Allkaruen: a Jurassic pterosaur from Patagonia

Allkaruen koi is a Jurassic pterosaur from Patagonia whose best-known fossil is a three-dimensionally preserved braincase. CT scans revealed a mosaic of anatomical features that helped place it near the evolutionary split leading toward later pterodactyloids. The name means “ancient brain” in Teushen, referring to the central specimen. Other isolated bones have been associated with the genus, but they were not found articulated with the braincase. Its profile belongs in the pterosaur catalogue as a rare South American record from early pterosaur evolution.

Quick facts

A braincase from the Cañadón Asfalto Formation

Laura Codorniú and colleagues named Allkaruen koi in 2016. The genus name combines Teushen words meaning “ancient brain”, while koi means “lake”. The name refers both to the well-preserved braincase and to the lake deposits at La Yesera in Argentina's Chubut Province. The principal specimen, MPEF-PV 3613, is held at the Egidio Feruglio Paleontology Museum.

The holotype preserves the braincase in three dimensions, an unusual advantage among early pterosaurs whose fossils are often flattened or represented by isolated limb bones. A lower jaw, MPEF-PV 3609, and a cervical vertebra, MPEF-PV 3615, were also referred to the taxon from the same locality. They are separate specimens, not parts visibly connected to the type. Their assignment is based on the description and comparisons, so they should not be presented as one articulated skeleton.

Internal anatomy and a mosaic of traits

Computed tomography allowed researchers to inspect the cavity around the brain and the inner-ear labyrinth without destroying the specimen. The shape of the semicircular canals, the rear of the skull and the openings for nerves revealed a combination of characters. Some resemble those of early long-tailed pterosaurs such as Rhamphorhynchus, while others occur in more derived pterodactyloids.

The original phylogenetic analysis placed Allkaruen close to the base of Monofenestrata, the group that includes pterodactyloids and their near relatives. In that sense it is often described as transitional: its anatomy combines character states associated with different parts of the pterosaur tree. “Transitional” does not mean it was the direct ancestor of later forms. It is a branch that preserves a useful combination, and the exact branching pattern can change as new fossils and character sets are analysed.

Later analyses have not always recovered the same relationship. Studies using broader pterosaur datasets have supported different positions among early monofenestratans, including comparisons with Darwinopterus. Such movement is expected when the taxon is represented chiefly by a braincase and the matrix includes many characters from the rest of the skeleton that cannot be scored. Its internal cranial features remain valuable even when its precise place in the tree is debated.

Dating a Jurassic pterosaur

The Cañadón Asfalto Formation has a complex chronology, and estimates for its fossil-bearing levels have changed. Literature has placed the material from the later part of the Early Jurassic to the early Middle Jurassic. The broad statement “Jurassic” is secure; selecting a narrower interval depends on which dating evidence and stratigraphic interpretation are followed. This uncertainty makes Allkaruen important without requiring a single exact age.

Its record also matters geographically. Early pterosaur fossils are unevenly distributed, and Patagonia provides a rare window into the southern part of the ancient world. The nearby lake setting is supported by the sedimentary context, but does not prove that the pterosaur routinely hunted over water. The teeth, food remains and full wing are not known well enough to identify a precise diet or flight style.

What remains outside the fossil

The braincase reveals more internal anatomy than an ordinary surface impression, but most of the body remains unknown. The full skull, crest, tail, wing span, skin covering and colour are not preserved in the type. The referred jaw and neck bone add observations but do not remove the uncertainty about how the animal looked as a whole.

Allkaruen is therefore most informative as a specimen for studying braincase anatomy and early pterosaur relationships. Its name, locality and three-dimensional preservation are well established. Its exact wingspan, diet, behaviour and direct ancestry are not. Reconstruction can fill those gaps for an illustration, but the distinction between observed bone and comparison should remain explicit.