Fossil Visions in the Two Medicine

(This post is the third in a series of three about my field work on the trace fossils of the Late Cretaceous (75 million-year-old) Two Medicine Formation, which I just completed a week ago. My previous two posts, which mostly explain the scientific importance of this field work, are Tracing the Two Medicine and Burrowing Wasps and Baby Dinosaurs.)

Looking back on three weeks of field work in the Late Cretaceous Two Medicine Formation, one of the realizations I had was how long it took before I could see more of what was there. The most frustrating part of this realization, though, is also knowing that I still missed plenty. This mix of satisfaction and unease is the duality that often accompanies the birthing and honing of search images, a visual training that enables paleontologists to find the fossils we want to find whenever we walk around a field site and look.

Tony-Martin-Searching-Fossils-Two-MedicineThis outcrop of the Late Cretaceous (75 mya) Two Medicine Formation in central Montana is chock-full of fossils, but you might not know that from just looking at this picture. That means you have to get out onto the rocks and look closely for them, but first make sure you have the right search images for finding them. (Photograph by Ruth Schowalter.)

The Two Medicine Formation in particular presents a major challenge for cultivating search images because of the variety of fossils in it. Moreover, most of these fossils require very different search images. For example, over my three weeks of prospecting, I looked for the following fossils:

  • Plant root traces
  • Invertebrate burrows and tracks
  • Insect cocoons and pupal chambers
  • Dinosaur tracks
  • Dinosaur nests
  • Dinosaur eggshells
  • Dinosaur coprolites
  • Dinosaur bones
  • Dinosaur toothmarks (on dinosaur bones)

I also found a few other fossils I didn’t expect to find, but there they were. This happenstance served as a good reminder that simply going out into the field with a bullet-point checklist of what you think you’ll find (like what you just read) isn’t good enough. In other words, you also need to see what’s there, rather than just what you expect to be there.

On top of looking for these fossils, I’m a geologist, too. This means I also paid close attention to the rock types in the Two Medicine Formation – sandstones, mudstones, conglomerates, limestones – and their physical sedimentary structures – such as cross-bedding or graded bedding. Moreover, Two Medicine strata in the field area are not necessarily in their original horizontal positions, but instead are bent, tilted, and faulted in places. This is where training I had in structural geology – the study of how rocks were deformed – came in handy.

Geologic-Anticline-Two-MedicineOriginally horizontal sedimentary strata were bent upward into a fold, which we geologists normally call an anticline. In such folds, the fossils in the center of the fold are geologically older, whereas the fossils on the outside of the fold are younger. That is, unless the strata were overturned, in which case we’d call it antiformal syncline, then the fossils would have the opposite age relations. Thank you for teaching this, structural geology professors! (Photograph by Anthony Martin.)

Geologic-Fault-Two-MedicineIt’s not my fault, so we’ll blame the Two Medicine Formation for this breakage of sedimentary rocks. Based on how it looks like the fault block on the right moved up relative to the one on the left, I think this is a reverse fault, which – like the anticline and almost everything else on earth – was caused by plate tectonics. (Photograph by Anthony Martin.)

Thus whenever I stepped into the field each day, I had to rapidly switch, combine, or otherwise tap into different types of vision. I’ve often jokingly referred to my ability to spot traces and trace fossils in the field as “ichnovision” (my most likely comic-book hero superpower), and my geological training means I’m using “geovision.” Yet in the Two Medicine Formation – a rock unit world-famous for its dinosaur bones and eggs – I also had to use “osteovision” (seeing fossil bones) and “oovision” (seeing fossil eggshells). These forms of fossil vision are tough for me, as I never see dinosaur bones or eggshells in the southeastern U.S., which is where I spend most of my time in the field.

So just to give you an appreciation of what it was like during my three weeks of looking for fossils in the Two Medicine Formation, here are a few photos and brief descriptions of some fossils I found. To be sure, there was much more than this, but at least I can share these for now so you can begin to see through my eyes.

Fossil-Plant-Root-Traces-Two-MedicineThese odd-looking structures weathering out of an outcrop in the Two Medicine Formation had variable diameters, central cores filled with calcite, and branched in places. I’m fairly sure these are fossil plant root traces, but they were the only ones I saw like them during three weeks of field work. So I remain a little skeptical of my identification, and remain open to their being some geological features I’ve just never seen before then. (Photograph by Anthony Martin.)

Horizontal-Burrows-Two-MedicineThese are longitudinal sections of horizontal burrows in a sandstone, showing off their beautifully expressed internal structures called meniscae. Meniscae are formed by burrowing invertebrates – such as beetle larvae or cicada nymphs – that pack their burrow with sediment behind them as they move. This means the convex side of the meniscae points in the direction the animal was moving. Go ahead, apply that principal and see what you figure out for yourself. (Photograph by Anthony Martin.)

Vertical-Burrows-Two-MedicineThese are more invertebrate burrows, but they’re vertically oriented, meaning you only see their circular cross-sections when you look at the top bedding-plane surface of this sandstone. Notice how some of them are open but others are filled with sandstone. The open ones were filled with mud originally, but that softer sediment has since weathered out, leaving them hollow. (Photograph by Anthony Martin.)

Limulid-Tracks-Two-MedicineThese are invertebrate tracks, and they form a distinctive enough pattern that I recognized them as a trackway, where the trackmaker (probably a freshwater horseshoe crab) turned. But they’re also preserved in positive relief (“sticking out”) because the original traces were filled with sand, which made a natural cast of the tracks. Think about how you have to reverse your concept of tracks to recognize these. (Photograph by Anthony Martin.)

Fossil-Cocoons-Two-MedicineOne of my main research interests in the Two Medicine Formation is its insect trace fossils, which include some of the best-preserved fossil insect cocoons I’ve ever seen in the geologic record. See where the patterns of their original weaves? These cocoons were likely made by wasps – or something acting very much like wasps – 75 million years ago. I usually prospected for these cocoons by looking for their distinctive oval shapes on the ground, then looked more closely for the weave pattern. (Photograph by Anthony Martin.)

Fossil-Cocoon-in-situ-Two-MedicineThis is what a fossil insect cocoon looks like in an outcrop. Sometimes a burrow would be connected to the cocoon, showing where the original mother insect dug a brooding chamber for its intended offspring. (Photograph by Anthony Martin.)

Dinosaur-Bone-Two-MedicineA rare piece of dinosaur bone that actually looks like a bone, even to an untrained eye. Although this one is white, the dinosaur bones in the Two Medicine Formation varied wildly in their colors. So spotting these fossils was more a matter of looking for both a shape and texture that translate into “bone.” (Photograph by Anthony Martin.)

Fragmented-Dinosaur-Bone-Two-MedicineThis is more what most dinosaur bones looked like when I found them in the field area. You probably spotted the big chunk right away, but how about the smaller ones that tend to blend in with the non-dinosaur-bone rocks around them? (Photograph by Anthony Martin.)

Adult-Hadrosaur-Track-Two-MedicineHere’s another example of how fossil tracks are not like modern ones in size, shape, and how it’s preserved. This is a three-toed dinosaur track (probably made by a hadrosaur), but it was originally made in mud, then sand filled in the track-sized hole to make a natural cast, which 75 million years later weathered out so that it’s sitting by itself on the eroded surface of a mudstone. What’s the scale? My boot’s a size 8 1/2 (men’s). Yes, I felt a little inadequate.  (Photograph by Anthony Martin.)

Hadrosaur-Track-in-situ-Two-MedicineWhat does a natural sandstone cast of a dinosaur track look like when it’s still in outcrop? Look for a lump on the bottom of a sandstone bed. From a side view, you might then see a couple of “toes” pointing in one direction, like in this one: the central toe is to the left and one of the outer toes is on the side, clser to you. Note how the sandstone bed also has a few open invertebrate burrows in it, too. Ichnobonus! (Photograph by Anthony Martin.)

Hadrosaur-Coprolite-Two-MedicineCheck out this big piece of, well, dinosaur coprolite. These trace fossils contained blackened (carbonized) wood fragments that originally passed through the gut of a dinosaur (probably a hadrosaur), and were later cemented by calcite. But you had to look at them doubly, because some of these trace fossils included their own trace fossils made by insects, namely dung beetle burrows. (Photograph by Anthony Martin.)

Field-of-Feces-Two-MedicineYou’ve heard of ‘Field of Dreams’? This is a ‘Field of Feces.’ The ground here is adorned with dinosaur coprolites, which are weathering out of the mudstone and breaking apart on the surface. This serves as a good example of how once you know what the dinosaur coprolites look like in this area, you’re less likely to just walk by them, singing “Where Have All the Coprolites Gone?”. (Photograph by Anthony Martin.)

Eggshell-Fragments-Two-MedicineThe Two Medicine Formation is famous for its dinosaur eggs and babies, but even more common than those are bits and pieces of dinosaur eggshells. These show up as black flakes on ground surfaces and sometimes in a rock, which you then must distinguish from all other black flakes that are not dinosaur eggshells. (Photograph by Anthony Martin.)

Find-Dinosaur-Eggshell-Two-MedicineCan you find the dinosaur eggshell in this photo? I’ll bet the answer was “yes,” but I made it a little easier for you by cropping the photo, placing the eggshell near the center of the image, and oh yea, showing you what typical eggshells look like in the previous photo. Now think about detecting this bit of eggshell from a standing height and while walking. (Photograph by Anthony Martin.)

After viewing the photos and reading the descriptions, do you think you could recognize each of these fossils if you were somehow magically transported to the Two Medicine Formation in Montana?

The likely answer to that question is, maybe, maybe not. For instance, despite all of my previous paleontological and geological field experience, it took me about two weeks of being in the field before I started accurately identifying dinosaur bones and eggshells. This humbling situation gave me a renewed appreciation for the people who regularly work in the Two Medicine Formation, but also imparted a lesson about taking the time to learn from misidentified burrows, cocoons, coprolites, bones, and eggshells in it. Most things I saw in the Two Medicine were not these fossils, meaning my ways of seeing had to become more discriminating over time.

Thus given enough practice and “dirt time” seeking fossil in the field and correcting your mistakes – preferably with an expert peer-reviewing your finds beside you – the fossil visions will come to you. Then, next thing you know, you start noticing more of what you didn’t see before, expanding your consciousness of the lives that preceded your own.

* * *

Many thanks to Dr. David Varricchio for inviting me to be part of his NSF-sponsored research project in the Two Medicine Formation this summer, and by extension, my deep appreciation to Montana State University and Museum of the Rockies for their logistical support at Camp Makela. May it have many more successful field seasons.

Seven-Samurai-PaleontologyThe Seven Samurai of paleontology at Camp Makela, ready for action in the Two Medicine Formation of central Montana. These ruffians/malcontents/Guardians of the Cretaceous Galaxy are otherwise known as (left to right): Ulf, Jared, me, Ashley, Emmy, Paul, and Eric. (Photograph and choreography by Ruth Schowalter.)

For more about these people and other human connections between the paleontological research that took place in the Two Medicine Formation – and told from a non-paleontological perspective – go to Cretaceous Summer 2014, which had links to four blog posts done on site by my wife Ruth Schowalter. Also be sure to check out Brad Brown’s blog post from the Burpee Museum of Natural History about his experiences at the field site, Just What the Doctor Ordered: Two Medicine Delivers High Biodiversity in a Low Profile Area.

Horseshoe Crabs Are So Much More Awesome Than Mermaids

Given all of the controversy over a recent cable-TV program, in which its broadcasting channel decided mythical marine animals deserved more air-time than real ones, I thought it was important to highlight one extant animal that never fails to surprise me. This animal’s lineage is more ancient than dinosaurs, reptiles, or even amphibians, with its oldest fossils dating from about 450 million years ago. It is also the largest living marine invertebrate animal you are likely to see on beaches of the eastern U.S. and Gulf Coast. And at this time of year, if you see it crawling around on a beach, it’s because of sex. For the past month or so, this animal has been participating in massive orgies. Pictures of this gamete-laden frenzy somehow made it past prudish censors of Facebook and other social-media sites, titillating prurient invertebrate enthusiasts everywhere and filling them with cockle-warming glee.

Juvenile-Limulid-SapeloBehold, a fine juvenile specimen of the Atlantic horseshoe crab (Limulus polyphemus)! Although it lives in the ocean, it can walk on land for hours, like some sort of reverse Aquaman, but totally cooler than him. And some day, if this one lives long enough, it will use those legs to walk on land again, but in pursuit of sex. Sounds to me like this animal deserves its own planet. (Photograph by Anthony Martin, taken on Sapelo Island, Georgia.)

As you already know from reading the title of this post, I’m talking about horseshoe crabs. More properly known as limulids by real marine biologists and paleontologists, these ultra-cool, über-hip, but totally retro critters are more closely related to spiders than they are to true crabs, but their common name is so, well, common, that scientists just sigh and begrudgingly go along with it for the sake of public communication.

Modern limulids are represented by four species, three of which are in Asia, but the grandest of them all is the Atlantic horseshoe crab, Limulus polyphemus. This species is at its largest here in Georgia, which may be a function of the Georgia Bight, an extensive offshore shelf that affords more food and habitat than other areas. How big? I’ve seen some as long as 70 cm (27 in) – tail included – and 40 cm (16 in) wide, big enough to scare both of our cats at home. They grow to these sizes after hatching as little limulids not much bigger than the period on this sentence, an astonishing increase in mass if they make it to adulthood (which most don’t).

Baby-Limulid-TrailThe circuitous trail of a baby limulid, made on a sandflat at low tide. Its body width can be estimated by the width of the interior of the trail, and its body length was slightly more than that, meaning it was smaller than my fingernail. See that central groove? That’s from its tail, but if you want to impress your friends, call it a telson. (Photograph by Anthony Martin, taken on Sapelo Island, Georgia.)

Horseshoe crabs are so astounding that I could go on endlessly about all sorts of facts about them. Fortunately for you, gentle reader, other folks have written entire books about them and heaps of popular and scientific articles. (For starters, try going here.) So I don’t want to needlessly duplicate what others have done, and done well. Instead, I’ll focus on my main interest in these animals – their traces – and will regale you with tales of the traces they can make with their tails.

Horseshoe crab tails are spiky projections called telsons. Based on lots of the traces I’ve seen on the Georgia coast and a few direct observations, the main function of a telson is to help a horseshoe crab to get back on its feet after being knocked onto its back. That is, whenever a limulid is upside-down, it immediately start using its telson as a sort of sideways pole vault to lever itself into a less vulnerable position.

Without a telson, an upside-down horseshoe crab is stuck; its legs run furiously, but to no avail. However, with a telson, it can put the pointy end into the sand or mud underneath its body, and push itself up from a surface. This gives a limulid a fighting chance to get back to where it once belonged and start walking. This strategy works best if it turns to its right or left side, as limulids are longer than wide. They may be wonders of nature, but they’re not doing back flips or somersaults.

Limulid-Telson-Windshield-Wiper-TraceA large adult horseshoe crab that was right-side-up when trying to get back to the sea, got tired, and tried to use its telson to move itself along. In this instance, it didn’t work, but the traces made by the telson show its range of motion, working like a windshield wiper. (Photograph by Anthony Martin, taken on Sapelo Island, Georgia.)

OK, all of the preceding information I already knew. After all, I have: coauthored an edited book chapter about juvenile limulid traces and their close resemblance to trace fossils made by trilobites; coauthored another article on the history of limulid-trace studies (which go back to the 1930s!) that’s now in review; and devoted a lengthy section of a chapter in my book to limulids as tracemakers. So you could say I’ve been feeling pretty cocky about what I knew about these animals as tracemakers. That is, until one horseshoe crab showed me how much I still need to learn about them and what they can make.

The humility-inspiring traces showed up in a photo on a Facebook page I follow (and so should you), the St. Catherines Island Sea Turtle Conservation Program. The program organizers – Gale Bishop and Robert (Kelly) Vance – regularly add photo albums showing sea turtle traces (trackways, body pits, nests), and otherwise report on other facets of natural history they observe on St. Catherines Island beaches. As a result, I live vicariously through these pictures while marooned in the metro-Atlanta area. But they also like to throw me ichnological stunners once in a while, such as the following photo that Kelly posted last week.

Limulid-Telson-Trace-1Who needs made-up animals on TV when traces like these, made by awesome invertebrates like horseshoe crabs, turn up on a Georgia beach? (Photograph by Robert Kelly Vance, taken on St. Catherines Island, Georgia; scale is about 15 cm (6 in) long.)

Kelly found these traces while patrolling the beaches of St. Catherines Island for other traces, namely those of expectant mother sea turtles. Although these distracted briefly from his mission, I was very happy he stopped to document these, as I had never seen anything like them, despite much looking at traces on Georgia beaches.

The holes in the sand, defining a nearly perfect circle, were made by the telson of an adult horseshoe crab that kept on trying to right itself after landing on its back. Each puncture mark shows where it inserted the telson into the sand and then pushed itself up and to its side. Based on the number of holes, direction of sand flung out of each hole, and little “commas” made by extraction of the telson, it tried to flip itself a minimum of 16 times, and all to the right. These separate actions culminated in a 360° clockwise rotation of its body. Also check out the central depression with smaller drag marks; this is where its head shield was in contact with the sand. To imagine the movement represented by these traces, think of a horseshoe crab doing a slow-motion, step-by-step, break-dance backspin.

Seeing the evidence for such persistence was wow-inducing in itself, but in my ichnologically influenced euphoria, I figured the limulid finally succeeded in righting itself. After all, the trackway just to the left of the trace, indicates where it walked away from the scene of its gravitationally challenged situation.

But then I realized there was no “impact mark.” This large horseshoe crab flipping itself onto the sandy surface should have registered an outline of its body before it started walking. Instead, the place where it started walking showed no such impression, meaning it must have made a soft landing, with only its legs and telson digging into the sand. What happened? Did it use mind over matter and levitate itself through telekinesis? Or was it gently picked up and placed on its feet by a merciful mermaid? (Or merman: let’s make sure we’re being inclusive when talking about made-up stuff.)

It turned out that Kelly was the dues ex machina that entered this limulid’s drama, providing divine intervention just when it was needed. When I expressed my puzzlement to Kelly about how this large arthropod finally turned itself over, he confessed to saving it, in which he lifted it and put it back on its feet, where it promptly walked away in a series of tight spirals. The spiraling is something I’ve seen before in their tracks, a method used to find the downslope direction, which normally leads horseshoe crabs to the low-tide mark and the comfort of a watery environment.

Limulid-Telson-Trace-2Another perspective of the “escape” traces made by the limulid’s telson (background), but this time with its tracks, showing how it started spiraling clockwise in an attempt to make its way back to the sea. Check out those telson drag marks in the trackway, doing a little bit of back-and-forth movement as its owner walked. (Photograph by Robert Kelly Vance, taken on St. Catherines Island, Georgia.)

Limulid-Telson-Trace-3OK everyone, start singing “Born Free!” The spiraling helped this limulid (arrow) to find a downslope direction, which took it in the right direction to the sea. But it’s not all sunshine and lollipops for other limulids, some of which are visible in the background, and look like they’re still stuck. Given the tidal range on the Georgia coast – 2.5-3 m (8.2-9.8 ft) – strong wave energy, and wide beaches, lots of big limulids that come in with the flood tide get knocked onto their backs by waves and left behind. It’s almost as if some sort of natural selection is taking place, and something similar might have happened in the geologic past, affecting the evolution of its lineage. (Photograph by Robert Kelly Vance, taken on St. Catherines Island, Georgia.)

In the last photograph, I was glad to see how the story told by these traces promised a happy ending for this limulid that had so stubbornly tried to put itself back on its feet. Yet when you also notice how many of its compatriots did not make it back into the life-nourishing sea, it also serves as a sobering reminder that storybook endings don’t always happen in nature, and what we wish to be true sometimes isn’t.

In this instance, I don’t know whether this horseshoe crab made it back into the sea to live another day or not. Still, the lesson it left for us in the sand lives on, and I am now slightly more confident that if any limulids were stuck on their backs at any point in their 450-million-year history, made similar traces with their tails, and these marks were preserved as trace fossils, we just might recognize them for what they are. For that alone, I am grateful. Thank you, horseshoe crabs, for being real, making traces, and continuing to share this planet with us today.

(Acknowledgement: Special thanks to Drs. Robert Kelly Vance and Gale Bishop for being my ichno-scouts on St. Catherines Island, and feeding my mind with such tasty treats while I am landlocked.)

Further Reading

Brockmann, H.J. 1990. Mating behavior of horseshoe crabs, Limulus polyphemus. Behaviour, 114: 206-220.

Martin, A.J. Life Traces of the Georgia Coast. Indiana University Press, Bloomington, Indiana, 692 p.

Martin, A.J., and Rindsberg, A.K. 2007. Arthropod tracemakers of Nereites? Neoichnological observations of juvenile limulids and their paleoichnological applications. In Miller, W.M., III (editor), Trace Fossils: Concepts, Problems, Prospects, Elsevier, Amsterdam: 478-491.

Shuster, C.N., Jr., Barlow, P.B., and Brockmann, H.J. (editors). 2003. The American Horseshoe Crab. Harvard University Press, Cambridge, Massachusetts: 427 p.