At last, the rhinogradentians (part I)

I am always surprised when I meet zoologists who aren't familiar with Harald Stumpke's* famous 1957 book Bau und Leben der Rhinogradentia, a volume translated into English by Leigh Chadwick in 1967 as The Snouters: Form and Life of the Rhinogrades (and referred to from hereon as 'Stumpke 1967'). For the few who don't know, this legendary text discusses in marvellous detail the biology, lifestyle and evolution of the snouters (aka rhinogrades or rhinogradentians), a bizarre and unique group of small mammals originally thought endemic to the south Pacific Hi-yi-yi archipelago, an island group discovered by Einar Pettersson-Skamtkvist (umlaut over the a) in 1941 as he escaped from Japanese imprisonment...

* Stumpke has an umlaut over the u: the scienceblogs publishing platform doesn't recognise accents however, hence its absence from my text here.

Little known is that Skamtkvist didn't only discover the rhinogradentians: the islands were also inhabited by the Hooakha-Hutchi people, the bizarre megaphone birds and an endemic shrew, Limnogaloides. A diverse endemic insect fauna was bizarre in including Palaeozoic relicts, and the flora also included archaic forms such as large clubmosses. Of great interest is that rhinogradentians were mentioned in late 19th century European literature, and indeed there is some indication that the Hi-yi-yi islands had actually been discovered by westerners long prior to 1941.

As evidenced by the bibliography of Stumpke 1967, a substantial number of publications devoted to rhinogradentian morphology, behaviour and evolution appeared during the 1940s and 50s and such was the interest in this group that the Darwin Institute was set up on Hy-dud-dye-fee (the largest island of the archipelago), and an international conference on rhinogradentians held there in 1953 [adjacent image shows conference delegates photographed at the meeting, from here]. In what must be ranked as one of the greatest man-made environmental catastrophes of all time, secretly conducted atomic tests occurring in the same region initiated an earthquake that caused the sudden subsidence and destruction of the entire archipelago. So not only were all the rhinogradentians known at the time made extinct, but all scientists formally specialising on the group were killed. As we'll see later, rhinogradentians have proved not to be endemic to the Hi-yi-yi archipelago as was assumed, and the discovery of new rhinogradentian species has continued to the present, making the group a constant presence in the zoological literature.

What has always bugged me about rhinogradentians is that information on many of the taxa that Stumpke described is impossible to come by, unless that is you're lucky enough to own a copy of Stumpke 1967 (14 family-level taxa and 189 species were named, and the possibility that additional, undiscovered species might have inhabited the archipelago was regarded as likely). Sure, anyone that's really interested in the group will have gone to the trouble of obtaining this work, but because the book is hard to get hold of in many parts of the world, the full diversity of rhinogradentians is not as well appreciated as it might be. A few internet sites discuss rhinogradentian diversity and figure a few of the rarely-discussed species (most notably, this excellent French site), but they fail to cover the more remarkable lineages within the group. In order to amend things, and to bring an understanding of rhinogradentian diversity to a greater audience, my aim here is to briefly review the group's taxonomic diversity. Let me emphasise that I am barely providing a fraction of the information published within Stumpke 1967 however, and if you want in-depth information on the members of the group you must get hold of the book yourself.

Archirrhinos haeckelii, Haeckel's primitive snouter [shown in adjacent image], was identified by Stumpke as the most basal rhinogradentian. A superficially shrew-like quadruped, it possessed a large nasarium that served as a support when the animal lunged forward to grab prey. Archirrhinos -like ancestors are thought to have given rise to the nasolimacids, the snail-like snouters. While primitively similar to Archirrhinos, nasolimacids evolved a specialised nasarium where the muscles and sinuses were extensively subdivided, thereby allowing the structure to function much like the foot of a snail. These remarkable mammals lived standing on their foot-like nasarium, their bodies sub-vertical and their limbs reduced or modified. They included armour-plated forms as well as the sugarmice Rhinolimaceus, a taxon so named as a gland at its tail base secreted a sweet-tasting fluid.

Probably closely related to the nasolimacids were the rhinocolumnids or pillar-nosed snouters. Again, what was probably the most basal member of the group - the snifflers Emunctator [see adjacent image] - were not all that different from Archirrhinos, though they differed notably from that taxon in using mucosal strands to trap and ingest prey from shallow water. Emunctator possessed a long, mobile tail equipped with a poisonous spur at its tip, and the same character was present in most other pillar-nosed snouters. Among these are the sedentary honeytails Dulcicauda, a group that - like the sugarmice - evolved caudal glands that secreted fragrant secretions that were attractive to insects [Dulcicauda shown at bottom of post]. The pillar-nosed snouters of the short-tailed genus Columnifax developed a remarkable symbiosis with various of the predatory snout leapers, of which we'll see more of later. Columnifax produced milk that the snout leaper drank, while the snout leaper would collect hermit crabs that were eaten by the Columnifax.

What particularly captured the attention of many of those who studied rhinogradentians is that, though descending from shrew-like terrestrial insectivores, they underwent one of the most remarkable adaptive radiations seen among mammals. In addition to the terrestrial forms we've seen so far, there were also fossorial, aquatic and arboreal forms. A morphologically unusual and phylogenetically isolated group, the hypogeonasidans, included both fossorial and aquatic forms. Among the fossorial species were the bizarre ribbon snouters Rhinotaenia: small, blind mole-like rhinogradentians with a bilobed, asymmetrical, tube-like siphonal nasarium. Predominantly animals of tidal sediments, some of them lived almost like parasites within the valves of large bivalves.

Ribbon snouters are generally regarded as closely related to the aquatic rhinostentorids, or trumpet snouters. These also possessed an elongate siphonal nasarium, but their absence of features associated with fossoriality suggests that they never went through a fossorial stage and may instead have descended directed from basal hypogeonasidans that lived at the water's edge. Indeed several derived characters, and a unique sort of mucin secretion, were common to the snifflers and the ribbon snouters, suggesting that the hypogeonasidans evolved from snifflers (Bromeante de Burlas 1952). Trumpet snouters are among the groups that are never mentioned whenever rhinogradentians are discussed. In these forms the nasarium was a funnel-like filtering apparatus, formed into a distal rosette fringed with water-repellent hairs. Stiff bristles lined the naked body. Some species hung suspended from the water's surface while others were pelagic.

Closely related to the hypogeonasidans were the even more remarkable georrhinidans or burrowing snouters, the only group in which the snout was markedly larger than the rest of the body. Basal members of this group are best exemplified by the fossorial mole snouters Rhinotalpa. Equipped with strongly reduced limbs as well as wreaths of stiff bristles around the snout and head, Rhinotalpa used inflation and deflation of its nasarium as well as erection and relaxation of the bristle wreaths to move within its burrows. These features were not present in all members of the genus however, and some (e.g., R. angustinasus) exhibited both tiny size and a highly simplified anatomy: besides a short, simple gut, reduced brain, and absent eyes and nares, they apparently relied on cutaneous respiration [adjacent life-sized Rhinotalpa model from Takeshi Tokiwa's site].

Georrhinida includes what must be among the most amazing of all tetrapods, the holorrhinids (allsnouters), as they were so structurally modified that even their tetrapod identity has been doubted (actually, this is not the first time that members of a rhinogradentian clade have been wrongly identified as invertebrates). The most specialised allsnouter was the Turbellarian-like dwarfsnouter Remanonasus menorrhinus. Just 2 mm long, this tiny worm-like mammal has been regarded by some as a free-living turbellarian flatworm, and in fact Stulten (1955) argued that turbellarians were highly derived rhinogradentians! [adjacent image shows a true turbellarian].

The forms that we've looked at so far all belong to the so-called monorrhinan or uni-snouter and asclerorrhinan or soft-nosed snouter divisions. Among the soft-nosed snouters, the most familiar and best known are the sclerorrhinans, or snout leapers. Equally well known are the members of the last division: the polyrrhinans, or multi-snouters. In the classic phyllogram produced by Bromeante de Burlas (1950), all the major divisions were posited as having originated from a common ancestor in a 'hub and spokes' type arrangement. This might have merit, especially given models that propose an explosive island-endemic radiation of rhinogradentians from a founding ancestor. However, the presence of fossil and extant rhinogradentians in the Old World has shown that all of the major divergences within Rhinogradentia had occurred during the Mesozoic, making the group an ancient one, with a relict distribution in modern times. This argues against the idea of an explosive island-endemic radiation.

Snout leapers and their relatives included numerous terrestrial, arboreal and even volant forms. The most basal forms, the arboreal snout leapers or perihopsids, were likened by Stumpke to arboreally modified kin of Archirrhinos. Their overall proportions were not extraordinary, but their large nasarium was jointed like a limb and possessed a distal plate-like organ. The tail was similarly equipped, providing the animal with similar spring-action pseudo-limbs at both end of the body. Accordingly, perihopsids were reportedly extraordinarily agile, able to leap both forwards and backwards with speed. Presumably derived from a perihopsid-like snout leaper, the hopsorrhinids or true snout leapers were characterised by an extraordinarily long, gracile, jointed nasarium that the animals used to make long backward leaps. The body was particularly short, hindlimbs were entirely absent, and the tail was long, slender and prehensile. Multiple species inhabited the beaches of Hi-yi-yi.

More on rhinogradentians in the next post... [UPDATE: now available here].

Refs - -

Bromeante de Burlas, J. 1950. A derivacao e a arvore genealogica dos Rhinogradentes. Boll. Braz. Rhin. 2, 1203.

- . 1952. The Hypogeonasidae. Bull. Darwin Inst. Hi. 5, Suppl.

Stulten, D. 1955. The Evolution of Turbellarians, a Review of New Aspects. Piltdown University Press.

Stumpke, H. 1967. The Snouters: Form and Life of the Rhinogrades. The Natural History Press, Garden City, New York.