Fossils can tell us what animals living in the distant past looked like. Over the centuries, palaeontologists have made incredible strides in reconstructing extinct life forms, helped along by cumulative experience, technological advances, and a steadily increasing body of rare but truly exceptionally preserved fossils. But reconstructing their behaviour—surely that is all just speculative? In Locked in Time, palaeontologist and science communicator Dean R. Lomax, with the able help of palaeoartist Bob Nicholls, presents fifty of the most exceptional fossils that preserve evidence of past behaviour: from pregnant plesiosaurs to a pterosaur pierced by a predatory fish. I was eagerly awaiting this book from the moment it was announced, but I was still caught off-guard by some of the astonishing fossil discoveries featured here.
Locked in Time: Animal Behavior Unearthed in 50 Extraordinary Fossils, written by Dean R. Lomax and illustrated by Robert Nicholls, published by Columbia University Press in May 2021 (hardback, 312 pages)
Locked in Time effectively consists of fifty vignettes organised around five themes. Perhaps surprisingly, this is not the first popular science book on behaviour revealed by the fossil record. Anthony J. Martin wrote Dinosaurs Without Bones in 2014, focusing on ichnology, the study of trace fossils such as fossil footprints, scratch marks, or nests. Lomax, however, casts his net far wider than just dinosaurs and has scoured the scientific literature for both trace and body fossils that reveal how these animals likely behaved in life. Some examples require careful inference, while others, cases that the pioneer Arthur Boucot called “frozen behaviour”, are blindingly obvious.
There are some truly astonishing fossils featured here. An ichthyosaur that died while giving birth proves beyond a doubt they were live-bearing. A pair of turtles was unfortunate enough to end up caught in the act of mating for eternity. There are well-known fossils such as the pregnant mother fish Materpiscis, the battle between a Protoceratops and a Velociraptor featured on the book’s cover, or the tall spiral structures once nicknamed Devil’s corkscrews by flummoxed fossil hunters that turned out to be burrows, some even containing fossilised beavers. There is violence: the skulls of two fighting male mammoths that died with tusks still interlocked; but also tranquillity: the fossil troodontid Mei long that was found in a posture reminiscent of modern-day birds resting or sleeping. There is the fossil that got Lomax started on this project: a metres-long trackway of a horseshoe crab scurrying over a lake bottom with the individual fossilised at the end of it! There is palaeopathology, parasites, and even fossil farts.
But make no mistake, Locked in Time is much more than a book of trivia and factoids. Palaeoethology, the study of behaviour of organisms in the fossil record, is a proper subdiscipline of palaeontology and has bearings on the study of palaeoecology. And for all the spectacular fossils and occasional puns, Lomax also subtly educates his reader on more serious topics. Three things struck me in particular.
“There is palaeopathology, parasites, and even fossil farts. But make no mistake, Locked in Time is much more than a book of trivia and factoids.”
First, as Lomax acknowledges, not all interpretations of behaviour are uncontested. Are the fossils of the primitive bird Confuciusornis sanctus that display exquisitely preserved ornamental feathers really males? Further research showed the fossils without these feathers to contain medullary bone, a temporary tissue associated exclusively with reproductively active females. The ornamented individuals showed no such evidence, strengthening the case for sexual dimorphism in this species. Or what of the 200-year-old claim that ichthyosaurs were cannibals? It took until the 1990s when further detailed studies of presumed stomach contents concluded that there were no signs of bite marks or etchings by stomach acids, strengthening the case that these were instead embryos. An important theme that Lomax highlights repeatedly is the utility of studying the behaviour of animals alive today. Although care is in order, comparisons with the behaviour of extant animals and the traces they leave can help demystify the behaviour of extinct animals.
Second, Lomax reveals the inner workings of palaeontology. It is not uncommon for fossils to linger for years or even decades after excavation pending the availability of funding and a skilled preparator. One specimen of the plesiosaur Polycotylus latipinnus was dug up in 1987 but not prepared until 2011, finally revealing its pregnancy. Sometimes technological advances breathe new life into old fossils, such as the burrow containing the therapsid Thrinaxodon liorhinus that was discovered in 1975. Not until 2013, when it was examined with powerful x-rays at a synchrotron facility, did the partially prepared fossil reveal a thus-far hidden injured amphibian that had crawled into the burrow and nestled itself against the likely dormant Thrinaxodon. In other cases, palaeontologists have to beware of frauds, as traders occasionally doctor fossils to make them fetch a higher price on the marketplace—some stories could just be too good to be true.
Third, Lomax proves himself to be a gentle educator. He will immediately explain jargon (e.g. Lagerstätten, ecdysis, or anamorphosis) and only introduce it where appropriate. And though this is popular science, Lomax is keen to bust myths. No, the large shark Megalodon is no longer alive, and there is zero evidence for either Dilophosaurus or other dinosaurs being capable of spitting acid, no matter what Jurassic Park tried to tell you. He beautifully channels deep time when writing “Before dinosaurs even appeared, trilobites were already fossils under their feet” (p. 110), and explains why trace fossils are much more common than body fossils: “Over its lifetime, an animal might leave behind countless footprints […] but only one skeleton” (p. 105).
“An important theme that Lomax highlights repeatedly is the utility of studying the behaviour of animals alive today. Although care is in order, comparisons […] can help demystify the behaviour of extinct animals.”
Finally, seeing is believing, and Locked in Time is richly illustrated. Most vignettes include photos and schematic drawings of the fossils, and my jaw dropped on numerous occasions. Given that disarticulated and fragmentary fossils are the norm, the selection that Lomax has curated here is truly breathtaking. Furthermore, all vignettes include a single or double-page spread with palaeoart from Bob Nicholls, tastefully reproduced in grayscale.
He was featured in Dinosaur Art and wrote the introduction to Dinosaur Art II where he discussed his Psitaccosaurus reconstruction mentioned here on p. 75. Nicholl’s artwork is heavily informed by science and adds much flavour to this book.
In an interview I did with Lomax, published at the NHBS Conservation Hub, he mentioned having made an initial selection of 100 fossils, so many fascinating examples did not make the cut. Other interesting studies were unfortunately published too recently to be considered for inclusion, such as the 2021 Caneer et al. paper discussing possible tracks made by a Tyrannosaurid rising from a prone position, or the 2021 Lockley et al. paper on the sand-swimming trace fossils left by a Pleistocene golden mole.
Locked in Time is an outstanding and highly original piece of popular science that overflows with Lomax’s enthusiasm and passion for his topic. Even if your shelves are already heaving with palaeontology books, make space for one more. Believe me, you have not seen a book like this before.
Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.
Other recommended books mentioned in this review:
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Following my two earlier reviews of Convergent Evolution and Convergent Evolution on Earth, this is the final of three reviews of MIT Press books in The Vienna Series in Theoretical Biology dealing with convergent evolution, something I consider to be one of evolutionary biology’s most exciting topics. Are evolutionary changes happy accidents, i.e. contingencies? Or is there a law-like repeatability underneath, explaining why some traits evolve time and again, i.e. convergence? Is it even a matter of either-or? And what lessons does this hold for life elsewhere in the universe? Philosopher Russell Powell wrestles with these questions in a manner that is as rigorous as it is intellectually rewarding. Evolutionary biologists will want to give this excellent book a very close read.
Contingency and Convergence: Toward a Cosmic Biology of Body and Mind, written by Russell Powell, published by MIT Press in February 2020 (hardback, 308 pages)
Powell starts by examining the legacy of Stephen Jay Gould, the famous evolutionary biologist and science populariser who tragically passed away in 2002. In his book Wonderful Life, Gould proposed the thought experiment of rewinding the tape of life to see how it would evolve on repeat plays. Based on the exceptional Cambrian fossils found in the Burgess Shale in British Columbia, he argued that lifeforms would be radically different, contingent on small variations that amplified with time. Others criticised this idea by pointing to the many examples of convergent evolution, the ubiquitous pattern of evolution repeatedly hitting on the same or similar solutions to a problem in different organisms.
Before proceeding, however, Powell offers a surprising little detour into astrobiology. Since we cannot rewind the tape of life, the next best thing would be to see how life evolved elsewhere. Just some of the interesting topics considered are the anomaly of life’s single origin, the timing of the emergence of intelligent, multicellular life compared to the lifespan of our Sun, and the problem of the observer selection effect. Where I found the previously reviewed Convergent Evolution on Earth lacking in its lessons for astrobiology, Powell engages with this topic in a far more thorough manner. Despite decades of effort, the stars remain eerily silent. Which brings us back to contingency and convergence as the third best option to answer Gould’s questions.
But what, exactly, did Gould have in mind with evolutionary contingency? And does the criticism from convergence really engage with his argument? These are the kinds of deeper questions that Powell explores in the first half of his book.
“Some, […] have interpreted contingency as meaning non-repeatability, others […] have equated it with unpredictability. Neither is correct, writes Powell.”
See, the problem is that Gould never clearly defined contingency, nor used it consistently. His idea remains, as philosophers put it, underspecified. Like others before him, part of Powell’s mission here is to piece together what Gould intended, based on a close reading of especially Wonderful Life and the magisterial The Structure of Evolutionary Theory. Laying bare the theoretical underpinnings of, as he dubs it, Gould’s “radical contingency thesis” is an incredibly instructive exercise. Also because it explains how Gould has been misinterpreted and why the critique from convergence often descends into “dismantling “straw man” versions of Gould’s thesis” (p. 101).
Some, prominently Simon Conway Morris and George McGhee, have interpreted contingency as meaning non-repeatability, others (myself included) have equated it with unpredictability. Neither is correct, writes Powell. Gould’s argument of contingency applied specifically to conditions in the early Cambrian: “small changes [here] would have led to a very different initial occupation of morphospace” (p. 43). But he acknowledged that, after this, many body plans became “developmentally entrenched”, something that has been confirmed by research in the field of evolutionary developmental biology, or evo-devo. Evolution prefers the easier option of reusing and repurposing existing structures. So, with time, the layers upon layers of interacting genes and regulatory networks laying out gross body morphology became too hot to touch for evolution, with mutations often being lethal.
Powell here makes a distinction between deep and shallow replays of the tape of life. The problem is that most examples of convergent evolution that are being wielded as overturning Gould’s contingency are of the shallow kind. Seen this way they are not that surprising, a point Jonathan Losos also raised in Improbable Destinies. According to Gould, convergent evolution cannot overcome developmentally entrenched body plans, their existence remains a matter of contingency—or so argues Powell. This points the way to the kinds of evidence required for a more convincing critique of contingency.
“Powell here makes a distinction between deep and shallow replays of the tape of life. […] most examples of convergent evolution […] are of the shallow kind.”
In my opinion, the first half of this book is obligatory reading for evolutionary biologists. The text will require your close attention, but it is engagingly written and incredibly rewarding. Do not be intimidated by the table of contents, but do expect to have to look up some words: I doubt that many outside of philosophy circles regularly speak of exegetical shortcomings or the “nomological vacuum of biology”.
The second part, which is more Powell’s personal challenge to Gouldian contingency, argues that, next to bodies, minds and cognition are highly convergent evolutionary outcomes. Partially for the sake of brevity, and partially because the first half is such a bombshell, I will be glossing over this second half. It is no less interesting, though it is the tougher part of the book, dealing as it does with slippery concepts such as cognition and consciousness.
Briefly, the starting point is the evolution of “image-forming sensory modalities”; a broad definition of vision that includes echo- and electrolocation. The ability of organisms to access real-time, detailed representations of their environment has been hailed by some as the kick-start for evolution, heralding the Cambrian explosion. It also led to the evolution of—borrowing and adapting the German term Umwelt from 19th-century biologist Jakob von Uexküll—Umweltian cognition and consciousness. Without going into the many subtleties Powell discusses here, this can be very coarsely defined as your “first-person portal on the world” (p. 212). This also entails fascinating discussions of the evolutionary history of brains and neurons, and behavioural evidence of sophisticated perception and cognition in invertebrates, specifically in cephalopods and insects.
Overall, I was very impressed with the depth and rigour of Contingency and Convergence. It provides the intellectual challenge and reward you would associate with this book series. Powell not only provides a valuable analysis of this debate but also challenges readers to engage better, both theoretically and empirically, with contingency and convergence in evolution. Without a doubt, this is a must-read for evolutionary biologists.
For those interested in second and third opinions, I recommend you also check out reviews by fellow bloggers bormgans and Jeroen Admiraal.
Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.
Other recommended books mentioned in this review:
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To outsiders, phylogenetics, the study of the evolutionary relationships between organisms, must seem like quicksand: the landscape is ever-changing and what you thought was solid ground can turn into contested and unstable territory overnight. Even so, we are getting an ever-clearer picture. In no small part this is due to new methods: the rapid technological progress in DNA sequencing has now made it both feasible and affordable to sequence whole genomes (all of a cell’s DNA) instead of selected genes for many taxa. And when you can bring multiple lines of evidence – morphological, developmental, genetic, and palaeontological – to bear on the question of evolutionary relationships, the resulting family trees become better supported and more credible. That is exactly what Gonzalo Giribet and Gregory Edgecombe, both experts in invertebrate biology and palaeontology, have done here in The Invertebrate Tree of Life – a work of dizzying scope since 96% of all known species are invertebrates. They have synthesized a truly monstrous amount of research to give an overview of our current thinking on invertebrate phylogeny, writing a new benchmark reference work for students of invertebrates.
The Invertebrate Tree of Life, written by Gonzalo Giribet and Gregory D. Edgecombe, published by Princeton University Press in February 2020 (hardback, 680 pages)
As the authors point out in their preface, it is exactly the pace of progress that makes this book relevant and gives it a reason to exist next to third editions of e.g. Nielsen’s Animal Evolution and Brusca’s Invertebrates – two works that have much inspired these authors. Those works date back to 2012 and 2016, respectively. The nearly 3,000 references here, packed in a 100-page bibliography, bring the reader bang up-to-date to mid-2019.
In 52 chapters, the authors navigate down the different branches of the invertebrate tree of life, for most groups summarising at least their systematics, genomics, and the fossil record where information is available. As they point out, that last one is often an either/or proposition. Either books discuss the biology of living invertebrates, or they focus on the fossil record. You might think that it is only invertebrates with shells or exoskeletons that would litter the fossil record. And they do. But several exceptional sites, so-called Lagerstätte, preserve invertebrate fossils in exquisite detail, revealing more than just the hard parts. Next to the well-known Burgess Shale in British Columbia, there is the Chengjiang biota found in the Chinese province of Yunnan.
For the better-studied groups, the authors provide additional information on morphology, physiology, neuroanatomy, reproduction, development, and life cycles. Given the scope of this work these sections are necessarily brief – this book is explicitly not intended as a complete textbook on invertebrate biology. But it will give you the basics with plenty of references to papers and books if you want to read more on, say organ systems or neuroanatomy. And if you want to go into the real technical details on any one group, the ongoing The Handbook of Zoology series published by De Gruyter is your next port of call.
“You might think that it is only invertebrates with shells or exoskeletons that would litter the fossil record. And they do. But several exceptional sites, so-called Lagerstätte, preserve invertebrate fossils in exquisite detail […]”
This brevity notwithstanding, there is plenty of absolutely fascinating material here even when invertebrate taxonomy is not your speciality. I knew that cnidarians have a complex life cycle, with free-swimming jellyfish alternating with sessile polyps. But wait, boloceroidid sea anemones can swallow their own tentacles that then grow into new polyps? Or what to think of all the invertebrate groups practising kleptocnidism: the stealing of the stinging nematocyst cells from Cnidaria, which are then used for their own defence? And what to make of the fact that in Platyhelminthes (flatworms) even a single cell can regenerate an entire animal?
The authors also do a good job balancing how much (or really, how little) information they provide for well-known groups such as arthropods and molluscs versus all the other lesser-known groups. Getting more familiar with the latter gives the reader a renewed appreciation of the diversity of life. So, yes, invertebrate organisms can be both bizarre 
and incredibly diverse, but communicating this is a but a secondary aim of this book.
The real value of The Invertebrate Tree of Life is the discussion of evolutionary relationships and how our thinking has shifted over time: how we used to think groups were related, what names have fallen by the wayside, and how other group names have come to encompass different groups. For example, Arthropoda used to include Onychophora (the velvet worm, pictured on the book’s cover), which is now considered a sister group. These shifts in nomenclature can make entering the literature a daunting prospect, so this historical review is invaluable.
“The real value of The Invertebrate Tree of Life is the discussion of evolutionary relationships and how our thinking has shifted over time […] These shifts in nomenclature can make entering the literature a daunting prospect, so this historical review is invaluable.”
Similarly, the authors provide a good overview of current competing schools of thought on evolutionary relationships. For example, some authors talk of the clade Neuralia (Cnidaria + Ctenophora + Bilateria, but excluding Porifera and Placozoa), while Giribet & Edgecombe support the clade Planulozoa (Cnidaria + Placozoa + Bilateria). Similarly, which groups are part of Xenacoelamorpha (a clade of mostly marine worms)? That depends on who you ask. They also guide the reader to areas of consensus, and the kinds of data in support of it. Take Nemertea (ribbon worms), which look like flatworms. The current consensus based on phylogenomic analyses places them closer to molluscs and annelids.
The book is illustrated with a selection of photos of extant and extinct species, detailed trees for the internal organisation of prominent groups such as molluscs and arthropods, and useful schematic drawings of morphology, anatomy, and life cycles of other notable groups. My only criticism is that, for a book that wishes to be a textbook for students, there is no glossary – the authors assume familiarity with terminology. Some of it is explained, but can you tell paraphyly from polyphyly, synapomorphy from plesiomorphy, or schizocoely from enterocoely? I admittedly had to look some of this up, 
and some diagrams in the introductory chapters to get everyone on the same page would have been useful. But this is a minor quibble.
The Invertebrate Tree of Life is an invaluable work for anyone entering or already working in the field of invertebrate evolution, taxonomy, and phylogenetics. By unlocking and reviewing a huge body of literature, identifying knowledge gaps, and providing a balanced overview of both current consensus views and disagreements, Giribet and Edgecombe provide an incredibly useful community service, making this work a benchmark for the future.
Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.
Other recommended books mentioned in this review:
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“Across the Bridge: Understanding the Origin of the Vertebrates“, written by Henry Gee, published by the University of Chicago Press in July 2018 (paperback, 312 pages)
Gee uses the first few chapters to bring the reader up to speed on what a vertebrate actually is, how evolution works and how scientists make hypotheses about relatedness, how certain genes (Hox genes and others) influence gross morphology, and how embryos develop. That last topic is a fluke of history, for scientists in the 18th and 19th century used to think that the way an embryo develops mirrors evolutionary progress from simple to complex organisms. It gave rise to that hackneyed expression that “ontogeny recapitulates phylogeny”, an expression inseparable from the name of German naturalist Ernst Haeckel. His drawings of developing embryos are iconic, contested, but ultimately not entirely wrong (See Haeckel’s Embryos: Images, Evolution, and Fraud for the definitive history of those, and my (long-winded) review of that book here).
The topic of embryology brings with it a lot of terminology that you may or may not remember from your zoology classes: the names of tissue layers (ecto-, endo-, and mesoderm), other structures (blastopore, coeloms, notochord), and processes (gastrulation, neurulation, enterocoely, and schizocoely). Luckily, Gee includes plenty of helpful schematic drawings.
Vertebrates, together with a number of invertebrate groups, belong to the deuterostomes, a distinction that is based on what an embryo develops first, the mouth or the anus. Adding more terminology, a substantial part of the book first reviews the morphology and development of those invertebrate groups closest to the vertebrates, including the echinoderms (think sea stars), hemichordates, tunicates, and Amphioxus. He then cuts the cake the other way and looks at typical vertebrate organs and features and asks what equivalents our closest invertebrate cousins have where, say, the heart, immune system, head, or brain is concerned.
“Would our thinking about evolution have such a close link with development if early naturalists had not been so interested in embryology?”
All this terminology notwithstanding, Gee does a good job of keeping his text as readable as can be reasonably expected. He inserts a touch of humour here and there. He makes observations in his footnotes that delighted both my inner five-year-old (comb jellies have several anuses!) and my inner 38-year-old (would our thinking about evolution have such a close link with development if early naturalists had not been so interested in embryology?). But most important and helpful: most chapters end with a summary. Who said that summaries should only be something found in student textbooks?
The other refreshing part of this book is how it corrects many potential misunderstandings about how evolution proceeds. There is no linear progression from simple to complex organisms. Some lineages evolve faster than others. Invertebrates today are not some frozen primitive snapshot of life hundreds of millions of years ago but have kept on evolving since they split off from the last common ancestor between us and them (you are more likely to have heard this in the context of the evolution of humans and other primates). And what can really throw a spanner in the works: organisms can lose complex traits again.
Technological advances over the last few decades have allowed detailed studies of genes and whole genomes, and have birthed the study of the evolutionary relatedness of genes between organisms – a discipline known as evolutionary developmental biology, or evo-devo for short (see Endless Forms Most Beautiful: The New Science of Evo Devo and the Making of the Animal Kingdom for an accessible introduction, and From Embryology to Evo-Devo: A History of Developmental Evolution for a deeper study). These advances, and the aforementioned spanner in the works (the loss of complex traits), have brought about at least one revolutionary change in our thinking which Gee relates here with gusto. Of all the invertebrate cousins described, tunicates (sessile or free-floating marine filter feeders such as sea squirts that look nothing like us) are more closely related to us than Amphioxus (segmented, vaguely fish-like invertebrates also known as lancelets that do resemble us). Closer inspection shows it makes sense, but also shows the remarkable evolutionary history of tunicates that have lost many traits. All this serves as a powerful reminder that evolution is far from a neat and tidy story of progression.
“Invertebrates today are not some frozen primitive snapshot of life hundreds of millions of years ago but have kept on evolving since they split off from the last common ancestor between us and them”
Gee closes out with a short look at fossils, which seems a bit of a non-starter for creatures that lack hard structures such as bones and teeth. The deuterostomes diversified in the Cambrian, between 541 to 485 million years ago, and there are a few localities on the planet that have yielded exceptional invertebrate fossils of the very distant ancestors of vertebrates and all the other deuterostome groups mentioned here so far. One of these is the Burgess Shale in British Columbia, made famous by Gould’s book Wonderful Life: The Burgess Shale and the Nature of History, but the Chengjiang biota of southern China is equally astounding, as documented in The Cambrian Fossils of Chengjiang, China: The Flowering of Early Animal Life.
If your interest in evolution goes well beyond extinct mammals and reptiles, you can handle a bit of zoological jargon, and Chordate Origins and Evolution: The Molecular Evolutionary Road to Vertebrates strikes you as too technical, then Across the Bridge is a wonderfully readable overview of how we vertebrates relate to our closest invertebrate cousins. Although much remains unclear and there is plenty of scope for further research, this book is a timely overview of the current state of knowledge on this neglected topic.
Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.
Other recommended books mentioned in this review:
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