Scottish geologist Charles Lyell quipped that the present is the key to the past. To say that the reverse also holds is more than just circular reasoning. Felisa Smith, a professor in ecology and evolutionary biology, studies extinct mammals and applies this knowledge to the present. This book is a neatly crafted package that gives the reader all the required background knowledge, while its case studies make for fascinating reading. (Spoiler alert: packrat middens are my new favourite discovery.)
Palaeoecology is a subdiscipline of palaeontology that studies the interactions between organisms and their environments across geologic timescales. Mammalian Paleoecology is divided into three parts: starting from fundamentals, Smith discusses how we obtain information from extinct mammals, before showing how this can be applied to current problems such as climate change and biodiversity loss. The overall read is very satisfying, each chapter logically following on the last and building towards some major payoffs at the end.
The fundamentals section shows Smith has another ace up her sleeve: excellent writing that makes technical topics accessible. She introduces the fossil record, ponders its quality and completeness, and gives a brief overview of mammal evolution. More technical, and one of my favourite aspects of palaeontology, is taphonomy: the study of how organisms become fossils. I learned more about its basics from this chapter than I did from the recent and rather eclectic Fossilization. Topics covered include the pre- and post-burial processes (biostratinomy and diagenesis, respectively), the biases that determine what is and is not preserved, and the work of one of its pioneers, Kay Behrensmeyer. I maintain that there is a gap in the market for an up-to-date text, as I only know of older books dedicated to taphonomy. Similarly insightful was the chapter on dating, covering both relative techniques, such as stratigraphy and index fossils, and absolute techniques, such as radiometric and amino acid dating.
“The overall read is very satisfying, each chapter logically following on the last and building towards some major payoffs at the end.”
With these basics in the bag, Smith then dives into the interesting part: how can we use fossils to learn about the ecology of extinct mammals? The focus is on five topics. Size influences biological processes and their rates, such as metabolism, and there are some interesting interactions between environment and body size. Teeth make up much of the fossil record and e.g. dental microwear and isotope analyses can reveal diet, while high-crowned teeth tell something about past plant communities. Stable isotopes come in various flavours and Smith discusses four examples and their applications. For instance, carbon isotopes can reveal past diets, while oxygen isotopes are useful for reconstructing past climate. Trace fossils can reveal past behaviour and Smith discusses unusual examples. The spores of Sporormiella fungi growing on animal dung are widespread and can help reconstruct past ecosystems. Smith’s speciality – I absolutely loved this bit – are palaeomiddens, refuse heaps constructed by generations of packrats that contain plant and animal fragments encased in masses of crystallized urine. They are an absolute treasure trove! (Something only a biologist would write about blocks of hardened rat pee). Trace fossils can also be potential sources of ancient DNA. Finally, there are various palaeoclimatological proxies such as tree rings, ice cores, and others that we can use to reconstruct past climates.
The last two chapters are the big payoff. Here, Smith takes everything covered so far and shows that we can apply this knowledge to current concerns regarding climate change and biodiversity loss. More specifically – and this is one take-home message from this book – palaeoecology can combat shifting baseline syndrome*. It “helps define the range of “normal” variation of ecosystems and set baselines against which changes in species composition, abundance, and richness in modern ecosystems can be compared” (p. 216).
“Smith’s speciality – I absolutely loved this bit – are palaeomiddens, refuse heaps constructed by generations of packrats that contain plant and animal fragments encased in masses of crystallized urine.”
The climate has always changed, but now we can start answering how mammals responded to this, whether via extinction, migration, or adaptation. Particularly interesting is how this helps answer a long-standing question in ecology: do communities respond in unison as a superorganism, as once argued by Frederic Clements, or are they assemblages of individualistic species, as once argued by Henry Allen Gleason? The analysis of large datasets on home ranges through deep time shows assemblages disassociating in response to climatic changes. Many past communities have no modern analogue. And how much of observed past adaptation was genetics versus phenotypic plasticity?
Similarly, extinction has always happened, but there is widespread concern that we are witnessing the sixth mass extinction. Palaeontology holds the answers and Smith explains why she reaches a conclusion I have encountered elsewhere: “while we are not quite at the level of a sixth mass extinction, we are firmly on the trajectory toward it” (p. 192). Recent extinctions are undeniably due to us, yet the cause of Pleistocene megafauna extinctions has been hotly debated, with hunting and climate change the main contenders. Though both play a role, Smith explains why the consensus leans towards humans. Her analyses on changes in average mammal body size over deep time show a clear decrease over the last 120,000 years that closely tracks human migration around the globe. Taking a step back, she observes that “being large-bodied or an herbivore did not increase extinction risk for most of mammal evolutionary history. The size and trophic bias to extinction were only present when hominins were” (p. 199). To her surprise, other analyses even suggest that temperature changes over the last 66 million years never enhanced extinction rates. Finally, she discusses how fossil data can reveal not just the cause of extinction but also the ecological consequences.
“one take-home message from this book [is that] palaeoecology can combat shifting baseline syndrome.”
Though the revelations come thick and fast in the last two chapters, Smith’s writing is incredibly accessible. I particularly appreciated her explanations of how certain technologies work, e.g. what the idea is behind mass spectrometry. How what sounds easy on paper, can be laborious in practice, such as collecting a packrat midden and counting its contents under a microscope. And how there are important caveats and limitations to keep in mind, whether working with Sporormiella spores or ancient DNA. This is all helped along by a generous serving of photos and diagrams that illustrate not just results, but also principles behind certain analyses. The only complaint I have is that they are all printed in greyscale while some were clearly designed with colour in mind. Since there is no colour plate section, some complex plots are hard to interpret.
Obviously, Mammalian Paleoecology comes highly recommended to readers interested in evolutionary biology and palaeontology. But even if the term palaeoecology means little to you, you are in safe hands with this book. It is an absolutely fascinating text that benefits from some fantastic writing, decades worth of Smith’s own experience, and an excellent structure that logically threads chapters together.
* This term was originally popularized by Daniel Pauly in the context of fisheries but has since found wide application in ecology.
Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.
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