Lowland kagu

The general proportions of the various bones of the lowland kagu are very similar to those of the kagu. They differ in the greater size of the extinct species in averaging about 15% larger, with no overlap between the hindlimb elements and only rare overlap between those of the wings. The describers postulate that R. orarius and R. jubatus were lowland and highland forms, respectively.[1]

In 2018, Jörn Theuerkauf and Roman Gula argued that R. orarius was not a valid species.[2] They claimed that Balouet and Olson had overstated the larger size of R. orarius, and assigned all their found specimens to R. orarius but none to R. jubatus, which would be rare if there were two kagu species coexisting in the same island; that the extant kagu is also found in the lowlands, making speciation unlikely, and that no other two kagu species in Oceania share the same island. Instead, they proposed that there was only one kagu species in the Holocene of New Caledonia, R. jubatus, which decreased in average size after human colonization as a result of hunters and introduced predators like dogs favoring the capture of larger animals. This very same possibility had been raised by Balouet and Olson in their original paper and discounted as unlikely, but Theuerkaf and Gula pointed that similar rapid size changes have been documented in other vertebrates when exposed to new competitors and predators.