Are Sumatran rhinos really ‘living fossils’?

One of my least favourite terms in the whole of natural history writing is ‘living fossil’, and its use and meaning are on my mind right now as Loren Coleman (of Cryptomundo) and I have just been debating it. What exactly do people mean when they talk of organisms being ‘living fossils’, and does this term actually mean anything at all?

The event that sparked this off is the announcement that Sumatran rhinos Dicerorhinus sumatrensis have just been filmed on Borneo. Given how elusive the animals are (see below), this is a big deal, and all the more so given that the presence of a living Bornean population was only announced in 1986 (the discovery actually occurred in 1983 but was kept secret until 1986). Historically, Sumatran rhinos occurred across Sumatra and Borneo as well as north-eastern India, Myanmar, southern Bangladesh, the Malay Peninsula and possibly Vietnam and elsewhere. They were reported from Yunnan, China, as recently as the 1930s.

Because they are elusive, often nocturnal, and inhabit thick, often mountainous forests, it stands to reason that they are good at disappearing and reappearing. This goes for their presence on the Asian mainland as well as that on Borneo and Sumatra. John MacKinnon, the zoologist best known for his involvement in the discovery of the Saola Pseudoryx nghetinensis, has reportedly never seen a wild Sumatran rhino, despite all his time in the field and efforts to find them. Camera traps installed at Way Kamblas National Park, northern Sumatra, succeeded in photographing wild rhinos in 1995, allegedly the first time this had been done since 1932 (Bristow 1997).

Indeed, from the point of view of zoological discovery, Sumatran rhinos are interesting, having only been officially named by German biologist Johann Gotthelf Fischer von Waldheim in 1814. Actually, a published description of a Sumatran rhino had appeared 20 years prior to this, when William Bell sent a description and some illustrations to Joseph Banks, the then-president of the Royal Society of London. Bell had examined the animal after it had been shot near Fort Marlborough, Sumatra, in 1793. Even earlier, a pair of horns described by Jacobeus (1696) have been regarded by some as of Sumatran rhino origin. Linnaeus assumed that Jacobeus had been writing about the Black rhino Diceros bicornis, and as a result assumed ‘India’ as the type locality for this species.

Fischer von Waldheim had named his new rhino as a species of the genus Rhinoceros, but in 1841 Constantin Wilhelm Lambert Gloger thought that the species deserved its own genus, Dicerorhinus. Actually, an older generic name – Didermocerus – was coined by Joshua Brookes in 1828. Mostly forgotten about until George Simpson discussed it in 1945, it has been proposed that the publication where Didermocerus appeared (A Catalogue of the Anatomical and Zoological Museum of Joshua Brookes) should be considered invalid for the purposes of nomenclature. This view has a lot going for it, but for the fact that Brookes is usually taken as the author of Acinonyx, the cheetah genus (Boylan 1967). During the 1870s the taxonomy of Sumatran rhinos became more confusing. Sclater (1872a, b) argued that there were two species, Rhinoceros sumatrensis and R. lasiotis. Gray (1872, 1873) then thought that R. lasiotis was the ‘typical’ Sumatran rhino, that R. sumatrensis was synonymous with a species he had named in 1854 (R. crossii, later Ceratorhinus crossii), and that Malaysian and Burmese rhinos represented the new species C. niger and C. blythii.

Few of these putative taxa have stood the test of time. R. lasiotis (now R. s. lasiotis) has, and has been recognised as the subspecies of mainland Asia (Groves 1967). It is generally thought to be extinct, but a few individuals might persist in Myanmar and in 1999 it was announced that Sumatran rhinos had been seen near the Indian border with this country. In 1991 it was thought possible that individuals might also survive in Thailand and Laos (Martin & Vigne 1991). Groves & Kurt (1972) noted that the status of R. crossii remains somewhat uncertain: it is based on a single unusual and very long (80 cm) horn that is probably (but not definitely) from D. sumatrensis. The Bornean population was named as a distinct subspecies, D. s. harrissoni, in 1965 (Groves 1965), which makes it the largest recently named terrestrial mammal.

A small, two-horned species, the Sumatran rhino has long, shaggy reddish-brown fur covering its body and limbs. ‘Small’ for a rhino means that it is about 3 m long, 1-1.5 m tall at the shoulder, and between 800 and 2000 kg in weight. It has large lower canines (but no upper canines) that it uses in combat and both horns are short, the second (aka frontal) may be so low that it is barely more than a bump. A few individuals have been recorded with very long nasal horns of nearly 40, and even nearly 70, cm long.

In terms of their global population, Sumatran rhinos are in big trouble, and the estimated world population of 300 (as of 2001) is thought to be the remnants of one that crashed by c. 50% during the 1990s, mostly due to illegal hunting and habitat loss. Captive breeding has unfortunately not helped in boosting numbers: until fairly recently it was thought that Sumatran rhinos did quite well in captivity – they were the first rhino species to breed in captivity (a female kept at Calcutta gave birth in 1889), and a specimen kept at London died at age 32 (this individual was, incidentally, the type specimen of D. s. lasiotis). By the 1990s however, it had to be concluded that 20^th century captive breeding had been a failure, with not one of the 39 zoo-kept individuals having bred (during 2004 however, one calf was born at Cincinnati Zoo). 18 of these 39 were dead by the late 1990s. Why the rhinos fare badly in captivity is not known, but it might be that they find small enclosures and exposure to sunlight too stressful. The solution to this problem might be the Sumatran sanctuary at Way Kambas National Park. European, American and Asian zoos are sending their rhinos to this park (Bristow 1997).

To get back to their current appearance in the global media, it seems inevitable that, whenever Sumatran rhinos are mentioned, that old chestnut about them being a ‘living fossil’ is trotted out. It is invariably stated that they are particularly close to the Pleistocene woolly rhino, Coelodonta, and it is often implied that their persistence to the presence is remarkable and that they should be regarded as an anachronism. Such comments aren’t restricted to the popular literature: Groves & Kurt (1972, p. 4) wrote ‘As presently defined, Dicerorhinus is the genus that gave rise to all living Rhinocerotidae; in this sense, and in that it closely resembles certain Miocene species, the Sumatran rhino may be regarded as a living fossil’ [some of these statements are arguable: read on].

This, I suppose, answers the question as to what a ‘living fossil’ is… it’s an archaic animal (i.e., one whose anatomy harks back to an early stage in its group’s evolution) that appears to have persisted for a long time, relatively unchanged. The problem is that this is so vague that it’s all but meaningless. What is a ‘long time’, given that different forms of life evolve at different paces? And what is ‘relatively unchanged’, given that the same sort of body shape can persist for tens of million of years?

I know that this ‘living fossil’ claim has a ‘long and useful educational tradition’, and that such august scientists as E. O. Wilson have employed the Sumatran rhino as such (go here for the quote). My point is that all of this is misleading, and that in fact Sumatran rhinos are no more ‘living fossils’ than many other living mammal species. Consider the following.

Is D. sumatrensis an old species?

No, the living species D. sumatrensis doesn’t have a fossil record extending beyond the Pleistocene. A few bone and teeth are known from the late Pleistocene of Borneo and a fossil subspecies, D. s. eugenei Sody, 1946, is known from the Holocene of Sumatra. So far as we know therefore, the Sumatran rhino isn’t a particularly old species. It’s apparently less than about 2 million years old, and thus utterly typical for a living mammal.

Is Dicerorhinus particularly old and/or conservative?

Dicerorhinus has a fossil history going back to the Miocene (and perhaps to the Late Oligocene). However, a great many living mammal genera have fossil records going back this far. Examples - picked at random - include Geomys (pocket gophers), Muscardinus (hazel dormice), Glis (edible dormice), Martes (martens), Genetta (genets), Viverra (civets), Tursiops (bottlenose dolphins), Orcinus (killer whales), Physeter (sperm whales), Balaenoptera (rorquals), Tragelaphus (bushbuck, kudus etc), and many others. As discussed in a previous post (Pleistocene refugia and late speciation: are extant bird species older than we mostly think?), some modern bird genera seem to have first appeared in the Miocene, and many thoroughly modern amphibians and reptile genera go back this far or further.

So if Sumatran rhinos should be regarded as ‘living fossils’, why aren't bottlenose dolphins, blue whales, edible dormice, sitatungas, gannets, barn owls or peafowl ever referred to as such? It seems either that we are surrounded by taxa that should be regarded as ‘living fossils’, or that the term is pretty much useless given that most modern animal species belong to groups that have a fossil history.

And was Dicerorhinus conservative throughout its evolutionary history? No, Dicerorhinus species were quite diverse. Among the many species, some (such as the Pleistocene Christol’s rhino D. megarhinus and Etruscan rhino D. etruscus) were gracile and long-legged compared to D. sumatrensis, others (like Merck’s rhino D. kirchbergensis) were large, while others (like the Steppe rhino D. hemitoechus) were apparently specialized grazers, with a downwardly-flexed head and neck. Incidentally, not all species traditionally placed in Dicerorhinus are still thought to belong there. Some belong to the closely related Lartetotherium for example (Cerdeño 1995) [the adjacent painting is Burian's restoration of an Etruscan rhino. Note the long legs].

Is D. sumatrensis anatomically archaic?

Interestingly (and in contradiction to that quote from Groves & Kurt 1972, p. 4, given above), most of the anatomical features that make Dicerorhinus appear ‘primitive’ seem to be reversals. That is, the genus has uniquely ‘switched back’ to primitive character states, but actually descended from ancestors with a more ‘modern type’ morphology (Cerdaño 1995). Furthermore, the genus seems not to be ancestral to other living rhinos, but a lineage that, within the rhinocerotid clade Rhinocerotinae, is closer to Rhinoceratina (containing Rhinoceros) than it is to Dicerotina (containing Ceratotherium and Diceros). As such, Dicerorhinus isn’t really any older than other extant rhino genera (Tougard et al. 2001) [image below features a reconstructed skeleton of a Steppe rhino. Borrowed from the La fauna del Quaternario site].

Everything restated, more simply… ish

Sumatran rhinos have been thought of as ‘living fossils’ because – supposedly – they belong to a particularly old group, the group they belong to was particularly conservative throughout its history, and they are anatomically archaic. Ignoring for a moment the fact that the species itself appears to be geologically young, these assumptions are no truer for Sumatran rhinos than they are for a great many other living tetrapods, and at worse they are just plain wrong. Dicerorhinus is NOT particularly old, it was NOT particularly conservative, and it is NOT particularly archaic in terms of anatomy! And if you want to argue that it is (in answer to all of the above), then I demand that Bottlenose dolphins and Peacocks and all those other tetrapods now be consistently referred to as ‘living fossils’ too, forever more.

Why then do we persist with this ‘living fossil’ twaddle? Mostly, I suppose, this is because some animals look ‘more ancient’ than others, and when it is found that they belong to a group with a reasonable fossil history… presto: living fossil. But as I have tried to show here, this term is essentially meaningless. Should we use it at all? If a single species could be shown to have persisted, unchanged, for a shockingly long length of geological time, then I suppose the term would be appropriate. But what is ‘shockingly long’. All in all, it has to be said that the whole concept of the ‘living fossil’ is utterly subjective, hence its uselessness.

Update: a response to this post has been written by Loren Coleman... Sumatran rhinos are living fossils. I think we'll have to agree to disagree. For the latest news on Tetrapod Zoology do go here.

Refs - -

Boylan, P. J. 1967. Didermocerus Brookes, 1828, v. Dicerorhinus Gloger, 1841, (Mammalia: Rhinocerotidae), and the validity of A Catalogue of the Anatomical and Zoological Museum of Joshua Brookes, 1928. Bulletin of Zoological Nomenclature 24, 55-56.

Britow, M. 1997. The rhino’s return. BBC Wildlife 15 (2), 68-69.

Cerdeño, E. 1995. Cladistic analysis of the family Rhinocerotidae (Perissodactyla). American Museum Novitates 3143, 1-25.

Gray, J. E. 1872. On the double-horned Asiatic rhinoceros. Annals and Magazine of Natural History 10 (series 4), 208-209.

- . 1873. On the dentition of rhinoceroses (Rhinocerotes) and on the characters afforded by their skulls. Annals and Magazine of Natural History 11 (series 4), 356-361.

Groves, C. P. 1965. Description of a new subspecies of rhinoceros, from Borneo, Didermoceros sumatrensis harrissoni. Säugertierk. Mitt. 13, 128-131.

- . 1967. On the rhinoceroses of southeast Asia. Säugertierk. Mitt. 15, 221-237.

- . & Kurt, F. 1972. Dicerorhinus sumatrensis. Mammalian Species 21, 1-6.

Jacobeus, O. 1696. Muséum Regium. Nürnberg.

Martin, E. B. & Vigne, L. 1991. The horn quintet. BBC Wildlife 9 (5), 356-357.

Sclater, P. L. 1872a. Untitled note. Proceedings of the Zoological Society of London 1872, 493-494.

- . 1872b. Untitled note. Proceedings of the Zoological Society of London 1872, 790-794.

Tougard, C., Delefosse, T., Hänni, C. & Montgelard, C. 2001. Phylogenetic relationships of the five extant rhinoceros species (Rhinocerotidae, Perissodactyla) based on mitochondrial cytochrome b and 12S rRNA genes. Molecular Phylogenetics and Evolution 19, 34-44.