This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Among the weirdest and most fascinating of rodents are the scalytails/scaly-tails, scaly-tailed squirrels or anomalures, properly termed Anomaluridae. For those of you that don’t know, this is a small group of exclusively* African, mostly gliding herbivores that have a weird method of supporting their gliding membranes. They look something like gliding squirrels, but they're not squirrels at all and belong to a completely separate branch of the rodent family tree.
Only three or four extant anomalure genera are recognised. Idiurus and Zenkerella are conventionally grouped together in a 'subfamily' termed Idiurinae or Zenkellerinae. Meanwhile, Anomalurus and Anomalurops (the latter is not recognised by all authors) are conventionally classified in a second 'subfamily' termed Anomalurinae.
* But read on.
On supporting science journalism
If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
Anomalures are also unusual in having proportionally enormous guts, a skull and dentition purportedly specialised for bark-eating, and – as you might have guessed – a (partially) scaly tail. They’re easily among the least frequently discussed group of rodents, and I bet most people without a special interest in rodents and/or mammalogy won’t even have heard of them. I’ve been meaning for a while to say at least a few brief things about them... let’s see how it turns out.
The first thing to say about anomalures is that they glide, sometimes covering distances exceeding 100 m and even 250 m in single glides. Large membranes (or patagia) span the space between the fore- and hindlimbs, and additional membranes connect the trailing edges of the hindlimbs with the proximal parts of the tail. Thin, leading-edge membranes (termed propatagia) extend from the wrist to the neck. All of these membranes are highly flexible and are partially contracted when the animals are climbing, clinging or doing other things that aren’t gliding. But during gliding, the limbs are stretched far out to the side and the membranes are fully extended.
For reasons that aren’t entirely clear (and, so far as I can tell, have never really been explored or discussed), anomalures have evolved a peculiar accessory structure that they use in supporting and spreading their extended patagia: it’s a large, spine-like cartilaginous elbow spur – sometimes called a spreader – that projects laterally like an extra limb segment (Johnson-Murray 1987). This can be extremely long. In some Anomalurus species (like Lord Derby’s anomalure A. derbianus), it’s equivalent in length to the lower arm plus at least half of the hand. Convergently similar cartilaginous spreaders are also present in some gliding squirrels and in the fossil gliding rodent Eomys. In those animals, however, it grows from the wrist, not the elbow. When the anomalure patagium is folded, the lateral tip of the spreader protrudes from the side of the forearm like a big, upward-curving spur (you can see it in my drawing above).
Anomalure hands and feet are narrow with strongly curved claws (variously likened to those of bats or cats) that seem to be specialised devices for vertical clinging. An additional, weirder structure also seems to have evolved in step with this habit of clinging on vertical woody supports: a patch of large, rough, overlapping scales with spine-like keels are arranged along the underside of the tail, located about a third of the way along the tail’s length. It’s said that these scales work as ‘anti-skid’ devices on smooth tree trunks, and also that they work as climbing irons when the animals are moving vertically on a trunk (Hanney 1977).
Anomalures might be bark specialists, gnawing away at the outside of tree trunks and also using their incisors to snip off small branches. This behaviour means that their residence in a given area can actually be revealed by looking for snipped-off branches that occur at the same height at which anomalures glide and forage: Kingdon (1997, p. 175) says that “This pruning activity can be directly correlated with habitual glide-lines radiating from dens”. However, stomach content data shows that their diet is more diverse than this, with berries, fruits, leaves, flowers and even sap all being among recorded food items. Especially intriguing is the fact that one specimen of Lord Derby’s anomalure had eaten large numbers of ants and termites (Julliot et al. 1998). Was this representative of a one-off, opportunistic event, or do some species routinely eat social insects?
What little data is available on life history and reproduction indicates that anomalures produce 1-3 young per litter, perhaps producing two litters per year. The production of a single young by a rodent mother may seem unusual given that we’re so used to the incredible fecundity of rats, mice and voles, but it’s actually quite typical for larger species. [Photos below by Daderot and Eveha.]
At least some anomalures (including the Pygmy scalytail or Zenker’s flying mouse Idiurus zenkeri) are highly social, roosting in tree hollows in numbers of 100 or more. When disturbed, these animals have been described “taking to the air in clouds, and ‘floating away among neighbouring trees like bits of soot from a chimney’” (Hanney 1977, p. 31). That last bit of descriptive prose had a rather familiar ring to it, and I wasn’t at all surprised on checking to find out that it came from the pen of Ivan T. Sanderson. Hmm. Anomalures seem to be quite vocal and capable of making hisses, growls, purrs, twitters and even loud, hooting calls. They also snap their teeth audibly when threatened.
Really big ones and really small ones
One thing that’s always struck me about this group is how big some of them are. The biggest Anomalurus species (like Pel’s anomalure A. peli) are almost 90 cm long in total and weigh almost 2 kg, making them among the biggest and heaviest of gliding mammals. Of course, these species are relative giants within the group, since those at the other end of the scale (like the Pygmy scalytail) are tiny things sometimes less than 20 cm in total length, weighing in at 14-17.5 g (Kingdon 1997).
Beecroft’s anomalure A. beecrofti is unusual compared to other Anomalurus species: it has a more projecting, narrower snout, a laterally concave toothrow and a longer, slimmer tail. It has hence been considered worthy of its own genus (Anomalurops) by some authors. Some phylogenetic studies have found it to be closer to Zenkerella and Idiurus than to Anomalurus (Maurivaux et al. 2011).
Several fossil anomalures are known*, the oldest of which are from the Oligocene. A rather unspecialised skeletal anatomy, combined with a fossil record which indicates a history of little evolutionary change, led to Kingdon’s (1997) promotion of the idea that anomalures might represent some sort of potential prototype group for squirrels, porcupines and other rodent lineages. Alas, this idea doesn’t have much going for it when we look at molecular studies, or at the morphological similarities shared between anomalures and both fossil and living rodents. A large amount of both molecular and morphological data links them (somewhat counter-intuitively) with springhares (Pedetidae), the two forming the clade Anomaluromorpha. When we look at fossil rodents, we see that anomalures are part of a more inclusive clade – Anomaluroidea – that includes several little known extinct groups.
* Kingdon (1997) says that over 20 genera are known. I’m not sure where he got that figure from.
Tell me about the fossil anomalures already!
So, what about those fossil anomalures? Actually, anomalures proper – that is, fossil species similar enough to living ones to be included in the group Anomaluridae – are ‘only’ known from the Oligocene onwards. Paranomalurus from the Oligocene and Miocene of Kenya is similar to Anomalurus and seems to be closely related to it and deeply nested within Anomaluridae proper. Anomalurus itself has a record that starts in the Miocene, as does the flightless anomalure Zenkerella.
If we look at the fossils allied with this group that pre-date the Oligocene, we see a number of taxa that, while anomalure-like, are archaic and unusual compared to modern species, and lack the derived characters that unite anomalurids proper. The fossils concerned, which include the zegdoumyids of Eocene Algeria and Tunisia and the nementchamyids of Eocene Algeria and Myanmar, thus appear to be stem-anomalures (Maurivaux et al. 2005, 2015).
Zegdoumyids – known almost entirely from teeth (some taxa were named on the basis of single teeth) – have been the subject of a reasonable amount of uncertainty but now appear to be a paraphyletic assemblage of stem-anomalures, outside the clade that includes nementchamyids and crown-anomalures (Maurivaux et al. 2005, 2015). This is significant biogeographically, as it means that an Afro-Arabian origin for Anomaluroidea as a whole is currently the best-supported hypothesis, with the Asian members of the group presumably representing an Eocene dispersal event out of Afro-Arabia. Time will tell whether this hypotheses stands up.
There are secondarily flightless rodents
One final thing warrants mention. Most extant anomalures are gliders, but Zenkerella (the Cameroon scaly-tail or Flightless scaly-tail squirrel) is not. What gives, and what does this mean for stem-anomalures? Within the crown-group, is flightless Zenkerella the sister-group to the gliding clade? In other words, does it represent a surviving relict of an ancestral, flightless bauplan?
There aren’t exactly a huge pile of studies on anomalure phylogeny, but what's been published so far seems to show that Zenkerella is deeply nested within Anomaluridae (Maurivaux et al. 2011). The idea that patagia and a gliding ability evolved independently three or more times within crown-anomalures looks exceedingly unlikely, so it currently seems that Zenkerella is secondarily flightless. Neat: a secondarily flightless rodent.
The fossils of stem-anomalures are, unfortunately, not good enough to tell us what these rodents were like overall. Sallam et al. (2010) noted that the contemporaneous Eocene Egyptian taxa Kabirmys and Shazurus have a body size difference reminiscent of that seen in extant, sympatric Anomalurus and the comparatively tiny Idiurus and Zenkerella. Kabirmys (though known only from teeth and jaw fragments) was evidently similar in size to both Anomalurus and to the Miocene taxon Paranomalurus. Furthermore, its teeth are enough like those of Anomalurus to suggest a similar feeding behaviour (Sallam et al. 2010). Might this possible evidence for niche partitioning and 'modern' dietary preferences indicate gliding abilities in the ancient taxa concerned? For now, we just can’t say, as frustrating as that is.
I ended up saying much more about anomalures than I thought I would. However, it’s worth saying that there’s more discussion of the fossil taxa mentioned here in my in-prep book on the vertebrate fossil record. And if you want to help out with my production of that gargantuan project, assistance at patreon is always gratefully received. And if you love rodents, check out the fetching Tet Zoo rodent-themed merchandise!
For previous Tet Zoo articles on other rodents, see...
Refs - -
Hanney, P. W. 1977. Rodents: Their Lives and Habits. David & Charles, Newton Abbot, London, Vancouver.
Johnson-Murray, J. L. 1987. The comparative myology of the gliding membranes of Acrobates, Petauroides and Petaurus contrasted with the cutaneous myology of Hemibelideus and Pseudocheirus (Marsupialia: Phalangeridae) and with selected gliding Rodentia (Sciuridae and Anomaluridae). Australian Journal of Zoology 35, 101-113.
Julliot, C., Cajani, S. & Gautier-Hion, A. 1998. Anomalures (Rodentia, Anomaluridae) in Central Gabon: species composition, population densities and ecology. Mammalia 62, 9-21.
Kingdon, J. 1997. The Kingdon Field Guide to African Mammals. Academic Press, San Diego.
Marivaux, L., Adaci, M., Bensalah, M., Rodrigues, H. G., Hautier, L., Mahboubi, M., Mebrouk, F., Tabuce, R. & Vianey-Liaud, M. 2011. Zegdoumyidae (Rodentia, Mammalia), stem anomaluroid rodents from the Early to Middle Eocene of Algeria (Gour Lazib, Western Sahara): new dental evidence. Journal of Systematic Palaeontology 9, 563-588.
Marivaux, L., Ducrocq, S., Jaeger, J.-J., Marandat, B., Sudre, J., Chaimanee, Y., Thura Tan, S., Htoon, W. & Naing Soe, A. 2005. New remains of Pondaungimys anomaluropsis (Rodentia, Anomaluroidea) from the latest middle Eocene Pondaung Formation of Central Myanmar. Journal of Vertebrate Paleontology 25, 214-227.
Marivaux, L., Essid, E. M., Marzougui, W., Ammar, H. K., Merzeraud, G., Tabuce, R. & Vianey-Liaud, M. 2015. The early evolutionary history of anomaluroid rodents in Africa: new dental remains of a zegdoumyid (Zegdoumyidae, Anomaluroidea) from the Eocene of Tunisia. Zoologica Scripta 44, 117-134.
Sallam, H. M., Seiffert, E. R., Simons, E. L. & Brindley, C. 2010. A large-bodied anomaluroid rodent from the earliest late Eocene of Egypt: phylogenetic and biogeographic implications. Journal of Vertebrate Paleontology 30, 1579-1593.
Vianey-Liaud, M., Jaeger, J.-J., Hartenberger, J.-L. & Mahboubi, M. 1994. Les rongeurs de l’Eocene d’Afrique nord-occidentale [Glib Zegdou (Algerie) et Chambi (Tunisie)] et l’origine des Anomaluridae. Palaeovertebrata 23, 93-118.
Vianey-Liaud, M. & Jaeger, J.-J.1996. A new hypothesis for the origin of African Anomaluridae and Graphiuridae (Rodentia). Palaeovertebrata 25, 349-358