Five reasons as to why Proapteryx is interesting
So 2013 was a big year for our understanding of extinct birds. We know that phorusrhacids terrorised Europe, that early birds didn’t have scaly feet, a new understanding of the enantiornithe cladogram, et cetera. One of these discoveries is that small, unassuming New Zealand bird known as Proapteryx micromeros, a bird the size of a rail (and actually compared to the Banded Rail, Gallirallus philippensis, in terms of leg proportions) that stalked Aotearoa’s forests in the early Miocene, where is now Otago. It lived in a New Zealand already populated by a strange and alien fauna, composed of animals like terrestrial mekosuchine crocodiles, turtles, pythons and even a mysterious, poorly understood non-chiropteran mammal. Birds were already dominant, however, and the site where it was found, the Saint Bathans Formation, already had a very recognisable avifauna, with ducks, seagulls, passerines, herons, owlet-nightjars, hawks and even recognisable forms of Moa and Adzebill running around. While there were a few oddbirds here and there, like a palaelodid flamingo, the avifauna of Saint Bathans was distinctively modern, conforming to the Neogene bird faunas, with the odd, ancient birds already rendered into flightless giants while more derived and familiar bird groups dominated the skies.
There was, however, one component of the familiar avifauna in Saint Bathans that was absent, and that was the kiwi, the bizarre, nocturnal, terrestrially probing, hairy ratite that we all know and love. As the name implies, Proapteryx was an early form of kiwi, but it was a rather different animal. Unlike the moas and adzebills, it wasn’t already a large, flightless giant, but a still very small animal that, based on limb proportions, was probably still capable of flight or at least had evolved from then very recently flying ancestors. This kiwi wasn’t therefore an early relic from a time when birds could afford to not be neognaths, but a recent coloniser of the island continent like the pigeons and herons it co-existed with, and by all accounts a typical member of the Australasian avifauna of the period, making the absence of volant palaeognaths from modern Oz all the more conspicuous. It’s discovery bears therefore a lot of meaning in the study of not only kiwis, but also palaeognaths as a whole, and of the changes in avifauna in a landmass once presumed to have been exceptionally conservative.
1. It confirms that the known Palaeognath clades are [mainly] not the result of vicariance
The most obvious conclusion advertised in articles about this bird is that the ages old hypothesis that ratites all evolved from a common ancestor that diverged as Gondwanna split apart is effectively disproven. Previously, it had already been under fire across the 20th century, with genetic data connecting kiwis to australian ratites instead of moas, and Hackett et al. 2008 that demonstrated that palaeognath diversity as a whole is independent from the progression of Gondwanna’s split up (i.e. tinamous are more closely related to Australasian ratites than to rheas, elephant birds are more closely related to either than the indo-african ostriches, et cetera.). Proapteryx not only offers evidence that palaeognaths cross the Tasman Sea in the Miocene, well after New Zealand had already become an island, but also that kiwis evolved independently from moas, as Saint Bathans already preserves fossils of large, flightless dinornithids while it’s sole kiwi is a dimininutive flying or then recently flightless bird.
2. It implies a higher diversity of mid-Cenozoic palaeognaths than previously thought
While palaeognath fossils are rare, period, it’s clear that these birds were quite diverse in the Palaeogene. Fossil sites in Eurasia and North America dating bear fossils of birds known as “lithornithids”, a diverse menagerie of flying palaeognaths tentatively classified as a monophyletic clade, but actually more likely to represent a paraphyletic assemblage of birds leading to modern ratites, something especially likely when one examines basal ostriches like Palaeotis. “Lithornithids” disappear from the fossil site in the Oligocene, alongside many other laurasian bird clades, and with their extinction it was presumed that the only flying palaeognaths that remained were the already poorly-flying tinamous, surrendering the skies to neognath birds for the rest of the Cenozoic.
With the discovery of a flying kiwi dating to the Miocene, this obviously wasn’t the case. Proapteryx is a small bird, actually smaller than the largest known “lithornithids” such as Paracathartes, and it clearly evolved from animals that were very competent flyers, if it wasn’t a competent flyer itself, indicating the presence of rather generalistic flying palaeognaths in the Miocene of Australia and New Zealand. Not only that, since kiwis are most closely related to autralian cassowaries and emus, the presence of the already large, flightless Emuarius in the Oligocene of Australia suggests that aussie ratites were undergoing an adaptative radiation in the Oligocene and early Miocene, branching in at least two radically different ecological niches, instead of being reduced to flightless forms as previously thought. More volant palaeognaths are expected to turn up in Oligocene and Miocene fossil sites in mainland Oz, if they haven’t already and simply have been misidentified (as sadly often the case when dealing with fragmentary bird remains).
3. It offers a possible behaviorial connection between kiwis and “lithornithid grade” palaeognaths, and perhaps an insight to the ancestral ratite condition
While Proapteryx is explicitly contrasted against Lithornis in Worthy et al. 2013 (namely, in the morphology of the quadrate bone and associated elements), there are possible similarities between the paraphyletic “lithornithids” and kiwis raised by it’s discovery. “Lithornithids”, while most likely subjected to a wide variety of lifestyles, generally possess long, slender jaws, which have been implicated as having been used in probing, often compared to those of shorebirds; given their hypothesised arboreal tendencies, they might have been ecologically analogous to hoopoes and woodcocks, introducing their jaws on the soil or in cavities in the trees to search for insects and other small prey.
This obvious raises a strong parallel to modern kiwis, which are birds specialised to probe on the New Zealand forest substrates for foraging, having sensor pits, the nostrils at the tip of the jaws, vibrissae and other adaptations that evolved to detect prey in this manner. Previously, these weird features were thought to be the product of island life, but the discovery of a volant kiwi might very well establish these as having predated flightlessness, presumably having evolved in “lithornithids” and having simple been retained, if slightly refined, in extant kiwis.
Indeed, it might very well suggest that kiwis are actually the most ecologically conservative of all extant palaeognaths, having remained in a probing, woodcock like lifestyle while rheas, emus and ostriches became plains dwelling herbivores, cassowaries became frugivores and tinamous became fowl-like omnivores and granivores.
4. It offers insight about the evolution of kiwi flightlessness
In the New Zealand that Proapteryx lived in, mekosuchine crocodiles offered predation to the native avifauna, while small mammals of uncertain affinities competed with them. It is easy to understand why moas, waterfowl and adzebills became massive and wingless, as there simply weren’t any other animals occupying the roles of large herbivores and of opportunistic carnivores respectively, but the ecology of Saint Bathans did not showcase the sheer variety of flightless birds that New Zealand is famous for, implicating that the competitive and predatorial pressure from the crocodiles, mammals and perhaps even the local lepidosaurs and turtles was a restricting factor in that biota. Certainly not the environment where one would expect a flightless insectivore, and yet the kiwi outlived the competing mammals in it’s island continent.
It is possible that one of the more bizarre aspects of kiwi biology, the proportionally massive eggs, evolved in response to flightless in such an environment, allowing larger and more independent chicks, both the eggs and juveniles being less vulnerable to mammal or mekosuchine predation. It is now clear that this isn’t the result of allometry, as kiwis evolved from small flying birds and aren’t dwarfs as previously thought. Thus, kiwis may have responded to a rather competitive environment, instead of being the result of 20-16 million years of lack of negative pressures.
5. It offers insight about the progression of palaeognath avifaunas in the Neogene
As showcased before, Proapteryx is a game changer in our understanding of palaeognaths, implicating diversity previously unheard off. It is also an indicator of a rather conspicuous faunal turn over, as there aren’t any more flying palaeognaths in Oz, in spite of the continent’s overall rather conservative bird faunas. Given the rather obvious diversity of many derived bird groups in the Oligocene and Miocene of Australia, and the definite presence of many recognisable birds in the Saint Bathans Formation, such as passerines, rails, shorebirds, owlet-nightjars, parrots and pigeons, competition and eventual ecological displacement by neognaths seem to be unlikely, at least on on a systematic, large scale, especially when “lithornithid grade” palaeognaths don’t seem to have an equivalent in modern australasian avifaunas. Rather, it is possible that Australia’s late Miocene/Pliocene climatic changes spelt doom for Proapteryx like birds, as they have for several other volant terrestrial insectivores like mystacine bats. Flying palaeognaths seem to generally have fared poorly in conditions of mass drying, as evidenced by the disappearence of “lithornithids” from Laurasia and possibly of non-aepyornithid palaeognaths from Madagascar, the only clade seemingly adapting well being nothurine tinamous, which diversified in South America’s Miocene grasslands. By contrast, flightless palaeognaths have basically prospered, albeit in rather different ecological niches, the patterns of which worth investigating in the near future.
Miocene fossils show that kiwi (Apteryx, Apterygidae)
are probably not phyletic dwarves (Worthy et al. 2013)
Paleognathous Birds from the Early Tertiary of the Northern Hemisphere (Peter W. 1988)
Jones et al. 2009
Miocene mammal reveals a Mesozoic ghost lineage on insular New Zealand, southwest Pacific (Tennyson et al. 2006)
Hackett et al. 2008
Turvey et. al 2005
Phillips et al. 2010