Poster P6 - Impact of Terrain and Gradient on Early Echinoderm Locomotion Using Paleobionic Approaches

Introduction: Understanding locomotion in early echinoderms has challenged paleontologists for decades. Species belonging to the enigmatic group of echinoderms - namely Pleurocystitidae, which existed 400 million years ago, do not have any suitable modern analogs to compare motion to and trace fossils that can provide significant evidence of movement. Studies with robots have been used to learn about the behavior of extinct animals, yet have primarily been conducted with rigid machines [3]. Some paleobionic studies of these early echinoderms have also explored the impact of stem morphology and gait on locomotion [1]. Because pleurocystitids are found in a range of substrates, from soft to firm ground, there has yet to be a paleobionic study into the effects of sediment texture and topographical gradient on their locomotion performance. Our team is implementing a paleobionic approach to investigate how different sediments, such as sand and mud, cause the pleurocystitid's behavior to change in terms of locomotion performance. Robotic studies that previously assessed substrate impact locomotion have focused predominantly on vertebrates [2]. Our team is also assessing how the topographical gradient could have affected the animal's locomotion.

Materials and

Methods: A robot reconstruction based on [1], known as the Rhombot V2 seen in Figure 1, is used for these studies. This version of the robot is designed to be slightly smaller than the original robot and has the ability to systematically change the angle the proximal stem makes with the theca. Indeed, in some glyptocystitid taxa, articulation between successive pairs of proximals may be differently oriented (even spirally arranged), allowing the stem to bend in various directions and angles. Images of the robot, as well as the flexural angle described, can be seen in Figure 1b. As seen in Figure 1d, the robot will be placed in an aquarium with AprilTags in the corners and a camera mounted above. By analyzing the movement of the AprilTag on the robot as it moves, the speed of the robot is acquired. The aquarium is also mounted on two jacks, as seen in Figure 1c. This would allow us to adjust the gradient the robot operates on, with a maximum angle of 17.5 degrees. We analyze the impact of sand, mud, and control surfaces on the robot's body lengths per second. We examine the effect of a flexural angle of 0, 10, and 20 degrees and adjust the test bed to have an angle of 0, 7.5, and 15 degrees.

Results, Conclusions, and Discussions: Results
The results of these experiments focus on comparing the cost of transport as well as body lengths per second to compare the impact of the main factors. The cost of transport indicates the amount of energy the robot had to expend in order to achieve the speed from the trial. Although this does not symbolize the metabolic energy the pleurocystitid would have used to accomplish this motion, this metric serves as a valuable means of comparing robot energy expenditure between different substrates and morphologies. Body lengths per second are necessary for determining which mediums and flexural angles the pleurocystitid moved the quickest compared to its size. This metric is used to compare soft robots to their biologically inspired counterparts.[4] The direction of motion is also a critical consideration. Although most evidence suggests the animal moved brachioles first, we want to see if moving in the stem direction to burrow is possible.
Conclusion
By comparing the variables of angle of surface, angle of stem in relation to theca, and medium properties, we can gain insights into the life of an extinct echinoderm that lived 400 million years ago and has no modern analogs or trace fossils. These insights will also serve soft roboticists who are interested in the behavior of single-limb locomotion under different terrain conditions.
Discussions
Paleobionics serves as a valuable tool for paleontologists to understand the motion of extinct animals and for engineers to learn from creatures that used to roam the earth. This study intends to further this field of study by illustrating how sediment texture, gradient, and morphology impact soft aquatic locomotion. Future work can include the paleobionic reconstruction of other enigmatic extinct echinoderm taxa, to understand their evolution of locomotion.

Acknowledgements (Optional): National GEM Consortium through the GEM Fellows Program (RD)

References
[1] Desatnik, R., Patterson, Z. J., Gorzelak, P., Zamora, S., LeDuc, P., & Majidi, C. (2023). Soft robotics informs how an early echinoderm moved. Proceedings of the National Academy of Sciences, 120(46), e2306580120.
[2] McInroe, B., Astley, H. C., Gong, C., Kawano, S. M., Schiebel, P. E., Rieser, J. M., ... & Goldman, D. I. (2016). Tail use improves performance on soft substrates in models of early vertebrate land locomotors. Science, 353(6295), 154-158.
[3] Nyakatura, J. A., Melo, K., Horvat, T., Karakasiliotis, K., Allen, V. R., Andikfar, A., ... & Ijspeert, A. J. (2019). Reverse-engineering the locomotion of a stem amniote. Nature, 565(7739), 351-355.
[4] Zhao, J., Zhang, J., McCoul, D., Hao, Z., Wang, S., Wang, X., ... & Sun, L. (2019). Soft and fast hopping–running robot with speed of six times its body length per second. Soft robotics, 6(6), 713-721.