Carboniferous Giants and Mass Extinction The Late Paleozoic Ice Age World

Not so long ago, the idea that giant reptiles once roamed the earth was novel, unbelievable to some, but their reign represents only one part of deep time. Go back further in time, to the Carboniferous (358.9 to 298.9 million years ago), and you will find a world of giants as bizarre and otherworldly as the dinosaurs must have once seemed to us. A world where clubmoss trees grew up to 50 metres tall, with scorpions as large as dogs and flying insects the size of seagulls. With Carboniferous Giants and Mass Extinction, palaeobiologist George McGhee, Jr. presents a scholarly but fascinating overview of the rise and fall of this lost world, and why it still matters to us.

McGhee has devoted some 40 years to studying the late Paleozoic extinctions. He has previously authored The Late Devonian Mass Extinctions (1996) and When the Invasion of Land Failed (2013), and seeing how frequently he references the latter, the book under review and the 2013 title deserve close reading together.

McGhee first sets the stage, primarily focusing on the Devonian, Carboniferous, and Permian Periods. This was a time of repeated planetary ice ages lasting millions to tens of millions of years. What characterises this section, and the book as a whole, is that it goes beyond a popular science account of the consensus view. The resolution of the information conveyed, if you will, is higher and McGhee goes into the nitty-gritty: the different explanations, competing models, and different datasets obtained by different scholars working on this. This is science in all its messy, real-world, complicated glory.

After outlining the different ideas of how these long ice ages came about, the book takes a roughly chronological approach, dropping down to the resolution of geologic Ages. McGhee has included reference tables throughout the book, but if, like me, stratigraphy is not part of your background, names such as the Famennian, Visean, or Serpukhovian will be new. If you don’t want to constantly be flipping back and forth between pages, you might want to have a print-out of the latest chronostratigraphic chart at hand. This is obviously not a complaint, but it will give you an indication of the level this book is pitched at.

The world McGhee reveals here is strange indeed. With flowering and woody plants yet to evolve, the land was dominated by rainforests of giant lycophyte, horsetail, fern, and other trees. Some of their descendants are still with us, others have gone extinct. But not only were the species different, they grew differently too. As McGhee describes vividly, rather than dense canopies of leaves, Carboniferous rainforests looked more like fields of giant green stalks without branches, topped by spore-bearing organs. As opposed to modern long-lived trees, Carboniferous trees grew trunks once, dispersed their spores, and then died.

Important, too, was the rise in atmospheric oxygen levels (hyperoxia). Water locked in ice sheets led to a drop in sea levels that are thought to have exposed vast areas of land that were prime territory for these new rainforests. The onset of photosynthesis on a massive scale pumped oxygen into the atmosphere. One intriguing line of evidence McGhee mentions are huge charcoal deposits that indicate that Earth, for the first time in its history, started experiencing wildfires (see Andrew Scott's Burning Planet, he is an authority on this topic). At atmospheric oxygen levels of 30% (vs. our current 21%) fires burn much hotter, even consuming wet plant material. This and other factors here discussed resulted in some 90% of the world’s coal reserves being deposited during this period (hence the name Carboniferous).

Hyperoxia has also been proposed to have driven the observed animal gigantism. McGhee enthusiastically introduces the many super-sized versions of insects, scorpions, and millipedes; vertebrates that were distant ancestors of today’s amphibians, reptiles, and mammals; and giant marine invertebrates. The illustrations reveal these animals to often be five, ten, or more times bigger than their modern counterparts.

And then the party ended. A first crisis at the end of the Capitanian Age some 265 million years ago removed not so much a large number of species, but it did remove important players in the ecosystems. But that was just a prelude to the mother-of-all-extinction events at the end of the Permian. This has been written about by noted scientists such as Douglas Erwin (Extinction) and Michael Benton (When Life Nearly Died). But the science is regularly updated and McGhee provides a detailed overview of the latest findings, revealing just how apocalyptic this event was.

Not only did it involve the largest ever instance of continental flood basalt volcanism, forming the Siberian traps, but the rising plume of magma responsible for it erupted in a place where earth’s crust contained thick deposits rich in sulfur, bromide, and chloride-containing compounds, making the area, in McGhee’s words, a giant petrochemical bomb. The volumes of carbon dioxide and other gases emitted are given in the tens to hundreds of trillions of tonnes, resulting in a rapid temperature increase of an estimated 16°C. The proposed kill mechanisms for the extinction encompass everything from heat death and suffocation, to acid, carbon dioxide, and radiation poisoning. A hugely impoverished world resulted, but life squeezed through, just, and the world entered the Triassic Period and the age of the dinosaurs.

In his final chapter, McGhee explores the relevance of the late Paleozoic world to us now, little known as it is outside of academia. The huge coal deposits from this time enabled both the Industrial Revolution and subsequent anthropogenic climate change. But it also saw major innovations in the history of life: the invasion of land by animals; while the end-Permian extinction and the subsequent drop in oxygen levels may, some argue, have driven the evolution of four-chambered hearts and even warm-bloodedness. McGhee engages in an interesting bit of speculative alternative history, asking how life would have been different if the late Paleozoic ice age had not happened or if the end-Permian mass extinction had panned out differently.

Carboniferous Giants and Mass Extinction is a fascinating and well written scholarly overview of this exceptional period in deep history. The subject matter is technical, and readers will certainly benefit from having some background in palaeontology. But McGhee’s entertaining and captivating writing prevents this book from being a hardcore geology snooze-fest. If you want to know more about what came before the dinosaurs, this, and his other books, come highly recommended as overviews of the academic debates and developments.