The Gordian Mouse

North American deer mouse from near Santa Fe, New Mexico, possibly Peromyscus sonoriensis. Copyright J. N. Stuart.

The North American deer mouse Peromyscus maniculatus has been recognised as one of the most widespread mammal species in North America. It has been recorded from most of the continent, with the main areas of absence being northern Canada and Alaska, the south-eastern corner of the United States (where it is replaced by a closely related species, the smaller Oldfield deer mouse Peromyscus polionotus) and coastal regions of Mexico. They are highly adaptable animals, eating a wide range of foods, and their tendency to gnaw just about anything available for food or nesting has not always endeared them to their human compatriots. In recent years, they have also received their fair share of unwelcome attention as vectors for pathogens such as the Sin Nombre hantavirus*, which causes a devastating (and commonly fatal) pulmonary disease in humans. It is therefore not surprising that the deer mouse has become one of the most extensively studied mammal species out there (not quite in house mouse or black rat territory, maybe, but still definitely up there). A search for 'Peromyscus maniculatus' on Google Scholar brings back well over 16,000 results. Surely with this degree of attention, this is at least one taxon for which I cannot deploy my usual 'the taxonomy is uncertain'?

*'Sin nombre' is, of course, Spanish for 'without name'. The reason for this coy appellation is that the virus in question was first labelled the 'Four Corners' virus after being identified from patients in that region of the United States in 1993, but was renamed after residents of the region protested.

Of course it isn't. Quite the opposite, in fact. With a range this large, it is not surprising that a fair amount of variability has been recognised in the North American deer mouse over the years. Over sixty subspecies of Peromyscus maniculatus have been recognised from various regions. This variability has also been part of the deer mouse's appeal as a study animal, as evolutionary scientists have examined the relationships between subspecies. The problem comes when you realise that in some places you may find two distinct 'subspecies' of deer mouse in a single locality. If a 'subspecies' is defined as a geographical sub-unit of a reproductively coherent species (as it traditionally has been), then that ain't right. Two taxa occupying a single range and not interbreeding should be separate species, surely? But travel to another location nearby, and you'll find the two 'species' interbreeding and merging into one. One case that has been cited as a classic example of a 'ring species' is around the Rocky Mountains, where the subspecies P. maniculatus artemisiae and P. m. nebrascensis do not interbreed with each other, but both interbreed with other subspecies to the north and south.

Part of the problem, of course, is that the 'species' is just a horrendously messy concept. There are a wealth of different species concepts out there, but the essential ideal underlying most is that a 'species' represents the point at which the reticulating relationships between interbreeding individuals become less important for understanding relationships than the branching relationships between population lineages. Which is not a point at all. Lineages diverge at multiple, quasi-independent levels in the process of speciation—they separate geographically, behaviourally, genetically, morphologically—and there is no magic point at which they suddenly change from 'one species' to 'separate species'. Nevertheless, as the recent trend has been to recognise species on a finer level than in the past, it seems likely that the future will see a subdivision of the current Peromyscus maniculatus.

Figure showing distributions of lineages of Peromyscus maniculatus from Kalkvik et al. (2012).

Over much of its range, Peromyscus maniculatus can be divided between two main morphotypes: a forest form with larger ears and a longer tail, and an open-country form with smaller ears and a shorter tail. Where the two forms are found in one region, they maintain their ecological distinctiveness. However, because of the aforementioned hybridisation between geographically adjacent populations, it is unlikely that any subdivision of P. maniculatus will be directly between these two morphotypes. One split that has already been widely accepted was proposed by Hogan et al. (1993), who found that certain populations in coastal north-west North America were genetically quite distinct from other P. maniculatus, and recommended the recognition of a separate species P. keeni. Peromyscus keeni is also generally larger and longer-tailed than P. maniculatus. However, more recent studies of the phylogeography of North American deer mice by Dragoo et al (2006) and Kalkvik et al. (2012) have found that even with the removal of P. keeni, P. maniculatus remains paraphyletic to both that species, to the south-west species P. polionotus (whose distinction from P. maniculatus has not generally been questioned), and possibly to the Mexican P. melanotis. Both the later studies identified six major lineages within P. maniculatus, and Dragoo et al. (2006) suggested that it may need to be divided between at least three species. The 'true' Peromyscus maniculatus, under this scheme, includes the two lineages found in the north-east of North America (the original type locality of the species being in Labrador). A coastal lineage found in the south-west was identified by both studies as related to P. keeni, and it remains to be seen whether it would be better included in that species or recognised as its own separate species. A lineage identified in southern New Mexico could be recognised under the name of Peromyscus blandus. The remaining two lineages, one found around the Rocky Mountains and one in the Great Plains, formed a clade that Kalkvik et al. (2012) identified as the sister lineage of P. polionotus. Dragoo et al. suggested that the name Peromyscus sonoriensis was available for this clade. However, Kalkvik et al. recognised that the Rocky Mountain lineage was a forest form and the Great Plains lineage a open-country form, so there may be grounds for their recognition as distinct species.

While the correlation between these studies appears promising, it must be stressed that both were analysing the same gene (cytochrome b) and it remains to be seen whether the lineages they identified continue to be supported by other sources of data. It also needs to be seen whether they stand up to the inclusion of further populations: western Canada and Mexico stand out as poorly sampled areas in both studies. Peromyscus maniculatus in its current form may represent one species, it may represent four, or it may yet refer to even more than that.

REFERENCES

Dragoo, J. W., J. A. Lackey, K. E. Moore, E. P. Lessa, J. A. Cook & T. L. Yates. 2006. Phylogeography of the deer mouse (Peromyscus maniculatus) provides a predictive framework for research on hantaviruses. Journal of General Virology 87: 1997-2003.

Hogan, K. M., M. C. Hedin, H. S. Koh, S. K. Davis & I. F. Greenbaum. 1993. Systematic and taxonomic implications of karyotypic, electrophoretic, and mitochondrial-DNA variation in Peromyscus from the Pacific Northwest. Journal of Mammalogy 74 (4): 819-831.

Kalkvik, H. M., I. J. Stout, T. J. Doonan & C. L. Parkinson. 2012. Investigating niche and lineage diversification in widely distributed taxa: phylogeography and ecological niche modeling of the Peromyscus maniculatus species group. Ecography 35: 54-64.

The Litopterns: Macrauchenia and More

Much has been made of the "splendid isolation" of South America for a large part of the Cenozoic. Finding itself girt by sea, South America became home to a number of endemic groups of animals: the 'terror-birds' of the Phorusrhacidae, notoungulates that were something like a rhino and something like a rabbit, and giant armadillos and anteaters. Among these uniquely South American animals were the subjects of today's post, the litopterns.

Digital reconstruction of Macrauchenia by Deskridge.

Litopterns are one of those groups of animals that tend to be represented in recent popular media by a single example, which many of you may recognise in the picture above. This was Macrauchenia patachonica, one of the latest surviving litopterns (like many other South American taxa, litopterns did not fare well during the so-called Great Faunal Interchange when South and North America became connected). But as with so many other under-represented groups, the popular exemplar is not necessarily a prime example. Macrauchenia was not only one of the last litopterns, it was also one of the largest, and the litopterns came in a whole range of appearances.

The earliest litopterns are known from the late Palaeocene. The basalmost members of the group are classified as the Protolipternidae, but the members of this family are united by primitive characters only. It is generally accepted that litopterns were closely related to two Palaeocene families of South American 'condylarths', the Didolodontidae and Sparnotheriodontidae, and there is a certain degree of arbitrariness about whether or not these families should also be treated as litopterns. Those who would exclude the didolodontoids from the litopterns do so on the basis of the latter's tarsal morphology, which has become adapted for a more cursorial lifestyle. The protolipternids bridge the gap between didolodontoids and other litopterns in that they possessed a litoptern-like tarsus, but retained teeth more like the didolodontoids. Protolipterna also retained five toes on the feet, while this number was reduced in later litopterns (Bastos & Bergqvist 2007). The other noteworthy feature of protolipternids was that they were not very big. Rose (2009) includes an illustration of partial upper and lower jaws of the protolipternid Asmithwoodwardia whose scale bar indicates that the complete skull must have been less than five centimetres in length, or about the size of a brown rat. Cifelli (1983) suggested on the basis of their small size that these animals may have been more leapers than runners, a suggestion not directly supported but not entirely ruled out by Bastos & Bergqvist (2003).

Reconstruction of Thoatherium minusculum by Charles R. Knight.

The remaining litopterns mostly belong to the families Proterotheriidae, Adianthidae and Macraucheniidae, united by specialisations of the dentition and reduction of the number of toes to three (a fifth family, the late Palaeocene Notonychopidae, are represented by dental remains only). The Palaeocene to Pleistocene Proterotheriidae have attracted a reasonable amount of interest in the past because of their convergences with the horses in the Northern Hemisphere. Like horses, proterotheriids centred locomotion on the middle toe only, with the toes on either side being reduced. In the Miocene proterotheriid Thoatherium, the side toes were almost completely lost, reduced to splints even smaller than those of the modern horse (being by this measure more horse-like than an actual horse, Thoatherium has also been a popular subject for books on evolution). In other respects, however, proterotheriids were not so horse-like. With relatively low-crowned teeth, proterotheriids and other litopterns were browsers rather than grazers, and they may have preferred more wooded terrain rather than grasslands. Ecologically, proterotheriids were probably more like deer or small antelopes than horses, and they resembled small antelopes in size. Only one proterotheriid survived into the Pleistocene, Neolicaphrium recens, and only in Uruguay and northern Argentina (Ubilla et al. 2011).

The Adianthidae were small litopterns (though not as small as the protolipternids) known from the Eocene to the Miocene. Most adianthids are known only from dental remains and/or jaw fragments, though some limb bones are known from the Miocene Adianthus godoyi (Cifelli 1991). These indicate a gracile form, probably more similar to proterotheriids than to macraucheniids, though Cifelli noted the similarities to the former were likely related to size rather than indicative of any deeper affinity.

Reconstruction of the cramaucheniine macraucheniid Theosodon garretorum, with the carnivorous metatherian Borhyaena tuberata, by Charles R. Knight.

The Macraucheniidae retained three functional toes, with the middle toe not substantially larger than the two side ones. They also differed from the proterotheriids in the development of a longer neck, and have usually been compared to camels in appearance (the name 'Macrauchenia' was originally coined to effectively mean 'big llama', in the mistaken belief that it represented an ancestor of that animal). They have been divided between to subfamilies, the Oligocene to Miocene Cramaucheniinae and the late Miocene to Pleistocene Macraucheniinae, though the latter are undoubtedly descended from the former. The cramaucheniines retain a plesiomorphic anterior nasal opening, but in the Macraucheniinae the nasal bones are reduced and the nasal opening has moved posteriad on the skull (Dozo & Vera 2010). It is this dorsal position of the nasal opening that has lead to the interpretation of Macrauchenia as having some form of proboscis, like that of a tapir. The combination of a long neck and a proboscis is, however, an unusual one, and I've wondered if it may have been more of a prehensile upper lip. The macraucheniids did better in the Pleistocene than the proterotheriids, with three species described from a large chunk of the continent, but eventually they two went the way of the toxodont.

REFERENCES

Bastos, A. C. F., & L. P. Bergqvist. 2007. A postura locomotora de Protolipterna ellipsodontoides Cifelli, 1983 (Mammalia: Litopterna: Protolipternidae) da Bacia de São José de Itaboraí, Rio de Janeiro (Paleoceno superior). Anuário do Instituto de Geociências 30 (1): 58-66.

Cifelli, R. L. 1983. Eutherian tarsals from the Late Paleocene of Brazil. American Museum Novitates 2761: 1-31.

Cifelli, R. L. 1991. A new adianthid litoptern (Mammalia) from the Miocene of Chile. Revista Chilena de Historia Natural 64: 119-125.

Dozo, M. T., & B. Vera. 2010. First skull and associated postcranial bones of Macraucheniidae (Mammalia, Litopterna) from the Deseadan Salma (late Oligocene) of Cabeza Blanca (Chubut, Argentina). Journal of Vertebrate Paleontology 30 (6): 1818-1826.

Rose, K. D. 2009. The Beginning of the Age of Mammals. JHU Press.

Ubilla, M., D. Perea, M. Bond & A. Rinderknecht. 2011. The first cranial remains of the Pleistocene proterotheriid Neolicaphrium Frenguelli, 1921 (Mammalia, Litopterna): a comparative approach. Journal of Vertebrate Paleontology 31 (1): 193-201.

Why Are There So Many Avahis?

Western woolly lemur Avahi occidentalis, photographed by Axeltelford.

A few years back, I wrote a post on the lemur family Indriidae: the indri, the avahis, the sifakas. One thing I briefly mentioned in that post is that recent years have seen an apparent avalanche of new indriid species being described. But why has this happened, and how sturdy are these new distinctions?

In 1982, Tattersall provided an overview of Malagasy lemurs that recognised just four species of indriid: the indri Indri indri, the avahi Avahi laniger, Verreaux's sifaka Propithecus verreauxi and the diademed sifaka P. diadema (Tattersall 2007). A fifth species was added in 2008, the golden-crowned sifaka P. tattersalli. But the real explosion has come in only the last ten years or so. Recent workers have proposed the recognition of seven species of sifaka (Mayor et al. 2004), and no less than nine species of avahi (Zaramody et al. 2006, Andriantompohavana et al. 2007, Lei et al. 2008). Each of the species within a genus is generally geographically separated from its congeners, and some species are recorded only from very small ranges.

In the case of the sifakas, none of the new 'species' is actually a new taxonomic entity per se. With the exception of P. tattersalli, all were previously recognised previously as subspecies of either P. verreauxi or P. diadema. The most obvious differences between the various varieties of sifaka is coloration. As noted in the earlier post linked to above, popular depictions of sifakas are heavily biased towards P. [verreauxi] verreauxi, found in the south-west of Madagascar, with a white body and black skull-cap (photo below by Jouan & Rius):

However, the sifakas are much more varied than you might think from watching David Attenborough documentaries alone. As well as the red-and-black Propithecus [diadema] diadema illustrated in the earlier post, sifakas vary from the almost entirely black P. [diadema] perrieri of the far north of Madagascar (photograph by Pete Oxford):

to the almost entirely white north-eastern P. [diadema] candidus (photo by Kevin Schafer):

The various sifaka subspecies were analysed by Mayor et al. (2004), who identified them as genetically distinct as well as distinct in appearance, and therefore recommended treating them all as separate species. However, other authors such as Tattersall (2007) have pointed out that morphological distinctions between populations may become less clear when overall variation is considered.

The above photos, from Rakotonirina et al. (2013), show variation in sifakas at a single site in central-west Madagascar, near the boundary between the ranges of Propithecus [verreauxi] deckeni and P. [verreauxi] coronatus and including individuals that might be assigned on grounds of coloration to either taxon.

In the case of the avahis, things are even more convoluted than for the sifakas. While the diurnal sifakas may vary noticeably in external appearance, the nocturnal avahis keep to a more or less basic brown. There are some slight differences between avahis on the western and eastern sides of Madagascar that had lead to the recognition of two separate subspecies, Avahi laniger laniger in the east and A. l. occidentalis in the west. A. laniger and A. occidentalis were subsequently treated as separate species on the basis of differences in their karyotypes. Each has been further subdivided into multiple species largely on the basis of genetic data alone (though vocalisation data was also a factor in separating A. unicolor from A. occidentalis). What is more, the genetic distinctions have mostly been made on the basis of mitochondrial data only, and some 'species' have only been represented in analyses by data from a few individuals. Markolf et al. (2011) suggested that genetic species could not be distinguished reliably on the basis of such small samples because of the increased risk of confusing individual variation for species-level distinctions. In the majority of cases, differences in mitochondrial genes between Avahi samples have correlated with geographical separation, but there is at least one notable exception. The central-east Malagasy location of Ranomafana has provided samples that fall into three distinct haplotype clusters. Though recognised as a single species A. peyrierasi on the basis of their common distribution, these three clusters do not form a monophyletic group in phylogenetic analyses, and the geographically separate taxa A. betsileo, A. meridionalis and A. ramanantsoavana are all nested between the A. peyrierasi haplotypes (Lei et al. 2008).

Eastern woolly lemur Avahi laniger, photographed by Inaki Relanzon.

None of the Avahi species as currently recognised overlap in range. However, in a landmass that has lost four-fifths or more of its original forest cover, it is worth asking how much of this isolation is original, and how much man-made relictualism. As always in questions of scientific research, we are left noting that further investigation is required.

REFERENCES

Andriantompohavana, R., R. Lei, J. R. Zaonarivelo, S. E. Engberg, G. Nalanirina, S. M. McGuire, G. D. Shore, J. Andrianasolo, K. Herrington, R. A. Brenneman & E. E. Louis Jr. 2007. Molecular phylogeny and taxonomic revision of the woolly lemurs, genus Avahi (Primates: Lemuriformes). Special Publications, Museum of Texas Tech University 51: 1-59.

Lei, R., S. E. Engberg, R. Andriantompohavana, S. M. McGuire, R. A. Mittermeier, J. R. Zaonarivelo, R. A. Brenneman & E. E. Louis. 2008. Nocturnal lemur diversity at Masoala National Park. Special Publications, Museum of Texas Tech University 53: 1-41.

Markolf, M., M. Brameier & P. M. Kappeler. 2011. On species delimitation: yet another lemur species or just genetic variation? BMC Evolutionary Biology 11: 216.

Mayor, M. I., J. A. Sommer, M. L. Houck, J. R. Zaonarivelo, P. C. Wright, C. Ingram, S. R. Engel & E. E. Louis Jr. 2004. Specific status of Propithecus spp. International Journal of Primatology 25 (4): 875-900.

Rakotonirina, L. H. F., F. Randriantsara, A. H. Rakotoarisoa, R. Rakotondrabe, J. Razafindramanana, J. Ratsimbazafy & T. King (in press, 2013). A preliminary assessment of sifaka (Propithecus) distribution, chromatic variation and conservation in western central Madagascar. Primate Conservation.

Tattersall, I. 2007. Madagascar's lemurs: cryptic diversity or taxonomic inflation? Evolutionary Anthropology 16: 12-23.

Zaramody, A., J.-L. Fausser, C. Roos, D. Zinner, N. Andriaholinirina, C. Rabarivola, I. Norscia, I. Tattersall & Y. Rumpler. 2006. Molecular phylogeny and taxonomic revision of the eastern woolly lemurs (Avahi laniger). Primate Report 74: 9-23.

The Wolf in Time

Black-backed jackal pup Canis mesomelas, photographed by Blake Matheson.
The dogs of the genus Canis include some of the most familiar of all mammals: the wolf Canis lupus, the coyote C. latrans, and of course the domestic dog Canis familiaris. I have already discussed in an earlier post how these three, together with the golden (Canis aureus) and the Simien (C. simensis) jackals, form a cluster of closely related species (that I'll refer to as the 'wolf group') that are not always clearly separated. Today, I'll take things a bit further and look at the fossil history of the genus Canis.

Coyote Canis latrans, from Ryan Photographic.
The earliest taxa assigned to the genus Canis are known from the late Miocene, about six million years ago (Tedford et al. 2009). Early Canis have been identified in both Europe (C. cipio) and North America (C. ferox), though there is some uncertainty about whether the European C. cipio should be treated as Canis or assigned to the related, slightly earlier fossil genus Eucyon. Whatever the case, it doesn't appear to have been long before Canis populations were well and truly established on both continents. The North American Canis ferox was, as far as I can tell, probably not dissimilar to a modern coyote in appearance, and early Canis species probably also resembled coyotes in being fairly generalist predators. In the evolutionary analysis by Tedford et al. (2009), C. ferox was suggested to have begat C. lepophagus at the beginning of the Pliocene, which in turn begat two lineages: one leading to the modern wolf group, the other leading to three North American Plio-Pleistocene species (C. thooides, C. feneus and C. cedazoensis) that were smaller than their ancestor and probably similar in appearance to modern jackals. It is somewhat unfortunate that Tedford et al.'s analysis did not include the African side-striped (C. adustus) and black-backed (C. mesomelas) jackals, which molecular and morphological analyses have generally agreed lie outside the wolf group. Biogeography alone suggests that the North American 'jackals' were probably convergent rather than directly related to the modern African species, but it would be nice to know.

Mounted skeleton of dire wolf Canis dirus, from lora_313. This species probably weighed between 50 to 80 kg, which is comparable in size to a very large dog such as a bullmastiff or great dane.
The modern wolf group diversified in the late Pliocene, including a number of fossil species as well as the modern. The rate of diversification and spread of wolf-group Canis was such that palaeontologists refer to their appearance in the fossil record as the 'wolf event', and use it as a marker of the development of the colder tundra climate of the Pleistocene ice ages. Higher diversity in Eurasia suggests that it was probably the centre of diversification, with North American species derived from repeated colonisation. Significant among these was the relatively large C. armbrusteri, a close relative of the grey wolf C. lupus. Canis armbrusteri is notable as the probable ancestor of the late Pleistocene dire wolf C. dirus, made famous by its appearances in the works of Robert E. Howard* and similar authors. As well as being a dominant predator in North America, the dire wolf spread into northwestern South America. A similar large Canis species, C. nehringi, is also known from the same time in Argentina, but the analysis of South American canids by Prevosti (2010) was unable to clearly determine whether C. nehringi was a southern relative of C. dirus or a convergent relative of the Xenocyon lineage.

*A man who spent far too much time thinking about oiled chests if ever there was one.

Dholes Cuon alpinus, from Rajnish Pradhan.
Xenocyon is itself relevant to the history of Canis: first appearing in the late Pliocene, Xenocyon lycaonoides is probably the ancestor of the modern African hunting dog Lycaon pictus and the Asian dhole Cuon alpinus, forming a hypercarnivorous lineage specialised for collaborative hunting of large prey. Phylogenetic analyses of modern taxa have varied as to whether Lycaon and Cuon are the sister group of modern Canis, or whether they are in fact more closely related to the wolf group than are C. adustus or C. mesomelas, rendering Canis paraphyletic. Removal of the latter two species from Canis into separate genera as Schaeffia adusta and Lupulella mesomelas to preserve monophyly has been suggested, but almost universally ignored (as well as failing to resolve the status of the non-wolf-group fossil Canis species). Tedford et al. (2009) even nested the Xenocyon lineage within the wolf group itself, as sister to the Canis lupus-C. dirus group, but one might suspect the influence of convergences to large size and hypercarnivory. Prevosti (2010) placed Lycaon and Cuon in a more standard position just outside the wolf group, but did not consider as many fossil Canis species as Tedford et al.

Remains of Cynotherium sardous (plus some smaller mammal), from here.
The Xenocyon lineage was undoubtedly Eurasian in origin, but the primarily Eurasian X. lycaonoides did spread into northern North America, and a second species X. texanus was found in the Pleistocene of (surprisingly) Texas. The modern dhole Cuon alpinus was also present in North America in the latest Pleistocene, with remains of at least four individuals found in a cave in northeastern Mexico, as well as being found in Europe (Tedford et al. 2009). Also a member of the Xenocyon lineage was the Pleistocene Cynotherium sardous, found on the Mediterranean islands of Sardinia and Corsica (which were a single island when the Mediterranean sea level was lower). Though descended from hypercarnivorous ancestors, Cynotherium became adapted in its island habitat to hunting smaller prey (such as the Sardinian lagomorph Prolagus sardus). Though it retained the simplified dentition of a hypercarnivore, it became smaller and the skull became less reinforced, as befits an animal no longer wrestling down large ungulates (Lyras et al. 2006).

REFERENCES

Lyras, G. A., A. A. E. Van Der Geer, M. D. Dermitzakis & J. De Vos. 2006. Cynotherium sardous, an insular canid (Mammalia: Carnivora) from the Pleistocene of Sardinia (Italy), and its origin. Journal of Vertebrate Paleontology 26 (3): 735-745.

Prevosti, F. J. 2010. Phylogeny of the large extinct South American canids (Mammalia, Carnivora, Canidae) using a "total evidence" approach. Cladistics 26: 456-481.

Tedford, R. H., X. Wang & B. E. Taylor. 2009. Phylogenetic systematics of the North American fossil Caninae (Carnivora: Canidae). Bulletin of the American Museum of Natural History 325: 1-218.

Beaver Fever

Eurasian beaver Castor fiber, from here.

Beavers are one of those animals that are familiar even to people who do not live in parts of the world where you can find beavers. The two living species of beaver are semi-aquatic rodents with one species each native to Eurasia (Castor fiber) and North America (C. canadensis) (though the North American beaver has been introduced to several parts of Europe). Differences between the two are slight: the Eurasian beaver is generally larger (up to 35 kg) and has a somewhat longer skull and a less rounded tail. Beavers are best known, of course, for their construction of elaborate subaquatic nests and dams*. Dams are generally about fifteen to seventy metres across, but have been recorded over 600 metres across (Rybczynski 2008). Beavers may also dig burrows connected to their dams, and construct canals over one hundred metres long (Rybczynski 2008).

*The original text here has been edited following Howard's comment below.

Phylogenetically, beavers are somewhat remote from other rodents, and represent the last survivors of a once more diverse lineage. First known from the late Eocene, the members of the beaver family Castoridae are divided in the most recent treatments between five subfamilies (Korth 2001, 2004). The plesiomorphic subfamilies Agnotocastorinae and Anchitheriomyinae are not well known, and the Agnotocastorinae in particular may be non-monophyletic (Rybczynski 2007). The remaining subfamilies fall into two distinct lineages: one containing the Palaeocastorinae (Oligocene-Miocene), the other the Castoridinae (Oligocene-Pleistocene) and Castorinae (Oligocene-present). Of these two lineages, only the latter are known to have been semiaquatic: the Palaeocastorinae are strictly terrestrial.

Preserved Daimonelix burrow in the American Museum of Natural History, with specimen of Palaeocastor fossor in the presumed nesting chamber, photographed by Inazakira.

The palaeocastorines, a strictly North American lineage, were specialised burrowers. Their incisors, which have rounded faces in modern beavers, became flattened and adapted for digging. Their burrows were distinctive helicoidal structures, described as trace fossils under the name of Daimonelix ('devil's spiral'), that could reach over 2.5 metres in depth and twenty centimetres in diameter. These burrows were constructed in 'towns' with multiple burrows in close proximity. Though each burrow was independent, without connections between adjacent burrows, such close positioning suggests that palaeocastorines may have had a well-developed social structure (Hugueney & Escuillié 1996). However, though the palaeocastorines were much more diverse at their apogee than the castoroidine-castorine lineage, they became extinct after a relatively short period.

Reconstructed skeleton of Castoroides ohioensis alongside that of (I presume) a modern beaver in Earlham College, from here.

The Castorinae and Castoroidinae may never have achieved the diversity at any one point in time of the palaeocastorines, they were more successful over the long haul: the more diverse of the two subfamilies, the Castoroidinae, only became extinct fairly recently. Castoroidines are commonly referred to as the 'giant beavers', and while not all castoroidines were giant (many, if not most, were smaller than modern beavers), the largest of them certainly were: the North American Castoroides reached an estimated size of about 100 kg, and would have been as large as a small bear. Whether the giant Castoroides produced similarly gigantic dams, however, is uncertain. Evidence of wood-chopping behaviour like that known for modern beavers (in the form of preserved wood bearing identifiable tooth marks, in association with beaver remains) is only well supported for one fossil species, the castoroidine Dipoides (suggested evidence for wood-chopping in Castoroides is more equivocal) (Rybczynski 2008). Phylogenetic bracketing between Dipoides and modern beavers would suggest that wood-chopping arose at the base of the castoroidine-castorine clade; alternatively, the absence of direct evidence of such behaviour may suggest convergence between these two species. Also, Dipoides was a less efficient wood-cutter than modern Castor, cutting with the rounded edges of its incisors while Castor uses the flattened ends, and if it used chopped wood to construct nests then they would have probably been correspondingly more simple (beavers also use chopped wood for food, eating the leaves and bark, so wood-chopping does not automatically indicate dam-building). There are other indications that fossil beavers may not have been as specialised aquatically as the modern species: the early castorine Steneofiber, for instance, did not possess the flattened tail of Castor (a flattened tail has been indicated for Castoroides but Castor and Castoroides probably developed such tails independently) (Hugueney & Escuillié 1996).

Reconstruction of Trogontherium cuvieri, from Fostowicz-Frelik (2008).

Perhaps the primary enigma among fossil beavers is the European Pleistocene Trogontherium. Although also referred to as a 'giant beaver', and often implied to be a European parallel to Castoroides, Trogontherium was a quite different animal. Fostowicz-Frelik (2008) argued that leg proportions and other features indicate that Trogontherium was a more terrestrial, cursorial animal than other beavers (in particular, its narrowed rather than flattened toe bones suggest that it lacked the webbed feet of modern beavers). The phylogenetic analysis of beavers by Rybczynski (2007) placed Trogontherium as closely related to Castoroides, but certain plesiomorphies in its tooth morphology lead Rybczynski to suggest that this position was probably an artifact of convergences due to large size, and that Trogontherium should perhaps be in a much more basal position.

REFERENCES

Fostowicz-Frelik, Ł. 2008. First record of Trogontherium cuvieri (Mammalia, Rodentia) from the middle Pleistocene of Poland and review of the species. Geodiversitas 30 (4): 765-778.

Hugueney, M., & F. Escuillié. 1996. Fossil evidence for the origin of behavioral strategies in early Miocene Castoridae, and their role in the evolution of the family. Paleobiology 22 (4): 507-513.

Korth, W. W. 2001. Comments on the systematics and classification of the beavers (Rodentia, Castoridae). Journal of Mammalian Evolution 8 (4): 279-296.

Korth, W. W. 2004. Beavers (Rodentia, Castoridae) from the Runningwater Formation (Early Miocene, early Hemingfordian) of western Nebraska. Annals of Carnegie Museum 73 (2): 1-11.

Rybczynski, N. 2007. Castorid phylogenetics: implications for the evolution of swimming and tree-exploitation in beavers. Journal of Mammalian Evolution 14: 1-35.

Rybczynski, N. 2008. Woodcutting behavior in beavers (Castoridae, Rodentia): estimating ecological performance in a modern and a fossil taxon. Paleobiology 34 (3): 389-402.

Groundhogs, Woodchucks and Other Big Squirrels

Thirteen-lined ground squirrel Ictidomys tridecemlineatus, photographed by Phil Myers.

The Holarctic ground squirrels of the Marmotini were the subject of one of my earliest posts at this site, before I really knew what I was doing*. So I'll have a go at improving it now.

*Not, of course, that I know what I'm doing now.

The Arctic ground squirrel Urocitellus parryii, photographed by Ianaré Sévi.

Marmotini is the clade of squirrels that includes ground squirrels (Spermophilus), antelope ground squirrels (Ammospermophilus), marmots (Marmota) and prairie dogs (Cynomys). Authors seem to differ on whether to also include the chipmunks (Tamias), but the question is somewhat semantic: agreement seems to be universal that the chipmunks represent the sister group to the remaining marmotins (Herron et al. 2004), so the only real question is how inclusive one wishes to make the term. The Chinese rock squirrels Sciurotamias may also belong to the Marmotini (Steppan et al. 2004). Except for the semi-arboreal chipmunks, marmotins are largely terrestrial in habits. They nest in underground burrows (including chipmunks), and some species form quite complex societies.

Père David's rock squirrel Sciurotamias davidianus, from here.

Ground squirrels previously assigned to the genus Spermophilus* have a wide range through Eurasia and North America. However, both morphological and molecular data indicate that Cynomys is derived from within 'Spermophilus', and molecular data indicate that Ammospermophilus and Marmota are as well (Herron et al. 2004). Helgen et al. (2009) divided the former Spermophilus between eight genera. Six of these genera are found in North America, one (Spermophilus proper) is found in Eurasia, and only one (Urocitellus) spans the divide between northeast Asia and North America. Whether the Marmotini as a whole are Eurasian or North American in origin is equivocal: of the three basalmost branches, Sciurotamias is definitely Eurasian, Tamias could be either (the Siberian chipmunk Tamias sibiricus is the sister to the remaining North American species) and the Spermophilus clade is probably North American in origin, with dispersals back to Eurasia in Marmota, Urocitellus and Spermophilus (Herron et al. 2004).

*Particularly in the European literature, it was not uncommon in the past to find the name Citellus being used in place of Spermophilus. Citellus Oken 1816 is indeed an older name than Spermophilus Cuvier 1825; however, the publication that the former derives from was not one that used the binomial system, and hence it has been declared invalid as a source of names (International Commission on Zoological Nomenclature 1956).

The woodchuck Marmota monax, from here.

Marmotins were the dominant squirrel group in North America during the Neogene; tree squirrels, though present, were exceedingly rare (Emry et al. 2005). The Pliocene Paenemarmota was the largest of all marmotins, reaching the size of a large beaver (Repenning 1962).

REFERENCES

Emry, R. J., W. W. Korth & M. A. Bell. 2005. A tree squirrel (Rodentia, Sciuridae, Sciurini) from the Late Miocene (Clarendonian) of Nevada. Journal of Vertebrate Paleontology 25 (1): 228-235.

Helgen, K. M., F. R. Cole, L. E. Helgen & D. E. Wilson. 2009. Generic revision in the Holarctic ground squirrel genus Spermophilus. Journal of Mammalogy 90 (2): 270-305.

Herron, M. D., T. A. Castoe & C. L. Parkinson. 2004. Sciurid phylogeny and the paraphyly of Holarctic ground squirrels (Spermophilus). Molecular Phylogenetics and Evolution 31: 1015-1030.

International Commission on Zoological Nomenclature. 1956. Opinion 417. Rejection for nomenclatorial purposes of volume 3 (Zoologie) of the work by Lorenz Oken entitled Okens Lehrbuch der Naturgeschichte published in 1815–1816. Opinions and Declarations Rendered by the International Commission on Zoological Nomenclature 14: 1–42.

Repenning, C. A. 1962. The giant ground squirrel Paenemarmota. Journal of Paleontology 36 (3): 540-556.

Steppan, S. J., B. L. Storz & R. S. Hoffmann. 2004. Nuclear DNA phylogeny of the squirrels (Mammalia: Rodentia) and the evolution of arboreality from c-myc and RAG1. Molecular Phylogenetics and Evolution 30: 703-719.

Little Yellow Bats

Specimen of an unidentified Rhogeessa, photographed by Kate Comis.
The Neotropical members of the genus Rhogeessa go by the incredibly imaginative vernacular name of 'little yellow bats'. On the face of it, this would seem to sum up the salient features of these animals pretty succinctly: they're small, they're yellow, and they're bats. Eleven species of Rhogeessa were recognised by Baird et al. (2008a), found from northern Mexico to southern Brazil. The species of Rhogeessa are rather difficult to distinguish from each other; in particular, the six species that referred by Baird et al. (2008a) to the 'Rhogeessa tumida complex' are all but indistinguishable. Nevertheless, recent authors have regarded them as good species, and it is because of the reasons why this is that the genus has attracted the most interest.

The only really reliable way to distinguish the species of the R. tumida complex is to take a look at their chromosomes. Despite their external similarity, the species have different chromosome numbers and arrangements from each other. This forms an interesting contrast to other bat genera, which may have more morphological variability but little chromosome variation. Comparison between Rhogeessa chromosomes has lead to the suggestion that the various species may have diverged as a result of a process called Robertsonian translocation.

Specimen of the Yucatan yellow bat Rhogeessa aeneus photographed by Alex Borisenko.

To explain Robertsonian translocation, I have to indulge in a bit of background terminology of chromosomes (skip this if you know all this stuff). Think of the classic picture of an X-shaped chromosome (this is actually a doubled chromosome that develops during cell division: two copies, called chromatids, have been produced of the chromosome that will be separated when the cell divides). The point where the two chromatids are joined is a region of the chromosome called the centromere: it provides an attachment point for the spindle fibres that will draw each individual chromatid apart. The centromere is not always positioned at the midpoint of the chromosome: those chromosomes in which it is are called metacentric, while other acrocentric chromosomes have the centromere close to one end so the conjoined chromatids look closer to V-shaped than X-shaped.

Translocation is a process where a piece of genetic material breaks off one chromosome and becomes attached to another. Robertsonian translocation is a particular type of translocation where two acrocentric chromosomes, by breaking at the centromeres, effectively become fused to form a single metacentric chromosome (as shown in the diagram below from here):

Robertsonian translocation has been observed in many species; it even happens occasionally in humans. Where it becomes interesting for evolutionary studies is that the resulting metacentric chromosome continues to function in the same manner as the original acrocentric chromosomes, with little or no negative effects (the short bit from each acrocentric chromosome that is lost rarely contains any functioning genes). The individual carrying the fused chromosome even remains fertile, because when meiosis occurs in any individual with both the fused and unfused chromosomes, the two unfused chromosomes will each line up with their matching arm on the fused chromosome. However, imagine a situation where one individual in a population experiences a Robertsonian translocation between two chromosomes (call them 1 and 2), but another individual has a translocation between one of those chromosomes and another chromosome (say, 2 and 3). The individual that carries chromosomes 1-2 and 3 will produce fertile offspring if mating with an unfused individual, as will that carrying 1 and 2-3. However, if the 1-2 individual mates with the 2-3, their offspring will carry both fused chromosomes. Because these chromosomes and the unfused 1 and 3 cannot easily match up in a way that allows them to be separated effectively during meiosis, the hybrid offspring will have significantly reduced fertility. This has been practically shown to be the case between Robertsonian races of mice (Capanna et al. 1977). If the two fused chromosomes each become more predominant in a population than the original unfused chromosomes (either by drift or hitchhiking), then gene flow will be slowed or stopped between individuals carrying one or the other. Hey presto, speciation!

Speciation as a result of Robertsonian translocation also provides a counter-example to those who, when objecting to the taxonomic recognition of 'cryptic' species, raise the Biological Species Concept to defend their viewpoint. Contrary to popular assumption, there is no essential correlation between speciation and morphological divergence. Even under the Biological Species Concept, two populations may be good species (i.e. non-interfertile) and yet morphologically indistinguishable.

Allen's yellow bat Rhogeessa alleni, photographed by Merlin Tuttle. Rhogeessa alleni is the most morphologically distinct species of Rhogeessa, and has been included in a separate genus or subgenus Baeodon. Baird et al. (2008a), on the basis of molecular phylogeny, recognised the subgenus Baeodon but also including R. alleni's sister species, R. gracilis.

Anyway, I have a vague memory that somewhere along the line I was talking about bats. The known karyotype numbers for Rhogeessa vary from 30 (in three species: R. alleni, R. gracilis and R. io) to 52 (in a specimen from Suriname that Baker et al. 1985 assigned to R. tumida but which almost certainly represents an undescribed species). The phylogeny for the genus that was recovered by Baird et al. (2008a, b) could be consistent with both Robertsonian fusions and fissions taking place during the genus' history. In an attempt to test whether the different karyotypes truly function as isolating mechanisms (and hence whether the chromosomal 'species' are actually species), Baird et al. (2008b) could only find genetic indicators of possible recent hybridisation between the two apparently least divergent species, R. tumida (34 chromosomes) and R. aeneus (32 chromosomes); all other species maintained reciprocal monophyly in each of the three gene types (mitochondrial, Y-chromosome and somatic chromosome) tested. Baker et al. (1985) referred to another 32-chromosome karyotype ('32N') that differed from R. aeneus (assuming, on the basis of geography, that R. aeneus corresponds to Baker et al.'s '32B') in terms of exactly which chromosomes had been fused and so would be reproductively incompatible with R. aeneus. However, this 32N form appears to be assigned by Roots & Baker (2007) to R. io, otherwise with 30 chromosomes. As 30-chromosome R. io and the 32N form differ only in a single pair fusion in the former (Baker et al. 1985), they would probably remain interfertile by the principles described earlier. In contrast, despite their apparent difference in chromosome number of only two, R. tumida and R. aeneus actually differ in five chromosome fusions (three on one side, two on the other), meaning their interfertility should be considerably lower.

And if you've gotten this far and you're still not sick of bats, Darren Naish covered Rhogeessa and its relatives as part of his mammoth series on vesper bats earlier this year.

REFERENCES

Baird, A. B., D. M. Hillis, J. C. Patton & J. W. Bickham. 2008a. Evolutionary history of the genus Rhogeessa (Chiroptera: Vespertilionidae) as revealed by mitochondrial gene sequences. Journal of Mammalogy 89 (3): 744-754.

Baird, A. B., D. M. Hillis, J. C. Patton & J. W. Bickham. 2008b. Speciation by monobrachial centric fusions: A test of the model using nuclear DNA sequences from the bat genus Rhogeessa. Molecular Phylogenetics and Evolution 50 (2): 256-267.

Baker, R. J., J. W. Bickham & M. L. Arnold. 1985. Chromosomal evolution in Rhogeessa (Chiroptera: Vespertilionidae): possible speciation by centric fusions. Evolution 39 (2): 233-243.

Capanna, E., M. V. Civitelli & M. Cristaldi. 1977. Chromosomal rearrangement, reproductive isolation and speciation in mammals. The case of Mus musculus. Bolletino di Zoologia 44 (3): 213-246.

Roots, E. H., & R. J. Baker. 2007. Rhogeessa parvula. Mammalian Species 804: 1-4.

Origins - A Day in the Broom Room

Welcome to something rather special: after nearly four years, this is Catalogue of Organisms' first ever guest post. Ted MacRae usually writes about tiger beetles and other insects at his own excellent site, Beetles in the Bush, but he has provided a post for this site after winning at 'Name the Bug'. For it, he has selected a topic with a history arguably far more complicated than it should have ever needed to be: human evolution. I hope you all enjoy, and anyone who isn't familiar with Ted's own site already should check it out.--Christopher Taylor.

I may be better known for my interest in entomology, having studied insects all of my adult life and much of my childhood. Entomology, however, was only one of many subjects that piqued my interest as a child, the other big ones being evolution and paleontology, and - especially - human evolution. Obviously, Insecta won out over Australopithecus as the focus of my career pursuits, but I've remained a bit of a closet paleoanthropologist ever since and try to stay abreast of the ever-increasing pace of significant fossil finds that the field enjoys. For the most part, staying abreast has, for me, involved reading both primary journal articles and popular books on the subject. I could rattle off the names and numbers of paleoanthropology’s most famous hominid fossils as easily as I could the genera of Buprestidae. Little did I realize that one day I would have the opportunity to behold, with my own eyes, some of the very fossils that I had read about for so many years.

In 1999, I had the opportunity to travel to South Africa to spend some time in the field with my friend and colleague, Chuck Bellamy, who at the time was serving as Senior Curator in the Coleoptera Department at the Transvaal Museum (now the Ditsong National Museum of Natural History) in Pretoria. During my visit, and learning of my interest in paleoanthropology, Chuck arranged for me a private tour of the 'Broom Room' with its curator, Dr. Heidi Fourie. The Broom Room houses some of the most important fossils of early hominids in the world, including the famous Sterkfontein (STS) 5 'Mrs Ples' and Swartkrans (SK) 48 crania. The bulk of the fossils were discovered by Robert Broom, Raymond Dart, and their successors at the famous Sterkfontein and Swartkrans hominid sites in the northeastern part of the country. Australopithecus africanus, Paranthropus robustus, and some of the earliest known members of the genus Homo (recently described as H. gautangensis) are represented among the fossils, which range from 1.5 to 2.8 million years in age.

When Robert Broom first arrived in South Africa in 1936 and saw Raymond Dart's recently discovered 'Taung child" - the first known Australopithecus fossil, he is said to have knelt at the edge of the table containing the fossil and exclaimed, "I behold my ancestor!" Such was the feeling I had when I first entered the Broom Room and saw the rich wooden cabinets and rows of fossils neatly arrayed on its red felt-lined shelves. I knew which fossil I wanted to see first - Mrs. Ples, the most complete Australopithecus africanus cranium known, discovered at Sterkfontein in 1947 by Robert Broom and John Robinson. Originally named Plesianthropus transvaalensis, it was subsequently regarded to be conspecific with the Taung Child and thought to represent an adult female (most researchers now regard it to represent a sub-adult male). As Dr. Fourie held the cranium for me to look at, I noticed the fossil was about 3.5 feet off the floor - about the presumed height for the species. I suddenly saw Mrs. Ples standing before me in life - a living, breathing being, not an animal, yet not quite human either. I may not have used Broom's precise words, but I whispered something along those lines to myself as the slender, hairy virtual creature stood before me. The Museum Gift Shop was selling plaster replicas of Mrs. Ples, one of which now sits on the desk in my office. I think about that experience at the Transvaal Museum almost everytime I look at it.

Among the other A. africanus fossils I noticed was a partial cranium, also discovered by Broom at Sterkfontein in 1947. While not as complete as Mrs. Ples (missing portions of its left side), it was subsequently associated with a mandible found in the same layer (STS 36) by matching wear patterns on the teeth - making it one of the most complete A. africanus skulls to have been found. Originally classified as a female, this 2.5 million year old cranium is smaller and less prognathous (forward projecting mouth) than other known A. africanus crania. However, its large post-canine teeth and indications of massive chewing muscles suggest it is a male.

Skulls are not the only cranial fossils represented in the collection. Endocranial casts as well have been found in the same deposits from which the crania were taken, and I actually got to hold STS 60 in my own hands! With a chimp-like volume of 428 cubic centimeters, it's a bit on the small side of normal for A. africanus (nearly 60 cc less than the brain capacity of Mrs. Ples, though equaling that of STS 71). Holding it in my hands, I mentally compared its size with that of my own brain and tried to imagine the cognitive differences implied by such difference (okay, no jokes here!).

Broom recognized that the australopithecine fossils he was finding in South Africa represented two distinct morphs - a "gracile" form now encompassed by A. africanus, and a more "robust" form that he described in 1938 as Paranthropus robustus. The fossils from Swartkrans conform to this latter type, with the most complete cranium being SK 48, discovered by Broom and Robinson in 1952 and dated to between 1.5 and 2.0 million years in age. It should be noted that the term "robust" refers not to the size of the body, but rather the characters of the skull that include a more prominent sagittal crest in males, greater sexual dimorphism in body size, and robust zygomatics and mandible with large, thickly enameled post-canine dentition. The Museum Shop had a plaster replica of SK48 as well, which also now sits on the desk in my office.

It should be noted that not everyone in the field accepts Paranthropus as a valid genus distinct from Australopithecus. There is little doubt that the included species represent a derived and specialized form, but whether P. robustus from South Africa and the two east African species P. aethiopicus and P. boisei form a monophyletic clade is still an open question. There is some evidence to suggest that P. robustus is descended from A. africanus because of similarity of some derived features with the latter, which if correct would render Paranthropus paraphyletic and not a valid taxon (Conry 1997). This view, necessarily, suggests also that the hyper-masticatory adaptations of robust australopithecines evolved independently in eastern and southern Africa. While this certainly could have happened, (Strait et al., 1997) argue that a more parsimonious interpretation of multiple morphological traits suggests Paranthropus is indeed monophyletic and that it should be retained as a valid genus. Either way, Paranthropus certainly represents a distinct “grade” of australopithecines, and until more convincing data to the contrary are produced I prefer to recognize it as a valid genus.

The massive mandibles that distinguish P. robustus from the gracile australopithecines are richly represented among the Swartkrans material (one can imagine that these robust bony structures are prone to preservation), including SK23 (discovered at the same time and dated to the same age as the SK 48 cranium) and SK 12 and the SK 52 partial skull (see photos). Presumably these morphological adaptations of the mandible/maxilla and associated musculature are related to a more specialized diet of tough plant material that required considerable chewing to process, compared to the more omnivorous A. africanus that pre-date them.



Also amongst the P. robustus fossils is SK 1585, the only endocranial cast known for the species with a volume of 530 cc. Combined with SK 48 and other partial and cranial remains that have been recovered for the species, it appears that the brain of P. robustus averaged slightly larger than that of the earlier A. africanus. Whether this translates to increased cognitive function is open for debate, although there are some structural differences in SK 1585 compared to A. africanus endocranial casts that suggest this could be the case.

Perhaps the most contentious fossil in the Broom Room is SK 847, a highly fragmentary partial cranium discovered at Swartkrans in 1969 by Ronald Clarke and dated to between 1.5 and 1.8 million years. Originally thought to represent P. robustus due to its association with other fossils of that species, it was eventually associated with a maxilla (SK 80) originally described as Telanthropus capensis and later included in Homo erectus. The affinities of the composite specimen were contentious, and at the time of my visit its classification remained unresolved (Johanson and Edgar 1996). More recent studies have suggested that this and other South African specimens represent a species not previously sampled in east Africa, and the specimen was eventually included as a paratype in the description of a new species, Homo gautangensis, the newest member and earliest representative of its - our - genus (Curnoe 2010).



REFERENCES:

Conroy, G. C. 1997. Reconstructing human origins: a modern synthesis. New York, Norton.

Curnoe, D. 2010. A review of early Homo in southern Africa focusing on cranial, mandibular and dental remains, with the description of a new species (Homo gautengensis sp. nov.). Journal of Comparative Human Biology 61(3):151–77.

Johanson, D. and B. Edgar. 1996. From Lucy to Language. New York: Simon and Schuster Editions.

Strait, D. S., F. E. Grine and M. A. Moniz. 1997. A reappraisal of early hominid phylogeny. Journal of Human Evolution 32:17-82.

Copyright © Ted C. MacRae 2011

Old Men of the Woods

The mandible of Khoratpithecus piriyai from Chaimanee et al. (2004). Scale bar equals 1 cm.

The subject of today's Taxon of the Week post is the Ponginae. Rather than comment directly on the prolonged, bitter and largely pointless arguments on ape beta taxonomy, I'll simply note that for this post I'm restricting Ponginae to the clade including the modern orangutans and fossil apes closer to orangs than other modern apes.

I say 'orangutans' because there are two distinct modern varieties that are listed as separate species by Groves (2005), the Sumatran Pongo abelii and the Bornean P. pygmaeus. During the Pleistocene, orangutans were also found in continental south-east Asia and southern China (Bacon & Long 2002). Previous to the Pleistocene, however, a gap of six or seven million years separates Pongo from their generally accepted relatives in the Miocene genera Sivapithecus and Khoratpithecus (Finarelli & Clyde 2004; Chaimanee et al. 2004). Other possible pongine genera whose position is less firm include Lufengpithecus, Ankarapithecus and Gigantopithecus. In the phylogenetic analysis by Finarelli & Clyde (2004), the Miocene Lufengpithecus and Ankarapithecus were initially placed on the orangutan stem on the basis of morphology, but switched over to the base of the stem of the African ape-human clade under an analytical method that attempted to reduce stratigraphic incongruence.


Sumatran orangutan Pongo abelii drinking while neatly concealing its baby. Photo from here.

Sivapithecus is the best-known of the fossil pongines, with a number of species assigned to it from India dating from about 12.8 to 7.4 million years ago. Though Sivapithecus was similar to modern orangutans in skull morphology, it differed in its dentition and postcranial morphology. Sivapithecus lacked the adaptations for brachiation of orangutans; when moving in trees, it would have run along the top of branches in the manner of a monkey rather than swinging underneath the branches like an orangutan. Arm-swinging was probably a later innovation on the orangutan line (and would have therefore developed independently from other arm-swingers such as gibbons). Khoratpithecus, currently known only from teeth and jaw bones, had dentition more similar to Pongo and is probably more closely related to modern orangs than Sivapithecus; unfortunately, we do not yet know whether it retained the plesiomorphic postcranial morphology of Sivapithecus.


Skull of Sivapithecus indicus. Photo by FunkMonk.

Ankarapithecus is a smaller ape that, as its name suggests, was found in Turkey. If it is a pongine, it would increase the known range for that clade. An even greater range has been suggested by the assignation of the Spanish Hispanopithecus to the pongine line (Cameron 1997), though it was instead assigned to a non-pongine clade of European apes by Begun (2009). As indicated above, Ankarapithecus and the Chinese Lufengpithecus show features that could be interpreted as showing relationships to either pongines or hominines. Indeed, some fossils now regarded as Lufengpithecus were initially assigned to Homo (Harrison 2002), though possibly that may say more about the expectations of Chinese palaeoanthropology than about Lufengpithecus itself.


Reconstruction of Gigantopithecus herd. Image from here.

The only potential pongine other than Pongo itself known from later than the Miocene is Gigantopithecus. Three species have been assigned to this genus: the Pleistocene Gigantopithecus blacki from China and two Miocene species from Indo-Pakistan (the Indo-Pakistan species were smaller than G. blacki, but still larger than most other apes). A significant time gap separates the Chinese and Indo-Pakistani species, and it has been suggested that they may represent two separate lineages that independently attained large size (in which case the Indo-Pakistani species are placed in the genus Indopithecus). Gigantopithecus blacki had the largest teeth of any known ape, but without any post-cranial remains it is difficult to know its overall size. Johnson (1979), assuming the proportions of Gigantopithecus blacki to be similar to those of a modern gorilla, suggested that it may have weighed around 270 kg (vs about 160 kg for a gorilla) with long bones about 20% longer than those of a gorilla. However, it has been suggested that the large teeth of Gigantopithecus were specialised for feeding on bamboo (Kupczik & Dean 2008), in which case it may have had larger teeth relative to body size than other apes (it is notable in this light that the incisors of Gigantopithecus were actually smaller than a modern great ape's: Woo 1962). At any case, estimates that Gigantopithecus was more than twice the size of a modern gorilla seem unlikely.

REFERENCES

Bacon, A.-M., & V. T. Long. 2001. The first discovery of a complete skeleton of a fossil orang-utan in a cave of the Hoa Binh Province, Vietnam. Journal of Human Evolution 41 (3): 227-241.

Begun, D. R. 2009. Dryopithecins, Darwin, de Bonis, and the European origin of the African apes and human clade. Geodiversitas 31 (4): 789-816.

Cameron, D. W. 1997. A revised systematic scheme for the Eurasian Miocene fossil Hominidae. Journal of Human Evolution 33 (4): 449-477.

Chaimanee, Y., V. Suteethorn, P. Jintasakul, C. Vidthayanon, B. Murandat & J.-J. Jaeger. 2004. A new orang-utan relative from the Late Miocene of Thailand. Nature 427: 439-441.

Finarelli, J. A., & W. C. Clyde. 2004. Reassessing hominoid phylogeny: evaluating congruence in the morphological and temporal data. Paleobiology 30 (4): 614-651.

Groves, C. P. 2005. Order Primates. In: Wilson, D. E., & D. M. Reeder (eds) Mammal Species of the World: a taxonomic and geographic reference, 3rd ed., vol. 3 pp. 111-184. John Hopkins University Press.

Harrison, T., Ji X. & D. Su. 2002. On the systematic status of the late Neogene hominoids from Yunnan Province, China. Journal of Human Evolution 43 (2): 207-227.

Johnson, A. E., Jr. 1979. Skeletal estimates of Gigantopithecus based on a gorilla analogy. Journal of Human Evolution 8 (6): 585-587.

Kupczik, K., & M. C. Dean. 2008. Comparative observations on the tooth root morphology of Gigantopithecus blacki. Journal of Human Evolution 54 (2): 196-204.

Woo, J.-K. 1962. Gigantopithecus and its phylogenetic significance. Scientia Sinica 11 (3): 391-396.