Birds Above All Else

There is an exchange between two characters in Tom Stoppard’s mind-bending play Arcadia (1993) that captures in a clever way the essence of science. In Act Two, Scene VII, two characters in the present day on the Coverly estate are discussing events that may or may not have occurred there in 1809. Hannah Jarvis, an author, is skeptical of the conclusions being advanced by another character, an academic, regarding what happened over a century ago. Valentine Coverly, a graduate student in mathematics, is the scion of the Coverly family that owned, and still owns, the estate.

Valentine It may all prove to be true.

Hannah It can’t prove to be true, it can only not prove to be false yet.

Valentine (pleased) Just like science.

Hannah’s statement, with its double negatives, does address a core tension in science: no conclusion, no matter how thoroughly tested, is above challenge. As Carl Sagan wrote, in The Demon-Haunted World: Science as a Candle in the Dark (1996), “Except in pure mathematics nothing is known for certain (although much is certainly false).”

But there are some propositions in science that have moved beyond Hannah’s cautionary statement into the realm of nearly universally accepted scientific facts. Evolution through natural selection is one of them. The subject of paleontologist Steve Brusatte’s masterful new book, The Story of Birds: A New History from Their Dinosaur Origins to the Present (2026) should be another, if it isn't already. Brusatte writes:

Birds are dinosaurs.

This realization is one of the greatest achievements in the history of paleontology.

(I read an e-book version of this book so I won’t be providing page numbers for quotations.)

The book reaches two conclusions: birds are dinosaurs, and birds are marvelous creatures. It is a heady and graceful work, serving not just the fossil lover, but the bird lover as well. It's also rich fare for someone in search of how science has been done in the past and is done now. Basically, it's a good read.

That he is completely smitten by birds, ancient and present, is, if possible, an understatement. To offer just one example of the language he uses when extolling birds, early on he writes that birds are

some of the most unusual, the best adapted, the most fine-tuned animals that live today, and that have ever lived, in the 4.5-billion-year history of the Earth.

Though Brusatte describes his personal connections to this work, he is restrained and does not fall victim to the temptation, succumbed to in much popular science writing, of becoming the hero of the story.

Undoubtedly and deservedly so, Archaeopteryx is the hero of the paleontology side of the story that Brusatte tells. The fossils of this species, first discovered in Germany in the early 1860s, show that the animal sported feathers. It is considered the oldest bird, dating from the late Jurassic (about 150 million years ago). The picture below shows the so-called “Berlin” fossil of the species. (This image is in the public domain one of the several fossils of the species. It is published under the Creative Commons Attribution-Share Alike 3.0 Unported license.)

There are human heroes in this story, as well. Though Brusatte profiles many of the scientists who have done significant, groundbreaking work exploring the evolution, development, and family ties of birds, I am most taken by two who were involved in either first enunciating that birds are dinosaurs, or reviving that position after years of willful neglect: biologist and Darwin champion Thomas Henry Huxley (1825-1895) and paleontologist John Ostrom (1928-2005).

Huxley is a fascinating personality, faithful to his working class origins and dogged in his commitment to what he perceived as true. Brusatte highlights the bold steps Huxley took in lectures in the 1860s to lay out why he believed: "Aves (birds) has its root in the dinosaurian reptiles." The key elements to his argument were the combination of bird and dinosaur attributes that he detected in two recent fossils discoveries; these approached the issue from different directions. First, there was the earliest bird, Archaeopteryx, which featured some characteristics of reptiles, and, then, there was Compsognathus, a reptile also from the late Jurassic that sported some avian features. Huxley was brilliant in building his argument to a striking and irresistible conclusion.

Brusatte describes how, in time, this conclusion about the relationship of birds to dinosaurs fell out of favor and faded into the background. Some hundred years after Huxley had staked this claim, paleontologist John Ostrom revived it in a profound way. Ostrom had found and named Deinonychus, a coelurosaur theropod dinosaur that lived during the early to late Cretaceous (115 to 98 million years ago). Ostrom argued that the coelurosaurs, unlike their lumbering, huge brethren (think T. rex), were small, agile, and quick predators. They were also light weight, given that their bones were hollow. (Coelo, the Greek root, for their genus name means “hollow,” and, by the way, birds have hollow bones.)

Ostrom knew the coelurosaurs from his find and others which dated back to the Jurassic.  After closely analyzing the four Archaeopteryx fossils known at the time, he came to a profound conclusion. In a brief, one-page article, The Ancestry of Birds, which appeared in the March 9, 1973, issue of Nature, Ostrom listed the 21 specific morphological attributes of Archaeopteryx that he posited were shared by coelurosaurs. He introduced the list by writing:

The following coelurosaurian features of Archaeopteryx collectively are here considered as prima facie evidence of a coelurosaurian (Theropoda) ancestry of birds.

He concluded that Archaeopteryx had gained these features through "direct inheritance from a small coelurosaurian ancestor."

Of Ostrom's article, Brusatte writes:

It is probably the single most important page ever written about dinosaurs.

He summarizes what Ostrom was demonstrating as follows:

Birds are a type of coelurosaur, which is a type of theropod, which is one of many types of dinosaurs.

In support of the book’s central conclusion, Brusatte delineates in detail how the pedigree of many typical bird traits and features is actually dinosaurian in nature. These traits range from hollow bones, to feathers, to nesting, to the laying of colorful, hard-shelled eggs, to an efficient respiratory system, and to relatively small size (true of many dinosaurs), as well.

Of course, what possibly most marks an animal as a bird is its ability to fly. Archaeopteryx, the most primitive bird, was a flyer, but only a crude one. Brusatte outlines how flight probably arose and, most importantly, uses this as one of several powerful examples in the book to illustrate a central aspect of evolution. A feature that arises for one purpose may, over time, come to serve another, different one. Flight is a clear example of this. Feathers, so critical to birds' flight, arose among dinosaurs, not for flight, but possibly initially as "sensory devices," and then for insulation. Indeed, Brusatte calls feathers a "default condition" for dinosaurs (think, hair for mammals). In this case, over time, evolution repurposed the early feathers into an essential, powerful component of a constellation of features, including the forearms evolving into wings, that would be necessary for birds to take flight. Light weight from hollow bones clearly contributed.

If one were foolish enough to skip the paleontological heart of this book with its treatment of how and why birds evolved and, instead, just read the last chapter, it would still be worth the price of admission. That chapter, titled Marvelous Modern Birds: Intelligence, Cognition, Birdsong, and How Birds Are Still Evolving Today, is a précis of much that makes extant birds so universally fascinating. In this chapter, Brusatte makes good on his assertion that "birds are among the smartest, cleverest, and most cognitively advanced animals on the planet." Some of today's birds, he shows, think about the future, engage in abstract thought, fashion tools, among other impressive mental and physical feats. I was particularly taken with the oscines, the songbirds, whose songs are not innate or instinctive, but are, instead, learned by immature birds from their elders. They are "vocal learners," no mean feat, considering the complexity of many of their songs.

Finally, birds have faced dramatic threats to their survival from their origins, threats that have included such events as the mass destruction at the end of the Cretaceous, and never-ending challenges from climate change. Sadly, in recent centuries, the impact of Homo sapiens on these dinosaurs has been profound, with many species lost to hunting and wanton destruction of the environment, and billions of individuals killed by the domestic cats we seem to be unable to keep indoors (but, despite claims to the contrary, only a piddling number are lost to wind turbines). Brusatte demonstrates how birds have endured these challenges, showing that they are truly survivors. He writes:

The future is uncertain. And paleontologists like me are always more comfortable looking to the past than predicting what might lie ahead. But as the world changes rapidly around us — because of us — I am willing to make a bet. The Age of Birds will persist, even if the Age of Humans does not.

The Surprising Story of Life on Earth

The inexorable movement of Earth’s tectonic plates, repeatedly creating massive continents and then tearing them apart, is an underlying plotline to the story of life on this planet.  Since the first appearance of life, a mere 100 million years after the birth of the planet 4.6 billion years ago, the influence of continental creation and destruction on the environment and, thus, on the fortunes of life has been profound.

In his relatively recent book, paleontologist Henry Gee, a senior editor at Nature, tells that part of the story well, but goes much further, painting with broad brush strokes a narrative of how life has evolved and survived, amid repeated extinction events.  The narrative he penned is informed and insightful, marked by humor and a steady stream of surprises.  This is a book that moves easily and quickly through the billions of years it covers.  Not for nothing did Gee title it:  A (Very) Short History of Life on Earth:  4.6 Billion Years in 12 Pithy Chapters (published in 2021).

Early on, he writes, “Perhaps the most amazing thing about life – apart from its very existence – is how quickly it began.”  (p. 5)  It strikes me that an ineluctable drive to exist undergirds life’s almost immediate appearance on the new planet, and that this also helps to explain why the history of life is the history of responses to change.  It’s a call and response pattern whereby life survives repeated episodes of devastation, episodes that Gee often ties to the creation of massive continents (e.g., creation of Pangea completed during the Permian) and to the breakup of those massive continents (e.g., breakup of Pangea beginning at the tail end of Triassic), all having profound effects on climate and habitable areas.

Evolution is part and parcel of this ebb and flow pattern.  Gee’s account offers up many examples of evolutionary change sparking still more change, all in the service of survival.  His narrative of the Cambrian is one of predator and prey engaged in a dance whereby new methods of attack spark new methods of defense which in turn . . . .   For instance, in the Cambrian, descendants of Saccorhytus (a very early enigmatic animal) chose not to build armor, but, instead, to evolve a means of escape though a “swishing tail.”  (p. 31)  This notochord would evolve into the spinal cord.  With the evolution of a spine came a cascade of changes, allowing animals to become more complex and bigger.  “All vertebrates are visible to the naked eye.”  (p. 36)  Of course, evolution doesn’t let a useful invention go to waste, and many vertebrates did evolve armor.

As I mentioned, Gee’s account is full of surprises.  One of the most startling, to me, was that of the import of the anus.  Who knew that the evolution of the anus in the Precambrian would have such far reaching effects?  He describes how plankton had simply let wastes flow everywhere out of their bodies, feeding oxygen-consumers in the water column.  But, once a discrete anus emerged “in some species of otherwise undistinguished worms,” the result was “a revolution in the biosphere.”  (p. 18)  In his telling, poop in concentrated packets was expelled from the anus and settled to the ocean floor, drawing the oxygen-consumers to the bottom, with the result that “the seas, once turbid and stagnant, became clearer and still richer in oxygen – enough to enable the evolution of larger life forms.”  (p. 18)

But, wait, that’s not all.  The anus in time lead to orientation to animals with a mouth at one end and an anus at the other, it became “a distinct direction of travel.”  And this led, eventually, to an arms race as animals began eating one another.  Yes, there is a leap there, but Gee is such a gifted writer that the reader, well, this reader, is happy to make that jump with him.

His is a very rich account of life on Earth, and it’s also idiosyncratic, each of its 12 chapters is centered on one or a few developments that, in Gee’s eye, are central to the broad portion of life’s history under review.  For instance, chapter 7, titled “Dinosaurs in Flight,” principally about the Jurassic and Cretaceous, is a wonderful exposition of the avian nature of dinosaurs. 

Flight, according to Gee, was in the works for dinosaurs from their beginning.

Dinosaurs and their immediate relatives spent millions of years accumulating everything needed for flight:  feathers, a fast-running metabolism, efficient air cooling to keep it under control, a lightweight frame, and a singular devotion to egg laying.  (p. 99)

Clearly, not all dinosaurs flew or kept flying once they did fly.  Flying is demanding of an animal’s physiological resources.  Notes Gee, it’s “an expensive habit . . . . so, it is not surprising that many flyers gave it up when the opportunity arose."  (p. 105)

Or consider that his chapter on mammals (succeeded by chapters that consider primates and then hominins).  Gee has embraced the view of mammals that I’ve expressed in this blog before (in the post titled Mammals All The Way Down, November 29, 2021) – they weren’t just biding their time in shadow of the dinosaurs, they were actively evolving.  Gee begins this chapter by discussing the evolutionary changes that equipped mammals with a hearing ability more sensitive than anything that had existed previously in vertebrates.  The physiological changes required to achieve this new level of hearing are, in Gee’s skilled telling, a product of complex evolutionary change repurposing bones.  Though mammals during the reign of the dinosaurs may have been away from center stage and, certainly, not the main act, they were evolving with gusto.  He stresses, “They were a venturesome bunch and not to be kept down.”  (p. 121)

After several chapters tracing the evolution of primates into hominins into humans, Gee ends with a final chapter (The Past of the Future) that leaves no doubt that ours is just a brief moment in the history of life on this planet and that we humans are fated to suffer the eventual fate of all life on the planet – extinction.  He writes:

Within the next few thousand years, Homo sapiens will have vanished. The cause will be, in part, the repayment of an extinction debt, long overdue. The patch of habitat occupied by humanity is nothing less than the entire Earth, and human beings have been making it progressively less habitable.  (p. 188)

He speculates on how the planet will change in the future and how life will be affected.  I was particularly taken by the humorous footnote he inserted covering the whole of this sobering exposition:

I should say that from this point onward, most of what I say is conjecture, or what scientist call making stuff up.  As someone once said, prediction is very hard, especially about the future.  (p. 268, footnote 17, note number appears on p. 189)

Bottomline, Gee posits that humanity won’t survive and all evidence of our existence, relative to the billions of years life existed on Earth will be negligible.  But, despite that lack of enduring legacy, or, actually, because of it, Gee counsels us that it’s important “to preserve what we have, to make our mayfly existence as comfortable as possible, for ourselves and our fellow species."  (p. 207)

This is an engaging, witty, and informative read.  I do have one quibble:  I think plants get short shrift in Gee’s telling of the story of life.  They’re not ignored, but the drama in his account has mostly to do with the trials, tribulations, successes and failures of animals.

The Challenge of Dinosaurs in Virginia

Most of my fossil hunting has been in Maryland with an occasional foray into other states, including neighboring Virginia, Delaware, and West Virginia.  So I thought I had some familiarity with the ground covered by geologist Robert E. Weems' new book:  The Age of Dinosaurs in Virginia and Nearby States (2022).  Actually, it turns out I didn't have a real clue as to the central challenge Weems faced in writing this book.

Weems tackles a sweeping and complex topic in this well-written, accessible volume.  (The Background Note below provides additional information about the author and describes a connection I have with him.)  The book is organized chronologically, describing the flora and fauna of the region across most of the Mesozoic Era that encompassed the Triassic, Jurassic, and Cretaceous Periods running from 252 to 66 million years ago (mya).  The many tables, graphs, maps, and illustrations that grace the book are clear and informative.  Sadly, though, the volume is missing an index which makes revisiting topics difficult.

Among the most important and welcome aspects of the book is Weems’ account of the deep impact that geological forces over time have had on the area’s geography and climate, factors shaping its flora and fauna.  This includes the influence of the ever northward movement of Virginia (that is, the geographic region we now call “Virginia”) from near the equator in the early Mesozoic, to the “low latitude desert belt” in the late Triassic, and to the “subtropical belt” in the Cretaceous.  (p. 14)  I think it is important to always keep in mind these broad, powerful forces shaping the environment within evolution is at work.

The highlight for me of this focus on the geological influence on the composition of the flora and fauna is his discussion of the late Cretaceous.  He delineates the effect of the separation of the Appalachia continent (of which the Virginia region was a part) from the Laramidia continent during the Cretaceous as shown in the map below of North America during late Campanian Stage of the Cretaceous (about 75 mya).  (This map is taken from Scott D. Sampson, et al., New Horned Dinosaurs from Utah Provide Evidence for Intracontinental Dinosaur Endemism, PLOS One, September 22, 2010.  It was modified from one prepared by Ron Blakey.  It is available on Wikimedia Commons and used here under Creative Commons Attribution 4.0 International License.)

According to Weems, this isolation of Appalachia in the late Cretaceous had important consequences for its terrestrial species, particularly its dinosaurs.  He posits that during the Campanian, the dinosaurs of Appalachia “diverged markedly” from those of Laramidia whose extensive dinosaur fauna, I would note, included the iconic Tyrannosaurus and Triceratops.  (p. 89)  As Weems writes

Because Appalachia was a small continent and isolated from the rest of the Campanian world, its dinosaurs were less diverse and on average smaller than the dinosaurs found elsewhere.  In most other parts of the Campanian world, dinosaurs continued to diversify and on average grow ever larger.  (p. 98)

Evolutionary changes marking the dinosaurs of Laramidia, such as the rise of the ceratopsians, could not be represented in Appalachia during much of the late Cretaceous when there was no terrestrial link.

The key challenge Weems set for himself in writing this book is significant:  focusing this story on Virginia.  An initial reflection of the impact of choosing this topic comes with the discussion early in the book of the phrase “Age of Dinosaurs.”  Clearly, it falls within the Mesozoic because that’s when dinosaurs lived and the most logical ending point for Weems and the majority of the rest of us is with the mass extinction at the end of the Cretaceous, 66 million years ago.  (Admittedly, the present day existence of birds – avian dinosaurs – muddies the water of the ending just a bit.)  An important question is:  when did it begin?  With the appearance of the first animal considered a dinosaur or when dinosaurs came to dominate the landscape?  My own take on it squares with that of paleontologist Steve Brusatte who argues that the “Age of Dinosaurs” started in the Jurassic when dinosaurs were no longer bit players, but clearly in line to be faunal stars.  (The Rise and Fall of the Dinosaurs:  A New History of Their Lost World, 2018, p. 99)  Yes, there were dinosaurs before that, during the Triassic, but they didn’t seem destined to larger roles until the mass extinction at the end of Triassic opened up vast niches into which dinosaurs stepped.  In contrast, Weems posits that the "Age" has its origins about 243 mya in the Triassic when the first dinosaurs came on stage, even though he acknowledges that those first representatives were “relatively small and unimposing animals” (p. 4)  So, why look that far back?  His answer is simple.  It’s because “a large part of that early record is represented in the rock record of Virginia.”  (p. 4)

This is an acceptable and understandable decision because those early years helped shape what came later.  It is also true that, without the Triassic fossil record, the Virginia-based evidence of the kinds of flora and fauna that lived here during the "Age of Dinosaurs" is dramatically limited.  Even with reaching back into the Triassic, Weems had no choice in telling this story but to look beyond Virginia to "nearby states."  Why?  Because the exposed geological formations of the Mesozoic in Virginia are much too limited.  The map of Virginia (Figure 3, p. 10-11) he includes showing where Mesozoic strata are exposed makes the situation abundantly clear.  There are 15 relatively small Mesozoic exposures in the state:  nearly all (12) from the Upper Triassic (it's no wonder Weems starts the story there), a single one from the Lower Jurassic, none from the Middle and Upper Jurassic, 2 from the Lower Cretaceous, and none from the Upper Cretaceous.  This map reflects the lengthy “unconformities” in the stratigraphic record of the state, that is, gaps where exposed geological strata are missing and, so, the fossil record in Virginia is absent there as well.  A figure depicting graphically the stratigraphic record of the Mesozoic in Virginia (Figure 2, p. 6) makes this starkly evident:  most of it is blank.

Weems is up front about this situation and his strategy for dealing with it:  use information where available about the flora and fauna of neighboring states to fill in the picture, arguing that what was going on there likely was true for Virginia as well.  That’s a perfectly reasonable approach, though one that he has to turn to repeatedly throughout the volume.  That does raise the question of why he choose Virginia as the ostensible focus in the first place, and not, say, a broadly defined Mid-Atlantic region as a whole.  In fact, his book is an excellent account of the "Age of Dinosaurs" as it played out in that broader region, albeit in the slightly uncomfortable guise of a story about Virginia.

Background Note

Robert Weems is a highly respected, much published geologist who had a long career with the U.S. Geological Survey.  His publications cover a wide variety of geological and paleontological topics.  Many can be downloaded in PDF format from the website of the Maryland Geological Society (MGS).  In the interest of full disclosure, I would note that Weems and I have both been members of the MGS, a club composed of amateur and professional geologists and paleontologists.  The club helped Weems defray part of the cost of publishing The Age of Dinosaurs.

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 Elsa 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 mammal 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.

David H. Koch Hall of Fossils - Deep Time ~ Come for the Dinosaurs, Absorb the Lessons

On June 8, 2019, after five years of work, $125 million, and, I’m certain, myriad headaches, the Smithsonian’s National Museum of Natural History opened its completely renovated fossil hall, the David H. Koch Hall of Fossils – Deep Time.  This post describes primarily what caught my eye on my first visit to this remarkable hall.  Given that it houses over 700 fossils and presents some 75,000 words on its signage, I clearly missed a great deal and return visits are in order.  At the outset, I would note that, for all intents and purposes, this is a new hall and that’s how I’ll refer to it below.

The prior incarnation of a Smithsonian fossil hall enveloped visitors in darkness that seemed to emanate from the crowded displays, the gloomy worn carpeting, the faint-hearted lighting, and even the outdated science that was referenced in the skeleton poses and the signage.  No natural light, no windows.

This newest hall greets the visitor with grace, space, and, above all (quite literally) light.  (Dear visitors, please embrace these attributes wholeheartedly.)  The picture below – a panorama shot of a  Diplodocus skeleton stretching nearly 90 feet from the tiny tip of its tail (curving off to the left) to its undersized head at the end of a wonderfully long neck – shows some of the new hall in much of its airy glory.

That’s where I have to begin, with the dinosaurs, because that’s where most who come to this hall will naturally first gravitate.  And the child in most of us will find irresistible the tableau in the center of the hall’s long room where a Tyrannosaurus rex and a Triceratops horridus portray a scene that may well mirror many that actually occurred some 66 million or more years ago.

Of note, the skulls of both skeletons shown above are casts because the actual skeletons (housed elsewhere in the museum) would have weighed too much to be posed in such close proximity with each other.  This is described in an article and illustrated map of the new hall that appeared in the Washington Post (Bonnie Berkowitz and Aaron Steckelberg, A New Old Home for the Nation’s Dinosaurs, June 2, 2019).  This is a highly informative piece on the new hall.  Well worth exploring.

Labels throughout the hall clearly identify what’s a cast and what’s not.  Material that fills in incomplete specimens are a different shade allowing the sharp eyed to distinguish real fossil from manufactured material.  I have to say that some of these differences in shading are too subtle for my eyes.

Back to the tableau of a teenage (it is assumed) T. rex beginning to munch on a T. horridus.  It fails to answer a question debated among paleontologists.  Was T. rex a predator or a scavenger?  The display is agnostic on this, though, in all likelihood, T. rex went both ways.  (Hear National Museum of Natural History’s director, Kirk Johnson, discuss this aspect of the T. rex on the NPR show 1A with Joshua Johnson (episode aired on June 11, 2019).  In this episode, Kirk Johnson waxes enthusiastic about many features of the new hall.)

Among other dinosaurs found here is this Camarasaurus skeleton positioned near the Diplodocus.

The Camarasaurus picture highlights one distinguishing feature of the new hall:  there’s no effort to fill spaces with recreated, lifelike foliage from the diverse time periods.  Rather, the vegetation representations are minimalist.  The hall does allow visitors to see the ecosystems of such worlds, depicting them in little “diorama” stands scattered throughout the hall.

Here’s a close-up of the diorama of a grassland scene some 19 million years ago, reconstructed on the basis of fossils found in the Harrison Formation, Nebraska.

How is this new hall organized?  The designers want visitors to enter from the museum's rotunda and, so they are to begin with recent history, not deep time.  Visitors are greeted by bronze statues of a human family being stalked by a saber-toothed tiger.  The former recognizable, the latter striking but still mostly familiar.  Walking from the entrance down the length of the main long gallery, visitors at first move somewhat slowly back in time, only hundreds of thousands of years in those initial steps, but then rather quickly millions of years go by until, angling off to the left from this first gallery, visitors come to the initial emergence of life on the planet.  This organization forces a rethinking of time as, in the process of walking through the exhibits, the familiar or somewhat familiar is replaced by the faintly familiar which in turn is replaced by the truly different, alien even.  Deep time, for sure.

There is a spiral motif that marks some of the hall’s signage which not only represents how Earth’s history reaches far, very far back in time, but also speaks to the connections among all things.  The past is, indeed, prologue.  No “special creations” here.  One such spiral sits high over the entrance to the hall accompanied by a quotation from Charles Darwin’s On The Origin of Species.  That quotation marks one of the recurrent themes that is traced throughout the hall’s displays – the history of life on this planet cannot be understood without a recognition of the fundamental role that evolution played and is still playing.

No shying away in this hall from evolution.  Indeed, a statue of a young Charles Darwin sits near the center of the hall with a Galapagos finch (?) perched on his shoulder.

This message about evolution’s central role in life on Earth is unavoidable as one’s path in the hall moves from the present into deep time.  Among the many exhibits and signage positing and elaborating on this theme, is a large sign featuring Smithsonian paleobiologist Gene Hunt.  An extensive set of quotations from him begins, “Species evolve by natural selection to meet the challenges of the world.”  Accompanying this is a fascinating (at least it was to me) display of two arrays of specimens of the bivalve mollusc Spondylus, commonly, though erroneously, referred to as “spiny oysters.”  The array on the left shows eight Spondylus specimens from a single species that lived in Florida some three million years ago.  Back then specimens of this species exhibited some, relatively marginal variations.  But, as the display notes, “Over time, variations like these can become the building blocks of new species.”  The eight Spondylus specimens on the right are from different species of this genus living in oceans today.

That evolution has had to contend with several major extinction events in deep time is unquestionably also a central element in the hall’s lesson on evolution.  One of the most striking ways this is conveyed comes when one gets back to the end-Permian extinction.  Here is a close-up of the key part of a graphic showing the wealth of diversity among plants and animals that marked the Permian in general (at the left of the event), but which was abruptly curtailed:

Massive volcanic eruptions 252 million years ago released gases that dramatically altered the climate, causing extinctions that rippled through food webs and devastated animal communities.

Emerging from that extinction event, life on Earth was diminished with a tremendous loss of diversity.

Embedded in the text of that sign is another fundamental message of the new hall that is repeated many, many times – Earth’s climate changed frequently in deep time and such changes had fundamental consequences for life on this planet.  Yet, as central as this message is to an understanding of how life as we now know it came to be (yes, the living are all connected in this great spiral of time), what impressed me perhaps most forcefully about the new hall was the way the content of that message is framed for visitors when they begin their journey.  Here is one way that message is delivered early in that journey.

"And humans are the cause."

And another:

One early kiosk loops a video discussing what can be learned about climate change over the past several hundred thousand years by studying ice cores.

Toward the very end of the short video, after it shows the strong correlation discovered between changes in the concentration of carbon dioxide and changes in the Earth’s temperature (more carbon dioxide, higher temperatures), the voice-over narrator says this:

Today CO2 is skyrocketing higher than any time in the past 800,000 years as we burn fossil fuels and cut down forests.  This time, humans are the reason Earth’s temperatures are rising.

Human’s lethal touch on life on Earth is felt in other ways as this display asserts.

To me, that’s the most amazing aspect of the new fossil hall – its robust assertion, clearly backed by science, that today’s climate change has human activities as its root cause and that it poses an existential threat.

Why is this amazing?  Because it shows that a bright line was drawn between David H. Koch, the core financial donor to the remaking of the fossil hall, and the content of the new hall.  David and his brother, Charles, head a multinational corporate empire founded by their father which, among other things, owns oil refineries and pipelines.  Writer Jane Mayer profiled the Koch brothers in a fascinating and detailed article in 2010 that ran in The New Yorker (Covert Operations, August 30, 2010).  In it, Mayer wrote:

The Kochs are longtime libertarians who believe in drastically lower personal and corporate taxes, minimal social services for the needy, and much less oversight of industry—especially environmental regulation.

Their attitude toward climate change?  As quoted by Mayer, a 2010 Koch Industries newsletter asserted that "fluctuations in the earth’s climate predate humanity . . . .  Since we can’t control Mother Nature, let’s figure out how to get along with her changes.”

That's why I think it quite striking that the new hall teaches its lesson about today’s climate change so strongly (stridently even), despite the beliefs of that donor.

I fear I've given the richness of the new hall short shrift because all I've highlighted has been the dinosaurs and the interwoven messages that I hope all visitors absorb.  There are hundreds of fossils on display, most much smaller than those awesome Mesozoic creatures.  I will close with a picture of what is one of the jewels of this new hall (I agree with director Kirk Johnson on this).  The piece of matrix pictured below comes from the Green River Formation (Wyoming) and features the 52-million-year-old, intact, incredibly beautiful skeleton of a tiny early horse, Protorohippus venticolum.  This is the most complete skeleton of this early horse species found to date.  The preserved detail is breath taking.  Given the fossil impressions of several fish in this matrix, evidently this horse died at a lake’s edge, floated out on the water for some distance, and then sank to the bottom.

Where Worlds Meet or Perhaps Collide

Several years ago on a whim, I purchased a packet of 100 worldwide stamps that mostly feature dinosaurs.  The Mystic Stamp Company originally assembled and sold this packet.  For reasons not relevant to this post, my philatelic interest from my early teen years has robustly revived and that dinosaur packet (found under a bed after a dusty search) now sits on my desk, the object of some study, offering a sense of two worlds – paleontology and philately – meeting.

Here are a few examples of these stamps.

I’ve concluded that this collection, regardless of how it was brought together, actually constitutes a fairly representative sample of how dinosaurs, and by extension, things paleontological, have been treated on postage stamps.

There are several sites on the web that allow me to make this kind of generalization beyond just my small sample of 100 stamps.  For instance, I consulted with Stamps2Go, a great marketplace for folks selling and those buying postage stamps, which currently has 750 stamps for sale that are nestled under the topic “Animals:  Extinct:  Dinosaurs.”  Admittedly, not all of them are dinosaurs, but most are.  (Later edit:  To be sure, among the 750 stamps are duplicates of the same issue being offered by different sellers.)

Then there’s another website that proves once again that if you can imagine it, it’s probably already on the web.  The Paleophilatelie site is the brainchild of Paleophilatelist in Munich, Germany, who married his interest in fossils with his stamp collecting, creating in the process a beautiful virtual collection of worldwide postage stamps (and related postal items such as first day covers and cancellations) with some relationship to paleontology.  It’s a source of endless fascination (though perhaps that may be true for me just because I’ve been sucked into the black holes of these two interests).  Anyway, I have found it great fun to go through his collection of stamps; one can either browse the full gallery or select stamps from specific countries.

So, based on my sample and what I see at sites like the two just described, I’ve reached the two following conclusions:

• The artwork and details in these stamps are mostly second rate.  No other way to say it (unless third rate is more appropriate).  Details often seem wrong.  Among the offending aspects are the proportions of various body parts of the animals, the structure of appendages, the animals’ posture, and their general environment.  Even if the details are right, the artwork mostly fails to bring these creatures to life.  Sad stuff.

• Fossils are missing from the vast majority of these stamps.  In general, postage stamps don’t depict the fossils that underlie our understanding of how extinct ancient animals (and plants) looked and lived.  In my sample of 100 stamps, only one shows a fossil skeleton of a dinosaur.  (I certainly won’t extrapolate from that and suggest that only one percent of postage stamps with dinosaurs or other things paleontological shows fossils.)  The one in my collection was the lowest denomination issue that was part of a five-stamp set released in 1991 to honor that nasty, ill-tempered British paleontologist Sir Richard Owen, a doyen of paleontology in the mid 19th century who coined the word dinosaur.  The stamps feature somewhat stylized portions of skeletons of various dinosaurs, including Iguanodon, the only one of the dinosaurs depicted on these stamps whose fossils Owen actually knew.  (The discussion about these stamps on the Paleophilatelie site is helpful.)

Although some countries do quite nicely with fossils on their stamps, such as Germany, the question remains why fossils are the general exception.  Are fossils harder to illustrate?  Are we (the general public, postage stamp users, or collectors) assumed to be more attracted to depictions of the living creatures or, perhaps, considered likely to be put off by fossilized bones on our stamps?  Maybe fossils are thought to be too static, failing to convey action very well.  Frankly, I don’t think that’s true of fossils, and the inferior artwork used for many dinosaur stamps certainly puts a lie to the notion that illustrating the living animals is necessarily the avenue to attractive, action-filled stamps.

How do U.S. stamps fare in this kind of discussion?  Most of the stamps in my dinosaur packet come from African and Asian nations.  None come from the U.S. though the U.S. has featured illustrations of living dinosaurs on a number of occasions.  For instance, here is a stamp issued in 1970 titled The Age of Reptiles.  (It is in the public domain and downloaded from Wikimedia Commons.)

The artwork on the U.S. stamps I’ve looked at is certainly passable, if generally not memorable.

What of fossils on U.S. stamps?  My search of Arago database of all U.S. stamps on the Smithsonian’s National Postal Museum website turned up exactly one stamp with fossils, featuring a fairly abstract illustration of a trilobite and some ferns.  It was issued in conjunction with the Knoxville World’s Fair in 1982, and bears the title Fossil fuels, one of four stamps in a block with an energy theme (another of the stamps was titled Breeder reactor).  It's telling that that's how fossils came to be on a stamp.  But is that it?  It’s what I could find though I’d be happy to be corrected.

[Well, in this later edit (that itself has been edited further), I will correct myself.  I've found at least two other instances in which fossils appear on U.S. stamps.  In 1955, the U.S. Postal Service issued a stamp commemorating Charles Willson Peale (1741 - 1827) and his museum.  The gifted Peale was, among other things, an artist, politician, and naturalist, and he turned his massive collection of natural history specimens into a museum.  He painted a portrait of himself lifting the curtain on a view of his museum and this is what the 1955 stamp depicts.  At his feet (on the right side of the stamp) are mastodon fossils.

It was the Paleophilatelist on his Paleophilatelie site in his Milestones Paleontology Related Philatelic Items who led me to this stamp which was already in my collection.  Also please see his comment below on this blog post.

And, as I discovered in conversation with a stamp collector, the Postal Service issued stamps in 1974 celebrating the abundance of minerals in the United States.  One of those minerals is, in fact, a fossil - petrified wood.  Here is that stamp:

(This image in the public domain and downloaded from Wikimedia Commons.)]

One final note which may relate to a place where the worlds of paleontology and philately do collide, at least in this country.  As I looked at many hundreds of paleontologically oriented postage stamps from across the globe, it was fairly easy to note when the bicentennial of Charles Darwin’s birth occurred (2009) because at roughly that point there was an explosion of Darwin-related stamps from many countries.  The Darwin OnLine website offers a selection of worldwide stamps featuring the great naturalist.  Conspicuously, though not unexpectedly, missing, is the U.S. where I conclude that, even though the published criteria for selection of individuals to be honored on U.S. stamps pose no particular barrier to the British Charles Darwin, the U.S. Postal Service appears to have shied away from offending the religious right.

Sexual Dimorphism ~ Another Challenging Variable

This post was prompted by the recent publication of a fascinating article by paleontologist Jordan C. Mallon about our ability or, more specifically, our inability to distinguish between males and females in the available fossils for several prominent dinosaur taxa.  (Recognizing Sexual Dimorphism in the Fossil Record:  Lessons from Nonavian Dinosaurs, Paleobiology, 2017.)

Sexual dimorphism is present when characteristics of the males and the females of a given species differ.  These differences may be reflected in male and female morphology including such features as body shape (e.g., display features such as crests), body size, or coloring, and may also be found in variations in behavior.  Sexual dimorphism is an important attribute that is central to evolutionary biology, particularly as it relates to sexual selection.

With extant animals, some aspects of sexual dimorphism may be obvious as with the African lion species (Panthera leo) whose males sport a distinctive mane and may be somewhat larger than the females.  Pictured below are mounted specimens on display in the Hall of Mammals at the Smithsonian’s National Museum of Natural History.

A word of caution.  Having living animals available for study offers no certainty that we will, in fact, accurately identify sexually dimorphic traits.  The lions in the Hall of Mammal, as posed, promote the widely accepted canard that the female lions do all of the hunting for the pride and the males just lounge around.  It was only in this decade that careful tracking of male and female lions in Kruger National Park (South Africa) using GPS and other technology showed that females hunt in open areas while males do their hunting in denser vegetation shielded from observation.  (Scott R. Loarie, et al., Lion Hunting Behaviour and Vegetation Structure in an African Savanna, Animal Behaviour, Volume 85, 2013.)

Certain patterns of sexual dimorphism have been detected in extant animals.  For example, with respect to size and morphology, biologists Ehab Abouheif and Daphne J. Fairbairn observe, “In most species of animals, females attain larger body sizes than do males (e.g., most spiders, insects, fish, amphibians, reptiles), whereas in most birds and mammals, males are the larger sex.”  (A Comparative Analysis of Allometry for Sexual Size Dimorphism:  Assessing Rensch’s Rule, The American Naturalist, Volume 149, Number. 3, March, 1997, p. 540, references omitted.)

But all bets are off with extinct animals known from the fossil record.  Mallon is not alone in positing that the barriers to identifying the sex of extinct species are particularly high.  Armed only with fossils, we find ourselves without much of what might help to distinguish males from females including gender-specific behavior as well as some morphological attributes such as sex organs and coloring.  Without the information that observation of living animals provides, paleontologists face a daunting challenge.  As evolutionary ecologist Robert J. Knell and his colleagues have written,

Identifying sexual dimorphism in animals known only as fossils is often difficult; specimens of a particular species are sometimes rare, unique, or unavailable, and reliably identifying sex in fossils is often difficult or impossible.”  (Robert J. Knell, et al., Sexual Selection in Prehistoric Animals:  Detection and Implications, Trends in Ecology and Evolution, 2012.)

On the one hand, if the sex-related differences preserved in the fossil record are slight, then, as Knell et al. note, one must work with large samples (IF such sample sizes are actually available).  Apparently, it hasn’t been unprecedented for claims to be made about the sexes of dinosaur fossils based on bones from just two specimens (which is clearly an inadequate sample size).  At times, it may be differences in the sizes of features that distinguish males from females, not the presence or absence of such features in either of the genders.

On the other hand, “when dimorphism is strong, there is a risk that different sexes will be described as different species.”  (Knell, et al.)  That is, relying on morphology to identify species, as is a common and necessary practice in paleontology, poses the risk that significant differences in fossils, possibly related to sexual dimorphism, might be treated as indications of the presence of different species, not the two sexes of the same species.

From his analysis, Mallon concludes that, with the dinosaur fossils at hand, paleontologists, for the most part, cannot tell whether an individual dinosaur was male or female.  In this vein, I would add that one must ignore the nicknames given some of the iconic dinosaur fossil skeletons such as the Field Museum’s Tyrannosaurus rex which is known as “Sue,” the Triceratops horridus cast on display in the Last American Dinosaurs exhibit at the Smithsonian’s National Museum of Natural History which is fondly called “Hatcher” and its companion T. rex cast in the same exhibit carrying the nickname “Stan.”  (The National Museum has Hatcher's fossil bones, while its Stan is just one of many casts that can be found worldwide.  Stan's original fossil bones are in the Black Hills Institute.)  These nicknames honor the collectors who found the original fossils – Sue Hendrickson, John Bell Hatcher and Stan Sacrison.  In all fairness, the Field Museum’s T. rex was nicknamed Sue by Peter Larson because he believes this specimen was, in fact, female.  (See, Jacqueline Ronson, Sue the Tyrannosaurus Has a Sexual Identity Crisis, Inverse Science, December 1, 2016.)

Yes, there are some dinosaur fossils that can be identified as female because they carry eggs or have a medullary bone which, in contemporary birds, supplies the calcium used to fashion egg shells.  Mallon concludes that, for the taxa he studied (nine prominent species including Tyrannosaurus rex, Allosaurus fragilis, a couple of Stegosaurus species, and Protoceratops andrewsi), the available evidence offers “no support for sexual dimorphism.”  Mallon takes pains to make it clear he is not saying that such dinosaurs were not sexually dimorphic, simply that, in his considered view, the fossil record is inadequate to make such a claim.

This is certainly a thought-provoking, revisionist conclusion and it’s not just relevant for dinosaurs.  Mallon lays out the conditions under which sexual dimorphism might be successfully detected in different fossil taxa.

For me personally, his article brought me up short.  I have to admit that for the years that I’ve collected vertebrate fossils (principally shark teeth), I have never pondered the gender of the animals and how that might be reflected in the fossils I have at hand.  So much for my intellectual curiosity and my efforts at taxonomy.

Considering just fossil shark teeth, several of the most important variables that might account for differences in the teeth under study are:

• natural variation among individuals within the same species (hey, this variability is the material that natural selection work with),
• chronological age of the specimens (not only might teeth from younger sharks be smaller, they might also differ in terms of shape),
• position of the teeth in the jaws (most shark families are heterodonts, that is, teeth vary by location in the jaws; depending upon the shark, teeth from the upper jaws can differ from those in the lower jaw, and/or they can vary by location within a specific jaw),
• differential influence of the environment on the health and development of individuals from the same species,
• taphonomic processes (i.e., what happens to an organism in death) which may introduce variability in the fossils that emerge millions of years later, and
• sexual dimorphism.

This list is based primarily on paleontologist Bretton W. Kent’s Fossil Sharks of the Chesapeake Bay Region (1994, p. 2).  Though he does include sexual dimorphism as a factor behind some variation in fossil shark teeth, as far as I can tell, it’s only in a very few species that sexual dimorphism has been identified.  He cites it in just two fossil shark species found in the Chesapeake Bay region – Hexanchus (cow sharks), and Rhizopriondon (sharpnose sharks).

Ultimately, it’s somewhat ironic that, despite his warnings and the unlikelihood of detecting sexual dimorphism in most of my collected fossils, Mallon’s piece has prompted me to be sensitive to that possibility.