Macroecology

Macroecology means different things to different people. For me, it encompasses  more synthetic studies that try to answer questions of broad interest in paleontology. Such studies frequently raise as many questions as they answer, but continue to refine our understanding of past communities. Because South America was effectively an island for much of the past 66 million years, it is a great place to tackle these types of questions. Islands are places of evolutionary innovation and have played a major role in the development of evolutionary theory. More than 150 years ago, careful study of island faunas by Charles Darwin and Alfred Wallace helped them to independently propose the idea of evolution via natural selection.

  • Paleoenvironments: Many types of data can be used to reconstruct past habitats, including information from mammals themselves. One example is a cenogram analysis, which correlates the distribution of species body masses with habitat and environmental attributes such as rainfall and vegetational structure.
    A typical woodland habitat in southeast Australia.

    A typical woodland habitat in southeast Australia. Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

    I applied this technique to South American fossil sites in 2001 and showed that habitat interpretations based on cenogram analysis often conflict with those based on other lines of evidence. I concluded that it may not be possible to apply these types of macroecological models to endemic faunas in places such as South America, at least using modern data sets from South America. Since then, I have been working with students and colleagues to expand the comparative database of modern faunas for these types of analyses to determine why this may be the case. It may partly be related to the unusual carnivorous mammals of this continent (see below). Another difficulty may be determining the proper scale of comparison between modern and fossil faunas, as I pointed out in a 2013 paper.

  • Predator-Prey Diversity: Just as Australia and ancient South America were both island continents, they also had similar types of predatory mammals: marsupials (metatherians). In modern mammal communities, predators are always rarer than their prey due to fundamental ecological principles. This was also true in the past. However,
    Side view of the skull of the saber-toothed sparassodont Thylacosmilus atrox (modified from Riggs 1933).

    Side view of the skull of the saber-toothed sparassodont Thylacosmilus atrox. Modified from Riggs (1933).

    in my fieldwork in South America, I noticed that remains of predatory mammals were even rarer than they were at North American fossil sites I had investigated. That started me wondering about possible causes and effects. In 2006, I published a paper that examined the relative diversity (richness) of marsupial predators in past South American faunas. I came to the conclusion these these South American predators (sparassodonts) were less successful in terms of species diversity than their placental counterparts in other parts of the world. I also provided quantitative data to support my impression of their scarcity in the fossil record. Now that these patterns have been established (at least in my mind), I am delving further into the issue of why this is case. It may be related to tooth replacement in marsupials, constraints on their forelimbs, or a combination of factors. Hopefully my studies will provide additional insights into how sparassodonts partitioned dietary resources among themselves and with so-called “terror-birds”( phorusrhacids).

  • Xenarthrans: Armadillos, sloths, and anteaters, which are collectively known as xenarthrans, are undoubtedly one of the most fascinating groups of South American mammals. One feature that they all have in common is a reduced
    Quebrada Honda from Velizar

    An extinct armadillo from the middle Miocene of Bolivia. Reconstruction by Velizar Simeonovski. Copyright D. Croft.

    dentition: they virtually all lack enamel on their teeth, and in xenarthrans that have teeth (anteaters do not), they tend to be simple compared to those of most other mammals. The group most likely originated in South America, but its origins are still shrouded in relative mystery; xenarthran fossils from the beginning of the Age of Mammals are rare and mostly fragmentary. What was the common xenarthran ancestor like, and why do xenarthrans have such a poor fossil record for much of the first 30 million years of the Cenozoic Era? These are just a few of the questions Tim Gaudin and I addressed in a 2015 Journal of Mammalogy paper on the origins of this quintessentially South American mammal group.

Comparison of foot bones (metatarsals) from a water buffalo (Bubalus bubalis; left), a tamaraw (B. mindorensis; center), and the extinct dwarf buffalo, B. cebuensis (right).

Comparison of foot bones (metatarsals) from a water buffalo (Bubalus bubalis; left), a tamaraw (B. mindorensis; center), and the extinct dwarf buffalo, B. cebuensis (right). Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

  • Island Dwarfism: One of the best examples of an evolutionary phenomenon that has occurred repeatedly through geologic time is that of island dwarfing: when a large mammal becomes smaller through evolutionary time after becoming isolated on an island. Examples of dwarf mammoths, hippos, deer, and other animals abound, but this phenomenon had never previously been reported in the cattle group (family Bovidae: subfamily Bovinae). Due to my past experience at The Field Museum, I had the opportunity to help describe some bones of an extinct species of dwarf water buffalo from the Philippines in 2006. These bones had been discovered almost 50 years earlier in a phosphate mine on Cebu Island, Philippines by a mining engineer named Michael Armas. He collected them and kept them safe for nearly four decades until they came to the attention of Dr. Hamilcar Intengan, a physician. Dr. Intengan recognized the importance of these fossils and brought them to the Field Museum in 1995 for scientific study, the results of which we published in 2006. This tiny buffalo would have stood only 2.5′ high at the shoulder and weighed a mere 350 lbs. Its exact age is not known, but it probably lived between 10,000 and 50,000 years ago.

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