Ground Sloths Spread Into North America Multiple Times

The manus of Mylodon darwinii Owen (Tardigrada, Mylodontidae) and its phylogenetic implications

Authors:

Haro et al

Abstract:

The first nearly complete and articulated manus of the ground sloth Mylodon darwinii, from the upper Pleistocene of Argentina, is described. It shares similarities with Mylodonopsis ibseni from Brazil, including a cuneiform with a distinct pisiform facet, an obliquely concave ulnar facet, and a prominent distolateral process, as well as a gracile metacarpal III. It shares a flattened pisiform with Glossotherium robustum. The trapezoid is unique in the obliquely elongate proportions of its dorsal surface. Shapes of the articular facets indicate different functions in digits II and III, with the former having a greater range of motion and the latter greater stability at the joints. Clear arboreal or fossorial adaptations are absent. A phylogenetic analysis recovered M. darwinii as closely related to M. ibseni and agrees with larger phylogenetic analyses of sloths based on craniomandibular evidence. Our data support more than two mylodontine dispersal events to North America.

A Remarkable Neogene Assemblage of Sloths in Colombia

Neogene sloth assemblages (Mammalia, Pilosa) of the Cocinetas Basin (La Guajira, Colombia): implications for the Great American Biotic Interchange

Authors:

Amson et al

Abstract:

We describe sloth assemblages from the Cocinetas Basin (La Guajira peninsula, Colombia), found in the Neogene Castilletes and Ware formations, located in northernmost South America, documenting otherwise poorly known biotas. The tentative referral of a specimen to a small megatherioid sloth, Hyperleptus?, from the early–middle Miocene Castilletes Formation, suggests affinities of this fauna with the distant Santa Cruz Formation and documents a large latitudinal distribution for this taxon. The late Pliocene Ware Formation is much more diverse, with five distinct taxa representing every family of ‘ground sloths’. This diversity is also remarkable at the ecological level, with sloths spanning over two orders of magnitude of body mass and probably having different feeding strategies. Being only a few hundred kilometres away from the Isthmus of Panama, and a few hundred thousand years older than the classically recognized first main pulse of the Great American Biotic interchange (GABI 1), the Ware Formation furthermore documents an important fauna for the understanding of this major event in Neogene palaeobiogeography. The sloths for which unambiguous affinities were recovered are not closely related to the early immigrants found in North America before GABI 1.

Quaternary glaciation and the Great American Biotic Interchange

Quaternary glaciation and the Great American Biotic Interchange

Authors:

Bacon et al

Abstract:

Recent geological studies demonstrate that the Isthmus of Panama emerged some 10 m.y. earlier than previously assumed. Although absent today in Panama, Central American savanna environments likely developed in connection with the onset of Northern Hemisphere glaciations. As is widely recognized, most of the mammals crossing the isthmus since 2.5 Ma lived in savannas. Could climate-induced vegetational changes across Panama explain the delayed migration of mammals, rather than terrestrial connectivity? We investigate the congruence between cross-continental mammal migration and climate change through analysis of fossil data and molecular phylogenies. Evidence from fossil findings shows that the vast majority of mammals crossed between South and North America after ca. 3 Ma. By contrast, dated mammal phylogenies suggest that migration events started somewhat earlier, ca. 4–3 Ma, but allowing for biases toward greater ages of molecular than geologic dating and uncertainties in the former, we consider this age range not to be significantly earlier than 3 Ma. We conclude that savanna-like environments developed in response to the vast Laurentide ice sheet at the first Quaternary glaciation triggered the initiation of the Great American Biotic Interchange in mammals.

Academic Bun Fight: Panama Isthmus Closed Earlier Than Currently Thought (or did it)

Biological evidence supports an early and complex emergence of the Isthmus of Panama
Authors:

Bacon et al

Abstract:

The linking of North and South America by the Isthmus of Panama had major impacts on global climate, oceanic and atmospheric currents, and biodiversity, yet the timing of this critical event remains contentious. The Isthmus is traditionally understood to have fully closed by ca. 3.5 million years ago (Ma), and this date has been used as a benchmark for oceanographic, climatic, and evolutionary research, but recent evidence suggests a more complex geological formation. Here, we analyze both molecular and fossil data to evaluate the tempo of biotic exchange across the Americas in light of geological evidence. We demonstrate significant waves of dispersal of terrestrial organisms at approximately ca. 20 and 6 Ma and corresponding events separating marine organisms in the Atlantic and Pacific oceans at ca. 23 and 7 Ma. The direction of dispersal and their rates were symmetrical until the last ca. 6 Ma, when northern migration of South American lineages increased significantly. Variability among taxa in their timing of dispersal or vicariance across the Isthmus is not explained by the ecological factors tested in these analyses, including biome type, dispersal ability, and elevation preference. Migration was therefore not generally regulated by intrinsic traits but more likely reflects the presence of emergent terrain several millions of years earlier than commonly assumed. These results indicate that the dramatic biotic turnover associated with the Great American Biotic Interchange was a long and complex process that began as early as the Oligocene–Miocene transition.

That was a bad analysis!!!

Appearance of an early closure of the Isthmus of Panama is the product of biased inclusion of data in the metaanalysis

Author:

Lessios

Extract:

In their PNAS article “Biological evidence supports an early and complex emergence of the Isthmus of Panama,” Bacon et al. (1) use data from molecular comparisons of terrestrial and marine organisms taken from the literature to estimate dates of rate shifts in migration. One of their conclusions is that “events separating marine organisms in the Atlantic and Pacific oceans [occurred] at ca. 23 and 7 Ma” (1). The authors base this conclusion on two kinds of molecular dating: (i) 31 dates from phylogenies with evolutionary rates calibrated from fossils at one or more nodes, and (ii) 52 dates from mitochondrial divergence between sister species on either side of the Isthmus taken from the review by Lessios (2) (note: complete data are available from the Dryad Digital Repository). For the latter, divergence was converted to time by assuming a mitochondrial DNA divergence rate of 2% per million years. Unfortunately, Bacon et al.’s (1) metaanalysis of separations of marine organisms contains unexplained omissions of data and mistakes. Nine of the fossil calibrated divergence values are wrong, and three are omitted (though present in publications used to derive other dates). Thirty-eight comparisons from Lessios (2) are excluded. Criteria for inclusion of data are not stated but, judging from the estimated dates, only data from Cytochrome c oxidase subunit 1 were taken into account, even though Lessios (2) presents data for multiple mitochondrial genes. This selectivity in the marine dataset of Bacon et al. (1) is hard to explain, because the terrestrial data come from various genes, and because eight comparisons of Cytochrome c oxidase subunit 1 of marine species are among those excluded.

no!  it wasn't, damnit!

Reply to Lessios and Marko et al.: Early and progressive migration across the Isthmus of Panama is robust to missing data and biases

Authors:

Bacon et al

Extract:

The emergence of the Isthmus of Panama left a major imprint on the biodiversity of the Americas. The connection between South and North America facilitated dispersal of terrestrial and freshwater organisms, while separating marine species between the eastern Pacific and Caribbean seas. Recent geological data have questioned the long-standing view of a Pliocene emergence of the Isthmus (1) and show that the Central American Seaway, defined as the deep oceanic seaway along the tectonic boundary of the South American plate and Panama arc, was already closed by 15–13 Ma (2). Caribbean–Pacific shallow water exchange probably continued, albeit intermittently, until a full closure at 3.5 Ma (1–3). Recently Bacon et al. (3) used molecular and fossil data to evaluate the timing, tempo, and directionality of biotic exchange and vicariance across the Isthmus, and tested whether biological data are congruent with recent geological evidence. Significant increases in terrestrial dispersals were found at ca. 20 and 6 Ma, and increases in marine vicariance at ca. 23 and 7 Ma. Similar patterns prevailed despite intrinsic differences among the taxonomic groups surveyed. This led Bacon et al. (3) to reject the assumption of a single closure of the Isthmus at ca. 3.5 Ma in favor of an older, more complex model of land emergence and biotic interchange.

Procyonids Dispersed Twice From North America to South America Prior to the Great American Biotic Interchange

Carnivorans at the Great American Biotic Interchange: new discoveries from the northern neotropics

Authors:

Forasiepi et al

Abstract:

We report two fossil procyonids, Cyonasua sp. and Chapalmalania sp., from the late Pliocene of Venezuela (Vergel Member, San Gregorio Formation) and Colombia (Ware Formation), respectively. The occurrence of these pre-Holocene procyonids outside Argentina and in the north of South America provides further information about the Great American Biotic Interchange (GABI). The new specimens are recognized in the same monophyletic group as procyonids found in the southern part of the continent, the “Cyonasua group,” formed by species of Cyonasua and Chapalmalania. The phylogenetic analysis that includes the two new findings support the view that procyonids dispersed from North America in two separate events (initially, previous to the first major migration wave—GABI 1—and then within the last major migration wave—GABI 4—). This involved reciprocal lineage migrations from North to South America, and included the evolution of South American endemic forms.

A New Miocene Neogene Dromomerycine Artiodactyl Suggests Earlier End to South America's 'Splendid Isolation'

New late Miocene dromomerycine artiodactyl from the Amazon Basin: implications for interchange dynamics

Authors:

Prothero et al

Abstract:

A new dromomerycine palaeomerycid artiodactyl, Surameryx acrensis new genus new species, from upper Miocene deposits of the Amazon Basin documents the first and only known occurrence of this Northern Hemisphere group in South America. Osteological characters place the new taxon among the earliest known dromomerycine artiodactyls, most similar to Barbouromeryx trigonocorneus, which lived in North America during the early to middle Miocene, 20–16 Ma. Although it has long been assumed that the Great American Biotic Interchange (GABI) began with the closure of the Isthmus of Panama in the late Pliocene, or ca. 3.0–2.5 Ma, the presence of this North American immigrant in Amazonia is further evidence that terrestrial connections between North America and South America through Panama existed as early as the early late Miocene, or ca. 9.5 Ma. This early interchange date was previously indicated by approximately coeval specimens of proboscideans, peccaries, and tapirs in South America and ground sloths in North America. Although palaeomerycids apparently never flourished in South America, proboscideans thrived there until the end of the Pleistocene, and peccaries and tapirs diversified and still live there today.

Pliodasypus vergelianus: A Dasypodini Armadillo From Pliocene Neogene Venezuela has Implications for the Great American Biotic Interchange

A new Dasypodini armadillo (Xenarthra: Cingulata) from San Gregorio Formation, Pliocene of Venezuela: affinities and biogeographic interpretations

Authors:

Castro et al

Abstract:

We describe Pliodasypus vergelianus gen. et sp. nov., a Dasypodini armadillo from the middle Pliocene of Venezuela (Vergel Member, San Gregorio Formation). Although scarce, the remains are remarkable because of their geochronologic proximity to the main phase of Great American Biotic Interchange (GABI). The cladistic analysis conducted reveals that Pliodasypus groups with Dasypus and both are sister taxa of Propraopus, whereas Anadasypus is at a basal position. With respect to the records of tribe Dasypodini, after its oldest representative (Anadasypus, middle and late Miocene), the chronologically subsequent form is Pl. vergelianus (middle Pliocene), followed by Dasypus bellus in higher northern latitudes (late Pliocene), and then by widespread occurrences in the Pleistocene of North America (D. bellus) and South America (Propraopus, Dasypus punctatus, and Dasypus novemcinctus). Thus, we infer that Dasypus differentiated in the late Pliocene at low latitudes in the northern South America. It leads to two alternative hypotheses of dispersal: (a) some early Dasypus remained cryptically in South America until the Pleistocene, whereas others dispersed to North America between 2.2 and 2.7 Ma, or (b) they dispersed to North America subsequently to the emersion of the Panamanian isthmus and D. bellus differentiated there; later, during the Pleistocene, D. bellus entered South America and experienced speciation. The same process of re-ingression has been proposed to other xenarthrans, breaking with the traditional assumption that the GABI was unidirectional.