Myths, mammals, and extinctions

It's interesting how the scientific version of urban legends can be incorporated into people's knowledge of how things work, and become so entwined that most folks don't even know which bits are true and which aren't.

Stephen Jay Gould riffed on this theme in his essay "The Case of the Creeping Fox Terrier Clone," which appeared first in Natural History and was later published in his essay collection Bully for Brontosaurus.  He looks at the claim that an early horse species, Hyracotherium, was "the size of a fox terrier" -- something that Gould found quoted in dozens of books on prehistoric animals (and which has therefore been used as a gauge of the animal's size in countless classrooms).  It turns out that it originated with a paleontologist, O. C. Marsh, who said Hyracotherium was the "size of a fox" -- a significant underestimate, as both foxes and fox terriers top out at around twenty pounds, and Hyracotherium weighed in at something closer to sixty.  But the analogy stuck, and people continued to pass it along without checking its veracity -- giving us the impression of tiny dog-sized horses, lo unto this very day.

Another example of this, from the same field, is that mammals were small, few in number, and low in biodiversity until along came a meteorite that for some reason selectively killed all the dinosaurs, leaving the mammals to throw a great big party and evolve like mad into the species we have around today.  This is incorrect on a variety of levels:

1. The K-T (Cretaceous/Tertiary) Extinction of 66 million years ago seems to have been caused by a double whammy -- the aforementioned meteorite, which left the Chicxulub Crater in what is now the Gulf of Mexico, and the formation of the Deccan Traps, a lava field from a colossal supervolcano eruption, all the way around the Earth in what is now India.
2. Dinosaur biodiversity had been decreasing for some time before the K-T Extinction, and in fact by some estimates was already down 40% from its peak during the mid-Cretaceous.
3. ...however...  All the dinosaurs didn't go extinct 66 million years ago, and I'm not talking about Nessie, Ogopogo, and Mokélé-Mbembe.  We still have dinosaurs around, we just call 'em birds.  The evidence is now incontrovertible.  Think about that next time you're putting out sunflower seeds for the chickadees.
4. The extinction hit pretty much every taxon that existed at the time.  The hardest-hit were large carnivores -- a vulnerable spot in the food chain at the best of times -- but no one escaped unscathed.  In fact, one group that got wiped out completely were the ammonites, a cephalopod mollusk that had thrived for 350 million years before getting clobbered during the K-T Extinction.
5. Most pertinent to this post, the mammals weren't just skulking around waiting for their opportunity; they'd been thriving alongside dinosaurs since the Triassic Period, 154 million years earlier.  This was the topic of a paper released a couple of months ago in Biology Letters by Tiago Bosisio Quental of the University of São Paulo and Mathias Pires of the University of Campinas.

What Quental and Pires did is a thorough survey of mammalian fossils, analyzing biodiversity as a function of time in three of the four big lineages of mammals -- Eutherians (most of the mammals you're familiar with), Metatherians (marsupials), and Multituberculates (an odd group of rodent-like mammals that were only distantly related to the rest of Class Mammalia, and which were one of the most common groups of mammals for almost two hundred million years).  They didn't include the fourth lineage -- the Monotremes, or egg-laying mammals -- only because they are extremely rare in the fossil record.

A late Cretaceous multituberculate, Catopsbaatar [Image licensed under the Creative Commons, Artwork by Bogusław Waksmundzki. Article by Zofia Kielan-Jaworowska and Jørn H. Hurum, Catopsbaatar, CC BY 2.0]

What they found -- predictably -- is that the dynamics of the extinction, and the years following it, is far more complex than it's usually represented.  "All these mass extinction episodes are heterogeneous," study co-author Pires said.  "They occurred for different reasons and unfolded in different ways.  Their impact on life forms was not absolute but relative.  Some groups suffered more, others less.  Some disappeared, while others took advantage of the new environmental conditions after the catastrophe to diversify rapidly."

Even within groups, the extinction didn't have uniform effects.  "Extinctions were concentrated among the specialized carnivorous metatherians and insectivorous eutherians," Pires said, "whereas more generalized eutherians and multituberculates survived and maintained higher diversity."

He added, "This means that studies of macroevolutionary phenomena focusing on broad taxonomic groups may miss a much richer macroevolutionary history, which can be perceived only at finer taxonomic scales."

Which can more generally be summed up as "the simple explanation is usually wrong."  It'd be nice if things weren't so complex, especially for we non-scientists.  But like Gould's fox-terrier-horse, many of these oversimplifications are flat-out incorrect -- and the truth is so much more interesting.

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This week's Skeptophilia book recommendation is one of personal significance to me -- Michael Pollan's latest book, How to Change Your Mind.  Pollan's phenomenal writing in tours de force like The Omnivore's Dilemma and The Botany of Desire shines through here, where he takes on a controversial topic -- the use of psychedelic drugs to treat depression and anxiety.

Hallucinogens like DMT, LSD, ketamine, and psilocybin have long been classified as schedule-1 drugs -- chemicals which are off limits even for research except by a rigorous and time-consuming approval process that seldom results in a thumbs-up.  As a result, most researchers in mood disorders haven't even considered them, looking instead at more conventional antidepressants and anxiolytics.  It's only recently that there's been renewed interest, when it was found that one administration of drugs like ketamine, under controlled conditions, was enough to alleviate intractable depression, not just for hours or days but for months.

Pollan looks at the subject from all angles -- the history of psychedelics and why they've been taboo for so long, the psychopharmacology of the substances themselves, and the people whose lives have been changed by them.  It's a fascinating read -- and I hope it generates a sea change in our attitudes toward chemicals that could help literally millions of people deal with disorders that can rob their lives of pleasure, satisfaction, and motivation.

[If you purchase the book from Amazon using the image/link below, part of the proceeds goes to supporting Skeptophilia!]