Clues to the Past – What fossils tell us about ancient animal behavior

Featured Image Caption: Trilobites are extinct marine animals that were around about 540 million years ago. Researchers have found trilobite fossils in tight spaces where they might have been hiding from predators. (Source: Tim Evanson via Wikimedia Commons)

Reference: Hsieh, S. & Plotnick, R. (2020). The representation of animal behaviour in the fossil record. Animal Behaviour, 169, 65-80. https://doi.org/10.1016/j.anbehav.2020.09.010

What Did Sue Do?
Sue is a Tyrannosaurus rex on display at the Field Museum in Chicago, IL. Sue lived about 67 million years ago! (Source:  Zissoudisctrucker via Wikimedia Commons)

Growing up in Illinois, some of my favorite memories involved trips to the Field Museum, a natural history museum in downtown Chicago. Visit after visit, I was greeted at the museum’s entrance by Sue, a fossil replica of a Tyrannosaurus rex. Sue – named after the fossil’s discoverer – is the largest and most complete T. rex fossil ever discovered. I always marveled at Sue’s size but never really considered that fossils can tell us more than just how an organism looked. Researchers also use fossils to determine how ancient animals behaved and interacted with their environment. For example, one of the biggest mysteries in T. rex behavior is the purpose of their small forelimbs. With Sue’s help, researchers at the Argonne National Lab found that T. rex arms were likely not used for much at all after they x-rayed Sue’s small forelimb.

Animals change their behaviors based on what is happening in their environment. Throughout time, animals have experienced climate change, extinctions, and significant disturbances to their environment. Animals face the same threats today, many of which are caused by humans. If we know how animals adapted to these changes in the past, we can determine how these challenges may affect animal survival today. Such a mystery requires investigators skilled to uncover the clues to the past. These investigators include paleontologists and ethologists. Paleontologists are people who study fossils to learn about ancient organisms and how they behaved. Ethologists are people who study how living organisms behave.

Behaviors and Bones
Trace fossils (top) are impressions made by animals, whereas frozen behaviors (bottom) include the animal performing the behavior (Sources: Greg Willis via Wikimedia Commons (top) and JD Cooper via Needpix.com (bottom))

           Ethologists define behavior as the way a living organism responds to internal cues (hunger, fear, etc.) or external cues (weather, predators, etc.). Ethologists study a broad range of animal behaviors, such as how they look for food, defend against predators, communicate, claim territory, mate, and care for their young. Behavior is vital for the survival and reproduction of animals. As such, studying animal behavior is very important for understanding how animals adapt to changing environments. Often, ethologists observe the actions of the same individuals over some length of time to study their interactions with other organisms and with the environment.

            On the other hand, paleontologists are detectives who can only infer how animals behaved based on well-preserved fossils. There are two different types of fossils that paleontologists study: frozen behaviors and trace fossils. Frozen behaviors occurred when the organism was performing an action – such as mating, fighting, or eating – at the time of death (for example, insects found in amber). Frozen behaviors are highly reliable interpretations because the behaviors and the organisms performing those behaviors are present. Trace fossils appear due to the activity of an animal (footprints, burrows, nests). Trace fossils are less reliable because it can be challenging to determine what animal made the trace and what they were doing.

The First Behaviors in the Fossil Record

            Researchers Shannon Hsieh and Roy Plotnick at the University of Illinois at Chicago recently reviewed the evidence of animal behavior in the fossil record to unite the two fields. They determined the earliest occurrence of many behaviors studied by ethologists today. Some of the earliest behaviors described in fossil records occurred during the Ediacaran (635 to 542 million years ago), Cambrian (541 to 520 million years ago), and Devonian (416 to 358 million years ago) periods. Organisms undoubtedly performed behaviors before these times, but their actions were likely not well-preserved in fossils.

Trace fossil of feeding scratches made by Kimberichnus, an extinct mollusk (Source: Aleksey Nagovitsyn via Wikimedia Commons)

Ediacaran (635-542 million years ago) – During the late Ediacaran period, trace fossils show fan-shaped feeding scratches in the seabed thought to be made by an extinct mollusk-like animal (Kimberichnus). Researchers believe an unknown predator created circular holes on an extinct coral-like worm (Cloudina). There is also evidence that organisms produced u-shaped burrows as protection from predators during the Ediacaran.

Cambrian (541-520 million years ago) – During the Cambrian period, a species of a brachiopod (clam-like marine animals) mimicked the appearance of a foul-tasting sponge to trick predators. Small marine animals, Kunmingella and Fuxianhuia, cared for their eggs and juveniles, respectively. Some Cambrian organisms had antennae used to smell pheromones for synchronized molting.

Devonian (416 to 358 million years ago) – The first evidence of mutualism, a beneficial relationship between two different species, occurred during the Devonian. Researchers found an aquatic filter feeder (Leioclema) interlocked with a coral species (Aulopoa).

When paleontologists and ethologists work together, we can better understand how behaviors evolved and how the environment has influenced those behaviors. Many prehistoric events, including extinctions, global warming patterns, and disturbances, can inform today’s conservation efforts. Fossils like Sue the T. rex have incited wonder and curiosity in researchers and museum visitors alike. This admiration and interest will drive many more discoveries for the fields of paleontology and ethology.  

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Brandi Pessman

I am a second-year Ph.D. student at the University of Nebraska-Lincoln in the School of Biological Sciences. Growing up on a farm in a small town in Illinois, I developed an early love for animals and a fascination with their behaviors. When I was younger, however, it never crossed my mind that I would be using spiders to investigate how human presence affects animal behavior, but I am loving every second of it. Studying the behaviors of animals can tell us a lot about the role that we play in their survival (or death), which is becoming increasingly important as human populations continue to grow. When I am not studying spiders, I enjoy playing with my cat or being outdoors!

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