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.

 

“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.

 

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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