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 proboscideans, Zoological 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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Stegodon: Does this ancient elephant have origins in Asia?

So much has been said in recent years about the wealth of fossils in China. Almost all of it about dinosaurs: exciting new species, feathered fossils, nest upon nest of dinosaur eggs.  There is no doubt that China holds exciting clues to the history of our planet; one has only to wait to hear of the next discovery.

Within the past few months, yet another exciting find was revealed, but this time about a little known mammalian ancestor: Stegodon.

 

Painting of a Stegodon by artist Hannah Stephens, hannahleestudio.com.

 

The name Stegodon, to me, evokes ‘dinosaur’, not ‘mammal,’ but this was, indeed, an ancient animal.  Its fossils resemble those of other similar mammals, from mastodons to mammoths to today’s elephants.

 

 

Figure 1: Reconstruction of four insular dwarf proboscideans with their respective mainland ancestors. Mainland proboscideans: 1, Palaeoloxodon antiquus; 2, Mammuthus columbi; 3, Stegodon zdanskyi [stegodon found in China]. Insular proboscideans: 4, Palaeoloxodon ‘mnaidriensis’; 5, Palaeoloxodon falconeri; 6, Mammuthus exilis; 7, Stegodon aurorae [a type of dwarf stegodon found in Japan]. Based on skeletons at Museo di Paleontología, University of Rome, Italy (1), American Museum of Natural History, New York (2), Taylor Made Fossils, U.S. (3), Museo di Paleontología e Geología G.G. Gemmellaro, Palermo, Italy (4), Forschungsinstitut und Naturmuseum Senckenberg, Frankfurt, Germany (5), Santa Barbara Museum of Natural History, Santa Barbara, U.S. (6), Taga Town Museum, Honshu, Japan (7). Photos 1–2, 4–7 George Lyras, photo 3 courtesy of TaylorMadeFossils.com, reproduced here with permission.

From The effect of area and isolation on insular dwarf proboscidea by Alexandra A. E. van der Geer et al; photo and caption courtesy of Dr. Alexandra van der Geer.

 

When Stegodon skulls with tusks attached have been found, many (but not all) of the tusks are close together–preventing the trunk (the ‘proboscis,’ from which this group gets its name; proboscis —> proboscidea) from hanging between them.

They lived in what is now Africa and Asia, causing continued debate over its place of origin. Until recently, the oldest known Stegodon fossil, a 6.5+ million-year-old partial molar from Kenya, was described by William J. Sanders in 1999.  That record changed this past December when Dr. Hong Ao and his colleagues published their results dating the sediment in the Lanzhou Basin, China, from which a number of fossils–including that of a Stegodon–were found.

And that Stegodon was found to be between 8 – 11 million years old.

 

Detail of the Geologic Time Scale, created by the Geological Society of America.  Stegodon is believed to have existed between the Miocene to the Pleistocene, a relatively small segment of time in Earth’s overall history, but still considerably longer than that of our own species!  (You can view the time scale in much better detail here.) 

 

The fossils of the Stegodon, along with at least 5 other species, were actually found in the 1980s by Professor Xing Zhang of Northwest University in China.  Given the length of time between the fossil excavations and the recent dating of these fossils, one might wonder why determining the fossil age took so long.

Dr. Ao, a scientist at the State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, and his colleagues state that western China does not have suitable material for radiometric dating, an oft-used method for this purpose.

When I asked why this was so, Dr. Ao replied by email, “Because volcanic activities are rare in western China during the late Cenozoic, it is difficult to find  in situ tephros or tuffs for radiometric dating (e.g., ⁴⁰Ar/³⁹Ar dating).”

Instead, they conducted a magnetostratigraphic study, one in which they determined the age of the rock through the polar reversal record.  Combining this with analysis of the fossils provided evidence that this Stegodon is 2 – 5 million years older than that of the Kenyan partial molar.

“We are indeed surprised by our dating results,” Dr. Ao continued, “which document that the Lanzhou Stegodon is the oldest Stegodon worldwide, although the Stegodon  fossil [was] not discovered by us. However, our dating results document it to be the oldest known Stegodon fossil.”

 

Images of Professor Yongxiang Li (from Northwest University) and his master student as well as several employed workers who helped to excavate mammal fossils, Lanzhou Basin, China; photos courtesy of Dr. Hong Ao.

 

Images of indricotherine fossils found in Lanzhou Basin, China; photos courtesy of Dr. Hong Ao.

 

Dr. Ao himself has been working with fossils in the Lanzhou Basin for 5 years.  When asked how  he and his co-authors chose to work together on this recent paper, he wrote, “I have collaborated with them on broad subjects before, thus I [invited] them to join [in] this research.”

Finding information about this extinct species is difficult.  Unlike mammoths or mastodons, Stegodon does not appear to be a popular ancient animal.

Fortunately, Dr. Alexandra van der Geer–paleontologist, indologist, ethno-zoologist and author –has not only studied this species, she very generously made herself available for questions.  Currently, Dr. van der Geer is an Associate Researcher at both the National and Kapodistrian University of Athens and the Naturalis Biodiversity Center in the Netherlands. She is part of the Isolario project, a project that studies biodiversity and cultural evolution within an island context.

When I asked why there was such a dearth of information on Stegodon, she wrote, “We can think of several reasons for this.”

“First of all, stegodons typically are the elephants of Southeast and East Asia, where most countries did not have the resources and opportunities most Western countries had when it comes to scientific research and excavations. These activities are costly, and since they don’t have a direct use in the sense that they don’t advance the medical, technical or economic levels of a country, they have, understandably, a lower priority. Furthermore, because of this region, half the publications (especially before the 1990s or so) you can find are in Chinese or Japanese, which is not very helpful to the [English-speaking] world.

“Secondly, stegodons are forest elephants. Forest areas are very unlikely places for the long-term preservation of organic materials: everything is eaten, digested or otherwise broken down into smaller components in no time. The tropical (warm, humid) climate of these forests is not helpful either, as decomposition is much faster here. Stegodon remains only have a chance to be preserved when (1) they are covered fast, such as with river sediments, volcanic ashes etc., (2) or are in an oxygen-free environment, such as sunken deeply into a swamp, (3) or were deposited in a natural fridge such as limestone caves where they are gradually covered in clayish sediment or [travertines]. The same is valid for Palaeoloxodon, the Old World fossil elephant, but Europe has many limestone caves, which are excellent for preservation (for a nice [travertine]-preserved negative skull, see Stuttgart museum: skull cast SMNS nr. 32888 from Bad Cannstatt).

“As you can deduct from these preservation issues, it is more likely to find molars and tusks than skeletal material, which is much softer. The vast majority of proboscidean findings all over the planet consists of molars and tusks, and that is not for nothing. Inherently this means that there is much more information about their dentition and diet than about their bodies.”

I was interested in understanding why Stegodons are portrayed as hairless animals, so very similar to contemporary elephants.  Was this just an artistic guess?

“The hairlessness of stegodons is not an artistic guess but a scientific guess instead,” Dr. van der Geer answered.  “Very large animals with thick skins (pachyderms) in a (sub)tropical environment are unlikely to have a significant hair covering. Elephants lost their hairs secondarily. The information for hair growth is not lost, and baby elephants still have a thin, woolly coat. Woolly mammoths lived in the cold, temperate zones, and needed hair, so they were covered in a thick layer of hairs, and for this is evidence (mummies preserved in the permafrost), but the other mammoths (M. meridionalis, M. columbi, M. exilis, etc.), [did] not, and it’s generally assumed that they had a light coat fitting to the temperate zones.

“Tropical and subtropical stegodons almost certainly did not have any coat that’s worth mentioning. Stegodons of temperate zones, however, may have been more hairy. Indeed, the lack of hairs makes them look more like today’s elephants.”

 

Photo of the two life-size models of Stegodon ganesa;photo courtesy of Dr. Gerrit van den Bergh (University of Woolongong, Australia); special thanks to Dr. Alexandra van der Geer.

 

“Note, however, that the proboscis is carried very differently. Their tusks are set very close to each other, so the proboscis doesn’t fit in between as in modern elephants, Asian and African alike. This means that the mobility of their proboscis was more restricted, relative to their living relatives.”

 

Fossils of Stegodon florensis insularis, from Flores, Indonesia; photo courtesy of Dr. John de Vos (Naturalis, the Netherlands); special thanks to Dr. Alexandra van der Geer.

 

Mandible (and holotype!) of  Stegodon timorensis; photo courtesy of Eelco Kruidenier (Naturalis, the Netherlands); special thanks to Dr. Alexandra van der Geer.  Anyone familiar with proboscidean teeth and jaws will recognize the similarities instantly.

 

But how do we know that Stegodon–a rather enormous animal–evolved into something smaller?

“[D]warfs and giants are relative. Something can be a dwarf, yet have a considerable size. When we speak of dwarf stegodons, we mean stegodons that are much smaller than their ancestors. For this, you have of course to have identical or otherwise similar elements from both the descendant and the ancestor in order to compare reliably,” she continued.

“The expectation is that dwarf stegodons must have existed on the islands, following the so-called island rule, according to which large animals get smaller in isolation. There is sound evidence that this rule still stands, and is even more pronounced for fossil species (see Lomolino et al., 2013, in Journal of Biogeography).

“Indeed, the many fossil molars from the Southeast Asian islands (‘Wallacea’) are all much and much smaller than the same molars from their mainland ancestors (see Van den Bergh, 1999). True, you first have to know what is the ancestor, and for this you need information about morphology, or how the molars, tusks, skulls and postcranial elements look like. After that, you compare the sizes.

“Note that if a molar is, for example, half the length of the same molar of its ancestral species, the body weight of that animal must have been a quarter of that of its ancestor! (the cubic law: linear reduction 50% means volume reduction 50% of 50%).”

Molar of Stegodon florensis; photo courtesy of Dr. Gerrit van den Bergh (University of Woolongong, Australia); special thanks to Dr. Alexandra van der Geer.  

 

“The most interesting dwarf stegodon is Stegodon sondaari, named after the Dutch palaeontologist Paul Yves Sondaar (1934-2003), expert in fossil insular mammals. This stegodon lived on the island of Flores about a million years ago, and weighted only about 15% of the weight of its ancestral species, S. elephantoides (see Van der Geer et al., 2016, in Journal of Biogeography, doi:10.1111/jbi.12743).

“Sondaar’s dwarf stegodon is not the smallest stegodon, that honour goes to the Sumba stegodon (S. sumbaensis), of only 8% of the original weight. Sondaar’s stegodon is interesting because it may have witnessed the arrival of early humans, possibly the ancestors of the Hobbit, or Homo floresiensis. Its fossils are contemporaneous with primitive lithic artefacts, dated to about a million years ago (see Brumm et al., 2010, in Nature 464, pp. 748–752).”

 

Molar (and holotype!) of Stegodon sompoenis; photo courtesy of Dr. Gerrit van den Bergh (University of Woolongong, Australia); special thanks to Dr. Alexandra van der Geer.  

 

“[R]ecently,” she concluded, “one of the island dwarf stegodons (S. timorensis of Timor) has been dated to about 130 thousand years ago (see Louys et al., 2016, in PeerJ 4:e1788). This excludes, according to the authors, an anthropogenic cause for its extinction, because humans had not yet arrived at the island.”

 

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So many people helped with this blog post!  (But please remember that any errors are my own.)

Many, many thanks to Dr. Hong Ao (Dr. Ao Hong) from the State Key Laboratory of Loess and Quaternary Geology (Chinese Academy of Sciences) for his fascinating responses and the great images of fossil excavations in the Lanzhou Basin.  I am thrilled that he was willing to answer questions about his research and that of his colleagues! It was a great honor and a pleasure connecting with him!

I am indebted to Dr. Alexandra Van der Geer, who very kindly (and so very quickly–despite everything else she has going on!!) answered specific questions about Stegodon that I could not find anywhere else and who provided pictures of dwarf Stegodon fossils.  It was an equally great honor and pleasure connecting with her!

A mastodon-sized thank you to the amazing Dr. Katy Smith for providing needed and hard-to-find material on Stegodon fossils!

And an enormous thank you to artist Hannah Stephens for her depiction of a Stegodon as it may have appeared in life.  I am particularly moved by the warmth of its intelligent-looking eyes, and I love the tones within its skin.  I adore this picture.  I am grateful to have it in this post;  I am thrilled to have the actual painting hanging on my wall!  Please be sure to check out her artwork at: http://hannahleestudio.com or http://hstephens.blogspot.com

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References from Dr. Alexandra Van der Geer:

1. Brumm A, Jensen GM, van den Bergh GD, Morwood MJ, Kurniawan I, Aziz F, Storey M (2010) Hominins on Flores, Indonesia, by one million years ago. Nature 464, 748–752.
2. Lomolino MV, van der Geer AAE, Lyras GA, Palombo MR, Sax DF, Rozzi R (2013) Of mice and mammoths: generality and antiquity of the island rule. Journal of Biogeography 40, 1427–1439.
3. Louys J, Price GJ, O’Connor S. (2016) Direct dating of Pleistocene stegodon from Timor Island, East Nusa Tenggara. PeerJ 4:e1788
4. van den Bergh GD (1999) The Late Neogene elephantoidbearing faunas of Indonesia and their palaeozoogeographic implications; a study of the terrestrial faunal succession of Sulawesi, Flores and Java, including evidence for early hominid dispersal east of Wallace’s line. Scripta Geologica 117, 1–419.
5. van der Geer AAE, van den Bergh GD, Lyras GA, Prasetyo UW, Due RA, Setiyabudi E, Drinia H (2016) The effect of area and isolation on insular dwarf proboscideans. Journal of Biogeography, doi: 10.111/jbi.12743

References used in this blog post:

1. New magnetochronology of Late Miocene mammal fauna, NE Tibetan Plateau, China: Mammal migration and paleoenvironments; by Hong Ao, Peng Zhang, Mark J. Dekkers, Andrew P. Roberts, Zhisheng An, Yongxiang Li, Fengyan Lu, Shan Lin, Xingwen Li; Earth and Planetary Science Letters; 1o December 2015
2. Oldest record of Stegodon (Mammalia: Proboscidea); by William J. Sanders; Journal of Vertebrate Paleontology; Vol. 19, No. 4, Dec. 13, 1999, pp. 793 – 797
3. Fossil elephantoids, Awash paleolake basins, and the Afar triple junction, Ethiopia; by Jon E. Kalb; Palaeogeography, Palaeoclimatology, Palaeoecology; 1995, pp. 357 – 368
4. The effect of area and isolation on insular dwarf proboscidea; by Alexandra A. E. van der Geer, Gerrit D. van den Bergh, George A. Lyras, Unggul W. Prasetyo, Rokus Awe Due, Erick Setiyabudi, and Hara Drinia; Journal of Biogeography; 11 March 2016.
5. Magnetostratigraphy – concepts, definitions, and applications, by Cor G. Langereis, Wout Krijgsman, Giovanni Muttoni, and Manfred Menning; Newsletter on Stratigraphy, Vol. 43/3: 207–233, April 2010
6. Mammoths and Mastodons of the Ice Age, by Adrian Lister, Firefly Books, 2014
7. Mammoths, by Adrian Lister and Paul Bahn, University of California Press, 2007
8. The Proboscidea: Evolution and Palaeoecology of Elephants and Their Relatives, Edited by Jeheskiel Shoshani and Pascal Tassy, Oxford Science Publications, 1996

• Stegodontidae: evolutionary relationships by Haruo Saegusa, pp. 178 – 190, The Proboscidea: Evolution and Palaeoecology of Elephants and Their Relatives
• Palaeobiogeography of late Neogene African and Eurasian Elephantoidea by Jon E. Kalb, David J. Froehlich, and Gordon L. Bell, pp. 117 – 123, The Proboscidea: Evolution and Palaeoecology of Elephants and Their Relatives