Locomotory Behaviour of Early Tetrapods from Blue Beach, Nova Scotia, revealed by microanatomical analysis
Kendra I. Lennie, Sarah L. Manske, Chris F. Mansky and Jason S. Anderson
Summarized by Makayla Palm
What data were used? Previous research has analyzed possible moving mechanics for the first tetrapods that lived on land (i.e., a four-limbed vertebrate), but most of the conclusions were made in inference, like by analyzing footprints. The researchers of this study aimed to find more direct evidence of how these early reptiles like Tiktaalik or Ichyostega moved in order to determine what lifestyles new fossils from Blue Beach, Nova Scotia had (aquatic/land). In order to test their hypothesis, they studied the limb bones of the new fossils and living creatures like cats and platypi in order to observe how these limb bones adapted to the stresses of gravity and hitting solid ground. The scientists used 3D scans of bones from both the modern and the fossil tetrapods; the living ones had a range of lifestyles from aquatic to terrestrial, for better comparison to the fossils.
Methods: The researchers took 3D scans and measured the volume of limb bones from eight extant (or living species) and five extinct species (the fossils from Blue Beach, Nova Scotia). This information would give them the ability to tell how, or if, these creatures walked. The extant species were studied in order to observe how and where muscles were stressed during walking (and what clues that left behind in bone) in living creatures to find what patterns to look for in the fossil specimens; this created what is called a compactness profile. The compactness profile summarizes how the different tissues in the bones react to stress over time by observing the amount of trabecular tissue in a certain part of the limb bone. Trabecular bone is a kind of bone tissue that is made of tiny plates meshed together. The trabecular tissue arranges itself where the bone experiences the most stress; this is the pattern being observed in the compactness profile. The bones from each specimen were digitally sliced in a cross-section to observe the internally visible trabecular bone. The researchers observed the trabecular bone in extant species first because their moving mechanics are known. Once they established the pattern of where the trabecular bone was in extant species, they applied it to the extinct species to determine their moving mechanics.
Results: The trabecular bone’s location shows where the most stress is being absorbed in the bones (think of swimming and the different muscle groups used in contrast to walking- a long walk and a long swim will leave one sore in different ways.) The study shows different stress in the trabecular bone across taxa, depending on if the creature was aquatic or land-living. They concluded that the aquatic species had trabecular bone in the midshaft, or middle of the bone because they would pump their legs while swimming. Terrestrial, or land-based tetrapods, had trabecular bone around the two ends of their femurs, indicating they walked. Some of the fossil tetrapods had less dense trabecular bone than some of the extant species, but it was at the ends of the limb bone; researchers concluded these fossils would have lived a semi-aquatic life (a modern alligator is semi-aquatic, for example). Based on the results of this study, it is likely that the Blue Beach tetrapods represented a range of different lifestyles, from fully aquatic, and semi-aquatic, to fully terrestrial, as all of the patterns of trabecular bone described above were found in the different taxa.
Why is this study important? The study of tetrapod locomotion, or movement mechanics, reveals how the earliest known walking creatures lived and moved. Previous research used proposed ideas on locomotion by inferring muscle and ligament placement on the limb bones of the tetrapods. This study uses direct evidence by looking at how the limb bones react to stress to determine how these creatures moved in various environments.
The big picture Researchers are using tissue evidence in order to better understand how the earliest walking tetrapods walked. The tissue, or trabecular bone, helps researchers see direct evidence of walking, rather than relying on inferred information about soft tissues. The analysis of trabecular bone is direct evidence for locomotion because it is re-arranged by stresses from gravity. The ability to observe these changes in soft tissue depending on lifestyle is a definitive classification of lifestyle for these early risers. Rather than saying “these creatures had the ability to walk”, the researchers are saying, “these creatures did walk.”
Article Citation: Lennie, K. I., Manske, S. L., Mansky, C. F., & Anderson, J. S. (2021). Locomotory behaviour of early tetrapods from Blue Beach, Nova Scotia, revealed by novel microanatomical analysis. Royal Society open science, 8(5), 210281.