Kelenken
Kelenken is a genus of phorusrhacid, an extinct group of large, predatory birds, which lived in what is now Argentina in the middle Miocene about 15 million years ago. The only known specimen was discovered by high school student Guillermo Aguirre-Zabala in Comallo, in the region of Patagonia, and was made the holotype of the new genus and species Kelenken guillermoi in 2007. The genus name references a spirit in Tehuelche mythology, and the specific name honors the discoverer. The holotype consists of one of the most complete skulls known of a large phorusrhacid, as well as a tarsometatarsus lower leg bone and a phalanx toe bone. The discovery of Kelenken clarified the anatomy of large phorusrhacids, as these were previously much less well known. The closest living relatives of the phorusrhacids are the seriemas. Kelenken was found to belong in the subfamily Phorusrhacinae, along with for example Devincenzia.
Phorusrhacids were large, flightless birds with long hind limbs, narrow pelvises, proportionally small wings and huge skulls, with a tall, long, sideways compressed hooked beak. Kelenken is the largest-known phorusrhacid, 10% larger than its largest relatives known previously. At long, the holotype skull is the largest known of any bird, and has been likened to the size of a horse's skull. The tarsometatarsus leg bone is long. Kelenken is thought to have been about tall and exceeded in weight. Kelenken differed from other phorusrhacids in features such as the length of its beak, in having a supraorbital ossification that fits into a socket of the postorbital process, and in having an almost triangular foramen magnum.
Phorusrhacids are thought to have been ground predators or scavengers, and have often been considered apex predators that dominated Cenozoic South America in the absence of placental mammalian predators, though they did co-exist with some large, carnivorous borhyaenid mammals. The long and slender tarsometatarsus of Kelenken suggests that it could run faster than had previously been assumed for large phorusrhacids, and would have been able to chase down small animals. Studies of the related Andalgalornis show that large phorusrhacids had very rigid and stiff skulls; this indicates they may have swallowed small prey whole or targeted larger prey with repetitive strikes with the beak. Kelenken is known from the Collón Curá Formation, and lived during the Colloncuran age of South America, when open environments predominated, which allowed more cursorial and large animals to occur. The formation has provided fossils of a wide range of mammals, with a few fossils of birds, reptiles, amphibians and fish.
Taxonomy
Around 2004, fossils of a phorusrhacid were discovered by Argentine high school student Guillermo Aguirre-Zabala between two houses, about from the railroad of Comallo, a small village in the north-west of the Río Negro Province in the Patagonia region of Argentina. The outcrops where the specimen was discovered belong to the Collón Curá Formation. Aguirre-Zabala prepared the specimen himself, and the discovery led him to shift from studying psychology to studying paleontology and Earth science.The specimen became part of the collection of the Museo Asociación Paleontológica Bariloche in Río Negro, where it was cataloged as specimen BAR 3877-11. Prior to the animal receiving a scientific name, the specimen was reported and discussed by the Argentine paleontologists Luis M. Chiappe and Sara Bertelli in a short 2006 article. In 2007, Bertelli, 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 honors its discoverer.
The holotype and only known specimen 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 bone 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. The describers concluded these bones belonged to a single specimen due to being collected together, because their general preservation was similar, and because they were morphologically consistent with belonging to a large phorusrhacid. The specimen possessed the most complete skull of a large phorusrhacid known at the time. Previously, such skulls were known only from the fragmentary Devincenzia and Phorusrhacos. The skull of the latter disintegrated during collection, which hampered comparison between phorusrhacid taxa of different sizes, until the discovery of Kelenken.
Evolution
In their 2007 description, Bertelli and colleagues classified Kelenken as a member of the family Phorusrhacidae, based on its enormous size, combined with its sideways compressed, strongly hooked beak, and convex culmen. Five phorusrhacid subfamilies were recognized at the time, though their validity had not then been confirmed through cladistic analysis, and the describers found Kelenken most similar to taxa that had traditionally been considered phorusrhacines. Features shared with phorusrhacines include that the hind part of the skull is low and compressed from top to bottom, a wide occipital table, a blunt postorbital process, and a tarsometatarsus that is similar to that of Titanis in that the supratrochlear surface of the lower end is flat. Further comparison was hampered by the lack of anatomical information about phorusrhacines.The Brazilian paleontologist Herculano Alvarenga and colleagues published a phylogenetic analysis of Phorusrhacidae in 2011 that found Kelenken and Devincenzia to be sister taxa, each other's closest relatives. While the analysis supported there being five subfamilies, the resulting cladogram did not separate Brontornithinae, Phorusrhacinae and Patagornithinae. In their 2015 description of Llallawavis, the Argentinian paleontologist Federico J. Degrange and colleagues performed a phylogenetic analysis of Phorusrhacidae, wherein they found Phorusrhacinae to be polyphyletic. The following cladogram shows the position of Kelenken following the 2015 analysis:
During the early Cenozoic, after the extinction of the non-bird dinosaurs, mammals underwent an evolutionary diversification, and some bird groups around the world developed a tendency towards gigantism; this included the Gastornithidae, the Dromornithidae, the Palaeognathae and the Phorusrhacidae. Phorusrhacids are an extinct group within Cariamiformes, the only living members of which are the two species of seriemas in the family Cariamidae. While they are the most speciose group within Cariamiformes, the interrelationships between phorusrhacids are unclear due to the incompleteness of their remains.
Phorusrhacids were present in South America from the Paleocene and survived until the Pleistocene. They also appeared in North America at the end of the Pliocene, during the Great American Biotic Interchange, and while fossils from Europe have been assigned to the group, their classification is disputed. It is unclear where the group originated; both cariamids and phorusrhacids may have arisen in South America, or arrived from elsewhere when southern continents were closer together or when sea levels were lower. Kelenken itself lived during the middle Miocene, about 15 million years ago. Since phorusrhacids survived until the Pleistocene, they appear to have been more successful than for example the South American metatherian thylacosmilid predators, and it is possible that they competed ecologically with placental predators that entered from North America in the Pleistocene.
Description
Phorusrhacids were large, flightless birds with long hind limbs, narrow pelvises, proportionally small wings and huge skulls, with a tall, long, sideways compressed hooked beak. Kelenken is the largest known phorusrhacid, about 10% larger than the largest phorusrhacids previously known, such as Phorusrhacos. The holotype skull is about long from the tip of the beak to the center of the sagittal nuchal crest at the upper back of the head, making it the largest skull of any known bird. The hind end of the skull is wide. The tarsometatarsus leg bone is long. The head height was up to, while modern seriemas reach in height. While the weight of Kelenken has not been specifically estimated, it is thought to have exceeded.Skull
Prior to the discovery of Kelenken, the skulls of incompletely known large phorusrhacids were reconstructed as scaled up versions of those of smaller, more complete relatives like Psilopterus and Patagornis, as exemplified by a frequently reproduced 1895 sketch of the destroyed skull of the large Phorusrhacos, which was itself based on that of Patagornis. These reconstructions highlighted their assumed very tall beaks, round, high eye sockets, and vaulted braincases, but Kelenken demonstrated the significant difference between the skulls of large and small members of the group. The holotype skull is very massive, and triangular when viewed from above, with the hind portion compressed from top to bottom. The upper beak 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 upper beak 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 upper beak is high and very robust, though apparently not as high as in patagornithines, such as Patagornis, Andrewsornis and Andalgalornis.The front end of the premaxilla 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, rather than the weaker projections of the smaller psilopterines. The underside of the upper beak'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. Patagornis had a similar morphology on the front part of the palate. Much of the upper beak's side is scarred by small, irregular pits, which functioned as nerve exits. The hindmost two thirds of the upper beak are excavated by a prominent furrow, which runs parallel to the margin of the tomium.
The nostrils are small, rectangular, and are located in the upper hind corner of the upper beak as in patagornithines. The nostrils appear to be 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 is not discernible. The quadrangular shape of the antorbital fenestra is clear despite it being crushed somewhat on both sides. The front border of this opening is approximately level with 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 fenestrae, and these bones were recessed in relation to the jugal bar and the outer side margin of each frontal bone. The antorbital fenestra is proportionally smaller than that of Patagornis.
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, 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, and flat from side to side. The jugal bone is about four times taller than thick by the lower center of the eyesocket, and its height is greater than in other phorusrhacids.
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 cannot 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 postorbital processes were probably mainly formed by the frontals. On their lower sides, each frontal forms a large depression where a jaw muscle attached. The postorbital process is separated narrowly from a robust 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 zygomatic process, along the side of the squamosal bone, which corresponds to a jaw closing muscle. The subtemporal fossa further behind is broad and its back is defined by a blunt, sidewards extension of the nuchal crest.
The maxillae form an extensive palate, with the side margins being almost parallel for most of the upper beak's length, and the palate becomes wider from the front back to the region of the eye sockets. Like in Patagornis, these bones are separated at the midline by a distinct, longitudinal depression running much of their length, and along the back half of the palate, this depression is flanked by portions of the maxillae. The side margin at the back of the maxilla has a sutured contact with the jugal which is well-defined, similar to Patagornis. The part of the skull roof behind the eye sockets is flat and scarred by the development of the temporal musculature. The occipital table is very wide, like in Devicenzia, and low, which gives it a rectangular appearance when viewed from behind. The occipital condyle is round with a vertical groove that originates on its upper surface, and reaches almost to the center of the condyle. The foramen magnum is almost triangular, uniquely for this genus, and has a blunt upper apex, and it is slightly smaller than the condyle. Above the foramen magnum is a crest-like prominence, vertically extending from the edge of the foramen to the transverse nuchal crest. A fossa under the condyle is not visible, differing from Patagornis and Devicenzia, whose fossae are distinct.