TheSt Bathans fauna is found in the lowerBannockburn Formation of theManuherikia Group ofCentral Otago, in theSouth Island ofNew Zealand. It comprises a suite offossilised prehistoric animals from the lateEarly Miocene (Altonian) period, with an age range of 19–16 million years ago.
The layer in which the fossils are found derives fromlittoral zone sediments deposited in a shallow, freshwater lake, with an area of 5600 km2 from present day Central Otago to Bannockburn and the Nevis Valley in the west; to Naseby in the east; and from the Waitaki Valley in the north to Ranfurly in the south. The lake was bordered by an extensivefloodplain containingherbaceous andgrassywetland habitats withpeat-formingswamp–woodland.[1] At that time the climate was warm with a distinctly subtropical Australian climate[2] and the surrounding vegetation was characterised bycasuarinas,eucalypts andpalms as well aspodocarps,araucarias andsouthern beeches.
The fossiliferous layer has been exposed at places along theManuherikia River and at other sites in the vicinity of the historic gold mining town ofSt Bathans. The fauna consists of a variety of vertebrates, includingfish, acrocodilian, arhynchocephalian (a relative oftuatara),[3]geckos,[4]skinks,[4]a primitive mammal,[5] several species ofbats,[6] and several kinds ofbirds, especiallywaterbirds.[7] Of tree-dwelling birds, parrots outnumber pigeons thirty to one.[8]Proapteryx, a basal form ofkiwi, is known from there. The Miocene ecosystem was recovering from the ‘Oligocene drowning’ a few million years earlier, when up to 80% of the current land area of New Zealand was submerged. The wildlife that lived in, on, and around the palaeolake Manuherikia was uniquely New Zealand, which strongly suggesting that some emergent land remained during this near drowning event.[9] Marked global cooling and drying during the Miocene, Pliocene and the Pleistocene Ice Ages resulted in the extinction of the 'subtropical' elements of the St Bathans fauna. Those that survived adapted to the dynamic geological and climatic changes, and would form part of the enigmatic fauna that characterised New Zealand when humans arrived in the late 13th century.[10]
Research on the St Bathans fauna is led byTrevor Worthy, a New Zealander based inFlinders University, Adelaide. Other key scientists involved includeJenny Worthy from Flinders University,Paul Scofield andVanesa De Pietri fromCanterbury Museum, andAlan Tennyson from theMuseum of New Zealand Te Papa Tongarewa.[11] In 2016 Vanesa De Pietri was awarded aRoyal Society of New ZealandMarsden Fast Start grant to study the shorebird fossils.[12] This long-running (since 2000) collaborative research programme also includes scientists from theUniversity of New South Wales in Sydney and from theUniversity of Queensland in Brisbane.[10]
Surprisingly, given modern New Zealand's dearth of land mammals, there is a basaltheriiform mammal, theSt Bathans mammal.[5] Several species ofmystacinebats are also known, as well as avesper bat and severalincertae sedis species.[6][13] This bat fauna includedVulcanops, a giant burrowing bat three times the size of today’s relatives, and more closely related to South American bats.[14] This suggests that small land mammals were a common component of New Zealand's fauna in the Miocene, with even bats being significantly more diverse than today.
New Zealand's two modernpalaeognath clades, thekiwi andmoa, have early representatives in the fauna. The former is represented by the diminutive, possibly volantProapteryx.[15] The latter is represented by several bones and egg shells of currently unnamed species, but already identifiable as true moa, being large sized and flightless.[16] The fact that moa are already recognisably modern in anatomy, and possibly ecology, while kiwis are fairly unspecialised and probably still flighted, confirms the previous suspicions that neither clade is closely related and that they arrived in New Zealand independently: moa arrived and became flightless earlier in theCenozoic, while kiwi were then recent arrivals.[15]
Anseriforms (waterfowl) dominate the fauna. At least nine species are recognised from St Bathans, making it the richest waterfowl fauna in the world.[9] All the waterfowl species are unique to New Zealand. Bones attributable to Cape Barren goose (Cereopsis spp.), thought to represent the ancestors of extinct Pleistocene-HoloceneCnemiornis goose, and those of a second possible goose species have been found.[17] In both instances, there is not enough material currently to erect species.[17] Stiff-tailed ducks dominate the fauna withManuherikia lacustrina,M. minuta,M. douglasi,Dunstanneta johnstoneorum and a further undescribed species ofManuherikia.[1][18][9] One species ofshelduck,Miotadorna sanctibathansi, has been found and is common.[1] The dabbling duckMatanas enrightii remains poorly known as only a few fossils have been found.[1]
Palaelodids are ancient relatives of flamingos. The new species from St Bathans (Palaelodus aotearoa) is smaller than, and morphologically distinct from, the Late Oligocene-Early MiocenePalaelodus wilsoni from Australia.[19]
Two pigeon species have been described.Rupephaps is a large fruit pigeon, possibly related to the modernHemiphaga species. The Zealandian dove is similar to theNicobar pigeon.[8]
SeveralGruiformes have been described. The St Bathans adzebill (Aptornis proasciarostratus) was only slightly smaller than its more recent descendants.[20] There were two flightlessrails: the commonPriscaweka parvales and uncommonLitorallus livezeyi.Priscaweka parvales was no bigger than a sparrow.[21]
Charadriiformes, including gulls, terns, noddies, snipes, dotterels, plovers, jacanas, oystercatchers, sheathbills and theplains-wanderer, are a large group of birds that are mostly found in marine or semi-marine environments. There are about 350 species, and they are mostly small to medium-sized. Two of these are known from St Bathans, the New Zealand lake-wanderer (Hakawai melvillei), a relative of the plains-wanderer, and Sansom's plover (Neilus sansomae), a plover-like bird of uncertain affinities but possibly related to sheathbills and the Magellanic plover.
Petrels are seabirds in the orderProcellariformes. This group includes albatrosses. Petrels today make up most of all species of seabird, and the order is the only order of birds to be entirely marine. One species of petrel is known from the St Bathans Fauna – adiving petrel in the same genus as modern diving petrels, the Miocene diving petrel (Pelecanoides miokuaka).
At least twoherons are known:Pikaihao bartlei andMatuku otagoense. The former is abittern, while the latter is a much larger species that appears to be basal withinArdeidae (the herons).[7]
Oneeagle, similar in size to awedge-tailed eagle, and another bird of prey, similar in size to a smallhawk, have been found, but await formal description.[9]
Twoparrot genera are represented.Heracles is represented by its sole species,Heracles inexpectatus, the largest known parrot, weighing 7 kilograms and standing 1 meter tall.Nelepsittacus is represented by at least four species. These vary drastically in size, suggesting that they occupied a wide variety of ecological niches, having diversified in the relative absence of other parrots.[22]
ANew Zealand wren,Kuiornis indicator, is known from these deposits, possibly similar to the modernrifleman.[23] Two or three otherpasserine species remain undescribed.
The St Bathans fauna is rich in reptile and amphibian remains. Several groups present in modern New Zealand are represented, such asleiopelmatidfrogs,[24] asphenodontian similar to the moderntuatara,[3]geckos, andskinks.[4] However, there are also several species not seen in modern-day New Zealand, such as amekosuchinecrocodile up to 3 metres in length[25] andpleurodire andmeiolaniidturtles.[26] This suggests that New Zealand'sherpetofauna was much richer in this epoch, probably because its climate was considerably warmer than today.[26]
The vast majority of the bones excavated from St Bathans are those of freshwater fish such as the ancient relatives of today'sbullies,galaxiids, and the extinctNew Zealand grayling.[27]
As well as fishes, shellfish, including freshwater mussels, and freshwater crayfish dominated the aquatic life in the palaeolake Manuherikia.[10] A new species of St Bathans freshwater limpet,Latia manuherikia, was described bymalacologistBruce Marshall in 2011. This was both the first known fossilLatia and the first record of this genus from the South Island.[28]
Notable examples of absent taxa includemarsupials,snakes,agamid andvaranid lizards,lungfish,eels,cockatoos, and all but one lineage (bellbirds andtūī) of the 80 species ofAustralian honeyeaters.[10]
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