A number of reptile species have been discovered in the Mesozoic fossil record, bearing feathers that were apparently used to support gliding locomotion, rather than true, powered flight as we see in present day birds.

Gliding is defined as descent through the air at an angle of less than 45° to the horizontal, and this mode of locomotion has been achieved in a surprising diversity of animals through a number of impressive morphological and behavioural adaptations that generate the required aerodynamic forces (upward ‘lift’ exceeding air resistance, or ‘drag’). Among the distinct types of gliding that are known, several provide excellent case studies in convergent evolution, as they have evolved repeatedly in very distantly related groups: here we focus on the repeated evolution of feathers from scales in extinct reptiles very well adapted to gliding locomotion.

A number of reptile species have been discovered in the Mesozoic fossil record, bearing feathers that were apparently used to support gliding locomotion, rather than true, powered flight as we see in present day birds. Feathered gliders were all archosaurs, unrelated to modern lizards but instead identified as a group of dinosaurs called the theropods, out of which the lineage(s) leading to modern birds (technically termed ‘Aves’) emerged. Mesozoic gliding theropods are (almost) exclusively from the advanced group ‘Maniraptora’ but belong to different sub-groups (Oviraptorosauria and Deinonychosauria) of maniraptors, indicating convergent evolution of feathers adapted for gliding down from trees. As an aside, there is great controversy about the origins of true flight and birds, although it is generally agreed to have evolved from feathered, reptilian ancestors. It is notable that the existence of multiple lineages of tree-dwelling, feathered gliders, not to mention terrestrial, feathered ‘runners’, may leave space for multiple possible ancestors. For example, the famous bird-lizard Archaeopteryx lithographica of the Late Jurassic (155-150 Ma) was apparently capable of true flight and is widely considered to be a true bird ancestor in the group ‘Avialae’, but another species in the closely related group Oviraptorosauria, Avimimus portentosus of the Late Cretaceous (75 Ma), has also been proposed as the ‘closest link’ between reptiles and birds, having feathered wings used not in flight but as an aid in high speed running along the ground.

Feathered wings

Within the last decade, a number of astonishingly preserved Mesozoic reptiles bearing bird-like traits have been unearthed in China. Two important examples among these are the small dromeosaur Microraptor gui (Early Cretaceous, 130-125 Ma) and the earlier archosaur Longisquama insignis (Late Triassic, 220 Ma). Both of these reptiles evolved heavily feathered bodies as a convergent adaptation to gliding between trees. As a dromeosaur, Microraptor is a member of the ‘Deinonychosauria’, adjacent to the Avialae (group containing true birds), and it possesses feathers with asymmetric vanes (as in birds) arranged to form paired forewings and hindwings, with a terminal diamond-shaped fan on the tail. Longisquama was adapted to gliding with scaly, overlapping ‘feathers’ arranged as an ideal aerofoil on the forelimb, paired clusters along the dorsal surface (back) of the body and as tufts on the chin and neck. The feathers of L. insignis may or may not be equivalent to those observed in Microraptor and Archaeopteryx: their detailed structure, and the relationship of Longisquama to other dinosaurs is still uncertain. The bird-like and impressively aerodynamic morphologies of Microraptor and Longisquama have led to the suggestion that both could be bird ancestors, meaning that avian flight would not represent a single innovation, but evolved by convergence on separate occasions.

‘Four-winged’ gliding, as in Microraptor, is reminiscent of the mechanism used by ‘flying’ fish, even in their capacity to actively move their ‘wings’, giving the impression of a transition between gliding and true, powered flight.  Flying fish comprise approximately 50 species of the fish family Exocoetidae (e.g. Cheilopogon, Exocoetus, Hirundichthys), and a few species of the closely related Hemirhamphidae (e.g. Euleptorhamphus viridis), all inhabiting tropical and subtropical waters of the Atlantic, Pacific and Indian Oceans . They travel on average 30-50m per glide, twice as fast as they are able to swim (up to 60km/hr), and manage to do so by spreading (and flapping) their enlarged pectoral and pelvic fins after launching themselves from the water using forward thrust from a deeply forked, fast-moving caudal (or ‘tail’) fin. Four-winged exocoetid gliding parallels a general mechanism shared with feathered, four-winged Microraptor, in which the fore- and hind-limbs are modified into effective aerofoil structures for aerial locomotion. Furthermore, two-winged species of excocoetid that glide only on enlarged pectoral (forelimb) fins effectively mirror the mode of gliding found in Longisquama, with its ‘feathered’ forelimb wings and tail fan. These are remarkable examples of convergent evolution between reptiles and fish, a pair vertebrate groups that have been evolving separately for hundred of millions of years.

Feathered tails

Some 10- 25 Ma before the appearance of four-winged Microraptor in the fossil record, we find evidence of some interesting gliding theropods in a group known as the ‘Oviraptorosauria’. Oviraptorosaurs are slightly less evolutionarily advanced than the so-called ‘Eumaniraptoria’ – which comprise the Deinonychosauria (e.g. Microraptor and Sinovenator, capable of gliding) and Avialae (e.g. Archaeopteryx, Jeholornis and Sapeornis, primitive ‘birds’ capable of flight). The oviraptor Protarchaeopteryx robusta had indisputably bird-like feathers as a tuft on the end of its tail, and Caudipteryx zoui had a well feathered tail and forelimbs; both species are Late Jurassic-Early Cretaceous (150-140 Ma) in age.

A dominant tail plume is hypothesised to stabilise gliding between trees, and as such represents an astonishing convergence with the morphological and functional adaptations of certain gliding mammals. For example, gliding in scaly-tailed anomalures (African rodents – e.g. Anomalurus, Idiurus) and feather-tailed possums (marsupials of the family Acrobatidae – e.g. Acrobates, Distoechurus) is dependent on either projecting rows of scales or stiff, feather-like hairs respectively, each located on either side of a flattened, broad tail. On a smaller evolutionary scale, Caudipteryx and Protarchaeopteryx  evolved a ‘tail-wing’ independently from one another within the Oviraptorosauria, representing convergent evolution of a gliding adaptation that appears to be only a small step behind the specialised ‘four-wings plus tail-fan’ of Microraptor.