Soft robotics informs how an early echinoderm moved

Abstract

The transition from sessile suspension to active mobile detritus feeding in early echinoderms (c.a. 500 Mya) required sophisticated locomotion strategies. However, understanding locomotion adopted by extinct animals in the absence of trace fossils and modern analogues is extremely challenging. Here, we develop a biomimetic soft robot testbed with accompanying computational simulation to understand fundamental principles of locomotion in one of the most enigmatic mobile groups of early stalked echinoderms-pleurocystitids. We show that these Paleozoic echinoderms were likely able to move over the sea bottom by means of a muscular stem that pushed the animal forward (anteriorly). We also demonstrate that wide, sweeping gaits could have been the most effective for these echinoderms and that increasing stem length might have significantly increased velocity with minimal additional energy cost. The overall approach followed here, which we call "Paleobionics," is a nascent but rapidly developing research agenda in which robots are designed based on extinct organisms to generate insights in engineering and evolution.

Keywords: Paleobionics; locomotion; paleontology; pleurocystitids; soft robotics.

Fig. 1.

“Rhombot” rhombiferan robot based on morphology of the pleurocystitid. The stem is composed of a soft elastomer embedded with shape memory alloy (SMA) muscle wire for flexural actuation. The soft robot stem is designed based on 3D scans from pleurocystitid fossils.

Fig. 2.

Soft robotics engineering the movement of early echinoderms. (A) Phylogenetic position of modern echinoderms [after (25)], including the extinct blastozoans as a sister group of crinoids. (B) Phylogenetic tree of the earliest blastozoans with the position of Pleurocystites shown in yellow (18) Free-living forms derive from a sessile, permanently fixed blastozoans. (C and D) Pleurocystitid specimens from the lower Katian (Upper Ordovician) of Brechin, Ontario, Canada; (E) Tomographic reconstruction of a proximal stem. (F) Differentiable discrete mechanics simulation; (G) Optimized trajectory from iLQR; (H) Simulation data of direction of motion for various morphologies; (I) Rhombot, soft robotic representation of the pleurocystitid untethered in a local pond; (J) Gait analysis data of the pleurocystitid robot using a rigid stem and wide sweeping gait.

Fig. 3.

Trajectory optimization results for the pleurocystitid body plan. (A) Rendering of the pleurocystitid body plan showing critical elements. (B) Kinematic representation used for the idealized pleurocystitids in simulation. (C) Snapshots of the brachioles-first gait (which is quite similar in character to the stem-first gait). (D) Comparison of optimized gaits for several configurations while attempting to move in either brachioles-first or stem-first directions. Configurations include a fully actuated stem (blue, yellow, light blue, and orange) and a proximally actuated stem (purple and green). (E) Comparison of optimal gait for stem-first (Top) and brachioles-first (Bottom) motion. The trials occurred over the same amount of time, and thus, the displacements reflect average velocity. (F) Velocity for various stem lengths based on gait depicted in C. (G) Velocity of optimal gaits optimized for each stem length.

Fig. 4.

Gait analysis of Rhombot with different stem sizes, sculling width, and distal stem flexibility. (A) Image of the robot showing its size scale. (B) Image of the Rhombot at different timesteps as it moves along the aquarium bed. (C) Graph of the differences in speed of the stiff stem in cm/min with a wide and narrow sweeping gait. (D) Graph of the speed of the Rhombot in the flexible stem configuration with a wide and narrow gait. (E) The normalized cost of transport (COT) measurements of the stiff stem layout. (F) The normalized COT measurements of the flexible stem.