Abstract
Island birds that were victims of anthropic extinctions were often more specialist species, having evolved their most distinctive features in isolation, making the study of fossil insular birds most interesting. Here we studied a fossil cranium of the ‘giant’ extinct scops owl Otus murivorus from Rodrigues Island (Mascarene Islands, southwestern Indian Ocean), to determine any potential unique characters. The fossil and extant strigids were imaged through X-ray microtomography, providing 3D views of external and internal (endocast, inner ear) cranial structures. Geometric morphometrics and analyses of traditional measurements yielded new information about the Rodrigues owl’s evolution and ecology. Otus murivorus exhibits a 2-tier “lag behind” phenomenon for cranium and brain evolution, both being proportionately small relative to increased body size. It also had a much more developed olfactory bulb than congeners, indicating an unexpectedly developed olfactory sense, suggesting a partial food scavenging habit. In addition, O. murivorus had the eyes placed more laterally than O. sunia, the species from which it was derived, probably a side effect of a small brain; rather terrestrial habits; probably relatively fearless behavior; and a less vertical posture (head less upright) than other owls (this in part an allometric effect of size increase). These evolutionary features, added to gigantism and wing reduction, make the extinct Rodrigues owl’s evolution remarkable, and with multiple causes.
Highlights
Avian extinctions on islands, because of human encroachment, have been considerable and w idespread[1,2,3], including those between the seventeenth and nineteenth centuries on Indian Ocean islands[4,5]
Among principal component (PC), LAT1 summarizes most of the deformation across all species except A. noctua, whereas DORS1 summarizes better deformation between O. sunia and its descendent O. murivorus (Supplementary Fig. 4)
The deformation observed between O. sunia and O. murivorus consists of the following morphological modifications
Summary
Because of human encroachment, have been considerable and w idespread[1,2,3], including those between the seventeenth and nineteenth centuries on Indian Ocean islands[4,5]. A recently extinct owl (eighteenth century) endemic of Rodrigues Island (Mascarene Islands, southwestern Indian Ocean), Otus murivorus, was formerly placed in its own genus, Mascarenotus[7], but a study based on ancient DNA found it to be derived from the much smaller continental Oriental scops owl Otus sunia lineage; its referral to the genus Otus[8] This insular scops owl had evolved gigantism, becoming twice as large and four times heavier than its continental ancestor, and had a slightly reduced wing length[8], all characteristics unseen in extant island owls but observed in some extinct ones, including those on the other Mascarene I slands[8,9]. Results are discussed in terms of paleoecology and evolution of the Rodrigues owl, in relation to absence of predators and reduction of interspecific competition, with fewer taxa than on the continent, including raptorial b irds[6], the owl being the top predator of its oceanic island[9]
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