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Bones, Clones, and Biomes
The History and Geography of Recent Neotropical Mammals
THE UNIVERSITY OF CHICAGO PRESS
Copyright © 2012 The University of Chicago
All right reserved.
ISBN: 978-0-226-64919-1
Chapter One
Introduction to the History and Geography of Neotropical Mammals Bruce D. Patterson and Leonora P. Costa
The Neotropics are home to more than 1500 species of living mammals, almost 30% of all extant species. These charismatic organisms include remarkable animals found nowhere else on earth: armadillos and anteaters, capybaras and capuchins, maned wolves and mouse opossums, sloths and sakis. Endemic groups include the platyrrhine monkeys (136 species), the phyllostomid bats (58 genera and 192 species, exploiting almost every dietary strategy used by bats worldwide), the caviomorph rodents (12 families and 288 species of guinea-pig relatives), and the sigmodontines (a subfamily of mice that explosively radiated into at least 88 genera and 399 species over the past 6–8 million years). Local diversity of mammals in the Neotropics is so great that biologists can capture representatives of 5–6 families and 30–35 species of bats in a single night by using mist nets. Individual reserves in the Neotropics can support more than 220 species of mammals, twice the number known from the eastern third of North America!
This biological bounty can be partly attributed to the striking diversity of Neotropical landscapes and biomes. The region supports lush rainforests (both temperate and tropical), parched deserts, sprawling savannas, thorny scrublands, alpine steppes, and towering peaks. A plethora of climatic and edaphic conditions accompany and shape this biotic diversity. Another agency fostering its endemism has been South America's isolation as an island continent for much of the last 65 million years (Ma). However, unlike its Gondwana neighbor Australia, South America's history of isolation has been interrupted by a succession of continental connections that permitted intermittent faunal exchanges with Africa, Antarctica and Australia, and North America at different times. These episodes of exchange, each involving distinctive sets of immigrants, appear as discrete "strata" in South America's extensive fossil record. To comprehend the development of modern Neotropical mammal faunas requires not only mastery of the Neotropics' substantial mammalian diversity, but also knowledge of lineages and landscapes stretching back to the Mesozoic. A synthesis of this information requires expertise that spans the disciplines of geology, paleontology, physical geography, systematics, and biogeography. Few modern workers claim this breadth.
As a result, the leading works on the evolution and biogeography of Neotropical mammals, books like Mammals on Southern Continents, Splendid Isolation, and the Great American Biotic Interchange (Keast, Erk, and Glass 1972; Simpson 1980; Stehli and Webb 1985) are grossly outdated. Although there are now excellent field guides (Emmons and Feer 1997; Reid 1997) and some regional treatises on the fauna (Eisenberg and Redford 1989, 1999; Redford and Eisenberg 1992), no recent work has treated the origins, interrelationships, and biogeography of living Neotropical mammals. Even the taxonomic foundations for this undertaking lack refinement. Angel Cabrera's classic Catálogo (Cabrera 1958, 1961) is a half-century old, and the three-volume series now destined to replace it, Mammals of South America, is only partly completed (Gardner 2008). In any event, that work has been organized around taxa (orders, families, genera, and species), not around time periods and regions. For this reason, a synthesis focused on regional and historical features of the modern Neotropical mammal fauna nicely complements that taxonomic series.
Since the last attempt at a synthesis, a host of practical and conceptual advances have been made in disciplines flanking historical biogeography. Continental drift has replaced hypothetical land bridges as a means of intercontinental dispersal (cf. Simpson 1980). Absolute chronometric methods have been developed and applied to precisely date faunal horizons (Flynn and Swisher 1995). Additional land mammal ages have been described (Flynn et al. 2003), while existing ones have been better delimited (Kay et al. 1999). Nearly a quarter of living Neotropical mammal species has been newly discovered or rediscovered (Patterson 1996, 2000), offering taxonomic and geographic refinement to biogeographic reconstructions. Cladistics has replaced earlier paradigms in systematics (Cracraft and Donoghue 2004; Felsenstein 2004), and explicit analyses such as phylogeography and dispersal-vicariance approaches have refined historical biogeographic reconstructions (Crisci, Katinas, and Posadas 2003; Ree et al. 2005). All of these advances contribute substantial new understanding to discussions on the origin, diversity, and distribution of Neotropical mammals, and a comprehensive synthesis of their findings is long overdue.
We addressed this need by assembling expert paleontologists and neontologists and asking them to detail their complementary spatiotemporal perspectives by focusing on a central question: How did changing climates and landscapes, intercontinental connections, and newly evolved lineages interact to populate Central and South America and the Antilles with almost 30% of the world's living mammals? The paleontologists were tasked with exploring the historical context of the modern fauna. Their descriptions and characterizations of Mesozoic, Paleogene, and Neogene landscapes and inhabitants offer depth and context to understanding of modern landscapes and faunas.
The neontologists were challenged to account for modern faunas region by region. Each of the regional chapters explores the historical development, diversification, and endemism of the mammal groups living there. We make no pretense of documenting modern diversity or its distribution, but rather try to summarize patterns reflected in those distributions and relationships, and do so to the extent that regional studies permit. Our treatment does not extend to aquatic and marine mammals, because very different physical and geographic issues are involved with those groups and, for most, even less information is available. Restrictions on manuscript length precluded a fuller exploration of Neotropical paleontology — although some intervals remain poorly known and studied, this decision was largely owing to the richness and complexity of modern faunas and the desire to treat them as comprehensively as possible. Even so, some distinctive biomes, such as the Gran Chaco and Valdivian Forest, are only touched upon by chapters devoted to neighboring biomes.
Earlier attempts to chronicle the development of Neotropical mammal faunas were plagued by labored arguments and controversy over plate tectonics and phylogenetic methods (Keast, Erk, and Glass 1972; Simpson 1980). Here, chapters employ these well-worked methods and conceptual frameworks to generate new and refined interpretations of how current faunas came to be. We devote two of the volume's chapters to Neotropical carnivore radiations: one is paleontological and documents spatial and temporal records, while the other chapter uses molecular trees to date key events in the radiation, especially those in the tropics and during their initial radiation, when fossils are generally lacking.
Chapters articulate research visions that are scattered widely across physical geography, geology, paleontology, systematics, mammalogy, and biogeography. Each of the contributors is an acknowledged authority in his or her field, and we were fortunate to secure the participation of virtually all the scientists that we approached with our plans. From the start, their participation was to be both oral and written. These contributions were first presented to the scientific community in two back-to-back symposia at the 10th International Mammalogical Congress in Mendoza, Argentina, on August 14, 2009. The symposia gave each contributor an overview of the ensemble and the approaches, analyses and results of other participants. We hope that this has helped us to integrate these papers into a coherent volume whose scope exceeds the expertise and conceptual and analytical range of any single person.
The various chapters were submitted to us by January 31, 2010, and were sent out for anonymous peer review; revised versions of the chapters were submitted to the publisher on July 15, 2010; updated citations were possible in the proof stage.
A number of people aided us in this project. We thank the organizers of IMC-10, especially Ricardo Ojeda, for the invitation to organize a symposium and subsequently giving us the latitude (and schedule) to make it last all day. A host of peer reviewers offered helpful criticisms and helped us shape and rework the contributed manuscripts. In alphabetical order, we thank Sergio Ticul Alvarez, Robert Anderson, Mariano Bond, François Catzefl is, Guillermo D'Elia, Albert Ditchfield, Analia Forasiepi, Alfred Gardner, Chris Himes, Christopher Johnson, Douglas Kelt, Thomas Lacher, Lucia Luna, Bruce MacFadden, Ross MacPhee, Kevin Murray, James Patton, Alexandre Percequillo, Bruce Shockey, Maria Nazareth da Silva, Richard Tedford, Robert Voss, Marcelo Weksler, Lars Werdelin, Michael Woodburne, and Neal Woodman. Our contributors all bore in good cheer (but variable timeliness) our efforts to orchestrate their scholarship. We thank three anonymous reviewers of the volume as a whole for their excellent feedback and suggestions; depth of knowledge and generous editorial assistance unmasked one of these (special thanks to Alfred Gardner). Finally, of course, we thank Christie Henry and Amy Krynak at the University of Chicago Press for their expertise in bringing this project forward in its current form and Rachel Cabaniss for her careful copy editing.
Bruce D. Patterson Chicago, USA
Leonora P. Costa Vitoria, Brazil
Chapter Two
Punctuated Isolation The Making and Mixing of South America's Mammals
Darin A. Croft
South America's fossil record of mammals from the Cenozoic Era (the past 65 million years or so) is far from perfect. No fossils have yet been discovered from some intervals of this time span, whereas only one or a few poorly sampled localities represent others. Moreover, most intervals are known only from a relatively restricted portion of the continent. In many respects, South America's mammalian record pales in comparison to that of northern continents.
On the other hand, South America overall probably has the best fossil record of mammals among southern hemisphere continents. Thanks to a handful of spectacular localities and an abundance of others, the broad picture of mammal evolution on the continent has been known for more than half a century (e.g., Simpson 1950). As detailed in the chapters that follow, the mammals of this continent evolved largely within the context of isolation, punctuated by rare episodes of waif dispersal and faunal interchange. This contrasts starkly with the history of mammal evolution on northern continents, in which episodes of faunal interchange were commonplace (Janis 1993). Whereas northern continents have acted as thoroughfares for numerous mammal groups, South America and other southern continents have acted more as cul-de-sacs: areas off the beaten path with limited access and little large-scale turnover (Keast, Erk, and Glass 1972). They have not been evolutionary dead ends for mammals (with the exception of Antarctica), but rather have produced communities of thriving, endemic species exhibiting mixtures of novel and convergent traits.
Although the general context of evolving mammal communities in South America has been known for many years, remarkable advances have taken place in recent decades in virtually every aspect of mammalian paleontology. These advances have been made by scores of researchers from South America and elsewhere using a variety of methods ranging from traditional to cutting edge. Some of these experts, representing many of the main areas of scientific investigation, have been brought together in the first part of this volume to summarize these recent advances and to provide a framework for the origins and biogeography of modern mammal communities.
The next chapter, by Goin and collaborators, offers a broad-stroke perspective on changing faunas and habitats of South America over the past 250 Ma. Of necessity, their observations are based primarily on the Patagonian fossil record, since it is the only region of the continent where nearly all major time intervals are represented. The excellent record in Patagonia is the result of fortuitous geological and climatic factors. For much of the Cenozoic, parts of Patagonia gradually accumulated water- and wind-borne sediments, conditions highly suitable for the preservation of teeth, bones, tracks, and other evidence of past life. Some of these sediments were sands and silts derived from the erosion of topographically higher areas, whereas others were ashes from volcanic eruptions. Occasionally, transgressions of the ocean onto the continent added marine sediments over the continental record. With the rise of the Andes and changes in global climate patterns, Patagonia developed the dry, windy, temperate conditions that characterize it today (Barreda and Palazzesi 2007). Because these conditions support little vegetation, the sediments that accumulated over millions of years, and the fossils within them, have been exposed to the elements, as well as to the eyes of paleontologists. Without such exposures, fossils would remain inaccessible and undiscovered, an observation that partly explains the scarcity of fossil localities in tropical lowlands, which are typified by abundant vegetation. Water and wind erosion in Patagonia continuously expose new fossils and, given the vast expanse of Patagonia, many new fossil localities undoubtedly remain to be discovered.
Based on the record from Patagonia, Goin and collaborators recognize five main phases in the evolution of South American mammal faunas. The transitions between these phases are recognized by more or less drastic changes in faunal composition. Among these transitions, the most significant might have been the one that occurred near the end of the Age of Dinosaurs (the Mesozoic Era, ending ~65 Ma). It is almost as if the list of mammals living in South America at the time had been wiped clean; the main constituents of Mesozoic ecosystems, the nontribosphenic mammals (which mostly lack living descendants) disappeared and were replaced by therians (marsupials and placentals), likely of North American origin. The fossil record documenting this remarkable change is relatively sparse, and the exact sequence of events and their causes remain unknown. Nevertheless, current evidence suggests that the results of the interchange between the Americas at this time were just as dramatic as those that occurred only a few million years ago (as a result of the Great American Biotic Interchange [GABI], discussed later). Goin and collaborators refer to this early event as the FABI (First American Biotic Interchange). The brief period following the FABI appears to have been the heyday for South American metatherians (marsupials) in terms of taxonomic diversity and abundance. It came to an end about 50 Ma as placentals definitively gained the upper hand, having diff erentiated into the main groups so characteristic of later South American paleocommunities.
Another major shift in South American mammal faunas (as well as those in many parts of the world) occurred ~35 Ma; Antarctica's land connections to South America and Australia were finally severed, isolating all three continents and initiating a cascade of events that, in combination with other factors, resulted in a substantial drop in global temperatures (Zanazzi et al. 2007). The immediate effects of the so-called Eocene-Oligocene transition in South America were unrecognized in the mammal fossil record until the discovery of a new locality in central Chile in the late 1980s (Wyss et al. 1993, 1994). The many taxonomic, paleogeographic, and paleoecological insights that have come from this fauna are highlighted in the chapter on Cenozoic Andean faunas by Flynn and collaborators (chapter 4, this volume). The story of the serendipitous discovery of this locality — and of additional localities in the region — is nearly as interesting as the science (see Flynn, Wyss, and Charier 2007). At last tally, the rock formation whose unusual conditions preserved these fossils had produced more than 1400 specimens, mostly skulls, mandibles, and teeth, ranging in age from 40–15 Ma. Several hundred other specimens have been collected from a remarkable succession of strata in a similar formation slightly farther south in Chile (Flynn et al. 2008). These assemblages represent an increasingly important and expanding record of extra-Patagonian mammal evolution.
(Continues...)
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