Kiwi are an enigmatic and threatened bird group, unique to New Zealand, with five living species. While modern kiwi taxa are well represented in Late Pleistocene/Holocene bone deposits, the fossil record earlier than this is extremely poor, and so very little is known about the origin and evolution of kiwi.
The Marton kiwi is known from a unique foot bone (tarsometatarsus) found in sediment north of Marton in 1998. This holotype bone (NMNZ S.36731) is held at the Museum of New Zealand Te Papa Tongarewa, Wellington. It is well-preserved but slightly damaged at each end.
The site where it was found has produced a small but significant fossil bird fauna, including a large rail (Fleming’s rail Pleistorallus flemingi), as well as unidentified remains of moa and another smaller rail. The bones were in a mid-Pleistocene pumice sand layer (about 1 million years old). At the time of its deposition, the site was coastal shoreline, being either an exposed beach or a more sheltered environment. The fact that bones at this site have been found as isolated elements with their extremities worn, indicates that they were washed around in water before final deposition. The specific name of the kiwi – littoralis – reflects that it was found on an ancient marine shore (= littoral) and that the species may have been restricted to a coastal region due to volcanic activity in the central North Island.
The fossil is very similar to the tarsometatarsi of living kiwi species, most closely resembling those of the North Island brown kiwi and the rowi in size and shape, but it differs in being stouter, with proportionally narrower ends. This indicates that the toes were less splayed than those of modern kiwi species.
The fossil’s overall similarity to that of modern species suggests that the Marton kiwi had a similar ecology to its modern relatives and that it was flightless. The contemporary terrestrial environment where the fossil was deposited probably included tall, wet, closed canopy forests – a habitat that modern kiwi commonly inhabit. Thus, it seems that some mid-Pleistocene kiwi were not only similar in size and appearance to modern kiwi but also lived in similar environments.
For flightless taxa, like kiwi, separation between the North and South Islands of New Zealand was/is a major barrier to dispersal. One theory is that the modern lineage of kiwi originated on the South Island, with the oldest colonisation of the North Island at either 1.6 or 1.1 million years ago, after the islands joined. Under this scenario, ancestors of the Marton kiwi could have arrived from the south a few 100,000 years earlier. The two islands were connected for long periods after the time that the Marton kiwi was alive. Thus, its descendants potentially could have walked between these landmasses before the current physical isolation between the islands occurred.
However, the last million years has been a tumultuous period in New Zealand’s geological history, with extensive volcanism, tectonic uplift and several glacial cycles. As kiwi are relatively poor dispersers, and the country’s history led to many allopatric kiwi lineages, these factors have made kiwi vulnerable to extinction. The basin where the Marton fossil site occurs would have been particularly vulnerable to volcanic events, e.g. the one million year old Potaka tephra (in which the kiwi fossil was found) was an ignimbrite that originated from the central North Island and left widespread deposits in the region. The eruption of the Taupo volcano 26,500 years ago covered much of the North Island in ash, with about 0.5 m falling in the Marton region. The habitat devastation that such events caused may have been enough to wipe out a local kiwi population and, if the range of the Marton kiwi was already small, then it is plausible that this resulted in its complete extinction. This might explain why the unusual features of the tarsometatarsus of the Marton kiwi are not found in modern taxa.
Two species of kiwi are now known from the fossil record: the Marton kiwi and the Miocene St Bathans kiwi. The St Bathans kiwi lived 19-15 million years ago; it was much smaller than any living species and possibly volant. The Marton kiwi is therefore the second oldest species of kiwi known and it fills in one further piece of the puzzle of kiwi evolution.
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