Dr. Advait Jukar – Solving Mysteries in South Asian Fossil Communities

Dr. Advait Jukar–Deep Time – Peter Buck Fellow at the Smithsonian Institute–wants to really understand ancient ecosystems in South Asia, but doing so means beginning with some of the very basics. Challenges include not just a lack of available fossils from the region, but also the lack of detailed records from early paleontologists and a dearth of contemporary research.

He is, in a sense, an explorer.  (All paleontologists are.)  If we think of paleontology as a sculpture in progress, many scientists are working on the fine details.  Dr. Jukar, on the other hand, has the clay and the tools, but the sculpture itself hasn’t even begun to take form.

Consider what he has to work with: isolated proboscidean teeth and skulls, for example, collected by Hugh Falconer and his crew in the early 1800s.  They didn’t record where the bones were found, let alone where each fossil was in relation to the other.  Those who later described these fossils made dubious claims regarding the species.  And few people to this day have revisited this data or expanded upon it. 

 

Image of Dr. Advait Jukar at work with an Elephas hysudricus molar; courtesy of Dr. Jukar

 

Compare this to Maiasaura fossils in the Northwest US.  So many fossils of this species have been excavated that Dr. Holly Woodward Ballard has created the Maiasaura Life History Project.  Its goal is to uncover more details about this particular species than any other currently known extinct creature.  She has a wealth of data at her disposal. Unlike Dr. Jukar, the fossils she can study have been found fairly well articulated, very well documented, and in remarkable abundance.  There are adults, sub-adults, juveniles and embryos.  She and her colleagues are able to add to existing scientific literature using the latest technology.  It’s an exciting project with absolutely fascinating possibilities.

 

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 for this post

 

 

 

But so, too, is Dr. Jukar’s intended research.   Focusing on the tail end of the Neogene, about 4 million years ago, through the Quaternary, he wants to understand herbivorous mammals—their community, their ecology, their biogeography.  It’s just a question of building the necessary foundation first.

“I started to compile all of these species lists,” he explained by phone, “and saw that there were lots of species of proboscideans in South Asia during that period of time.  We have gomphotheres; we have stegodons; we have elephants.”

One way to understand an animal’s impact on its environment is to assess its body mass.  How big (or small) were these animals?  And therefore, how much did they need to eat?  A larger animal would presumably need to eat a larger amount of vegetation.  Similarly, a larger animal might reproduce less frequently than smaller animals.  Body mass reveals clues about how an animal fits into the ecosystem.

However, he continued, “I hit a wall because there was no way for me to estimate how big these elephants were.  The problem was they were largely known either from skulls or teeth, and the traditional methods to estimate the weight of an extinct elephant were using shoulder height or the length and circumference of the long bones. So if I have a skull but I don’t have long bones, I’m sort of in a bind, because now I can’t estimate how much this animal weighed when it was alive.”

He looked to methods that others have used in the past. One method used by his colleagues at Howard University seemed to be a promising fit.  They used the occipital condyle breadth of seacows—a proboscidean relative–as an indicator for body mass.  Dr. Jukar’s PhD advisor, Mark Uhen, mentioned that this method had also been used on yet another large mammal: the whale. 

The occipital condyle is a bone found at the base of the back of the skull, connecting the skull to the spinal column.  It’s a relatively small bone.  Why would this have an impact on determining body mass?

“If occipital condyle breadth is correlated with the size of the animal,” Dr. Jukar said, “and if the occipital condyle is the point where the skull attaches to the rest of the skeleton, then maybe the size of the skull scales with the size of the overall body. And if that’s true, then maybe the occipital condyle breadth will scale with the size of the limb bones as well.”

 

Image of the back of a mastodon (nicknamed ‘Max’) skull at the Western Science Center in California displaying the occipital condyle bone resting on the metal stand; photo taken by Jeanne Timmons

 

In other words, if a paleontologist has but one skull of an extinct proboscidean and no other related fossils, can that person measure the breadth of the occipital condyle as a way to determine the size and weight of that animal?

To test this theory on proboscideans, he researched available scientific literature and visited a number of museum collections.  Ultimately, he and his two co-authors, S. Kathleen Lyons and Mark Uhen, compared the occipital condyle breadth to the length and circumference of leg bones within extant elephants and extinct relatives.  Two elephant species were studied, as were six gomphotheres, three mastodons and one stegodon.  

Image of a fossil Moeritherium at the Yale Peabody Museum; photo taken by Jeanne Timmons

 

While careful to note that this method has its limitations, the results were promising.  The equations are different for each proboscidean family (gomphothere body structure and size is not the same as that of a mastodon) and they do not work for some of the smaller proboscidean species, such as Moeritherium.  In layperson’s terms, this research works for taller, lumbering proboscideans, not those with much shorter limbs and a perhaps waddling gait. Their paper and its results were published in the Zoological Journal of the Linnean Society: A cranial correlate of body mass in proboscideans.

This, though, is just the tip of the iceberg in terms of Dr. Jukar’s research.  An enormous collection of fossils from India resides in the Natural History Museum of London.  Found in an area referred to as the Siwalik Hills (or the “Siwaliks”) at the base of the Himalayas, Scottish paleontologist Hugh Falconer and his team collected them in the 1800s.  Among them are several stegodon teeth and skulls. 

Image of Dr. Advait Jukar measuring a Stegodon ganesa fossil in the Natural History Museum of London collection; courtesy of Dr. Jukar

 

The two species of stegodon excavated from the Siwaliks are, to this date, known as Stegodon insignis and Stegodon ganesa.  The species have very similar teeth, but their skulls seem to differ greatly.  The skull of S. insignis, according to Dr. Jukar, is “almost triangular in shape with relatively small tusks,” which Falconer chalked up to sexual dimorphism.

“Which I just thought was the weirdest thing to ever say about stegodons because the skulls are clearly different. They’re clearly not sexually dimorphic.”

Moreover, there seems to be confusion regarding which fossils Falconer assigned to which stegodon species that continues to this day.  

“So what was going on in his mind? I have no idea.  It’s a problem! Because since then, people have said that both of these must be the same species without really truly investigating them. 

“Any Stegodon tooth that they’re finding in the Siwaliks, they’re calling Stegodon insignis or Stegodon ganesa or a hyphenated version of the two: Stegodon insignis-ganesa, which is taxonomic heresy.” 

And here Dr. Jukar was emphatic: “You CANNOT do that with the taxonomic code.”

“And that was Osborn’s fault.”

He was referring to Henry Fairfield Osborn, former professor then curator of the American Museum of Natural History in the late 1800s.

“Osborn [is known to have asserted], ‘I agree with what Falconer said, so I’m going to hyphenate these two words.’  Which created such a mess.  So we have no idea what’s going on there. 

“There’s a lot of work to be done with elephant taxonomy, biogeography and systematics and comparisons between China, the Levant, East Africa and India.”

 

Image of the Levant (Public Domain)

 

Dr. Jukar and other colleagues have also recently published papers on the earliest known fossil of Hexaprotodon, an extinct hippo, from South Asia, and the first record of a Hippaprionine horse (Plesiohipparion huangheense) from the Indian Pliocene.

He is currently working with Dr. Adrian Lister of the Natural History Museum in London to further understand the various proboscidean fossils in the Siwalik collection.

This is important work, but Dr. Jukar pondered its reception to the wider world.

“For a long time paleontologists have been criticized as being mere stamp collectors because we find things and then we name them and then we try to figure out in what larger group they belong to.  But that is the basis of our data.

“Only when I have a comprehensive sense of what the species are, when they lived and where they lived can I start doing these more complicated community-level analyses.

“But because the basic science of naming a fossil might not be very exciting, [as it doesn’t directly impact] human life very much, it doesn’t get a lot of attention. 

“I am definitely interested in the big picture questions of dispersal from Africa into South Asia, about the ecology of these groups, about how communities have changed through time, but I can’t really do a rigorous analysis until I figure out who the [basic] players are in this place.”

Image of Dr. Advait Jukar with a Mammuthus columbi (Columbian mammoth) skull; courtesy of Dr. Jukar

 

References:

  1. Colbert E. (1996). Henry Fairfield Osborn and the Proboscidea. In:  Shoshani J, Tassy P. The Proboscidea : evolution and palaeoecology of elephants and their relatives, Oxford: Oxford University Press, xxii – xxv
  2. Dr. Advait Jukar’s website: https://advaitjukar.weebly.com
  3. Jukar, Advait M., Lyons, S. Kathleen, Uhen, Mark D. (2018.  A cranial correlate of body mass in proboscideansZoological Journal of the Linnean Society, Volume 184, Issue 3, 20 October 2018, Pages 919–931, https://doi.org/10.1093/zoolinnean/zlx108
  4. Jukar, Advait M., Patnaik, Rajeev, Chauhan, Parth R., Li, Hong-Chun, Lin, Jih-Pai (2019). The youngest occurrence of Hexaprotodon Falconer and Cautley, 1836 (Hippopotamidae, Mammalia) from South Asia with a discussion on its extinction, Quaternary International, January 2019, https://doi.org/10.1016/j.quaint.2019.01.005
  5. Jukar, Advait Mahesh, Sun, Boyang, Bernor, Raymond Louis, (2018). The first occurrence of Plesiohipparion huangheense (Qiu, Huang & Guo, 1987) (Equidae, Hipparionini) from the late Pliocene of India,  Bollettino della Società Paleontologica Italiana; 57(2):125-132 · August 2018
  6. Saegusa H. (1996). Stegodontidae: Evolutionary Relationships. In:  Shoshani J, Tassy P. The Proboscidea : evolution and palaeoecology of elephants and their relatives, Oxford: Oxford University Press, xxii – xxv

 

It was a GREAT pleasure and honor speaking with Dr. Advait Jukar.  Many, many thanks for your time, Advait, your help, your fascinating insight and your gorgeous images!! I cannot wait to read your future scientific papers!

 

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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