Xibalbaonyx
| Xibalbaonyx | ||||||||||||
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| Temporal occurrence | ||||||||||||
| Upper Pleistocene to Lower Holocene | ||||||||||||
| 11,000 years | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Xibalbaonyx | ||||||||||||
| Stinnesbeck , Frey , Olguín , Stinnesbeck , Zell , Mallison , González , Núñez , Morlet , Mata , Sanvicente , Hering & Sandoval , 2017 | ||||||||||||
Xibalbaonyx is an extinct genus of sloths from the Megalonychidae family. It is documented by several skulls from Mexico that come from two different regions: on the one hand, various cenotes from the Yucatán peninsula, on the other hand, from the Lake Chapala region further west. Both skulls date to the end of the Pleistocene . The finds testify to a large representative of the family, who reached roughly the size of Megalonyx . The very small front canine-like teethare special features, as well as the lack of a crest , which is otherwise typical of numerous extinct members of the Megalonychidae. The animals fed on soft leafy food, but individual anatomical differences suggest regional deviations. The genus was introduced in 2017. Another large representative of the Megalonychidae from Mexico named in the same year could be identical to Xibalbaonyx .
features
Xibalbaonyx was a large representative of the Megalonychidae. It reached the maximum size of Megalonyx , which in turn was about the size of a cattle . Xibalbaonyx is documented through several skull finds . Their length varied from 29 to 39 cm with a width of 12 to 14.6 cm (without zygomatic arch ) and a height of 15.7 to 21.4 cm. In side view, the skull was regularly arched at the forehead line. Only at the level of the nasal bone did the skull draw in markedly, clearly separating the blunt rostrum . This is a striking difference to today's two-toed sloths ( Choloepus ), in which the skull merges smoothly into the snout. In addition, the skull is more bulging at the back. The nasal bones of Xibalbaonyx were almost horizontal. Overall, they took up around a third of the total length of the skull. The nostrils measured 7.2 cm in width and 8.5 cm in height in large individuals and were almost vertical. It did not have the comparatively large dimensions of Ahytherium . The relatively large height of the nostril compared to the width also meant that the upper jaw was very high. It protruded slightly beyond the nasal bone, in side view it had a trapezoidal shape with the larger long edge at the base. The frontal bone also took up about a third of the length of the skull. At its front end, the nasal bone pushed far back. Two distinctive temporal lines were formed on the strongly arched parietal bones , which ran almost parallel. A crest , as it was found in Megalonyx , Ahytherium or some Caribbean representatives of the Megalonychidae, did not exist. As with many sloths, the zygomatic arch did not form a completely closed arch. The anterior arch section typically consisted of three processes: one long ascending and descending, and one short horizontally. The ascending and descending processes were at right angles to each other, so the opening between the two was wider than that of Megalonyx . The posterior arch attachment was also long, it began at the posterior section of the temporal lines that crossed the temporal bone here . The cheekbone also formed part of the orbital margin . In the posterior view, the occiput was round in shape. The articular processes for connection with the cervical spine were convex and oriented downwards. There was also a prominent horizontal bulge on the occiput. At the base of the skull, the glenoid pit for the joint with the lower jaw appeared flat and broad in the transverse direction and narrow in the longitudinal direction of the skull. The wing bone region was inflated and hollow and thus elongated in the longitudinal direction and of a rhombic shape. At least 40 small bone openings (foramina) with a diameter of 0.2 to 1.5 mm were found on the palate .
The lower jaw was massive and compact. It measured between 21 and 30.3 cm in length. When viewed from the side, it was shaped like an hourglass . The spoon-like extension of the symphysis at the front end of the lower jaw, which is characteristic of sloths, was comparatively short at 1 to 3 cm in Xibalbaonyx . It tapered towards the tip. In terms of its dimensions, the extension was roughly the same as that of Nohochichak , but it was longer than that of Megalonyx and shorter than that of Pliometanastes . The lower edge of the horizontal bone body was sinusoidal with the largest protuberance below the row of teeth. In the area of the diastema there were numerous foramina, each of which had openings of 0.5 mm; they are partly interpreted as mental foramen . The ascending branch was wide and up to 13 cm high. When viewed from the outside, its leading edge completely covered the rearmost tooth. The crown process rose about 5 cm above the upper edge of the horizontal bone body. The articular process sat lower, which in turn is a difference to Meizonyx . The joint itself was long hourglass-shaped when viewed from above and arched when viewed from the side. The distinctive angular process at the rear end of the lower jaw formed a blunt finish. At the base of the crown process, the mandibular canal opened on the outside of the lower jaw to a width of 10 mm. It reappeared on the mandibular foramen below the crown and articular processes on the inside of the lower jaw. The foramen itself was partially doubled in contrast to Meizonyx and Nohochichak with their simple mandibular foramen.
The dentition had the structure typical of sloths, consisting of a canine-like ( canine - shaped ) tooth in the front section and several molar-like ( molar-shaped ) teeth in the rear. There were four molar-shaped teeth in the upper dentition and three molar-shaped teeth in the lower half , so that the dentition consisted of a total of 18 teeth. The caniniform tooth had an oval to triangular cross-section, in the upper row of teeth it was curved backwards, in the lower row it was more or less straight. However, it only protruded slightly beyond the rear teeth and was therefore comparatively small. There was a shear edge at each tip; the upper canine-shaped tooth pointed backwards and the lower one forward. There was a diastema in the posterior segment of the teeth. In the upper jaw this took up about a quarter of the length of the skull, in the lower jaw it was about the length of the symphysis process. The molar-shaped teeth were rectangular, trapezoidal or oval in cross-section and had high tooth crowns. Their chewing surfaces were, in agreement with other representatives of the Megalonychidae, typically bilophodontic (with two transverse ridges on the chewing surface). The tooth was deeply cut between the individual ridges. The entire upper row of teeth measured 16.8 cm in length in large individuals including the approximately 8 cm long diastema. The corresponding value for the lower row of teeth was 13.1 cm (including a 4.9 cm long diastema), here the rear teeth were 7.8 cm.
Of the few postcranial skeletal elements known so far , the humerus should be emphasized, which was short and robust with a length of 32 cm. The shaft was characterized by strong muscle marks and bone ridges. They gave the surface of the shaft a rough character. The lower end of the joint was wide.
Fossil finds
The previously known fossil material of Xibalbaonyx comes from two more distant regions in Mexico . The first find used to describe the genus came to light on the Yucatán peninsula . It was discovered there in the cenote of El Zapote west of Puerto Morelos and about 36 km south of Cancun and 90 km north of Tulum in the Mexican state of Quintana Roo . El Zapote is part of an underground karst system over 7000 km long , of which only a small part has been investigated so far. The access to El Zapote is now under water and consists of a 30 m long and 10 m wide chimney that leads to a 40 m wide and 60 m high chamber. This is covered with lime-rich speleothems , which are known as "hell bells" due to their large, 2 m and more measuring and characteristic shape. Inside the chamber, the remains of Xibalbaonyx were found at a depth of 50 to 55 m. They consist of a complete skeleton that is spread over an area of 3 m. So far, only the skull has been recovered from this. Radiocarbon dates give this an age of 10,647 to 10,305 years BP . The data indicate that the zenote, like much of the karst system in the transition from the Pleistocene to the Holocene, was flooded by rising sea levels as a result of the glacial melt of the last glacial period . A lower jaw, a cervical vertebra and an upper arm bone also came to light in the Cenote Tortugas on the Yucatán, locally also known as Cementerio de Xenartros ( Spanish for "cemetery of the collateral animals"). This is only 2 km southwest of El Zapote. It has a diameter of 25 m and a depth of 55 m, with the remains of the sloth only 1 m above the lowest point. There is no access to other cenotes in the region. Also from the peninsula, Nohochichak is another large representative of the Megalonychidae with almost simultaneous occurrence.
Another skull has been reported from Zacoalco west of Lake Chapala in the Mexican state of Jalisco . The 70 km long and 30 km wide Lake Chapala is the largest lake in Mexico, but it only represents the remainder of a much larger inland body of water that existed in the Pleistocene. This filled the Chapala rift system, a tectonic depression between the Sierra Madre Occidental and the Jalisco block . The region, which has been the focus of scientific interest since the 1950s, is rich in fossil finds from the end of the Pleistocene with remains of fish, amphibians, reptiles, birds and mammals. The latter also include several representatives of the secondary articulated animals . Eremotherium , one of the largest known representatives of the sloth, has been described from Zacoalco . Before it was scientifically processed in 2018, the skull of Xibalbaonyx was partially considered to be a remnant of a member of the Mylodontidae or Megalonychidae.
Paleobiology
In general, a specialization in leafy plant food is assumed for the extinct representatives of the Megalonychidae analogous to the recent two-toed sloths ( browsing ). This can be seen, among other things, from the generally narrow snout including the narrow, spoon-like symphysis extension on the lower jaw. Xibalbaonyx shows individual special adjustments in this aspect. The lack of a crest indicates that the temporalis muscle probably played a smaller role than other members of the family. This suggests that plants are less fiber-rich as a source of food. The elongated wing bone advocates a more developed masseter muscle or pterygoideus muscle, the former is extended over the zygomatic arch, the latter over the articular and angular process. Both cause the mouth to close as well as lateral and forward or backward chewing movements. The dominance of the masseter muscle speaks against increased biting force and thus in favor of very soft plant food. The caniniform teeth are smaller in Xibalbaonyx than in related forms. This is to be interpreted as an indication that they were only used to a minor extent during food intake.
In addition to these general differences to other representatives of the Megalonychidae, the regional forms of Xibalbaonyx also show certain deviations from one another. The skull from the Lake Chalapa region to the west has more developed temporal ridges than that of the more eastern Yucatán Peninsula. In the former, the crown process on the lower jaw is more like a hook. Both indicate gradual differences in the development of the temporalis muscle. In contrast, the skull from the Yucatán Peninsula has multiple bone openings on the lower jaw (foramen mental and foramen mandibulae) compared to the skull of Lake Chalapa. This could be due to a more perfused snout region in the animals of Yucatán, which may have resulted in a more flexible tongue. The latter is a sign of a more selective food intake. The regional differentiations may be an expression of different landscape conditions. At the end of the Pleistocene, the Lake Chalapa region consisted of a landscape rich in water and vegetation with gallery forests . Yucatán, on the other hand, was characterized by dry, almost desert-like conditions.
The robust, short humerus with its rough surface advocates strong arm muscles, which primarily control the movement of the forearm by stretching and bending. According to this, Xibalbaonyx could have digged for food, but could also have been capable of climbing locomotion. Since similar movement patterns are also generated when swimming, a partial movement in the water is not excluded, especially since today's tree sloths are very good swimmers.
Systematics
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Internal systematics of the Megalonychidae based on skeletal features according to Stinnesbeck et al. 2020
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Xibalbaonyx is a genus from the extinct family of Megalonychidae within the suborder of the sloths (Folivora) and the suborder of the sibling animals (Xenarthra). The Megalonychidae represent a very diverse group. The closest relatives of the Megalonychidae are the Megatheriidae and the Nothrotheriidae . The former include the largest known representatives of the sloth, the latter consist of rather smaller members of the sloth. All three families together are referred to the superfamily of the Megatherioidea . Within the sloth, the Megalonychidae form a very old line, their first appearance is documented in the Oligocene in Patagonia . Characteristic features can be found in the canine ( canine - shaped ) or incisiform ( incisiform ) design of the foremost tooth as well as in the molar-like ( molar-shaped ) rear teeth. The latter are characterized by two transverse ridges ( bilophodont ) on the chewing surface, which indicates a rather leaf-eating diet of the Megalonychidae. In contrast to the Megatheriidae and the Nothrotheriidae, the rear foot is plantigrad and not rotated, which means that it has retained its original shape. The Megalonychidae were once widespread, fossil remains can be found in South America as well as in Central America as well as in North America up to the arctic north. In a classic view based on skeletal anatomical comparisons, the Megalonychidae originally also included the sloths of the West Indies and the two-toed sloths ( Choloepus ) still alive today . Molecular genetic investigations together with protein analyzes could not establish any closer relationships between these individual groups.
Much of the fossil material of the Megalonychidae is fragmentary and largely incomplete. The systematic relationships between the individual representatives of the group have therefore so far only been inadequately worked out. However, due to the wealth of shapes, different lines of development can be demonstrated. One includes largely South American representatives such as Megistonyx or Ahytherium or Ortotherium , another consists of the North American forms Megalonyx and Pliometanastes (here, based on skeletal features , also Caribbean sloths such as Megalocnus or Neocnus ). At the moment it is not possible to determine direct ancestors for the North American representatives of the Megalonychidae. As a result, their connection to the South American forms is rather unknown. According to phylogenetic analyzes, Xibalbaonyx has a closer relationship to other Central American forms such as Meizonyx or Nohochichak . Above all, Meizonyx mediates more strongly to the South American Megalonychidae. The forms there are largely found in the tropical landscapes. The forms that occur mainly in the temperate regions of North America, such as Megalonyx or Pliometanastes , on the other hand, are more distant relatives.
Three types of Xibalbaonyx are known:
- X. exinferis Stinnesbeck , Stinnesbeck , Frey , Olguín & González , 2020
- X. microcaninus Stinnesbeck, Frey & Stinnesbeck , 2018
- X. oviceps Stinnesbeck, Frey, Olguín, Stinnesbeck, Zell , Mallison , González, Núñez , Morlet , Mata , Sanvicente , Hering & Sandoval , 2017
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Relationship of the three species of Xibalbaonyx according to Stinnesbeck et al. 2020
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X. microcaninus is occupied from the Chapala region and represents a smaller species. It stands out from shorter eckzahnähnliche teeth and some other features. X. oviceps was found in Yucatán and comprises a larger representative with dimensions comparable to Megalonyx . It is also the nominate form of the genus. X. exinferis also comes from Yucatán and has very small canine-like teeth and a double mental foramen on the extremely short symphysis process of the lower jaw.
The first scientific description of Xibalbaonyx was presented by a research team led by Sarah R. Stinnesbeck in 2017. The basis was the find from the cenote of El Zapote on the Yucatán peninsula. The skull and lower jaw recovered there represents the holotype of the genus (copy number Za2014-01 or Za2014-05). The genus name Xibalbaonyx indicates Xibalbá , the underworld in Maya mythology . On the one hand it honors the divers who recovered the fossil material in the underground cenote under sometimes dangerous conditions, on the other hand it refers to the Yucatán as the home of the Maya in general. The second part of the name onyx is of Greek origin ( όνυξ ) and means something like "claw". Only shortly after the first description of Xibalbaonyx was published, H. Gregory McDonald and colleagues presented another large representative of the Megalonychidae based on a partial skull and a lower jaw, also from Yucatán, and named it Nohochichak . Like Xibalbaonyx, it comes from a cenote not far from El Zapote and has a similar stratigraphic age. The two genera are similar in their characteristics, but there are also certain differences. In Nohochichak , for example, the postorbital process is relatively strong, but this is absent in Xibalbaonyx . Other deviations concern the position of certain foramina, the shape of the zygomatic arch or the shape of the nasal bone. The so far limited finds of the two genera make an assessment of these differences difficult. It is therefore unclear whether they are due to a generic differentiation or to species differences within a genus. There is also the possibility that it is a sexual dimorphism or pathologies . Due to this lack of clarity, the two genres have not been combined so far.
literature
- Sarah R. Stinnesbeck, Eberhard Frey, Jerónimo Avíles Olguín, Wolfgang Stinnesbeck, Patrick Zell, Heinrich Mallison, Arturo González González, Eugenio Aceves Núñez, Adriana Velázquez Morlet, Alejandro Terrazas Mata, Martha Benavente Sanvicente, Fabio Heronyx : Carmen Xicibepsalba , a new megalonychid ground sloth (Folivora, Xenarthra) from the Late Pleistocene of the Yucatán Peninsula, Mexico, and its paleobiogeographic significance. Paläontologische Zeitschrift 49 (2), 2017, pp. 245–271 doi: 10.1007 / s12542-017-0349-5
- Sarah R. Stinnesbeck, Eberhard Frey and Wolfgang Stinnesbeck: New insights on the paleogeographic distribution of the Late Pleistocene ground sloth genus Xibalbaonyx along the Mesoamerican Corridor. Journal of South American Earth Sciences 85, 2018, pp. 108–120 doi: 10.1016 / j.jsames.2018.05.004
Individual evidence
- ↑ a b c d e f g Sarah R. Stinnesbeck, Eberhard Frey, Jerónimo Avíles Olguín, Wolfgang Stinnesbeck, Patrick Zell, Heinrich Mallison, Arturo González González, Eugenio Aceves Núñez, Adriana Velázquez Morlet, Alejandro Terrazas Mata, Martha Benavente Sanvicente, Fabio Hering and Carmen Rojas Sandoval: Xibalbaonyx oviceps, a new megalonychid ground sloth (Folivora, Xenarthra) from the Late Pleistocene of the Yucatán Peninsula, Mexico, and its paleobiogeographic significance. Paläontologische Zeitschrift 49 (2), 2017, pp. 245–271 doi: 10.1007 / s12542-017-0349-5
- ↑ a b c d e f g h i j k Sarah R. Stinnesbeck, Eberhard Frey and Wolfgang Stinnesbeck: New insights on the paleogeographic distribution of the Late Pleistocene ground sloth genus Xibalbaonyx along the Mesoamerican Corridor. Journal of South American Earth Sciences 85, 2018, pp. 108–120 doi: 10.1016 / j.jsames.2018.05.004
- ↑ a b c d e f g Sarah R. Stinnesbeck, Wolfgang Stinnesbeck, Eberhard Frey, Jerónimo Avilés Olguín and Arturo González González: Xibalbaonyx exinferis n. Sp. (Megalonychidae), a new Pleistocene ground sloth from the Yucatán Peninsula, Mexico. Historical Biology. An International Journal of Paleobiology, 2020, doi: 10.1080 / 08912963.2020.1754817
- ↑ Wolfgang Stinnesbeck, Eberhard Frey, Patrick Zell, Jerónimo Avilés, Fabio Hering, Norbert Frank, Jennifer Arps, Anna Geenen, Johannes Gescher, Margot Isenbeck-Schröter, Simon Ritter, Sarah Stinnesbeck, Eugenio Aceves Núñez, Vicente Fito Dahne, Arturo González González and Michael Deininger: Hells Bells - unique speleothems from the Yucatán Peninsula, Mexico, generated under highly specific subaquatic conditions. Palaeogeography, Palaeoclimatology, Palaeoecology 489, 2018, pp. 209-229
- ↑ a b c H. Gregory McDonald, James C. Chatters and Timothy J. Gaudin: A new genus of megalonychid ground sloth (Mammalia, Xenarthra) from the late Pleistocene of Quintana Roo, Mexico. Journal of Vertebrate Paleontology 37 (3), 2017, p. E1307206
- ↑ Spencer George Lucas: Late Cenozoic fossil mammals from the Chapala rift basin, Jalisco, Mexico. New Mexico Museum of Natural History and Science Bulletin 44, 2008, pp. 39-49
- ↑ Spencer George Lucas: Late Cenozoic vertebrate fossil assemblages from Jalisco, Mexico. New Mexico Museum of Natural History and Science Bulletin 44, 2008, pp. 51-64
- ^ Alfredo A. Carlini and Gustavo J. Scillato-Yané: The oldest Megalonychidae (Xenarthra: Tardigrada); phylogenetic relationships and an emended diagnosis of the family. New Yearbook for Geology and Paleontology Abhandlungen 233 (3), 2004, pp. 423–443
- ^ A b H. Gregory McDonald and Gerardo de Iuliis: Fossil history of sloths. In: Sergio F. Vizcaíno and WJ Loughry (eds.): The Biology of the Xenarthra. University Press of Florida, 2008, pp. 39-55
- ↑ a b Timothy J. Gaudrin: Phylogenetic relationships among sloths (Mammalia, Xenarthra, Tardigrada): the craniodental evidence. Zoological Journal of the Linnean Society 140, 2004, pp. 255-305
- ↑ Frédéric Delsuc, Melanie Kuch, Gillian C. Gibb, Emil Karpinski, Dirk Hackenberger, Paul Szpak, Jorge G. Martínez, Jim I. Mead, H. Gregory McDonald, Ross DE MacPhee, Guillaume Billet, Lionel Hautier and Hendrik N. Poinar : Ancient mitogenomes reveal the evolutionary history and biogeography of sloths. Current Biology 29 (12), 2019, pp. 2031-2042, doi: 10.1016 / j.cub.2019.05.043
- ↑ Samantha Presslee, Graham J. Slater, François Pujos, Analía M. Forasiepi, Roman Fischer, Kelly Molloy, Meaghan Mackie, Jesper V. Olsen, Alejandro Kramarz, Matías Taglioretti, Fernando Scaglia, Maximiliano Lezcano, José Luis Lanata, John Southon, Robert Feranec, Jonathan Bloch, Adam Hajduk, Fabiana M. Martin, Rodolfo Salas Gismondi, Marcelo Reguero, Christian de Muizon, Alex Greenwood, Brian T. Chait, Kirsty Penkman, Matthew Collins and Ross DE MacPhee: Palaeoproteomics resolves sloth relationships. Nature Ecology & Evolution 3, 2019, pp. 1121-1130, doi: 10.1038 / s41559-019-0909-z
- ↑ H. Gregory McDonald, Ascanio D. Rincón and Timothy J. Gaudin: A new genus of megalonychid sloth (Mammalia, Xenarthra) from the Late Pleistocene (Lujanian) of Sierra de Perija, Zulia State, Venezuela. Journal of Vertebrate Paleontology 33 (5), 2013, pp. 1226-1238