Of dwarfs and dragons

Stegodon

Little and large. Two different Stegodon species; the large mainland species on the left, with the smaller dwarf species on the right. (Art by Matt Salusbury)

Stegodons were just one branch of the rather diverse probiscidean family tree (the family that includes elephants, mastodons, gomphotheres and mammoths). The stegodons, like the mammoths and elephants, are generally thought to have arisen in Africa, and ended up in Asia, migrating all the way to what’s now Japan before this line eventually became extinct. The preponderance of stegodon remains suggests they some species may once have been very common and numerous.

The conventionally-sized stegodons of mainland Asia were probably slightly bigger than the biggest of today’s elephants. While they would have looked quite a lot like elephants, it is likely that they probably had their tusks closer together than their modern counterparts. These enormous tusks would have been so close, that they couldn’t have let their trunk dangle down between them like modern elephants do, so they may have draped their trunk over the edge of one of their tusks. Most stegodon species were massive, up there with the tallest of the mammoths, somewhere around 3.9 metres (13 feet) in height at the shoulder, almost as tall as a double decker bus.

The largest Stegodon species reaching the size of the woolly mammoth. (Image from here)

The largest Stegodon species reaching the size of the woolly mammoth. (Image from here)

The stegodons’ gently curving tusks were longer than their trunks, and some fossils have been found which were 3 metres (well over 9 feet) long. They may have had longer front legs than back legs, with a sloping back. But most of the differences between stegodons, mastodons, mammoths and elephants were down to the teeth or the ridges or layers of enamel on their teeth, or even just the thickness of the layers of enamel on their teeth. “Stegodon” means “roof tooth,” and these giants had very distinctive teeth for a different diet than that of modern elephants.

Stegodons included the earliest known examples of island dwarfism among the Proboscidea, dating all the way back to around 16 million years ago, long before the modern elephants arose. From the landmass that became Japan (imaginatively called ‘proto-Japan’), the gompothere Stegolophodon lived long before the emergence of the Elephantidae (the family that includes the forest elephant, the savannah elephant and the Asian elephant, as well as mammoths). It is thought that Stegolophodon gave rise to all later Stegodon species.

There was a “dwarf” version of Stegalophodon, which like its huge ancestor had four tusks, two coming out of the top jaw and also two emerging from the bottom jaw. The fossils so far discovered don’t show for sure whether the bottom-jaw tusks protruded from the gum enough to have been visible. It may have taken Stegalophodon several million years to evolve into its proto-Japanese dwarf version. This is incredibly slow for island dwarfism to evolve, and very likely indicates that the ancestors were not isolated from other populations and only truly evolved dwarfism when they were completely separated.

Then there was the-not-actually-all-that-small dwarf stegodon Stegodon aurorae. Found at two sites in Japan from about 1.8 million years ago, this species reached a still very impressive height of 8 or 9 foot (up to 2.7 metres) at the shoulder. This big dwarf was still pretty small compared to it’s mainland ancestor (which was around 12ft, or 3.6metres).

The shorter legs of some of the pygmy elephants and pygmy stegodons may have been an adaptation for what palaeontologist Paul Sondaar described as “low gear locomotion”, allowing these pygmies to scramble up and down cliffs and over rocks to reach upland pastures. Sondaar has even suggested that the tendency of island elephants to evolve into pygmies had more to do with the rugged terrain of the islands and the need to scramble up and over such terrain on the islands than any food shortages elephants may have encountered there. Size estimates for another dwarf stegodon, Stegodon sondaari, like all the prehistoric pygmy probosicidians, are forever being revised. For the moment, it’s generally held to have been 20-30% smaller than the Java version of Stegodon from which it was descended.

The Indonesian island of Flores is well known for the miniature human species Homo floresiensis (also known as the ‘hobbit’). In the past, this island has been home to other wonderful beasts, including another pygmy stegodon, Stegodon sondaari. This shrunken beast also had short legs in proportion to the rest of its body, and a very short jaw. Stegodon sondaari is thought to have been smaller than a water buffalo, making it about half the size of its presumed mainland ancestor, the slightly pointy-headed Stegodon trigoncephalus.

Homo (Image from here)

Homo floresiensis skull, roughly about half the size f a modern human skull.  (Image from here)

There’s a confusing array of different fossil stegodons, stegolophodons and elephants from Indonesian islands of Flores and nearby Sulawesi and from other islands of the Indonesian archipelago. We’re even unsure about whether some of these were elephants or stegodons. One of the many fossil species from Sulawesi, for example, is currently known as Elephas celebensis because – for the moment – we think it was more likely to have been an elephant than a stegodon. Elephas celebensis was an earlier arrival to Flores than the later “mid-sized” Stegodon sondaari, which was the dwarf descendant of a second wave of proboscidian arrivals to the island in Mid-Pleistocene period – between 780 and 120 thousand years ago.

Stegodon sondaari is presumed to have died out about a million years ago, before the arrival of anything even vaguely human-like on Flores. There is some puzzlement about the still living Komodo dragon, named after the Indonesian island of Komodo, much smaller than Flores and immediately to its West. Komodo dragons, at up to three metres long from head to tail, are the world’s biggest living land reptile. These humungous lizards still live on Komodo, and on three other equally tiny islands close by. There are even believed to be populations of them left in isolated pockets along the coast of much bigger Flores immediately to the East.

The bewilderment surrounding the Komodo dragons is how it could have grown so big on local islands, where there aren’t any reasonably big indigenous animals by way of prey. (Its current diet of deer, goats and pigs was introduced much later.) Biologist Walter Auffenberg speculated in the early 1970s, as have others since, that when the large monitor lizards that were the presumed ancestors of the Komodo dragons arrived on the islands the original proto-Komodo dragon food would have been some kind of pygmy probiscidian like the little Stegodon sondaari. While large mammals tend to get smaller over time on islands, it was thought the Komodo dragons had evolved into bigger forms.

Relatively recently, thoughts have changed again about Komodo dragons. In 2004, palaeontologists showed that the the dragons kept more or less the same size as their ancestors, which were from the mainland Australia. The fossils show that their rage was much bigger than it is currently, and that these incredible reptiles are not an example of island gigantism (the opposite of island dwarfism).

Another, later wave of arriving stegodons clambered ashore on the Flores in the Late Pleistocene, eventually evolving into Stegodon florensis insularis. The dwarf hominids, Homo floresiensis, that shared the island, along with cat-sized giant cave rats and giant flightless storks, may have killed and eaten Stegodon florensis. The various reconstructions of this species that I’ve seen put it somewhere between the size of a modern water buffalo or slightly taller at the shoulder than the height of a modern human.

Comparasion of Stegodon and Homo fle (Image from here)

Comparasion of the mini-Stegodon and Homo floresiensis with modern human an African elephant.  (Image from here)

However, caution needs to been read into reconstructions, as  nearly all the stegodon bones of the butchery site at Flores’ Liang Bau caves were juvenile. The current thinking  is that Homo floresiensis killed and ate the juveniles of yet another stegodon – a larger, more conventional-sized and more recent arrival. Given their own size and primitive weaponry (we’re unsure whether they had discovered how to make fires or not) the “hobbits” may have wisely decided only to go for the juveniles of this species and to stay away from the bigger adults where possible.

Sometime around 17,000 years ago, the dry climate of the island of Flores suddenly started to get wetter, and then after that came a big volcanic eruption. These two events seem to have finished off all of these dwarf and giant exotics that had been unique to Flores. Homo floresiensis – probably along with the  dwarf and conventionally-sized stegodons it hunted – had disappeared by the time modern humans showed up shortly later on Flores around 11,000 years ago.

Note from the author:

This text is adapted from my book Pygmy Elephants (CFZ Press, 2013, http://pygmyelephants.blogspot.co.uk, @Pyg_Eleph), and the attached drawings are all from Pygmy Elephants too.  I drew the “Stegodon sondaari and monitor lizard” illustration back in 2010, showing an S. sondaari running away from a solitary monitor lizard. Since then I’ve seen photos of a sort of Komodo dragon feeding frenzy over a dead deer, which suggests the monitors that were the ancestors of Komodo dragons may have hunted in packs.

Written by Matt Salusbury (@Pyg_Eleph)

Edited by Jan Freedman (@janfreedman)

Further Reading:

Giant fossil bird found on ‘hobbit’ island of Flores, BBC News, 

http://news.bbc.co.uk/earth/hi/earth_news/newsid_9261000/9261713.stm quoting Zoological Journal of the Linnean Society.

Cavaretta, C., Gioia, P., Mussi,M., & Palombo, M. R. (2001), ‘La Terra degli Elefanti Alti del 10 Congresso Internazionale’, Consiglio Nazionale della Richerhe, Rome. [Full article]

Shoshani, J., & Tassy, P. (1996), ‘The Proboscidea’, Oxford University Press. [Book]

Sondaar, P. (1994), ‘Paleoecology and evolutionary patterns in horses and Island mammals’, Historical Biology, Volume 8, Issue 1-4. (Paul Sondaar proposed the “low gear locomotion” idea in this and other works. [Full article]

van der Geer, A, Lyras, G., de Vos, J., & Dermitzakis,M, (2010), ‘Evolution of Island Mammals: Adaptation and Extinction of Placental Mammals on Islands’, Wiley-Blackwell. [Book]

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2 Responses to Of dwarfs and dragons

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