There is a world somewhere between reality and fiction. Although ignored by many, it is very real and so are those living in it. This forum is about the natural world. Here, wild animals will be heard and respected. The forum offers a glimpse into an unknown world as well as a room with a view on the present and the future. Anyone able to speak on behalf of those living in the emerald forest and the deep blue sea is invited to join.
The systematics of Madagascar’s extinct elephant birds remains controversial due to large gaps in the fossil record and poor biomolecular preservation of skeletal specimens. Here, a molecular analysis of 1000-year-old fossil eggshells provides the first description of elephant bird phylogeography and offers insight into the ecology and evolution of these flightless giants. Mitochondrial genomes from across Madagascar reveal genetic variation that is correlated with eggshell morphology, stable isotope composition, and geographic distribution. The elephant bird crown is dated to ca. 30 Mya, when Madagascar is estimated to have become less arid as it moved northward. High levels of between-clade genetic variation support reclassifying Mullerornis into a separate family. Low levels of within-clade genetic variation suggest there were only two elephant bird genera existing in southern Madagascar during the Holocene. However, we find an eggshell collection from Madagascar’s far north that represents a unique lineage of Aepyornis. Furthermore, divergence within Aepyornis coincides with the aridification of Madagascar during the early Pleistocene ca. 1.5 Ma, and is consistent with the fragmentation of populations in the highlands driving diversification and the evolution of extreme gigantism over shorts timescales. We advocate for a revision of their taxonomy that integrates palaeogenomic and palaeoecological perspectives.
Introduction
The elephant birds of Madagascar (Aves: Aepyornithidae) were large, flightless ratites that became extinct around a millennium ago. The relatedness of elephant birds to other birds remained a mystery until several genetic studies discovered that they are sister to New Zealand’s kiwi1,2,3, revolutionising our understanding of avian diversification. However, the biodiversity and evolutionary relationships within elephant birds have been uncertain and unstable since they were first described over 150 years ago4, as most species are known only from few incomplete Pleistocene-Holocene post-cranial skeletal remains from south and central Madagascar5,6,7 (Fig. 1a and Supplementary Data 1). About eight species of elephant birds across two genera were generally accepted based on morphological comparison of skeletal fossils4 (Fig. 1c), but a recent morphometric re-evaluation of skeletal material6,7 reclassified elephant birds into four species across three genera (Aepyornis, Mullerornis and a new genus, Vorombe). However, this revision remains questionable: homoplasy in morphological characters that has arisen via convergent evolution means post-cranial skeletal morphology poorly distinguishes species limits within extinct ratite taxa8 as well as the evolutionary relationships between them. Alternatively, the use of ancient DNA (aDNA) has proven to be highly successful in the delineation of extinct bird species boundaries, phylogenetic relationships, and geographic ranges8,9,10,11,12, and corroboration of elephant bird systematics by molecular methods is long overdue. Although the warm, humid environment of Madagascar is suboptimal for the preservation of aDNA in bone13, it has been retrieved from elephant bird eggshell3,14, which is found in abundance while skeletal fossils are less common15. Aided by eggshell micro-morphology, stable isotope geochemistry and palaeoproteomics, here, we detail the first phylogeographic survey of elephant birds using eggshell whole mitochondrial aDNA, in order to revisit elephant bird taxonomy and evolutionary history. As an island with high levels of endemism, Madagascar is a model system for studying the mechanisms underlying evolution and extinction, and the lack of resolution around the life history of the world’s largest birds presents a major gap in our understanding.
