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Wednesday, 26 August 2020

The "palaeontological folklore" of mastodon hair

The American mastodon Mammut americanum is one of the most iconic members of the North American megafauna. A frequent subject of museum displays, books and technical research for over two centuries, we can all immediately conjure mental images of this long-bodied, relatively short-legged elephant-like animal. Nearly all of us will imagine mastodons the same way: that is, covered with a thick layer of brownish hair in a fashion reminiscent of its even more iconic cousin, the woolly mammoth. This is simply how we've come to understand mastodon life appearance through centuries of artistic and literary reinforcement (examples in familiar books include Špinar and Burian 1972; Benton 2015; Prothero 2017). Many texts don't even bother citing academic sources evidencing the claim: the hairy mastodons concept has been repeated often enough and long enough to be established fact. Zebras have stripes, lions have manes, and mastodons had a thick, brown covering of hair.

Zdenek Burian's 1964(?) take on the American mastodon: an entirely typical restoration of this species that would pass as a credible restoration at any point during the last two centuries.

It might come as a shock, therefore, to learn that the foundation of evidence behind our shaggy mastodons was actually entirely baseless for almost two centuries, and that this widely accepted concept has only gained a small amount of support within our lifetimes. This isn't a new revelation, either. American anthropologist and author Loren C. Eisley, a key figure in unravelling the strange history of mastodon soft-tissues, held no punches when describing mastodon hair as "items of palaeontological folklore" in 1945 (p. 108). Eisley's interest in mastodons was driven by ideas of their survival into recent centuries, where so-called mastodon soft-tissues discovered in the 19th century were interpreted as evidence of mastodons dying out just hundreds of years ago. Eisley published rebuttals to this concept several times during the 1940s and, in his 1946 Science article Men, Mastodons, and Myth, he specifically delved into the peculiar history of mastodon soft-tissue discoveries. Through historical detective work, Eisley uncovered a series of erroneous interpretations, failed corrections, and even deliberate subterfuge from the early days of American fossil exploration. A brief summary of his findings are provided below, but be sure to check out Eisley's account yourself for the full picture.

Lies, damned lies, and mastodon hair

The story of mastodon soft-tissue begins in 1800 Newburgh, New York, where a mastodon tooth and associated sample of coarse, 'dun brown' hair was extracted from a bog on the farm of a Mr. A. Colden. This specimen, said to have been so rotten that it decayed to dust within days, was a significant find. It was not only the first alleged mastodon hair, but also the first indication that some Pleistocene giants might have been covered in fur. It's among the more credible accounts of mastodon hair from the 19th century and, for many, was the best evidence of mastodon skin even as new finds were made later in the same century.

Charles Knight's awesome 1897 Mammut americanum painting. So far as I can tell, Knight's work is among the earliest restorations of this species and establishes the shaggy coat we'd come to associate with it. This is one of my favourite Knight paintings: just look at that landscape.

Shortly after the Colden Farm discovery, reports of mastodon soft-tissue then came thick and fast. In 1805 a mastodon was reported by Shawnee native Americans as having a long nose and mouth - interpreted, naturally, as a fossilised trunk. A report of a fossil mastodon stomach with gut content was reported at around the same time, as were more specimens with small amounts of hairy skin. Especially large sheets of skin were reported in 1839. These were reportedly so well preserved that they included arteries and sinew, but were too fragile to collect in one piece - only small fragments could be extracted. Collectively, such remains formed a significant dataset regarding mastodon soft-tissues, from internal organs to external features. They are the seed from which the concept of hairy, brown mastodons grew, and by the end of the century hairy mastodons were described in textbooks (e.g. Hutchinson 1893 - note that Hutchinson's book contains a rare Joseph Smit illustration of a nearly hairless mastodon) and featured in influential Charles Knight artworks (above),

But if this evidence is so great and extensive, why are none of these specimens better known? Are they not on display in museums, or at least illustrated in a book or paper? It turns out that there's a good reason you've never seen them: they're all completely bogus. Not a single example of mastodon soft-tissue reported in the 19th century was accessioned to a museum, virtually none were examined by individuals with paleontological expertise, and no detailed reports were ever made. Some clearly never existed and those that did were almost certainly erroneous takes of otherwise unremarkable objects.

Eisley's investigation assumes that some of these misinterpretations were honest mistakes by inexperienced parties, or perhaps over-interpretations of field reports (e.g. the 'long nose' specimen account makes no actual mention of soft-tissues, and could pertain to osteological observations alone). He attributes these errors to the early 1800s being a time of great excitement about the then newly-discovered Siberian frozen mammoths, and anticipation that fossil American proboscideans would return remains of similar quality. This might explain why some cases, such as with the alleged mastodon stomach specimen, are just outright strange. Gut tissues are among the first organs to decay when animals die and it would be very odd for a stomach to survive, alone, after the rest of the animal had rotted away. This discovery sounds a lot like the sort of over-zealous interpretation that might be made by someone naive about taphonomy and, indeed, this specimen was quickly subjected to rebuttals and corrections from more experienced scholars. ;

But other accounts were likely dishonest from the start - tall-tales to excite interest in fossil specimens set for display in private museums and touring shows. The early 19th century was a time before public museums existed in the USA, so fossil remains were exhibited to the public through private enterprise: hype and publicity-seeking were important to making such efforts financially viable. It's in this context that we have to view the 1839 Missouri mastodon reported to have extensive amounts of preserved skin, arteries and sinew, which was reported by non-other than Albert Koch: the same museum proprietor responsible for reconstructing Basilosaurus as a sea serpent and a Missouri mastodon as the 'Missourium' - a monstrous assemblage of mastodon and wooden blocks toured for public display in the mid-1800s. Koch's exaggerating, fraudulent approach to the conveyance of paleontological data robs all credibility from his accounts, and no-one can take his unverified claim of giant skin samples seriously. The fact that he never followed up on these seemingly remarkable soft-tissue remains is further evidence that they never existed.

Albert Koch's "Missourium" - a composite and distorted mastodon skeleton augmented with bits of wood to make the vertebral column longer. The Missourium specimen was collected around 1840, and I wonder if Koch's accounts of giant skin samples were associated with its discovery.

But what of the Colden Farm find which - while still entirely anecdotal - at least mentioned the unstable nature of its mastodon hair, and thus accounted for its absence in current collections? This 1800 discovery gains additional credibility in pre-dating our knowledge of frozen mammoths in Russia, and must therefore have been an unbiased, honest interpretation of alleged mastodon material. But, again, particulars of this find are peculiar: isn't it strange that an entire mastodon disappeared to leave only a solitary tooth and a patch of hair behind? This is taphonomically very odd, but was not an isolated incident: other fragmentary bones with patches of hair were also recovered from swamps in the same area. The American geologist James Hall provided an explanation of such sites which is far more consistent with our understanding of animal decay patterns. As part of a wider survey of the geology of New York state, Hall found the swamps yielding these fragmentary mastodon remains were filled with a hair-like algae known as conferva. It was a good match, size and morphology wise, for the alleged mastodon hair, and when desiccated, it turned - you guessed it 'dun brown'. When describing a mastodon site in 1843, he wrote:

In a small muck swamp in Stafford, Genesee County, a small molar tooth was found several years since. Its situation was beneath the muck, and upon a deposit of clay and sand. A large quantity of hair-like confervae, of a dun brown color, occurs in this locality; and so much does it resemble hair, that a close examination is required to satisfy one's self of its true nature.

Hall 1843 (from Eisley 1946, p. 522)

Hall was one of the few truly experienced and qualified individuals to write about mastodon soft-tissues in the 19th century, so his assessment is of real interest to this story. This is not to discount the insight of the pastors, farmers and businessmen behind other accounts, but Hall's explanation certainly sits better with our modern understanding of taphonomy, as well as the fact that those New York swamps have - even today - yet to yield a single scrap of incontrovertible mastodon hair. For Eisley, if there's any truth to these early takes on mastodon hair, Hall nailed it: the New York mastodon hair samples were simply misidentified dry algae.

'Conferva' is not a widely used term nowadays, but once pertained to a great number of filamentous green algae species. It's easy to see how examples like the above might be mistaken for mastodon hair by naive parties. Image by Anne Dixon, from 1843-45, borrowed from Getty Museum.

Without any specimens to examine, we cannot be certain today whether Hall and Eisley were correct, but their work clearly shows that 19th century claims of mastodon hair are suspicious. This is the line taken by at least some modern authors writing about mastodon hair (e.g. Hallin 1989; Haynes 1991; Larramendi 2015) but, as we know from history, most people ignored both Hall in the 19th century and Eisley in the 20th to perpetuate the discredited concept of hairy mastodons. How did such questionable data become the established, unquestioned truth about mastodon life appearance? Eisley (1946) attributed this to the illusory truth effect, where the repetition of a claim by perceived authorities makes it seem factual and truthful, regardless of the underlying evidence. In this case, enough scientists, museums, books and other media have towed the hairy mastodon line to transform folklore into 'fact', seemingly without anyone wondering where the real evidence of mastodon hair was. As Eisley put it:

In the midst of this constant repetition of what, through the sheer prestige of age, has come to be accepted as undeniable fact, it has never been pointed out that American institutions of science do not possess the tangible evidence which alone could justify such wholehearted faith in the exact appearance of this long-vanished beast.

Eisley 1946, p. 517.

I struggle to think of a case where interpretations based on a comparably feeble palaeontological dataset have been rehashed so uncritically for so long, so regularly and so publically. There is ordinarily some pushback against wholly baseless ideas of extinct animal life appearance, even if only among specialists, but I can only find a handful of articles promoting non-hairy takes on mastodon life appearance from the last century. In light of dedicated efforts by the likes of Eisley to set the record straight - published in Science, no less - it's really quite baffling that we've unwaveringly promoted hairy, brown mastodon for so long.

Finally: a real specimen

The situation around mastodon soft-tissues has changed somewhat today. In the 1980s, Pleistocene mammal expert Kurt Hallin published two abstracts and one popular article about the first genuine chunks of mastodon skin, both of which were covered in hair (Hallin and Gabriel 1981; Hallin 1983, 1989). But if you're hoping that this finally gives us real insight into mastodon life appearance, you're out of luck. To my knowledge, this specimen has never been described or illustrated beyond these short works, and a scanning electron microscope image of a single hair is all that's been figured of it (below). This short paragraph by Haynes (1991) provides one of the more detailed overviews I could find:

Specimens of what appears to be carbonized skin holding together bundled and fine hairs interspersed with hollow, coarser hairs may be the only preserved Mammut soft tissue currently known. These specimens were recovered by Krut Hallin in association with cranial fragments found near Milwaukee, Wisconsin (K. Hallin 1989 personal communication). The preserved guard hairs are hollow, a common enough trait in mammals, including woolly mammoths and African elephants. The underfur appears similar to that of semi-aquatic mammals such as the otter and beaver (Hallin 1983, 1989; Hallin and Gabriel 1981), in that it is very fine and wavy, and grows in dense bundles.
Haynes 1991, p. 34.

30 years on, these brief reports remain our only direct evidence of mastodon hair (Haynes 1991; Larramendi 2015), leaving the life appearance of the American mastodon not significantly advanced from Eisley's dismantling of 19th century discoveries from 75 years ago. While the Wisconsin hair specimen represents a potential step forward, it remains in a scientific grey area for having never being described or illustrated in detail, and Hallin's interpretation has never been subjected to peer review. Its significance to mastodon palaeobiology and life appearance thus remains an open question, as does the nature of mastodon skin in general. After all, the Wisconsin specimen only represents skin from the cranium, and we really need skin from different regions of extinct animal bodies to be confident about their full appearance. ;With such little data to hand, the whole body integument of mastodons remains mysterious (Haynes 1991; Larramnedi 2015).

Hallin's (1989) SEM shot of mastodon hair from Wisconsin - that's it on the left. To my knowledge, this is the only published image of mastodon hair.

Could mastodon still be hairy, though?

But this is not to say that we should just give up on restoring mastodon life appearance, however. These are relatively well-understood fossil animals, and we can surely say something about their skin from details of their anatomy, ecology and palaeobiogeography. If we are to play this predictive game, perhaps the most important first step is to realise that woolly mammoths - which are clearly historic inspiration for mastodon art - are not great analogues for American mastodon. The familiarity of woolly mammoths makes it easy to forget they were genuinely strange, specialist species adapted to extreme cryoarid Pleistocene habitats (Boeskorova et al. 2016). They relied on thick, three-tiered hairy coverings, generous adipose layers, shoulder fat humps, small ears, shortened tails, and trunk tip 'hoods' to protect themselves from extremely cold winters and periodic food shortages (Boeskorova et al. 2016). American mastodon, by contrast, were not Ice Age specialists. They actually evolved during the warmer Pliocene conditions pre-dating the Pleistocene glaciations and, even during the Ice Age, avoided the extreme climates endured by woolly mammoths. The distinct habitat preferences of mastodon and woolly mammoths are demonstrated by their remains being very rarely found in the same fossil horizons (Graham 2001; Hodgson et al. 2008). Woolly mammoths were denizens of steppe, tundra and forest habitats in northern regions (specifically, Alaska, the upper midwestern and northeastern United States, and the northern Atlantic coastal plain), while mastodon preferred wetter woodlands in more southerly locations (the eastern US (extending as far south as Florida), southeastern Canada, and parts of Mexico) (Haynes 1991; Graham 2001; Newsom and Mihlbachler 2006). Mastodon thus enjoyed a range of habitats and climates: northernmost populations inhabited boreal forests, living alongside moose and beavers, while those of Florida and Mexico inhabited relatively balmy swamps and woodlands, sharing their environment with reptiles and amphibians (Hine et al. 2017). Without soft-tissue mastodon fossils we can't truly assess their cold adaptations, but Larramendi (2015) noted that Mammut tails are long for proboscidians. This contrasts with the especially short tails of woolly mammoths and might have implications for mastodon thermal energetics - in other words, they weren't feeling the cold enough to shorten their tails (Larramendi 2015). These details of ecology, biogeography and anatomy demonstrate how different mastodon and woolly mammoths were, and caution against our porting of mammoth skin to their stockier cousins.

Woolly mammoths are very familiar to us, but we should not forget that they were not 'normal' giant animals. They were specialists adapted to life in extremely cold, dry habitats and, like most specialists, their lifestyle required a glut of anatomical weirdness. We should not readily assume that they are suitable model species for other giants. This 2018 image shows M. primigenius meeting a family of Neanderthals.

Having taken mammoths out of the picture, we can focus on a key question about mastodon: would they have benefitted from a full-body covering of hair? To answer this, we have to consider their giant size, and what that means for their thermal energetics. I'm increasingly of the opinion that that, when considering the life appearance of gigantic, multi-tonne extinct animals, we should be justifying the presence of thick layers of insulation, not the removal of it, and I feel that approach has merit here. American mastodon averaged body masses of 8 tonnes, making them larger than not only their distant woolly cousins (Larramendi 2015) but also considerably heavier than an average living elephant of any species. As discussed at length in previous posts, even at these smaller masses, elephants are simply so big that they really don't feel cold that much, to the extent that some African populations endure months of sub-freezing nights despite their lack of hair (Haynes 1991). Indeed, we know that elephants likely rely on these cooler periods to survive, as their size and low surface area-volume ratio present them with numerous challenges related to heat loss during the day. Among other issues, they cannot shed heat as quickly as it is generated during exercise, so their bodies regularly warm to dangerous hyperthermic temperatures. With little direct control over their body temperatures - they cannot even sweat or pant - they rely on features of their environment - bodies of water for drinking, trunk spraying, wallowing, cool nighttime temperatures etc. - to remain cool (Wright and Luck 1984; Weissenböck et al. 2012; Rowe et al. 2013).

Given that mastodon represent an even stockier, heavier and shorter-legged variant on the elephant body plan, they surely faced similar, or even more pronounced, challenges on this front. Cooler Pleistocene temperatures would have alleviated these concerns somewhat, but were unlikely to solve them outright. Elephants generate their own climate envelope around their bodies when exercising so that, even away from their very hot natural ranges, they overheat when exercising for long periods (Rowe et al. 2013). The insulative effects of fibrous integuments compound considerably with body size - a layer of fuzz over a one-tonne animal has a significantly greater insulative effect than the same layer on a 1 kg animal (Fariña 2002; Porter and Kearney 2009). Thus, even if smaller mammals living alongside mastodon required fur coats, mastodon themselves may not have needed them. Based on the thermal tolerances of living animals, we can predict that a blanket of fur all over a mastodon would have a significant impact on their thermal physiology, likely pushing their thermal neutrality (crudely explained, their ambient temperature 'comfort zone') many degrees below zero. Consider that Fariña (2002) calculated thermal neutrality for a naked 4 tonne Megatherium as -17°C*: I not sure what the thermal neutrality of a mastodon is, but their additional 4 tonnes of mass would surely push their thermal neutral figure even lower, maybe much lower. These values are only indicative because they assume dry, windless conditions, and both wind and rain make animals more vulnerable to cold, but they give a sense of how cold-resistant multi-tonne animals are. I'm guessing that Floridan mastodons felt pretty hot a lot of the time.

*Before you ask, this calculation also assumed a placental-typical metabolic rate, so the comparison to mastodon is apt.

Another question we might ask is how prone animals inhabiting generally cold habitats, such as boreal forests, are to heat stress. Does the cold in these settings negate any risk of overheating? Weighing up to 750 kg, moose and elk - members of Alces - are some of the largest modern animals to inhabit boreal forest, and their thermal energetics are well studied. They thus provide useful insight into how large animals cope with cold climates. Winters in boreal forests can get seriously cold - an average day might be -20°C - but moose are actually still prone to heat-stress during these months (Dussault et al. 2004; van Beest and Milner 2013). These aren't just occasional occurrences, either: moose have to manage their heat-stress continually via habitat choice and behavioural modification. If they don't, the associated energy drain to cool their bodies dramatically impacts their mass and health over the winter months (van Beest and Milner 2013). If these cervids find their whole-body fur coats stifling in boreal forest winters, how would mastodon - creatures ten times their size or more - cope with similarly hairy skin? I appreciate that this comparison is very crude - deer are not proboscidians, and modern boreal forests might be climatically different to those of the Pleistocene - but it demonstrates that we should not automatically equate cold climates with cold-stressed animals, and that we should think about the physiological implications of insulating fossil giants, even in frosty settings.

An American mastodon in the cypress swamps of Pleistocene Florida, sporting a hairy face and shoulders, but a largely hairless body. This looks odd compared to our standard fully-hairy restoration, but is consistent with our cranial mastodon hair sample and the thermal physiology of very large proboscideans. Happily, there's enough hair left for an obvious Simpsons gag. Mmm... mastodon fresh.

Putting all this together, there are clear grounds to question whether mastodon needed full hair coats. Consider the evidence: we know that elephant-like mammals already struggle more than other species with heat loss; that an 8 tonne animal is going to have very low thermal neutrality even without a fur coat; that mastodons were not universally denizens of especially cold settings; and that at least some parts of their anatomy - their tails - show opposing conditions to hairy, cold-adapted relatives. Based on this, I'm wondering if parts of the mastodon body - maybe even large portions of it - had reduced or absent hair to negate heat stress (above). This may have been especially important for mastodon living in warmer southern regions or areas with hot summers. Perhaps seasonal or geographic variation in hair growth and distribution was also employed, so that mastodon living in the warm south were almost as hairless as living elephants, and those exposed to cold winters were a little hairier? Of course, we cannot ignore the data from the Wisconsin hair specimen, which shows at least some mastodon faces were hairy, but we should be cautious about extrapolating this to a full-body covering. Proboscidian faces are actually one of the few parts of their bodies which are vulnerable to the cold, as their ears and trunks can develop frostbite (Haynes 1991). The face is thus one of the regions we might expect hair to be present in mastodon enduring cold winters, and it may not reflect the thermal demands of the rest of the body. Clearly, what's needed here is a dedicated study factoring mastodon mass, body surface area, metabolism, and heat loss against a nuanced consideration of Pleistocene climates: there are lots of equivalent studies on other fossil animals, and it would be great to get some real data and investigation on this. Until then, I find Asier Larramendi's (2015) comment on this matter an excellent summary: "...the relatively long tail... and the massive body of M. americanum suggest that the prevalent ideas that these animals were covered with a thick coat of fur are probably exaggerated".

From folklore to parable?

Of course, the main point of this article isn't to explore mastodon appearance in detail, but to stress that the nature of their skin is nowhere near as well-evidenced or understood as we've historically implied. I feel that those of us involved in education, research and palaeoart have a job on our hands to revise our misplaced confidence about mastodon life appearance, and to become more open-minded and explorative about what these iconic animals may have looked like. We should also take a moment to think about how this matter differs from our conventional issues with depictions of fossil animals. While it's not uncommon to bemoan how prehistoric animals are discussed and communicated to the public, we mostly do so because of pop culture influences - a film, TV show or book that perpetuates old tropes or fabricates something ridiculous about a prehistoric species. But there's no sensationalised media behind the perpetuation of hairy mastodons: these misleading depictions have come directly from decades of academic texts, officious museum displays and professionally produced palaeoartworks repeating the same unverified facts over and over, all the while ignoring direct rebuttals of the sketchy data behind this depiction. This one's on us, in other words. Perhaps it's time to turn this the "palaeontological folklore" of mastodon hair into a "palaeontological parable": a story to remind us to check and verify even the most basic and well-known information about our subjects from time to time, to ensure we're actually communicating science, and not rehashing mastodon-sized tall tales.

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