The Evolution Underground – Part 1: Book Review

Not all scholars write with the playfulness or the open curiosity found in books written by Dr. Anthony Martin, professor at Emory University.

In his second work with Pegasus Books, “The Evolution Underground: Burrows, Bunkers, and the Marvelous Subterranean World Beneath Our Feet,” he opens with an anecdote about an outdoor class on an island off of the Georgia coast.  If you have any question about whether this book is for you, read those first several pages.

He, his colleague, Michael Page, and several students were mapping alligator dens.  While they’d witnessed many active dens from a safe distance, in this instance, they were exploring those long abandoned by their former occupants.  They were, he explained to the reader, in the middle of the forest where a now-nonexistent canal once ran.  Without water, there would, of course, be no alligators.

Only he was wrong.  And this was pointed out when a student noticed teeth within the den.

Embed from Getty Images

Picture of alligators by Michael Leggero, courtesy of Getty Images

You will need to read the book to find out what happens, but this first chapter perfectly encapsulates how Dr. Martin writes. If you want to learn about any aspect of our world from a scientific and curious lens, here is an author you might want as your guide.  He is no stranger to presenting enormous volumes of information in an easily digestible way, nor is he one to make it cumbersome. His wit and sense of adventure make learning fun.  Moreover, there is no arrogance in his books.  The words “so far,” “unknown,” and “as yet” are sprinkled throughout the text.  He is not afraid to admit when science (or, indeed, when he himself!) has been mistaken, when theories are disproven, educational assumptions found incorrect. He writes with the understanding that our scientific knowledge–like life itself–is still evolving. And like so much of his writing, it only serves to prompt the reader into thoughtful reverie: where might science take us in the future? What will be revealed years, decades, centuries from now, and how will this impact the world?  The creative and wondrous question “What if?” floats like a butterfly through its chapters.

Dr. Martin describes how these seemingly abandoned alligator dens may have indeed been dug when water was present, but that even despite drought, parts of their internal structures may connect with the groundwater table.  Water within the den may have also attracted thirsty birds and animals on the island.  He and his students later found the ravaged corpses and bones from such unsuspecting creatures both in and outside of other forest dens.

“All of this trace evidence told us the alligators could switch from aquatic to terrestrial predation if necessary, like a shark deciding it was going to turn into a lion.  This surprising behavioral transformation and adaptability in alligators was made possible through their dens, which during times of environmental change became all-purpose hunting lodges.” – page 7, “The Evolution Underground: Burrows, Bunkers, and the Marvelous Subterranean World Beneath Our Feet,” by Dr. Anthony J. Martin, Pegasus Books

And thus begins his exploration of the animals—including humans!—worms, insects and birds that have created sanctuaries below ground.  Burrows, he posits, have made survival possible throughout Earth’s history, and these underground homes have made and continue to make enormous impact on life above ground.

“The bigger picture behind these everyday observations of many holes in the ground, however, is that the long history of these burrowing invertebrates completely altered global environments, from the deepest sea to the highest mountains, and even affected the atmosphere and climate.  In short, the entire surface of our planet is built upon one big complex and constantly evolving burrow system, controlling the nature of our existence.” – page 14, “The Evolution Underground: Burrows, Bunkers, and the Marvelous Subterranean World Beneath Our Feet,” by Dr. Anthony J. Martin, Pegasus Books

Dr. Martin encourages us to take a closer look at a generally overlooked part of our world. That closer look involves fascinating details about creatures and places one may not have realized existed.  Burrowing owls–with their photogenic and often amusing images–may be familiar, but perhaps not so much the charming fairy penguins of Tasmania, or the alarming assassin flies associated with gopher tortoise burrows, who both kill and start digesting their hapless victims with an injection of neurotoxins and enzymes.

Embed from Getty Images

Image of burrowing fairy penguins, courtesy of Getty Images
Embed from Getty Images

Slideshow of burrowing owls, courtesy of Getty Images

 

Perhaps the most powerful section of the book—one that really drives home his point about survival underground—involves the eruption of Mount St. Helens in Washington State thirty-seven years ago.

Whether you’ve only read about it or whether you’ve actually visited, Mount St. Helens is a stark reminder of how devastating Nature can be.  After a couple of months of earthquakes, the volcano erupted in the morning of May 18th, 1980. Not only did it obliterate everything in its path, the eruption and its aftermath killed 57 people and all of the wildlife within about 150 square miles.

Embed from Getty Images
Image of Mount St. Helens before the eruption of 1980, photo by Jeff Goulden, courtesy of Getty Images

Embed from Getty Images

Image of Mount St. Helens today, courtesy of Getty Images

Here, Dr. Martin uses creative nonfiction (or ‘narrative nonfiction’) to help illustrate how, despite this traumatic event, the entire area made a comeback.  Loowit, a sweet little fictional pocket gopher, takes the reader through some of the natural events that transformed devastation into renewal and rebirth.

He describes her home: a branching set of underground tunnels and rooms that can reach up to 500 feet long, complete with food storage areas, latrines, and other chambers. Although undeterred by snow, she was, at the time of the eruption, comfortably ensconced in her burrow.  This saved her.  He takes us through how she emerges after the eruption, her confusion, her tentative steps back into a new world above ground, how she and other survivors may have eventually formed communities.

In sum, in a world that now knew mostly death and destruction, these pocket gophers not only survived, but kept surviving, and in so doing, helped bring life back to an area that did not outwardly appear to contain much.

…these little ecosystem engineers began terraforming the previously desolate landscape, first by helping plants take root and grow. Each individual pocket gopher was capable of overturning more than a ton of soil each year…” – page 262, “The Evolution Underground: Burrows, Bunkers, and the Marvelous Subterranean World Beneath Our Feet,” by Dr. Anthony J. Martin, Pegasus Books
Embed from Getty Images

Image of a pocket gopher, courtesy of Getty Images

Of the 55 mammal species in the area of Mount St. Helens in May 1980, only 14 survived the volcanic eruption and its collateral damage. Surface-dwelling elk, deer, black bears…and all other large- to medium-size mammals perished. On the other hand, nearly all the small mammals that lived were burrowing rodents…One of the few non-rodent survivors was the tiny Trowbridge’s shrew (Sorex trowbridgii), which (not coincidentally) is also a burrower.  Pocket gophers are active year round, but many other small-mammal species were both underground and still hibernating when the eruption took place.  The fortuitous timing of this disaster at the transition between winter and spring thus greatly enhanced the chances of these minutest of mammals to emerge and thrive.  Of the rodents that had already come out of hibernation, nocturnal species were doubly lucky to have already retired for the day in their burrows when the blast occurred.  Had the volcano erupted at night, many more would have died.” – page 264, “The Evolution Underground: Burrows, Bunkers, and the Marvelous Subterranean World Beneath Our Feet,” by Dr. Anthony J. Martin, Pegasus Books

For the pocket gopher populations that survived the eruption of Mount St. Helens in 1980, their collective actions were the key to turning a desolate, monochromatic landscape back into a vibrant and verdant one.  From a geological perspective, their effects were astoundingly quick, with partial ecological restoration apparent within just five years of the eruption. Consequently, pocket gophers and other burrowing animals that lived beyond May 18, 1980, send a powerful message about the benefits of burrows for surviving such an ecologically traumatic events, as well as for their role in restoring an ecosystem after it is nearly destroyed.” – pages 266-267, “The Evolution Underground: Burrows, Bunkers, and the Marvelous Subterranean World Beneath Our Feet,” by Dr. Anthony J. Martin, Pegasus Books

 

I want more books like “The Evolution Underground” and “Dinosaurs Without Bones.”  Books that tickle my intellect and my sense of humor.  Books that pull me in with their interesting anecdotes, their engaging playfulness, their sensitivity to all genders (ie: not referring to all humans as “mankind” or simply “man”), and their ability to make me think outside the pages.

When I read a book and am left not only with the satisfaction that comes from something that I’ve enjoyed but also an eagerness for more, I know I’ve found a talented author.

Dr. Anthony Martin is, indeed, a talented author.

 

******

A sincere and enthusiastic THANK YOU to Dr. Anthony Martin for his willingness to connect by phone and for his generous responses to my questions!  It was a pleasure and an honor to be able to speak with him, and—like his writing—he made it fun!  I eagerly (if impatiently) await any possible future work.  

FULL DISCLOSURE: The author of this blog loved Dr. Martin’s previous book with Pegasus, “Dinosaurs Without Bones,” and thus, jumped at the chance to review his latest work (fully predisposed to embrace it) by requesting a review copy from the publisher.  I am very grateful to Pegasus Books for the opportunity to do so. I am specifically grateful to Deputy Publisher, Jessica Case, with whom it was wonderful to work!

Dinosaurs Without Bones

Advertisements

Maiasaura Life History Project: The Art of Scientific Research (Part 2)

It’s one thing to be a detective. It’s another to be an artist: shifting expectations, making unlikely comparisons, causing one to consider entirely new perspectives.

Comparing elements of extant alligators and red deer to an extinct hadrosaur certainly changes how one views paleontology.  There is something unifying about it, connecting traits of living species—creatures that share the world with us today—to species that died out millions of years ago.  Instead of a scientific field one might put into a box labeled “the study of the past,” it becomes an increasingly complex vine weaving the past with the present.  And if animals as seemingly disparate as alligators, red deer and hadrosaurs share similarities, what else among us does?

Maiasaura HWB - Maiasaura replica

Maiasaura peeblesorum model; courtesy Dr. Holly Woodward Ballard

This connection was made all the more apparent in speaking with Dr. Holly Woodward Ballard about her background and her recent paper.  Her love of dinosaurs and microscopes were a perfect match for osteohistology, a field she pursued during her Masters.

Dr. Jim Farlow and Dr. Jack Horner—both members of her PhD committee and who have experience studying the bone microstructure of alligators and Maiasaura respectively—contributed to her Maiasaura peeblesorum research. They acknowledge that comparing alligator bone growth to dinosaurs has been done before; alligator bone growth has been studied extensively.

Embed from Getty Images

Red deer on the Isle of Rum, however, have been studied even longer. Dr. Woodward Ballard and her colleagues found similarities to Maiasaura in their survivorship rates, as well as within their bone microstructure.

Embed from Getty Images

Just as the red deer in Scotland, Maiasaura seem to have experienced a high mortality rate in the first year.  If, however, they survived that first year, they seemed more likely to live through sexual maturity, which may have been between 2-3 years of age. Eight or nine years marks another difficult year for both species. This is when their bodies appear to decline, or senesce, and they are at greater risk for mortality at this age.  Dr. Woodward Ballard and her colleagues note that one Maiasaura tibia with 10 lines of arrested growth (“LAGs”, indicating 10 years of life) appeared to still be growing.

“We have to understand the biology of modern animals and how it works before we can make any kind of hypotheses or inferences into extinct animals,” she explained. “The most important thing I learned from this experience was that we really don’t know as much as we should know about how modern animals grow and the life history details that are stored their bone tissue.”

“It’s sort of circular in that the more we learn about modern animals to apply it to the extinct ones, the more we learn about how bone biology works, how bone grows, and that has direct applications to the medical field, to veterinary biology, and to all kinds of modern fields where bone biomechanics and that sort of thing play a big role.”

Studying bones was only part of the research.  The other involved applying statistical models to the data compiled.  There are advantages to so many fossils from what the authors of the paper described as a  “monodominant” bonebed.  As mentioned in the previous post, the Maiasaura bones originate from three bonebeds in Montana, but these bonebeds are from the same stratigraphy across 2 km.  This means that the scientists can be relatively sure these animals experienced the same environmental stresses.  Differences in the bones, therefore, would indicate differences within each animal instead of being caused by external factors.

And the number of tibia studied in this paper was highly significant.

“There was one paper that came out about the mortality rates–survivorship curve distribution,” said Dr. Liz Freedman Fowler of Montana State University, co-author of Dr. Holly Woodward Ballard’s paper, “and the math in that was fairly complicated. Holly wanted to make sure that she did it right, and so that’s where I came in. It is quite complicated math making sure that you get all the different steps right.  Because the paper was critiquing and criticizing a previous paper that had done it wrong slightly, we wanted to use the methods of this kind of revision paper to make sure that we analyzed things appropriately.”

Dr. Liz Freedman Fowler new dinosaur

Dr. Liz Freedman Fowler with a painting of an entirely separate (and new!) species of hadrosaur she helped discoverProbrachylophosaurus bergei; photo by Sepp Janotta of the Montana State University News Service

 

“[A sample size of 50] was their suggestion,” she explained further, “because the previous histology papers that have been looking at mortality rates, they’ve been using a much smaller sample size: 10-15 individuals, [for example], which is still big for paleontology. But, you know, the smaller your sample size, the greater the chance that what you’re seeing is just random variation in your sample.  Whereas when you get a larger sample size, you can be more confident that you’re more accurately representing the population.

“Normally with dinosaurs you only have maybe two or three examples of a single species. So there’s really not much you can do mathematically because there’s just not enough data to run statistics on.”

Referenced throughout their paper was one published in Paleobiology in 2011 by David Steinsaltz and Steven Hecht Orzack.  The Steinsaltz/Orzack paper was a response to one published in Science in 2006.

“Based on [Steinsaltz and Orzack’s] modeling,” Dr. Woodward Ballard explained, “they recommended that the minimum sample size of 50 is what you would need for an extinct population in order to figure out what the shape of the survivorship curve is.  It’s not really a hard-and-fast rule.  But this is the only time that mathematicians have actually suggested a minimum number for producing statistically robust survivorship curves for dinosaurs. The fact that we were able to then meet their suggested requirements was pretty important.”

Upon first reading the paper by Dr. Woodward Ballard et al, I believed that one needed a sample of at least 50 fossils of a species in order to estimate a statistically-significant survivorship curve.  But—of all numbers—why 50? And why so many when most bones of extinct species are not as abundant as those found so far for Maiasaura?

Over the course of a conversation with Dr. Steven Orzack, I learned that what he and his co-author offered was a way to decrease potential misclassification errors in statistical calculations.

In simplest terms, they were raising the bar.

The 2006 paper by Erickson et al had used a sample size of 22 different Albertosaurus skeletons to calculate a convex survivorship curve. Convex, in other words, means that the survival rates decrease with age.

Yale - Albertosaurus side great

Cast of Albertosaurus libratus from (appropriately for this post) Red Deer River Valley, Alberta, Canada at the Yale Peabody Museum; image taken by the author of this blog

 

By using computer simulation to repeatedly “resample” that estimated curve, as well as a survivorship curve that was not convex (one in which some survival rates increased with age), Steinsaltz and Orzack found that about 10% of the simulated samples of size 22 taken from the non-convex sample would look convex. Such a result would mislead a scientist to misclassify the underlying survivorship curve as being convex when, in fact, it was non-convex.  When they repeated this process by more than doubling it to a sample size of 50, they discovered the misclassification error rate fell to less than 1%.

Paleontologists don’t always have access to a wealth of fossils from the same species.  This is something Dr. Orzack—trained as both a paleontologist and a neontologist—knows all too well.

HMNH - Deionychus

HMNH - Deionychus skull

Images of a partial Deinonychus skeleton, discovered in Montana in 1974 by Dr. Steven Orzack and a team of Harvard researchers, now at the Harvard Museum of Natural History; images taken by the author

 

“I don’t have any problem with sample sizes of 22 in the sense that if that’s the best you have, that’s fine,” he said. “What would have been better is [if Erickson et al had done] the statistics better.”

“Convexity,” he stated, “is a very specific claim.”

“[There are] weaker conclusions you can make about how survival rates change with age than [those published in the paper by Erickson et al.] If you boost your sample size to 50, you have a much lower probability of saying incorrectly that there is convexity when there isn’t,” he concluded.

“Paleontology is moving in a much more mathematical and analytical direction,” Dr. Freedman Fowler explained. “ We’re trying to be more rigorous and treat it more like a modern science.  That’s why we often use the term ‘paleobiology,’ instead of just ‘paleontology’ now. We’re trying to use the science and the tools of modern biology to look at how fossil organisms lived and kind of reconstruct their lives.”

And certainly, the math contained within the paper by Dr. Woodward Ballard, Dr. Liz Freedman Fowler and their colleagues is—to someone like myself—a bit overwhelming.

When speaking with Dr. Freedman Fowler, I asked her if her mathematical skills were rare within the field.

“I wouldn’t say ‘rare’,” she replied, “but it’s certainly not all of us. There are quite a lot of other paleontologists that use R and use math and things. But it’s a minority that goes in that direction.”

Maiasaura HWB - Maiasaura life history

FIGURE 6. Survivorship curve for Maiasaura. Sample size of 50 tibiae was standardized to an initial cohort of 1000 individuals (assumes 0% neonate mortality). Survivorship is based on the number of individuals surviving to reach age x (the end of the growth hiatus marked by LAG x). Age at death for individuals over 1 year old was determined by the number of LAGs plus growth marks within the EFS, when present. Error bars represent 95% confidence interval. Mean annual mortality rates (μ^) given for age ranges 0–1 years, 2–8 years, and 9–15 years. Vertical gray bars visually separate the three mortality rate age ranges; courtesy Dr. Woodward Ballard.

 

“Paleontology is very collaborative because it’s such a broad and interdisciplinary field. Nobody can be an expert in everything.”

When I asked her whether the sub-fields within paleontology have always been so diverse, she responded, “It is certainly a more recent development, and that’s true for many sciences.”

“[Looking back at] papers written 50 years ago, they’re almost all single authors. They’re also much more simple. These papers were just ‘I found this new species. Here’s what it looks like.’  There wasn’t much analysis.

“But now, as all these different branches of science have grown–all the different subfields within biology and geology and chemistry–we’re getting so many more tools that we can use to analyze fossils and look at them in all these different ways.  We’re also having a much larger sample size of fossils. We’re constantly out in the field collecting new specimens and that’s filling in gaps.  Between two species, [for example], we now find the intermediate species.  And we’re getting more complete growth series—the ontogenetic series—of animals. We’re out there finding juvenile dinosaurs and sub-adult dinosaurs and comparing them to the adult dinosaurs.

“Because we’re always adding this data, we always have more and more to work with. So we’re able to do types of analyses that we couldn’t 50 years ago. It was just impossible.”

And this paper is only the beginning. Dr. Woodward Ballard explained that she wants to “really make Maiasaura the dinosaur that we know the most about and really use it as a model to compare to other dinosaurs.”

In a moment of reflection, she said, “I get this question a lot:  ‘Well, great, you’re studying dinosaurs, but what’s that going to do for me?’”

She hopes that the interest in dinosaurs will pull people into science in general, describing a scenario in which the kids—wanting to see dinosaurs—visit a museum with their parents.  While there, the family may learn of other scientific discoveries, prompting even more interest in various scientific fields.

“The more we can make dinosaurs these realistic animals, [not just animals that are no longer around], I think it’s really going to get [kids] interested in science and the world around them.  Being able to continue to add more information to Maiasaura, I think, is going to be the way to really draw people in.”

“The big thing for me,” she said, “is not only collecting fossils, but [also] bringing college-aged kids to Montana to see a different part of the United States, [especially those] kids who might not [otherwise] have the opportunity to be exposed to science.”

“There’s still so much that can be done with the Maiasaura bonebed,” she continued, “with Maiasaura as an animal, so [many] opportunities for outreach and scientific investigation. I spoke with Jack Horner about this during my dissertation work and afterwards; I told him that I would really like to be able to work on Maiasaura potentially for the rest of my career. He thought it was a great idea.  I’ll do other research, too, but I plan to get out to Montana every summer.

“There’s just so much work that I decided to call it the ‘Maiasaura Life History Project’ and every paper that comes out will just be adding to what we already know about Maiasaura.”

At this time, there is no overall funding for the project. Dr. Woodward Ballard is currently writing grant proposals for future expeditions.

 

Holly Woodward-WCA-Branvold Quarry-Aug5-2015

Dr. Holly Woodward Ballard; photo by Dr. Karen Chin, courtesy of Dr. Woodward Ballard

 

 

References:

  1. Maiasaura, a model organism for extinct population biology: a large sample statistical assessment of growth dynamics and survivorship; Holly N. Woodward, Elizabeth A. Freedman Fowler, James O. Farlow, John R. Horner, Paleobiology, October 2015
  2. Statistical methods for paleodemography on fossil assemblages having small numbers of specimens: an investigation of dinosaur survivorship rates; David Steinsaltz, Steven Hecht Orzack, Paleobiology, Winter 2011
  3. Largest dinosaur population growth study ever shows how Maiasaura lived and died, Montana State University, MSU News Service
  4. MSU team finds new dinosaur species, reveals evolutionary link, Montana State University, MSU News Service
  5. Tyrannosaur Life Tables: An Example of Nonavian Dinosaur Population Biology; Gregory M. Erickson, Philip J. Currie, Brian D. Inouye, Alice A. Winn

 

**I need to stress that the methods used in this paper and the overall research by Dr. Woodward Ballard and Dr. Liz Freedman Fowler were extremely complex. Dr. Woodard Ballard, Dr. Freedman Fowler and Dr. Orzack graciously walked me through scientific and statistical elements that I had trouble understanding. If there are any errors in this post, they are my own.

Also, while comparisons between extant and extinct species may be normal to those in the field, it was not as dramatically apparent to me until this paper. 

I would like to extend, again, an enormous THANK YOU to Dr. Holly Woodward Ballard. I would also like to extend that same thank you to Dr. Liz Freedman Fowler and Dr. Steven Orzack.  It was a great pleasure and honor speaking with each of them–not to mention fun!–and I am profoundly grateful for their generosity!  

I am very eager to learn more as the Maiasaura Life History Project continues!! 

Maiasaura Life History Project: Paleontology at an Entirely New Depth (Part 1)

I envy the future.

I really do.

Every time I read a dinosaur book—whether a kids’ book with my nieces and nephews or otherwise—I am reminded just how much we’ve learned since I was young. It is staggering, the amount of information available to dinosaur enthusiasts. Whether it is in the number of new species discovered each year, the unbelievable details paleontologists glean (from teeth alone!), or the new technology that helps scientists unravel the once unknowable.

If this is what we know now, and in the relatively brief time since paleontology was first established, what are we going to know fifty years from now? A century? A millennium?

I think about the future almost as much as I marvel at the past. Assuming our knowledge base only increases, the future of paleontology promises to reveal what can only be—at this point in time—imagined.

Which is why when I learned of the Maiasaura Life History Project, I had to know more.

Dr. Holly Woodward Ballard wants to flesh out one particular species of dinosaur such that we know it almost as intimately as living animals today.  That species is a type of hadrosaur, an extinct herbivore from the late Cretaceous. Thanks to almost 40 years of excavation in Montana, we have thousands of its fossils from which to extract information and this, according to Dr. Woodward Ballard, is to be her life’s work.

Holly Woodward-WCA-Branvold Quarry-Aug5-2015

Dr. Holly Woodward Ballard at Branvold Quarry, August 2015; Photo taken by Dr. Karen Chin, courtesy of Dr. Woodward Ballard

Maiasaura peeblesorum was inadvertently discovered in the late 1970s, both by the people who initially found the bones and by the paleontologists who eventually described them.  “Inadvertently” because Marion and John Brandvold, the people who found the bones, didn’t know what they’d found, and because Dr. Jack Horner and Bob Makela—who had done extensive research prior to their expedition—did not expect to find the object of their search in a local fossil shop they visited on a whim.

The 1988 book “Digging Dinosaurs” by Jack Horner and James Gorman describes this discovery. In it, there is a fascinating anecdote: Prior to 1978—the year Maiasaura peeblesorum was found—they say that the number of adult fossils found globally could be listed in a volume the size of a book. The number of juvenile fossils could be listed in something the size of a pamphlet.  But the number of known baby fossils could fit on an index card.

All of that changed thanks to Dr. Horner and Bob Makela. The Brandvold bones gave them specific clues about where to look and what to look for.  Their subsequent excavations revealed not only numerous baby dinosaurs, but actual nests. These significant discoveries prompted the following revolutionary ideas: that some dinosaurs may have cared for their young and that they may have been warm-blooded. The latter hypothesis continues to be debated today.

Paleontologists have been digging in the area ever since.  Their efforts have produced one of the few species of dinosaur to be so well represented in the fossil record, a fact that inspired Dr. Woodward Ballard in her research at Montana State University.

Maiasaura field site Montana

Maiasaura field site in Montana, photo courtesy of Dr. Woodward Ballard

Jack Horner, her PhD advisor, proposed the idea that she focus on population histology—revealing the growth history of a specific dinosaur species.  Given her interest in osteohistology and the wealth of Maiasaura fossils, this seemed a perfect fit.  Her dissertation was but a prelude to the work that followed.

This past October, Dr. Woodward Ballard, now of Oklahoma State University, Dr. Liz Freedman Fowler and Dr. Jack Horner of Montana State University and Dr. Jim Farlow of Indiana Purdue University published a paper in Paleobiology on the growth and survivorship rates of Maiasaura peeblesorumThe paper was unique in that, unlike most dinosaur species, they had 50 bones with which to analyze and sample.

Bone microstructure, much like trees or proboscidean tusks, records the growth of an animal in rings. In this case, Dr. Woodward Ballard was able to identify the “lines of arrested growth” (or “LAGs” for short).

“A LAG,”she explained by phone, “represents a period of missing time.”

Growth rings in Maiasaura bone

Growth rings in Maiasaura bone, courtesy of Dr. Woodward Ballard

The paper is a fascinating glimpse into the depth of detective work paleontologists must do in order to understand long extinct species. Comparing bone growth in extant reptiles and mammals to these fossil bones, using complicated statistical models, and analyzing bone structure under the microscope, the authors offer an extraordinary view into the life of Maiasaura.  It is, to date, the largest sample set of a single dinosaur species analyzed to such a degree.

Fifty Maiasaura tibiae from three Montana bonebeds provided the details. This specific leg bone was chosen for analysis because it displays histology so clearly.  The same is not true, for example, of a hadrosaur femur.

“The femur,” Dr. Woodward Ballard said, “is special in all hadrosaurs, [not just] Maiasaura. It has this big flange coming off of it, and it’s this spur bone that a fairly large tail muscle was attached to.”

“Because bone responds to stress and remodels based on the stress that’s applied to it, this flange of bone is always changing and getting larger as the [animal grows.] The remodeling that occurs within [this] bone overprints–or erases–the original signal that was there. So it’s very hard to get at that same record of growth in the femur because it’s constantly being erased in that particular area.”

One of the things they discovered through lines of arrested growth (LAGs) was that most of the tibiae in this study belonged to Maiasaura younger than a year old.

But deciphering this required understanding bone growth in living species.

“We have to use modern animals and use what we see in their bones as a basis for what we say in the fossil record,” she replied when asked about this. “We have to assume that the same processes today were working back in the Cretaceous (in this case).”

So they looked to previously published alligator studies and those of the red deer on the Isle of Rum, Scotland—one of the most extensively studied mammals anywhere in the world.

Acknowledging that these inferences should be treated with some caution, they note similarities in tibia bone growth between alligators and Maiasaura. Growth marks within the bone and lines of arrested growth (LAGs) are similar in red deer and this species of dinosaur.

“When the growth is being kept track of from year-to-year, we find that one LAG appears every year for every year of growth,” she explained.

Hence, if there are no LAGs in the bone, it indicates that the animal was less than a year. And the high mortality rate among such young animals—considerably smaller than their enormous parents and therefore not as able, perhaps, to aptly defend themselves—is not necessarily surprising.  The paper also calculates survivorship rates among Maiasaura, enabling us to know how old the dinosaur was at sexual maturity, how long it tended to live, the age at which it was at higher risk for mortality among its species.

“Once I compiled the data from Maiasaura,” she said, “got all the bone measurements, got all the LAG circumference measurements within the bones—I realized that I wanted this paper to be more than just quantitative and simple growth curve graphs. I mean, I could do that much, but I really wanted it to be statistically strong, very robust, something that followed the rules put forth by other papers, such as the Steinsaltz and Orzack paper. [Dr. Liz Freedman Fowler] was just a natural choice to have to help me figure out what to do with all this data.”

————–

In Part 2: more detail about the Maiasaura peeblesorum survivorship curves, as well as applying complicated statistical methods to paleontological data.

An enormous and sincere thank you to Dr. Holly Woodward Ballard for her generosity: her time, her patience, her willingness to go over points I had difficulty understanding and for the beautiful pictures accompanying this post!

References:

  1. Maiasaura, a model organism for extinct population biology: a large sample statistical assessment of growth dynamics and survivorship; Holly N. Woodward, Elizabeth A. Freedman Fowler, James O. Farlow, John R. Horner, Paleobiology, October 2015
  2. Digging Dinosaurs, John R. Horner and James Gorman, 1988, Workman Publishing Ltd
  3. Largest dinosaur population growth study ever shows how Maiasaura lived and died, Montana State University, MSU News Service

Digging Dinosaurs book cover

Jack Horner - inscription for post

Treasured copy of “Digging Dinosaurs”, the book that details the discovery of Maiasaura peeblesorum and its nests, signed by Jack Horner at the Boston Museum of Science when the author of this blog met him in 2013