The Rise of Reptiles 320 Million Years of Evolution

Reptiles are an incredibly diverse animal group with a long and complex evolutionary history, conquering land, skies, and seas multiple times. Continued discoveries of both living reptiles and fossil material are adding more details and layers to the story of their evolution. A review of how they all relate to each other has been long overdue, and geologist and curator of Vertebrate Paleontology Hans-Dieter Sues here takes on that challenge. The resulting The Rise of Reptiles is a technical and heavily illustrated reference work for the serious zoologist and palaeontologist.

This book is part of an unofficial series. Over the years, Johns Hopkins University Press has published eight books with titles like this, each of which aims to offer a current picture of our thinking on the evolutionary relationships of the members, or clades, that make up these groups.

Taxonomists nowadays generally turn to phylogenetic classification to build family trees. Sues shortly introduces both the history and the logic behind this method, explaining how the shared presence of certain features is used to determine which groups descended from a common ancestor. These features can (and ideally do) encompass as much data as possible, including molecular, morphological, behavioural, and ecological traits. A keyword here, which explains why taxonomy is always in flux and why a book like this can only ever be a snapshot in time, is “hypothesis”. Taxonomists construct family or phylogenetic trees that are most likely given the available body of data. When the data changes, for example when we discover new fossils or start using a new source of information such as DNA, so does the likelihood of these family trees. Phylogenetic trees are thus constantly being revised, and often there are multiple possible interpretations of the available data.

When you want to elucidate the deep evolutionary history of (extinct) organisms, all you have are fossils remains. In the case of reptiles and other vertebrates that means skeletal morphology is your feature of choice. Sure, amazing fossils have been unearthed showing body outlines, skin impressions, and traces of hair and feathers, but these remain exceptional and rare. For palaeontologists, most material comprises fossilised skeletons that are incomplete and disarticulated (read: a jumbled mess).

So, the other important task of Sues’ short introduction is to familiarise the reader with the basics of skeletal morphology, especially of skulls, teeth, and vertebrae. There are useful illustrations here, and the book comes with a glossary, but to really get the most out of this book, you need to be familiar with the terminology – this is no pop-science book.

This becomes clear once you get to the bulk of the book: ten chapters over some 270 pages that very systematically go through the whole reptile family tree. These chapters consist almost exclusively of short sections that describe exactly what shared features of fossilised skeletons have been used to define certain groups and their relationships, and how opinions differ over what skeletal features are informative and should be included when building trees. Most group names are shown in phylogenetic diagrams throughout the book, though some names are left out (I am not sure why), and only comparison of text and image clarify what they refer to.

To give you an example of what to expect, take the description for Paripelvia, a group of marine reptiles:

"Paripelvia [...] comprises all post-Triassic euichthyosaurians along with a few Late Triassic taxa [...]. Motani (1999) listed several shared derived features for this clade: scapular blade straight; radius broader than long; and pubis more slender than ischium. C. Ji et al. (2015) used different synapomorphies including the absence of a dorsal lamina on the maxilla and the absence of a contact between the maxilla and external naris. [...]"

As I have said elsewhere, this is not intended as a criticism, but to give you an idea of the level this book is pitched at. This is the hard science, the raw material, the data and the analyses on which pop-science books (indirectly) rely.

However, if you are in that target audience, this book is a gold mine. Most palaeontologists and zoologists will only be intimately familiar with small parts of the reptile tree. There is simply too much information out there for any one person to master. So, to have Sues zoom out, synthesise vast quantities of data, and provide a big-picture view of reptile evolution is incredibly valuable.

But even if you are just a curious and adventurous reader (I include myself in that group), this book is informative and revealing. Three things stood out for me. First, the illustrations that grace this book: hundreds of colour photos of fossils and living reptiles, plus line drawings of skeletons. Unbelievable and stunning are two words that are fighting with each other to come out of my mouth first.

Second is that Sues stands on the shoulders of those who came before him (who in turn stood on the shoulders... etc.). For some groups there is, for now, a clear consensus (Gauthier et al.’s 300-page monograph Assembling the Squamate Tree of Life is frequently mentioned). For other groups, Sues has to draw on many smaller papers and on interpretations that sometimes disagree with each other. The reference section is very valuable if you want to dig deeper and need a starting point. Interesting side note: Baron et al.‘s 2017 Nature paper, proposing radical changes to the dinosaur family tree, is not yet taken as gospel and here mentioned as part of "the diversity of competing phylogenetic hypotheses [that] underscores our still inadequate understanding of the fossil record of early dinosaurs".

The last thing that struck me was the incredible diversity of reptiles past and present: Parareptilia, Testudinata (includes turtles), Sauropterygia (includes ichthyosaurs and other marine reptiles), Lepidosauromorpha (includes lizards and snakes), and Archosauromorpha (includes groups leading to both crocodiles (Pseudosuchia) and pterosaurs, dinosaurs and birds (Avemetatarsalia)). What a wondrous diversity to behold! Although some basic information on biology is included for all groups, there is no space for much detail, and Sues refers readers to other reference works.

For advanced students and professionals working in the fields of herpetology, evolutionary biology, vertebrate zoology, or palaeontology this technical reference work is a must-have. Overviews like these do not come along often as they require a tremendous amount of knowledge, time, and patience to write. Sues is to be commended for taking on this herculean effort, as is the publisher for supporting and publishing this kind of scholarship.