Kelenken

Location in Argentina where the holotype was found

Fossils of a phorusrhacid "terror bird" specimen were discovered by the Argentine student Guillermo Aguirre-Zabala, about 100 metres (330 ft) from the train station of Comallo, a small village in the north-west of the Río Negro Province in Patagonia, Argentina (coordinates: 41° 01' 59.4" S, 70° 15' 29.7" W). The outcrops where the specimen was discovered are thought to belong to the Collón Curá Formation, though the stratigraphy of the region has only been studied preliminarily. Aguirre-Zabala prepared the fossils, and they became part of the collection of the Museo Asociación Paleontológica Bariloche in Río Negro, where it was cataloged as specimen BAR 3877-11. In 2007, the Argentine paleontolgiusts Sara Bertelli, Luis María Chiappe, and Claudia Tambussi made the specimen the holotype of Kelenken guillermoi; the genus name refers to a spirit in the mythology of the Tehuelche people of Patagonia which is represented as a giant bird of prey, and the specific name honours its discoverer.[1]

The holotype consists of a nearly complete skull which is somewhat crushed from top to bottom, with most of the eye sockets, skull roof, braincase, and left quadrate preserved, while most of the palatal bones behind the eye sockets are missing. The specimen also includes an associated left tarsometatarsus, a small upper portion of a foot phalanx bone, and some indeterminate fragments. These bones were thought to belong to a single specimen based on being collected next to each other, with nothing else collected at the site, in that they all agreed in their general preservation such as color, texture, etc., and because they all fit morphologically with belonging to a large phorusrhacid.[1]

Evolution

Kelenken is assigned to the subfamily Phorusrhacinae, a subfamily of terror birds up to 3.2 metres (10 ft) high, but somewhat slender and decidedly more nimble than the Brontornithinae, together with: Devincenzia, Late Oligocene to Early Miocene (Deseadan) Fray Bentos Formation of Uruguay, Kelenken, Colloncuran of Río Negro Province, Argentina; largest known phorusrhacid, Phorusrhacos, Early to Middle Miocene (Santacrucian) Santa Cruz Formation of Argentina, Titanis, Early Pliocene to Early Pleistocene (Blancan) of Florida and Texas. [2][3]

Reconstructed skeleton of Paraphysornis, a relative from Brazil

The following cladogram follows the analysis of Degrange and colleagues, 2015:[4]

Phorusrhacidae

Mesembriornithinae     Mesembriornis incertus        Mesembriornis milneedwardsi        Llallawavis scagliai      Procariama simplex          Psilopterinae         Psilopterus affinis      Psilopterus bachmanni          Psilopterus colzecus      Psilopterus lemoinei                Kelenken guillermoi        Devincenzia pozzi        Titanis walleri          Phorusrhacos longissimus      Andalgalornis steulleti      Andrewsornis abbotti      Patagornis marshi      Physornis fortis        Paraphysornis brasiliensis

Life restoration

Kelenken is the largest known phorusrhacid; the holotype skull is about 716 mm (28.2 in) long from the tip of the beak to the center of the sagittal nuchal crest, making it the largest skull of any known bird. The hind end of the skull is wide 312 mm (12.3 in). The tarsometatarsus lower leg bone of the holotype is 437.14 mm (17.210 in) long.[1][5][6] One published size diagram has shown it with the head held above 2 m (6.6 ft).[5]

The skull is very massive, and triangular when viewed from above, with the hind portion compressed from top to bottom. The rostrum (or snout) is very long, exceeding half the total length of the skull, unlike in Mesembriornis and Patagornis, and is longer than that of Phorusrhacos. The ratio between the rostrum and the skull of Kelenken is 0.56, based on the distance between the bony nostril and the front tip. In spite of the crushing from top to bottom, the rostrum is high and very robust, though apparently not as high as in the patagornithines such as Patagornis, Andrewsornis, and Andalgalornis. The front end of the premaxilla (the frontmost bone of the upper jaw) prominently projects as a sharp, downturned hook. Such a strong downwards projection resembles most closely the condition seen in large to medium sized phorusrhacids such as Phorusrhacos, Patagornis, Andrewsornis, and Andalgalornis than the weaker projections of the smaller psilopterines.[1]

Illustrations of the holotype skull from the right (top) and left (bottom)

The underside of the rostrum's front portion forms a pair of prominent ridges that are each separated by a groove from the tomium, or sharp edge of the beak. These ridges are also separated from a broader central portion of the premaxilla by a longitudinal groove (the (rostral premaxillar canal). Patagornis had a similar morphology on the front part of the palate. Much of the rostrum's side is scarred by small, irregular pits, which functioned as nerve exits. The hindmost two thirds of the rostrum are excavated by a prominent furrow, which runs parallel to the margin of the tomium.[1]

The nostrils are small and are located in the upper hind corner of the rostrum as in patagornithines (the size and location of the nostrils is unknown in the larger phorusrhacines and brontornithines). The nostrils appear to longer from front to back than high, though this may be exaggerated by crushing, and their hind margin is formed by the maxillary process of the nasal bone. Whether the nostrils are connected to each other at the middle (lacking a septum as in other phorusrhacids) is not discernible. The quadrangular shape of the antorbital fenestra (an opening in front of the eye socket) is clear despite it being crushed somewhat on both sides. The front border of this opening is approximately at the level of the hind margin of the nostril, and its lower margin is straight when viewed from the left side. Robust lacrimal bones form the hind margins of the antorbital fenestra, and these bones were recessed in relation to the jugal bar and the outer side margin of the frontal bone. The antorbital fenestra is proportionally smaller than that of Patagornis.[1]

While the shape of the eye sockets may be slightly affected by compression from top to bottom, it is likely they were low, almost rectangular in shape, with a concave upper margin and a slightly convex lower border. The upper part of the eye socket is delineated by a thick, rounded edge (a supraorbital ossification), the hind part of which appears to overhang downward as seen from the side. In Patagornis, a similar structure has been suggested to be a process of the lacrimal bone, and while the connection between these is not clear in Kelenken, this structure was probably also an extension of the lacrimal. The supraorbital ossification fits within a socket formed by a part of the frontal bone that forms the postorbital process, a configuration unknown in other phorusrhacids. The lower margin of the eye socket is formed by a robust jugal bar which is very tall (larger than that of Devincenzia), and flat from side to side. The jugal is about four times taller than thick by the lower center of the eyesocket, and its height is greater than in other phorusrhacids.[1]

Illustration of the holotype left tarsometatarsus lower leg bone; the upper side is on the left

The frontal bones appear to have been flat on their upper side. The area where the frontals would have contacted the premaxillae is damaged so that their sutures can not be identified, but the sutures between the frontals and the nasals and parietals are fully fused. This fusion makes it difficult to identify how these bones were part of the skull roof, but the blunt, robust postorbitalprocess was probably mainly formed by the frontal. On their lower sides, each frontal forms a large depression where a jaw muscle attached. The postorbital process is separated narrowly from a well-developed zygomatic process, and these two projections enclose a narrow temporal fossa. The postorbital process contains scars left by massive jaw muscles, parts of which invaded most of the skull roof at the level of the parietal bones. There is a well developed depression behind the robust zygomatic process, along the side of the squamosal bone, which corresponds to a jaw closing muscle. The subtemporal fossa is brad and its back is defined by a blunt, sidewards extension of the nuchal crest.[1]

The maxillae form an extensive palate, with the side margins being almost parallel for most of the rostrum's length, and the palate becomes wider from the front back to the region of the eye sockets.[1]

Kelenken lived in the Langhian stage of the Miocene, approximately 15 million years ago. Fossils were found in tuff beds of the Colloncuran Collón Curá Formation in the Cañadón Asfalto Basin of western Patagonia, Argentina.[8]

The Collón Curá Formation represents a transition from a forested ecological biome to more open pampa-type of vegetation. The formation has provided a rich assemblage of mammals, among others the rodent Guiomys unica and the sparassodont Patagosmilus goini and reptiles, mainly snakes and tortoises.[9]

Wikimedia Commons has media related to Kelenken.

• Dalla Salda, Luis, and Juan Franzese. 1987. Las megaestructuras del Macizo y Cordillera Norpatagónica argentina y la génesis de las cuencas volcano-sedimentarias terciarias. Revista Geológica de Chile 31. 3–13. Accessed 2018-09-04.
• Escosteguy, Leonardo, and Mario Franchi. 2010. Estratigrafía de la región de Chapelco, Provincia del Neuquén. Revista de la Asociación Geológica Argentina 66. 418–429. Accessed 2018-09-10.
• Kramarz, Alejandro; Alberto Garrido; Analía Forasiepi; Mariano Bond, and Claudia Tambussi. 2005. Estratigrafia y vertebrados (Aves y Mammalia) de la Formación Cerro Bandera, Mioceno Temprano de la Provincia del Neuquén, Argentina. Revista Geológica de Chile 32. 273–291. Accessed 2017-10-20.