Here I am again, derailing the Tet Zoo publishing schedule, but while I have them on my mind I may as well deal with them now. It’ll be brief (no, it wasn’t). GOLDEN MOLES!!!1!, or chrysochlorids. If you think true moles, or talpids, are weird (think about it for a minute: they are), you ain’t seen nothin’ yet. Golden moles are an entirely African group of shiny-furred, fossorial mammals with vestigial eyes (sometimes grown over with haired skin), concealed ears that lack pinnae, a robust snout that terminates in a fleshy pad, and extremely unusual, specialised forelimbs. How ‘extremely unusual’? The shoulder joint projects the upper arms laterally, giving golden moles sprawling humeri, but the robust, broad humeri are twisted such that the lower arms and hands are not directed laterally as they are in true moles, but instead can only be moved antero-posteriorly. The hands are tetradactyl, with particularly big claws present on digits II and III (digits I and IV are usually small or vestigial). A prong-like olecranon process projects posterodorsally like a curving spike from the ulna, and a unique third lower arm bone – the ‘flexor bone’ – is present and runs parallel to the radius and ulna. It’s thought to be an ossified flexor tendon (Gasc et al. 1986) [image of Eremitalpa forelimb below from palaeos.com]. The sides of the thorax are deeply hollowed to make space for the enlarged forelimbs. The hindlimbs are not specialised which, incidentally, very much contrasts to the condition present in marsupial moles (which I won’t be discussing here)…
As usual, there’s no such thing as ‘the Golden mole'; instead we’re talking about a group of species (21 or so) classified in nine genera. Two of these, Carpitalpa and Eremitalpa, are monotaxic (for C. arendsi, Arend’s golden mole, and E. granti, the Desert golden mole, respectively), but the others each consist of two or more species: the yellow golden moles Calcochloris, narrow-headed or South African golden moles Amblysomus, forty-toothed golden moles Chlorotalpa, cape golden moles Chrysochloris, large or giant golden moles Chrysospalax, Gunning’s golden moles Neamblysomus, and cryptic or secretive golden moles Cryptochloris. As you’d expect for obscure subterranean mammals, many species are very poorly known, and in fact some are only known from the holotype (this is the case for Visagie’s golden mole Cryptochloris visagiei, for example, according to some [possibly outdated] sources). Furthermore, new species are sometimes recognised, with the most recent addition to the group being the Robust golden mole Amblysomus robustus Bronner, 2000 (Bronner 2000, 2006) [species shown at the top is Cryptochloris zyli].
There are fossil golden moles now?
There are a few fossil golden moles, and some extant species also have a fossil record extending back into the Pleistocene or Pliocene. Proamblysomus antiquus Broom, 1941 and Chlorotalpa spelea Broom, 1941 are from the Late Pliocene or Pleistocene of South Africa, but a good partial skull of Pr. antiquus, reported in 1948, has since been lost. Prochrysochloris miocaenicus Butler & Hopwood, 1957 is from the Lower Miocene of Kenya (it’s also been reported from Namibia) and differs from other golden moles in having less molar-like premolars. This difference (and others) have led some to put it in its own little Prochrysochlorinae ‘subfamily’.
A far older fossil golden mole, Eochrysochloris tribosphenus, was described from the Lower Oligocene Jebel Qatrani Formation of northern Egypt by Seiffert et al. (2007), but is only known from lower jaw material. A Palaeocene fossil that has similarities with golden moles and might be a stem member of the group is also known but remains unnamed so far as I know. Unfortunately all of these fossils shed little on the early evolution of the group, but do read on. Incidentally, don’t get confused if you see the name Pseudochrysochloris: this Oligocene animal is not a golden mole, but a fossorial palaeanodont (palaeanodonts are enigmatic, but the best known kinds were something like proto-pangolins… in fact they may be stem pangolins). Little known is that there was a radiation of tiny little fossorial palaeanodonts, superficially resembling fairy armadillos and golden moles. One made a guest appearance in my 2006 Christmas card, and I’ve been meaning since then to write about them.
Afrosoricida, Afroinsectivora, Afroinsectiphillia… (I kid you not)
What exactly are golden moles? Most people smart enough to consider this question will already know everything I’m about to say, but what the hell. Conventionally, golden moles were allied with tenrecs, shrews, hedgehogs and true moles in a group termed either Insectivora or Lipotyphla. What are the characters that support this classification? Well, golden moles are small, and… insectivorous, like shrews, right? Butler (1988) proposed reduction of the jugal, presence of a mobile proboscis, absence of caecum and a few other characters as insectivoran/lipotyphlan synapomorphies, but golden moles only possess a few of the relevant characters and mostly seemed classified in Insectivora/Lipotyphla by default. In fact golden moles have been considered distinct enough to get their own special insectivoran ‘sub-order’ or ‘super-family’, variously termed Chrysochloroidea Broom, 1915, Chrysochloroida Heim de Balsac & Bourlière, 1954, Chrysochlorida Butler, 1972, or Chrysochloromorpha MacPhee & Novacek, 1993.
Molecular analyses came along in the mid-late 1990s and mostly scattered Insectivora/Lipotyphla to the four winds, with data strongly indicating that golden moles and tenrecs were in fact part of Afrotheria, a clade that also includes aardvarks, hyraxes, sirenians and proboscideans. Afrotherian monophyly was established solely on the basis of molecular support, frustrating (but not defeating) efforts to reconcile the tree for extant placentals with that established on fossils and/or morphology. Morphological characters for afrotherians are starting to come in: Sánchez-Villagra et al. (2007) showed that afrotherians have higher numbers of thoracolumbar vertebrae (20 and above) than other mammals (mostly 19). Golden moles have 22-24 thoracolumbars and fit with this proposal [afrotherian phylogeny below borrowed from Tabuce et al. 2008. Image above – call it ‘Afrotheria, the logo’ – from microecos].
Golden moles and tenrecs appear to be close relatives, forming a clade usually termed Afrosoricida Stanhope et al., 1998 (though this is essentially synonymous with Tenrecoidea McDowell, 1958, see Asher (2001)). Afrosoricidans might be closely related to sengis (and a sengi + afrosoricidan clade has already been named Afroinsectivora Waddell et al., 2001), and in turn afroinsectivorans might be close to aardvarks (and an aardvark + afroinsectivoran clade has already been named Afroinsectiphillia Waddell et al., 2001). However, a lot more work needs doing before we can be more confident about these proposals (Asher et al. 2003, Seiffert 2007, Tabuce et al. 2008).
Strange ears, neat teeth
Surface-foraging golden moles have been reported to repeatedly submerge their heads and shoulders in the sand (in some species, this behaviour has been likened to the rooting performed by pigs), and this behaviour, combined with the presence of particularly large, stout ear ossicles* and with the absence of contractile middle ear muscles, have led many to suggest that golden moles detect prey by sensing subterranean vibrations, and conduct sound through their skull bones (Mason 2003a) [for images see Matthew Mason’s homepage]. However, some golden moles (notably Amblysomus) have ear bones more typical for mammals (Mason 2003b) [Eremitalpa granti shown in adjacent image].
* Golden mole ear ossicles are, proportionally, the biggest of any mammal.
We saw recently that true moles have dilambdodont teeth. Golden moles share with tenrecs, solenodons and various fossil groups the zalambdodont condition: this is where the ectoloph (the crest on top of the molar) is large and V-shaped, with the valley of the V being on the lingual side of the tooth. Given that both golden moles and tenrecs share zalambdodonty, it would seem reasonable to assume that this condition was present in their common ancestor. But Widanelfarasia, a Late Eocene Egyptian fossil that is either a basal tenrec or a close relative, has a molar morphology intermediate between dilambdodonty and incipient zalambdodonty, suggesting that zalambdodonty was convergently aquired by golden moles and tenrecs (Seiffert et al. 2007) [skull of Cape golden mole Chrysochloris asiatica shown below, from palaeos].
Golden moles are mostly insectivorous, but the larger species eat giant earthworms and lizards (including geckos and burrowing skinks). With their sharply cusped teeth well suited for cutting, they can probably bite hard, but they are not reported to bite when handled, and in fact some species feign death. I once saw a well known palaeontologist complaining that the only images he could find of live golden moles showed them shoving centipedes or lizards into their mouths. It does seem to be the case that their feeding behaviour is often captured in photos, but it isn’t always. Stuart & Stuart’s (1988) field guide to South African mammals has five photos of golden moles simply looking bemused or reticent about being exposed on the surface in daylight, for example, and there are lots of photos of live ones, often looking surprisingly cute, in Walker’s Mammals of the World.
As always, there’s lots more that could be said, in particular about their physiology, respiration, burrowing habits and peculiar vocalisations. But here I am wasting time when I could be, I dunno, writing a paper or earning proper money or something.
Refs – –
Asher, R. J. 2001. Cranial anatomy in tenrecid insectivorans: character evolution across competing phylogenies. American Museum Novitates 3352, 1-54.
– ., Novacek, M. J. & Geisler, J. H. 2003. Relationships of endemic African mammals and their fossil relatives based on morphological and molecular evidence. Journal of Mammalian Evolution 10, 131-194.
Bronner, G. N. 1996. Geographic patterns of morphometric variation in the Hottentot golden mole, Amblysomus hottentotus (Insectivora: Chrysochloridae): a multivariate analysis. Mammalia 60, 729-751.
– . 2000. New species and subspecies of golden mole (Chrysochloridae: Amblysomus) from Mpumalanga, South Africa. Mammalia 64, 41-54.
Butler, P. M. 1988. Phylogeny of the insectivores. In Benton, M. J. (ed) The Phylogeny and Classification of the Tetrapods. Clarendon Press, Oxford, pp. 117-141.
Gasc, J. P., Jouffroy, F. K. & Renous, S. 1986. Morphological study of the digging system of the Namib Desert Golden mole (Eremitalpa granti namibensis): cineflurographical and anatomical analysis. Journal of Zoology 208, 9-35.
Mason, M. J. 2003. Morphology of the middle ear of golden moles (Chrysochloridae). Journal of Zoology 260, 391-403.
– . 2003. Bone conduction and seismic sensitivity in golden moles (Chrysochloridae). Journal of Zoology 260, 405-413.
Sánchez-Villagra, M. R., Narit, Y. & Kuratani, S. 2007. Thoracolumbar vertebral number: the first skeletal synapomorphy for afrotherian mammals. Systematics and Biodiversity 5, 1-7.
Seiffert, E. R. 2007. A new estimate of afrotherian phylogeny based on simultaneous analysis of genomic, morphological, and fossil evidence. BMC Evolutionary Biology 2007, 7: 224 doi:10.1186/1471-2148-7-224
– ., Simons, E. L., Ryan, T. M., Bown, T. M. & Attia, Y. 2007. New remains of Eocene and Oligocene Afrosoricida (Afrotheria) from Egypt, with implications for the origin(s) of afrosoricid zalambdodonty. Journal of Vertebrate Paleontology 27, 963-972.
Stuart, C. & Stuart, T. 1988. Field Guide to the Mammals of South Africa. New Holland Publishers, London.
Tabuce, R., Asher, R. J. & Lehmann, T. 2008. Afrotherian mammals: a review of current data. Mammalia 72, 2-14.