McDonald and Carranza-Castañeda—New Miocene ground sloth from Mexico
mafic volcanic rocks related to the Trans Mexican volcanic belt are 6–3Ma old in the area near Tecolotlán (Rosas-Elguera et al., 1996; Gómez-Tuena et al., 2007). The origin of the Tecolotlán Basin has not been studied, but we note that on both sides of the elongated depression there exist several conspicuous NE- trending lineaments that suggest the existence of a late Miocene– Pliocene graben in the area. Extensive outcrops of Pliocene mafic lavas south of the town of Tecolotlán (Paizanni-Herrera et al., 1999) suggest that drainage in the grabenmay have been blocked periodically by volcanic products. The basin is delimited to the east by the oldest sedimentary
rocks, limestones, exposed near San José de Los Guajes, and to the west by a series of interbedded late Cretaceous volcanic and sedimentary rocks informally called the Juchitlán beds (Kowallis et al., 2017). Before 1997, there was no previous work related to the
(Kowallis et al., 1998, 2003). Kowallis et al. (2017) recently finished the most complete study of the area.
Stratigraphy.—The Tecolotlán Basin is filled with fluvial- lacustrine sediments, consisting of poorly consolidated muds, clays, and sandstones that contain an abundance of North American mammalian faunas, many of which include South American immigrants, along with reptiles and fishes. Interbedded with these sediments are several important volcanic ashes that can be radiometrically dated. For the time being, this interesting late Hemphillian sequence has been informally called the San José beds (Kowallis et al., 2017). Resting at the upper part of the sequence, above an
erosional unconformity, is a more recent unit composed of sands, gravel, and sandy mudstones. This part of the section is named the San Buenaventura beds (Kowallis et al., 2003, 2017) and the age is assigned to the latest Blancan–Irvingtonian. In the southeast of the basin near the town of Juchitlán, another late Blancan deposit, Jal Teco 47 Los Pitahayos, has been described and contains the remains of two South American immigrant taxa: Glyptotherium cylindricum Brown, 1912 and Neochoerus occidentalis Carranza Castañeda, 2016. Because the distance between the late Hemphillian localities
in which the fossil materials were collected is so great within the basin, for the purpose of this work, it was decided to consider two areas of major importance, taking into consideration that one third of the sequence is referred to the late Blancan (Fig. 2).
The Santa María area.—This locality is named for its location near a small ranch called SantaMaría, at the southeast portion of the basin (20°07'24''N; 104°03'39''W). This general area lies along a complex system of deep arroyos, and the exact fossil site corresponds to the major canyon, situated just south of the ranch. Here, the thickness of the sediments measures 28–30 m. At the base of the section are layers of gravel covered by clayswith sands of different sizes. This alternation of clays with sands is only interrupted by a horizon of compacted and characteristically
reddish clay. Above this clay is a paleo-channel composed of fine-grained sand from which several rodent molars, along with the two megalonychid molariforms referred to the new taxon described in this work, were found. At the top of the fossiliferous
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paleo-channel, there are several layers of clays and gravel, and the sequence ends with a series of lacustrine sediments and layers of clay that are 2–6m thick and show evidence of soil development and roots. Above the whole section there are sandy outcrops cemented by carbonates in which freshwater gastropods are common. It is interesting to note that the cemented sands occur along the entire basin, separating the lower fluvial-lacustrine sediments of the Santa María sequence from the higher San José beds.
biostratigraphy of the Tecolotlán Basin. The first contributions were made by B. Kowallis of Brigham Young University
The San José beds area.—This sequence is well represented at the locality called Jal Teco 20 La Hacienda (20°08'56''N; 104°04'56''W). Access to the locality is by Highway 81 from Tecolotlán. The site is near the ruins of an old mansion called La Hacienda, from which the locality name is derived. The thickness of this section, measured from the top of the
consolidated carbonate sands, is 20–30m. It consists of layers of clays and sands at the base, and is covered by clayish sediments. It is from this sequence that the jaw of the newmegalonychid taxon described in this paper was collected. Stratigraphically, it was found about one meter below a layer of volcanic ash and 12m above the base of the unit. The tuff from the San José beds produced some grains of sanidine that were dated by 40Ar/39Ar and gave an age of 4.95±0.02Ma (Kowallis et al., 2017). More recently another analysis of the same layer from a different site was analyzed by theU/Pb method using zircons, which produced a date of 4.85±0.1Ma for the ash (LA-ICPMS), by Laboratorio de Estudios Isotópicos,Centro de Geociencias,
UNAM.The results of these two radiometric assessments are consistent and correspond with the estimated biostratigraphic ages based on the mammalian species present in the San José beds. Resting on the upper part of the sequence, above an
erosional unconformity, there is a more recent unit composed of sands, gravel, and sandy claystones. This part of the section has been named “The San Buenaventura beds.” These sediments are thought to be Blancan in age, and their thickness varies at different sites. The most complete section is at the Jal Teco 7 Las Gravas locality, with at least 30m of exposed sediments. The basal bed is irregular and its lower contact with the San José beds is not exposed. In general, the lithology of the sequence consists of layers of sandy clay, paleo-channels with gravels, some sandy layers of variable thicknesses, and other strata with sands and many paleo-channels.
Locality Jal Teco 47 Los Pitahayos.—In the southeastern part of the basin, near the town of Juchitlán, another late Blancan deposit was described only 300m away from the Los Corrales ranch, the Jal Teco 47 Los Pitahayos locality (20°05'02''N; 104° 05'38''W). The thickness of the sediments at the site is ~20 m. The
fauna includes two South American immigrants, Neochoerus occidentalis and Glyptotherium cylindricum, along with Equus simplicidens Cope, 1892, aff. Stockoceros, Platygonus sp.,
osteoderms and teeth of crocodiles, parts of turtle carapace, and fish remains (operculae). The late Blancan age assigned to this unit was estimated through the analysis of a pyroclastic fall tuff,
discovered within the sediments, which was dated using 40Ar/39Ar and gave a date of 2.62±0.03Ma (Kowallis et al., 2017).
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