Flipper bone distribution reveals flexible trailing edge in underwater flying marine tetrapods

Skin, scales, and cells in a Jurassic plesiosaur

Plesiosaurs are an iconic group of Mesozoic marine reptiles with an evolutionary history spanning over 140 million years (Ma).1 Their skeletal remains have been discovered worldwide; however, accompanying fossilized soft tissues are exceptionally rare.2 Here, we report a virtually complete plesiosaur from the Lower Jurassic (∼183 Ma)3 Posidonia Shale of Germany that preserves skin traces from around the tail and front flipper. The tail integument was apparently scale-less and retains identifiable melanosomes, keratinocytes with cell nuclei, and the stratum corneum, stratum spinosum, and stratum basale of the epidermis. Molecular analysis reveals aromatic and aliphatic hydrocarbons that likely denote degraded original organics. The flipper integument otherwise integrates small, sub-triangular structures reminiscent of modern reptilian scales. These may have influenced flipper hydrodynamics and/or provided traction on the substrate during benthic feeding. Similar to other sea-going reptiles,4,5,6,7,8,9,10 scalation covering at least part of the body therefore probably augmented the paleoecology of plesiosaurs.

How does the shape of the wing and hindlimb bones of aquatic birds relate to their locomotor abilities?

Aquatic birds represent diverse ecologies and locomotion types. Some became flightless or lost the ability for effective terrestrial locomotion, yet, certain species excel in water, on land, and in air, despite differing physical characteristics associated with each medium. In this exploratory study, we intend to quantitatively analyze the morphological variety of multiple limb bones of aquatic birds using 3D geometric morphometrics. Morphological variation is mainly driven by phylogeny, which also affects size and locomotion. However, the shape of the ulna, including the proportion and orientation of the epiphyses is influenced by size and aquatic propulsive techniques even when phylogeny is taken into consideration. Certain trends, possibly linked to functions, can be observed too in other bones, notably in cases where phylogenetic and functional signals are probably mixed when some taxa only englobe species with similar functional requirements: penguins exhibit the most distinctive wing bone morphologies, highly adapted to wing-propulsion; advanced foot-propellers exhibit femur morphology that reduces proximal mobility but supports stability; knee structures, like cnemial crests of varied sizes and orientations, are crucial for muscle attachments and efficient movement in water and on land; taxa relying on their feet in water but retaining terrestrial abilities share features enabling swimming and walking postures. Size-linked changes distinguish the wing bones of non-wing-propelled taxa. For hindlimbs, larger size relates to robust bones probably linked to terrestrial abilities, but robustness in femora can be connected to foot-propulsion. These results help us better understand birds' skeletal adaptation and can be useful inferring extinct species' ecology.

The Role of Locomotory Ancestry on Secondarily Aquatic Transitions

Land-to-sea evolutionary transitions are great transformations where terrestrial amniote clades returned to aquatic environments. These secondarily aquatic amniote clades include charismatic marine mammal and marine reptile groups, as well as countless semi-aquatic forms that modified their terrestrial locomotor anatomy to varying degrees to be suited for swimming via axial and/or appendicular propulsion. The terrestrial ancestors of secondarily aquatic groups would have started off swimming strikingly differently from one another given their evolutionary histories, as inferred by the way modern terrestrial amniotes swim. With such stark locomotor functional differences between reptiles and mammals, we ask if this impacted these transitions. Axial propulsion appears favored by aquatic descendants of terrestrially sprawling quadrupedal reptiles, with exceptions. Appendicular propulsion is more prevalent across the aquatic descendants of ancestrally parasagittal-postured mammals, particularly early transitioning forms. Ancestral terrestrial anatomical differences that precede secondarily aquatic invasions between mammals and reptiles, as well as the distribution of axial and appendicular swimming in secondarily aquatic clades, may indicate that ancestral terrestrial locomotor anatomy played a role, potentially in both constraint and facilitation, in certain aquatic locomotion styles. This perspective of the land-to-sea transition can lead to new avenues of functional, biomechanical, and developmental study of secondarily aquatic transitions.

Tailoring the bending pattern of non-uniformly flexible pitching hydrofoils enhances propulsive efficiency

We present new measurements of non-uniformly flexible pitching foils fabricated with a rigid leading section joined to a flexible trailing section. This construction enables us to vary the bending pattern and resonance condition of the foils independently. A novel effective flexibility, defined as the ratio of added mass forces to elastic forces, is proposed and shown to provide a scaling for the natural frequencies of the fluid-structural system. Foils with very flexible trailing sections ofEI< 1.81 × 10^-5N m²do not show a detectable resonance and are classified as 'non-resonating' as opposed to 'resonating' foils. Moreover, the non-resonating foils exhibit a novel bending pattern where the foil has a discontinuous hinge-like deflection instead of the smooth beam-like deflection of the resonating foils. Performance measurements reveal that both resonating and non-resonating foils can achieve high propulsive efficiencies of around 50% or more. It is discovered that non-uniformly flexible foils outperform their rigid and uniformly flexible counterparts, and that there is an optimal flexion ratio from 0.4 ⩽λ⩽ 0.7 that maximizes the efficiency. Furthermore, this optimal range coincides with the flexion ratios observed in nature. Performance is also compared under the same dimensionless flexural rigidity,R*, which highlights that at the same flexion ratio more flexible foils achieve higher peak efficiencies. Overall, to achieve high propulsive efficiency non-uniformly flexible hydrofoils should (1) oscillate above their first natural frequency, (2) have a flexion ratio in the range of 0.4 ⩽λ⩽ 0.7 and (3) have a small dimensionless rigidity at their optimal flexion ratio. Scaling laws for rigid pitching foils are found to be valid for non-uniformly flexible foils as long as the measured amplitude response is used and the deflection angle of the trailing sectionβ is < 45°. This work provides guidance for the development of high-performance underwater vehicles using simple purely pitching bio-inspired propulsive drives.

Kinematics and hydrodynamics analyses of swimming penguins: wing bending improves propulsion performance

Penguins are adapted to underwater life and have excellent swimming abilities. Although previous motion analyses revealed their basic swimming characteristics, the details of the 3D wing kinematics, wing deformation and thrust generation mechanism of penguins are still largely unknown. In this study, we recorded the forward and horizontal swimming of gentoo penguins (Pygoscelis papua) at an aquarium with multiple underwater action cameras and then performed a 3D motion analysis. We also conducted a series of water tunnel experiments with a 3D printed rigid wing to obtain lift and drag coefficients in the gliding configuration. Using these coefficients, the thrust force during flapping was calculated in a quasi-steady manner, where the following two wing models were considered: (1) an 'original' wing model reconstructed from 3D motion analysis including bending deformation and (2) a 'flat' wing model obtained by flattening the original wing model. The resultant body trajectory showed that the penguin accelerated forward during both upstroke and downstroke. The motion analysis of the two wing models revealed that considerable bending occurred in the original wing, which reduced its angle of attack during the upstroke in particular. Consequently, the calculated stroke-averaged thrust was larger for the original wing than for the flat wing during the upstroke. In addition, the propulsive efficiency for the original wing was estimated to be 1.8 times higher than that for the flat wing. Our results unveil a detailed mechanism of lift-based propulsion in penguins and underscore the importance of wing bending.

High-Efficiency Can Be Achieved for Non-Uniformly Flexible Pitching Hydrofoils via Tailored Collective Interactions

New experiments examine the interactions between a pair of three-dimensional (AR = 2) non-uniformly flexible pitching hydrofoils through force and efficiency measurements. It is discovered that the collective efficiency is improved when the follower foil has a nearly out-of-phase synchronization with the leader and is located directly downstream with an optimal streamwise spacing of X*=0.5. The collective efficiency is further improved when the follower operates with a nominal amplitude of motion that is 36% larger than the leader’s amplitude. A slight degradation in the collective efficiency was measured when the follower was slightly-staggered from the in-line arrangement where direct vortex impingement is expected. Operating at the optimal conditions, the measured collective efficiency and thrust are ηC=62% and CT,C=0.44, which are substantial improvements over the efficiency and thrust of ηC=29% and CT,C=0.16 of two fully-rigid foils in isolation. This demonstrates the promise of achieving high-efficiency with simple purely pitching mechanical systems and paves the way for the design of high-efficiency bio-inspired underwater vehicles.

Convergent evolution of forelimb-propelled swimming in seals

Modern pinnipeds (true and eared seals) employ two radically different swimming styles, with true seals (phocids) propelling themselves primarily with their hindlimbs, whereas eared seals (otariids) rely on their wing-like foreflippers.^1,2 Current explanations of this functional dichotomy invoke either pinniped diphyly^3-5 or independent colonizations of the ocean by related but still largely terrestrial ancestors.^6-8 Here, we show that pinniped swimming styles form an anatomical, functional, and behavioral continuum, within which adaptations for forelimb swimming can arise directly from a hindlimb-propelled bauplan. Within phocids, southern seals (monachines) show a convergent trend toward wing-like, hydrodynamically efficient forelimbs used for propulsion during slow swimming, turning, bursts of speed, or when initiating movement. This condition is most evident in leopard seals, which have well-integrated foreflippers with little digit mobility, reduced claws, and hydrodynamic characteristics comparable to those of forelimb-propelled otariids. Using monachines as a model, we suggest that the last common ancestor of modern seals may have been hindlimb-propelled and aquatically adapted, thus resolving the apparent contradiction at the root of pinniped evolution.

Ecophysiological steps of marine adaptation in extant and extinct non-avian tetrapods

Marine reptiles and mammals are phylogenetically so distant from each other that their marine adaptations are rarely compared directly. We reviewed ecophysiological features in extant non-avian marine tetrapods representing 31 marine colonizations to test whether there is a common pattern across higher taxonomic groups, such as mammals and reptiles. Marine adaptations in tetrapods can be roughly divided into aquatic and haline adaptations, each of which seems to follow a sequence of three steps. In combination, these six categories exhibit five steps of marine adaptation that apply across all clades except snakes: Step M1, incipient use of marine resources; Step M2, direct feeding in the saline sea; Step M3, water balance maintenance without terrestrial fresh water; Step M4, minimized terrestrial travel and loss of terrestrial feeding; and Step M5, loss of terrestrial thermoregulation and fur/plumage. Acquisition of viviparity is not included because there is no known case where viviparity evolved after a tetrapod lineage colonized the sea. A similar sequence is found in snakes but with the haline adaptation step (Step M3) lagging behind aquatic adaptation (haline adaptation is Step S5 in snakes), most likely because their unique method of water balance maintenance requires a supply of fresh water. The same constraint may limit the maximum body size of fully marine snakes. Steps M4 and M5 in all taxa except snakes are associated with skeletal adaptations that are mechanistically linked to relevant ecophysiological features, allowing assessment of marine adaptation steps in some fossil marine tetrapods. We identified four fossil clades containing members that reached Step M5 outside of stem whales, pinnipeds, sea cows and sea turtles, namely Eosauropterygia, Ichthyosauromorpha, Mosasauroidea, and Thalattosuchia, while five other clades reached Step M4: Saurosphargidae, Placodontia, Dinocephalosaurus, Desmostylia, and Odontochelys. Clades reaching Steps M4 and M5, both extant and extinct, appear to have higher species diversity than those only reaching Steps M1 to M3, while the total number of clades is higher for the earlier steps. This suggests that marine colonizers only diversified greatly after they minimized their use of terrestrial resources, with many lineages not reaching these advanced steps. Historical patterns suggest that a clade does not advance to Steps M4 and M5 unless these steps are reached early in the evolution of the clade. Intermediate forms before a clade reached Steps M4 and M5 tend to become extinct without leaving extant descendants or fossil evidence. This makes it difficult to reconstruct the evolutionary history of marine adaptation in many clades. Clades that reached Steps M4 and M5 tend to last longer than other marine tetrapod clades, sometimes for more than 100 million years.

Wing Musculature Reconstruction in Extinct Flightless Auks (Pinguinus and Mancalla) Reveals Incomplete Convergence with Penguins (Spheniscidae) Due to Differing Ancestral States

Despite longstanding interest in convergent evolution, factors that result in deviations from fully convergent phenotypes remain poorly understood. In birds, the evolution of flightless wing-propelled diving has emerged as a classic example of convergence, having arisen in disparate lineages including penguins (Sphenisciformes) and auks (Pan-Alcidae, Charadriiformes). Nevertheless, little is known about the functional anatomy of the wings of flightless auks because all such taxa are extinct, and their morphology is almost exclusively represented by skeletal remains. Here, in order to re-evaluate the extent of evolutionary convergence among flightless wing-propelled divers, wing muscles and ligaments were reconstructed in two extinct flightless auks, representing independent transitions to flightlessness: Pinguinus impennis (a crown-group alcid), and Mancalla (a stem-group alcid). Extensive anatomical data were gathered from dissections of 12 species of extant charadriiforms and 4 aequornithine waterbirds including a penguin. The results suggest that the wings of both flightless auk taxa were characterized by an increased mechanical advantage of wing elevator/retractor muscles, and decreased mobility of distal wing joints, both of which are likely advantageous for wing-propelled diving and parallel similar functional specializations in penguins. However, the conformations of individual muscles and ligaments underlying these specializations differ markedly between penguins and flightless auks, instead resembling those in each respective group's close relatives. Thus, the wings of these flightless wing-propelled divers can be described as convergent as overall functional units, but are incompletely convergent at lower levels of anatomical organization-a result of retaining differing conditions from each group's respective volant ancestors. Detailed investigations such as this one may indicate that, even in the face of similar functional demands, courses of phenotypic evolution are dictated to an important degree by ancestral starting points.

Fingers zipped up or baby mittens? Two main tetrapod strategies to return to the sea

The application of network methodology in anatomical structures offers new insights on the connectivity pattern of skull bones, skeletal elements and their muscles. Anatomical networks helped to improve our understanding of the water-to-land transition and how the pectoral fins were transformed into limbs via their modular disintegration. Here, we apply the same methodology to tetrapods secondarily adapted to the marine environment. We find that these animals achieved their return to the sea with four types of morphological changes, which can be grouped into two different main strategies. In all marine mammals and the majority of the reptiles, the fin is formed by the persistence of superficial and interdigital connective tissues, like a 'baby mitten', whereas the underlying connectivity pattern of the bones does not influence the formation of the forefin. On the contrary, ichthyosaurs 'zipped up' their fingers and transformed their digits into carpal-like elements, forming a homogeneous and better-integrated forefin. These strategies led these vertebrates into three different macroevolutionary paths exploring the possible spectrum of morphological adaptations.

Convergent Evolution of Swimming Adaptations in Modern Whales Revealed by a Large Macrophagous Dolphin from the Oligocene of South Carolina

Modern whales and dolphins are superbly adapted for marine life, with tail flukes being a key innovation shared by all extant species. Some dolphins can exceed speeds of 50 km/h, a feat accomplished by thrusting the flukes while adjusting attack angle with their flippers [1]. These movements are driven by robust axial musculature anchored to a relatively rigid torso consisting of numerous short vertebrae, and controlled by hydrofoil-like flippers [2-7]. Eocene skeletons of whales illustrate the transition from semiaquatic to aquatic locomotion, including development of a fusiform body and reduction of hindlimbs [8-11], but the rarity of Oligocene whale skeletons [12, 13] has hampered efforts to understand the evolution of fluke-powered, but forelimb-controlled, locomotion. We report a nearly complete skeleton of the extinct large dolphin Ankylorhiza tiedemani comb. n. from the Oligocene of South Carolina, previously known only from a partial rostrum. Its forelimb is intermediate in morphology between stem cetaceans and extant taxa, whereas its axial skeleton displays incipient rigidity at the base of the tail with a flexible lumbar region. The position of Ankylorhiza near the base of the odontocete radiation implies that several postcranial specializations of extant cetaceans, including a shortened humerus, narrow peduncle, and loss of radial tuberosity, evolved convergently in odontocetes and mysticetes. Craniodental morphology, tooth wear, torso vertebral morphology, and body size all suggest that Ankylorhiza was a macrophagous predator that could swim relatively fast, indicating that it was one of the few extinct cetaceans to occupy a niche similar to that of killer whales.