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.
Dr Prothero is a man of many talents. He co-wrote the introductory textbook Sedimentary Geology : an introduction to sedimentary rocks and stratigraphy that hooked me into specializing in sedimentology in my graduate geology work.
You can download a chapter and some plates of the mentioned book "Evolution: What the Fossils Say and Why It Matters" for eva(o)luation purpose from the homepage of Columbia University Press:
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.
Also, crocodiles depend on their mothers only for a short time. You can kill off all adults, and the next generation grows up anyway.
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).
As far as I can tell, this overlooks the fact that water in areas with limestone underground is buffered, and the fact that some lissamphibians did die out, for example the gracile-snouted Albanerpeton clade and the noterpetontid ?sirenids.
I also can't see how acid rain could be avoided given an impact on carbonates and sulfates. Evidence like etched sand grains exists, too.
many more (ammonoids, marine reptiles) were in decline
There has been a persistent trend since the 1980s: the more closely people look, the shorter and steeper this decline becomes. We seem to be looking at the Signor-Lipps effect even in those cases where the decline hasn't vanished altogether yet.
In short, a more comprehensive model invokes the Deccan eruptions as a long-term climatic stressor
But how is this possible, when the main phase of the eruptions was over 100,000 years before the boundary and produced nothing but a transient increase in the global average temperature?
Peter Wilf, Kirk R. Johnson & Brian T. Huber: Correlated terrestrial and marine evidence for global climate changes before mass extinction at the Cretaceous-Paleogene boundary, PNAS 100(2), 599 -- 604 (21 January 2003)
G. Ravizza & B. Peucker-Ehrenbrink: Chemostratigraphic evidence of Deccan Volcanism from the Marine Osmium Isotope Record, Science 302, 1392 -- 1395 (21 November 2003)
Afrotheria is a clade based entirely on molecular evidence. There is no supporting evidence from paleontology or morphology, so far as I know.
It's not that bad. Two or three morphological characters have been proposed as autapomorphies of Afrotheria; there's the name-giving biogeography; Eocene hyraxes look considerably less like perissodactyls than extant hyraxes do; the idea of a Glires-Macroscelidea relationship has been replaced by a more Afrotheria-like arrangement in recent morphological analyses (of which the biggest is the Maelestes paper); and the latest JVP shows that the golden moles and the tenrecoids evolved their zalambdodont teeth independently from each other and from the solenodontids, thus removing one of the few characters that had held them in Lipotyphla.
the molecular people have been very loud and persistent
No wonder. All afrotheres have LINE insertions in the exact same places -- that's a character that is almost completely immune to convergence. And that's just the most dramatic piece of evidence. Molecularly, the clade is really robust.
Yes, there are STILL unsolved mysteries in our business!
The biggest reason for this particular mystery is:
1) there still is no all-encompassing morphological cladistic analysis of Placentalia and all the possible placentals of the Paleogene;
2) to make such an analysis would be a complete PhD thesis, or two, or perhaps three. The Maelestes paper was a good start, but nothing more than that. (And it didn't order any characters. Moan, groan.)
Anyway! I'd like to read more on the rhino and camel "stories"! :-)
Concerning the marine extinctions, the impact predicts the sterilization of the uppermost layer of the oceans, and this is probably the only way how to kill all ammonites (they had planktonic larvae called ammonitellae, and famously survived the Frasnian-Famennian, Devonian-Carboniferous, Permian-Triassic, and Triassic-Jurassic mass extinctions, even though they took heavy losses). This also fits the fact that the nautiloids and the coleoids, which don't have planktonic larvae, did not die out.
I laughed when I read the Afrotheria question, Brian. What a great interview. I love how diverse your interviewees are. I like the dinotheres too, but they never get much press. I wasn't aware that rhinos and camels had such complete fossil records, although I shouldn't be surprised. Many mammal groups have great records.
This is a question for Dr. Prothero: Why do you think sabertooths, mammoths, and a few other beasties are so popular in public perception? Is it just because they were around so recently, being hunted by our ancestors? It seems to me there are dozens of MORE interesting mammalian groups (like whales! Or elephants! Or desmostylids!).
I might be a little late, but this is an excellent interview (again), so I think I add my two cents.
> and even the largest were only cat-sized
> They mostly have teeth adapted for insectivorous diets.
Most species, yes; but judging from modern conditions*, the gnawers, like haramyids, gondwies and above all the multis might have outnumbered the insectivorous mammaliforms on the individual level.
> [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?
For its sheer coolness factor - and probably a bit of euarchontogliran chauvinism - I back the anagalid theory, although I have to admit that the evidence is pretty thin.
> I like the dinotheres too
dinotheres or dinoceratans? Both are among the more interisting and charismatic prehistoric** mammals.
* wich might not be identical with mesozoic ones. We have grasses
today, and they are cultivated by humans, this favours gnawers.
And some of the insectivorous niches held by mammals or at least
mammaliforms in the mesozoic are held by squamates and avian dino-
** Well, the prehistoric part might be questionable when it comes to
the dinotheres. There are cryptids...
You may find interesting a proposal as to why the herbivore dinosaurs declined in armor, teeth, and quite a bit in bony structure across the Cretaceous. I suggest it was due to a phosphorus famine created by erosion of the soil of the runways of plant smothering termites. You may see this discussed in more detail starting in http://www.angelfire.com/nc/isoptera/termites.html and its links, which links explore the possible affect that ant evolution had upon them.
I saw the discussion of dinosaur predators on the history channel. They portrayed the dinosaurs as holding their bodies horizontally and their legs vertical. I doubt very much if they were stupid enough to hold their bodies horizontally, but they could not hold their legs vertically. If they tried to do so, they would fall forward and either break their thin ribs or break their fall with that big head, because their center of gravity is forward of their hips. Of course they could walk horizontally if they thrust their hips backward. You can verify this by bending forward at the hips (but I doubt if you have any intention of walking this way or that they did).
It is conceivable that you would also find interesting a hypothesis of my son explaining the Decca (or Deccan) lava flows as disruption of the crust by the Yucatan meteor impact in http://charles_w.tripod.com/decca_traps.pdf .
Sincerely, Charles Weber
try as i might, nothing comes up when looking for "camel evolution." why?
amazing.....amazed to see the insatiable passion of prothero.....he have really indulged me in paleontology for the past few months.....i really admire the interview and the great part i felt was when i got my question put up about the afrotheria(was having headache when i found tethyteria and afrotheria together)....wohooo....sir u rock....:)
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.