Presbyornis: an early waterbird with long legs

Presbyornis was an early Cenozoic waterbird with a combination seldom seen today: a broad, flattened bill, webbed feet and exceptionally long legs. Its body was about the size of a small goose, while its extended legs raised it to around a metre in height. Associated skeletons show that it could both walk and swim, and its wings and breastbone confirm powered flight.

The genus was first known from isolated bones that resembled those of several unrelated wading birds. Better skulls and skeletons placed it among early anseriforms, although its precise position near the base of the duck-and-goose branch remains debated. The extinct bird catalogue places Presbyornis among other fossil lineages without treating a similar body shape as proof of close relationship.

Quick facts

From scattered leg bones to associated skeletons

Alexander Wetmore named Presbyornis pervetus in 1926 from incomplete early Eocene material in the United States. Its long leg bones initially invited comparisons with shorebirds and flamingos. Those comparisons made sense from the proportions alone, but the evidence changed when skulls and associated skeletons revealed a broad bill, a characteristic shoulder girdle and other features of waterfowl anatomy.

The fossils do not make Presbyornis an ordinary modern duck stretched upward on stilts. Presbyornithids were an extinct branch of early waterbirds. Analyses generally place them within or close to the base of Anseriformes, but their exact position relative to screamers, magpie geese, ducks and geese depends on which skeletal characters and fossil taxa are included. The square bone of the skull retains primitive features that caution against placing the genus inside any living duck family.

Flamingos are a useful comparison for wading posture, not a demonstrated close relative. The Cretaceous Hesperornis also lived in water, but it was a toothed diving bird with a very different skeleton. Similar habitat alone does not link these birds on the evolutionary tree.

A bill for shallow-water feeding

The bill was broad and flattened. Rows of fine plates, or lamellae, ran along its edges and could retain small particles as water drained from the mouth. This anatomy is consistent with filtering or picking food from the water column and the soft surface of lake sediment. The fossils do not preserve a complete menu. Small invertebrates, seeds and soft plant material are plausible, but their relative importance cannot be measured from the bill alone.

Long lower legs allowed the bird to wade farther into shallow water without immersing its body. Its toes were connected by webbing, as shown by track evidence, so it was not simply a terrestrial wader with narrow shorebird feet. It could walk over soft lake margins and swim. Some sediment-surface traces resemble probing or sweeping movements, but not every track or feeding mark can be assigned securely to Presbyornis.

The wings and a well-developed breastbone show that it retained powered flight despite its tall stance. Lightly built legs would not have imposed the same burden as heavy, flightless limbs. No fossil records its migration route, seasonal range or flight distance. Migration is ecologically possible, not directly demonstrated.

What the large bone beds mean

Green River deposits contain abundant Presbyornis bones, and some localities also preserve eggs and eggshell. Together these finds support nesting in colonies near lakes and a social way of life. They do not provide a count of one living flock. Bones can accumulate during separate events, move with water and become mixed before burial.

Researchers have proposed toxic algal blooms, disease, volcanic ash and changes in lake level as possible contributors to mass mortality at particular sites. None should be applied automatically to every bone bed. Establishing a cause requires the sediment, preservation pattern and associated fossils from each layer to be assessed on their own.

Living in groups may have helped individuals locate productive shallows and reduced danger to any one bird. It could also have increased competition for food, and colonies may have shifted as water levels changed. These are reasonable ecological interpretations of the deposits, not preserved details of daily social behaviour.

An early experiment in waterfowl ecology

Presbyornis lived after non-avian dinosaurs had disappeared, when modern bird groups were diversifying. It shows that early anseriforms already occupied a range of feeding and habitat roles. The filtering bill and webbed toes fit waterfowl, while the long legs are an unusual combination that later became uncommon.

Compared with the much older Archaeopteryx, Presbyornis had a toothless beak and a modern bird-like flight apparatus, with neither a long bony tail nor teeth. Among Cenozoic birds it occupied the role of a lake-margin waterbird, but its fossil record still leaves colour, courtship, exact diet and seasonal movement unknown.