Saturday, September 30, 2006

Did ichthyosaurs fly? Probably not, no

For various reasons, I haven’t had the chance lately to do any blogging. So in order to at least add something new, I’ve decided to recycle the text of a very old article, first published in 1997. I’ve made no effort whatsoever to update it, so please keep this in mind (though, because the original article didn’t include any citations or references, these have been added). Here we go.

It is often difficult to visualise the life appearance of extinct animals: quite simply because many of these animals are profoundly different from any we have living with us today. Perhaps, out of all the extinct types of Mesozoic reptile, ichthyosaurs are the easiest to visualise. Certain living marine animals - the speedy, open-water dolphins, lamnid sharks [Great white and relatives] and scombroid fishes [the tuna-mackerel group] - possess the ichthyosaur body shape and surely move in an ichthyosaur-like way. So alike are all of these groups in their shape and presumably their behaviour that is has proved irresistible for artists to depict ichthyosaurs as ‘Mesozoic dolphins’. They are shown travelling in schools, porpoising from the waves, and smoothly powering themselves beneath the surface with their forked, shark-like tails. But try and imagine now how different things might look if ichthyosaurs did not swim in this way at all, but propelled themselves with their powerful forefins instead. They did not swim by fast side-to-side movement of the tail, but flew underwater like turtles or penguins.

This idea must sound unusual to the uninitiated. In recent years, however, it has been seriously proposed and has won a fair deal of acceptance in the community*. For my third year dissertation at the Southampton Oceanography Centre (University of Southampton), I decided to investigate this suggestion and assess for myself how likely, or unlikely, it really was. Were ichthyosaurs really underwater fliers?

* That's not true, and it wasn't in 1997 either.

The suggestion that ichthyosaurs may have flown owes itself to German palaeontologist Jurgen Riess who published his ideas in a 1986 study on ichthyosaur biomechanics and phylogeny (Riess 1984, 1985, 1986, Riess & Frey 1991, Riess & Tarsitano 1989). Riess presented new, wing-like reconstructions of ichthyosaur fins. Previous discoveries, some described as long ago as 1841, had shown that the soft tissue of ichthyosaur fins extended well beyond the fin skeleton. There seem to have been rigid plate-like structures and fibres along the fin’s leading edge, creating a stiff and narrow extension along the anterior-most border. Stiffening rods apparently supported a gently tapering, flexible posterior-most border to the fin that was much wider than the leading edge. In cross-section, these fins may have been hydrofoil shaped. In order that such fins could flap up and down to enable propulsion, constricted fin bases would be required. Riess argued that this was indeed the case [the drawing at the top, depicting the Triassic ichthyosaur Shonisaurus as an underwater flier, is by Dino Frey and from Riess (1986)].

Though aspects of the fin’s internal structure may have been common to all (or nearly all) ichthyosaurs, ichthyosaur fns do vary a great deal in shape. However, one trend that might be observed in the evolution of the ichthyosaur fin is the addition both of more and more elements (hyperphalangification), and of more and more digits (hyperdactylification). A plausible advantage of both conditions might be that a limb surface would become broader if new elements were added to its sides, or longer if more elements were added to its distal-most end. Both patterns would, in theory, be advantageous to an underwater flier.

Greater broadening of a fin could provide it with more propulsive power, and an increased length could help create a more wing-like plan: by the creation, say, of a slim, pointed fin tip. Certain ichthyosaurs carried the two patterns to the extreme, Platypterygius, a cosmopolitan genus from the Cretaceous, had up to 10 digits, the longest of which contained more than 30 phalanges, per forefin. Extra bones were added also to the forearm, making that part of the fin broader too. The evolution of these wide, elongate wing-like fins may be interpreted as increasing specialisation toward the underwater flight mode of propulsion. So far so good, but to ‘fly’ animals need more than a wing - they also need a powerful, well braced pectoral girdle as well as strong muscles. Did ichthyosaurs possess such attributes?

Articulated ichthyosaurs demonstrate that the pectoral girdle was a robust construction with vertical clavicles [collar bones] and scapulae [shoulder blades], so there was a firm base for the articulation of powerful forefins and their musculature. In some ichthyosaurs, muscle attachment sites on the pectoral girdle were quite pronounced, and fin muscles may even have extended onto the ribs too. So, not only were ichthyosaur fins wing-like in shape with a sometimes broad surface area, they were connected to a robust, torque-resistant skeletal framework and were controlled by well-developed muscles. A morphological configuration that, in its basic principles, is not unlike that of underwater-flying turtles or penguins, or indeed from airborne birds, bats and pterosaurs. The case for ichthyosaur flight looks good.

Or, it would do, were there not some modern-day forms that possess many of the same features as the ichthyosaurs, but which do not fly. Part of Riess’ case for flight in ichthyosaurs was based on analogy with two living animals: the Australian lungfish (Neoceratodus) and the Amazon river dolphin (Inia). Both animals have wing-like fins strikingly similar to those of some ichthyosaurs and Riess mistakenly believed that both forms were underwater fliers. His reasons for thinking so are complex and buried deep in the labyrinthine depths of technical zoological literature, but having surveyed the literature, discussed the issue with colleagues, and spent many hours observing Inia on video, I am certain that the idea of either flying lungfishes or dolphins has been based on misinterpretation. Mary Wade, a palaeontologist who has published some of the most important work on Platypterygius, came to the same conclusion in a 1990 paper of hers (Wade 1990).

So what does this all mean? Well, if there are living animals that possess wing-like fins yet do not fly, then a direct correlation between wing-like fins and flight does not exist. Wing-like fins, big fin muscles and robust pectoral girdles can instead be explained as adaptations for manoeuverability - something very much evident in the behaviour of Inia. Lengthening and broadening of fins is not proof of flight, either. The hyperphalangic condition occurs in certain whales (it is particularly marked in the Longfin pilot whale (Globicephala melas)), animals that most definitely are not fliers. Furthermore, some underwater fliers (e.g., sealions) do not exhibit hyperphalangy. So there is no clear correlation between hyperphalangy and flight.

Hyperdactyly - the broadening of fins - could well evolve as an aid to small-scale local movements. Most types of fish use their fins extensively for this purpose, as do whales with their flippers. As an animal that spends a great deal of time making small-scale local manoeuvres in its complex, three-dimensional watery home, Inia is especially telling in that it may actually be taking part in a trend toward hyperdactyly, as it bears an extra bone in its flipper that is to all intent and purposes a sixth digit. Those ichthyosaurs with extra-wide fins were probably spending a great deal of time moving slowly, like Inia, perhaps while investigating prey on the seafloor.

Furthermore, despite Riess’ fin reconstructions, ichthyosaurs with preserved soft tissues show that the fin attachment to the body was broad, rather than narrow. Up and down movement was therefore somewhat restricted, and not suited for vigorous flapping.

One clear correlation that does appear to be true for swimming vertebrates concerns that tail. Essentially, if an animal has a propulsive surface on the end of its tail, it uses it. Riess thought that some ichthyosaurs, his example was the Lower Jurassic genus Leptonectes*, both flapped with the forefins, and used the shark-like vertical tail to steer. However, studies of ichthyosaur locomotion, notably those by Christopher McGowan and Michael A. Taylor, have demonstrated that the ichthyosaur tail was a powerful organ of propulsion. Indeed, it is hard to explain the evolution of this important and superbly hydrodynamic feature if it was used merely for steering rather than for active and constant use.

Further confirmation of tail-propelled swimming in shark-shaped ichthyosurs comes from work by Motani, You and McGowan (Motani et al. 1996). They plotted ichthyosaur fineness ratio [body length/body height] against tail height/length ratio for different kinds of ichthyosaur and shark. Not surprisingly, the advanced shark-shaped ichthyosaurs (e.g., Stenopterygius) grouped tightly with lamnid sharks - exactly what we would expect if both groups were hydrodynamically adapted for the same style of propulsion. The result would surely have been different if these ichthyosaurs were not shark-like in their style of swimming.

* This genus was still known as Leptopterygius Huene, 1922, when Riess was writing, a name now known to be preoccupied by a fish named by Troschel in 1860.

For a previous post on ichthyosaurs see Ichthyosaurs wars and marvellous mixosaurs and Life in the Oxford Clay sea.

Incidentally, given that Neoceratodus is mentioned in the above text I am morally obliged to direct you to Pharyngula’s Neoceratodus campaign. This animal (yes I know it’s not a tetrapod) is under significant environmental pressure.

Coming soon… those long-promised posts on agamas, domestic dog origins and rhinogradentians. For the latest news on Tetrapod Zoology do go here.

Refs - -

Motani, R., You, H. & McGowan, C. 1996. Eel-like swimming in the earliest ichthyosaurs. Nature 382, 347-348.

Riess, J. 1984. How to reconstruct palecology? – Outlines of a holistic view and an introduction to ichthyosaur locomotion. In Reif, W.-E. & Westphal, F. (eds) Third Symposium on Mesozoic Terrestrial Ecosystems, Short Papers. Attempto Verlag (Tübingen), pp. 201-205.

- . 1985. Biomechanics of ichthyosaurs. In Riess, J. & Frey, E. (eds) Principles of Construction in Fossil and Recent Reptiles. Konzepte SFB 230 Heft 4, pp. 199-205.

- . 1986. Locomotion, biophysics of swimming and phylogeny of the ichthyosaurs. Palaeontographica Abteilung A 192, 93-155.

- . & Frey, E. 1991. The evolution of underwater flight and the locomotion of plesiosaurs. In Rayner, J. M. V. and Wootton, R. J. (eds) Biomechanics and Evolution. Cambridge Uni. Press (Cambridge), pp. 131-144.

- . & Tarsitano, S. F. 1989. Locomotion and phylogeny of the ichthyosaurs. American Zoologist 29, 184A (abs).

Wade, M. 1990. A review of the Australian Cretaceous longipinnate ichthyosaur Platypterygius, (Ichthyosauria, Ichthyopterygia). Memoirs of the Queensland Museum 28, 115-137.

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Thursday, September 21, 2006

Even more recently extinct, island dwelling crocodilians

In the previous post we looked at the small, island dwelling crocodilians of the south-west Pacific. I personally find it exciting that such animals were (in the case of at least some of the species) alive until just a few thousand years ago, that they were encountered by people, and that their remains have eluded detection until recent decades. The odds are high that further species await discovery.

All of the island dwelling crocodilians I discussed in the previous post were members of the predominantly Australasian mekosuchine radiation. But there is one recently extinct crocodilian of the south-west Pacific that I didn’t mention, and which isn’t a mekosuchine. First reported by Charles De Vis in 1905, it’s a long-snouted form known from Pleistocene remains discovered at Busai on Murua, one of the Solomon Islands. Because of its long, slender jaws, De Vis regarded this animal as a gharial and named it Gavialis papuensis. It then languished in obscurity until 1982 when Ralph Molnar published a redescription.

Molnar (1982) concluded that the Murua crocodilian almost certainly didn’t belong in the genus Gavialis, and that it was more likely closely related to Charactosuchus, Euthecodon or Ikanogavialis, with a relationship with the last named taxon being deemed most likely. That’s good news, because Ikanogavialis (best known for I. gameroi from Upper Miocene* Venezuela [a jaw segment from this taxon is figured at left]) is – while not the same thing as Gavialis – still undoubtedly a member of the gharial family, Gavialidae. South American Miocene Charactosuchus, while gharial-like, has been regarded as a highly unusual crocodylid of uncertain affinities (Langston 1965, Langston & Gasparini 1997), while Euthecodon – a uniquely African taxon, some species of which approached 10 m in length – is also a crocodylid, and perhaps a close relative of the living dwarf crocodiles (and we’ll discuss those more in a moment). Most recently, Rauhe et al. (1999) listed the Murua gharial as belonging to Ikanogavialis, and if this has been accepted then presumably we should refer to it as Ikanogavialis papuensis. The presence of this genus in both Venezuela and the south Pacific might seem odd given that living gharials are freshwater animals, but the fossil record shows that gharials formerly occurred widely in marine environments around the world.

* And, according to Langston & Gasparini (1997), not from the Pliocene as usually stated.

Like the south Pacific mekosuchines, the Murua gharial was again fairly small, at 2-3 m long. Its fossils were associated with those of sea turtles and sirenians, so it was almost certainly marine. To the list of small crocodilians that inhabited the south-west Pacific, we can add gharials then. Whether the Murua gharial became extinct before humans colonised the region, or whether its extinction was caused by people, we again don’t know. Indeed the only known specimen's exact geological age is unknown. Given this, and the many anthropogenic extinctions that occurred in the region, I can’t help but speculate that the Murua gharial survived into the Holocene, and that humans killed it off, but there’s no direct evidence for this. Regardless, it’s surprising that small, marine gharials survived so relatively late [the adjacent photo figures the living gharial species Gavialis gangeticus].

Besides mekosuchines and gharials, we also know of a third crocodilian group that included island dwelling forms, and again the species concerned became extinct geologically recently. To look at the members of this group we need to move over to the Indian Ocean. And it’s here that we find the most recently named of all the crocodilians we’ve looked at: Aldabrachampsus dilophus, from Aldabra (Brochu 2006). Though recently named, Aldabrachampsus was actually first described in 1976, though at this time it was misidentified as representing a dwarf population of Crocodylus niloticus (Arnold 1976). In being of Pleistocene age, Aldabrachampsus is like both Volia from Fiji (see previous post) and the Murua gharial in being too old to have its extinction indisputably linked to the arrival of people. Arnold (1976) discussed environmental changes that occurred on Aldabra that may have caused the extinction of endemic reptiles, among them geckos, iguanas and skinks, with the most notable of them being the breaching of the atoll rim and subsequent habitat degradation that occurred about 4000 years ago.

Various skull features make Aldabrachampsus unusual, including the shape of its premaxillae, and the orientation of its tooth row and external nostrils. However, its most obvious feature would almost certainly have been the convex crests that grew from the dorsolateral edges of the squamosal bones at the back of its skull. Some living crocodiles have crest-like projections in this region, but none have the prominent, elongate structures present in Aldabrachampsus. These crests explain the specific name, ‘dilophus’ meaning ‘with two crests’.

Given that Aldabrachampsus was comparable in size to the smallest living crocodilians – that is, between 2 and 2.5 m long – it is again tempting to assume that, like so many island dwelling tetrapods, it was a dwarf. This would actually be odd for a crocodilian, given that other island dwelling forms are not dwarfed relative to their mainland relatives (and, as we saw in the previous post, this seems to go for island dwelling mekosuchines as well). Indeed the stratigraphic occurrence of Aldabrachampsus doesn’t provide support for the idea that it’s a dwarf: it’s from sediments that were deposited shortly after a period of Aldabran submergence, and the species is therefore unlikely to have evolved on the island. It must therefore have swum in from elsewhere.

What sort of crocodilian was Aldabrachampsus? It was a crocodylid, but there’s no indication that it was anything to do with the mekosuchines: instead, there are reasons for thinking that it was an osteolaemine. That is, a member of the same crocodylid clade as the west African dwarf crocodiles (Osteolaemus*) - see picture at top - and the extinct Madagascan species ‘Crocodylus’ robustus (Brochu 1997, 2006). Some phylogenetic studies find that the osteolaemines also include Euthecodon, the bizarre gharial-like African taxon we met above, as well as Rimasuchus (Brochu 1997), a broad-snouted east African taxon that grew to 7 m or more in length. The African slender-snouted crocodile Crocodylus cataphractus might also be an osteolaemine, a view that would be in agreement with data suggesting that it needs removing from Crocodylus (the old generic name Mecistops Gray, 1844 is available: see McAliley et al. 2006), and additional fossil African crocodylids also seem to belong to this group. If this is all valid, then little Osteolaemus is a sorry remnant of a once diverse group that included several enormous species. Anyway, within this group, an affinity between Aldabrachampsus and ‘Crocodylus’ robustus is particularly plausible given that both taxa share a vaulted palate and large squamosal crests.

* Though conventionally thought to include just a single living species (O. tetraspis), new data has caused some workers to regard a second species as valid (McAliley et al. 2006). This is O. osborni, a taxon from the Congo (first described in 1919 and given its own genus, Osteoblepharon) until recently regarded as a subspecies of O. tetraspis.

Unlike the crests of Aldabrachampsus, those of ‘Crocodylus’ robustus were large horn-like growths (see photo above, my hands for scale), and unlike both Aldabrachampsus and Osteolaemus, this species was large and comparable in size to a Nile crocodile Crocodylus niloticus. Originally described in 1872, ‘Crocodylus’ robustus has been mostly considered synonymous with C. niloticus, but ‘this synonymy results from an inadequate initial description and from subsequent misidentifications of living C. niloticus from Madagascar as C. robustus’ (Brochu & Storrs 1995). The ‘true’ ‘Crocodylus’ robustus was a broad-snouted species sharing a list of skull characters with Osteolaemus, so it doesn’t belong in the genus Crocodylus and needs a new name (hence the use of quotes).

It wasn’t as big as Euthecodon or Rimasuchus, reaching 4-5 m in length (Burness et al. (2001) estimated its weight as 170 kg). This size would have made it the largest predator on Madagascar, and given that prehistoric Madagascar was also home to giant eagles and fossas, the lemurs, elephant birds and other animals of the island would certainly have lived in fear of formidable predators.

Again, what fascinates me most about ‘Crocodylus’ robustus is how recently it was alive. So far as I can tell from the literature, an exact date for its extinction is unknown, and I’d be interested to know if it disappeared as part of the anthropogenic wave of extinctions that occurred on the island. Chris Brochu is due to publish on this species in the near future, so more information will appear soon.

For a previous post on island dwelling non-avian reptiles see Tortoises that drink with their noses. For the latest news on Tetrapod Zoology do go here.

Refs - -

Arnold, E. N. 1976. Fossil reptiles from Aldabra Atoll, Indian Ocean. Bulletin of the British Museum (Natural History). Zoology 29, 85-116.

Brochu, C. A. 1997. Morphology, fossils, divergence timing, and the phylogenetic relationships of Gavialis. Systematic Biology 46, 479-522.

- . 2006. A new miniature horned crocodile from the Quaternay of Aldabra Atoll, western Indian Ocean. Copeia 2006, 149-158.

- . & Storrs, G. W. 1995. The giant dwarf crocodile: a reappraisal of ‘Crocodylus’ robustus from the Quaternary of Madagascar. In Patterson, Goodman & Sedlock (eds) Environmental Change in Madagascar, p. 70.

Burness, G. P., Diamond, J. & Flannery, T. 2001. Dinosaurs, dragons, and dwarfs: the evolution of maximal body size. Proceedings of the National Academy of Sciences 98, 14518-14523.

Langston, W. 1965. Fossil crocodilians from Colombia and the Cenozoic history of the Crocodilia in South America. University of California Publications in Geological Sciences 52, 1-169.

- . & Gasparini, Z. 1997. Crocodilians, Gryposuchus, and the South American gavials. In Kay, R. F., Madden, R. H., Cifelli, R. L. & Flynn, J. J. (eds) Vertebrate Paleontology in the Neotropics: The Miocene fauna of La Venta, Colombia. Smithsonian Institution Press (Washington, D.C.), pp. 113-154.

McAliley, L. R., Willis, R. E., Ray, D. A., White, P. S., Brochu, C. A. & Densmore, L. D. 2006. Are crocodiles really monophyletic? – Evidence for subdivisions from sequence and morphological data. Molecular Phylogenetics and Evolution 39, 16-32.

Molnar, R. E. 1982. A longirostrine crocodilian from Murua (Woodlark), Solomon Sea. Memoirs of the Queensland Museum 20, 675-685.

Wednesday, September 20, 2006

The small, recently extinct, island-dwelling crocodilians of the south Pacific

Given that I’ve lost the will to live lately, I haven’t been involved in as much research as is usual. The big project on British dinosaurs trundles on, I’ve been writing lately about the giant Gallotia lizards of the Canary Islands, and an alleged recent manatee sighting off the coast of Washington State has caused me to become involved in a discussion on manatee mobility (go here for more on this subject) and the extinction date of Steller’s sea cow Hydrodamalis gigas. Stoat packs, hominid origins and Britain’s changing herpetofauna have all been on my mind recently, plus I’ve been reading up on the origin of tetrapods: next month I begin an adult education class on tetrapod evolution, and this is one of the areas that I’m going to cover in the first session.

The research I’ve been doing on the Gallotia lizards (I’ll blog about them some time in the future, perhaps) got me into the subject of recently extinct, island dwelling reptiles. And that’s good, because I’ve been meaning for a while to write about island dwelling crocodilians (note the mention in the first babirusa post). We now know that small crocodilians inhabited tropical islands in the South Pacific and elsewhere until just a few thousand years ago: judging by these discoveries, small terrestrial crocodilians seem to have been an ordinary component of many tropical island groups until they were made extinct by people.

The first of these animals to be discovered was Mekosuchus inexpectatus from New Caledonia, a species that most interested people have heard about due to its coverage in popular books (e.g., Jean-Christophe Balouet’s Extinct Species of the World, Tim Flannery’s The Future Eaters and Charles Ross’ Crocodiles and Alligators [the big Merehurst encyclopedia]). Discovered in 1980, this species entered the literature in 1983 when Eric Buffetaut described its remains (teeth and skull bones) from a site on the Isle of Pines, just off New Caledonia. It was a small crocodilian, around 2 m long, and crushing teeth at the back of its jaws suggest that it ate molluscs on occasion. Based on the apparently archaic nature of its postorbital bar, Buffetaut (1983) speculated that this animal might be a late-surviving relict form from the Cretaceous. By 1987, Buffetaut and colleague Jean-Christophe Balouet had enough material (now from mainland New Caledonia as well as the Isle of Pines) to name the species. They regarded it as distinctive enough for its own family, Mekosuchidae, and they proposed that Mekosuchidae might be a relict group outside of the clade formed by the living crocodilian species (Balouet & Buffetaut 1987).

All of this was made more surprising by the fact that M. inexpectatus was essentially modern, with its remains coming from deposits that are certainly less than 4000 years old. The exact age of M. inexpectatus is unsure, but it may have been around as recently as 1670 years ago (Mead et al. 2002). Humans arrived on New Caledonia about 4000 years ago, so it is likely that M. inexpectatus was among the several New Caledonian endemics that were hunted to extinction. Beside the crocodile, these include large megapodes, rails, meiolaniid turtles and a monitor lizard. An association of M. inexpectatus remains with kitchen waste at Nessadiou (200 km north of Noumea) has been reported (Balouet 1989).

When first documented, M. inexpectatus was unique and without any apparent close relatives. Thanks to the research of Paul Willis and his colleagues however, we now know that this species was merely among the youngest of a predominantly Australasian radiation of Cenozoic crocodilians, the mekosuchines. To date this group includes generalised Kambara from Eocene Queensland and Australosuchus from Oligocene-Miocene South Australia, short-skulled Miocene Trilophosuchus from Queensland, the broad-snouted Oligocene-Miocene Baru* species, broad-snouted Pliocene-Pleistocene Pallimnarchus, and ziphodont** Miocene-Pleistocene Quinkana from Northern Territory and Queensland (Salisbury & Willis 1996, Willis 1993, 1997, Willis & Mackness 1996, Willis & Molnar 1991, Willis et al. 1990, 1993). A few unnamed Australian species, including some peculiar long-snouted, gharial-like forms, are also probably members of the mekosuchine radiation, and we’ll meet more members of the group in a minute. Mekosuchines aren't regarded as a distinct 'family' anymore, incidentally (as was proposed by Buffetaut and Balouet). Rather, they seem to be a clade within the larger group Crocodylidae [adjacent image is a life restoration of Baru darrowi, by me].

* There are several Baru species, but the first one to be named is B. darrowi. The specific name honours Paul Darrow, the British actor ‘best known for his role in the television series ‘Blake’s Seven’, in recognition of his support of continuing palaeontological investigation of the Riversleigh deposits’ (Willis et al. 1990, p. 522).

** Ziphodonty describes a tooth type where the teeth are recurved and laterally compressed. It is most often associated with theropod dinosaurs.

Some mekosuchines probably behaved much like living amphibious crocodiles – indeed they evolved in an Australia that was initially devoid of the modern Crocodylus species that first appeared there in the Pliocene, but others, like Quinkana, appear to have been terrestrial predators that might have behaved like giant monitor lizards. Judging by its limb bone morphology and the places where its remains are found, M. inexpectatus was a terrestrial form and it has even been suggested that it might have been scansorial: that is, able to climb trees. This isn’t regarded as likely by all fossil crocodilian experts. In the initial draft of an article I wrote about crocodilian history (Naish 2001) I mentioned that Mekosuchus might have been a tree climber. My reviewer (a noted crocodilian expert) crossed this out, writing in the margin ‘Do you want to remain a credible scientist’? Anyway, it is an idea still worth considering.

So M. inexpectatus was a member of a previously diverse group, the mekosuchines. Furthermore, recent discoveries from the Oligocene and Miocene of Queensland have revealed a fossil history for Mekosuchus extending well back, on mainland Australia, into the Neogene (Willis 1997). This is significant as it shows that Mekosuchus must have gotten to New Caledonia after evolving on Australia (it therefore wasn’t an island endemic, unique to New Caledonia). Furthermore, M. inexpectatus is comparable in size to the Australian Mekosuchus species, so its small size probably isn’t a specialisation for island life [the image at the top of the page depicts the Australian Mekosuchus species M. sanderi. It's one of Paul Willis' photoshop reconstructions. Visit reconstructing dead Aussie crocs for more].

Another island-dwelling Mekosuchus species, M. kalpokasi, was described in 2002 for skull material from the archaeological Arapus site of Efate island, Vanuatu. Discovered associated with pottery fragments, bivalve shells, and bones of fish, fruit bats, turtles and birds, M. kolpokasi comes from a layer radiocarbon dated at about 3000 years old. Again, its association with human waste and evidence for hunting strongly indicates that its extinction was caused by humans. The maxilla of M. kalpokasi clearly comes from an adult individual and has a tooth row of just 88.7 mm long, so again this was a small animal perhaps less than 2 m long. The shape of the maxilla also shows that M. kalpokasi was a short-skulled species, as were probably all Mekosuchus species. So Mekosuchus wasn’t just found on Australia and New Caledonia – it also got as far west as Vanuatu. Might mekosuchines prove to have been even more widely distributed?

Yes, for in 2002 Molnar et al. (2002) described another island dwelling form, Volia athollandersoni. Its name commemorates Voli Voli Cave (one of the discovery sites) and Atholl Anderson, well known for his many contributions to the prehistory and palaeoecology of south-west Pacific islands. Volia is from Viti Levu, Fiji, and again it was apparently a relatively small (2-3 m long), terrestrial form. It is Pleistocene in age, with one cave deposit that yielded it being dated at between 10,000 and 20,000 years old, though it can’t be ruled out that some remains of this species are younger than this. People have been on Fiji since about 3000 years ago, so at the moment Volia seems too old to have succumbed to human hunters. I would predict, however, that it actually survived to the time of human occupation, and only then became extinct. A giant frog, a terrestrial turtle, an iguana, rails, a snipe, pigeons and megapodes all became extinct on Fiji as a result of human intervention.

How did mekosuchines get to these islands, and what does this mean for their total distribution? There are two competing views here. View one – the dispersal model – states the following: given that mekosuchines as a group are younger than the rifting events which separated Fiji and New Caledonia from Australia, it seems most likely that they colonised the islands in their range by dispersal (Mead et al. 2002). That is, they swam to them. This means that the mekosuchines ancestral to the island dwelling forms must have had some marine swimming/drifting capabilities (much as do some living Crocodylus crocodiles).

This dispersal model suggests that island dwelling mekosuchines might have occurred on various islands where they have yet to be discovered, such as Tonga, Samoa, Santa Cruz, the Loyalty Islands, and various of the islands that form Vanuatu and Fiji (Mead et al. 2002). We have only recently (viz, post-1980s) discovered that Fiji and Tonga were home to recently extinct big frogs, iguanas, giant pigeons and other tetrapods, so it is quite conceivable that the fossils of such mekosuchines await discovery on some of these islands. It’s also worth checking native traditions and old historical accounts to see if the people who lived on, or visited, these islands ever reported anything that sounded like a terrestrial crocodile. I’m not aware of any such accounts but then I can’t pretend to have checked the relevant anthropological or historical literature. Balouet (1989) did report that New Caledonian people lacked any traditions of an animal that sounded at all like Mekosuchus.

View two – the vicariance model – proposes that mekosuchines may have been present on New Caledonia, Fiji and elsewhere prior to their separation of these land masses from the Australian plate. A few pieces of evidence suggest that this is plausible. We now know that crocodilians were present on New Zealand as recently as the Miocene, and it was intimated by Molnar et al. (2002) that this might support a vicariant origin of island dwelling crocodilians in the south-west Pacific. Molnar et al. (2002) also pointed to the presence of the frog Platymantis on Fiji as evidence for a vicariant origin of at least some of the tetrapods on these islands, though it’s worth noting that the old chestnut about frogs being incapable of dispersal across salt water is no longer viable (Vences et al. 2003). If vicariance was the major control on mekosuchine distribution in the south-west Pacific, then mekosuchines ‘might be expected to be found on other islands in the New Zealand-New Caledonia-Fiji-Solomons region, but not as far east as Tonga or Samoa’ (Molnar et al. 2002, p. 626).

So whether mekosuchines owe their distribution to dispersal or vicariance, we can be quite confident that they inhabited islands in the south-west Pacific where their presence has yet to confirmed.

This isn’t the end of the story. Mekosuchines weren’t the only recently extinct crocodilians that inhabited the island groups of the south-west Pacific. Furthermore, island dwelling crocodilians also inhabited islands in the Indian Ocean until very recently. And that’ll have to wait until next time. For the latest news on Tetrapod Zoology do go here.

Refs - -

Balouet, J.-C. 1989. New Caledonian crocodile (Mekosuchus inexpectatus). In Ross, C. A. (consulting ed.) Crocodiles and Alligators. Merehurst Press (London), p. 36.

- . & Buffetaut, E. 1987. Mekosuchus inexpectatus, n. g., n. sp., crocodilien nouveau de l’Holocene de Nouvelle Calédonie. Comptes Rendu de l’Academie des Sciences, Paris, Serie II 304, 853-856.

Buffetaut, E. 1983. On the late occurrence of an archaic crocodilian in the Pleistocene of the Isle of Pines (New Caledonia) and its biogeographical significance. Comptes Rendu de l’Academie des Sciences, Paris, Serie II 297, 89-92.

Mead, J. I., Steadman, D. W., Bedford, S. H., Bell. C. J. & Spriggs, M. 2002. New extinct mekosuchine crocodile from Vanuatu, South Pacific. Copeia 2002, 632-641.

Molnar, R. E., Worthy, T. & Willis, P. M. A. 2002. An extinct Pleistocene endemic mekosuchine crocodilian from Fiji. Journal of Vertebrate Paleontology 22, 612-628.

Naish, D. 2001. Fossils explained 34: Crocodilians. Geology Today 17 (2), 71-77.

Salisbury, S. W. & Willis, P. M. A. 1996. A new crocodylian from the Early Eocene of eastern Queensland and a preliminary investigation of the phylogenetic relationships of crocodyloids. Alcheringa 20, 179-226.

Vences, M., Vieites, D. R., Glaw, F., Brinkmann, H., Kosuch, J., Veith, M. & Meyer, A. 2003. Multiple overseas dispersal in amphibians. Proceedings of the Royal Society of London B 270, 2535-2442.

Willis, P. 1993. Trilophosuchus rackhami gen. et sp. nov., a new crocodilian from the early Miocene limestones of Riversleigh, northwestern Queensland. Journal of Vertebrate Paleontology 13, 90-98.

- . 1997. New crocodilians from the late Oligocene White Hunter Site, Riversleigh, northwestern Queenslands. Memoirs of the Queensland Museum 41, 423-438.

- . & Mackness, B. S. 1996. Quinkana babarra, a new species of ziphodont mekosuchine crocodile from the Early Pliocene Bluff Downs Local Fauna, northern Australia with a revision of the genus. Proceedings of the Linnean Society of New South Wales 116, 143-151.

- . & Molnar, R. E. 1991. A new middle Tertiary crocodile from Lake Palankarinna, South Australia. Records of the South Australian Museum 25, 39-55.

- ., Molnar, R. E. & Scanlon, J. D. 1993. An early Eocene crocodilian from Murgon, southeastern Queensland. Kaupia 3, 27-33.

- ., Murray, P. & Megirian, D. 1990. Baru darrowi gen. et sp. nov., a large, broad-snouted crocodyline (Eusuchia: Crocodylidae) from mid-Tertiary freshwater limestones in Northern Australia. Memoirs of the Queensland Museum 29, 521-540.

Monday, September 11, 2006

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 20th 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.

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Friday, September 08, 2006

Literally, flying lemurs (and not dermopterans)

Mention ‘flying primate’ and most zoologists will think you’re referring to the well known, controversial theory of John Pettigrew of the University of Queensland. And if that sentence sounds familiar, it’s because I used it previously to introduce a post all about Pettigrew’s controversial theory that megabats are not close relatives of microbats, but are instead close relatives of us primates (go here). As reviewed in that post, currently the data does not favour the idea that the two bat groups evolved independently, and the ‘flying primate’ theory has not won widespread acceptance. The term ‘flying primate’ might be better attached however to another group of mammals, and this time they really are undeniable primates. They are lemurs, and they fly. Well… fly sort of.

And, no, I am not talking about the flying lemurs, aka colugos, aka dermopterans: they aren’t primates, though they are close relatives.

Among the 50-odd living lemur species, among the most charismatic and distinctive are the sifakas or simponas, a group of relatively large, long-tailed arboreal indriids. If you’re wondering, ‘sifaka’ is an onomatopoetic reference to their alarm call, and is pronounced ‘shee-fak’ (or something similar if you use your imagination). With tail, they get to about 1 m long, and they weigh up to 7 kg. Most sources will tell you there are three species - two of which, the Diademed sifaka Propithecus diadema and Verreaux’s sifaka P. verreauxi, have been scientifically known since the 1800s, while the third, the Golden-crowned sifaka P. tattersalli (yes, named after primatologist Ian Tattersall), wasn’t named until 1988. The latter is one of several lemur species that have been named very recently. They include Cleese’s woolly lemur Avahi cleesei, Seal's sportive lemur Lepilemur seali, the Mitsinjo sportive lemur L. mitsinjonensis, Goodman’s mouse lemur Microcebus lehilahytsara and the Northern giant mouse lemur Mirza zaza, all named in 2005, and Mittermeier’s mouse lemur Microcebus mittermeieri, Jolly’s mouse lemur M. jollyae, Simmons’ mouse lemur M. simmonsi, and the new sportive lemurs Lepilemur sahamalazensis, L. aeeclis and L. randrionasoli, all named in 2006.

However, several taxa conventionally regarded as Diademed and Verreaux’s sifaka subspecies are extremely distinct, and in view of my previous comments on ‘unlumping’ (see Giraffes: set for change, Giant furry pets of the Incas, and The many babirusa species: laissez-faire lumping under fire again) you probably won’t be surprised to learn that some mammalogists now recognize these ‘subspecies’ as valid species. Of the forms previously included within P. diadema, Milne-Edward's sifaka P. edwardsi, the Silky sifaka P. candidus and Perrier’s sifaka P. perrieri are now regarded by some as distinct species, and among those taxa previously included within P. verreauxi, we now have Coquerel’s sifaka P. coquereli, the Crowned sifaka P. coronatus and Decken’s sifaka P. deckeni raised to species level by some primatologists. Taxonomic revisions have thus upped the number of sifaka species from three to nine.

Actually, there may be a few more. A population discovered at Tsinjoarivo in 1999, originally thought referable to P. diadema, is morphologically distinctive and might represent a new species. Known informally as the Tsinjoarivo sifaka, it is small, with distinctive black patches on its limbs, and with duller yellow-orange on its limbs, and less white facial hair, than definite P. diadema.

Sifakas are what is known as ‘arboreal clingers and leapers’, a locomotor category first identified by Napier & Walker (1967). Climbing mostly on vertical trunks, including those of the horrendously spiky Didiereaceae trees, sifakas are able to propel themselves with their long and powerful hindlimbs for considerable distances: up to 10 m and perhaps more in cases. As they leap, they extend their arms forwards and outwards, and flaps of furry skin extend around their arms. Most people interested in animals know this because they’ve seen it depicted on TV, most memorably perhaps in episode 12 (A Life in the Trees) of Attenborough’s BBC series Life on Earth (funnily enough, previously mentioned in The Cultured Ape and Attenborough on gorillas). The comical prancing behaviour that sifakas employ when on the ground is also often featured on TV documentaries. Anyway, the flaps of skin that extend as a sifaka jumps appear to function in gliding, and indeed the entire arm seems to function as an airfoil. They are gliding primates.

You might be surprised to learn that there is an extensive literature discussing the apparent gliding behaviour of lemurs and other primates, much of it reviewed in Demes et al.’s 1991 paper ‘They seem to glide’. Demes and colleagues were quoting J. J. Petter’s 1962 article in which Petter noted of indris* that ‘Pendant le saut leur corps est parfaitement horizontal et ils semblent planer’. In fact suggestions that the extendable skin around the arms of indriids might have an aerodynamic function can be traced back to French explorer Alfred Grandidier who thought as much in 1875. Grandidier was among the first to properly study and document Madagascan wildlife, publishing (with A. Milne-Edwards) his observations in the 32-volume Histoire Physique, Naturelle, et Politique de Madagascar.

* The Indri Indri indri is of course not a sifaka, but it is a close relative, and a large one given that it reaches 70 cm in total length and 7.5 kg. Unlike sifakas it has only a vestigial tail, but like sifakas it adopts a spread-eagle position when leaping.

Most popular and semi-technical works on primates fail to mention any of this stuff however, which amazes me given that it’s rather interesting to say the least. I am happy to report that Nowak (1999, p. 527), at least, says of sifakas ‘the short arms are limited in their movement by small gliding membranes’. This is the sixth edition of Walker’s Mammals of the World: I checked the fourth edition (1983), which I also own, and this also contains the same line.

Other strepsirrhine primates also seem to slow their leaps by spreading their arms and using skin membranes, including galagos (Charles-Dominique 1977). So to test the idea that these leaping primates might really be enjoying an aerodynamic effect from their arms and associated membranes, Demes et al. (1991) estimated the effects of lift and drag on these primates based on velocity at takeoff, body mass, surface area and other variables. They found that aerodynamic lift and drag could significantly affect the flight path of leaping indriids, and that this was partly due to the relatively large surface area to body mass ratio they exhibited. This large surface area comes predominantly from the skin membranes. However, they weren’t able to go beyond theory and do anything practical like wind-tunnel tests, mostly because there aren’t that many spare sifakas kicking around for use in laboratory tests. Indeed the species involved here are endangered and protected.

As hinted at by the fact that it’s not much mentioned in the books, the supposed gliding habits of sifakas and other primates are not as well known among mammalogists as they might be. In fact these habits have been most widely brought to attention in the literature on bird origins. Rightly or wrongly, the debate over avian origins has long been dichotomized into a ‘ground up’ school, and a ‘trees down’ school. It is absolutely wrong to argue – as some workers have – that the ‘trees down’ theory is at odds with the very robust and well supported body of evidence showing that birds are theropod dinosaurs, given that basal birds, and the theropods closest to birds, were apparently small-bodied proficient tree climbers, and not big cursorial Deinonychus-like predators as some would have it. If small, scampering scansorial predators were the ancestors of birds, I find the evidence to better support the idea that flight evolved in the trees, and I’ve argued such in some not particularly good, and much overlooked, articles (Naish 2000a, b).

What have gliding lemurs got to do with all this, I hear you ask. Alan Feduccia, the ornithologist who should be best known for his work on Neotropical passerines but is unfortunately far better known for his various attempts to poke holes in the bird-dinosaur theory, has repeatedly used sifakas and other gliding primates as models for the early stages in the development of avian flight (Feduccia 1993, 1995, 1996, Geist & Feduccia 2000). In other words, Feduccia proposed that sifakas might serve as an analogy illustrating how feathers and flight might have evolved from leaping arboreal prototypes.

Hold on: feathers? Well, Feduccia not only drew attention to the presence and role of the skin membranes in sifakas, he also discussed the presence of a thick, posteriorly projecting mat of hair on the sifaka forearm. In some specimens this mat provides c. 64% of the total width of the combined forearm + mat surface, and during leaping it appears to form the trailing edge of what is effectively an airfoil (the stiff leading edge of which is formed by the bony forearm and its associated membrane). The hair mat, Feduccia suggested, might give us an insight into how enlarged scales on the trailing edge of a proto-bird’s arm might, by incremental enlargement as they became increasing feather-like, have provided an aerodynamic advantage. Feduccia (1993, p. 162) stated that ‘there can be no doubt that an airfoil is produced by the sifaka’s arms and the partial incorporation of a “lift mechanism” would advantageously augment the horizontal extent of a “leap”’. The current data from theropods shows that quill-like integumentary structures were present in theropods before one lineage gave rise to scansorial proto-birds. Furthermore, there are reasons for thinking that proto-birds were flapping their wings from the start, and it’s debatable as to whether or not they ever went through a leaping and gliding phase. Even so, with their arboreal behaviour, skin membranes and specialized brachial integument, sifakas might still be informative.

In birds, bats and other volant tetrapods, the skin membranes that function in flight are termed patagia. The membrane that extends along the leading edge of the arm (usually from the shoulder to the wrist) is the propatagium while that connecting the trailing edge of the upper arm to the body is the plagiopatagium. While the extendable flaps of indriids and some other primates are proportionally smaller than the patagia of most gliding and flying tetrapods, they are in the same place and seem to serve the same function, so it seems appropriate to give them this name. Some workers have done this: Charles-Dominique (1977) likened the skin membranes of galagos to incipient patagia, and Feduccia (1993) used the term patagium in connection with sifakas.

Now, even without all this stuff on patagia, gliding behaviour and bird origin theories, sifakas are pretty cool and interesting mammals. But I still can’t help thinking that the gliding behaviour makes them particularly interesting, and I’m therefore surprised that more isn’t made of it. From the point of view of the big picture, it perhaps has significance in suggesting that gliding, or proto-flight or whatever, is actually reasonably easy to evolve, even in relatively large animals. Given that most mammals have flexible skin extending from the upper arm to the body that might provide drag and/or lift in leaping, it’s even conceivable that more mammals ‘seem to glide’ than we presently acknowledge. Indeed there are anecdotal accounts of semi-gliding, or parachuting, in such things as palm civets. Among primates, Feduccia (1996) and Geist & Feduccia (2000) also pointed to gliding behaviour in the sakis (Pithecia), a group of frugivorous South American monkeys that can reportedly ‘maneuver accurately while airborne to a target tree trunk, often adjusting their bodies so that they glide upwards at a steep angle just before contact’ (Geist & Feduccia 2000, p. 668-669) [The accompanying photo, taken from Graeme Elliott’s flickr site, depicts a White-faced saki P. pithecia].

So – getting back to sifakas and other lemurs – I think it’s not just that they ‘seem to glide’. It seems that they really do. There’s something else really neat I wanted to say about sifakas: the fact that they’re transitional creatures, caught in the middle of an evolutionary change that they’re struggling to adapt to. It’s a story that involves the extinction of other lemurs, giant fossas and eagles, and a social system that has yet to be properly ironed out. But it’ll have to wait to another time. For the latest news on Tetrapod Zoology do go here.

Refs - -

Charles-Dominique, P. 1977. Ecology and Behaviour of Nocturnal Primates. Duckworth (London).

Demes, B., Forchap, E. & Herwig, H. 1991. They seem to glide. Are there aerodynamic effects in leaping prosimian primates? Zeitschrift fur Morphologie und Anthropologie 78, 373-385.

Feduccia, A. 1993. Aerodynamic model for the early evolution of feathers provided by Propithecus (Primates, Lemuridae). Journal of Theoretical Biology 160, 159-164.

- . 1995. The aerodynamic model for the evolution of feathers and feather misinterpretation. Courier Forschunginstitut Senckenberg 181, 65-77.

- . 1996. The Origin and Evolution of Birds. Yale University Press (New Haven & London).

Geist, N. R. & Feduccia, A. 2000. Gravity-defying behaviors: identifying models for Protoaves. American Zoologist 40, 664-675.

Naish, D. 2000. Theropod dinosaurs in the trees: a historical review of arboreal habits amongst nonavian theropods. Archaeopteryx 18, 35-41.

- . 2000. 130 years of tree-climbing dinosaurs: Archaeopteryx, ‘arbrosaurs’ and the origin of avian flight. The Quarterly Journal of the Dinosaur Society 4 (1), 20-23.

Napier, J. & Walker, A. C. 1967. Vertical clinging and leaping – a newly recognized category of locomotor behaviour of primates. Folia Primatologica 6, 204-219.

Nowak, R. M. 1999. Walker’s Mammals of the World, Sixth Edition (two volumes). The Johns Hopkins University Press (Baltimore and London).