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EMBRYOS IN DEEP TIME: THE ROCK RECORD OF BIOLOGICAL DEVELOPMENT by MarceloSánchez University of California Press, Berkeley, CA, 2012, 256 p, $39.95

Gerardo Antonio Cordero*, * Ecology, Evolution, and Organismal Biology, Iowa State University, Ames, Iowa

A phylogeny of characters or organisms is arguably a static view of evolution, as only adult stages are typically considered. In Embryos in Deep Time: The Rock Record of Biological Development (University of California Press), Marcelo Sánchez reminds us that organisms are subjected to dramatic anatomical and morphological transformations over the course of their lives, an insightful but often overlooked principle in evolutionary biology. Indeed, developmental processes and the history of life cannot be ignored if we seek to gain a complete understanding of the diversity of life on Earth—I believe this statement best summarizes the book. Together with a colorful definition of the role of paleontology in the context of understanding the evolution of developmental processes, Sánchez presents a comprehensive summary of cutting-edge evolutionary research in an academically rigorous but refreshingly accessible manner.

It is clear that macroevolutionary change in anatomy and morphology cannot be easily linked to genetic change. That is, “novel” phenotypes cannot always be explained by changes in allele frequencies. Which leads to the famous or infamous topic of “ontogeny recapitulates phylogeny” set forth by Ernst Haeckel. Sánchez points out that more people learned about the topic of evolution from the work of Haeckel in comparison to Charles Darwin during the 19th century. He then discusses the classical example of mammalian jaw articulation to illustrate Haeckel's concept and points out that “ontogeny recapitulates phylogeny” is rarely observed. Sánchez goes on to explain that organisms consist of integrated modules that are themselves subject to evolutionary change, but can also respond to changes in the environment (Chapter 2). It was positively surprising for Sánchez to emphasize the importance of developmental plasticity or environmental effects on ontogeny. He emphasizes that simple genotype-phenotype relationships promoted by classical evolutionary thought have often been shown to be incorrect across multiple levels of biological organization. Indeed, during development, genes inevitably interact with the environment, thereby affecting growth rates and differentiation of animal tissues. It is striking that even developmental plasticity can be inferred by means of the examination of fossilized animals.

Overall, considerable attention is paid to the biology of bone and cartilage tissues in postnatal individuals. Sánchez notes that examining embryogenesis is rare in paleontology given that mineralized animal tissues (i.e., bone, dental) tend to fossilize more effectively during postnatal stages (Chapter 3). Thus, most examples are not of fossilized embryos as alluded to by the title of the book. This reminds us that paleontology is, in some ways, limited. Nevertheless, the fossils discussed by Sánchez provide extraordinary glimpses into the past. These studies are not necessarily limited to answering questions related to skeletal development. Remarkably, fossils have helped to clarify the evolutionary origins of life-history traits such as viviparity and even parental care in vertebrate animals. Examination of fossils, in addition to answering traditional questions related to anatomy, histology, morphology, and taxonomy (Chapters 4 through 6), is highly informative to the study of ecology, physiology, and life-history evolution in vertebrates including humans (Chapter 9). But Sánchez does not forget to elaborate on the importance of invertebrate fossils (Chapter 10).

Perhaps some of the most noteworthy contributions of paleontology to evolutionary developmental biology come by means of the study of fossilized animals that mirror phenotypes in extant taxa. For instance, developmental genes studied in present day model systems could hypothetically explain phenotypic patterns observed in fossilized adult animals. Although such comparisons are feasible in few instances, this topic is arguably one of the most exciting points of focus in the book (Chapter 7). It is fascinating to imagine the function of homeobox genes during the morphogenesis of segmented skeletal structures (i.e., vertebral segments) in ancient organisms. It is tempting, as some have, to hypothesize that expression patterns of bone morphogenetic proteins, ectodysplasin, and fibroblast growth factors, played a role in the patterning of skeletal traits of animals that are only known to us as fossils. However, gene expression patterns in themselves are rarely enough evidence to decipher the evolution of novel body plans, as demonstrated by Sánchez's discussion on how the recent discovery of a “missing link” fossil provided key anatomical information to modern developmental studies on the body plan of turtles (Chapter 8).

Embryos in Deep Time: The Rock Record of Biological Development by Sánchez serves an excellent purpose in framing the importance of paleontology within modern evolutionary and developmental biology. However, it is clear that the links among these disciplines are not fully established. Some may argue that modern developmental biology strives to understand mechanisms (cell and molecular) that drive embryogenesis while placing less emphasis on postnatal growth and macroevolution. Study of fossilized ontogenies is primarily feasible during the postnatal stage, but it is astonishing when phenotypic patterns in anatomy or morphology are convergent with those studied in extant species. As our knowledge of developmental genetics of extant taxa continues to increase, we will soon be in a position where we can reconstruct hypothetical gene expression patterns in ancient convergent phenotypes. Such exceptional endeavors will undoubtedly depend on the discovery of fossils. Marcelo Sánchez uses a style that is accessible to general audiences to wonderfully illustrate this fact. Thus, the book is an excellent learning resource for undergraduate and graduate students alike.