Journal Articles by Darin A Croft
We provide a synopsis of ~ 60 million years of life history in Neotropical lowlands, based on a c... more We provide a synopsis of ~ 60 million years of life history in Neotropical lowlands, based on a comprehensive sur- vey of the Cenozoic deposits along the Quebrada Cachiyacu near Contamana in Peruvian Amazonia. The 34 fossil- bearing localities identified have yielded a diversity of fossil remains, including vertebrates, mollusks, arthropods, plant fossils, and microorganisms, ranging from the early Paleocene to the late Miocene–?Pliocene (N20 succes- sive levels). This Cenozoic series includes the base of the Huchpayacu Formation (Fm.; early Paleocene; lacus- trine/fluvial environments; charophyte-dominated assemblage), the Pozo Fm. (middle + ?late Eocene; marine then freshwater environments; most diversified biomes), and complete sections for the Chambira Fm. (late Oligocene–late early Miocene; freshwater environments; vertebrate-dominated faunas), the Pebas Fm. (late early to early late Miocene; freshwater environments with an increasing marine influence; excellent fossil re- cord), and Ipururo Fm. (late Miocene–?Pliocene; fully fluvial environments; virtually no fossils preserved). At least 485 fossil species are recognized in the Contamana area (~250 ‘plants’, ~212 animals, and 23 foraminifera). Based on taxonomic lists from each stratigraphic interval, high-level taxonomic diversity remained fairly constant throughout the middle Eocene–Miocene interval (8-12 classes), ordinal diversity fluctuated to a greater degree, and family/species diversity generally declined, with a drastic drop in the early Miocene. The Paleocene–?Pliocene fossil assemblages from Contamana attest at least to four biogeographic histories inherited from (i) Mesozoic Gondwanan times, (ii) the Panamerican realm prior to (iii) the time of South America’s Cenozoic “splendid isolation”, and (iv) Neotropical ecosystems in the Americas. No direct evidence of any North American terrestrial immigrant has yet been recognized in the Miocene record at Contamana.
The early middle Miocene (Langhian age) site of Cerdas in the southern Bolivian Altiplano has pro... more The early middle Miocene (Langhian age) site of Cerdas in the southern Bolivian Altiplano has produced a diverse fauna of extinct mammals (15 species in seven orders and 11 families). In this study, we use paleosols and ichnofossils to reconstruct its paleoenvironment and the conditions in which its fossils were preserved. The described paleosols represent three pedotypes and three distinct landscape surfaces in an alluvial system. Type 1 paleosols are interpreted as Haplusteps (Inceptisols) that formed on a proximal floodplain in a subhumid to humid, patchy shrubland with seasonal variation in precipitation and associated changes in soil moisture conditions (Landscape 1). Type 2 paleosols are interpreted as Dystrudepts that formed on a well-vegetated, distal floodplain in a seasonal, humid climate with ground covering shrubland vegetation (Landscape 2). Type 3 paleosols are interpreted as Calciustolls (Mollisols) that formed in a shifting alluvial environment in a seasonal, sub-humid to semi-arid open environment (Landscape 3). Ichnofossil assemblages of Cerdas include Skolithos, Planolites, Macanopsis, Parowanichnus, rhizohaloes, and rhizotubules. These were produced by detritivorous, herbivorous, and faunivorous soil arthropods as well as plant roots and represent soil communities not normally preserved as body fossils that were living within a heterogeneous alluvial environment. The physical and geo-chemical properties of the paleosols and associated ichnofossil assemblages indicate that the paleolandscapes were composed of shrublands and open environments that experienced changes in moisture regimes due to seasonal precipitation and flooding events and had varying degrees of temporal stability. Our analysis is the first detailed study of pre-Pleistocene Cenozoic paleosols and trace fossils from the southern tropics (mid-latitudes) of South America and one of the few focused on important fossil-mammal bearing sediments.
We provide new and revised identifications of mammals from the early middle Miocene (Langhian age... more We provide new and revised identifications of mammals from the early middle Miocene (Langhian age, Colloncuran South American Land Mammal Age [SALMA]) of Cerdas, Bolivia. We also formally name a new typothere notoungulate, Hegetotherium cerdasensis, sp. nov., that can be distinguished by the absence of an external talonid sulcus on m3 and its small size (15–25% smaller than Hegetotherium mirabile). We refer several typothere specimens from Nazareno, Bolivia, to H. cerdasensis, which suggests that the two sites are of similar age. We report the first sparassodont and astrapothere remains from Cerdas. Sparassodont remains include an associated basicranium and mostly complete mandible; the species appears to represent a new, small-bodied borhyaenoid. Astrapothere remains consist of many tooth fragments from a new species of the subfamily Uruguaytheriinae. A partial sloth dentary from Cerdas likely pertains to the subfamily Megatheriinae and is the first report of the family Megatheriidae from the site. A newly discovered peltephilid armadillo specimen includes a partial articulated carapace that supports recognition of the Cerdas taxon as a new species. The two dasypodids of Cerdas (one Euphractini, one Eutatini) represent two new species closely related to undescribed species from the late middle Miocene (Serravallian age, Laventan SALMA) of Quebrada Honda, Bolivia. The mammals of Cerdas indicate that (1) the middle latitudes (southern tropics) contributed significantly to the diversity of Miocene mammal communities in South America; and (2) the Middle Miocene Climatic Optimum was a key factor in the differentiation of South American mammal assemblages.
Although sexual size dimorphism is an important component of the biology of many living New World... more Although sexual size dimorphism is an important component of the biology of many living New World metatherians, little is known about sexual dimorphism in extinct members of this group. Here, we describe morphometric variation in two traditionally recognized species of the paucituberculatan Acdestis from the early Miocene Santa Cruz Formation of Patagonia, A. owenii and A. spegazzinii (=A. lemairei). The results of this morphometric study show that specimens of Acdestis from the Santa Cruz Formation form a single morphological continuum, with no features that clearly distinguish the two recognized morphotypes besides overall body size and the depth of the mandibular ramus, both features which are sexually variable in extant marsupials. This suggests that the two species of Acdestis from the Santa Cruz Formation are better regarded as gender morphs of a single dimorphic species rather than two highly similar monomorphic species. The degree of sexual dimorphism inferred here for Acdestis is comparable to that seen in living sexually dimorphic marsupials. The identification of sexual dimorphism in Acdestis suggests that extinct paucituberculatans may have exhibited much greater variation in their ecology and life history than currently predicted based on extant members of this group. Resumen. EVIDENCIAS DE DIMORFISMO SEXUAL EN EL PALAEOTHENTIDAE DEL MIOCENO TEMPRANO ACDESTIS OWENII (MARSUPIALIA: PAUCITUBERCULATA). Aunque el dimorfismo sexual es un componente importante de la biología de muchos metaterios actuales del Nuevo Mundo, se sabe poco sobre el dimorfismo sexual en miembros extintos de este grupo. Aquí, describimos la variación morfométrica en dos especies tradicionalmente reconocidas del paucituberculado Acdestis de la Formación Santa Cruz (Mioceno de Patagonia), A. owenii y A. spe-gazzinii (=A. lemairei). Los resultados de este estudio morfométrico indican que los ejemplares de Acdestis de la Formación Santa Cruz consti-tuyen un único continuo morfológico, no habiendo rasgos que permitan distinguir claramente los dos morfotipos reconocidos aparte del tamaño general y la profundidad de la rama mandibular –rasgos que pueden variar con el sexo en los marsupiales vivientes. Esto sugiere que es pre-ferible considerar las dos especies de Acdestis de la Formación Santa Cruz como morfos de una sola especie dimórfica antes que dos especies monomórficas muy parecidas. El grado de dimorfismo sexual en Acdestis que se infiere aquí es comparable al visto en marsupiales vivientes sexualmente dimórficos. La identificación de dimorfismo sexual en Acdestis sugiere que los paucituberculados extintos podrían haber mostrado mucha más variabilidad en su ecología y ciclo biológico que lo que se predice actualmente basado en los miembros vivientes del grupo. Palabras clave. Argentina. Variación Individual. Dentición. Metatheria. Neógeno. Paleobiología. Plagiaulacoide. Santa Cruz.
Here we describe two new notoungulate taxa from early Oligocene deposits of the Abanico Formation... more Here we describe two new notoungulate taxa from early Oligocene deposits of the Abanico Formation in the eastern Tinguiririca valley of the Andes of central Chile, including a notosty-lopid (gen. et sp. nov.) and three basal toxodontians, cf. Homalodotheriidae, one of which is formally named a new species. The valley's eponymous fossil mammal fauna became the basis for recognizing a new South American Land Mammal " Age " intervening between the Mustersan and Deseadan of the classical SALMA sequence, the Tinguirirican. As a temporal intermediate between the bracketing SALMAs (Deseadan and Mustersan), the Tinguirirican is characterized by a unique cooccurrence of taxa otherwise known either from demonstrably younger or more ancient deposits, as well as some taxa with temporal ranges restricted to this SALMA. In this regard, two of the notoungulates described here make their last known stratigraphic appearances in the Tinguiririca Fauna, Chilestylops davidsoni (gen. et sp. nov.), the youngest notostylopid known, and Periphragnis vicentei (sp. nov.), an early diverging toxodontian, the youngest representative of the genus. A second species of Periphragnis from the Tinguiririca valley is provisionally described as Periphragnis, sp. nov., but is not formally named due to its currently poor representation. A specimen referred to Trigonolophodon sp. cf. T. elegans also is described. This taxon is noteworthy for also being reported from Santiago Roth's long perplexing fauna from Cañadón Blanco, now considered Tinguirirican in age. A phylogenetic analysis of notostylopids identifies Chilestylops as closely related to Boreastylops lumbrerensis from northern Argentina.
We describe the first Miocene turtle remains from Bolivia, which were collected from the late mid... more We describe the first Miocene turtle remains from Bolivia, which were collected from the late middle Miocene (13.18-13.03 Ma) of Quebrada Honda, southern Bolivia. This material includes a large scapula-acromion and fragmentary shell elements conferred to the genus Chelonoidis (Testudinidae), and a left xiphiplastron from a pleurodire or side-necked turtle, conferred to Acanthochelys (Chelidae). The occurrence of a giant tortoise and a freshwater turtle suggests that the paleoelevation of the region when the fossils were deposited was lower than has been estimated by stable isotope proxies, with a maximum elevation probably less than 1000 m. At a greater elevation, cool temperatures would have been beyond the tolerable physiological limits for these turtles and other giant ectotherm reptiles.
Ameghiniana, 2014
Despite being the dominant group of South American mammalian carnivores for much of the Neogene, ... more Despite being the dominant group of South American mammalian carnivores for much of the Neogene, most post-early Miocene sparassodonts (Metatheria) are poorly known. Here, we describe new specimens of the hathliacynid sparassodont Acyon myctoderos Forasiepi et al. from the middle Miocene locality of Quebrada Honda, Bolivia, including a juvenile specimen that preserves the first known hathliacynid DP3. This specimen is unique among sparassodonts in preserving both upper and lower deciduous premolars and suggests that hathliacynids may have differed from borhyaenoid sparassodonts in having simultaneous eruption of M3 and m4 rather than M4/m4. Another specimen from Quebrada Honda tentatively assigned to A. myctoderos preserves the first known long bones (femur, tibia) of this genus, allowing us to estimate its body mass (13–17.5 kg) based on postcranial data. These new specimens document further morphological and ontogenetic variation within A. myctoderos as well as the Sparassodonta as a whole.
Recent studies show Xenarthra to be even more isolated systematically from other placental mammal... more 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. Los estudios sistemáticos recientes muestran que, a nivel sistemático, los xenartros están aún más aislados de otros mamíferos placentarios de lo que se pensaba tradicionalmente. El grupo no sólo representa una de las cuatro ramas principales de los Placentalia, sino que también se han refutado las hipótesis previas de posibles conexiones con otros taxones de mamíferos fosoriales (por ejemplo Pholidota, Palaenodonta). No se conocen restos fósiles inequívocos de xenartros del Paleoceno y los restos provenientes del Eoceno consisten casi exclusivamente de osteodermos aislados de cingulados y restos postcraneanos aislados de origen sistemático incierto. No se conocen cráneos razonablemente completos de cingulados hasta finales del Eoceno medio; los cráneos más antiguos de perezosos y osos hormigueros provienen del Oligoceno temprano y del Mioceno temprano, respectivamente; y no existen esqueletos completos o casi completos de ninguno de los 3 linajes hasta el Mioceno temprano. Reconstruimos la ecología de los primeros xenartros basándonos en las especies actuales y lo que se sabe de la paleobiología del Mioceno y de los taxones extintos más recientes. Nuestros resultados sugieren que los primeros xenartros eran mirmecófagos y poseían adaptaciones para cavar y tal vez para trepar. Los primeros cingulados eran cavadores terrestres y probablemente mirmecófagos, pero pronto divergieron en numerosos linajes omnívoros. Nuestras reconstrucciones indican que los primeros perezosos eran herbívoros con preferencia de hábitats boscosos, tal vez exhibiendo adaptaciones tanto para cavar como para trepar. Atribuimos la rareza de restos de los primeros xenartros a varios factores: bajas densidades poblacionales asociadas a hábitos mirmecófagos; falta de dientes duraderos y cubiertos de esmalte; y una escasez general de localidades de mamíferos tempranos de las latitudes tropicales de América del Sur. La derivación de numerosos linajes omnívoros y herbívoros de un ancestro mirmecófago es un rasgo curioso y único de la historia de los xenartros y puede deberse a la peculiar ecología de la fauna de mamíferos sudamericanos. Los nuevos avances en la comprensión de la evolución temprana de los xenartros podrían depender de la localización de nuevos sitios fósiles paleógenos en áreas de tierras bajas poco accesibles del norte de América del Sur.
The Sparassodonta (Mammalia, Metatheria) are a group of carnivorous mammals that dominated the ma... more The Sparassodonta (Mammalia, Metatheria) are a group of carnivorous mammals that dominated the macropredatory guild of South America during the Cenozoic. Here, we describe a new sparassodont based on a single specimen from the middle Miocene Quebrada Honda local fauna of southern Bolivia. This specimen (UF 27881) does not clearly correspond to any major sparassodont group (e.g., Hathliacynidae, Borhyaenidae, etc.) and represents a morphotype previously unknown among the Sparassodonta. UF 27881 is distinguished from other sparassodonts by its short, broad, borhyaenid-like rostrum and small size, among other features. However, we decline to coin a new name for UF 27881 due to the fragmentary nature of this specimen and the absence of most of its dentition. This specimen suggests that the appearance of the Sparassocynidae and several hypercarnivorous didelphid taxa (including Thylophorops, Thylatheridium, Lutreolina, and Hyperdidelphys) represent an evolutionary response to the decline in small, predatory sparassodont taxa during the late Cenozoic. This study documents new morphological diversity among the Sparassodonta and highlights the value of fossils from traditionally undersampled parts of South America.SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at http://www.tandfonline.com/UJVP.
The late early Miocene Santa Cruz Formation of southern Argentina is one of the most productive f... more The late early Miocene Santa Cruz Formation of southern Argentina is one of the most productive fossil mammal-bearing formations in South America. Hundreds of species have been named from this formation, resulting in a clear overestimation of Santa Cruz mammal diversity. Recent paleoecological studies have attempted to minimize this problem by analyzing more restricted faunal lists based only on recently collected fossil samples. In some cases, this approach has excluded clearly recognizable Santa Cruz taxa. In this study, I use a family-level rarefaction analysis of the Yale Peabody Collection of Santa Cruz fossils to test whether such absences are likely due to smaller sample size or to other factors. I further attempt to reconcile these absences using distributional data from recent and historical Santa Cruz Formation collections. The rarefaction analysis indicates that most single-locality samples from the Santa Cruz Formation have lower familial diversity than expected based on the taxonomic distribution of specimens in the Princeton University Collection at Yale Peabody Museum (YPM-PU) as a whole. However, I conclude that single-locality samples are not necessarily more appropriate than multi-locality samples for paleoecological analyses of ancient Santa Cruz Formation mammal communities given the large regional extent of the Santa Cruz Formation and the large geographic area encompassed by most modern communities used for comparison. This study highlights the need for more precise stratigraphic correlations among Santa Cruz localities and integration of important historical collections into a modern stratigraphic framework.
Journal of Anatomy, 2013
The phylogenetic relationships of notoungulates, an extinct group of predominantly South American... more The phylogenetic relationships of notoungulates, an extinct group of predominantly South American herbivores, remain poorly resolved with respect to both other placental mammals and among one another. Most previous phylogenetic analyses of notoungulates have not included characters of the internal cranium, not least because few such features, including the bony labyrinth, have been described for members of the group. Here we describe the inner ears of the notoungulates Altitypotherium chucalensis (Mesotheriidae), Pachyrukhos moyani (Hegetotheriidae) and Cochilius sp. (Interatheriidae) based on reconstructions of bony labyrinths obtained from computed tomography imagery. Comparisons of the bony labyrinths of these taxa with the basally diverging notoungulate Notostylops murinus (Notostylopidae), an isolated petrosal from Itabora ı, Brazil, referred to Notoungulata, and six therian outgroups, yielded an inner ear character matrix of 25 potentially phylogenetically informative characters, 14 of them novel to this study. Two equivocally optimized character states potentially support a pairing of Mesotheriidae and Hegetotheriidae, whereas four others may be diagnostic of Notoungulata. Three additional characters are potentially informative for diagnosing more inclusive clades: one for crown Placentalia; another for a clade containing Kulbeckia, Zalambdalestes, and Placentalia; and a third for Eutheria (crown Placentalia plus stem taxa). Several other characters are apomorphic for at least one notoungulate in our study and are of potential interest for broader taxonomic sampling within Notoungulata to clarify currently enigmatic interrelationships. Measures of the semicircular canals were used to infer agility (e.g. capable of quick movements vs. lethargic movements) of these taxa. Agility scores calculated from these data generally corroborate interpretations based on postcranial remains of these or closely related species. We provide estimates of the lowfrequency hearing limits in notoungulates based on the ratio of radii of the apical and basal turns of the cochlea. These limits range from 15 Hz in Notostylops to 149 Hz in Pachyrukhos, values comparable to the Asian elephant (Elephas maximus) and the California sea lion (Zalophus californianus) when hearing in air, respectively.
Proceedings of the Royal Society B: Biological Sciences, 2012
The long-term isolation of South America during most of the Cenozoic produced a highly peculiar t... more The long-term isolation of South America during most of the Cenozoic produced a highly peculiar terrestrial vertebrate biota, with a wide array of mammal groups, among which caviomorph rodents and platyrrhine primates are Mid-Cenozoic immigrants. In the absence of indisputable pre-Oligocene South American rodents or primates, the mode, timing and biogeography of these extraordinary dispersals remained debated. Here, we describe South America's oldest known rodents, based on a new diverse caviomorph assemblage from the late Middle Eocene (approx. 41 Ma) of Peru, including five small rodents with three stem caviomorphs. Instead of being tied to the Eocene/Oligocene global cooling and drying episode (approx. 34 Ma), as previously considered, the arrival of caviomorphs and their initial radiation in South America probably occurred under much warmer and wetter conditions, around the Mid-Eocene Climatic Optimum. Our phylogenetic results reaffirm the African origin of South American rodents and support a trans-Atlantic dispersal of these mammals during Middle Eocene times. This discovery further extends the gap (approx. 15 Myr) between first appearances of rodents and primates in South America.
American Museum Novitates, 2012
Herein we describe and name two new species of leontiniid notoungulates, one being the rst known ... more Herein we describe and name two new species of leontiniid notoungulates, one being the rst known from Chile, the other from the Deseadan South American Land Mammal Age (SALMA) of Patagonia, Argentina. e Chilean leontiniid is from the lower horizons of the Cura-Mallín Formation (Tcm 1 ) at Laguna del Laja in the Andean Main Range of central Chile.
Journal of Mammalian Evolution, 2011
The rodents of the middle Miocene fauna of Quebrada Honda Bolivia are described. The most abundan... more The rodents of the middle Miocene fauna of Quebrada Honda Bolivia are described. The most abundant rodent is the chinchillid Prolagostomus sp. More precise identification of this species will require revision of early to middle Miocene lagostomines, taking into account variation in modern populations. The next most common rodents are the tiny octodontoid Acarechimys, sp. nov.?, and the caviid Guiomys unica. The Acarechimys species may be unique to Quebrada Honda, but verification awaits revision of this geographically and temporally widespread genus. Guiomys unica is a recently described species otherwise known only from two Patagonian localities, El Petiso and Río Chico. Two rodents are unique to Quebrada Honda. Mesoprocta hypsodus, gen. et sp. nov., is a dasyproctid distinguished by its very hypsodont, cement-covered cheek teeth. Quebradahondomys potosiensis, gen. et sp. nov., is an adelphomyine echimyid distinguished by the less oblique lophids of its trilophodont cheek teeth, among other features. The rodents of Quebrada Honda are more similar to those of Patagonia than those of northern South America, paralleling patterns seen in other mammal groups from this fauna.
Journal of Mammalogy, 2011
Rodents are important components of most modern ecosystems. Understanding their roles in paleocom... more Rodents are important components of most modern ecosystems. Understanding their roles in paleocommunities requires robust methods for inferring diet and other autecological characteristics. This pilot study tests whether a relationship between incisor morphology and diet exists among extant rodents that might be used to infer diets of extinct species. We focused on 11 genera of caviomorph rodents classified in 3 dietary categories: fruit-leaf, fruit-seed, and grass-leaf. For each genus 6 variables describing morphology of the upper incisor were measured on 5 specimens. Data were analyzed using a series of stepwise discriminant analyses. Discriminant analyses correctly predicted diets of nearly all training cases (,95%) using 4 incisor characteristics. Five additional species (1 caviomorph and 4 noncaviomorph), treated as unknowns, also were classified correctly. Jackknife analyses correctly predicted diets of approximately two-thirds of training cases. Our study indicates that incisor morphology is related to diet in extant caviomorph rodents. Incisor data therefore might be useful for inferring diets of extinct species.
Journal of Anatomy, 2010
We provide the first detailed description of the inner ear of a notoungulate, an extinct group of... more We provide the first detailed description of the inner ear of a notoungulate, an extinct group of endemic South American placental mammals, based on a three-dimensional reconstruction extracted from CT imagery of a skull of Notostylops murinus. This description provides new anatomical data that should prove to be phylogenetically informative, an especially significant aspect of this research given that both the interrelationships of notoungulates and the position of Notoungulata within Placentalia are still unresolved. We also assess the locomotor agility of Notostylops based on measurements of the semicircular canals. This is the best available data on the locomotion of a notostylopid because significant postcranial remains for this group have not been described. The cochlea of Notostylops has 2.25 turns, and the stapedial ratio is 1.6. The stapedial ratio is one of the lowest recorded for a eutherian, which typically have ratios greater than 1.8. The fenestra cochleae is located posterior to the fenestra vestibuli, a condition previously only reported for some stem primates. The separation of the saccule and utricule of the vestibule is visible on the digital endocast of the bony labyrinth. The posterior arm of the LSC and the inferior arm of the PSC are confluent, but these do not form a secondary crus commune, and the phylogenetic or functional significance of this confluence is unclear at this time. Locomotor agility scores for Notostylops suggest a medium or 'average' degree of agility of motion compared to extant mammals. In terms of its locomotion, we tentatively predict that Notostylops was a generalized terrestrial mammal, with cursorial tendencies, based on its agility scores and the range of locomotor patterns inferred from postcranial analyses of other notoungulates.
Palaeontologia Electronica
We describe a large collection of mesotheriid notoungulate specimens, including a well-preserved ... more We describe a large collection of mesotheriid notoungulate specimens, including a well-preserved cranium, from the early to middle Miocene locality of Cerdas, Bolivia. We attempt to classify the specimens into dental wear groups (i.e., ontogenetic groups) based on tooth size, morphology, and position of the posterior mental foramen, and we compare these specimens to those from other middle-latitude localities in Chile (Chu-cal) and Bolivia (Achiri, Choquecota, Quebrada Honda, and Nazareno). Our metric analyses, using bivariate plots and coefficients of variation (CVs), indicate that nearly all Cerdas specimens pertain to a single species. Our phylogenetic analysis indicates that this species, 'Plesiotypotherium' minus Villarroel 1978, should be referred to a dif-ferent genus, more closely related to Altitypotherium Croft et al. 2004 (Chucal) and/or Microtypotherium Villarroel 1974b (Choquecota). We do not refer 'P.' minus to a new genus at this time, pending revision...
Journal of Mammalian Evolution, 2009
We provide the first faunal report for the early/middle Miocene fauna of Cerdas, Bolivia (16.
This study characterizes the primary locomotor habit of Protypotherium, an inter- atheriid notoun... more This study characterizes the primary locomotor habit of Protypotherium, an inter- atheriid notoungulate from the early Miocene of Santa Cruz, Argentina. Locomotor habit was assessed both qualitatively (i.e., by noting osteological features correlated with specific locomotor habits in extant mammals) and quantitatively (i.e., by using mul- tivariate analyses to compare limb element lengths, widths, and ratios to those of extant
Palaeogeography, Palaeoclimatology, Palaeoecology, 2008
Notoungulates were the most diverse and abundant clade of endemic South American ungulates, prese... more Notoungulates were the most diverse and abundant clade of endemic South American ungulates, present in nearly all Tertiary faunas. Hypsodont dentitions evolved in several notoungulate lineages by the early Oligocene, and by the middle Miocene, four families were characterized by hypselodont (ever-growing) cheek teeth. The presence of hypsodont and hypselodont dentations in notoungulates has generally been interpreted as indicating grazing and/or open habitat feeding, but few studies have attempted to test this assumption using other methods of dietary inference. The present study uses mesowear analysis to explore the correlation between crown height and diet in the three most common notoungulates of the late Oligocene (Deseadan SALMA) fauna of Salla, Bolivia: a mesotheriid (Trachytherus alloxus), an 'archaeohyracid' (Archaeohyrax sp. nov.), and an interatheriid (Federicoanaya sallaensis). Mesowear analysis followed standard protocols. Specimens were digitally photographed in labial view so that cusp shape and occlusal relief could be scored. Cusp shape was scored subjectively as sharp, round, or blunt. Occlusal relief (measured digitally) was calculated as cusp height divided by intercusp distance and was categorized as 'high' or 'low' using three scenarios. The percentage of individuals of each species displaying sharp, blunt, and high cusps was calculated, normalized using the arcsine transformation, and used as variables in subsequent analyses to infer dietary preference. Statistical analyses included discriminant function (DFA) and hierarchical cluster (HCA). Mesowear scores were also explored. Analyses were executed using a comparative mesowear dataset of 64 modern ungulates of known diet (classified as browsers, grazers, or mixed feeders) as well as a subset of 27 'typical' ungulates. Trachytherus was classified as a grazer in most (5/6) DFAs; it primarily grouped with grazing ungulates (e.g., Alcelaphus, Connochaetes, Damaliscus, Equus) in both HCAs. Archaeohyrax was classified as a grazer in the DFA of 'typical' ungulates and as a mixed feeder or grazer in the expanded dataset DFA; it mostly clustered with grazers in the 'typical' HCA and with grazers (Alcelaphus buselaphus, Connochaetes, Hippotragus equinus) and mixed feeders (Axis spp., Rucervus) in equal proportions in the expanded HCA. Federicoanaya was classified as a mixed feeder in all DFAs; it grouped with grazers and mixed feeders in the 'typical' DFA and with all dietary types in the expanded HCA including two grazers (Hippotragus equinus and Ourebia), a mixed feeder (Tetracerus), and a browser, (Litocranius). Mesowear scores for the three species generally overlapped those for extant grazers, extending into the upper range of mixed feeders. These analyses suggest: (1) all three notoungulates included some grass in their diet and/or fed at least part of the time in open habitats; (2) these notoungulates were partitioning available dietary resources by foraging in different areas and/or varying the composition of their diets; and (3) Trachytherus had the most abrasive diet of the three notoungulates, and Federicoanaya had the least abrasive. Based on these analyses and the diversity of other mammals present at Salla, the fauna probably represents a semi-open woodland community.
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Journal Articles by Darin A Croft