Ancient DNA reveals elephant birds and kiwi are sister taxa and clarifies ratite bird evolution

The evolution of the ratite birds has been widely attributed to vicariant speciation, driven by the Cretaceous breakup of the supercontinent Gondwana. The early isolation of Africa and Madagascar implies that the ostrich and extinct Madagascan elephant birds (Aepyornithidae) should be the oldest ratite lineages. We sequenced the mitochondrial genomes of two elephant birds and performed phylogenetic analyses, which revealed that these birds are the closest relatives of the New Zealand kiwi and are distant from the basal ratite lineage of ostriches. This unexpected result strongly contradicts continental vicariance and instead supports flighted dispersal in all major ratite lineages. We suggest that convergence toward gigantism and flightlessness was facilitated by early Tertiary expansion into the diurnal herbivory niche after the extinction of the dinosaurs.

平胸总目鸟类（ratite birds）的演化长期以来被归因于白垩纪超大陆冈瓦纳（Gondwana）裂解所驱动的异域成种（vicariant speciation）。非洲与马达加斯加的早期孤立，提示鸵鸟及已灭绝的马达加斯加象鸟科（Aepyornithidae）物种应为最古老的平胸类群支系。我们对两例象鸟的线粒体基因组（mitochondrial genomes）进行了测序，并开展系统发育分析（phylogenetic analyses），结果显示这些鸟类与新西兰几维鸟的亲缘关系最近，而与鸵鸟所在的基部平胸类群支系相距较远。这一出人意料的结果与大陆分隔成种假说强烈相悖，转而支持所有主要平胸类群支系均曾借助飞行实现扩散。我们提出，恐龙灭绝后第三纪早期对昼行植食生态位的开拓，推动了平胸类群向巨型化与飞行丧失的趋同演化。