Fossil Visions in the Two Medicine

Posted on August 4, 2014 by TMartin

(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.

This 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.

Originally 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.)

It’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.

These 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.)

These 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.)

These 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.)

These 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.)

One 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.)

This 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.)

A 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.)

This 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.)

Here’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.)

What 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.)

Check 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-Medicine You’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.)

The 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.)

Can 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.

The 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.

Burrowing Wasps and Baby Dinosaurs

Posted on July 21, 2014 by TMartin

Anyone who knows a little bit about dinosaurs knows that some of them made nests, took care of their young, and that their parenting skills must have been more like birds, rather than most reptiles. If pressed, most dino-enthusiasts can further say this concept is exemplified by two dinosaurs, the large ornithopod Maiasaura and the small theropod Troodon, both of which lived at the same time and place, 75 million years ago and in what we now called Montana.

But what animals lived beneath the nests and feet of those dinosaur parents and their babies? What behaviors did these animals express 75 million years ago? Would the behaviors of these animals have resembled those of ones living today, or did they reflected evolutionary dead-ends? And did these animals also take care of their young?

Whoa, check out this female Carolina sand wasp (Stictia carolina), energetically digging an inclined burrow into a Georgia coast dune! Why is she digging a burrow? To make a brooding chamber for her babies (larvae), who will hatch from their eggs and chow down on paralyzed prey stuffed into that chamber by their thoughtful mama. Gee, I wonder if any wasps did this in the geologic past? (Photograph by Anthony Martin, taken on Tybee Island.)

Why, yes, they did. That’s a fossil cocoon connected to an inclined burrow, reflecting a behavior much like that of modern sand wasps, but preserved in the Late Cretaceous Two Medicine Formation of central Montana. (Photograph by Anthony Martin.)

The answers to these questions are, in order: insects (wasps and beetles; most likely), burrowing and reproduction; they behaved very much like modern insects, and they likely did take care of their young by making brooding chambers and leaving food for their offspring. In my experience, these revelations surprise many people, who may not be aware of how many of the insects we live with today are descended from insects lineages that shared the same ecosystems with dinosaurs throughout the 165-million-year history of the latter animals.

This summer, for me to learn more about life underground way back then, I had to go to the same site in central Montana where our understanding of dinosaur parenting became better defined, but also where I first learned how insect parenting related to dinosaur parenting. Where I am now is the same general location where the first known dinosaurs nests in North America were found in the late 1970s by Jack Horner and his friend Bob Makela (mentioned in my previous blog post).

One of the most productive and interesting of these nest sites, which are all in the Late Cretaceous Two Medicine Formation, was informally dubbed “Egg Mountain.” The “Egg” part of the moniker is easy to understand, but the “Mountain” part is more of an exaggeration, as it’s an isolated and modest hill on the high-plains landscape of central Montana. Anyway, I’m working there now, along with a dedicated crew of rubble pickers being led by the ever-intrepid Dr. David Varricchio.

A snapshot of science in process at Egg Mountain in central Montana. Dr. David Varricchio (center, with jackhammer) has been leading an NSF-sponsored study of the fossils at this site, with the hope of understanding more about nesting dinosaurs and the animals that lived around them. Rubble pickers for scale. (Photograph by Anthony Martin.)

So why would an ichnologist like me care about a site that is famous for its mere body fossils, consisting of many dinosaur eggs, eggshells, and bones? I’ll start with three words: dinosaur nest structure. This is where the first known dinosaur nest structure – which is a trace fossil – was recognized. The structure was a rimmed depression about the size of a kiddie pool, but a little more shallow. In the center of this depression was a clutch of eggs belonging to the small theropod Troodon. The width of the nest was perfect for accommodating an adult Troodon, which probably sat above the egg clutch to protect and incubate it.

Here’s the first known dinosaur nest structure, as it looked soon after its discovery in the mid-1990s. The rim is composed of limestone, but originally was soil compacted and shaped by either one or both Troodon parents. The white part is plaster of Paris covering the egg clutch, which was aligned with the dead center (pun intended) of the structure. Tape measure shows 1 m (3.3 ft). Photograph was probably taken by David Varricchio, and is from Varricchio et al. (1999), Journal of Vertebrate Paleontology, v. 19, p. 91-100.

My artistic recreation of this same rimmed Troodon nest structure with its egg clutch in the middle. The inner part of the structure – inside the rim – is about a meter wide. (Artwork by Anthony Martin, from Dinosaurs Without Bones (2014), which you should buy so I can better afford to do more research like this and blog about it for you.)

What’s even better about this find – ichnologically speaking – is how the parent dinosaurs must have moved the eggs after the mother laid them, and then partially buried them upright in soil. These eggs are elongate, which means they would have reclined if laid by a mother Troodon. Instead, they were nearly vertical, which means either the mother or father dinosaur manipulated these eggs after they emerged from the mother dinosaur. Thus this orientation is also a trace fossil of parental dinosaurs that were greatly increasing the chances their future offspring would stay alive.

Bottom view of the Troodon egg clutch from that nest structure, with these elongate eggs in nearly vertical positions, and aligned along a central axis. These arrangements of the eggs are trace fossils, too. Want to see this clutch for yourself? It’s is on display in the Museum of the Rockies in Bozeman, Montana. (Photograph by Anthony Martin.)

Now let’s leave dinosaurs for a moment and talk about something that really matters, like insect trace fossils. What is well known by those who have worked at Egg Mountain is that the dinosaurs there were not alone. Just below the dinosaurs’ nests, egg clutches, and feet were insects, and lots of them, shown by numerous cocoons. In a few places near Egg Mountain, these exquisitely preserved cocoons – most with their spiraled weave patterns still visible – are so common, you can close your eyes and scoop up a handful of them.

Fossil insect cocoons from the Two Medicine Formation and a locality near Egg Mountain. The cocoons on the left and right are ichnological two-for-one specials: the left one has a partial burrow attached to it, and the right one has an emergence trace (top) from where the adult insect said goodbye to its cocoon 75 million years ago. (Photograph by Anthony Martin.)

In an article I coauthored with David Varricchio in 2011, we concluded that most of these insect cocoons were likely from burrowing wasps, and the rest may have been from beetles. The trace fossils reflect a unexpectedly modern behavior in these Cretaceous wasps, which dug inclined tunnels that led down to enlarged brooding chambers. These insects laid eggs in the chambers and stocked them with provisions, which may have been paralyzed prey, such as other insects or spiders. Later, larvae hatched in the chambers, ate whatever Mother Wasp left for them, made cocoons around themselves once they decided to stop being so larval, pupated, burst out of their cocoons when they became adults, and emerged on the surface.

My simple depiction of a burrow and pupal chamber made by the solitary Carolina sand wasp (Stictia carolina). These traces consist of inclined tunnels that end in enlarged chambers, the latter of which accommodate eggs, food, and eventually larvae and cocoons. Scale = 10 cm (4 in). (Illustration by Anthony Martin, which is in Life Traces of the Georgia Coast (2013), which you should buy so I can better afford to do more research like this and blog about it for you.

Close-up of the burrow end – filled with sediment, but now rock – leading to a cocoon, still preserved in its pupal chamber in the Two Medicine Formation, from about 75 million years ago. Compare this to my illustration of a typical modern sand-wasp burrow, especially the end part of it. Notice the resemblance? (Photograph by Anthony Martin.)

However, most of the fossil cocoons in the Two Medicine Formation did not make it past the pupal stage. How do we know this? Because some of these outcrops have thousands of cocoons that are perfectly preserved as beautiful ellipsoids, with no sign that an adult insect emerged from them. One of the axioms of paleontology is that each animal’s tragedy of the past can some day fulfill a paleontologist’s dreams. Thus these thousands of dead Cretaceous wasps are providing me with much joy this summer, as I study these trace fossils for more clues about their lives and how they related to the ecosystems they shared with adult and baby dinosaurs.

A picture of one happy ichnologist, who is giving thanks for all of those insects that died and had their burrows and cocoons fossilized in the Two Medicine Formation for him to study. Thanks, insects! Thanks, geology! (Photograph taken by Ruth Schowalter in central Montana.)

But here’s what really cool about Egg Mountain: it has both dinosaur nests and insect nests, implying that wherever these insects nested, so did the dinosaurs. As a result, their co-occurrence gives us a glimpse of the ecology of those places at that time, a window into the past landscapes in which they lived and bred. This makes sense when you imagine how both these dinosaurs and insects wanted to keep their eggs out of water, so they placed them in high-and-dry areas, such as well-drained soils well above the water table. So as we gather more information from this site, we get ever-better insights in the cycles of life for both Cretaceous insects and the dinosaurs that happened to live in their world.

Life Traces of the Georgia Coast

Unseen lives of the Georgia barrier islands

Tag Archives: insect traces

Fossil Visions in the Two Medicine

Burrowing Wasps and Baby Dinosaurs

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