Paleontological Profiles: Donald Prothero

This past fall my friends Julia and Neil were kind enough to obtain a signed copy of the new book Evolution: What the Fossils Say and Why It Matters for me from the annual SVP meeting. Although I had not heard of the author, Dr. Donald Prothero, prior to reading the book I was certainly impressed with the wide selection of subjects he ably covered in the text. Little did I know that he is been a prolific author and researcher, some of his other recent published work being After the Dinosaurs and The Evolution of Artiodactyls. From microfossils to mammals, Prothero has studied a wide variety of ancient organisms, and I was certainly pleased to have the opportunity to interview him for this series.

• [Brian Switek] How did you first become interested in paleontology? More specifically, what was it about ancient mammals that captured your interest?

[Donald Prothero] I'm one of those kids who got hooked on dinosaurs at age 4 and never grew up. I knew I wanted to be a paleontologist as soon as I knew what the term meant, although ancient mammals were just as cool as dinosaurs, as far as I was concerned. By 10th grade I was planning my collegiate career in paleontology. When I was an undergrad studying under Mike Woodburne at UC Riverside, it was apparent that there were many more opportunities for research in fossil mammals than in the overcrowded world of dinosaurs. Thus, I chose to study under Malcolm McKenna at the American Museum, and soon found all those amazing opportunities in the Frick Collection, especially with rhinos, horses, and camels.

• [Switek] It has often been said that the extinction of the non-avian dinosaurs cleared the way for the "rise of mammals," but the topic is usually given short shrift. This has sometimes led to some confusion about when mammals first evolved and what happened after the end-Cretaceous extinction; could you describe what mammals were like during the Mesozoic and their early evolution after the extinction of the non-avian dinosaurs?

[Prothero] Mammals and dinosaurs both first appeared in the Late Triassic. During the Mesozoic (130 million years of Jurassic and Cretaceous time, the first two-thirds of mammalian history) most mammals were shrew-sized, and even the largest were only cat-sized, although one specimen of a larger Mesozoic mammal apparently ate dinosaur babies! They mostly have teeth adapted for insectivorous diets. Once the non-avian dinosaurs vanished, mammals radiated rapidly in the Paleocene into a variety of herbivorous and carnivorous groups, and by the Eocene, all the modern orders were present, from bats to whales.

• [Switek] Speaking of the end-Cretaceous extinction event (during which many different groups of animals became extinct, not just the non-avian dinosaurs), there is still a lot of active debate over the cause of that extinction. Although there is hard evidence for extinction mechanisms like the impact of an asteroid at Chicxulub and intense volcanic activity in the Deccan Traps, arguments over which was more important are still being fought. What are some of the big questions that remain about how these events might be related to the end-Cretaceous extinction, and why might mammals have survived (and subsequently thrived) while other groups of organisms died out?

[Prothero] This topic is extensively discussed in my 2006 book, After the Dinosaurs: The Age of Mammals. In a nutshell, the big problem with the asteroid impact model is that it fails to explain how so many groups went through the KT extinction with no effect that can be explained by the "nuclear winter" of the impact: crocodilians, turtles, amphibians, lizards, snakes, birds, and only a slight change in the mammalian fauna. The attempt to dismiss this problem by claiming that they all hid in burrows or under water simply doesn't work, especially for crocodilians, which were larger than many dinosaurs. And if the "acid rain" scenarios were anywhere close to reality, there would be no amphibians on the planet today (since they are sensitive even to the slight acid rain we are now producing). The problem with the marine record is similar: many groups (especially corals, gastropods and bivalves, most groups of microfossils) march through without a major abrupt extinction that the asteroid might explain, and many more (ammonoids, marine reptiles) were in decline and may never have seen the rock from space. The inoceramids and rudistids were gone completely long before the impact. In short, a more comprehensive model invokes the Deccan eruptions as a long-term climatic stressor, with the impacts as a final coup-de-grace for a few groups.

Interestingly, although the geochemists and geophysicists think the asteroid model is sufficient, most paleontologists do not. In a recent poll, 75% of vertebrate paleontologists thought that the impact had little or no explanatory power for the KT extinctions.

• [Switek] Along with Scott Foss, you recently edited a volume called The Evolution of Artiodactyls. In the introduction to that work it is said that future works covering the same groups may have to be called "The Evolution of the Cetartiodactyla" and include whales. What is the relationship between artiodactyls and whales, and how did this understanding come about?

[Prothero] A number of people since the 1960s have noticed the similarities between primitive whales and artiodactyls. Many different cladistic analyses (Prothero et al., 1988; Novacek, 1991) have placed whales as close sister-taxa to artiodactyls. But the most recent discovery was the molecular evidence that places whales within artiodactyls (as sister group to the hippo-anthracothere clade), and the simultaneous discovery in 2001 by two different groups of scientists of two different primitive whales that had the characteristic "double-pulley" astragalus of artiodactyls. As you will see in our new book, that revolution in understanding swept through paleontology very quickly and with little resistance, since so much evidence converged on a common answer so quickly.

• [Switek] Another book that you recently published was Evolution: What the Fossils Say and Why It Matters. The book is filled with evidence for evolution from the fossil record, although it also tackles creationist arguments throughout. With so many books about the evolution/creationism issue on the market, what makes this book different? What were you trying to get at with this book that had not been done elsewhere?

[Prothero] Nearly all the other books about creationism focus strictly on the philosophical or political issues, or on conventional Neo-Darwinian biology, but none have up-to-date discussions of all the evidence from paleontology, especially with the wealth of new fossil "transitional forms" that have been discovered in the past 20 years (and completely ignored by creationists, who attack only old out-of-date sources). That's why 2/3 of the book focuses on transitional fossils, or clarifies common misunderstandings about fossils not only by creationists, but also by legitimate scientists who have not kept up with all the revolutionary developments in paleontology of the past few decades.

• [Switek] One of the questions that continuously comes up on this blog involves the clade of mammals called the Afrotheria, specifically how can aardvarks, tenrecs, and elephants (just to name a few members) all form a monophyletic group? From what I understand there is some amount of disagreement over whether Afrotheria is a valid clade or not; could explain what the current arguments involving this group center on?

[Prothero] Afrotheria is a clade based entirely on molecular evidence. There is no supporting evidence from paleontology or morphology, so far as I know. It includes the tethytheres (proboscideans and sirenians) plus some African groups like hyraxes, golden moles, tenrecs, etc. Most paleontologists remain skeptical of a group that has no anatomical shared derived characters, but the molecular people have been very loud and persistent and not heavily resisted by paleontologists, so they have all the books following them right now. We'll see...

• [Switek] Some of my favorite extinct mammals are members of the Dinocerata, specifically the knobby-headed and saber-toothed Uintatherium. It may be a little self-indulgent of me to ask, but how are members of this group related to other mammals? What group of mammals did they evolve from?

[Prothero] STILL no consensus on this. You can look at the Lucas and Schoch chapter in the Janis (1998) volume for a quick summary, but there's no overwhelming anatomical evidence in support of any one group of animals as their sister group. Yes, there are STILL unsolved mysteries in our business!

• [Switek] Although extinct mammals are certainly fascinating, they often don't get as much attention as do dinosaurs. As a result it seems that there is still much to be learned about ancient mammals, even involving groups that have been known for some time. What are some of the groups that have been neglected, and how might study of these groups help fill in our knowledge of the mammal family tree?

[Prothero] Virtually all fossil mammals other than sabertooths, mammoths, and a few others, are neglected and unknown to the general public. My particular favorite is the evolution of fossil rhinos, which have a record just as complete and diverse as the evolution of the horse, but have never gotten the publicity they deserve. Or the evolution of camels, which is likewise just as good as the horse story, but virtually unpublicized.

• [Switek]During the course of your career you've done a lot of work on a number of different groups of animals, ranging from radiolarians to dinosaurs to mammals. Among the groups you have studied, do you have a particular favorite genus or family of creatures that you find especially fascinating?

[Prothero] I got started on rhinos pretty early in my grad career, and they remain my favorites after 25 years of working on them. I also love camels, and they are the subject of my next big project.

• [Switek]Finally, are there any particular paleontologists (past or present) that have served as an inspiration as you have pursued your own career?

[Prothero] My undergrad advisor, Mike Woodburne, got me off to a good start, and I was definitely inspired by my grad advisor, Malcolm McKenna. However, the most influential of all was my friend Earl Manning, who was only a scientific assistant at the AMNH when I started, but knew more about fossil mammals that any person I've ever met. He could identify an isolated foot bone of almost any taxon at sight, and was bold enough to make cladograms of the entire Mammalia long before anyone else tried (1976, 1977). He was unjustly treated at the AMNH, and has suffered a lot of hard knocks since then, but he DID finally finish his Ph.D. at Tulane last year. I still ask his advice when I want comments from someone who really knows their fossils and will make the effort to critique your paper in detail.