Andrés Rinderknecht

The largest fossil rodent

The discovery of an exceptionally well-preserved skull permits the description of the new South American fossil species of the rodent, Josephoartigasia monesi sp. nov. (family: Dinomyidae; Rodentia: Hystricognathi: Caviomorpha). This species with estimated body mass of nearly 1000 kg is the largest yet recorded. The skull sheds new light on the anatomy of the extinct giant rodents of the Dinomyidae, which are known mostly from isolated teeth and incomplete mandible remains. The fossil derives from San José Formation, Uruguay, usually assigned to the Pliocene–Pleistocene (4–2 Myr ago), and the proposed palaeoenvironment where this rodent lived was characterized as an estuarine or deltaic system with forest communities.

AN ENIGMATIC CINGULATA (MAMMALIA: XENARTHRA) FROM THE LATE MIOCENE OF URUGUAY

Cingulates, the armadillos (Dasypodidae), glyptodonts (Glyptodontidae) and pampatheres (Pampatheriidae), are perhaps the most distinctive xenarthrans, as their bony carapaces make them easily recognizable. Dasypodids are known from the late Paleocene to the Recent and glyptodonts and pampatheres from the late Eocene (following Kay et al., 1999) and Miocene (Carlini et al., 1989; Carlini and Scillato-Yand, 1993) to the Pleistocene-early Holocene of South America (ScillatoYan6, 1995). They were abundant, and in the Pliocene all three families dispersed into North America as part of the Great American Biotic Interchange (Stehli and Webb, 1985). They are taxonomically diverse; systematically complex, and distinguished mainly on characters of ornamentation of the carapace and caudal tube. Herein we report the presence of an enigmatic cingulate from the late Miocene of Uruguay (collected in late 1988 by Luis R. Castiglioni) that we tentatively consider as a glyptodont belonging to the genus Neoglyptatelus, of the poorly known subfamily Glyptatelinae. Paula Couto (1979) recognized only four subfamilies within the Glyptodontidae, Propalaeohoplophorinae, Hoplophorinae, Doedicurinae and Glyptodontinae. However, a fifth, the Glyptatelinae, which includes the earliest and most primitive glyptodonts, is also generally recognized (Castellanos, 1932; Hoffstetter, 1958; Scillato-Yan6, 1977; Carlini et al., 1997; McKenna and Bell, 1997). Glyptatelines are known from a minimum of three genera, Glyptatelus (late Eocene to late Oligocene; Mustersan and Deseadan LMAs, following Kay et al., 1999), Clypeotherium (late Oligocene; Deseadan LMA), and Neoglyptatelus (middle to late Miocene; Laventan and Huayquerian LMAs). Abbreviations-LMA, Land Mammal Age; x, average; s, standard deviation; n, sample size; OR, observed range; MNHN, Museo Nacional de Historia Natural de Montevideo, Uruguay.

The youngest record of phorusrhacid birds (Aves, Phorusrhacidae) from the late Pleistocene of Uruguay

We report the youngest record of a phorusrhacid bird based on a distal portion of a right tarsometatarsus. This fossil comes from late Pleistocene sediments of Uruguay. The age determination was based on lithological features, biostratigraphical studies and absolute dating. The evidence indicates that these groundbirds co-occurred with the typical Pleistocene South American megafaunal mammals. The so far youngest fossils of phorusrhacids stem from the Pliocene or lower Pleistocene of South and North America.

A mitogenomic timetree for Darwin’s enigmatic South American mammal Macrauchenia patachonica

The unusual mix of morphological traits displayed by extinct South American native ungulates (SANUs) confounded both Charles Darwin, who first discovered them, and Richard Owen, who tried to resolve their relationships. Here we report an almost complete mitochondrial genome for the litoptern Macrauchenia. Our dated phylogenetic tree places Macrauchenia as sister to Perissodactyla, but close to the radiation of major lineages within Laurasiatheria. This position is consistent with a divergence estimate of ∼66 Ma (95% credibility interval, 56.64–77.83 Ma) obtained for the split between Macrauchenia and other Panperissodactyla. Combined with their morphological distinctiveness, this evidence supports the positioning of Litopterna (possibly in company with other SANU groups) as a separate order within Laurasiatheria. We also show that, when using strict criteria, extinct taxa marked by deep divergence times and a lack of close living relatives may still be amenable to palaeogenomic analysis through iterative mapping against more distant relatives.

New genus of giant Dinomyidae (Rodentia: Hystricognathi: Caviomorpha) from the late Miocene of Uruguay

Abstract In this contribution we describe the fossil remains of a new genus and species of a giant rodent, Arazamys castiglionii (Mammalia, Rodentia, Dinomyidae), from Arazatí beach in San José Department, southern Uruguay. The specimen was exhumed from pelitic sediments of the Camacho Formation, biostratigraphically assigned to the late Miocene Huayquerian South American Land Mammal Age. The remains include an incomplete skull (braincase, auditory region, and nearly full dentition) and the atlas. Based on comparative studies of the anatomy of the auditory region, we describe 2 morphologies for the subfamily Eumegamyinae, 1 characterized by a short meatus acusticus externus, a great development of the foramen stylomastoideum, and a conspicuous ectotympanic cavity, and a 2nd characterized by a long meatus acusticus externus and lack of the ectotympanic fossa. The potential taxonomic and systematic value of the auditory region in Dinomyidae is discussed.

A New Genus and Species of Mylodontidae (Mammalia: Xenarthra) from the Late Miocene of Southern Uruguay, with Comments on the Systematics of the Mylodontinae

ABSTRACT New associated cranial and postcranial remains of a new genus and species of Mylodontinae: Lestobradys sprechmanni, gen. et. sp. nov (Mammalia: Xenarthra), were found on the coast of Kiyú beach (San José, southern Uruguay). The specimens described are from the late Miocene (Huayquerian SALMA) sediments of Camacho Formation. The mandibular configuration of this new taxon differs from all well-known members of the family in presenting a first alveolus strongly projected toward the labial region that is separated from the rest of the alveoli of the dental series by a marked diastema. It additionally differs in its combination of a subtriangular second alveolus, subquadrangular third alveolus, and a bilobed last alveolus. An incomplete skull, three thoracic vertebrae, and two caudal vertebrae were associated with the mandibular remains. We also refer a complete and well-preserved right astragalus to the new species. Aspects of the history, systematics, and taxonomy of the Mylodontidae and their proposed subfamilies are discussed.

Optimal swimming speed estimates in the Early Permian mesosaurid Mesosaurus tenuidens (Gervais 1865) from Uruguay

Abstract Mesosaurid biology has been subject of continuous debate since the first description of Mesosaurus tenuidens by Paul Gervais in 1867. Controversy surrounds their environmental and feeding preferences. Most studies suggested that mesosaurids were marine reptiles and perhaps piscivorous predators. Nonetheless, recent work suggests that they inhabited a salty, eventually hypersaline shallow epicontinental sea and that pygocephalomorph crustaceans were their preferred food item. Here, we present results of the first biomechanical study about optimal swimming capabilities in Mesosaurus tenuidens, which along with the comparative analysis of the limb morphology support the hypothesis that these animals were slow swimmers living in shallow waters. The study is based on the revision of several almost complete mesosaurid specimens and isolated, well-preserved bones housed in palaeontological collections in Uruguay, Brazil, France and Germany. We studied the relative size and proportions of the bones, as well as their morphology and anatomical position to produce a three-dimensional reconstruction of the original appearance of an undamaged, complete skeleton. Our results suggest a fairly low optimal swimming speed for mesosaurids, which is consistent with capture of fairly slow prey like pygocephalomorphs, possibly by filter-feeding, rather than by active pursuit of fast prey.

ESTIMATION OF HEARING CAPABILITIES OF PLEISTOCENE GROUND SLOTHS (MAMMALIA, XENARTHRA) FROM MIDDLE-EAR ANATOMY

Extant sloths are represented by only two genera (Bradypus and Choloepus) both of which are semi-arboreal, slow-moving mammals. However, fossil Bradypoda are much more diverse and were traditionally considered to be “ground sloths,” although some of them are now recognized as having been arboreal. This group was well represented in the American Pleistocene fauna and among the larger representatives of the extinct sloths is the genus Megatherium with a body mass of 4 to 6 tons (Farina and Blanco, 1996; Farina et al., 1998). In this work we studied the middle-ear morphology of two Pleistocene taxa of the Mylodontidae family: Glossotherium robustum with an estimated body mass of 1500 kg and Lestodon armatus with an estimated body mass of 4100 kg (Bargo et al., 2000). In the mammalian middle-ear, sound energy is transferred from the external auditory meatus to the cochlea. The bone mechanism responsible for this transfer acts as a transformer allowing the low impedance of the auditory meatus to be matched to the much higher impedance of the cochlear fluid. The sound is transmitted from the tympanic membrane to the cochlea by the three middle-ear ossicles: the malleus, the incus, and the stapes. Impedance is composed of resistance and reactance forces. The main resistive component of the ear is related to the viscosity of the inner ear fluid causing resistance against the motion of the stapes footplate and to a lesser degree, the friction caused by the moving components of the middle ear. Two types of reactance are involved in middle-ear impedance (Plassmann and Brandle, 1992; Webster and Plassmann, 1992 and references therein). One is the mass reactance that is due to the mass of middle-ear structures. This mass reactance affects high frequencies more than low frequencies, so the impedance due to mass reactance increases with increasing frequency. The other reactance component of the middle-ear is the spring-like property of the ligaments and muscles. This spring reactance mainly affects low frequencies. At the ideal frequency range these two reactive components (mass and spring) partially cancel each other, and the resistive component, which does not vary with frequency, dominates the middle-ear impedance. Therefore, larger middleear ossicles, producing high mass-reactance, are well adapted to transfer low frequency signals (Plassmann and Brandle, 1992;

Body Mass Estimations and Paleobiological Inferences on a New Species of Large Caracara (Aves, Falconidae) from the Late Pleistocene of Uruguay

Abstract The caracaras belong to a group of falconids with widespread geographical distribution in the Western Hemisphere, particularly in South America. Here we report fossil remains of a new species attributed to the genus Caracara from the late Pleistocene of Uruguay. This bird would have had an estimated body mass of 3700 grams, a value that greatly exceeds the maximum body mass reported for living falconids. Apparently, it would have had flying capabilities, in contrast to another paleospecies recently described from the Holocene of Jamaica. This fossil bird was found in association with mammal megafaunal remains and could offer new insights about the role of carnivorous birds in late Pleistocene environments of South America.

A new Mylodontinae (Mammalia, Xenarthra) from the Camacho Formation (late Miocene), Uruguay

The xenarthrans of the sloth subfamily Mylodontinae are conspicuous members of South and North American Neogene faunas. They occupied a large latitudinal range from central North America to Tierra del Fuego in the austral extreme of South America (Latorre, 1998). The first undoubted representative of the subfamily is Glossotheriopsis pascualli, from the Friasian SALMA (middle Miocene) of Argentina. Beginning in the late Miocene (Huayquerian SALMA) the mylodontines are represented by numerous taxa that exhibit a range of morphological heterogeneity (Esteban, 1988) in contrast to the other subfamilies. Most of the mylodontine taxa described for the Neogene are based on very poor or fragmentary material making a comprehensive comparative description of the genera within the subfamily difficult. The best and most useful single bone that can be used to characterize mylodontid species is the mandible as it is commonly preserved and contains a complex suite of characters that makes it systematically informative (Perea, 1992). It is for these reasons that the majority of the Neogene sloth taxa are based on mandibular remains as they do allow an adequate morphologic comparison and subsequent phylogenetic conclusions that document the diversity within this subfamily of sloths. In this work we describe a right mandibular ramus originating from the Camacho Formation at Kiyu beach, Department of San Jose, Uruguay (Fig. 1). While the fossil shows some affinities with Pleistocene genera it also shares characters with older mylodontines as well. On the basis of its distinctive suite of morphological features it can be distinguished from all other known mandibular morphologies within the subfamily, indicating it represents a new genus and species. The material described here is deposited at the Paleontological Collection of Vertebrates, Facultad de Ciencias, Uruguay (FCDPV).