Monday, November 29, 2021

Mammals All The Way Down

Well, not quite all the way down.  Still, mammals and their immediate ancestors have been in the vertebrate mix for a very, very long time.  Fossils of true mammals date back to the Triassic Period (252-201 million years ago).  True dinosaurs make their appearance at the same time.  I suspect the roots of the mammalian public relations problem lies in that coincident arrival of mammals and dinosaurs on the scene.  What’s the PR issue that mammals face?  In particular in the Mesozoic Era (comprising the Triassic, Jurassic, and Cretaceous Periods – 252 to 66 million years ago), to paraphrase comedian Rodney Dangerfield:  “Mammals don’t get no respect.”

Paleontologist Else Panciroli wants to remedy that and give mammals in that era their due.  Her new book is titled Beasts Before Us:  The Untold Story of Mammal Origins and Evolution (2021) and it’s an engrossing read.  She’s an amiable and funny companion in the journey across deep time tracing the appearance and evolutionary development of mammals from before and through the Mesozoic.  Although she on occasion appears in the narrative doing her work as a paleontologist, she avoids the trap in popular science writing – making herself the hero of the story.  (I’ve complained about this in previous posts on this blog.)

It's best to begin with the typical narrative about mammalian development in the Mesozoic.  In this version of the story, at the beginning of the era in the Triassic, early mammals and early dinosaurs are merely extras in the cast of vertebrate characters.  Through a great deal of luck after a period of environmental distress, dinosaurs come center stage as the storyline takes us into the Jurassic; dinosaurs dominate the scene through to the end of the Cretaceous.  A physiological arms race between plant-eating dinosaurs and predator dinosaurs drives each dinosaur group to become bigger and bigger.  Mammals, given only brief lines in the text, are small and seemingly quiescent extras, trying to go unnoticed by the thundering reptilian stars.  Only when the Cretaceous ends and non-avian dinosaurs are banished from the story, are mammals free to assume a central place.  (This plot arc is reprised often, including quite recently in geologist Andrew H. Knoll’s A Brief History of Earth:  Four Billion Years in Eight Chapters (2021), a superb book, no matter its treatment of Mesozoic mammals.)

The storyline I’ve just recounted is not wrong in its broad sweep.  But, after reading Panciroli’s volume, I recognize that it comes up short in its nearly sole emphasis on dinosaurs, not acknowledging the dynamic evolutionary action and experimentation taking place among those small animals just off center stage.  Panciroli makes it abundantly clear that marvels were being fashioned in those shadows.  That’s a central part of the “untold story” she has been moved to tell.

If you believed mammals – we fur-ball milk-givers – merely scooted underfoot like terrified snacks for most of the time of the dinosaurs, you are dead wrong.  If you have always repeated the tale that mammals come from reptile stock, wash your mouth out.  We mammals are a lineage all our own.  Our branch stretches away from the others, tied through time by our anatomy and physiology to the first backboned animals on land.  Long ago we struck out in a flowerless Eden, and we made good.  (p. 11 of the print version of the book)

Panciroli traces the evolution of mammals and their ancestors from the tetrapods that, in the Late Devonian Period (perhaps around 360 million years ago), first emerged from the water to live on land.  The path runs through myriad groups, from the synapsids to the therapsids to the cynodonts and so on.  She describes how and when some of those characteristics that have ultimately come to distinguish mammals first appeared.  (Although many core traits of mammals are identified in the course of the narrative, these are not (as far I found) pulled together in the book in a single, comprehensive statement of what defines a mammal.)  

The roots of some of those characteristics lie farther back in time than I realized.  Panciroli sees that, during the Permian Period (299 to 252 million years ago), mammal ancestors were major players, experimenting with different modes of earning a living, from herbivory to “hyper-carnivory,” even as some were acquiring the “key traits we associate with mammals, including warmer blood, higher energy lifestyles, and perhaps even fur.”  (p. 97)

Though the mass extinction at the end of the Permian knocked mammalian ancestors for a loop as it did most animals, it was during the Mesozoic Era when early mammals came into their own.  As Panciroli makes clear, for mammals size mattered, but not in the salacious way (wink, wink, nudge, nudge) comics would have it.  Paleontologists, she notes, long equated large body size with evolutionary success.

Mammals, we are told, did nothing during the time of dinosaurs, so we skip over them in our narrative of life.  Most books on mammal evolution only begin when they are finally free of their masters, and get bigger.  Because bigger is always better.  Right?  (p. 162)

Here Panciroli is making her strikingly revisionist argument regarding mammals, arguing that during the Mesozoic they were a significant source of evolutionary experimentation and development.  Central to her argument is the power of that attribute that has long been held against them, their small size.

The first mammals of the Late Triassic and Early Jurassic were pioneers.  They did something no dinosaur could do:  they shrank and exploited an untapped night-time niche.  To this day, of the 5,500 or so small species alive on Earth, 90 per cent are small-bodied, most of them rodents.  The median body mass for mammals today is less than 1kg (2.2lb).” (p. 170)

Rather than a handicap, the shrinking of early mammalian bodies (quite a useful development in a world with monstrously large reptiles) actually spurred the evolution of profoundly significant physiological traits boasted by mammals.  Equipped with those attributes first emerging in the Permian (a faster metabolism and warmer blood), these shrinking animals became nocturnal (their metabolism enabling them to function in the cold night) and evolved eyes that could see particularly well in low light.  The majority of mammals today are nocturnal.

Further, as mammals shrank, their ability to smell and hear was enhanced.  Panciroli writes, “Although most animals can smell and hear, mammals are among the smelliest and noisiest.”  (p. 165)  There is a direct relationship between size and these attributes.  For instance, Panciroli describes how smallness spurred development of the effective and unique mammalian ear structure, particularly with regard to the bones in the middle ear.  In a diminutive jaw, a strong bite could be accomplished more easily and, as result, several bones at the back of the jaw were no longer necessary.  These were repurposed by natural selection to be incorporated in the mammalian ear to enhance hearing.

The smaller mammalian jaw carried on the modifications in dentition that these animals had been experimenting with for a long time.  Mammalian teeth increasingly varied in the same jaw:  canines, incisors, premolars, and molars were structured to deal with food in different ways.  Further, teeth were increasingly likely to be replaced only twice, instead of continuously so that occlusion between upper and lower teeth was enhanced.

The combination of these attributes in these small animals, according to Panciroli, had another, even more profound effect.

Putting their newfound bite to good use, mammals became the scourges of the insect world.  Hunting in the relative safety of night with senses enhanced, the first mammals grew bigger brains, paving the way for increasingly complex social interaction and behaviour.  (p. 171)

The book is a corrective in more ways than I’ve described above.  Yes, Panciroli’s primary objective is to rescue the early history of mammals, but she also seeks to set several other records straight.  To wit, paleontology, long dominated by white men who have imposed their stamp on the field, has been hostile to women, failing to acknowledge their accomplishments or opening its ranks sufficiently to them.  In the course of laying out the real history of the development of mammals, she highlights the many contributions of women, from Jennifer Clack (whose work has been described in a previous post) to Zofia Kielan-Jaworowska who, Panciroli writes, “should be remembered as one of the greatest scientists of all time.”  (p. 268)  Further, the book also offers an accessible tutorial on modern paleontological methods, from the extensive use of statistical analysis of big data sets to the application of advanced technologies, such as CT scans.  Clearly, it’s not all field work.

If there is a geographical hero of this book, it’s Scotland and the Isle of Skye.  Much of Panciroli’s own work has been on the early mammal fossils of Scotland and she describes several fossil hunting trips there.  It’s clearly mostly cold, wet work.  The Scottish mammal fossils are tiny (fossil hunters spend lots of time on their knees) and typically embedded in matrix.  As a result, current study of those fossils owes a great deal to the application of those changing paleontological methods.

I’ll end with a critical note (perhaps more of my reading than of the book) and a warning.  The critical note is that, despite the overall chronological organization of the book (from deep time to the present), I found it quite easy to get lost, unsure of where we were in the story and who the characters were.  Quite frequently, Panciroli observes, parenthetically, that some subject or person she’s just mentioned will appear later in the book.  Perhaps this simply reflects the fits and starts of the actual evolution of mammals (as she frequently stresses, evolution doesn’t have an endgame in mind) and the course of paleontological work.  Nevertheless, I did lose my way at times, but the grace of her writing and the power of the narrative pulled me through.

The warning is this:  Don’t read this book in the Kindle version.  My first reading of it was Kindle-based.  Only when I came to research this post and acquired the print version, did I discover the book’s beautiful and informative photographs, and, most importantly, the endpapers that provide a wonderfully useful cladogram showing graphically the relationships of most of the major taxa that appear in this story.  Sadly, all of that is missing from the Kindle version.

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