Manuherikia: a Miocene diving duck from New Zealand

Manuherikia is an extinct genus of diving ducks from the Early to Middle Miocene St Bathans fauna of New Zealand. The original species, M. lacustrina, is represented by hundreds of bones, enough to compare much of its skeleton rather than infer the bird from one isolated element. Limb anatomy identifies it as a foot-propelled diver, while a spathulate bill with lamellae and enlarged salt glands suggests it could use both freshwater and more saline habitats. Additional species expanded the genus, and M. primadividua revealed a stratigraphic division in the fossil bird assemblage. The extinct bird catalogue includes Manuherikia as one of the best documented Miocene waterfowl of the Southern Hemisphere.

Quick facts

A diverse fossil duck from St Bathans

The genus was described from the St Bathans fauna of Central Otago, on New Zealand's South Island. Its fossils occur in the Manuherikia Group, especially the Bannockburn Formation, deposited around an ancient lake system between about 19 and 16 million years ago. What is now an upland interior landscape once included broad water margins, inflowing rivers and vegetated shorelines. The sediments preserve a varied community of fish, reptiles, bats and birds.

The type species Manuherikia lacustrina became much better known after additional material was referred to it. Trevor Worthy and Michael Lee's 2008 study described 551 specimens, including skeletal elements that had not been available when the species was first named. A large referred sample does not mean that hundreds of skeletons remain articulated: it consists of separate bones that researchers identify and compare. Still, the range of elements allows a more complete anatomical assessment than is possible for many fossil ducks.

Several duck lineages occurred in the St Bathans fauna, including Dunstanetta and Miotadorna. Their presence indicates that waterfowl diversity was substantial, but does not mean the birds all occupied the same feeding niche. Size and skeletal differences support ecological variety, while details of competition or daily interactions are not directly preserved.

Anatomy of a foot-propelled diver

The skeleton of M. lacustrina includes features associated with specialised diving. Researchers noted an expanded fibular condyle and deep popliteal fossa on the femur, a prominent cnemial crest on the tibiotarsus, and a distinctive orientation of the second toe's trochlea on the tarsometatarsus. In combination these structures support powerful leg-driven propulsion underwater. This is stronger evidence than simply calling the bird a diver because it lived near a lake.

Comparisons with living ducks help interpret how those joints and muscle attachments functioned. The 2008 study considered M. lacustrina similar in size and possibly ecological niche to the modern Australian blue-billed duck, Oxyura australis. That is a comparison, not a claim that the fossil bird behaved identically or belonged within the living stiff-tailed duck genus. Phylogenetic placement varies with the character set and taxa included.

The skull is fragmentary, but the available bill material shows a spatulate form with lamellae, structures that help filter or retain food. Enlarged salt glands were also described. These findings suggest that the bird could have used saline waters as well as the freshwater palaeolake. They do not prove its usual habitat was coastal or marine, and the exact foods are not preserved as gut contents.

More species and a changing lake fauna

Several species have been assigned to Manuherikia, including M. lacustrina, M. minuta, M. douglasi and the later-described M. primadividua. They differ in size and skeletal proportions, but the completeness of their material is uneven. Some species were initially based on humeri, while later discoveries provided other bones that could be referred. This uneven record means that one species cannot stand in for every member of the genus.

The description of M. primadividua in 2022 added a new species and a valuable stratigraphic signal. Its fossils occur in horizons above those yielding M. lacustrina in the Manuherikia River section. The species are not found together in the sampled beds, and the upper and lower occurrences have been used to recognise separate bird-fauna zones within the Bannockburn Formation. This is evidence of turnover through time, rather than simply more ducks being discovered at one level.

Stratigraphic absence should nevertheless be handled carefully. A species may fail to appear in a particular sample because fossils are rare, layers are incompletely exposed or collecting effort differs. The observed separation is meaningful because it recurs across mapped fossil horizons, but it is not a census of every bird living around the lake in each interval.

Food, appearance and what remains unknown

Foot-propelled diving ducks commonly pursue aquatic invertebrates or plant material, and the bill of Manuherikia may have helped handle small items. These are comparisons with living waterfowl, not direct observations of fossil feeding. The bill is less completely represented than the limb skeleton, and no meal contents identify a precise diet for the genus. Different species may also have used the lake in different ways.

St Bathans sediments preserve the setting of the bones but not the entire life of each bird. A fossil from lake-margin deposits shows where it entered the sedimentary record; it does not prove the animal never visited rivers or other waters. Likewise, the existence of multiple duck species suggests ecological diversity, but does not by itself reveal territorial behaviour, flock size or competition for specific food.

No feather impressions or skin preserve colour, pattern or display. A reconstruction can reasonably depict a compact duck with a specialised diving leg, but borrowing the exact plumage of a modern stiff-tailed duck goes beyond the evidence. Manuherikia is most informative when its rich bones and changing stratigraphic record are kept distinct from details that remain conjectural.