Today, Madagascar is home to a mosaic of different habitats--a lush rainforest in the east and a dry deciduous forest in the west, separated by largely open highlands. But the island off the southeast coast of Africa hasn't always been like that--a new study published in the Proceedings of the National Academy of Sciences announces that these two ecologically different portions of the island were once linked by a patchwork of forested areas. And to figure it out, the scientists analyzed the DNA of some of the cutest animals on earth--mouse lemurs.
"For a long time, scientists weren't sure how or why Madagascar's biogeography changed in very recent geological time, specifically at the key period around when humans arrived on the island a few thousand years ago. It has been proposed they heavily impacted the Central Highland forests," says Steve Goodman, MacArthur Field Biologist at The Field Museum in Chicago, who co-authored the study and has been studying Malagasy animals for thirty years. "This study shows the landscape was changing thousands of years before humans arrived."
So scientists wanted to learn about the history of Madagascar's landscape--why study mouse lemurs? The tiny primates are the perfect combination of fast-breeding, hardy, and unique to the island. "They reach reproductive maturity within a year, and that means that a lot of generations are produced very quickly," explains Goodman. "That enables us to see evolution at work faster than we would in an animal that took, say, five years to first reproduce." The lemurs, which are found only on Madagascar, live across much of the island, even forested areas that have been damaged by humans. That means that for scientists studying how the island changed over time, mouse lemurs are a jackpot. "The mouse lemurs are forest dependent--as the forest changes, they change. By studying how mouse lemurs evolved in different areas of the island, we're able to glimpse how the island itself changed and learn whether those changes were caused by humans," says Goodman.
By analyzing DNA from five different mouse lemur species, the scientists were able to tell when the different kinds of lemurs branched out from each other. "We were able to characterize tens of thousands of changes in the genomes of mouse lemurs that are now isolated and form separate species. By analyzing these DNA changes, we were able to understand when the species diverged from each other, and by inference, identify the ecological forces that might have driven them apart," says Anne Yoder, Director of the Duke University Lemur Center and lead author on the paper.
Ancient crops provide first archaeological signature of the westward Austronesian expansion
Authors:
Crowther et al
Abstract:
The Austronesian settlement of the remote island of Madagascar remains one of the great puzzles of Indo-Pacific prehistory. Although linguistic, ethnographic, and genetic evidence points clearly to a colonization of Madagascar by Austronesian language-speaking people from Island Southeast Asia, decades of archaeological research have failed to locate evidence for a Southeast Asian signature in the island’s early material record. Here, we present new archaeobotanical data that show that Southeast Asian settlers brought Asian crops with them when they settled in Africa. These crops provide the first, to our knowledge, reliable archaeological window into the Southeast Asian colonization of Madagascar. They additionally suggest that initial Southeast Asian settlement in Africa was not limited to Madagascar, but also extended to the Comoros. Archaeobotanical data may support a model of indirect Austronesian colonization of Madagascar from the Comoros and/or elsewhere in eastern Africa.
Archaeologists studying the distribution of ancient rice believe they may be close to solving one of the enduring mysteries of the ancient world - how people of South East Asian origin ended up living on the African island of Madagascar, 6,000 km away.
Teams from the universities of Bristol, Oxford and Queensland excavated 18 sites in Madagascar, Comoros, and along the East African coast and recovered over 2,500 seed remains. The researchers were able to show that typically South East Asian crops, such as rice and mung bean, are found on Madagascar and Comoros, while contemporary sites in Africa largely relied on indigenous crops.
A-type stratoid granites of Madagascar revisited: age, source and links with the breakup of Rodinia
Authors:
Nédélec et al
Abstract:
The stratoid granites of Madagascar are sheet-like granite sills emplaced conformably to the schistosity of the Antananarivo and Ikalamavony domains of the Precambrian basement. Most of them are fine-grained rocks with A-type affinities. They belong to two different plutonic suites: the Kiangara Suite north of Antananarivo and the Imorona-Itsindro suite recognized in the whole central Madagascar. Stratoid granites are everywhere characterized by the same structural pattern evidencing two deformation events. The first event (D1 characterized by foliations mildly dipping to the west and lineations trending WSW) was coeval with synkinematic magma emplacement. The second one, responsible for local reworking in ductile conditions, is regarded as the result of Late Pan-African transcurrent tectonics along N-S striking shear zones. New in situ U-Pb zircon dating from both suites yields similar results evidencing two group of ages: upper intercept ages of 790-780 Ma regarded as crystallization ages, and younger ages of ca. 555 Ma corresponding to a metamorphic overprint. These new data suggest that both plutonic suites in central Madagascar were emplaced at 790-780 Ma. New Sm-Nd isotopic data from the Kiangara Suite support the existence of two granite groups. The first group has εNd(t) at -15 and is likely derived from partial melting of the Late Archaean crust. The second group made of granites with a more pronounced A-type nature and comagmatic syenites has εNd(t) at -12 evidencing the contribution of mantle melts. Mafic enclaves with εNd(t) ranging from -5 to -11 encompass the Nd isotopic signature of coeval Itsindro gabbros, which are regarded as derived from a subcontinental lithospheric mantle source. The interpretation of both structural and isotopic data is consistent with crustal thinning and extension during a Cryogenian Rodinia rifting stage at 790-780 Ma, that left a major imprint in central Madagascar. Separation from a conjugage margin represented by the North China craton is suggested. The Pan-African influence is limited to reheating, tectonic reworking and fluid transfer in the vicinity of Late-Neoproterozoic (∼550 Ma) shear zones.
Precocity in a tiny titanosaur from the Cretaceous of Madagascar
Authors:
Rogers et al
Abstract:
Sauropod dinosaurs exhibit the largest ontogenetic size range among terrestrial vertebrates, but a dearth of very young individuals has hindered understanding of the beginning of their growth trajectory. A new specimen of Rapetosaurus krausei sheds light on early life in the smallest stage of one of the largest dinosaurs. Bones record rapid growth rates and hatching lines, indicating that this individual weighed ~3.4 kilograms at hatching. Just several weeks later, when it likely succumbed to starvation in a drought-stressed ecosystem, it had reached a mass of ~40 kilograms and was ~35 centimeters tall at the hip. Unexpectedly, Rapetosaurus limb bones grew isometrically throughout their development. Cortical remodeling, limb isometry, and thin calcified hypertrophic metaphyseal cartilages indicate an active, precocial growth strategy.
Implications of lemuriform extinctions for the Malagasy flora
Authors:
Federman et al
Abstract:
Madagascar’s lemurs display a diverse array of feeding strategies with complex relationships to seed dispersal mechanisms in Malagasy plants. Although these relationships have been explored previously on a case-by-case basis, we present here the first comprehensive analysis of lemuriform feeding, to our knowledge, and its hypothesized effects on seed dispersal and the long-term survival of Malagasy plant lineages. We used a molecular phylogenetic framework to examine the mode and tempo of diet evolution, and to quantify the associated morphological space occupied by Madagascar’s lemurs, both extinct and extant. Using statistical models and morphometric analyses, we demonstrate that the extinction of large-bodied lemurs resulted in a significant reduction in functional morphological space associated with seed dispersal ability. These reductions carry potentially far-reaching consequences for Malagasy ecosystems, and we highlight large-seeded Malagasy plants that appear to be without extant animal dispersers. We also identify living lemurs that are endangered yet occupy unique and essential dispersal niches defined by our morphometric analyses.
Madagascar's climate at the K/P boundary and its impact on the island's biotic suite
Authors:
Ohba et al
Abstract:
The K/P boundary, 66 Ma, was a critical time in the history of the planet's biota. On the island of Madagascar, few Late Cretaceous species survived the associated extinction, and distance from the various mainland sources then acted as a strong filter for the number of over-water arrivees. Reconstructing the climate of the early Paleogene is vital for understanding the environments to which new colonizers landed and established a toe-hold on the island. Beginning with a “global” air-sea-coupled climate model simulation using the land–sea distribution at 66 Ma, we used dynamical downscaling to construct a scenario for how the atmospheric and oceanic fluid envelopes washed over and around the Madagascar at the start of the Paleocene. Dynamical downscaling of the global model yields a climate model with much finer resolution. We used this method to reconstruct the habitats for known fossil localities from that general period, better understand the ecological diversity of plants and animals that have been hypothesized to have then been present, and evaluate hypotheses regarding the evolutionary history of lemurs—a vast clade of primates endemic to Madagascar whose last common ancestor dates to this time. Our results show an island with a climate notably different from that of today, but not nearly as arid as others have suggested: the spiny thicket biome did not exist; temperatures were lower; rainfall less seasonal.
Postcranial Osteology of Azendohsaurus madagaskarensis (?Middle to Upper Triassic, Isalo Group, Madagascar) and its Systematic Position Among Stem Archosaur Reptiles
Authors:
Nesbitt et al
Abstract:
During the Triassic, archosauromorphs became one of the first groups of diapsid reptiles to diversify in terms of body size and morphological disparity in both terrestrial and marine ecosystems across Pangaea. This seemingly rapid divergence, and the numerous unique body plans stemming from it, concomitantly has confounded reconstructions of archosauromorph relationships. Teasing apart homology from homoplasy of anatomical characters in this broad suite of body types remains an enormous challenge with the current sample of taxa. Here, we present the postcranial anatomy of Azendohsaurus madagaskarensis, an early archosauromorph from ?Middle to Upper Triassic strata of Madagascar. Azendohsaurus madagaskarensis is known from nearly the entire skeleton in an ontogenetically variable sample. The holotype locality consists of a monotypic bone bed; preservation ranges from complete but disarticulated bones to articulated sections of the skeleton. Azendohsaurus madagaskarensis embodies an aberrant constellation of archosauromorph features, including an elongated neck, a short, stocky tail, robust limbs, and unexpectedly short digits terminating in large recurved unguals on the manus and pes. Together with the cranium, the postcrania reveal A. madagaskarensis to be another representative of a growing coterie of highly apomorphic and bizarre Triassic archosauromorphs. At the same time, recovery and description of the full anatomy of A. madagaskarensis helps to identify a monophyletic grouping of specialized taxa that includes the North American Late Triassic–aged archosauromorphs Trilophosaurus, Spinosuchus, and Teraterpeton, Indian Pamelaria, and Moroccan Azendohsaurus laaroussii. Moreover, information derived from the skeleton of A. madagaskarensis solidifies the systematic position of these taxa among other archosauromorphs. Using the most comprehensively sampled phylogenetic analysis of early archosauromorphs, we found the clade encompassing the aforementioned taxa as the nearest outgroup of Prolacerta broomi + Archosauriformes. The newly recognized clade containing Azendohsaurus, Trilophosaurus, Spinosuchus, Pamelaria, and Teraterpeton demonstrates high morphological disparity even within a closely related group of archosauromorphs, underscores the polyphyly of protorosaurs ( = prolacertiforms), and suggests that most major divergences within this group occurred in the Triassic. Furthermore, our results indicate that craniodental character states ascribed to a herbivorous diet were much more pervasive across Triassic Archosauromorpha than previously conjectured.
The Malagasy cynodont Menadon besairiei (Cynodontia; Traversodontidae) in the Middle–Upper Triassic of Brazil
Authors:
Melo et al
Abstract:
The traversodontid cynodont Menadon besairiei, previously known from the ‘Isalo II’ group of Madagascar, is reported for the first time from the Triassic of southern Brazil. New material referable to M. besairiei was collected in the Schoenstatt outcrop (Santa Cruz do Sul municipality), which belongs to the Santacruzodon Assemblage Zone (Carnian) in the Santa Maria Supersequence. Their attribution to the Malagasy taxon is based on the presence of a deep snout; four upper incisors, with procumbent first and second incisors, and three lower, procumbent incisors; five upper postcanines, the first one ‘peg-like’; six lower postcanines, the two most anterior and the most posterior being reduced; the quadrangular form of the postcanines; a pterygoid reaching the jugal and excluding the maxilla from the suborbital fenestra; a mandible with a tall coronoid process covering the last lower postcanine laterally; and a posteriorly projected angular process. A cladistic analysis of traversodontid cynodonts was performed based on a matrix composed of 30 taxa and 78 characters. In the resulting trees, M. besairiei nested within the clade Gomphodontosuchinae, the only traversodontid subclade reasonably well supported. Thus, the presence of M. besairiei is established in the Santa Cruz do Sul fauna, constituting the first record for South America and confirming the previously proposed biostratigraphic correlation between the ‘Isalo II’ and the Santacruzodon Assemblage Zone.
Deep below the surface of a water-filled cave in Madagascar, divers and paleontologists have uncovered a boneyard full of extinct giant lemurs.
Hundreds of bones dot the silty bottom of Aven Cave in Tsimanampetsotse National Park. The remains include exotic species such as the extinct elephant bird, a flightless giant similar to an ostrich, but the most numerous bones are from long-lost giant lemurs.
The largest of the extinct lemurs were as big as gorillas, and paleontologists sometimes refer to the different types as sloth lemurs, koala lemurs, and monkey lemurs to describe their different lifestyles and the living animals they most closely resemble. Sometime between 2,000 and 500 years ago, all these giants disappeared, possibly at the hands of humans.
The underwater caves offer an unprecedented look at these lost species. "The preservation is really incredible," says Brooklyn College anthropologist Alfred Rosenberger, a National Geographic grantee who is leading the project.
Enamel Microstructure of Vintana Sertichi (Mammalia, Gondwanatheria) From the Late Cretaceous of Madagascar
Authors:
von Koenigswald et al
Abstract:
The enamel microstructure of the molariform cheek teeth of Vintana sertichi (Gondwanatheria, Mammalia) from the Late Cretaceous of Madagascar retains many of the plesiomorphic characteristics of mammalian prismatic enamel (e.g., single-layered schmelzmuster; non-decussating, small prisms). However, V. sertichi exhibits a relatively derived condition and resembles Lavanify miolaka (Late Cretaceous, Madagascar), Bharattherium bonapartei (Late Cretaceous, India), and an indeterminate sudamericid (Late Cretaceous, Madagascar) in possessing modified radial enamel, consisting of prisms separated by prominent interrow sheets of interprismatic matrix. A second group of gondwanatherians (Ferugliotherium windhauseni, Gondwanatherium patagonicum, Sudamerica ameghinoi, Greniodon sylvaticus), all from the Late Cretaceous and Paleogene of Argentina, retains relatively primitive, ‘normal’ radial enamel, in which the interprismatic matrix anastomoses around the prisms and does not form interrow sheets. The prisms of gondwanatherian molariforms generally become wider from the enamel-dentine junction to the outer enamel surface, dramatically so in Gondwanatherium. Seams are very rare; tubules are more common. Prism decussation has not been found in any gondwanatherian enamel. Gondwanatherian molariforms lack or have only a thin layer of external prismless enamel, although it is thick in the incisors of Sudamerica. Despite their apparent utility in differentiating among gondwanatherian taxa, or gondwanatherians as a whole from other clades, many of the derived characteristics of gondwanatherian enamel microstructure are found in other mammalian groups and may reflect a high degree of homoplasy.
Dental Morphology of Vintana Sertichi (Mammalia, Gondwanatheria) from the Late Cretaceous of Madagascar
Author:
Krause
Abstract:
The cranium of Vintana sertichi preserves the first associated upper dentition of a gondwanatherian mammal. Gondwanatherians are known almost exclusively from isolated teeth, particularly molariforms. As such, referral of V. sertichi to the Gondwanatheria, differentiation of V. sertichi from other gondwanatherians, and determination of the relationships of V. sertichi to other gondwanatherians must rely heavily on dental morphology. The hypsodont nature, gross morphology, enamel microstructure, and wear pattern of its molariform teeth serve to unequivocally identify the cranium of V. sertichi as that of a sudamericid gondwanatherian. Based on available morphology, V. sertichi appears to have had an upper dental formula of 2.0.1.4. There are two long, curved alveoli in each premaxilla for enlarged, buccolingually compressed, procumbent incisors that were well separated from the cheek teeth by a long diastema. There is no evidence of a canine. The single premolariform tooth appears to have been small and two-rooted, but neither the left nor the right crowns are preserved. Of the eight upper molariform alveoli, four are preserved with teeth in situ, three in the left maxilla (antepenultimate, penultimate, and ultimate) and one in the right (penultimate). The molariform cheek teeth have several salient characteristics: large size, hypsodont crowns, roughly quadrangular outlines, occlusal surfaces worn essentially flat, numerous cementum-filled infundibula, cementum-filled furrows that invaginate from the buccal side but do not extend to the base of the crown, and multiple, short root apices. The relative amounts of wear on the molariform cheek teeth of V. sertichi indicate a mesial-to-distal eruption sequence.
Endocranial and Inner Ear Morphology of Vintana Sertichi (Mammalia, Gondwanatheria) from the Late Cretaceous of Madagascar
Authors:
Hoffmann et al
Abstract:
We present the first digital reconstruction of the endocranial cavity and endosseous labyrinth of the Late Cretaceous gondwanatherian mammal Vintana sertichi from the Maevarano Formation of Madagascar. The Malagasy specimen is exceptionally well preserved and represents the only described cranium known for Gondwanatheria, an enigmatic clade from the Late Cretaceous and Paleogene of Gondwana. The endocranial cast of Vintana is relatively small for an animal of its estimated body mass. Its encephalization quotient is 0.28–0.56 for a range of body mass estimates, which is similar to that of basal mammaliaforms. The olfactory bulbs are very large, occupying over 14% of the endocranial volume. The cerebral hemispheres are only slightly expanded, more similar to the condition in Morganucodon than to that in multituberculates and monotremes. Unlike the condition in other Mesozoic mammaliaforms, the endocast is greatly flexed at the circular fissure. The osseous labyrinth displays a mixture of derived and primitive features. The cochlear canal is only slightly curved and short compared with that of extant therians. The ratio between total cochlear canal length and maximum cranial length is smaller than in basal mammaliaforms and approximates that of non-mammaliaform cynodonts. By contrast, the presence of both primary and secondary osseous laminae, the tractus foraminosus, and Rosenthal's canal represent derived characteristics of the mammalian inner ear typical of cladotherians. A modern innervation of the cochlea has either evolved independently in Vintana and cladotherians or was already present in the last common ancestor of both clades.
Sensory Anatomy and Sensory Ecology of Vintana Sertichi (Mammalia, Gondwanatheria) from the Late Cretaceous of Madagascar
Authors:
Kirk et al
Abstract:
Living mammals are distinguished from other extant tetrapods by adaptations for improved senses of hearing, touch, and smell. These adaptations, and concomitant reductions in visual anatomy, evolved during the Mesozoic in the mammalian and therian stem lineages. Here, we present a comparative study of the sensory anatomy of the Late Cretaceous gondwanatherian mammaliaform Vintana sertichi in order to draw inferences regarding its sensory abilities and sensory ecology. Our analyses demonstrate that Vintana has relatively large orbits that may have accommodated large eyes. Vintana also possessed cochlear primary and secondary osseous laminae and a cochlear canal that was relatively longer than in non-mammaliaform cynodonts but shorter than in extant therians. These features suggest that Vintana had some capacity for high-frequency hearing (i.e., graeter than 20 kHz), but that its cochlea may have encoded a more limited range of frequencies than the cochleas of most extant therians. The semicircular canals of Vintana have large radii of curvature and are nearly orthogonal, suggesting high sensitivity to angular head accelerations. These vestibular features may have evolved in order to stabilize large eyes during rapid and/or agile locomotion. Combined with evidence for large olfactory bulbs and a large trigeminal endocast, these data reveal that Vintana possessed a unique suite of sensory adaptations that distinguish it from other Mesozoic mammaliaforms. If these inferences are correct, then Vintana was probably a large-eyed and agile species, with a keen sense of smell and better high-frequency hearing than most other Mesozoic mammaliaforms.
Ancient DNA extracted from the bones and teeth of giant lemurs that lived thousands of years ago in Madagascar may help explain why the giant lemurs went extinct. It also explains what factors make some surviving species more at risk today, says a study in the Journal of Human Evolution.
Most scientists agree that humans played a role in the giant lemurs' demise by hunting them for food and forcing them out of habitats. But an analysis of their DNA suggests that the largest lemurs were more prone to extinction than smaller-bodied species because of their smaller population sizes, according to this team of American and Malagasy researchers.
By comparing the species that died out to those that survived, scientists hope to better predict which lemurs are most in need of protection in the future.
The African island of Madagascar has long been known as a treasure trove of unusual creatures. More than 80 percent of the island's plants and animals are found nowhere else. But not long ago, fossil evidence shows there were even more species on the island than there are today. Before humans arrived on the island some 2,000 years ago, Madagascar was home to 10-foot-tall elephant birds, pygmy hippos, monstrous tortoises, a horned crocodile, and at least 17 species of lemurs that are no longer living -- some of which tipped the scales at 350 pounds, as large as a male gorilla.
Using genetic material extracted from lemur bones and teeth dating back 550 to 5,600 years, an international team of researchers analyzed DNA from as many as 23 individuals from each of five extinct lemur species that died out after human arrival. They looked at a giant ruffed lemur, a baboon lemur, a koala lemur and two sloth lemurs -- all housed in the collections at the University of Antananarivo and the Duke Lemur Center at Duke University. The study also included genetic data from eight extant species, including the three largest lemur species still alive today.
The researchers found that the species that died out had lower genetic diversity than the ones that survived -- a hallmark of small population size.
Dental Function and Diet of Vintana Sertichi (Mammalia, Gondwanatheria) from the Late Cretaceous of Madagascar
Authors:
Schultz et al
Abstract:
The nearly complete and well-preserved cranium of Vintana sertichi provides an opportunity to investigate its dietary adaptations. We used a combination of comparative morphological and biomechanical analyses to reveal the direction of its power stroke during mastication, reconstruct the positions and relative sizes of its muscles of mastication, and predict its capacity to produce bite forces during incision and molariform occlusion. The orientation of dental wear striations on the upper molariform teeth of Vintana, in combination with the orientation of the enamel islets and synclines and the position of leading and trailing edges, demonstrates that the power stroke was primarily palinal (distally directed), with a significant buccally directed component that is absent in multituberculates, haramiyidans, and other gondwanatherians. The large palinal component of jaw movement was corroborated by biomechanical analyses of the moments generated by the primary jaw adductors around the dentary-squamosal joint axis. Similar analyses also confirmed the previously documented proal (mesially directed) power stroke of the extant rodent Myocastor. Finite element analyses predict that Vintana was capable of producing bite forces that were more than twice as high as the similarly sized Myocastor. Vintana was almost certainly an herbivore, as claimed previously for other sudamericid gondwanatherians. Its size and capacity to generate high bite forces at both the incisors and the molariform teeth suggest that it was a mixed feeder whose diet may have included relatively large, hard food items such as roots, seeds, twigs, or nut-like fruits.
Introduction, Systematic Paleontology, and Geological Context of Vintana Sertichi (Mammalia, Gondwanatheria) from the Late Cretaceous of Madagascar
Authors:
Krause et al
Abstract:
Vintana sertichi is a sudamericid gondwanatherian mammal known only from the Upper Cretaceous (Maastrichtian) Maevarano Formation in the Mahajanga Basin of northwestern Madagascar. It is based on a single specimen, a well-preserved and virtually complete cranium discovered in 2010 near Lac Kinkony. The cranium is superficially bizarre and constitutes the only cranial remains of the poorly known, phylogenetically enigmatic Gondwanatheria, which are otherwise known only from isolated teeth and fragmentary dentaries. Gondwanatheria are represented by seven other monotypic genera assigned to two families (Ferugliotheriidae and Sudamericidae). Historically, this clade was assigned to Xenarthra, Paratheria (as a sister group to Xenarthra), Multituberculata, Allotheria (as a sister group to Multituberculata), Mammalia incertae sedis, and, most recently, back to Multituberculata or a close relative of Multituberculata. The craniodental evidence provided by Vintana supports inclusion in Sudamericidae, the monophyly of Gondwanatheria, and the position of Gondwanatheria as nested within or sister to Multituberculata. In addition to briefly reviewing the taxonomic composition and phylogenetic history of Gondwanatheria, this introductory chapter sets the stage for the other chapters in the volume by (1) briefly summarizing the inferred life habits of gondwanatherians; (2) reviewing the systematic paleontology of V. sertichi; and (3) providing overviews of the discovery of the holotypic specimen, its preservation, its preparation, and the imaging and measurement techniques used to study it. The chapter closes with an overview of the geological context of V. sertichi, which indicates that the species lived in a coastal floodplain environment and in a highly seasonal, semiarid climate.
Geochronology and geochemistry of Cryogenian gabbros from the Ambatondrazaka area, east-central Madagascar: Implications for Madagascar-India correlation and Rodinia paleogeography
Authors:
Zhou et al
Abstract:
Tectonic setting of the ca. 840-760 Ma Imorona-Itsindro Suite throughout central Madagascar is critical to understanding Madagascar-India correlation in pre-Gondwana time and the paleogeography of the rifting Rodinia. Here we present new geochronological and geochemical data for gabbros of this suite from the Ambatondrazaka area in east-central Madagascar. SIMS U-Pb zircon dating reveals that the gabbros emplaced between ca. 797 and 772 Ma. They define a typical tholeiitic trend on the AFM and FeOT/MgO vs. SiO2 plots, which is confirmed by the occurrence of layered Fe-Ti-V oxide mineralization. Owing to the high-K nature, they were previously misinterpreted as a “calc-alkaline” association produced in a mature continental arc. Indeed, it implies in return an intra-plate affinity for tholeiitic rocks. Their high Zr/Y, La/Y and Ti/V ratios differ from those of typical arc and back-arc mafic suites but are compatible with an intra-plate setting. Additionally, they are characterized by low AlZ/TiO2 ratios in clinopyroxene, further supporting an intra-plate environment. The negative ɛNd(t) (-8.4 to -5.1) and zircon ɛHf(t) (-6.84 to -3.08) values, together with enriched trace element signatures, indicate their derivation from an enriched lithospheric mantle source. Considering the initial 87Sr/86Sr ratios (0.705409 to 0.706892) and Nd modal ages (∼2.1-2.4 Ga), we postulate that this mantle reservoir has EM1-type isotopic characteristics and was enriched by a Late Neoarchean “super accretion event”. Taken together, our results question the existence of the Betsimisaraka Suture but support the proposal that the Malagasy Shield and the Dharwar Craton of India are parts of the same entity–the Greater Dharwar Craton. Within this framework, the widely-cited hypothesis that an extensive Andean-type arc was developed along the northwestern margin of the rifting Rodinia is challenged, at least that the magmatic arc did not extend to the Malagasy Shield.
First cranial remains of a gondwanatherian mammal reveal remarkable mosaicism
Authors:
Krause et al
Abstract:
Previously known only from isolated teeth and lower jaw fragments recovered from the Cretaceous and Palaeogene of the Southern Hemisphere, the Gondwanatheria constitute the most poorly known of all major mammaliaform radiations. Here we report the discovery of the first skull material of a gondwanatherian, a complete and well-preserved cranium from Upper Cretaceous strata in Madagascar that we assign to a new genus and species. Phylogenetic analysis strongly supports its placement within Gondwanatheria, which are recognized as monophyletic and closely related to multituberculates, an evolutionarily successful clade of Mesozoic mammals known almost exclusively from the Northern Hemisphere. The new taxon is the largest known mammaliaform from the Mesozoic of Gondwana. Its craniofacial anatomy reveals that it was herbivorous, large-eyed and agile, with well-developed high-frequency hearing and a keen sense of smell. The cranium exhibits a mosaic of primitive and derived features, the disparity of which is extreme and probably reflective of a long evolutionary history in geographic isolation.
The 580–520 Ma Gondwana suture of Madagascar and its continuation into Antarctica and Africa
Authors:
Boger et al
Abstract:
U–Pb age data from southwest Madagascar provides a compelling case that the pre-Gondwana Indian plate was stitched with the arc terranes of the Arabian Nubian Shield along a suture that closed between 580 Ma and 520 Ma. The key observations supportive of this interpretation are: (1) metamorphism dated to 630–600 Ma is manifested only on the west side of the suture in rocks that have affinities with the oceanic and island arc terranes of the Arabian Nubian Shield, or represent continental rocks welded to these terranes prior to the amalgamation of Gondwana, and (2) orogenesis at 580–520 Ma is manifest in rocks on both sides of the suture, an observation taken to mark the timing of collision and reflect spatial continuity across the suture. In southwest Madagascar the distribution of metamorphic ages places the suture along the Beraketa high-strain zone, the tectonic boundary between the Androyen and Anoysen domains. Similar age relationships allow for the extrapolation of this tectonic boundary into both East Antarctica and Africa.
