Albertonykus

Albertonykus borealis was a very small alvarezsaurid from the early Maastrichtian Horseshoe Canyon Formation of Alberta, Canada. Its forelimb and hind-limb bones represent at least two individuals, but no skull, spine or articulated skeleton is known. A powerful one-clawed arm led to a feeding hypothesis that researchers tested against insect traces in fossil wood from the same formation.

Quick facts

Discovery in the Albertosaurus bonebed

The main fossils came from the Albertosaurus bonebed at Dry Island Buffalo Jump. Barnum Brown’s expedition discovered the locality in 1910, and Philip Currie relocated it in 1997. During excavations in the early 2000s, small limb bones entered the Royal Tyrrell Museum collections among far more numerous tyrannosaur remains. Nicholas Longrich recognised the alvarezsaurid claw and associated elements, then named the genus with Currie in 2009.

Albertonykus combines Alberta with Greek onyx, claw; borealis means northern and referred to its position relative to most Asian and South American alvarezsaurids then known. TMP 2001.45.91, a left ulna, is the holotype. Referred remains include a large first-finger claw, two tibiae, metatarsal parts and toe phalanges.

Repeated elements and size differences show at least two individuals in the main bonebed, while two toe bones came from nearby Horseshoe Canyon localities. The diagnosis therefore rests on a combination of isolated limb features, not one complete skeleton. Shared burial with Albertosaurus does not establish a predator-prey interaction or simultaneous death.

Arm, legs and body size

The ulna was short and robust, with a developed olecranon and muscle attachments arranged to generate strong elbow extension. As in specialised alvarezsaurids, one main hand digit carried a hooked claw. Such an arm had limited reach and was poorly suited to grasping large prey. Its mechanics favour repeated short movements close to the chest.

The arms were too short for extensive whole-body burrowing like a specialised digging mammal. Local disruption of bark, rotten wood, soil crust or an insect shelter is mechanically more plausible.

Tibiae and metatarsals indicate a light, elongated hind limb. Better-known relatives had an arctometatarsal foot, with the middle element pinched between its neighbours to distribute loads. Comparison supports an active ground-dwelling animal, but no trackway or complete leg gives a speed.

An early popular estimate suggested about 75 centimetres and 3.6 kilograms. Later scaling commonly gives roughly 0.7–1.1 metres and up to several kilograms. The range depends on which relative restores the missing vertebral column, tail and head, so no exact nose-to-tail value is available.

The termite hypothesis

Large surface termite mounds are not documented for this time, so Longrich and Currie considered wood-dwelling social insects. Fossilised Horseshoe Canyon wood contains galleries resembling those of modern dampwood termites. This matters because it demonstrates a suitable local food source instead of relying only on analogy with living anteaters.

The match is still circumstantial. No digestive remains or wood with attributable claw marks belongs to Albertonykus. It may have eaten several kinds of invertebrates or other small prey. Its very short arms also favour opening a local patch of bark or rotten wood over excavating a deep burrow.

Age, environment and dispersal

Sandstones, mudstones and coals of the upper Tolman Member of the Horseshoe Canyon Formation record rivers, floodplains and estuarine settings near the retreating Western Interior Seaway. The locality lay near 60° north and experienced marked seasonality. The bonebed dates to the early Maastrichtian, about 68.5 million years ago.

Bonebed accumulation may reflect a catastrophic event, but the small Albertonykus bones need not have entered it exactly with every tyrannosaur. Rare hadrosaur and other terrestrial vertebrate remains also occur in the assemblage.

The 2009 analysis placed Albertonykus near derived Asian alvarezsaurids and supported a then-popular dispersal route from South America through North America into Asia. Older Chinese transitional forms later made the early history more complex. Albertonykus remains a secure North American record, but one genus cannot establish a single migration route.

What is known about its appearance

Only limb elements directly define its appearance. A small body, strong short arms and slender legs are well supported. A narrow skull with small teeth and a feathered covering are inferred from relatives such as Mononykus and Shuvuuia. Neck length, tail shape, colour and soft tissues remain artistic choices because no skull or feather impression belongs to Albertonykus.

Evidence and reconstruction