''Allkaruen'' (meaning "ancient brain") is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of "

rhamphorhynchoid The Rhamphorhynchoidea forms one of the two suborders of pterosaurs and represents an evolutionary grade of primitive members of flying reptiles. This suborder is paraphyletic unlike the Pterodactyloidea, which arose from within the Rhamphorhync ...

pterosaur Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 ...

from the Early Jurassic Cañadon Asfalto Formation in

Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, th ...

. It contains a single species, ''A. koi.''

Description

As demonstrated by a CT scan, the braincase of ''Allkaruen'' exhibits a unique set of traits that are intermediate between more basal pterosaurs such as ''

Rhamphorhynchus ''Rhamphorhynchus'' (, from Ancient Greek ''rhamphos'' meaning "beak" and ''rhynchus'' meaning "snout") is a genus of long-tailed pterosaurs in the Jurassic period. Less specialized than contemporary, short-tailed pterodactyloid pterosaurs such ...

'' and more derived

pterodactyloid Pterodactyloidea (derived from the Greek words ''πτερόν'' (''pterón'', for usual ''ptéryx'') "wing", and ''δάκτυλος'' (''dáktylos'') "finger" meaning "winged finger", "wing-finger" or "finger-wing") is one of the two traditional ...

pterosaurs such as ''Anhanguera''. These traits are the position of the

anterior semicircular canal The semicircular canals or semicircular ducts are three semicircular, interconnected tubes located in the innermost part of each ear, the inner ear. The three canals are the horizontal, superior and posterior semicircular canals. Structure The ...

of the inner ear, the relative orientations of the

occiput The occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone overlies the occipital lobes of the cereb ...

and

occipital condyle The occipital condyles are undersurface protuberances of the occipital bone in vertebrates, which function in articulation with the superior facets of the atlas vertebra. The condyles are oval or reniform (kidney-shaped) in shape, and their anteri ...

, the relative positions of the

lateral Lateral is a geometric term of location which may refer to: Healthcare *Lateral (anatomy), an anatomical direction * Lateral cricoarytenoid muscle * Lateral release (surgery), a surgical procedure on the side of a kneecap Phonetics *Lateral co ...

margins of the flocculus and

cerebral hemisphere The vertebrate cerebrum (brain) is formed by two cerebral hemispheres that are separated by a groove, the longitudinal fissure. The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres ...

s, and the ratio between the length of the brain and the height of the hindbrain. In addition, the relative orientations of the

frontal bone The frontal bone is a bone in the human skull. The bone consists of two portions.'' Gray's Anatomy'' (1918) These are the vertically oriented squamous part, and the horizontally oriented orbital part, making up the bony part of the forehead, pa ...

and

lateral semicircular canal The semicircular canals or semicircular ducts are three semicircular, interconnected tubes located in the innermost part of each ear, the inner ear. The three canals are the horizontal, superior and posterior semicircular canals. Structure The ...

are more similar to ''Rhamphorhynchus'' than ''Anhanguera'', while the optic lobes are positioned lower than the

forebrain In the anatomy of the brain of vertebrates, the forebrain or prosencephalon is the rostral (forward-most) portion of the brain. The forebrain (prosencephalon), the midbrain (mesencephalon), and hindbrain (rhombencephalon) are the three primary ...

as in pterodactyloids. This unique combination of characters indicates that the anatomy of the braincase in pterodactyloids evolved through

mosaic evolution Mosaic evolution (or modular evolution) is the concept, mainly from palaeontology, that evolutionary change takes place in some body parts or systems without simultaneous changes in other parts. Another definition is the "evolution of characters ...

. The parietals of ''Allkaruen'' were long, being 60% of the length of the frontals; the frontals themselves were broad, flat, and extensively pneumatized. The lower jaw was about 3.5 times the length of the section of the skull that is preserved, and it is curved upwards at its tip. The

dentary In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

bore four or five tooth sockets in its front half, and in the latter half they were replaced by a groove; this combination is unique among pterosaurs. ''Allkaruen'' was a small pterosaur. The holotype specimen would have been an adult, since its skull was entirely fused.

Discovery and naming

The

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of sever ...

of ''Allkauren'' consists of a braincase ( MPEF-PV 3613), a mandible (MPEF-PV 3609), and a cervical vertebra (MPEF-PV 3615); a similar cervical vertebra (MPEF-PV 3616) was also referred to the genus. These elements were in 2000 discovered in the Cañadón Asfalto Formation, Cerro Cóndor, Chubut,

, which has been dated to the Middle-Late

Toarcian The Toarcian is, in the ICS' geologic timescale, an age and stage in the Early or Lower Jurassic. It spans the time between 182.7 Ma (million years ago) and 174.1 Ma. It follows the Pliensbachian and is followed by the Aalenian. The Toarcian ...

, 179.17 ± 0.12-178.07 ± 0.21 m.a (although earlier estimates have suggested an earliest-

Bathonian In the geologic timescale the Bathonian is an age and stage of the Middle Jurassic. It lasted from approximately 168.3 Ma to around 166.1 Ma (million years ago). The Bathonian Age succeeds the Bajocian Age and precedes the Callovian Age. Str ...

age). Other elements, mostly disarticulated and consisting of a number of limb bones as well as another mandible, were also discovered at the site. The name ''Allkaruen'' is Tehuelche in origin, being derived from ''all'' ("brain") and ''karuen'' ("ancient"). The specific name, ''koi'', originates from the Tehuelche word for "lake", in reference to the fact that the type locality would have been a saline lake.

Classification

In 2016, two phylogenetic analyses were conducted based on the dataset from the description of ''

Darwinopterus ''Darwinopterus'' (meaning "Darwin's wing") is a genus of pterosaur, discovered in China and named after biologist Charles Darwin. Between 30 and 40 fossil specimens have been identified, all collected from the Tiaojishan Formation, which dates t ...

'' in order to determine the phylogenetic position of ''Allkaruen''. One analysis was restricted to the holotype braincase, while the other included the holotype mandible and cervical vertebrae as well. ''Allkaruen'' was consistently recovered as the sister taxon of the Monofenestrata; the consensus of the 360 most parsimonious trees is shown below.

References

{{Taxonbar, from=Q26985345 Middle Jurassic pterosaurs Aalenian life Bajocian life Toarcian life Pterosaurs of South America Middle Jurassic animals of South America Jurassic Argentina Fossils of Argentina Cañadón Asfalto Formation Fossil taxa described in 2016 Novialoids