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1. Introduction
The sloths (suborder Folivora) are representatives of a singular group in the Xenarthra that includes eight families: Entelopidae, Bradypodidae, Scelidotheriidae, Mylodontidae, Nothrotheriidae, Megalonychidae and Megatheriidae (Roseet al., 2009). The last Family includes the largest ground sloths that lived at the end of Pleistocene,MegatheriumandEremotherium.
Eremotheriumhas three species with morphological differences, timing and distribution:E. eomigrans, found in Late Pliocene-Early Pleistocene of Florida, USA;E. sefvei, restricted to the Pleistocene of Bolivia, andE. laurillardi, from the Late Pleistocene from Brazil to USA, being this the most commonly found species in the North and South American continents (Cartelle and De Iuliis, 1995; De Iuliis and St-André, 1997; De Iuliis and Cartelle, 1999).
Eremotherium laurillardiwas very common during the Rancholabrean of North America and Lujanian of South America (Cartelle and De Iuliis, 2006). It is believed that this species reached a body mass of 3900 to 5000 kilograms and a length of 6 m including the tail (Cartelle, 2000; McDonald, 2012). Isotopic data indicate that this species was herbivorous, and it fed both herbaceous plants and shrubs (Francaet al., 2014; Pérez-Crespoet al., 2015).
In MexicoE. laurillardihas been mainly reported from localities in the central and southern part of the country (Polaco-Ramos, 1981; McDonald, 2002). However, despite the numerous records, only a little of this material has been formally described, so that the intraspecific variations in individuals found in Mexico are unknown. Another problem is that since the work of McDonald (2002), there has been no updated compilation of new localities for this genus in Mexico and therefore the paleobiogeographic history of the species has gaps.
Herein, we describe a femur referred toE. laurillardicollected from undifferentiated fluvial sediments (sand and gravel) that crop out at the edge of a natural irrigation channel near Constitución 27 town, in the Socoltenango municipality, State of Chiapas, southern Mexico (Figure 1). In addition, an extensive review of the records ofE. laurillardiin Mexico is presented, adding new locations to the list of McDonald (2002). Finally, the distribution and paleobiogeography of this species in Mexico is discussed.
Figure 1 Location of the Constitución 27 (red star), in the Socoltenango municipality, Chiapas, southeastern Mexico.
Institutional abbreviations: IHNFG, Instituto de Historia Natural, Fósil Geográfico, historically is the acronym used for the Paleontological Collection of the Secretaría de Medio Ambiente e Historia Natural of Chiapas, Chiapas, Mexico;FMNH, Field Museum of Natural History, Chicago, USA;MCL, Museu de Ciências Naturais da Pontifícia Universidade Católica de Minas Gerais, Belo Horizonte, Brazil;MNP, Museo Nacional de Panama, Panama City, Panama;MNRJ, Museu Nacional do Rio de Janeiro, Rio de Janeiro, Brazil;ROM, Royal Ontario Museum, Toronto, Canada;UCMP, University of California Museum of Paleontology, Berkeley, USA;USNM, National Museum of Natural History, Smithsonian Institution, Washington, USA;ZMUC, Zoologisk Museum Universitat Copenhagen, Copenhagen.
2. Material and Methods
The material reported here consists of a right femur with excellent preservation, collected by Mario Roberto Pérez Moreno, a local resident of the Constitución 27 town. The specimen is housed in the paleontological collection of the Museo de Paleontología “Eliseo Palacios Aguilera”, Paleontology Department, Secretaría de Medio Ambiente e Historia Natural of Chiapas, under the catalog number IHNFG-5324. The bone showed signs of dragging on the medial and lateral edges in both proximal and distal ends. The measurements were taken with a tape measure following to De Iuliis (1996) and are shown in Figure 2. The photographs were taken with a digital camera and processed in Corel Photo-Paint X8.
Figure 2 Schematic drawing of the femur showing the measurements taken. 1) Greatest proximodistal length, 2) greatest proximal transverse width, 3) greatest distal transverse width, 4) minimum width at midshaft, 5) distance between the distal articular facets.
Eremotherium laurillardirecords from Mexico were compiled based on the compilations of Polaco-Ramos (1981) and McDonald (2002), complemented with the recent reports published in scientific journals, books chapter and academic meeting’s abstracts. With this data, we produced an updated distribution map ofE. laurillardiin Mexico, using the hypsometric range map available at the Comisión Nacional para el Conocimiento y Uso de la Biodiversidad.
3. Results
3.1. SYSTEMATIC PALEONTOLOGY
Superorder Xenarthra Cope, 1889
Order Pilosa Flower, 1883
Suborder Folivora Delsuc, Catzeflis, Stanhope, and Douzery, 2001
Family Megatheriidae Gray, 1821
Subfamily Megatheriinae Gill, 1872
GenusEremotherium Spillmann, 1948
Eremotherium laurillardi (Lund, 1842)
Type specimen: ZMUC 1130, isolated molariform tooth of a juvenile specimen from the Pleistocene of Lagoa Santa, Minas Gerais, Brazil (Cartelle and De Iuliis, 1995).
Synonymy.Megatherium americanum(Palacios, 1950);Eremotherium mirabile(Polaco-Ramos, 1981);E. quanajuatense(Dugès, 1896);E. quanajuatense(Monés, 1973);E. rusconii(Polaco-Ramos, 1981);E. carolinense(Hoffstetter, 1952);E. elenense(Hoffstetter, 1952).
3.2. DESCRIPTION
IHNFG-5324 is a complete right femur (Figure 3). Its total length is 780 mm (see Table 1). Its contour is rectangular in cranial and caudal view (Figures 3A and 3B), with the proximal and distal ends parasagittally aligned. The shaft is thin in the anteroposterior view and laterally it has a small twist that gives it a sigmodial appearance (Figure 3C). The proximal and distal epiphyseal sutures are fused. The medial edge of the femur is almost straight, while the lateral edge is slightly concave. The head of the femur is robust and is proximomedially oriented with an inclination of 40° in relation to the longitudinal central axis. The base of the articular condyle is short and wide, so it hardly differs from the rest of the femur. The greater trochanter is not as prominent and does not reach the level of the head of the femur, as inM. americanum. At the distal end, the medial condyle and the lateral condyle are present, which have large joint surfaces. The medial condyle is larger than the lateral condyle, the latter being more convex. The patellar trochlea extends transversely to the medial edge of the lateral condyle (Figure 3E).
Figure 3 Right femur of specimen IHNFG-5324 from Constitución 27 town, in A) anterior, B) posterior, C) lateral, D) proximal and E) distal view. Abbreviations: gt, great trochanter; hf, head of femur; lc, lateral condyle; mc, medial condyle; pt, patellar trochlea. Bar scale equals 10 cm.
Table 1 Measurements (in mm) of IHNFG-5324 and selected specimens ofE. laurillardi. Specimens marked with asterisk (*) taken from De Iuliis (1996).
| IHNFG- 5324 | MCL 9524* | MNRJ 3866* | ROM 19788* | FMNH F26970* | USNM 11650* | MNP 45* | UCMP V4201* | |
|---|---|---|---|---|---|---|---|---|
| 1. Greatest proximodistal length | 780 | 826 | 714 | 723 | 684 | 816 | 828 | 895 |
| 2. Greatest proximal transverse width | 404 | 447 | 365 | 385 | 360 | 416 | 420 | 447 |
| 3. Greatest distal transverse width | 412 | 432 | 359 | 405 | 356 | 445 | 455 | 476 |
| 4. Minimum width at midshaft | 260 | 320 | 275 | 280 | 258 | 300 | 255 | 330 |
| 5. Distance between the distal articular facets | 44 | 44 | 14 | 38 | 26 | 40 | 26 | 32 |
3.3. RECORDS IN MEXICO
Eremotherium laurillardiis the most widespread ground sloth in Mexico with 31 localities distributed in 13 states (Figure 4, Table 2). Until now, the northernmost record came from El Cedral, San Luis Potosí (Polaco-Ramos, 1981), followed by Arperos, Guanajuato (Dugès, 1896). Chiapas and Jalisco are the states with most localities withE. laurillardiremains, eight and seven, respectively. In Chiapas all the localities are distributed in the Central Basin, and are: El Bajión locality, Suchiapa municipality; Nandachuquí, Chiapa de Corzo municipality; Ignacio Zaragoza, La Concordia municipality; Road to Monterrey town and La Simpatía localities, Villa Corzo municipality; Gliptodonte and El Sabino localities, Villaflores municipality, and recently Constitución 27, Socoltenango municipality (Palacios, 1950; Santamaríaet al., 1983; Than-Marchese, 2009; Than-Marcheseet al., 2012; Gómez-Pérez and Carbot-Chanona, 2012). In Jalisco state, the localities are Zacoalco, Venustiano Carranza, Chapala lake, El Limón, El Grullo, Tonaya, and Rancho El Aguacate, Amacueca (Polaco-Ramos, 1981; McDonald, 2002; Viramontes, 2015; Aguilaret al., 2015). Other localities in western Mexico are Arteaga, Aguacate and Hihuitlán, Michoacán; and Cerro Guayabilla, Colima (Polaco-Ramos, 1981; McDonald, 2002). Chimalacatlán, Morelos (Torres-Martínez, 2004), Tlatlaya, Estado de México (Polaco-Ramos, 1981), and San Mateo Huexoyucán, Tlaxcala (Sánchezet al., 2016), are localities located in the south-central region. In eastern Mexico the localities are Atoyac and Veracruz, Veracruz (Peña-Serrano and Carbot-Chanona, 2010; Alvarado-Mendozaet al., 2013), although remains of the genus are known from at least four more locations in Veracruz state, but they have not been formally reported (Peña-Serrano, 2016, com. pers.). Besides those from Chiapas, other locations in southern Mexico are Puerto Escondido; Etla, and Barranca del Muerto (Santiago Chazumba), in Oaxaca state (Polaco-Ramos, 1981; Guerrero-Arenaset al., 2016; Viñas-Vallverdúet al., 2017); and Apaxtla, Guerrero (Torres, 1981). In southeastern Mexico there is a unique locality in Teapa, Tabasco (Polaco-Ramos, 1981; McDonald, 2002).
Figure 4 Localities with Eremotherium laurillardi in Mexico. For references of the localities see Table 2. The arrows show the biological corridors sensu Ceballos et al. (2010): temperate corridor Transvolcanic Belt - Sierra Madre del Sur (1), and the tropical corridors Tamaulipas - Central America Gulf Lowlands (2), and Sonora - Central America Pacific lowlands (3).
Table 2 Localities with records ofEremotherium laurillardiin Mexico.
| ID locality | Locality | State | NALMA/Age | Reference |
|---|---|---|---|---|
| 1 | Etla | Oaxaca | Pleistocene | Polaco-Ramos, 1981 |
| 2 | Puerto Escondido | Oaxaca | Late Pleistocene | Guerrero-Arenas et al., 2016 |
| 3 | Barranca del Muerto, Santiago Chazumba | Oaxaca | Rancholabrean/Late Pleistocene | Viñas-Vallverdú et al., 2017 |
| 4 | Chimalacatlán | Morelos | Late Pleistocene | Torres, 2004 |
| 5 | Apaxtla | Guerrero | Pleistocene | Torres, 1981 |
| 6 | Tlatlaya | Estado de México | Pleistocene | Polaco-Ramos, 1981; McDonald, 2002 |
| 7 | Arteaga | Michoacán | Pleistocene | Polaco-Ramos, 1981; McDonald, 2002 |
| 8 | Hihuitlán | Michoacán | Pleistocene | Polaco-Ramos, 1981 |
| 9 | Aguanuato | Michoacán | Pleistocene | Polaco-Ramos, 1981 |
| 10 | Cerro Guayabilla | Colima | Pleistocene | Polaco-Ramos, 1981 |
| 11 | Venustiano Carranza | Jalisco | Pleistocene | Polaco-Ramos, 1981; McDonald, 2002 |
| 12 | El Limón | Jalisco | Pleistocene | Viramontes, 2015 |
| 13 | El Grullo | Jalisco | Pleistocene | Viramontes, 2015 |
| 14 | Tonaya | Jalisco | Pleistocene | Viramontes, 2015 |
| 15 | Rancho El Aguacate, Amacueca | Jalisco | ----- | Aguilar et al., 2015 |
| 16 | Zacoalco | Jalisco | Pleistocene | Polaco-Ramos, 1981; McDonald, 2002 |
| 17 | Chapala lake | Jalisco | Pleistocene | Polaco-Ramos, 1981; McDonald, 2002 |
| 18 | El Cedral | San Luis Potosí | Late Pleistocene | Polaco-Ramos, 1981; McDonald, 2002 |
| 19 | Arperos | Guanajuato | ---- | Dugès, 1896 |
| 20 | San Mateo Huexoyucán | Tlaxcala | Rancholabrean/Late Pleistocene | Sánchez et al. (2016) |
| 21 | Atoyac | Veracruz | Late Pleistocene | Peña-Serrano and CarbotChanona, 2010 |
| 22 | Buenavista, Veracruz, | Veracruz | Late Pleistocene | Alvarado-Mendoza et al., 2013 |
| 23 | Teapa | Tabasco | Late Pleistocene | Polaco-Ramos, 1981; McDonald, 2002 |
| 24 | El Bajión, Suchiapa | Chiapas | Late Pleistocene | Than-Marchese et al., 2012 |
| 25 | Nandachuquí, Chiapa de Corzo | Chiapas | Late Pleistocene | this work |
| 26 | Socoltenango | Chiapas | Late Pleistocene | this work |
| 27 | Ignacio Zaragoza, La Concordia | Chiapas | Late Pleistocene | Than-Marchese, 2009 |
| 28 | Road to Monterrey town, Villa Corzo | Chiapas | Late Pleistocene | Santamaría et al., 1982 |
| 29 | La Simpatía locality, Villa Corzo | Chiapas | Rancholabrean/Late Pleistocene | this work |
| 30 | Gliptodonte locality, Villaflores | Chiapas | Rancholabrean/Late Pleistocene | Gómez-Pérez and CarbotChanona, 2012 |
| 31 | El Sabino, Villaflores | Chiapas | Late Pleistocene | Palacios, 1950 |
4. Discussion
4.1. TAXONOMIC ASSIGNMENT
Among Pleistocene ground sloths, the femora of Megatheriidae are the only rectangular shaped bones (De Iuliis, 1996). The specimen IHNFG-5324 has the typical rectangular shape, which indicates that is it a Megatheriinae. Two species of Megatheriinae ground sloths are known from the Late Pleistocene,Megatherium americanum, restricted to Brazil and Argentina, andEremotherium laurillardi, which had a distribution from the southeastern of the United States of America to Brazil (Cartelle and De Iuliis, 1995). According to De Iuliis and St-André (1997), the femur ofM. americanumandE. laurillardican be differentiated by the following features: 1) The greater proximal and distal transverse widths relative to the midshaft width inM. americanum; 2) lateral and medial margins are more medially convex inM. americanum, whereas inE. laurillardiare more rectilinear and parallel; 3) patellar trochlea inE. laurillardiis transversely elongated, and the medial anterior margin extends substantially towards the sagittal plane of the femur, while the trochlea ofM. americanumis reduced essentially to an anterodorsal tibial facet lateral extension. The patellar trochlea is a less variable character in the femur of Megatheriinae and therefore is useful for differentiating betweenE. laurillardiandM. americanum(De Iuliis, 1996). The specimen IHNFG-5324 has all the features known for theE. laurillardifemur (Figure 3) and therefore we refer it to this species.
4.2. PALEOBIOGEOGRAPHY
Figure 4 shows that majority of the localities withE. laurillardiare distributed in the central and southern portion of Mexico, ranging at heights between 0-200 up to 2000-2500 meters above sea level. Contrarily, in USA,E. laurillardihas been found in localities restricted to the Atlantic and Gulf Coastal Plain, south of the Piedmont portion of the Appalachian Highlands and the Balcones escarpment in Texas, where elevation is low (below of 200 meters above sea level) and the relief is minimal (McDonald and Lundelius, 2009). In Central America there are records ofEremotherium laurillardiin Guatemala, El Salvador and Honduras (Webb and Perrigo, 1984; Cisneros, 2005; Dávilaet al., 2019). In Guatemala this species has been mainly recorded in localities with low elevation, in La Estanzuela (160 m.a.s.l.), Petén (170 m.a.s.l.), and Chiquimula (424 m.a.s.l.); the site with highest elevations is Guatemala City (1500 m.a.s.l). Similarly, the Río Tomayate locality, San Salvador city, El Salvador,E. laurillardihas records in medial elevations (~500 m.a.s.l.). In South America,E. laurillardiis recorded mainly in the Atlantic coast of Venezuela and Brazil, whereas in Perú and Ecuador it has a distribution over the Pacific coast (Cartelle and De Iuliis, 1995). Contrarily, in Cundinamarca, Colombia, a record ofE. laurillardicame from altitudinal range above 3000 m.a.s.l. (De Porta, 1961).
El Cedral, San Luis Potosí, is the northernmost locality in Mexico, and the southernmost localities are Constitución 27 and Ignacio Zaragoza, in the State of Chiapas. Of the 31 localities, only two (Buenavista, Veracruz and Puerto Escondido, Oaxaca) are at sea level. The low occurrence ofE. laurillardiin sea level sites in Mexico is different to the occurrence seen in USA, where the species seems to has a coastal plain environment preference (McDonald and Lundelius, 2009).
The wide altitudinal distribution ofE. laurillardiin Mexico suggests that it inhabited diverse types of ecological regions, from coastal plains to open forests.Eremotherium laurillardiwas distributed throughout Mexico using the temperate corridor of the Transvolcanic Belt - Sierra Madre del Sur, and the tropical corridors Tamaulipas - Central America Gulf Lowlands, and Sonora - Central America Pacific lowlands (sensuCeballoset al., 2010). This shows thatE. laurillardiused the same routes and habitat as the Harlan’s ground slothParamylodon harlani(Carbot-Chanonaet al., 2021) to move from south to north. The presence of a greater number of localities withE. laurillardiremains in Chiapas, compared to other states of Mexico, could be due its geographical position, where the climate was more agreeable for this species, given to its preference for tropical or semitropical areas. The several records in Guatemala (Dávilaet al. 2019), and the abundant individuals recovered in the Río Tomayate, El Salvador (Cisneros, 2005), areas close to Chiapas, support this idea.
Ferrusquía-Villafrancaet al. (2010) proposed that migration routes from South America toward North America, in Mexico where: (1) along the Pacific coast, bordering the Sierra Madre de Chiapas; and (2) along the Gulf coast. However, theE. laurillardifossil record in Chiapas, as well as of other megamammals (e.g.glyptodonts, capybaras, mammoths, gomphotheres, horses, camels) found in the region (Gómez-Pérez and Carbot-Chanona, 2012; Jiménez-Hidalgoet al., 2019; Carbot-Chanonaet al., 2020), indicate that probably the major migratory route from South America was through the Central Basin of Chiapas.
5. Conclusions
The right femur IHNFG-5324 described in this work, documents a new specimen of the giant ground slothEremotherium laurillardiin the State of Chiapas. The record from Constitución 27 town, adds the eighth locality with remains of this species in Chiapas. Its wide occurrence in Chiapas could be indicative of the tropical or semitropical climate in the region. The presence ofE. laurillardiand other megamammals in several localities in the Chiapas Central Basin, shows that this area was the principal migratory route from Central America to the central and northern parts of Mexico, and vice versa, denoting the importance of southern of Mexico in the understanding of the migratory routes of megamammals across the country.
In Mexico,E. laurillardiis the Late Pleistocene ground sloth with most records, distributed mainly in the central and southern part of the country. It has been recorded in 31 localities, with altitudinal ranges between 0-200 up to 2000-2500 meters above sea level, suggesting that it inhabited diverse types of ecological regions.
