deposits, Cabretta Island, Georgia. Senckenbergiana Maritima, 13: 111-155.

Frey, R.W., Basan, P.B. and Scott, R.M. 1973. Techniques for sampling salt marsh benthos and burrows. American Midland Naturalist, 89: 228-234.

Letzsch, W.S. and Frey, R.W. 1980. Deposition and erosion in a Holocene salt marsh, Sapelo Island, Georgia. Journal of Sedimentary Research, 50: 529-542.

Morris, R. W. and H. B. Rollins. 1977. Observations on intertidal organism associations on St. Catherines Island, Georgia. I. General description and paleoecological implications. Bulletin of the American Museum of Natural History, 159: 87-128.

Smith, J.M., and Frey, R.W. 1985. Biodeposition by the ribbed mussel Geukensia demissa in a salt marsh, Sapelo Island, Georgia. Journal of Sedimentary Research, 55: 817-825.