Zacatzontli

Zacatzontli
Temporal occurrence
Upper Miocene ( Hemphillium )
6.9 to 5.9 million years
Locations
Central America ( Mexico )
Systematics
Sub-articulated animals (Xenarthra) Tooth arms (pilosa) Sloths (folivora) Megatherioidea Megalonychidae Zacatzontli
Scientific name
Zacatzontli
McDonald & Carranza-Castañeda , 2017

Zacatzontli is an extinct genus of sloths from the Megalonychidae family. It represents a rather small representative of the family group, but has so far only been detected via a lower jaw and individual isolated teeth. The finds were made in the Tecolotlán Basin in southwest Mexico . They are more than 4.9 million years old, whichbelongsto the transition from the Miocene to the Pliocene . From a phylogenetic point of view, Zacatzontli ismore closely related to the South American forms of the Megalonychidae,despite its occurrence in Central America . The genus is one of the first representatives of the sloths thatreached northern regionsin the course of the Great American Faun Exchange .

features

Zacatzontli is a small member of the sloth that reached about the size of Neocnus from the island of Hispaniola . So far, however, only a lower jaw and individual isolated teeth are occupied by the shape. The lower jaw, which is 12.2 cm long, is only completely preserved with the left branch, but damaged at the joint end. The right branch has broken off. The horizontal bone body had only a slight protuberance at the lower edge at the level of the molars, which is a difference to Megalonyx . The height of the lower jaw on the first molar-like tooth was about 2.9 cm, in front of it, in the area of ​​the symphysis , and behind, on the third molar-like tooth, it was 2.5 cm. A typical characteristic of a sloth's lower jaw is the spatula or spoon-like extension of the symphysis towards the front. this was triangular in shape at Zacatzontli and thus resembled that of Megalonyx . In Zacatzontli it reached about 30% of the length of the entire row of teeth, which is significantly more than in Megalonyx (20%), but less than in Pliometanastes (40%). In the latter, it was also rectangular in outline. The surface of the process was concave and merged evenly into the symphysis. This in turn ended shortly before the first molar-like tooth and thus had a total length of 3.1 cm. At the symphysis the lower edge of the lower jaw rose steeply and ended at the anterior extension. There was a mental foramen to the side of the lower midline of the extension . At the rear end of the lower jaw was a low-lying angular process.

The dentition of the lower jaw of Zacatzontli consisted of a front canine-like ( caniniform ) tooth and three molar-like ( molar-shaped ) teeth, which means that four teeth were formed per branch of the jaw , typical of megalonychid sloths . A 0.9 cm long diastema separated the anterior tooth from the posterior row of teeth. The caniniform tooth rose forward at an angle of 45 °. It had a triangular cross-section with the tip pointing backwards. The molar teeth themselves are not preserved, but their shape and size are reproduced by the respective alveoli . Accordingly, they all had about the same size and an oval to triangular outline. In the first two molar-shaped teeth, the longitudinal axis of the teeth was perpendicular to the lower jaw, in the last, in the direction of the lower jaw. The entire length of the row of teeth was 5 cm, of which the molar teeth took up 3 cm.

Reference

The genus Zacatzontli has so far only been documented through the lower jaw and two isolated teeth. The fossil remains come from the sites Jal-Teco 20 and Jal-Teco 8 in the Tecolotlán Basin about 80 km southwest of Guadalajara in the Mexican state of Jalisco . The Tecolotlán Basin is a 20 km long and 10 km wide, roughly north-south trending depression. It may be due to a rift structure that existed in the Miocene and Pliocene and was partly under the influence of volcanic activity. Geological and paleontological exploration of the Tecolotlán Basin began in the 1990s. The Juchitlan formation , which belongs to the Upper Cretaceous and mostly consists of limestone , forms the subsurface of the basin . A series of sediments of continental origin that formed under fluvial- lacustrine conditions are deposited on it. Various volcanic ashes are incorporated into these. The lower part of the sequence is called the San José Beds , the sediments here are composed of barely solidified muds , clays and sandstones . They formed in the transition from the Miocene to the Pliocene (locally stratigraphically Upper Hemphillium ), which was determined by radiometric analyzes on an activated ash layer, the age of which dates to about 4.95 million years. The area is separated from the upper series of sediments, the San Buenoventura Beds , by a hiatus . These in turn consist of sand , gravel and sandy clay / silt stones . Their formation period covers the Upper Pliocene, the age has been geophysically determined to be about 2.6 million years.

Both sequences are very rich. The lower jaw of Zacatzontli was recovered from the Jal-Teco 20 site, the two isolated teeth from the Jal-Teco 8 site. These are each located in the south of the Tecolotlán basin and belong to the San José Beds . The sequence of deposits on Jal-Teco 20 reached a thickness of 20 to 30 m, the lower jaw was about 12 m above the base and 1 m below a volcanic ash band. In addition to reptiles , numerous mammals also came to light as accompanying finds . These include Megalonyx as another representative of the sloth, but also Teleoceras as a member of the rhinos, various horse forms ( Neohipparion and Dinohippus ), in addition also Hemiauchenia and other cloven-hoofed animals as well as predators and with Rhynchotherium also a representative of the proboscis . Various rodents and rabbits have been identified among the small mammals . With the exception of sloths and animal species, these are of North American origin.

Systematics

Internal systematics of the Megalonychidae based on skeletal features according to Stinnesbeck et al. 2020  Megalonychidae    Eucholoeps      Pliometanastes      Pliomorphus      Megalocnus      Parocnus      Neocnus      Acratocnus      Choloepus      Megistonyx      Ahytherium      Nohochichak      Meizonyx      Zacatzontli      Xibalbaonyx      Megalonyx      Megalonychotherium

Zacatzontli is an extinct genus of the family of Megalonychidae . The Megalonychidae, in turn, form a very diverse group within the subordination of the sloths (Folivora) and the subordination of the secondary animals (Xenarthra). The closest relatives of the Megalonychidae include the Megatheriidae with the largest known representatives of the sloths and the Nothrotheriidae , which mainly comprise smaller members of the sloths. Together they form the superfamily of the Megatherioidea . Within the sloth, the Megalonychidae represent a very old line that was first detectable in the Oligocene in Patagonia . Characteristic features of the Megalonychidae can be found in the canine ( canine- shaped ) or incisiform ( incisiform ) design of the foremost tooth and in the molar-like ( molar-shaped ) rear teeth. The latter have two transverse ridges ( bilophodont ) on the chewing surface, which distinguishes the Megalonychidae as predominant leaf-eaters. In contrast to the Megatheriidae and the Nothrotheriidae, the hind foot is not rotated, but designed in a plantigradic manner and thus retained its original shape. In their phylogenetic past, the Megalonychidae achieved a wide distribution, ranging from South America through Central America to North America . According to skeletal anatomical studies, the sloths of the West Indies as well as the two-toed sloths ( Choloepus ) that are still alive today were incorporated into the Megalonychidae . However, molecular genetic and protein-based analyzes did not reveal any closer relationships between the individual groups.

The systematics of the Megalonychidae is complex and incomplete, which is due to the often fragmented and incomplete finds. Due to their wealth of forms, however, various lines of development can be demonstrated. One consists largely of South American representatives such as Megistonyx or Ahytherium or Ortotherium , another from North American forms such as Megalonyx and Pliometanastes (plus, according to skeletal anatomical characteristics, Caribbean representatives such as Megalocnus or Neocnus ). At the moment it is not possible to determine direct ancestors for the North American representatives of the Megalonychidae, which also complicates the connection to the South American forms. A first phylogenetic study for Zacatzontli revealed a closer position to the South American Megalonychidae due to the characteristics of the lower jaw, although there are limitations due to the existing fossil material. This could be confirmed in further studies and in comparison with newly described forms from Central America, such as Xibalbaonyx and Nohochichak from the Yucatán peninsula . Accordingly, the Central American members of the Megalonychidae form a common clade , which in turn are closer to the South American forms. In contrast to the two younger genera, Zacatzontli is significantly smaller, and it also deviates more strongly in its lower jaw and tooth morphology. The differences can, however, be explained by the existing time gap of more than 4 million years between Zacatzontli and both Nohochichak and Xibalbaonyx . The related relationship of Zacatzontli shows that the division into a South American and a North American or a Caribbean branch of the Megalonychidae had already taken place in the late Miocene .

The genus Zacatzontli was first scientifically described in 2017 by H. Gregory McDonald and Oscar Carranza-Castañeda . The holotype (specimen number MPGJ 3126) forms the lower jaw from Jal-Teco 20 in the Tecolotlán Basin in the Mexican state of Jalisco , the site is now 1177 m above sea level. The name Zacatzontli comes from the Nahuatl and denotes the Aztec god of travelers; it thus reflects the presence of a South American sloth form in Central and North America. The only recognized species is Zacatzontli tecolotlanensis . The specific epithet refers to the city of Tecolotlán in the basin of the same name. This is also taken from the Nahuatl and means “place of the owls”.

Biogeographical meaning

With Zacatzontli besides Pliometanastes , Meizonyx and Thinobatistes another representative of sloths is that already very early in the American Great faunal took part. Other forms such as Eremotherium , Paramylodon or Nothrotheriops followed much later. The Great American Fauna Exchange began with the closure of the Isthmus of Panama and the formation of a land bridge between North and South America in the course of the late Pliocene around 2.7 million years ago. However, some animal species managed to cross the strait before that. In addition to the representatives of the sloths, other sidelobes such as the anteaters , the armadillos , the Glyptodontidae and the Pampatheriidae were also involved in the Great American Fauna Exchange. Overall, however, only a few species established themselves further north, most of the secondary articulated animals remained limited to the areas south of the 40th parallel north. The exception is Megalonyx , which penetrated almost to the 70th parallel north. The high diversity of the articular animals in general and of the sloths in particular from the late Miocene to the Pleistocene in Central America reflects today's conditions. According to this, Central America has the highest diversity of species in addition to the South American core area of ​​the secondary articular animals, while the number of representatives in North America is falling sharply.

literature

• H. Gregory McDonald and Oscar Carranza-Castañeda: Increased xenarthran diversity of the Great American Biotic Interchange: a new genus and species of ground sloth (Mammalia, Xenarthra, Megalonychidae) from the Hemphillian (late Miocene) of Jalisco, Mexico. Journal of Paleontology 91 (5), 2017, pp. 1069-1082 doi: 10.1017 / jpa.2017.45

Individual evidence

1. ↑ ^{a b c d e f g h i} H. Gregory McDonald and Oscar Carranza-Castañeda: Increased xenarthran diversity of the Great American Biotic Interchange: a new genus and species of ground sloth (Mammalia, Xenarthra, Megalonychidae) from the Hemphillian (late Miocene) of Jalisco, Mexico. Journal of Paleontology 91 (5), 2017, pp. 1069-1082
2. ↑ ^{a b} Bart J. Kowallis, Eric H. Christiansen, Oscar Carranza-Castañeda, Wade E. Miller, Keryn T. Ross and David G. Tingey: The Geology of the Tecolotlan Graben, Jalisco, Mexico. Geological Society of America Digital Map and Chart 22, 2017, pp. 1–37 doi: 10.1130 / 2017.DMCH022.TXT
3. ^ 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
4. ^ 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
5. ^ ^{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.
6. ↑ ^{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
7. ↑ 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
8. ↑ 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
9. ↑ 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
10. ^ ^{A b} Sarah R. Stinnesbeck, Eberhard Frey and Wolfgang Stinnesbeck: New insights on the palaeogeographic 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