Minute Trace Fossils Offer Major Implications For Extinction Recovery

Large body fossils of extinct creatures capture our imagination.  It’s understandable.  These were fascinating behemoths, and we can see something of their life in the bones that remain. While our collective attention might be focused on these very big things, researchers published a paper this past November that centered on some very tiny things.  And what they found has enormous implications for our understanding of ancient life.

Fossi leaf with insect damage - Michael Donovan

Insect feeding damage on a fossil leaf, including holes and a leaf mine (bottom right), made by a larval insect that fed on tissue within the leaf. The fossil is 67-66 million years old and from the Lefipán Formation in Patagonia, Argentina; photo and caption courtesy of Michael Donovan.

 

The authors Michael Donovan, Ari Iglesias, Peter Wilf, Conrad Labandeira, and N. Rubén Cúneo studied trace fossils of insect feeding damage on over 3000 fossil leaves from Patagonia (an area that encompasses the southern part of Argentina and Chile).

Remarkably, fossil leaves number in the tens of thousands in the Western Hemisphere alone.  But studying them for insect damage during the end Cretaceous and early Paleocene is relatively new.  Keep in mind that the end Cretaceous marked the last mass extinction this planet has known thus far.  The early Paleocene marks the time when life was, however slowly, working its way back into existence.

There is a preponderance of fossil leaves in the western interior North America (WINA) from this time period, and they have been studied.  In “Rapid recovery of Patagonian plant-insect associations after the end-Cretaceous extinction” published in Nature Ecology and Evolution, the authors compared the relatively smaller number of fossil leaves in Patagonia to the much larger numbers of such leaves from WINA.

What interested them was the diversity of insect damage to these Patagonian plant leaves.

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Tiny insect piercing and sucking marks on a fossil leaf from the fossil locality Palacio de los Loros 2 in Patagonia, Argentina (approximately 64 million years old). Piercing and sucking damage is made by insects that use their straw-like mouthparts to feed on fluids from within plants; photo and caption courtesy of Michael Donovan.

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Close up of the picture above; photo and caption courtesy of Michael Donovan.

The type of insect damage—the different ways insects fed upon a leaf–relates to the diversity of insects. That diversity of herbivorous insects, in turn, relates to a much larger food web.  In other words, the traces these ancient insects made indicate that there was a growing population of different types of insects. That growing population suggests a growing, thriving food web.  Life in Patagonia, after the last mass extinction, may have been returning at a much faster rate than its northern counterpart.

Embed from Getty Images

 

“If we’re just looking at the raw numbers, there are way more fossils, but less insect-damage diversity,” explained Michael Donovan in a phone interview referring to the WINA fossil leaf damage.  “In the Western US, there’s around almost 20,000 leaves included in those data sets. Maybe a little less.” He chuckled. “And that’s compared to the 3,646 [fossil leaves] in Patagonia. So, it’s a big difference!”

“We can’t always say exactly what insects were making the damage,” he wrote earlier in an email.  “During this study, we found many different kinds of damage representing the work of a wide range of plant-eating insects. Some types of damage can be made by a variety of insects. For example, many different kinds of insects with chewing mouthparts, such as beetles or grasshoppers, can create holes in leaves by feeding through the plant tissue. Other types of insect damage provide more specific information about the culprit. Leaf mines, for example, are made by larvae of some species of moths, flies, wasps, and beetles. The mines act as a detailed record of the behavior of the insect, which we can use to infer the type of insect that may have made the mine.”

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View of an excavation at the Palacio de los Loros 2 fossil plant locality in Chubut, Patagonia, Argentina. The fossils there were formed in the early Paleocene around 64 million years ago;photo and caption courtesy of Michael Donovan.

 

Michael was the one responsible for studying these 3,646 fossil leaves to see if any had any damage to begin with, and then to see whether that damage may have been insect-related.  (In a nod to how I may have organized such things, I wondered whether museum collections separate out fossils with traces of damage.  They do not. Or rather, as Michael explained, “How they are organized usually depends on the collector or museum. The collections used in this study are organized by plant morphotype/species. To collect the data, I inspected all of the leaf fossils under a microscope for insect damage.”)

But how can one determine the difference between disease-related traces and insect-related traces in a fossil leaf?

“One good thing to look for is reaction from the plant to the insect damage,” he answered.  “So, for example, if an insect chews through a leaf and makes a hole, [scar] tissue [will form] around the edges of the hole. On the fossil, it looks like a little dark area surrounding the hole.  That’s where the plant healed itself after the damage was made, and that shows that [the insect ate the leaf] when the plant was still alive. If it happened when the leaf was dead, it wouldn’t form that scar tissue. So if there’s something like a tear that was made when the leaf was already dead, reaction tissue wouldn’t form. Then some other types of damage are very distinctive, such as leaf mines, and look very similar to damage we see on modern leaves.”

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Skeletonization (feeding on leaf tissue between leaf veins but leaving the veins intact) caused by a plant-feeding insect. The leaf is from the Palacio de los Loros 2 fossil plant locality in Patagonia, Argentina (approximately 64 million years old); photo and caption courtesy of Michael Donovan.

 

Their research determined that there is a greater diversity of insect-damage to fossil leaves in Patagonia, and that this diversity occurred 4 million years after the meteorite crashed into Earth at Chicxulub, Mexico.  Contrast this to the western interior North America, in which insect-damage indicates that same recovery took 9 million years.

“The fossil plant collections that we studied were collected relatively recently by my coauthors (Ari Iglesias, Peter Wilf, and Rubén Cúneo) and other scientists as part of a larger research program on Patagonian fossil floras from the end of the Cretaceous through the Eocene,” Michael described. “The Paleocene floras have been dated with a variety of methods, which show us that the fossil sites were formed during three time slices in the early Paleocene. Using these dates, we were able to observe how plant-insect associations in Patagonia recovered in the 4 million years after the end-Cretaceous asteroid impact.”

Co-authors Conrad Labandeira and Peter Wilf were part of a 2014 study published in PLOS One (“Insect Leaf-Chewing Damage Tracks Herbivore Richness in Modern and Ancient Forests,” also by Mónica R. Carvalho, Héctor Barrios, Donald M. Windsor, Ellen D. Currano, and Carlos A. Jamarillo) in which extant insect leaf damage was correlated to the larger food web of two tropical rainforests.  The variety of insect traces on today’s leaves represents a healthy variety of insect species.  Like keystone species in any ecosystem, these traces indicate a thriving web of life.

Embed from Getty Images Embed from Getty Images

 

How remarkable to then extrapolate that insects so many millions of years ago, simply eating the leaves available to them in the Southern Hemisphere, can offer important clues to the state of life after the devastation our planet endured.  The traces of these tiny creatures—and the fragile plants that survived fossilization—are extraordinarily significant.

“It was pretty exciting to see what was happening in another part of the world,” Michael enthused.

When asked why fossil leaves and insects interested him, he responded, “Plants and insects are the most diverse multi-cellular organisms on Earth, and their interactions are important components of food webs on land. By studying insect feeding damage on fossil leaves, we can learn how insects and plants responded to major environmental changes in the past and have a better idea of how they may be affected in the future.”

“This is what I’m interested in continuing doing. This is a relatively newer field within paleontology, so there are lots of projects to pursue, lots of periods of time in the ancient past where we don’t know much about how insects and plants were interacting.”

“The Cretaceous-Paleogene extinction was a major event in the history of life and the most recent of the big mass extinctions. The plants and animals that we see today are all descended from organisms that survived this asteroid impact. We observed a faster recovery of plant-feeding insects in the Southern hemisphere—in Patagonia—compared to the Northern hemisphere—[in WINA.]  These patterns from the early Paleocene may be related to biodiversity patterns that we see today.”

 

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Leaf mine made by a larval insect that fed on tissue within the leaf. The fossil is ~65 million years old and from the Palacio de los Loros 1 fossil site in Patagonia, Argentina; photo and caption courtesy of Michael Donovan. 

 

An absolutely ENORMOUS thank you to Michael Donovan for making so much time to answer my questions, both in email and by phone.  The number of pictures he sent, and their detailed captions, was AMAZING.  I did not include them all here. I encourage you to read the paper done by him and his colleagues to see how many and beautiful they are. THANK YOU, MICHAEL!!

 

References:

  1. Donovan, M. P., Iglesias, A., Wilf, P., Labandeira, C. C. & Cúneo, N. R. Rapid recovery of Patagonian plant–insect associations a er the end-Cretaceous extinction. Nat. Ecol. Evol. 1, 0012 (2016).
  2. Carvalho MR, Wilf P, Barrios H, Windsor DM, Currano ED, Labandeira CC, et al. (2014) Insect Leaf-Chewing Damage Tracks Herbivore Richness in Modern and Ancient Forests. PLoS ONE 9(5): e94950. doi:10.1371/journal.pone.0094950
  3. Monocots versus Dicots, University of California Museum of Paleontology
  4. Museo Paleontológico Egidio Feruglio, Trelew, Argentina
  5. Check out more research done in Patagonia! Patagonia Paleofloras Project

 

Museo Paleontológico Egidio Feruglio

Museo Paleontológico Egidio Feruglio, home to the fossil leaves used in this paper and many other exciting fossils; photo by Pedrochubut (Template:MEF Photo) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)%5D, via Wikimedia Commons

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Meet Dr. Katy Smith – Mastodon Detective

If you imagine the Great Lakes region over 10,000 years ago, you might see large, hairy beasts with relatively straight tusks grazing around boggy areas or moving within dense forests.  Their fur and overall appearance might cause you to confuse them with woolly mammoths, but these are the mammoths’ shorter, stockier cousins.  And if any of them would let you get close enough to inspect their mouths, you’d see in an instant that their teeth are completely different than those of mammoths.

 

[image of contemporary boggy area in Alaska, courtesy Getty Images]

 

Whereas mammoths are believed to have eaten grasses and even flowers, mastodons needed teeth suited to the mastication of hardier stuff: shrubs, parts of trees, perhaps pinecones?   Mastodon teeth, with the bumps and ridges one might associate with carnivores, are easily recognizable as ‘teeth.’  Mammoths, in contrast, needed to grind food, producing teeth with spherical lengths of ridges across each tooth.

ISM - Mastodon tooth

 

[image courtesy of Ron Richards, Indiana State Museum, for this post: Mammoths and Mastodons in Indiana – Part 1.  Can you tell which tooth belongs to which species?]

 

ISM - Mammoth tooth

 

[image courtesy of Ron Richards, Indiana State Museum, for this post: Mammoths and Mastodons in Indiana – Part 1.]

And while woolly mammoths pervade popular culture and interest, there are some, like Dr. Katy Smith, Associate Professor of Geology at Georgia Southern University and Curator of the Georgia Southern Museum, who prefer their lesser-known cousins and have made fascinating contributions to our understanding of them.

Mastodon discoveries usually produce the fossils of a single animal, and rarely a complete skeleton. Rarer still, finding skeletal remains of multiple mastodons at the same site.

Such a unique discovery occurred in 2005, when more than 300 fossils were found in Hebron, Indiana.  Now known as the “Bothwell site,” it was originally going to be the location of the landowner’s pond.  Instead, Indiana State Museum paleobiologist Ron Richards and his crew uncovered bones that included numerous mastodons (Mammut americanum), giant beaver (Castoroides) and hoofed animals with even-toes (artiodactyls).

ISM - 2005 Bothwell Mastodon 2

 

ISM - 2005 Bothwell Mastodon 1

[images of the Bothwell site dig, courtesy of Ron Richards, Indiana State Museum, for this post: Mammoths and Mastodons in Indiana – Part 2.]

 

Four years later, the Bothwell site became the focus of Katy Smith, her dissertation, and two subsequent papers she co-wrote with Dr. Daniel Fisher at the University of Michigan.

But let’s take a moment to consider what paleontologists uncover. However rudimentary this may seem, it is important to note that bones are generally not discovered in neat order, intact and with each skeletal component attached where it would have been in the life of the animal.

Consider, too, that not all bones survive.  And those that do are often broken or in terrible condition.

So even at a site such as Bothwell, which produced lots of fossils, a paleontologist’s job is no less challenging.  The pieces of information are incomplete, mere clues to the animals that died there.

The questions, however, are profuse.

Why were so many animals found in that one spot?

If, as it is currently debated, mastodons shared behavioral traits with modern-day elephants, was this a family unit?

If so, was this group—like elephants–comprised largely of female and juvenile mastodons?

And why were other unrelated animals discovered among them?

Did a sudden disaster kill them all?  Were humans involved?

 

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Sexual dimorphism is another way of referring to the traits that make an animal either female or male.  Some of us would assume, since mastodon pelvic bones were not among the Bothwell fossil assemblage, that the sex of these animals would remain unknown.

There were 13 mastodon tusks, only four of which were complete. And this, remarkably, is what prompted Katy Smith’s research.

“I wanted to know if I just had tusks, what can I do to figure out if I’m looking at a male or a female,” she explained by phone.

Katy Smith - measuring an African elephant tusk

 

[image of Dr. Katy Smith measuring an African elephant tusk in (what this author believes must be one of the greatest places on earth) the basement and fossil collection of the University of Michigan; courtesy of Dr. Katy Smith]

 

“Other people have looked at [sexual dimorphism], but I wanted to look at it specifically with the Bothwell mastodons, because they were inferred to be female, and female mastodons are less common in the fossil record than males.

“When I presented preliminary results from my research in a paleontology class, the professor said, ‘Why don’t you try multivariate analysis?’ And it just kind of spiraled from there.”

Multivariate analysis,’ as the name implies, means using more than one type of measurement or observation towards a hypothesis.  In other words, rather than simply using size as a determination of sexual dimorphism, applying numerous methods and statistics that support or disprove it.

Already, the amount of information scientists have pulled from tusks alone is fascinating.

Tusks are teeth.  They are described, in Dr. Smith’s dissertation as “hypertrophic incisors.” And, unlike human teeth, they continue to grow the entire life of the animal. So where we can simply look at a human tooth and know immediately whether it is from an adult or a child, the same cannot be done with tusks.

What their hardy structure records includes the age of the animal, growth in winter or summer months each year, their overall diet, and periods of nutritional stress.  (As described in an earlier post, Proboscidean molars can even provide details regarding where they roamed during life.)

But much of this information can only be gleaned from well-preserved, intact tusks, as well as from cutting into and examining their chemical composition.

“If you don’t know what the sex of the animal is before you look at tusk microstructure,” she said, “it can be hard to interpret what you’re looking at.”

Part of what Dr. Smith hoped to discover were similarities in the tusks where sex and age had already been determined.  If certain structural elements were the same across female mastodon tusks, such that they tended to differ from male mastodon tusks, this might help determine sexual dimorphism in future tusk discoveries.

She also hoped to discover any similarities between the tusks of extant elephants and mastodons.

Katy Smith -longitudinally bisected tusk

 

[image of longitudinally bisected tusk, courtesy of Dr. Katy Smith] 

 

Thus, she studied and measured tusks of both species from numerous museum collections. (Asian elephant tusks were not used, as female elephants of this species tend to have either tiny tusks or no tusks at all.)  She rather amusingly refers to the approximate amount of tusks involved as “5,000 pounds of tusk.”

Her dissertation and the two papers describe the type of analysis performed in detail.  Among them were canonical variates analysis (CVA) and discriminant function analysis (DFA).

“Fortunately, we didn’t have to cut into the tusks to do those measurements. You just insert a stiff wire into the pulp cavity.”

“We think about tusks sometimes as stacks of sugar cones, because they actually grow in a kind of [layered] cone structure. So you think about one sugar cone, and then you put another one inside that one and then another one inside that one and so on and so forth. And the last sugar cone is empty. There’s nothing in it. That represents the pulp cavity.”

“[Analyzing the] pulp cavity is probably one of the best single measurements that you can use to distinguish between male and females. [I]n females, that pulp cavity will terminate before the gum line, and in males, it will terminate after the gum line, closer to the tip.

“This is something that we saw in almost every mastodon. So it was kind of cool.”

 

Katy Smith - female mastodon

 

[image of female mastodon skull and tusks, courtesy of Dr. Katy Smith]

 

“If we could have cut every tusk, I would have,” she admitted, and laughed. “But it was a matter of collecting these measurements at different museums. And so I would just go there and collect all of them, and that was how we’d get the pulp cavity depth.”

“I’ve always been interested in paleontology,” she said when I asked her how she got started.

“I’m one of those kids who just never grew out of it. My parents used to take me to the museum all the time, and I used to spend hours and hours staring at the dinosaur dioramas there, just loving it.  I told my kindergarten teacher I wanted to be a paleontologist. I never changed! My 5-year-old self grew up and became a paleontologist.”

But her interests moved away from dinosaurs when she realized that their fossil record in Wisconsin, her home state, was rare to nonexistent.

After all, she said, “I started just wanting to explore what was underneath my feet.”

It wasn’t until grad school at Michigan State, where she met the late Dr. Alan Holman, that she realized her passion for mastodons.  His own interest in the species was infectious, and it was through him that she learned of the numerous mastodon (Mammut americanum) fossil discoveries in the area.

“Wow!” she said, recalling her initial reaction. “There are over 300 mastodons in Michigan. This is exciting!”

Katy Smith - male mastodon

[image of male mastodon skull and tusks, courtesy of Dr. Katy Smith]

Not surprisingly, she did her PhD work at the University of Michigan, home to Proboscidean expert Dr. Daniel Fisher, who was her advisor.

“I wanted to work with him,” she explained, “because I wanted to continue working on mastodons, and he had a couple of ideas for projects. One of them included this assemblage of mastodons from Indiana, which were—supposedly—all female.”

What she discovered regarding the Bothwell site is both thought-provoking and fascinating:

  • 8 tusks were determined to be female; the other 5 are unknown
  • the ages of the mastodons range between 19 and 31 years old
  • there is evidence that at least one juvenile might have been among them (a “juvenile tooth crown” was found)
  • given that two mastodons died in winter, and another two died either in late summer or early autumn, this indicates that the collective deaths of these animals didn’t happen at the same time (hence, not a single event)
  • none of the mastodons appeared to be under nutritional stress when they died
  • members of a family unit would be expected to have the same “isotope profiles”–chemical signatures in their teeth–but these do not

Based on the evidence provided, Dr. Smith wonders whether these animals were part of a meat cache for humans (members of the Clovis culture) that co-existed at that time.

But perhaps the single most remarkable result of her research is helping other paleontologists–who often have nothing more than a single tusk–determine the sex of that animal using her different types of analysis.

Prior to her dissertation, only one female mastodon tusk had been analyzed for growth rate.  To date, I am unaware of any other publication (paper or book) that helps detail the sexual dimorphism in mastodons by tusks alone.

When I remarked upon this, I asked her if others had cited her work.  Her response, after stating that others had, was equally fascinating to me.

“It’s always the hope as a scientist that you’re contributing in some way,” she said, “and you know that you’re contributing if somebody else is using what you’ve done.”

 

An enormous and sincere THANK YOU to Dr. Katy Smith for her generous and fascinating answers to my many questions, her gracious help when I had trouble understanding certain points, and for being so much fun with whom to connect! I cannot express how much I wish I could attend her classes, nor how fascinating I found her dissertation. I am profoundly grateful that she shared it with me!

A sincere thank you to my Dad, as well, for helping me understand tooth components (i.e.: dentin, cementum)!

**A quick reminder that I am neither a scientist nor a paleontologist, so any errors in this post are my own.

Bothwell Mastodont Dig, courtesy of Indiana State Museum; many thanks to Bruce Williams and Leslie Lorance!

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References:

 

Other references:

 

Cohoes mastodon size comparison

[image of sign in the NY State Museum illustrating the size difference between an extant elephant, a woolly mammoth and the Cohoes mastodon; picture taken by the author]