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Many features of animal locomotion, from burrowing to gliding, show clear evidence of convergence. Read below or explore examples in the topic list to find out more.
In a key example, the overall action of walking legs in insects and mammals is strongly convergent, although the vertebrate limb and articulated legs of insects are very far from convergent. The parallel involves the neurobiology and motor action during walking, whereby posterior legs are primarily propulsive whereas the anterior set have a more complex function that includes braking.
Over the course of evolutionary time numerous animals have independently acquired membranes, flaps or feathers allowing them to glide between trees. Specialised lizards (e.g. geckos) and two unrelated groups of 'flying' frogs glide on flaps between their digits, enhanced by lateral body flaps, an aerodynamic body form and stereotyped aerial maneuvers. Remarkably, three families of tropical tree-dwelling ants are also capable of controlled gliding, 'steering' abdomen-first, aided by a flattened body shape with lateral flaps.
Many tree-dwelling mammals and reptiles have arrived at a refined style of gliding, using extensive membranes or 'patagia'. Mammals that glide skilfully on patagia stretched between fore- and hind-limbs include placentals such as the charismatic flying squirrels, anomalures and colugos as well as marsupials such as the flying possums (or phalangers), the Greater Glider and feather-tailed possums (or acrobatids). The 'flying dragon' Draco glides on a membrane supported by extended ribs, as did several extinct reptiles such as Xianglong and Icarosaurus. Unlikely and yet impressive gliders are tree snakes (Chrysopelea). These snakes jump from trees, pushing out their ribs to flatten the body somewhat and use stereotyped movements to maximise lift. Feathered tails proposed to aid gliding are known from fossils of two oviraptors (Protarchaeopteryx robusta and Caudipteryx zoui) and are convergent on the tail plumes of anomalures and feather-tailed possums. Tantalising in terms of the evolution of flight, the theropod raptor Microraptor gui had feathered fore and hind limbs that look like flight wings but in fact probably assisted only in gliding (its four-winged strategy reminds us of the four-winged exocoetid 'flying' fish that can glide for hundreds of metres).
Although some extinct, feathered reptiles clearly used their wings to glide, other species are even more surprising in suggesting multiple independent acquisitions of true flight. The birds that we know today all evolved from one lineage of flying feathered reptiles from among the theropods, and yet there seem to be several other lineages of flying reptile that evolved around the Cretaceous and did not survive to the present. For example, the famous Archaeopteryx and Vorona are from two separate clades of 'early birds', while more importantly Rahonavis is a flighted reptile from a distinct group, the dromeosaurids.
Adhesive toe-pads make life easier for tree-dwelling animals such as geckos and even allow them to crawl over walls and ceilings given the chance. Geckos, certain anolids and skinks have each independently evolved hairy adhesive pads, somewhat reminiscent of hairy adhesive 'scopula' of some spiders and insects. Tree-frogs have converged on adhesive pads but theirs tend to be smooth, similar to those of many insects, a few mammals (e.g. feathertail gliders and vespertilionid bats) and even echinoderm tube feet.
In considering insects, the first masters of flight, an important convergence occurs between the lolly-pop like balancing organs, or 'halteres' of flies and those of an independent group, the strepsipterans. A few beetles and hemipterans (coccoideans) also seem to have haltere-like structures for balance during flight. Turning to the moths, some moth larvae produce fine threads of silk to catch air currents and carry them away. This parallels exactly the strategy of 'ballooning' on lines of silk commonly observed in juvenile spiders and spider mites.
Ever since the earliest tetrapods learned to walk on land they have, at various times, lost their four legs in favour of a burrowing mode of life. Early Carboniferous tetrapods known as aistopods show limb reduction indicative of a burrowing adaptation, and among living amphibians the caecilians offer an extreme example, typically with total limb loss and earthworm-like locomotion. Among reptiles, the amphisbaenians or 'worm-lizards' have converged on an earthworm-like form and interestingly several other lineages have independently evolved limblessess. Most famous of the limbless reptiles must be the snakes, within which only the atractaspids hunt underground, and others include the snake-like Pygopodidae, burrowing skinks (showing a range of degrees of limb loss) and slow-worms (Anguidae). Certain species of skink as well as other unrelated desert lizards can 'dive' into and burrow easily through sand, thanks to convergent adaptations such as a shovel-like snout and toe fringes.
Turning to animals that must travel through water, thunniform swimming offers a clear case of convergence. Tuna fish and their very distant relatives the lamnid sharks both achieve high-speed swimming while keeping their trunk and head relatively still, helping them to focus on prey. Their lunate tails are joined to the body at a narrow region and rapid tail movement is driven by powerful, specialised musculature. Ichthyosaurs, predatory marine reptiles from the Mesozoic, had a body form similarly adapted to high-speed thunniform swimming through the oceans.