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Xibalbaonyx oviceps, a new megalonychid ground sloth (Folivora, Xenarthra) from the Late Pleistocene of the Yucatán Peninsula, Mexico, and its paleobiogeographic significance
Abstract
Here we describe a new genus and species of giant ground sloth, Xibalbaonyx oviceps (Megalonychidae, Xenarthra), from the drowned cave system of the northeastern Yucatán Peninsula. The specimen is Late Pleistocene in age and was discovered in the Zapote sinkhole (cenote) near Puerto Morelos in the Mexican state of Quintana Roo. Xibalbaonyx oviceps differs significantly from all hitherto known Megalonychidae including those from the Greater Antilles and South America. The new taxon suggests a local Caribbean radiation of ground sloths during the Late Pleistocene, which is consistent with the dispersal of the group along a Mexican corridor.
... A diverse faunal assemblage was also discovered, including peccaries (Muknalia minima), tapirs (Tapirus sp.), camels (Hemiauchenia sp.), ground sloths (Nothrotheriops shastensis, Xibalbaonyx oviceps, Nohochichak xibalbakah), proboscideans (Cuvieronius hyodon), giant armadillos (Glyptotherium sp.), and horses (Equus sp.) of late Pleistocene and early Holocene ages (e.g. peccaries and tapirs; Chatters et al., 2014;González González et al., 2008b;McDonald et al., 2017;Stinnesbeck et al., 2017aStinnesbeck et al., , 2017bStinnesbeck et al., , 2017c. ...
... Photographs were made with an Olympus E 620 SLR camera with a Zuiko digital lens, 14-42 mm, 1:3.5-5.8. The peccary mandibular ramus has been described in detail and assigned to an extinct genus and species of peccary, Muknalia minima, by Stinnesbeck et al. (2017a). The peccary mandible was scanned with an Artec three-dimensional (3D) scanner. ...
... A left mandibular ramus of the peccary Muknalia minima, Stinnesbeck et al. (2017a), was discovered in the Muknal remote siphon in 2012 by JAO and EAN and collected in the same year. The mandibular ramus was found at about 197 m distance from the Jailhouse cenote entrance and <13 m south of the 'Muknal Grandfather' in the cave tunnel terminating in the 'Charcoal Vault' (Figure 1), at a water depth of 30 m. ...
Here, we report on an incomplete human skeleton, soot patches related to anthropogenic fireplaces, and cut marks on the mandible of an extinct peccary, from the submerged Muknal cave southwest of Tulum on the Mexican Yucatán peninsula. The human individual, here named ‘Muknal Grandfather’, was identified as a male based on cranial parameters. The age at the time of death was estimated to be between 40 and 45 years. We propose that the human bones have been brought to the cave during the latest Pleistocene or early Holocene, but not later than 8600 14C yr BP (ca. 9600 cal BP), as a secondary burial of a partial skeleton. The peccary mandible was placed close to the burial site, possibly as part of the same ritual. The Muknal cave therefore served as a place for funeral rituals.
... New paleontological investigations from the Yucatán Peninsula Stinnesbeck et al., 2017a) and the Mexican basin (McDonald and Carranza-Castañeda, 2017) have markedly expanded the knowledge on the Mexican ground sloth fossil record. Here, we add a new input to this knowledge by describing a new ground sloth taxon from the central-western Mexican federal state of Jalisco, of which the skull is housed and exhibited in the Museo Regional de Guadalajara (MRG). ...
... It has therefore been subsequently assigned to Megalonyx jeffersonii by Lucas (2008a). Here we describe the skull and mandible of this specimen and assign the material to the recently established genus Xibalbaonyx (Stinnesbeck et al., 2017a), of which the holotype was found in the El Zapote cenote in the federal state of Quintana Roo in south-eastern Mexico. The new species of Xibalbaonyx expands the occurrence of the genus to centralwestern Mexico. ...
... The term "nasion" refers to the point where the nasofrontal and internasal sutures meet (Groves, 2003), but it is also the deepest depression on the nasal bridge (Martin, 1914). We here use the term nasional impression (Stinnesbeck et al., 2017a), as this feature is characteristic and diagnostic in Xibalbaonyx, including the new species of this genus described here. ...
Here we describe the skull of a new species of Megaloychidae (Xenarthra) Xibalbaonyx microcaninus from the federal state of Jalisco in west-central Mexico based on a complete skull exhibited in the Museo Regional de Guadalajara (MRG). The specimen was originally collected in the Upper Pleistocene sediment of the Zacoalco paleolake (Jalisco) and is here assigned to the recently established genus Xibalbaonyx from the Late Pleistocene of Quintana Roo in south-eastern Mexico. The genus Xibalbaonyx thus had a wider geographic distribution than previously considered and ranges from central to southern Mexico.
... Late Quaternary giant sloths are among the most iconic members of the mammalian fauna of the Americas and, at present, there are at least 40 species in four families: Megalonychidae, Nothrotheriidae, Mylodontidae, and Megatheriidae (e.g. Hoffstetter 1952;Matthew and Paula-Couto 1959;Cartelle 1991;Pujos et al. 2007;De Iuliis et al. 2009McDonald et al. 2013McDonald et al. , 2017McDonald et al. , 2020Stinnesbeck et al. 2017;McAfee et al. 2021). ...
... All measurements were made using ImageJ software (Abràmoff et al. 2004) on photographs of Megalonyx jeffersonii, Nothrotheriops shastensis, and Paramylodon harlani, as well as published figures of other sloth taxa (Matthew et al. 1959;MacPhee et al. 2000;McDonald 2006;Pujos et al. 2007;Lucas2008a, b;De Iuliis et al. 2009Gaudin 2011;Corona et al. 2013;McDonald et al. 2013McDonald et al. , 2017Lindsey and Lopez 2015;Stinnesbeck et al. 2017;McAfee et al. 2021; Online Resource 1: Table S1). ...
The sloths that dispersed into Central and North America were most likely derived from tropical taxa, as indicated by their high occlusal surface area (OSA) in comparison with body mass, as in South American tropical species. Relative muzzle width and hypsodonty indices were used to infer the dietary adaptation of several Late Pleistocene/Early Holocene giant ground sloths from the Americas. Nothrotheriidae and Megatheridae taxa were adapted to a browser or browsing mixed feeder diet, Mylodontidae taxa to a variety of dietary types, and Megalonychidae taxa to a browsing mixed feeder diet. However, these are generalized dietary adaptations and do not necessarily reflect the consumption of specific food resources. In general, Megalonychidae and Nothrotheriidae were climbing species, Mylodontidae were digging species, and Megatheriidae were fully terrestrial species.
... A taphonomical issue of bone preservation can be excluded, as neither the facial area of the skull, the mandible, nor postcranial elements, are affected by this form of bone alteration. Furthermore, the osteological remains of Chan Hol 1 and 2 from the same cave system, as also all other human and megafauna [51] remains from nearby caves in the area, are extremely well preserved and their bone surfaces are smooth. They allow for a reliable comparison of bone preservation levels based on the different find localities and from both fresh-and salt-water (e.g. ...
... Chan Hol 1 to 3 skeletons were contained in fresh-water, most other sites are salt-water). This excludes disintegration of bones by chemical reaction, producing holes, as seen for example on the dorsal surface of the cranium of the ground sloth Xibalbaonyx from the El Zapote cenote [51]. However, even at El Zapote where bones are extremely fragile due to heavy dissolution, the bone texture and surface are not deformed. ...
Human presence on the Yucatán Peninsula reaches back to the Late Pleistocene. Osteological evidence comes from submerged caves and sinkholes (cenotes) near Tulum in the Mexican state of Quintana Roo. Here we report on a new skeleton discovered by us in the Chan Hol underwater cave, dating to a minimum age of 9.9±0.1 ky BP based on ²³⁰Th/U-dating of flowstone overlying and encrusting human phalanges. This is the third Paleoindian human skeleton with mesocephalic cranial characteristics documented by us in the cave, of which a male individual named Chan Hol 2 described recently is one of the oldest human skeletons found on the American continent. The new discovery emphasizes the importance of the Chan Hol cave and other systems in the Tulum area for understanding the early peopling of the Americas. The new individual, here named Chan Hol 3, is a woman of about 30 years of age with three cranial traumas. There is also evidence for a possible trepanomal bacterial disease that caused severe alteration of the posterior parietal and occipital bones of the cranium. This is the first time that the presence of such disease is reported in a Paleoindian skeleton in the Americas. All ten early skeletons found so far in the submerged caves from the Yucatán Peninsula have mesocephalic cranial morphology, different to the dolicocephalic morphology for Paleoindians from Central Mexico with equivalent dates. This supports the presence of two morphologically different Paleoindian populations for Mexico, coexisting in different geographical areas during the Late Pleistocene-Early Holocene.
... The species is considered to have become extinct at the end of the middle Pleistocene (Alvarez and Polaco 1982;Bravo-Cuevas et al. 2016). However, the entire faunal assemblage found so far in the cenotes is of late Pleistocene to early Holocene age (González et al. 2008b;Stinnesbeck et al. 2017aStinnesbeck et al. , 2017b. It might therefore well be that Smilodon gracilis survived on the Yucatán Peninsula because of the extremely diverse habitat structure in this region. ...
... The new felid P. balamoides from El Pit is another faunal element identified as endemic to the northern Yucatán Peninsula (YP). It thus lines up with other recently described hitherto unique and thus probably endemic late Pleistocene mammals from the region, i.e. the peccary Muknalia minima (Stinnesbeck et al. 2017b) and the ground sloths Xibalbaonyx oviceps (Stinnesbeck et al. 2017a) and Nohochichak xibalbakah (McDonald et al. 2017). The new felid provides further evidence for the hypothesis that the region must have passed periods of ecological isolation, which lasted long enough to allow for a hitherto unexpected diversification of mammals in the area. ...
Here we describe a new species of a Pleistocene felid based on the distal third of a right humerus from the submerged El Pit cenote (sinkhole) near Tulum in Quintana Roo, Mexico. The new taxon, Panthera balamoides sp. nov., is characterized by a large entepicondylar foramen, a gracile and straight humeral shaft with a prominent supracondylar ridge with a small depression on the lateral epicondyle and a distal articular surface located medially with respect to the long axis of the shaft.
Two felid clavicles from the same locality have been assigned to Panthera atrox, while a humerus fragment from the Kim Ha cave near Tulum likely corresponds to Smilodon gracilis. Panthera balamoides lines up with other likely endemic mammals in the region, which suggest that at least northern Quintana Roo, if not the entire Yucatán peninsula, may have been ecologically isolated during the Pleistocene, due to the repeated expansion of grassland.
... Topheavy disparity profiles are associated with clades terminating at mass extinctions, indicating the clade was prematurely truncated (Brusatte et al. 2008;Hughes et al. 2013;MacLaren et al. 2017). Thus, our results indicate sloths were still reaching higher levels of morphological variety late in their evolutionary history, as indicated by the recently described genera from tropical Mexico, Xibalbaonyx and Nohochichak (Stinnesbeck et al. 2017;) with the Late Pleistocene extinction leaving only the two, morphologically specialized and highly convergent, extant genera. ...
... The later Late Miocene megalonychid dispersal into North America and the subsequent re-ingression into South America seen in our results may represent an artifact due to the low sampling of megalonychids in South America after the Santacrucian. This may be further evaluated by considering fragmentary megalonychids from the Late Miocene of Argentina (Brandoni 2011) and the presence of four Central and North American sloths not included in our analysis; Zacatzontli tecolotlanensis from the late Hemphillian of Mexico (McDonald and Carranza-Castañeda 2017), Meizonyx salvadorensis, from the Pleistocene of El Salvador (Webb and Perrigo 1985), and the newly described Xibalbaonyx oviceps (Stinnesbeck et al. 2017) and Nohochichak xibalbahkah ) from the late Pleistocene/early Holocene of Mexico. All of these taxa may be more closely related to South American taxa than to the other North American megalonychids. ...
Sloths, like other xenarthrans, are an extremely interesting group of mammals that, after a long history of evolution and diversification in South America, became established on islands in the Caribbean and later reached North America during the Great American Biotic Interchange. In all three regions, they were part of the impressive Pleistocene megafauna. Most taxa became extinct and only two small, distantly related tree-dwelling genera survived. Here we incorporate several recently described genera of sloths into an assembled morphological data supermatrix and apply Bayesian inference, using phylogenetic and morphological clock methods, to 64 sloth genera. Thus, we investigate the evolution of the group in terms of the timing of divergence of different lineages and their diversity, morphological disparity and biogeographical history. The phylogeny obtained supports the existence of the commonly recognized clades for the group. Our results provide divergence time estimates for the major clades within Folivora that could not be dated with molecular methods. Lineage diversity shows an early increase, reaching a peak in the Early Miocene followed by a major drop at the end of the Santacrucian (Early Miocene). A second peak in the Late Miocene was also followed by a major drop at the end of the Huayquerian (Late Miocene). Both events show differential impact at the family level. After that, a slight Plio-Pleistocene decline was observed before the marked drop with the extinction at the end of the Pleistocene. Phenotypic evolutionary rates were high during the early history of the clade, mainly associated with Mylodontidae, but rapidly decreased to lower values around 25 Ma, whereas Megalonychidae had lower rates at the beginning followed by a steady increase, peaking during the Late Miocene and the Pliocene. Morphological disparity showed a similar trend, with an early increase, followed by a slowly increasing phase through the Late Oligocene and Early Miocene, and ending with another increase beginning at the middle of the Miocene. Biogeographic analysis showed southern South America as the most probable area of origin of the clade and the main region in which the early diversification events took place. Both Megatheriinae and Nothrotheriinae basal nodes were strongly correlated with Andean uplift events, whereas the early history of Mylodontidae is closely associated with southern South America and also shows an early occupation of the northern regions. Within Megalonychidae, our results show Choloepus as a descendant of an island dispersing ancestor and a probable re-ingression to South America by a clade that originated in Central or North America.
... To test the importance of anthropic vs climatic importance in explaining late-Quaternary megafauna extinction, a dataset of late-Quaternary large mammals spanning all six biogeographic realms and encompassing 487 species was assembled, 152 of which went extinct before 1000 BCE, to focus on pre-Modern Era losses (SI1 and SI 2). Continuing advances in taxonomy (Larson et al., 2016;Paijmans et al., 2017), fossil dating (Vislobokova et al., 2020;Yang et al., 2019), and the discovery of new species (Stinnesbeck et al., 2017;Vrba et al., 2015) amongst both extant and extinct megafauna necessitated a thorough review. This dataset is unique in its coverage of current megafauna taxonomy and occurrence data and is accompanied by another dataset detailing changes in climate at 10 ky intervals from the late-Quaternary (SI3) at a local scale (Taxonomic Databases Working Group countries, TDWG) (Brummitt et al., 2008). ...
The Earth has lost approximately half of its large mammal species (≥45 kg, one-third of species ≥9 kg) over the past 120,000 years, resulting in depauperate megafauna communities worldwide. Despite substantial interest and debate for over a century, the reasons for these exceptionally high extinction rates and major transformation of the biosphere remain contested. The predominant explanations are climate change, hunting by modern humans (Homo sapiens), or a combination of both. To evaluate the evidence for each hypothesis, statistical models were constructed to test the predictive power of prehistoric human and hominin presence and migration on megafauna extinction severity and on extinction bias toward larger species. Models with anthropic predictors were compared to models that considered late-Quaternary (120–0 kya) climate change and it was found that models including human factors outperformed all purely climatic models. These results thus support an overriding impact of Homo sapiens on megafauna extinctions. Given the disproportionate impact of large-bodied animals on vegetation structure, plant dispersal, nutrient cycling and co-dependent biota, this simplification and downsizing of mammal faunas worldwide represents the first planetary-scale, human-driven transformation of the environment.
... There are no radiometric dates for any of the species, but all are presumed to be late Pleistocene in age. The types of Xibalbaonyx oviceps and X. exinferis were recovered from adjacent cenotes on the Yucatán Peninsula [61] while X. microcaninus is known from the state of Jalisco. While all three species are from localities that are about the same latitude, there is a significant difference in the elevation of the Jalisco specimen at 1372 m. ...
Late Pleistocene sloths were widely distributed and present in a diversity of habitats in South, Central, and North America and some Caribbean Islands. Late Pleistocene sloths include 27 genera in four families Megatheriidae, Megalonychidae, Mylodontidae, and Nothrotheriidae. There is no consensus on the number of valid species. Some sloths have wide geographic distributions and are present on multiple continents while others have a much smaller distribution. Our knowledge of the paleoecology and natural history of the different sloths varies greatly depending on their relative abundance. The wide distribution of sloths and adaptations to different habitats results in several “sloth” faunas with different taxonomic compositions. These generalized faunas can be distinguished geographically as Temperate North America (five genera), Southern Mexico and Central America (five genera), Northern South America (two genera), West Coast of South America (four genera), the Andes and Altiplano (four genera), Brazilian Intertropical Region (nine genera), Pampas-Patagonia and the Caribbean Islands (Cuba, Hispaniola, and Puerto Rico, four genera). Some genera may occur in multiple regions but are represented by different species. These regions also have differences in other mammalian taxa, so the sloths are often in ecological competition with different megaherbivores or preyed on by different carnivores.
... Colonial palaeontology practices in Brazil and Mexico are not limited to Sabinas, La Popa, Parras and Araripe basins. In Mexico, important Pleistocene mammal specimens from the Yucatán Peninsula have been targeted [111][112][113][114][115][116], as have the fossil deposits from the Miocene strata of the Acre Basin in Brazil [117,118]. A 2012 study found that foreign-led research has extensively been conducted in several northern Mexican states (Baja California Sur, Coahuila and Nuevo León) [119]. ...
Scientific practices stemming from colonialism, whereby middle- and low-income countries supply data for high-income countries and the contributions of local expertise are devalued, are still prevalent today in the field of palaeontology. In response to these unjust practices, countries such as Mexico and Brazil adopted protective laws and regulations during the twentieth century to preserve their palaeontological heritage. However, scientific colonialism is still reflected in many publications describing fossil specimens recovered from these countries. Here, we present examples of ‘palaeontological colonialism’ from publications on Jurassic–Cretaceous fossils from NE Mexico and NE Brazil spanning the last three decades. Common issues that we identified in these publications are the absence of both fieldwork and export permit declarations and the lack of local experts among authorships. In Mexico, access to many fossil specimens is restricted on account of these specimens being housed in private collections, whereas a high number of studies on Brazilian fossils are based on specimens illegally reposited in foreign collections, particularly in Germany and Japan. Finally, we outline and discuss the wider academic and social impacts of these research practices, and propose exhaustive recommendations to scientists, journals, museums, research institutions and government and funding agencies in order to overcome these practices.
... The karstic Yucatan Peninsula of Mexico contains many submerged caves, from which recent diving technology has made possible the recovery of Pleistocene vertebrate remains, including new megalonychid sloths. Nohochichak xibalbahkah from Sac Actun cave system in the state of Quintana Roo, Mexico, was described by , and another member of the family, Xibalbaonyx oviceps (Stinnesbeck et al., 2017) from Cenote Zapote, and X. exinferis (Stinnesbeck et al., 2020) from the adjacent Cenote Tortugas near Puerto Morelos, Quintana Roo was described along with a third species from Zocoalco Lake, Guadalajara Mexico named Xibalbaonyx microcaninus (Stinnesbeck et al., 2018). ...
We report here a new record of a megalonychid sloth from a late Pleistocene mammal assemblage from Cueva de Iglesitas (Mi.50), Caracas, Venezuela. This new site, the first with a Pleistocene fauna found in the vicinity of Caracas, is in a pristine geological and stratigraphic area and preserves previously untouched sediments containing fossils that provide important information about the late Pleistocene paleofauna and paleoenvironment of Caracas and the southwestern Caribbean region. A preliminary assessment of the relationship of this new sloth to other members of the Megalonychidae is presented.
... Four families of ground sloths reached North America in two principal migration events; first, Megalonychidae and Mylodontidae during the late Miocene, and later Megatheriidae, Mylodontidae and Nothrotheriidae during the Pliocene-Pleistocene interval, being an important part of the Great American Biotic Interchange (Marshall 1988;Webb, 1989Webb, , 2006. The record of ground sloths in the late Pleistocene of North America comprised taxa of the four families, the megathere Eremotherium laurillardi, the nothrothere Nothrotheriops shastensis; the megalonychids Megalonyx jeffersonii, Meizonyx salvadorensis, Xibalbaonyx oviceps, X. microcaninus, and Nohochichak xibalbahkah, as well as mylodont Paramylodon harlani (Webb and Perrigo, 1985;McDonald and De Iuliis, 2008;McDonald and Carranza-Castañeda, 2017;Stinnesbeck et al., 2017;2018); the later species is well known from several Irvingtonian and Rancholabrean localities in North America (Kurtén and Anderson, 1980;McDonald and Naples, 2008). ...
Paramylodon harlani was a large ground sloth recorded across North America, from Canada to Mexico. In Mexico, it is known from several late Pleistocene localities, but most of these records just mention the taxon in passing and few specimens have been described or illustrated. In this work, we describe a left tibia from the Valsequillo Basin, Puebla state. Its morphology and measurements allowed us to identify it as Paramylodon harlani, adding a new record for Mexico. In Mexico, P. harlani occurred mainly in the Trans-Mexican Volcanic Belt, central Mexico, with some records in the north and southeastern partof the country. Most localities are located between 1500 to 2000 m.a.s.l. Paleoenvironmental and paleoclimatic inference in some localities of Mexico where P. harlani occurred, showed heterogenous vegetation dominated by grasslands, and agree
with the preferred habitat proposed for this species based on localities in the United States. This indicates that P. harlani could inhabit different environments, from grasslands to more wooded areas, and this adaptation allowed it to extend its range from the north to the southeast of Mexico.
... The recent description of three new genera and four new species of megalonychid sloths from the tropical portion of Mexico (south of 23 27'N) clearly demonstrates there existed a greater diversity of members of this family in the northern Neotropics than previously suspected based on the records from El Salvador. These species -Zacatzontli tecolotlanensis from the late Miocene (latest Hemphillian) (McDonald & Carranza-Castañeda 2017), and Nohochichak xibalbahkah , Xibalbaoyx oviceps (Stinnesbeck et al. 2017) and X. exinferis (Stinnesbeck et al. 2020) from the late Pleistocene (Rancholabrean)present the challenge of determining their evolutionary history and phylogenetic relationships to each other and to Meizonyx, as well as to the other members of Megalonychidae. While recent revisions of the phylogenetic relationships of sloths based on aDNA (Delsuc et al. 2019) and palaeoproteomics (Presslee et al. 2019), have elucidated the broader relationships of sloths, the limited number of fossil specimens that preserve DNA or proteins means that resolution of the relationships for most sloth taxa will still have to be based primarily on morphology to determine the phylogenetic relationships of members of the family in South, North and Central America, and the Antillean taxa as well. ...
The megalonychid sloth Meizonyx salvadorensis was previously known only from the holotype mandible from El Salvador. Here, we describe the first record of M. salvadorensis from the late Pleistocene of Mexico, examine its relationship to other members of the family Megalonychidae and discuss the palaeobiogeographical and palaeoecological implications of this new record. The specimen consists of an associated skull and mandible, and parts of the postcranial skeleton, recovered from the Sistema Huautla cave complex located in the Sierra Mazateca, a part of the Sierra Madre Oriental del Sur, east of Huautla de Jiménez, in the state of Oaxaca, Mexico. Two phylogenetic analyses were carried out utilizing maximum parsimony and Bayesian approaches, and included the new material within the data matrix used for previous studies of members of Megalonychidae. The phylogenetic hypothesis obtained from the Bayesian analysis indicates the closest relationship of Meizonyx salvadorensis is to Xibalbaonyx. The recent discoveries of multiple new members of Megalonychidae in southern Mexico and their proposed relationships to each other and other members of the family resulting from this analysis suggest successive dispersal events complemented by endemic radiation in Central America and southern Mexico, possibly facilitated by the variety of habitats present in the region that allowed for their diversification.
... Cenote Zapote (Fig. 2) is a typical vertical pit-cenote, about 20 km inland in Quintana Roo state, México. It was first surveyed by Vicente Fito, who discovered the speleothems and named them as "Hells Bells" (Stinnesbeck et al., 2017). ...
Folia are speleothems that resemble bells, inverted cups, or bracket fungi, and whose origins are still controversial. Cenote Zapote (an underwater cave) in the Yucatán Peninsula (México), is home to some of the largest folia reported to date. These speleothems are currently growing in an active underwater system, meaning this site offers an excellent opportunity to constrain the different formation models proposed for folia, which have traditionally relied on inactive examples. In Cenote Zapote, folia are closely related to bubble trails and cupolas, suggesting an underwater CO2-degassing process. In thin section, they display a succession of columnar-open and columnar-elongated endings in micrite-dendritic fabrics. Our petrographic and geochemical results demonstrate the abiotic origin of these folia and indicate carbonate precipitation from cold water by CO2 degassing below the water table that started at least 5,210 yrs BP. We conclude that these folia formed as a result of subaqueous calcite precipitation around CO2 bubbles trapped below overhanging walls of the cave. The sequential alternation of columnar and micritic fabrics can be explained by changes in the position of the halocline and H2S-rich water mass while the exceptional size is the result of carbonate precipitation from waters saturated in CaCO3 during thousans of years. Then we propose the classification of these speleothems as a subtype of folia. This subtype could be named Hells Bells, respecting its original description.
... Exploration of submerged caves by technical divers in the Yucatán of Mexico has resulted in a number of fossil discoveries and recent publications. This includes early humans (Homo sapiens) in the Americas (e.g., Chatters et al., 2014;González González et al., 2013;Stinnesbeck et al., 2017a), and extinct fauna: 1) new types of giant ground sloths, Nohochichak xibalbahkah (McDonald et al., 2017), Xibalbaonyx oviceps (Stinnesbeck et al., 2017b), and Xibalbaonyx exinferis (Stinnesbeck et al. 2020), 2) a new peccary, Muknalia minima (Stinnesbeck et al., 2017c), 3) a new jaguar, Panthera balamoides (Stinnesbeck et al., 2018a; but questioned as a possible ursid by Schubert et al., 2019 andRuiz-Ramoni et al., 2020), and 4) short-faced bears (Arctotherium wingei) and canids (Protocyon troglodytes) that were previously only known from the South American fossil record (Schubert et al., 2016;Schubert et al., 2019). ...
Ongoing investigation of peccary remains from fossiliferous deposits in the Yucatán resulted in re-examination of previously identified tayassuid fossils from the region. This included the recently described new genus and species of peccary, 'Muknalia minima', which is based on a dentary from Muknal Cave near Tulum, Quintana Roo, Mexico. Diagnostic characters of this taxon include a concave notch along the caudal edge of the ascending ramus and a ventrally directed angular process. Our assessment of the holotype indicates that these characteristics are not a reflection of the original morphology, but are instead the result of breakage and polishing of the posterior aspect of the dentary. Measurements and intact morphological features indicate the Muknal Cave specimen belongs to the extant collared peccary, 'Pecari tajacu'.
... M. obtusidens can be precluded, since all occlusal surfaces are oval to rounded Perrigo, 1984, 1985). Both the endemic Meizonyx salvadorensis from El Salvador (Webb and Perrigo, 1985) and Xibalbaonyx oviceps from Puerto Morelos, Mexico (Stinnesbeck et al., 2017), can also be excluded, since their occlusal molariforms are more rectangular than subtriangular. No upper molariforms have been found from Nohochichak xibalbakah , but the overall cranial morphology and lower molariforms resemble that of X. oviceps (Stinnesbeck et al., 2018). ...
We present a revision, dating and interpretation of the Late Pleistocene megafauna of Guatemala based on paleontological material located in collections in the country and other fossils housed in the American Museum of Natural History, New York. Assemblages are dominated by proboscideans (Cuvieronius) and xenarthrans (Eremotherium, Glyptotherium), while co-occurring Equus and Mixotoxodon are significantly less frequent, and Holmesina, Palaeolama, Tapirus, Neocherus, Mammuthus and other ground sloth taxa (Paramylodon; Megalonyx) are rare. Contrary to published records the faunal assemblage is dominated by North American faunal elements. The underrepresentation of typical South American fauna therefore suggests a more southernly located biogeographic juncture between the two subcontinents, at least for the Late Pleistocene. The biogeographical barrier was either formed by the high mountain chains, or alternating periods of low and high precipitation that triggered the intermittent expansion of either grass-or woodland, thus leading to an alternating filter for either grazers or browsers. The presence of an oak-dominated forest vegetation with Mixotoxodon, Eremotherium and Cuvieronius supports high precipitation rates during MIS 3 and 2, followed by drought during the Late Pleistocene deglaciation. The expansion of grassland during the Younger Dryas period favored the migration of Mammuthus along the Mesoamerican Corridor, which is otherwise absent in Guatemala. Our data also suggest a survival of Cuvieronius into the early Holocene in the southeastern lowlands of Guatemala along the Motagua river. Our review is important as Guatemala is key to understanding migrations along the Mesoamerican Corridor that acted as a bridge but also as a filter of faunal interchange between North-and South America.
... There are no apparent passages or conduits that connect El Zapote cenote to a cave system. Additional details on El Zapote cenote are given in Stinnesbeck et al. (2017b) and in Stinnesbeck et al. (2017a), who described the new genus and species of a giant ground sloth, Xibalbaonyx oviceps, from an individual that was found on the floor of El Zapote cenote. ...
Unique bell-shaped underwater speleothems were recently reported from the deep (∼ 55 m) meromictic El Za-pote sinkhole (cenote) on the Yucatán Peninsula, Mexico. The local diving community has termed these speleothems as Hells Bells because of their shape and appearance in a dark environment in ∼ 28-38 m water depth above a sulfidic halocline. It was also suggested that Hells Bells form under water, yet the mystery of their formation remained un-resolved. Therefore, we conducted detailed hydrogeochemi-cal and geochemical analyses of the water column and Hells Bells speleothems including stable carbon isotopes. Based on the comprehensive results presented in this study we deduce that both biogeochemical processes in the pelagic redoxcline and a dynamic halocline elevation of El Zapote cenote are essential for Hells Bells formation. Hells Bells most likely form in the redoxcline, a narrow 1-2 m thick water layer immediately above the halocline where a pelagic chemolithoau-totrophic microbial community thrives from the upward diffusion of reduced carbon, nitrogen and sulfur species released from organic matter degradation in organic-rich debris. We hypothesize that chemolithoautotrophy, in particular proton-consuming nitrate-driven anaerobic sulfide oxidation , favors calcite precipitation in the redoxcline and hence Hells Bells formation. A dynamic elevation of the halocline as a hydraulic response to droughts, annual tidal variability and recharge events is further discussed, which might explain the shape of Hells Bells as well as their occurrence over a range of 10 m water depth. Finally, we infer that highly stagnant conditions, i.e., a thick halocline, a low-light environment and sufficient input of organic material into a deep meromictic cenote are apparent prerequisites for Hells Bells formation. This might explain their exclusivity to only a few cenotes in a restricted area of the northeastern Yucatán Peninsula .
... New discoveries from submerged caves in the Yucatán are changing this, bringing to light an underworld of exquisitely preserved fossils from the late Pleistocene, when sea level and the water table were significantly lower during glaciations. Publications on these underwater discoveries have focused on early human skeletons [16,17], new mammalian genera (two ground sloths and a peccary) [18][19][20] and a new species interpreted to be a jaguar-like cat [21]. ...
The Great American Biotic Interchange is considered to be a punctuated process, primarily occurring during four major pulses that began approximately 2.5 Ma. Central America and southeastern Mexico have a poor fossil record of this dynamic faunal history due to tropical climates. Exploration of submerged caves in the Yucatán, particularly the natural trap Hoyo Negro, is exposing a rich and remarkably well-preserved late Pleistocene fauna. Radiometric dates on megafauna range from approximately 38 400-12 850 cal BP, and extinct species include the ursid Arctotherium wingei and canid Protocyon troglodytes. Both genera were previously thought to be indigenous to and confined to South America and appear to represent an instance of large placental mammals, descended from North American progenitors, migrating back north across the Panama Isthmus. This discovery expands the distribution of these carnivorans greater than 2000 km outside South America. Their presence along with a diverse sloth assemblage suggests a more complex history of these organisms in Middle America. We suggest that landscape and ecological changes caused by latest Pleistocene glaciation supported an interchange pulse that included A. wingei, P. troglodytes and Homo sapiens.
... There are no apparent passages or conduits that connect El Zapote cenote to a cave system. Additional details on El Zapote cenote are given in Stinnesbeck et al. (2017b) and in Stinnesbeck et al. (2017a) who described the new genus and species of a giant 30 ground sloth, Xibalbaonyx oviceps, from an individual that was found on the floor of El Zapote cenote. Biogeosciences Discuss., https://doi.org/10.5194/bg-2018-520 ...
Unique bell-shaped underwater speleothems were recently reported from the deep (~55 m) meromictic El Zapote sinkhole (cenote) on the Yucatán Peninsula, Mexico. The local diving community has termed these speleothems as Hells Bells because of their shape and appearance in a lightless environment in ~28-38 m water depth above a sulfidic halocline. It was 20 also suggested that Hells Bells form under water, yet the mystery of their formation remained unresolved. Therefore, we conducted detailed hydrogeochemical and geochemical analyses of the water column and Hells Bells speleothems including stable carbon isotopes. Based on the comprehensive results presented in this study we deduce that both, biogeochemical processes in the pelagic redoxcline and a dynamic halocline elevation of El Zapote cenote, are essential for Hells Bells formation. Hells Bells most likely form in the redoxcline, a narrow 1-2 m thick water layer immediately above the halocline 25 where a pelagic chemolithoautotrophic microbial community thrives from the upward diffusion of reduced carbon, nitrogen and sulfur species released from organic matter degradation in organic-rich debris. We hypothesize that chemolithoautotrophy, in particular the proton consuming nitrate-driven anaerobic sulfide oxidation, favors calcite precipitation in the redoxcline and hence Hells Bells formation. A dynamic elevation of the halocline as a hydraulic response to recharge events, e.g. hurricanes, is further discussed, which might explain the shape of Hells Bells as well as their 30 occurrence over a range of 10 m water depth. Finally, we infer apparent prerequisites for Hells Bells formation considering the exclusivity of these underwater speleothems to only a few cenotes of a restricted area of the northeastern Yucatán Peninsula.
... Underwater environments can also produce unique bone surface damage that might be diagnostic for these environments. For example, Stinnesbeck et al. (2017) reported on fragmentation and potential dissolution of parts of the cranial vault of Xibalbaonyx oviceps in response to exposure to cave water. González González et al. (2008) also reported on the poor preservation of their fossil remains, attributing this to salt water corrosion. ...
Underwater deposits, especially those in phreatic caves, often contain exquisitely preserved fossils, and many represent Quaternary Konservat-Lagerstätten. Nevertheless , they are unrecognised as such by most practicing palaeontologists. This review highlights the unique contributions to palaeontology made by underwater deposits as well as the technical and practical challenges facing underwater palaeontologists. Recovery of fossils from such deposits requires specialist training, equipment, and procedures unique to these environments. Taphonomic studies of underwater assemblages are rare and hampered by difficulties in fossil recovery. Neotaphonomic experiments and observations of modern accumulations in underwater settings should be a priority for future research. Regions where such techniques might provide important new insights into Quaternary faunas and environments, not accessible through traditional palaeontological approaches, include low-lying flooded continental shelves and soil-poor karstic landscapes. Underwater palaeontology represents a largely unex-plored yet significant source of fossils, further study of which will expand and enrich traditional approaches in the study of ancient organisms.
... Trees (e.g., Ceiba pentandra) collapsed onto the debris mount at a time when the cave was already partially flooded. Hells Bells speleothems cover the cave ceiling at water depths of 29-35 m and small primordial specimens also cover some of the trees (Figure modified from S. Stinnesbeck et al., 2017). ...
We here report on a type of meter-sized pendant speleothem growing under water in the submerged El Zapote sinkhole (cenote) west of Puerto Morelos on the Mexican Yucatán Peninsula. These conical, mantle-shaped downward expanding and diverging calcareous structures, here termed as Hells Bells, are yet unreported in the scientific literature. They are characterized by bell- or trumpet shaped longitudinal and circular, elliptical or horse-shoe-like horizontal cross-sections. Hells Bells grow downward, based on the downward divergence of the structures and the horizontally laminated internal texture of both blade-shaped spar calcite and microspar laminae. Age dating confirms that Hells Bells are young (<. 4500. yr) and formed in a subaquatic environment. They grow under lightless conditions in a stratified water body, which is characterized by a fresh water body overlying a salt water body with a stagnant transition zone (halocline) of several meters. We hypothesize that the growth of these structures is mediated by specific physical and biogeochemical conditions above and in the halocline. Stagnant hydraulic conditions led to extensive diffusion profiles of several nutrients including calcium originating from the salt water body. Dissolved organic carbon from the fresh water is microbially oxidized in the upper part of the halocline, where a distinct redox zonation was identified from oxic to anoxic conditions. Degradation processes combined with slightly alkaline pH values as well as the diffusive transport of calcium into this zone may induce an increase in calcite oversaturation. Phylogenetic analysis of the community on the surface of the Hells Bells suggests the presence of microorganisms involved in the nitrogen-cycle, from which some potentially have the capability to increase the pH by autotrophic growth and denitrifying activity, thus supporting calcite precipitation. The growth of Hells Bells is strictly dependent on the elevation of the halocline. This offers a wide potential for the use of Hells Bells as archives of paleo-hydrological conditions during the Holocene, e.g. the variation of thickness of the fresh water lens on the Yucatán Peninsula.
The Occlusal Surface Area index together with the body mass contribute to better understanding the paleoecology and evolution of the giant sloths in the Americas. The giant sloths from southern South America probably had higher levels of fermentation/lower metabolic requirements, whereas the tropical taxa had higher metabolic requirements. The sloths that dispersed into Central and North America were most likely derived from tropical taxa based on the patterns observed in this study. Relative Muzzle Width and the Hypsodonty indices were used to infer the dietary adaptation of several late Pleistocene/early Holocene giant ground sloths from the Americas. The Nothrotheriidae and Megatheridae taxa were adapted to a browser or mixed-feeder "browser" diet, the Mylodontidae taxa to a variety of dietary types, and the Megalonychidae taxa to a mixed-feeder browser diet. However, these dietary adaptations do not necessarily re ect food niches. In general, Megalonychidae and Nothrotheriidae were climber species, Mylodontidae taxa were digger species, and Megatheriidae taxa were fully terrestrial species.
Phylogenetic relationships among sloths (Folivora) have been extensively studied in the past few decades using maximum parsimony approaches. Recently, Bayesian phylogenetic methods also began to be employed for this task, with advances in methods for data partitioning and tip-dating analyses leading to exciting new possibilities in morphological phylogenetics. In this context, we assembled the largest morphological data set ever applied to sloths and reassessed their phylogeny and divergence times, evaluating alternative models of partitioning and dating in a Bayesian framework. The updated phylogeny of sloths is largely in agreement with previous morphological studies, with Bradypus recovered as sister to Eufolivora, the presence of two major sloth clades (Mylodontoidea and Megatherioidea) and Choloepus among Megalonychidae. However, the present study yields some important advances in understanding the relationships of genera with historically unresolved or controversial allocations. The major sloth clades diversified from the Late Eocene to the Early Miocene. Homoplasy-based partition models outperformed anatomical partitioning and unpartitioned analyses, with considerable impacts on topology and posterior probabilities. Estimates obtained using homoplasy-partitioned models with Bayesian analyses were in strong agreement with those of maximum parsimony. We emphasize the importance of model comparison with Bayes factors and the assessment of synapomorphies in Bayesian morphological phylogenetics.
A reevaluation of the late Miocene (early late Hemphillian) megalonychid fossils found at San Gerardo de Limoncito, Coto Brus Valley, Costa Rica is presented. These specimens which had previously been referred to Pliometanastes cf. P. protistus, based on a partial mandible and associated teeth material are now considered to belong to the genus Zacatzontli, previously only known from the late Miocene of Mexico and here reported for the first time from Costa Rica. The material is considered to be a new species of Zacatzontli based on a comparison with the type and other members of the Megalonychidae from the Hemphillian of North America as Zacatzontli tecolotlanensis, Pliometanastes protistus and Megalonyx spp.
A reevaluation of the late Miocene (early late Hemphillian) megalonychid fossils found at San Gerardo de Limoncito, Coto Brus Valley, Costa Rica is presented. These specimens which had previously been referred to Pliometanastes cf. P. protistus, based on a partial mandible and associated teeth material are now considered to belong to the genus Zacatzontli, previously only known from the late Miocene of Mexico and here reported for the first time from Costa Rica. The material is considered to be a new species of Zacatzontli based on a comparison with the type and other members of the Megalonychidae from the Hemphillian of North America as Zacatzontli tecolotlanensis, Pliometanastes protistus and Megalonyx spp.
McDonald HG 2021. Yukon to the Yucatan: Habitat partitioning in North American Late Pleistocene ground sloths (Xenarthra, Pilosa). Journal of Palaeosciences 70(2021): 237-251. The late Pleistocene mammalian fauna of North America included seven genera of ground sloth, representing four families. This cohort of megaherbivores had an extensive geographic range in North America from the Yukon in Canada to the Yucatan Peninsula in Mexico and inhabited a variety of biomes. Within this latitudinal range there are taxa with a distribution limited to temperate latitudes while others have a distribution restricted to tropical latitudes. Some taxa are better documented than others and more is known about their palaeoecology and habitat preferences, while our knowledge of the palaeoecology of taxa more recently discovered remains limited. In order to better understand what aspects of their palaeoecology allowed their dispersal from South America, long-term success in North America and ultimately the underlying causes for their extinction at the end of the Pleistocene more information is needed. A summary overview of the differences in the palaeoecology of the late Pleistocene sloths in North America and their preferred habitats is presented based on different data sources.
This work outlines an underwater laser scanner (ULS) operational readiness test (ORT)
demonstrating the efficacy of ULS-200 in response to National Aeronautics and Space
Administration (NASA) Planetary Science and Technology through Analog Research (PSTAR) Program knowledge gaps. This geographic information science and technology (GIST) project advises stakeholders on extravehicular activity (EVA) design and engineering (D&E) via cave diving. Analog surveys define strengths, weaknesses, opportunities, risks, and threats (SWORT) in three-dimensional (3D) remote sensing (RS) detection and ranging (DAR) via light (LiDAR) and photogrammetry (PhoDAR). Sidemount cave diving procedures and life-support systems (LSS) facilitate paleontological, hydrogeological, and microbiological evidence sampling, mitigating crew resource management (CRM) risks. 3D geographic information systems (GIS) toolkits produce LiDAR and PhoDAR digital terrain models (DTM) that require British Cave Research Association (BCRA) GIST quality assessment and control (QAC) modernization. Research outcomes included survey cost reductions, a < .15 cm precision ≈2,000m3 karst photoplethysmogram (volumetric LiDAR cavity system measurements) scan completed in <5 days and a GIST human-machine (H-M) CRM PSTAR D&E SWORT ORT. Products included geohazard maps, a regional karst network 3DGIS, a LiDAR photonic quasicrystal-vacuum orbifold indicatrix, and 3D underwater imaging artifact characterizations. Analog extraterrestrial environmental (ETE) analysis occurred in Cloudcompare, datasets were unable to be uploaded for virtual reality laboratory (VRL) simulation in Esri City Engine. This work provides PSTAR D&E references in high-fidelity EVA simulations, H-R ergonomics, quantum physics, and area of potential effect (APE) planetary protection design and engineering (PPDE).
Ongoing investigations in submerged cave systems of Quintana Roo in south-eastern Mexico reveal a rich Late Pleistocene megafaunal assemblage, among them the megalonychid ground sloth Xibalbaonyx oviceps. The taxon has been described based on a complete skull and mandible from El Zapote cenote west of Puerto Morelos. We here add hitherto unreported postcranial material from El Zapote, attributed to the holotype. This new material allows us to reconstruct unexpected locomotion capabilities for Xibalbaonyx oviceps including steep slope and rock climbing. This may have enabled the ground sloth to use the sinkholes and underground caverns as water resource and shelter. The Late Pleistocene age of the fossil allows for a co-existence with early human settlers on the Yucatán Peninsula.
Recent palaeontological research in submerged caves of the north-eastern Yucatán Peninsula (YP) in Mexico has resulted in the identification of a diverse megafaunal assemblage in the area, among them Late Pleistocene ground sloths assigned to the family Megalonychidae (Xenarthra). Here we report on a new species of Megalonychidae, Xibalbaonyx exinferis n. sp., from a new fossil locality named cenote Tortugas, located west of Puerto Morelos in the federal state of Quintana Roo. The taxon is based on a fragmentary left mandibular ramus, an atlas, and a left humerus. The new taxon is diagnosed by the presence of two mental foramina on the short symphyseal spout, a caniniform being the smallest mandibular tooth, and the anterolaterally directed aperture of the mandibular foramen on the lateral surface of the mandible. Xibalbaonyx exinferis is the third endemic megalonychid documented for the north-eastern Yucatán Peninsula and thus provides increasing evidence for an ecological isolation of the area from the rest of Mexico during the Pleistocene.
Numerous charcoal accumulations discovered in the submerged Chan Hol cave near Tulum, Quintana Roo, Mexico, have been 14C‐dated revealing ages between 8110 ± 28 14C a BP (9122–8999 cal a BP) and 7177 ± 27 14C a BP (8027–7951 cal a BP). These charcoal concentrations, interpreted here as ancient illumination sites, provide strong evidence that the Chan Hol cave was dry and accessible during that time interval. Humans used the cave for at least 1200 years during the early and middle Holocene, before access was successively interrupted by global sea level rise and flooding of the cave system. Our data thus narrow the gap between an early settlement in the Tulum area reaching from the late Pleistocene (∼13 000 a) to middle Holocene (e.g. 7177 14C a BP), and the Maya Formative period at approximately 3000 a bp. Yet, no evidence has been presented to date for human settlement during the ∼4000‐year interval between 7000 and 3000 a. This is remarkable as settlement in other areas of south‐eastern Mexico (e.g. Chiapas, Tabasco) and in Guatemala was apparently continuous.
https://www.academia.edu/3788867/CURRENT_RESEARCH_IN_THE_PLEISTOCENE_2008_Gonzalez
This paper presents radiocarbon results from modern South Pacific corals from the Marquesas Islands, Vanuatu, Papua New Guinea (PNG), and Easter Island. All of the measurements are from pre-bomb Porites corals that lived during the 1940s and 1950s. The data reflect subannual to multiannual surface ocean 14 C variability and allow for precise, unambiguous reservoir age determinations. The results are compared with published values from other coral records throughout the South Pacific, with striking consistency. By comparisons with other published values, we identify 3 South Pacific regions with uniform pre-bomb reservoir ages (1945 to 1955). These are 1) the Central Equatorial South Pacific (361.6 − 8.2 14 C yr, 2 σ); 2) the Western Equatorial South Pacific (322.1 − 8.6 14 C yr, 2 σ); and 3) the subtropical Pacific (266.8 − 13.8 14 C yr, 2 σ).
The Pleistocene fossil sloth Australonyx aquae De Iuliis, Cartelle, and Pujos, 2009 (Mammalia, Xenarthra, Megalonychidae) was described from the intertropical region of Brazil. However, its mandible was not known and only cursory descriptions of the ear ossicles were included. The mandible was subsequently recognized among the remains originally collected from the type locality, and belongs to the holotype individual. As a particularly important skeletal element for specific recognition, it requires description to complement our understanding of this species. The ossicles, usually poorly represented in the fossil record, require further description to allow differentiation from those of other sloths. Comparisons of the mandible and ossicles are conducted with homologous elements of the contemporaneous and sympatric Ahytherium aureum
Cartelle, De Iuliis, and Pujos, 2008, the only other megalonychid sloth known from intertropical Brazil, and reinforce the distinction between these two species detailed in their initial descriptions. Comparisons with other sloths (e.g., Acratocnus, Megalonyx, Neocnus ) also reveal differences with Au. aquae
in such features as form and size of the caniniform tooth, angular process, and mandibular condyle. Differences among the malleus and incus of Au. aquae and several species of other sloth clades reveal clade level distinctions among Megatheriidae, Nothrotheriidae, and Megalonychidae. A well-preserved skull from the Brazilian state of Rondônia is noted as probably belonging to Au. aquae . This skull cannot be assigned formally to this species because it is not deposited in a recognized institution, but it does extend considerably the known range of the species.
Modern sloths are among the more characteristic mammals of South and Central American faunas. Recent discovery in four Paleogene, 22 Neogene, and dozens of Pleistocene fossiliferous localities in the tropics has revealed an unexpected paleobioversity constituted by some 81 fossil sloth species. Probably originating in southern South America near the Eocene/Oligocene transition, sloths were represented in the tropics during the late Oligocene by Pseudoglyptodon, Mylodontidae, and Megalonychidae. The latter occupied the West Indies between at least the late early Miocene and late Pleistocene, and two mylodontid clades, Octodontobradyinae and Urumacotheriinae, were characteristic of Amazonian localities from the Colhuehuapian and the Laventan periods, respectively, until the end of the Miocene. Megatheriinae and Nothrotheriidae appeared during the middle Miocene, colonizing the tropics and then North America, where Mylodontidae and Megalonychidae had already been present since the early late Miocene. Nothrotheriids are more abundant and diversified during the late Miocene in the tropics than in southern South America. Remains closely related to either of the modern sloths are absent from the fossil record, including those in the tropics. The characteristic suspensory posture of Bradypus and Choloepus appeared independently and likely after the Miocene epoch, and thus well after the hypothesized split suggested by molecular studies of the respective clades of these genera. Given their current widespread distribution in and reliance on the tropics, prospecting efforts for the direct fossil kin of suspensory sloths should concentrate on deposits in the Amazonian region, as this area has shown promise in producing fossil sloths.
Prehistoric evidence from submerged caves and sinkholes (cenotes) on the Yucatan peninsula provides strong evidence for the existence of an early preceramic human settlement in southern Mexico. During our ongoing paleoanthropological research we have already documented three well-preserved human skeletons as old as 13,000 and 9000 years from these sites in Quintana Roo (Gonzalez et al. 2008a, Gonzalez et al. 2008b). The findings were associated with hearths and a diverse megafaunal assemblage of late-Pleistocene age. A fourth human skeleton was discovered in 2009, two more in 2010, and two others in 2011. Here we provide a first register of these additional five skeletons, bringing the total assemblage to eight. A ninth skeleton was informally reported from the same area by INAH researchers. These findings thus constitute one of the largest databases on bones of early humans in Mexico. © 2014 by Kelly E. Graf, Caroline V. Ketron, and Michael R. Waters. All rights reserved.
The origins and sister groups of basal Xenarthran lineages are either controversial or still unknown. The absence of most major groups of Xenarthra in South American mammal-bearing sediments of Late Cretaceous and Early Paleocene age is the greatest enigma in the study of this Superorder. The aim ofthis paper is to describe a new genus and species of Megalonychidae, Deseadognathus riggsi, from the type locality of the Deseadan Land-Mammal Age, Oligocene of Patagonia, Argentina, the oldest megalonychid thus far known. Using 17 characters of the mandible, a phylogenetic analysis of 14 genera of megatherioids yielded two equally parsimonious and highly congruent trees of 49 steps. The polarity of several characters is supported by ontogeny, e.g., progressively increasing divergence of dental series, increase in size of caniniforms, its sub-circular to sub-triangular section, and shortening of the rostrum. The results of our phylogenetic analysis confirm the monophyletic status of the Megalonychidae with an enlarged membership. Available evidence indicates a South American origin for the Megalonychidae, followed by later radiations at different times to the West Indies and North America.
In the Mexican state of Jalisco, south of Guadalajara, Lake Chapala (Lago de Chapala) is the largest natural freshwater lake in Mexico and is located in the inner graben of the Chapala rift basin. Most of the outcrops in the Chapala rift basin are Miocene-Pleistocene volcanic rocks, but more than 600 m of lacustrine sediments are exposed on its northern flank. Some of these sediments also underlie Lake Chapala and indicate that a large, permanent and relatively deep lake was present since the early Pliocene and subsequently dismembered by tectonics and volcanism. Fossil vertebrates from the Chapala rift basin are fishes, reptiles, birds and (mostly) mammals. Those collected from the bottom of Lake Chapala or its shorelines are an extensive assemblage of late Pleistocene (Rancholabrean) age that includes xenarthrans (
Recent studies show Xenarthra to be even more isolated systematically from other placental mammals than traditionally thought. The group not only represents 1 of 4 primary placental clades, but proposed links to other fossorial mammal taxa (e.g., Pholidota, Palaeanodonta) have been contradicted. No unambiguous Paleocene fossil xenarthran remains are known, and Eocene remains consist almost exclusively of isolated cingulate osteoderms and isolated postcrania of uncertain systematic provenance. Cingulate skulls are unknown until the late middle Eocene, and the oldest sloth and anteater skulls are early Oligocene and early Miocene age, respectively; there are no nearly complete xenarthran skeletons until the early Miocene. Ecological reconstructions of early xenarthrans based on extant species and the paleobiology of extinct Neogene taxa suggest the group's progenitors were myrmecophagous with digging and perhaps some climbing adaptations. The earliest cingulates were terrestrial diggers and likely myrmecophagous but soon diverged into numerous omnivorous lineages. Early sloths were herbivores with a preference for forested habitats, exhibiting both digging and climbing adaptations. We attribute the rarity of early xenarthran remains to low population densities associated with myrmecophagy, lack of durable, enamel-covered teeth, and general scarcity of fossil localities from tropical latitudes of South America. The derivation of numerous omnivorous and herbivorous lineages from a myrmecophagous ancestor is a curious and unique feature of xenarthran history and may be due to the peculiar ecology of the native South American mammal fauna. Further progress in understanding early xenarthran evolution may depend on locating new Paleogene fossil sites in northern South America.
Geomorphological and biological archives of relative sea-level change in the western North Atlantic-Caribbean region following the Last Glacial Maximum have traditionally supported the hypothesis of a punctuated rise towards the present sea level. Such records, however, are often at insufficient resolution to discern centennial-scale changes. In caves where the water table is closely controlled by sea level, active periods of speleothem growth constraining maximum sea level, used in combination with marine overgrowths constraining minimum sea level, are a promising alternative archive recording sea-level variability at higher resolution. Here, we present a U-Th-dated early–middle Holocene speleothem record from a submerged cave on the tectonically
stable Yucatán Peninsula, Mexico. Our record shows that polychaetes (Annelida, Polychaeta) colonised a sub-aerially deposited stalagmite during four individual submergence events. Submergence events occurred at approximately 8.9, 8.6, 8.4 and 6.0 ka, which we attribute to previously unrecognised minor sea-level oscillation events (OE1–OE4) above and below −6.12 ± 0.1 m relative to present sea level (r.s.l.). Combining these results with mangrove-derived relative sea-level constraints from another submerged cave on the Yucatán Peninsula, we are able to suggest that OE1 and OE2 did not reach as high as −5.26 m r.s.l., but that OE3 and OE4 exceeded −5.22 m r.s.l. We conclude that subsidence of the North American ice-load bulge was the main cause of relative sea-level rise. Superimposed on the glacio-isostatic adjustment were periods of widespread northern hemisphere cooling and ice margin re-advance, resulting in a relative sea-level fall on four occasions during the early–middle Holocene.
This paper presents radiocarbon results from modern South Pacific corals from the Marquesas Islands, Van- uatu, Papua New Guinea (PNG), and Easter Island. All of the measurements are from pre-bomb Porites corals that lived dur- ing the 1940s and 1950s. The data reflect subannual to multiannual surface ocean 14C variability and allow for precise, unambiguous reservoir age determinations. The results are compared with published values from other coral records through- out the South Pacific, with striking consistency. By comparisons with other published values, we identify 3 South Pacific regions with uniform pre-bomb reservoir ages (1945 to 1955). These are 1) the Central Equatorial South Pacific (361.6 ± 8.2 14C yr, 2 σ); 2) the Western Equatorial South Pacific (322.1 ± 8.6 14C yr, 2 σ); and 3) the subtropical Pacific (266.8 ± 13.8 14C yr, 2 σ). The question of how much, and how fast, South Pacific reservoir ages might have varied in the past is addressed by examining a published record from a Pleistocene coral from Vanuatu that lived over a 700-yr period during the Younger Dryas. The average reservoir age at that time was larger than today, by ~150 yr, and exhibited reservoir age variability on a decadal timescale not seen in modern times. Measured paleo-reservoir ages increase sharply in this record by as many as 300 14C yr in 3 decades. These increases are punctuated by smaller reservoir age decreases, on the order of 150 yr. This reservoir age variability provides a rare picture of active ocean ventilation and ocean-atmosphere exchange at the close of the Pleistocene. © 2009 by the Arizona Board of Regents on behalf of the University of Arizona Celebrating 50 Years of Radiocarbon.
Radiocarbon 14C dating provided New World archaeological research with the first continent-wide common chronometric scale that transcended the mostly relative site- and region-specific chronological sequences that had been assembled during the preceding century of fieldwork. 14C data continue to play a critical role in establishing a chronometric framework for the 5-century-long debate concerning the timing of the initial peopling of the New World. Other issues where 14C results have been of particular importance include the origins and development of New World agriculture and determination of the relationship between the Western and Classic Maya long-count calendar. The introduction of a third-generation measurement technology of accelerator mass spectrometry (AMS) beginning in the late 1970s has provided a means of obtaining analyses on milligram and microgram amounts of carbon permitting more detailed critical approaches to increasing the accuracy of 14C values on certain sample types-particularly human skeletal materials. It also provided a more effective means of allowing greater dating precision in situations where such data had an important bearing on the validity of inferences about the rates of cultural evolutionary change in New World societies. © 2009 by the Arizona Board of Regents on behalf of the University of Arizona.
Choloepus hoffmanni Peters, 1858 (Hoffmann’s two-toed sloth) is 1 of 2 extant two-toed sloths. A high-canopy folivore, C. hoffmanni has a disjunct range with a northern population in Central America and northern South America and a southern population in South America. C. hoffmanni is listed as “Least Concern” by the International Union for Conservation of Nature and Natural Resources because of its wide distribution.
Uranium-lead geochronology in detrital zircons and provenance analyses in eight boreholes and two surface stratigraphic sections in the northern Andes provide insight into the time of closure of the Central American Seaway. The timing of this closure has been correlated with Plio-Pleistocene global oceanographic, atmospheric, and biotic events. We found that a uniquely Panamanian Eocene detrital zircon fingerprint is pronounced in middle Miocene fluvial and shallow marine strata cropping out in the northern Andes but is absent in underlying lower Miocene and Oligocene strata. We contend that this fingerprint demonstrates a fluvial connection, and therefore the absence of an intervening seaway, between the Panama arc and South America in middle Miocene times; the Central American Seaway had vanished by that time.
Copyright © 2015, American Association for the Advancement of Science.
Extant sloths present an evolutionary conundrum in that the two living genera are superficially similar (small-bodied, folivorous, arboreal) but diverged from one another approximately 30 million years ago and are phylogenetically separated by a radiation of medium to massive, mainly ground-dwelling, taxa. Indeed, the species in the two living genera are among the smallest, and perhaps most unusual, of the 50+ known sloth species, and must have independently and convergently evolved small size and arboreality. In order to accurately reconstruct sloth evolution, it is critical to incorporate their extinct diversity in analyses. Here, we used a dataset of 57 species of living and fossil sloths to examine changes in body mass mean and variance through their evolution, employing a general time-variable model that allows for analysis of evolutionary trends in continuous characters within clades lacking fully-resolved phylogenies, such as sloths.
Results:
Our analyses supported eight models, all of which partition sloths into multiple subgroups, suggesting distinct modes of body size evolution among the major sloth lineages. Model-averaged parameter values supported trended walks in most clades, with estimated rates of body mass change ranging as high as 126kg/million years for the giant ground sloth clades Megatheriidae and Nothrotheriidae. Inclusion of living sloth species in the analyses weakened reconstructed rates for their respective groups, with estimated rates for Megalonychidae four times higher when the extant genus Choloepus species was excluded.
Conclusions:
Analyses based on extant taxa alone have the potential to oversimplify or misidentify macroevolutionary patterns. This study demonstrates the impact that integration of data from the fossil record can have on reconstructions of character evolution and establishes that body size evolution in sloths was complex, but dominated by trended walks towards the enormous sizes exhibited in some recently extinct forms.
New well-preserved remains of the megalonychid sloth Eucholoeops Ameghino, 1887 recovered under strict stratigraphic control from late Early Miocene Santa Cruz Formation (c. 19 to 14 Ma; Santacrucian Age), together with analysis of older collections, consideration of intraspecific variation in extinct and extant sloths, and assessment of the validity of the early literature on Santacrucian sloths, permit revision of the status of the numerous species erected for this genus. The current contribution deals with the systematics of E. ingens Ameghino, 1887, but its methodology provides a basis for revision of other Eucholoeops species, as well as other sloth genera recovered from the Santa Cruz Formation. The failure to make progress on the systematics of the Santacrucian taxa since their first description is shown to be due mainly to a combination of the poor quality of many of the specimens, which are often fragmented and incomplete and from older collections, as well as inadequate stratigraphic and geographic control of their recovery, an overly rigid reliance on the early literature that accompanied their descriptions, and lack of consideration for intraspecific variation. A neotype is designated for E. ingens, as the original specimen is no longer available. The species E. latirostris Ameghino, 1891, E. externus Ameghino, 1891, and E. curtus Ameghino, 1894 are considered as junior synonyms of E. ingens.
Photogrammetry is increasingly becoming the gold standard for surface digitizing in paleontology. We present techniques for specimen handling, photography and image handling in photogrammetry software that are specially adapted to typical use cases in paleontology, but are also applicable in other science disciplines like archaeology and art history.
Because of differences in craniofacial morphology and dentition between the earliest American skeletons and modern Native
Americans, separate origins have been postulated for them, despite genetic evidence to the contrary. We describe a near-complete
human skeleton with an intact cranium and preserved DNA found with extinct fauna in a submerged cave on Mexico’s Yucatan Peninsula.
This skeleton dates to between 13,000 and 12,000 calendar years ago and has Paleoamerican craniofacial characteristics and
a Beringian-derived mitochondrial DNA (mtDNA) haplogroup (D1). Thus, the differences between Paleoamericans and Native Americans
probably resulted from in situ evolution rather than separate ancestry.
A cladistic investigation of the phylogenetic relationships among 21 extinct and extant genera of sloths (Mammalia, Xenarthra, Tardigrada) was performed on the basis of characteristics of the bony anatomy of the auditory region. This study was undertaken in order to evaluate specific hypotheses of relationship within the group. Questions of particular interest include the relationship among the three traditional family groupings of extinct ground sloths and the monophyletic or diphyletic origin of the two genera of extant tree sloths. Eighty-five discrete morphological characters were analyzed using the computer program PAUP. Characters were polarized via comparisons to the following successive outgroups, all members of the supraordinal grouping Edentata: the Vermilingua, or anteaters; the Cingulata, or armadillos and glyptodonts; the Palaeanodonta; and the Pholidota, or pangolins. Three most parsimonious trees result (for 21 ingroup taxa and 5 outgroup taxa; Length = 304 steps, CI = 0.405, RI = 0.712). The results of this analysis provide characters which support the monophyly of the Xenarthra as a whole, as well as its dichotomous division into the clades Cingulata and Pilosa. In addition, it strongly corroborates the monophyly of the Vermilingua and the Tardigrada. Within tardigrades, the living three-toed sloth Bradypus is suggested to represent the sister-taxon to all other sloths. The results support the monophyly of the three traditional ground sloth families Megatheriidae, Megalonychidae, and Mylodontidae (the latter two quite strongly), and suggest that the subfamily Nothrotheriinae may represent a paraphyletic stem group for the remaining sloths. A novel relationship between the families Megalonychidae and Mylodontidae is proposed which contrasts with previous hypotheses of a close relationship between megalonychids and megatheriids. Lastly, this analysis strongly supports the derivation of the extant two-toed sloth Choloepus from within the West Indian megalonychid ground sloths, contradicting the traditional systematic grouping of the extant tree sloths into a monophyletic family Bradypodidae and suggesting that the living tree sloths represent one of the most remarkable examples of convergent evolution known among mammals.
The new genus and species Ahytherium aureum (Mammalia, Xenarthra, Megalonychidae) from the Quaternary of Poço Azul (Bahia, Brazil) is described. It is the first Brazilian megalonychid known from reasonably complete and well-preserved remains. Purported Brazilian megalonychids described in the past, such as Ocnopus gracilis and Xenocnus cearesis, are noted as belonging to other sloth clades, and the acceptance by past paleontologists of the existence of ‘strange’ megalonychids in Brazil is shown to be erroneous. Ahytherium aureum, in fact, exhibits typical megalonychid morphology. It differs from other known members of Megalonychidae in several characters, including a markedly shortened, but high rostral region, with dorsally inflated frontals, wide zygomatic processes of the frontal, narrow, blade-like and anterolaterally oriented lacrimals, curved, slender and oval caniniforms, gracile humerus with less developed deltopectoral shelf, and relatively distal position of the greater trochanter of the femur. A second specimen from São Paolo state is tentatively assigned to the new genus and species.
RESUMO - Nos últimos 25 anos ocorreram inúmeras descobertas de mamíferos neoterciários na região central do México, sendo que muitas vêm das bacias localizadas na Faixa Vulcânica Transmexicana. Em contraste com a maioria dos registros anteriores, estes achados têm sido cuidadosamente inseridos num contexto estratigráfico. Datações radiométricas em camadas de cinzas vulcânicas, juntamente com estudos paleomagnéticos, permitem o posicionamento do limite das Idades Mamíferos Terrestres da Ámerica do Norte Hemphillian - Blancan em 4,8 Ma. As pesquisas sobre os mamíferos neoterciários da região central do México também têm fornecido valiosas informações sobre novos táxons, ampliação da distribuição geográfica de muitos animais e mudanças ambientais na região. Vários mamíferos que imigraram da Ámerica do Sul durante o Neoterciário são também agora conhecidos na região central do México, adicionando novas e importantes informações concernentes à formação da ponte terrestre do Panamá e do Grande Intercâmbio Biótico Americano. Táxons como as preguiças Megalonyx e Glossotherium, os Cingulata Glyptotherium e Plaina e o roedor Neochoerus proveram dados relevantes a este evento. É geralmente aceito que a ponte terrestre do Panamá formou-se entre 2,5 e 3 milhões de anos atrás. Entretanto, as idades radiométricas e paleomagnéticas obtidas na região central do México associadas às informações aportadas pelos mamíferos imigrantes sul-americanos, indicam que esta ponte terrestre poderia ter-se originado mais de um milhão de anos antes. Por outro lado, o movimento tectônico, as mudanças de caráter insular e o vulcanismo na América Central durante o Neoterciário, com conseqüências eustáticas e na composição vegetal, tornam muito difícil a determinação cronológica precisa tanto da formação da ponte terrestre quanto do intercâmbio biótico resultante. Palavras-chave: Hemphillian, Blancan, Faixa Vulcânica Transmexicana, Grande Intercâmbio Biótico Americano. ABSTRACT - Many new discoveries of late Tertiary mammals have been made across central Mexico in the past two and one-half decades. Most came from basins located in the Transmexican Volcanic Belt. Unlike a majority of earlier finds, these have been carefully placed in a stratigraphic context. Associated volcanic ash layers have permitted a number of radiometric ages and, coupled with recent paleomagnetic studies, allow placement of the Hemphillian - Blancan North American Land Mammal Age boundary at 4.8 Ma. Research on late Tertiary mammals from central Mexico has also provided valuable information on new taxa, significant geographic range extensions for many animals, and environmental changes.Several South American immigrant mammals of late Tertiary age are now known from central Mexico. They add important new information concerning formation of the Panamanian land bridge and the Great American Biotic Interchange. Taxa providing relevant data regarding this event are the ground sloths Megalonyx and Glossotherium, the glyptodont Glyptotherium, the pampathere Plaina, and the capybara Neochoerus. It has been generally accepted that the Panamanian land bridge formed between two and one-half and three million years ago. However, radiometric and paleomagnetic age determinations in central Mexico associated with South American immigrants, indicate that this land bridge might have originated more than one million years earlier. Active plate movement in Middle America during the late Tertiary with attendant volcanism and insular changes, coupled with concurrent eustatic and vegetational changes, make chronologic precision regarding isthmus formation and the resultant biotic interchange difficult at best.
Field and Laboratory Methods in Primatology is a manual for students and researchers studying wild primates. Technological advances allow fieldworkers to collect a wide range of data, store samples for later analysis, and collect information remotely. These methods open up opportunities to gain new insights on previously studied populations and are the means of collecting data on species that have, until now, been difficult to study. However, information on the practicalities of using such methodology in the field has largely been lacking. Here, in this indispensable reference, experienced fieldworkers provide the first comprehensive guide to the wide variety of techniques available for the study of wild primates. Covering everything from pre-trip planning to laboratory analysis of endocrine and genetic samples, packed full of tips and emphasising practicalities and ethics throughout, it is a must-have for all field primatologists and others studying free-ranging animals.
Radiocarbon ( 14 C) dating provided New World archaeological research with the first continent-wide common chronometric scale that transcended the mostly relative site- and region-specific chronological sequences that had been assembled during the preceding century of fieldwork. 14 C data continue to play a critical role in establishing a chronometric framework for the 5-century-long debate concerning the timing of the initial peopling of the New World. Other issues where 14 C results have been of particular importance include the origins and development of New World agriculture and determination of the relationship between the Western and Classic Maya long-count calendar. The introduction of a third-generation measurement technology of accelerator mass spectrometry (AMS) beginning in the late 1970s has provided a means of obtaining analyses on milligram and microgram amounts of carbon permitting more detailed critical approaches to increasing the accuracy of 14 C values on certain sample types—particularly human skeletal materials. It also provided a more effective means of allowing greater dating precision in situations where such data had an important bearing on the validity of inferences about the rates of cultural evolutionary change in New World societies.
The Geological Society of London Proposal for "...ending the distinction between the dual stratigraphic terminology of time-rock units (of chronostratigraphy) and geologic time units (of geochronology). The long held, but widely misunderstood distinction between these two essentially parallel time scales has been rendered unnecessary by the adoption of the global stratotype sections and points (GSSP-golden spike) principle in defining intervals of geologic time within rock strata." Our review of stratigraphic principles, concepts, models and paradigms through history clearly shows that the GSL Proposal is flawed and if adopted will be of disservice to the stratigraphic community. We recommend the continued use of the dual stratigraphic terminology of chronostratigraphy and geochronology for the following reasons: (1) time-rock (chronostratigraphic) and geologic time (geochronologic) units are conceptually different; (2) the subtended time-rock's unit space between its "golden spiked-marked" lower and upper boundaries, actually corresponds to the duration of the time-rock unit's defining geologic s.l. events-set; therefore, in no way can physical time (instants or intervals) be directly defined by GSSPs, (3) combining in a single system of "chronostratigraphic units" the time-rock and geologic time units as currently understood, leads to the epistemological error of uniting evidence (rock successions) with inference (the interpreted duration of chosen defining events); (4) the redundancy of the terms eonothem, erathem, system, series, and stage with eon, era, period, epoch and age lacks support, given that they are conceptually different; in fact, referring to "eon," "era," etc. as terms uniting both time-rock and geochronologic connotations will produce needless nomenclatorial confusion, attaching different meanings to already well known and widely used geologic terms; and (5) the reversion of 'geochronology' to its main stream and original meaning of numerical dating has no foundation, just by considering that the use of geochronolgy precedes numerical dating, which became practical by the 1960's. We endorse the following: (1) the GSSP network needs to be improved through the use of reference sections at high latitude sites, and in sedimentary continental rock successions of achievable, dependable positioning in the global standard timetable; and (2) to attend to researchers using astronomically-forced sedimentary systems, the designation of unit stratotypes needs to be reinstated as a valid and as a, complementary means of defining chronostratigraphic units, particularly at the stage and lower chronostratigraphic rank.
Sloth remains recovered from Actun Lak, Cayo District, Belize, during archaeological excavations by the Pacbitun Regional Archaeological Project include numerous though mainly fragmentary elements. Among the identifiable remains are a lower molariform tooth and several elements of the manus and pes. Although no complete cranial and long bone postcranial remains are available, the sloth is confidently assigned to the nothrotheriid species Nothrotheriops shastensis (Sinclair). This is the most southern certain record of the North American genus Nothrotheriops Hoffstetter and the first record of a nothrotheriid from Central America. Based on our knowledge of the species' diet and habitat preference from dry caves in the southwestern United States, its presence in Belize has paleoecological and biogeographic implications. The Belize remains clearly indicate that sloth diversity in northern Central America was greater than previously suspected. They also suggest that a greater variety of habitats existed in this region during the Pleistocene, and that vegetation communities underwent more dramatic changes during this interval than previously thought.
Rates of sea-level fall at the termination of the last interglacial are poorly defined and archives from which sea-level elevations are constrained during marine isotope stage (MIS) 5 in the western North Atlantic–Caribbean region are restricted spatially to a few key sites. Here, growth periods of presently submerged but subaerially deposited speleothems from the Yucatan Peninsula, south-east Mexico, are dated using the 230Th/234U multi-collector inductively coupled plasma mass spectrometric method. The growth periods, used to constrain maximum elevations of relative sea level, are in agreement with glacio-isostatic adjustment models for the near to intermediate region of the former North American ice sheets. Furthermore, our data provide additional constraints on the timing of peak relative sea levels during MIS 5a and 5c, and the timing of the regression following the last interglacial highstand. Combining our data with coral archives of minimum relative sea level from the same region provides a novel approach to calculating the rate of relative sea-level fall, although initial estimates are poorly constrained because of the wide range in ages observed for the most suitable corals. Copyright © 2013 John Wiley & Sons, Ltd.
Superficial or buried soils are important palaeostratigraphic indicators that have been poorly studied in the Basin of Mexico. This paper summarizes soil genesis and evolution based on the study of 15 soil profiles with palaeosol layers. An environmental morphogenetic and pedological sequence was established, based on granulometric, micromorphological, geochemical and field soil studies to identify the properties and characteristics of the pedogenetic environments in which they were formed. Results indicate two continuous and recurrent phenomena which altered the genesis of the soils, they are: (a) Torrential erosive processes under tectonic influences and arid conditions, and (b) Volcanic deposits that buried and fossilized soils. Both processes interrupt the evolution and sequence of soil profiles.
Consequently, from the upper talus slopes to the lower portions stretching the length of the central lowest plains there are many palaeo-soil discontinuities, that have not been considered into stratigraphic studies of the lacustrine plains.
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Mexico's Pleistocene terrestrial mammal record includes 13 orders, 44 families, 147 genera and 280 species, thus is comparable to the Recent one, but shows greater ordinal and family diversity. Post-Pleistocene extinction chiefly involved meso- and megabaric species. The mammal, palynologic and paleosol records are strongly time and space biased in favor of Late Rancholabrean data from a few morphotectonic provinces; hence, only broad climatic trends could be delineated, which approximately coincide with those known for the Wisconsinan; they disclose by 25–12ka, greater moisture and cooler temperature conditions than at present, coinciding too with a larger mammalian diversity and local faunas disharmony. Climate fluctuations impacted the fauna, causing species distribution changes and extinctions. The Recent fauna's complex biogeographic pattern reflects this; it includes tropical and temperate species associations outside their respective latitudes. Combining geologic and Recent mammal distribution data with the Pleistocene record, possible dispersal routes were detected: high and low land, southward corridors for temperate species, and low land, northward ones for tropical species. Finally, the existence of a single Mexican Rancholabrean Faunal Province is incompatible with mammal record's makeup and distribution, which calls for a multiprovince scheme to better understand Mexico's Pleistocene mammal biogeography and faunistics.
A striking diversity in limb morphology among fossil sloths suggests that the traditional distinction between extant “tree” sloths and fossil “ground” sloths merits closer scrutiny. Morphologically, xenarthrans (sloths, anteaters, and armadillos) are divergent from other mammalian orders. However, morphological shape indices of aspects of the elbow, hip, and knee joints that clearly discriminate locomotor modes in primates, carnivores, and bovids also succeed in differentiating the fully arboreal, semiarboreal, and terrestrial genera of anteaters. When compared to this extant data base, fossil sloths from both the Pleistocene radiation of the Antilles and the Santacrucian radiation of Argentina exhibit a range of values for these indices comparable to those of primates and anteaters. These results indicate that the diversity of limb structure in fossil sloths is associated with variation in posture and locomotor habits including arboreality, semiarboreality, and terrestriality.