In the previous post we looked at the diversity of the rhinogradentians (aka rhinogrades or snouters) belonging to the so-called monorrhinan or uni-snouter division, and we also started to go through the asclerorrhinan or soft-nosed snouter division. Here, in the second post on this much-discussed and highly popular subject, we finish our tour of asclerorrhinans before going on to look at the last and most anatomically complex group, the polyrrhinans or multi-snouters. We finish by looking at the modern-day renaissance in rhinogradentian research: an endeavour which has resulting in the scarcely believable discovery of living relict species. Needless to say, the previous post is required reading before you tackle this one…
Conventionally regarded as the most recently evolved of the soft-nosed snouters were the snout leapers, and in the previous post we looked at the diverse arboreal and terrestrial members of this clade. Primitively quadrupedal, and with a jointed nasarium that functioned as a spring-action pseudo-limb, they also included the short-bodied true snout leapers or hopsorrhinids, a group where hindlimbs were absent.
The best known and most studied snout leaper – and indeed one of the best known and most studied of all rhinogradentians – was the Earwing Otopteryx volitans [skeleton shown above: sketch of live animal shown in adjacent image]. While conventionally awarded its own ‘family’ it was more likely a derived hopsorrhinid as it certainly possessed all the derived peculiarities of this group. When terrestrial hopsorrhinids leapt, they were sometimes observed using their large ears to help steer their progression. This perhaps gives us a clue as to how the flight ability of Otopteryx evolved, as this remarkable rhinogradentian used its enormous wing-like ears to fly with. The enormous aural cartilages and their hypertrophied muscles – particularly the m. aeroplano-jugalauris – allowed Otopteryx to flap its ears in rapid, skillfull flight and it was even able to avoid avian predators on the archipelago (Stumpke 1967). Jurassic specimens of Otopteryx were reported from France in 1995 [see image below: from here] and are so similar to the Hi-yi-yi population that they were initially referred to the same species. This is possible of course but would be unprecedented for a mammal, and the possibility remains that the fossil should be named as a distinct taxon.
Finally among the snout leapers, there were the specialised orchidiopsids or orchidsnouters. Indeed these rhinogradentians are so unusual that their classification among the snout leapers was initially not suspected. Entirely arboreal, they lacked hindlimbs and hence probably descended from hopsorrhinids (in fact, it is probably accurate to regard them as part of Hopsorrhinidae, rather than as a separate ‘family’), but unlike hopsorrhinids, they possessed a large, flattened rhinarium that resembled a leaf or petal. The large ears were also petal-like. Standing vertically on their powerful tail, and climbing slowly with their prehensile hands, they mimicked leaves and flowers and preyed on insects. At least one species, the Glowing lily snouter Liliopsis thaumatonasus, was bioluminescent: this apparently being due to symbiotic bacteria.
Polyrhiny – the condition in which the nasarium is divided into several or multiple snouts – was unique among rhinogradentians to the polyrrhinans, or multi-snouters. These are the most familiar of rhinogradentians, but they are also the most mysterious, as their origins remain obscure and they so differed from other rhinogradentians in so many details that they are generally thought to have diverged as early as the Cretaceous (Stumpke 1967). This has been validated by the discovery of hopsorrhinids in the Jurassic as just mentioned.
Polyrrhinans were divided by Stumpke into the tetrarrhinidans (four-snouters), hexarrhinidans (six-snouters) and dolichoproatans (tasselsnouters). Tetrarrhinidans include the well-studied snout walkers or nasobemes and the amazing Predacious snouter Tyrannonasus imperator [in adjacent image, a Predacious snouter is attacking a Nasobema. Image originally from Stumpke 1967 but taken from reproduction here]. The frugivorous nasobemes were able to walk on their four long snouts thanks to an internal skeleton of corpora spongiosa and a hydraulic system that provided turgidity when needed. The tail of these animals was equally unusual: it was distally hollow and boneless and connected to the caecum. By forcing intestinal gas into the tail, nasobemes were able to shoot out the ribbon-like tail for a distance of up to 3 m (this in a rhinogradentian with a rhinarium and body length of about 1 m: they were among the largest members of the group). Tyrannonasus preyed upon nasobemes, capturing them following pursuit on-nose and killing them with a poisonous tail spike. Various characters of the digestive tract indicate that Tyrannonasus evolved from frugivorous species and it has even been proposed that its predatory lifestyle developed from an original habit of stealing fruit from nasobemes, this piracy developing over time into predation.
Among the most specialised rhinogradentians, the hexarrhinidans (six-snouters) include the ribbon snouters, six-flowered snouters and flower-faced snouters. Ribbon snouters (Eledonopsis) lived in burrows and were shrew-like, but for the ribbon-like snouts that extended along the ground, some distance (up to c. 30 cm) away from the burrow. A ciliated epithelium extended along the entire dorsal surface of each snout, and small insects that became stuck here were carried by the cilia toward the mouth. Six-flowered snouters (Hexanthus) presumably descended from Eledonopsis-like forms but, instead of long ribbons, their proboscides took the form of long tubes, each sprouting a flower-like tip that the animal would wind about the stems of adjacent plants. Flying insects, deceived by these flower-like structures, would settle on them and be caught, and then moved by peristalsis along the tubes.
An apparent deviation from the cryptic lifestyle of other hexarrhinidans was adopted by flower-faced snouters (Cephalanthus). Equipped with petal-like fleshy growths surrounding the mouth, the best known members of this group remained sessile as remarkable flower-mimics. While young Cephalanthus had a ‘normal’ mammalian tail, in adults it was a long, stiff structure rooted in the ground. After birth, a young Cephalanthus would climb to the top of a tall flower, bite off the flower head, and then take up life as a flower-mimic, an exudate from the mouth attracting flying insects. The sessile attraction of insects, aided by oral exudates, may not be unique to rhinogradentians among tetrapods, as it has been suggested that frogmouths may also employ this technique (Diamond 1985). As discussed in my previous article on flightless future bats, Cephalanthus is also strikingly similar to the sedentary Flooer Florifacies mirabila of the Batavian forests (Dixon 1981).
While some of the polyrrhinans, such as the nasobemes and Predacious snouter, were quite large, they are dwarfed by the Bearlike shaggyfaced snouter Mamontops ursulus [shown in adjacent image], a herbivorous six-snouted species that attained a vertical height of 1.2 m in the males. Equipped with a thick woolly coat and short limbs that were well tucked-in to the pelage, it was a highland specialist that lived in small herds. Tasselsnouters, the dolichoproatans, were also large and possessed the most elaborate nasarial morphology of any rhinogradentian, with a ridiculous 18 snout pairs.
When Stumpke 1967 was published, rhinogradentians were regarded as unique to the Hi-yi-yi archipelago. As discussed earlier however, Jurassic fossils show that snout leapers – members of an apparently derived (and thus comparatively young) rhinogradentian clade – were formerly present in what is now continental Europe, suggesting that rhinogradentians were originally of very wide distribution. Several even more exciting recent discoveries have shown, amazingly, that relict rhinogradentians have survived to the present in various Old World locations. In 1999 a live rhinogradentian was discovered in Slovenia. Representing a new taxon, Acrorrhinos, it appears to be particularly archaic and closely related to Haeckel’s primitive snouter [image below from here: this site also provides an account of this taxon's discovery].
In 2004, a new, extant colonial rhinogradentian was discovered (Kashkina 2004), and later in the same year it was realised that various hermit crab parasites were also highly modified, tiny parasitic rhinogradentians (Bukashkina 2004). As discussed in the previous post, it was remarkable enough that the tiny turbellarian-like allsnouters were originally mistaken for invertebrates but, as ridiculous as it sounds, we now know of two rhinogradentian lineages that diverged from the mammalian body plan to such an unprecedented degree. It’s well known that a parasitic or infaunal lifestyle can lead to an overall simplification in form and function, and these incredible tiny boneless mammals provide what might be the best example of this in the whole of zoology.
Given that very little rhinogradentian material exists in collections, many species are only known from field sketches. Several workers have attempted in recent years to reconstruct some of the species three-dimensionally: Japanese model-maker Takeshi Tokiwa has produced models of many species (viewable here), as has Michaela Fortuber of the Naturhistorisches Museum at Braunschweig, Germany. Naturally, rhinogradentians have made many apperances in the technical literature and their affinities and origins continue to be debated. It remains tragic that the vast majority of species are now extinct, especially when they were such unique, remarkable creatures.
I’d like to thank everyone who has supported my academic interest in rhinogradentians over the years (particularly Dr Vector), and to all my readers for being so patient in waiting for these articles to appear. Rhinogradentians have been mentioned here at Tetrapod Zoology for some considerable time now (check the archives) and, despite requests that I should keep making the promises but perpetually fail to publish the promised articles, I always planned to deliver. At last, my work is done.
Refs – –
Bukashkina, V. V. 2004. New parasitic species of colonial Rhinogradentia. Russian Journal of Marine Biology 30, 150.
Diamond, J. 1985. Filter-feeding on a grand scale. Nature 316, 679-680.
Dixon, D. 1981. After Man: A Zoology of the Future. Granada, London.
Kashkina, M. I. 2004. Dendronasus sp. – a new member of the order nose-walkers (Rhinogradentia). Biol. Morya 30 (2).
Stumpke, H. 1967. The Snouters: Form and Life of the Rhinogrades. The Natural History Press, Garden City, New York.