in size and shape, but with shorter wings and possibly darker and/or duller plumage and a higher bill The great Oʻahu rail was the larger of two species of rail found on the island of
Oʻahu. Very little of its
skeleton is known – a
humerus, the upper and lower parts of the billtip, and a handful of legbones, none of which except the
holotype tarsometatarsus is complete – no
distal humerus and
proximal tibiotarsus is documented as of 2023. and most rails have rather similar habitat and habits, even the few bones allow for considerable insight when
"P." ralphorum is compared to other Rallidae: In life, it must have been roughly the size of an
ash-throated crake or half again as large as the
Hawaiian rail, about 20–25 cm (8–10 in) altogether, with a rather high and almost perfectly straight beak some 2 cm (slightly less than 1 inch) long, and weighing roughly 110 grams (almost 4 oz). It was flightless due to its small wings – the entire arm, from fingertips to shoulder joint, was only about 6 cm (some 2 in) long, with the single known
humerus about 10% larger in all dimensions than in the
Great Maui crake which was similarly sized and also flightless. The legs, on the other hand, were possibly slightly less well-developed as in the Maui species, with the holotype
tarsometatarsus measuring 35.7 mm. At the time of its description, the great Oʻahu rail was placed in
genus Porzana, which at that time was already suspected to be a
polyphyletic assemblage of rails which consisted of at least 2 different lineages.
Morphological cladistic analyses were conducted for the species, but the results are not consistent, and as of 2023 are not corroborated with
DNA sequence data either. In 1998 it was analyzed in a combined group together with the other extinct Hawaiian rails in a morphology-based analysis. This analysis could only place it with certainty within a group which roughly corresponds to
subfamily Himanthornithinae as it is circumscribed today; as later analyses revealed that the Hawaiian rails are almost certainly not a
monophyletic group, it is unsurprising that the
chimeric combined pseudo-"
taxon" could not be placed more certainly. In the absence of meaningful quantitative data, authors generally held the great Oʻahu rail to be part of a radiation of "
crakes", possibly even monophyletic, within the part of
"Porzana" nowadays recognized as a well-distinct himanthornithine genus
Zapornia, as these were the only rails known from that part of the world. No
ancient DNA was successfully recovered from
"P." ralphorum as of 2023, but in 2021 it was included in a study which utilized
DNA sequence data from living rails to create a well-supported framework aiding placement of prehistoric species for which no molecular data was available. Here, the species unexpectedly wound up far away from
Porzana. Though with much uncertainly due to the limited material available, it resolved as
sister species to the
weka (
Gallirallus australis), the
type and perhaps only remaining living species of
genus Gallirallus, a former "
wastebin taxon" much like
"Porzana", but from the other extant rail subfamily
Rallinae, and containing somewhat larger and longer-billed species. The weka and
Hypotaenidia are part of a badly resolved group of mainly
Melanesian taxa including an extremely high amount of flightless island
endemics, many of which (unlike those in
Zapornia) still extant today. Possibly,
"P." ralphorum was the most far-flung Pacific member of an
evolutionary grade in
tribe Rallini, containing mostly mid-sized to large strong- to long-billed rallines such as weka,
Calayan rail,
Chatham rail,
Chestnut rail,
Invisible rail,
Hawkins's rail,
New Caledonian rail,
Snipe-rail and possibly the
Fiji rail. Perhaps, even the
Mascarenes rails –
Rodrigues rail and the
Red rail of
Mauritius, both flightless and extinct – were part of this successive branching-off of species that became sedentary on islands from a volant population situated on the lands separating the southern Pacific and
Indian Oceans, ultimately giving rise to
Hypotaenidia which expanded across Polynesia to
Wake Island and possibly beyond. This has not been studied in a modern phylogenetic context accounting for the
polyphyly of
"Porzana" and
"Gallirallus", however. At any rate, the great Oʻahu rail is highly unlikely to belong to genus
Porzana as it is understood today. An interesting aspect of the proposed "galliralline" origin of
"P." ralphorum is that in this scenario, its smaller relative with which it shared its habitat would also be a "galliralline" convergent with
"Porzana" crakes, but representing a more recent arrival to Oʻahu, and most likely a true
Hypotaenidia, hitherto unknown for the Hawaiian Islands and unexpected because the genus is unattested from the
Line Islands eastwards. Such a double colonization by close relatives – a
basal "galliralline" followed by a typical
Hypotaenidia, both subsequently becoming flightless – was demonstrated for the Chatham Islands, where the more ancient lineage (the Chatham rail) evolved to smaller size, while the later arrival (
Dieffenbach's rail) became more robust than its volant ancestors; on Oʻahu, it would have been the other way around. ==Ecology and extinction==