The Role of the Tail or Lack Thereof in the Evolution of Tetrapod Aquatic Propulsion

Abstract

Synopsis Secondary aquatic vertebrates exhibit a diversity of swimming modes that use paired limbs and/or the tail. Various secondarily aquatic tetrapod clades, including amphibians, reptiles, and mammals employ transverse undulations or oscillations of the tail for swimming. These movements have often been classified according to a kinematic gradient that was established for fishes, but may not be appropriate to describe the swimming motions of tetrapods. To understand the evolution of movements and design of the tail in aquatic tetrapods, we categorize the types of tails used for swimming and examine swimming kinematics and hydrodynamics. From a foundation of a narrow, elongate ancestral tail, the tails used for swimming by aquatic tetrapods are classified as tapered, keeled, paddle, and lunate. Tail undulations are associated with tapered, keeled, and paddle tails for a diversity of taxa. Propulsive undulatory waves move down the tail with increasing amplitude toward the tail tip, while moving posteriorly at a velocity faster than the anterior motion of the body indicating that the tail is used for thrust generation. Aquatic propulsion is associated with the transfer of momentum to the water from the swimming movements of the tail, particularly at the trailing edge. The addition of transverse extensions and flattening of the tail increases the mass of water accelerated posteriorly and affects vorticity shed into the wake for more aquatically adapted animals. DPIV (Digital Particle Image Velocimetry) reveals differences were exhibited in the vortex wake between the morphological and kinematic extremes of the alligator with a tapering undulating tail and the dolphin with oscillating wing-like flukes that generate thrust. In addition to exploring the relationship between shape of undulating tails and swimming performance across aquatic tetrapods, the role of tail reduction or loss of a tail in aquatic-tetrapod swimming was also explored. For aquatic tetrapods, reduction would have been due to factors including locomotor and defensive specializations and phylogenetic and physiological constraints. Possession of a thrust-generating tail for swimming, or lack thereof, guided various lineages of secondarily aquatic vertebrates into different evolutionary trajectories for effective aquatic propulsion (i.e., speed, efficiency, acceleration).

Spinosaurus is not an aquatic dinosaur

“…In contrast, the hypertrophied neural spines of the tail in S. aegyptiacus are ambiguous as an ‘aquatic adaptation’ because expanded tails can function both as aquatic propulsors and terrestrial display structures. For the expanded tail to be an ‘aquatic adaptation,’ its morphological construction and biomechanical function must unequivocally show primary utility and capability in water, as is the case with extant tail-powered primary or secondarily aquatic vertebrates (e.g., newts, crocodylians, beavers, otters; Fish et al, 2021 ). The same must be shown or inferred to be the case in extinct secondarily aquatic vertebrates ( Gutarra and Rahman, 2022 ).…”

FIRST SKELETAL REMAINS OF <em>HELVETICOSAURUS</em> FROM THE MIDDLE TRIASSIC ITALIAN OUTCROPS OF THE SOUTHERN ALPS, WITH REMARKS ON AN ISOLATED TOOTH

“…6 we propose various options to estimate its length. The presence of a middle caudal vertebra with a tall (although incomplete) neural spine and no transverse processes, preserved in PIMUZ T 4353, suggests that, in any case, the tail was dorsoventrally tall and laterally compressed at its base, a morphology that potentially grants to this appendage a valuable role in generating thrust while undulating laterally (e.g., Fish et al 2021).…”

Propulsive performance of oscillating plates with time-periodic flexibility

“…It stands to reason that swimmers may be able to utilize their muscles on the time scale of the oscillation of their propulsors to tune performance, e.g. by dynamically changing the stiffness of their propulsors; however, there is no definitive observation or consensus in the biological community that swimmers take advantage of their muscles in this way (Fish & Lauder 2021). In this work, we will show that, from a purely hydrodynamic perspective, time-varying stiffness leads to propulsive benefits over constant-stiffness propulsors.…”