Field of Science

The Strangest of Spiders

In the comments to an earlier post, I promised to write a post sometime on micro-spiders. As alluded to in that post, some of the smallest spiders are mind-bogglingly tiny - the smallest known male spider, Patu digua, reaches all of 0.37 mm in length as an adult, but at least one other species known as yet only from females could potentially have a male even smaller. If one of these spiders crawled into your ear while you were sleeping, it could probably slip into your Eustachian tubes and tap on the back of your eyeballs. But even more remarkable than their small size is the bizarre morphologies on show among the micro-spiders. And no group of micro-spiders is more bizarre than the Archaeidae.

Archaeids are a bit bigger than Patu, but still pretty small - the largest examples reach about six millimetres. The name "Archaeidae", of course, means "old", and archaeids received their name because they were first described in 1854 from fossils in Baltic amber from northern Europe. In Europe, the archaeids are long gone (they may have disappeared along with the amber forests), but nearly thirty years after their initial description living examples were found in Madagascar. They are also known from Australia, while a specimen from Cretaceous Burmese amber has been placed in a living genus from South Africa and Madagascar (Penney, 2003). A species has also been described from the Jurassic of Kazakhstan, but it is uncertain whether this species is an actual archaeid or belongs to another micro-spider family such as Pararchaeidae.

Many micro-spiders show relatively long chelicerae (the fangs and their base) relative to body size, but in Archaeidae this is taken to the extreme, as can well be seen in the photo by Jeremy Miller at the top of this post. Because the trochanter (base) of the chelicerae is a rigid structure, lengthening them in spiders requires that the carapace as a whole be raised, otherwise the fangs would not be able to get anywhere near the mouth. Archaeids have developed a long "neck" supporting the eyes and chelicerae. The distinct shape of the cephalothorax together with the long chelicerae gives them an unmistakeable profile, and one common name used for the group is "pelican spiders". Despite their small size, archaeids are active hunters and voracious exclusive predators of other spiders (another common name is "assassin spiders"). It has been suggested that the lengthened chelicerae are directly related to their araneophagous diet, allowing them to strike their prey without getting too close, but as I already noted archaeids are not the only small spiders with lengthened chelicerae (though they are still the most dramatic), and I'd be interested to know if there is a correlation between small size and long chelicerae.

I'd also like to share this diagram from Wood et al. (2007) showing a molecular-derived phylogeny of the endemic Madagascan genus Eriauchenius. As can be seen, there is a fair amount of variation in the thickness of the "neck" (the darkness of the bars reflects the mean carapace height/length ratio for whichever group they subtend), and it had been suggested that those species with a particularly slender neck formed a derived clade. Wood et al. (2007) found that this does not appear to be the case, with at least two extreme narrow-neck groups - E. workmani in one and E. gracilicollis and E. lavatenda in the other - at quite divergent points in the tree. I also looks to me like at least one group - E. tsingyensis and its allies - may have gone the other way. To paraphrase a Rocky Horror Picture Show audience member - that spider has no neck.

Penney, D. 2003. Afrarchaea grimaldii, a new species of Archaeidae (Araneae) in Cretaceous Burmese amber. Journal of Arachnology 31 (1): 122-130.

Wood, H. M., C. E. Griswold & G. S. Spicer. 2007. Phylogenetic relationships within an endemic group of Malagasy ‘assassin spiders’ (Araneae, Archaeidae): ancestral character reconstruction, convergent evolution and biogeography. Molecular Phylogenetics and Evolution 45 (2): 612-619.

Linnaeus' Legacy: Legs Eleven

The next edition of Linnaeus' Legacy, the taxonomy and systematics blog carnival, will be coming up shortly at The Other 95%. Last month's edition at A DC Birding Blog matched its host by holding lots of birds, so my challenge to you all would be to try to make this month follow suit and bring out the marine invertebrates. Submissions can be directed to Eric Heupel via eric.heupel at, or use the BlogCarnival submission form.

More Juvenilia

Chris M at The Echinoblog has put up a list of odd things that sit in his laboratory, and notes that people look askance at the pile of toilet paper (a vital tool in drying specimens). Well, I can top that:

K-Y jelly is actually fantastic stuff for preparing temporary slide mounts. It's transparent, it holds things in place reasonably well (though it does heat up and start flowing a bit if you leave it under the light for too long) and it's water-soluble, so you can just take a specimen preserved in alcohol out of its vial and put it straight onto the slide then return it straight to the vial when finished without needing to wash or prepare it in any way. Putting the specimen in alcohol on a concave slide is still preferable, because alcohol is more optically clear than K-Y, but alcohol won't hold the specimen at an angle in any way if you need to look at the specimen in a particular position. Seeing as how this is invertebrate systematics we're talking about, the thing I most commonly need to look at on a slide are reproductive organs. Which leads to an actual exchange that took place:

Colleague: "Why do you have K-Y jelly in your office?"

Me: "I use it for mounting... I mean, I put genitalia in it... Crap."

Offhand, I have also now discovered that if you do an image search for K-Y jelly, it pays to have the SafeSearch option turned on.

Ye Gods I'm Immature

Psathyropus mysoreanus.

Inevitable Spandrels on a Biology Blog

Nearly thirty years ago, a paper was published that almost every student of evolutionary science will end up reading at some point in their career. Despite being only eighteen pages long and containing no original research, many people see it as marking something of a revolution in biology. Like many a revolutionary document, it says little of substance that is not completely obvious, but entire books have been written that derive their subject matter directly from it. I speak of Gould and Lewontin's 1979 paper "The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme".

In architectural terminology, a "spandrel" is the space formed where two arches meet each other, or where an arch meets a wall. The title of Gould and Lewontin's paper refers to such structures spaced around the dome in the Basilica di San Marco in Venice which sits on four arches (technically, the Venice structures are pendentives rather than spandrels, but this terminological error is irrelevant to Gould and Lewontin's argument). The iconography decorating the dome radiates towards those four pendentives. An observer completely ignorant of the requirements of architectural stability might feel that their focal significance in the iconography indicates that the pendentives were specifically designed to support the iconography, but this is simply not the case. Rather, the pendentives are simply a side-effect of building a stable dome, and the iconography has been designed to take advantage of their presence rather than vice versa. Gould and Lewontin used the Basilica di San Marco as a metaphor to criticise the over-reliance in evolutionary biology on adaptation as an explanation for characters of organisms. Wasn't it possible, claimed Gould and Lewontin, that at least some features of organisms were not selective traits in their own right, but merely biological "spandrels", architectural side-effects of the development of other traits?

Let me give a couple of examples. One that Gould and Lewontin refer to themselves involves the famous midget arms of Tyrannosaurus rex. Much speculation has taken place on what the function of these relatively tiny appendages could have been - props in raising the animal up from a lying position, grappling hooks in mating, etc. but as pointed out by G & L these "explanations" all rather missed the point. Tyrannosaurus did not evolve its tiny arms de novo, but inherited them from a proportionately longer-armed ancestor. While Tyrannosaurus arms could well have fulfilled any or all of the functions ascribed to them, none of the suggestions actually explained why Tyrannosaurus arms became so small in the first place. This may well have been a consequence of the rest of the animal increasing in size faster than the arms did, in which case the question is not "why did the arms become small", but "why did the rest of the dinosaur get so big?". Gould later brought up another example - giant pandas have an enlarged protruding wrist-bone on their forelimbs that functions as a crude thumb in manipulating the bamboo they eat. They also possess a similar enlarged ankle-bone on their hindlimbs that serves no obvious purpose, and indeed may be something of a nuisance. Gould suggested that the enlarged bone on the hindlimbs was a side-effect of the development of that on the forelimbs, due to the same processes underlying patterning in the development of both sets of limbs.

Unarguable as this all might seem, a certain frustration tends to set in towards the end of a reading of "Spandrels", as one eventually feels the need to rejoin with a "So what's your point, Vanessa?" Gould and Lewontin claimed, probably accurately, that over-atomising organism traits might lead to the failure to recognise linkage between features - looking at the spandrels only without considering the entire dome - and called for a whole-organism approach. But in practice, a certain degree of atomisation is necessary if our understanding is to go anywhere at all. It would seem ridiculous to claim that we cannot explain the evolution of the eye without also understanding the evolution of the big toe. Also, at the risk of stating a truism, we cannot possibly know everything about an organism, otherwise we would not be conducting the research in the first place.

Also, there is no way of distinguishing a priori a trait that has arisen as a result of being selectively advantageous from one that has "piggy-backed" on something else. It could well be argued that selective function for a given trait is a more fruitful base assumption that non-function because that is the basis that leads to further research, while assuming non-function is a bit of a show-stopper. As a result, while the concept of biological "spandrels" has caused a great deal of debate over the years on a theoretical basis, it is arguable whether it has had much direct practical effect. A similar fate befell another related Gould neologism, when Gould and Vrba (1982) coined the term "exaptation" for an organismal trait that had been co-opted for another use, just as the pendentives had been co-opted for use in decoration, in contrast to "adaptations" that had evolved specifically for the function they currently fulfilled. The distinction never caught on because, while it might seem theoretically significant, in practice it was fairly pointless. After all, evolution does not create things entirely from whole cloth but works through the alteration of pre-existing features, so all "adaptations" are at some level "exaptations".

Nevertheless, if the "Spandrels" paper had been so pointless as I've just implied, it wouldn't hold the iconic position in modern biology that it does. And agin like many another revolutionary document, this wasn't so much as a direct result of its own propositions, but of the effect it had on how people saw everything else. Gould and Lewontin were protesting against the assumption they saw in many biology studies that an adaptive function for any given trait must be in there somewhere, and if one could not be found it merely meant that we hadn't looked hard enough. As a result, the corollary assumption arose all too often that if an adaptive explanation could be derived for something, then it must be true. Gould and Lewontin argued that mere plausability was not sufficient support in itself for a proposition, because the possibility always existed that an equally plausible explanation had been overlooked - a logical error Voltaire had parodied in 1759 when he had his Dr. Pangloss in Candide, ou l'Optimisme argue that the nose had developed for the purpose of holding up spectacles. It was the recognition that adaptationist hypotheses were hypotheses that required further investigation as much as any other scientific proposition that was Gould and Lewontin's ultimate legacy to biology.

Gould, S. J., & R. C. Lewontin. 1979. The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme. Proceedings of the Royal Society of London Series B – Biological Sciences 205: 581-598.

The Calcareous Heart

Acanthocardia tuberculata. Photo by Hans Hillewaert.

Is Taxonomy a Science?

A Brief Moment of Politics

The speaker? Condoleeza Rice.

But Is It Art?

Art is Science, Science is Art

Guardando nel suo Figlio con l'Amore
che l'uno e l'altro etternalmente spira
lo primo e ineffabile Valore
quanto per mente e per loco si gira
con tant' ordine fé, ch'esser non puote
sanza gustar di lui chi ciò rimira.

Gazing on His Son with the Love
the One and the Other eternally breathe forth,
the inexpressible and primal Power
made with such order all things that revolve
that he who studies it, in mind and in space,
cannot but taste of Him.

Amaurobioidea: Rummaging through a Wastebasket

A representative of the strikingly-coloured Nicodamidae from Australia. Photo by Nick Monaghan. While such spiders were previously identified as Nicodamus bicolor, there are no less than 23 species in seven genera that have previously been included under that name.

One term that you may come across in discussions of phylogeny is the concept of a "wastebasket" taxon. As the name suggests, a wastebasket taxon is one into which authors tend to throw everything that they can't really deal with. Often, a wastebasket will include the members of a group that are relatively unspecialised, often primitive, and united less by their shared characters than their lack of distinct features to connect them to one or another of the specialised subgroups that the author may recognise within the parent group. Phalangodidae among short-legged harvestmen, Sylviidae among passerine birds and Perciformes among spiny-finned fishes are all examples of taxa that have become wastebaskets in the past. Some wastebasket taxa are explicitly established as such, like the 'Deuteromycota' that included asexual fungi before techniques were developed that made it significantly easier to relate asexual and sexual fungal taxa. More often, though, a taxon originally based on a certain combination of features will develop into a wastebasket over time as phylogenetic studies show that the original basis characters for that taxon represent plesiomorphies (ancestral characters). This week's highlight taxon, the spider superfamily Amaurobioidea, perhaps belongs to the latter group.

Tegenaria gigantea (Agelenidae). Photo from Wikipedia. Agelenids build funnel-shaped webs and are apparently often called some variant of "funnel spiders" in North America, but such names are likely to cause confusion here in Australia with a certain notorious mygalomorphs. Some species of Tegenaria such as the hobo spider are also known for being toxic, but nowhere near as toxic as the Australian funnel-web.

In an earlier post, I included a quick overview of basal spider phylogeny, going as far down as the clade Araneoclada that unites those spiders that have only a single pair of book lungs (ancestrally, at least - many families of Araneoclada have lost the book lungs entirely, or evolved tracheae in their place). Members of the Araneoclada are further divided between the Haplogynae and the Entelegynae, originally based on the presence (Entelegynae) or absence (Haplogynae) in females of paired copulatory ducts opening on a sclerotised plate called the epigyne. While the absence of such ducts in the Haplogynae is obviously a primitive character and no longer regarded as uniting them, the group has funnily enough been supported as monophyletic based on a number of other characters (except for a small number of 'haplogyne' taxa that are phylogenetically entelegynes) (Coddington & Levi, 1991). However, the Amaurobioidea belong to the Entelegynae, which is by far the larger of the two clades. Within the Entelegynae, the primary division was long based on whether or not a species possessed a cribellum, a plate-like structure among the spinnerets that bears hundreds of tiny silk-producing spigots. As these spigots exude silk simultaneously, the spider uses a specialised arrangement of bristles on the fourth pair of legs to weave them together to form a woolly thread (see here for a more detailed description). Because this woolly thread is composed of multiple tangled strands, it can effectively entangle prey such as small insects that get caught among the strands. Unfortunately, as knowledge of entelegyne spiders improved it became clear that possession of a cribellum did not define a phylogenetically coherent group. A number of cases were identified of pairs of taxa clearly related by other characters in which one taxon possessed a cribellum and the other did not. The eventual conclusion was that the cribellum was an ancestral character for the Entelegynae (as also supported by its presence in one haplogyne family, the Filistatidae) that had been lost on numerous occassions.

Ctenus floweri (Ctenidae), from Singapore. Photo by David Court. Ctenids are active hunters.

In general, the Amaurobioidea included cribellate spiders with unbranched abdominal median tracheae, as opposed to Dictynoidea with branched abdominal median tracheae (Coddington & Levi, 1991). Families that have been assigned to Amaurobioidea include (among others) Amaurobiidae, Agelenidae, Ctenidae, Amphinectidae and Nicodamidae, but relatively little unites these families. Most of them are generally ground-dwellers (which may explain the common name of one of the best-known members, the hobo spider Tegenaria agrestis). Many members build small sheet-webs, but others are active hunters. Both the characters referred to above have since been shown to represent plesiomorphies of larger clades, with the alternative conditions arising multiple times. The phylogenetic analysis of entelegyne spiders by Griswold et al. (1999) found the 'Amaurobioidea' to fall within a clade that was sister to the clade including the orb-weavers, but the same clade included the Dictynoidea and Lycosoidea (wolf spiders and such) nested within 'amaurobioids'. Indeed, not even the type family of Amaurobiidae was monophyletic, with some members closer to the lycosoids while others were closer to the agelenoids. The Amaurobioidea, it seems, was a bust.

Coming up - science and art, whether taxonomy is science, why family names are so awful, micro-spiders, and Parapseudoleptomesochrella almoravidensis.

Coddington, J. A., & H. W. Levi. 1991. Systematics and evolution of spiders (Araneae). Annual Review of Ecology and Systematics 22: 565-592.

Get Kunstformen!

E Pluribus Unum

For many people, the name "Ernst Haeckel" is most associated with slightly dodgy illustrations of vertebrate embryos that have doomed his memmory to be quote-mined by people with an agenda to push for all eternity. For others, though, the epitome of Haeckel's work lies in the many spectacular illustrations of invertebrates and protozoa he produced in such works as his reports on the biological material collected by the HMS Challenger expedition, and his 1899-1904 Kunstformen der Natur ("Artforms in Nature"). With their awe-inspiring detail and spectacular presentation, the plates he produced are more than just technical illustrations, they are true works of art. Perhaps among the greatest of his productions were the plates of siphonophores, an example of which is shown above. Baroque tentacled horrors, they loom out of the page threatening to engulf Dunwich. I wouldn't be able to tell you whether Lovecraft had ever seen one of Haeckel's illustrations to inspire him in his descriptions of the twisted hybrid offspring of Yog-Sothoth, but the resemblance is uncanny.

Siphonophores are planktonic cnidarians (the group that includes corals and jellyfish), distantly related to hydras (a good online reference on siphonophores has been put together by Casey Dunn). The most familiar member of the group is Physalia, the Portuguese man of war (so-called because of a supposed resemblance to that form of ship), but on the whole Physalia is not very typical of the order. All siphonophores are colonial, in their way - incomplete budding leads to the production of a colony of generally large numbers of metabolically interconnected zooids that are developmentally homologous to the more independent polyps of other cnidarians. However, the individual zooids of siphonophores are each highly specialised for separate divided functions such as feeding, reproduction or motility, meaning that siphonophore zooids are incapable of living independently of the colony. Perhaps more than any other group of organisms, the siphonophores challenge the question of what defines an individual or a colony, which has led to their description as "superorganisms".

Siphonophores have been divided into three main groups, the Cystonectae, Physonectae and Calycophorae, but the phylogenetic analysis of Dunn et al. (2005) found calycophores to be nested within physonects, the two together forming a clade they named the Codonophora. Cystonects (which include Physalia) form the sister-group to the codonophores, and share a colony morphology characterised by a division between a terminal pneumatophore (float) and the siphosome, the region of the colony containing feeding and reproductive zooids coming off a central stalk (in Physalia the central stalk is relatively short, but other siphonophores will have exceedingly long colonies). In the "physonects", the pneumatophore and siphosome are separated by the nectosome, a region of generally bell-shaped zooids called nectophores specialised for motility. In the calycophores, the pneumatophore has been lost and the colony is composed of the nectosome and siphosome. The illustration at the top of the post represents the physonect Physophora hydrostatica - the pneumatophore is the bulb-shaped structure at the top, with the zooids of the nectosome between the pneumatophore and the tentacle-like structures representing the top of the siphosome. These latter structures are not actually tentacles (the tentacles are the filaments radiating from the siphosome) but palpons, zooids whose function remains unknown but has been suggested to be related to excretion or defense. Underneath the palpons are the gonophores, the reproductive zooids, with separate male and female forms (males and females may both be present in a single colony, or there may be colonies of separate sexes). The large funnels like the horn of an old gramophone are gastrozooids, the feeding individuals. The clubbed side-branches on the trailing tentacles are tentilla, and contain concentrations of nematocysts for capturing prey. Most codonophores (but not cystonects) also have shield-like gelatinous bracts protecting the siphosome. Cystonects also have structures called gonodendra, which are concentrations of gonophores, palpons and also specialised nectophores that can propel a detached gonodendron through the water. Many codonophores are bioluminescent - the bracts may contain luminescent cells, and at least one member of the genus Erenna has flashing red tentilla that probably function as lures. The Physonecta illustrated above has only one iteration of the siphosome, but in other forms (such as the one illustrated below in another Haeckel plate) the clusters of palpons, gastrozooids and gonophores may form iterative elements that repeat continuously down the growing stem.

Despite what can only be described as their inherent coolness, siphonophores as a group are poorly known. Like other planktonic cnidarians, their gelatinous structure makes them quite frail and difficult to collect. The entire colony may be only loosely connected by the slender stem, such as in the example just above. Some siphonophores reach spectacularly large sizes - species of Apolemia may be more than 30 m in length, yet only a few centimetres in diameter. Attempts to net such specimens using conventional means would be lucky to retrieve anything more than disassociated mush.

Dunn, C. W., P. R. Pugh & S. H. D. Haddock. 2005. Molecular phylogenetics of the Siphonophora (Cnidaria), with implications for the evolution of functional specialization. Systematic Biology 54: 916-935.

Haeckel, E. 1899-1904. Kunstformen der Natur. Bibliographisches Institut: Leipzig & Wien.

It's Baaack!

After a brief hiatus, the palaeontology blog carnival The Boneyard has returned and can be found at Laelaps. But take care - Amphicyon is guarding it...

Sacred Monkeys

Hanuman langurs - photo by Kamal Kumar Dua. Though identified on the source site as Semnopithecus entellus, this taxon has been divided between up to seven species in recent publications.

Todays' Taxon of the Week is the primate genus Semnopithecus. Once again, that's a sentence that's a bit easier to glibly write than it is to define. Semnopithecus includes the langurs, and together with the surelis (Presbytis) and leaf monkeys (Trachypithecus) forms a generally-accepted clade within the Colobinae, a group that also includes the colobus and odd-nosed monkeys and is characterised by a number of adaptations to a higher proportion of leaves in their diet than most other primate groups - most notably, a division of the enlarged stomach into an upper neutral region and a lower acid region, with leaves being broken down by fermenting bacteria in the upper region. Within the langur clade, however, there has been disagreement on the best way to treat the three subgroups taxonomically. Some authors have included all three groups in Presbytis, others have restricted Presbytis to the surelis and combined the langurs and leaf monkeys as Semnopithecus, while others have recognised three separate genera. Because this is purely a question of ranking and there doesn't seem to be any disagreement that langurs and leaf monkeys are more closely related to each other than either are to surelis, there is no "correct" answer here. For the purposes of this post, I'm going to treat langurs and leaf monkeys as two subgenera of Semnopithecus, for no reason whatsoever other than it allows me to cover both groups, though it is worth noting that the phylogenetic analysis of Osterholz et al. (2008) did not confirm the monophyly of Trachypithecus relative to Semnopithecus sensu stricto.

No consensus seems to exist on the number of species within Semnopithecus. The langurs may represent as little as one or as many as seven species, depending on how the various populations around the Indian subcontinent are divided up. The leaf monkeys are even worse - Trachypithecus is the largest generic grouping in the Colobinae, and includes more than ten species scattered through south-east Asia. Many leaf monkey populations are poorly studied and species boundaries within the group are often unclear. Osterholz et al. divided Trachypithecus into fifteen species in five species groups as apparently recognised by Groves (2001) (which I haven't read), one of which (the Semnopithecus vetulus group with two species found in Sri Lanka and southernmost India) they found to cluster polyphyletically within Semnopithecus sensu stricto and transferred into the latter genus as a result. The Trachypithecus pileatus group, found on the boundary between the Indian subcontinent and south-east Asia, clustered with Semnopithecus in analysis of mitochondrial DNA but with Trachypithecus in analysis of Y chromosome data, leading Osterholz et al. to suggest the possibility of ancient hybridisation in the origin of the group.

The golden langur (Trachypithecus geei). Photo from

Most leaf monkeys live in small groups of about six to eighteen individuals (Brandon-Jones, 1984). Compared to some other primates, colobines apparently show relatively little social interaction among members of a troop (though still being fairly social compared to many other mammals, of course), which Brandon-Jones (1984) suggested may be an indirect consequence of their diet. Almost all colobines include a certain proportion of young leaves in their diet, but few can eat a significant amount of mature leaves. As this is fairly low-nutrition fare, colobines must spend a higher proportion of their time feeding than other primates, while the scattered distribution of young shoots requires individuals to spread themselves fairly thinly through a foraging site. Most colobines do eat fruit and other plant parts in addition to leaves, and langurs have a fairly varied diet that also includes such things as insects, roots, gum and sap. Indeed, langurs are noted for being able to readily stomach toxin-bearing foods such as Strychnos fruit that other herbivorous mammals would find inedible or even fatal. Langurs may be found in larger groups than leaf monkeys, with up to seventy individuals recorded in a troop (the largest size referred to by Brandon-Jones is a group of 120 individuals, though this may have been a temporary cluster of troops seeking water rather than a single troop). This may reflect their more varied diet, and/or it may reflect the fact that langurs are looked on favourably in most parts of India due to their supposed connection with the monkey god Hanuman (indeed, the name Semnopithecus means "sacred monkey") and are tolerated by humans or even actively encouraged and fed. Langur social structure varies significantly between different areas, possibly also as a result of food availability and population density. Like lions in Africa, langur troops are based on related females, with male offspring being evicted as they reach maturity, often forming nomadic all-male clusters. In some areas, breeding troops may include a number of mature males co-existing relatively peacefully, but in many areas most troops generally include only a single mature male. Also like lions, eviction of the incumbent male by another male in single-male areas is also often followed by the entering male killing any young already present in the troop in order to favour the raising of his own young. Interestingly, females who are pregnant at the time of takeover will engage in "pseudo-oestrus" behaviour - to completely anthropomorphise things, they fake sexual interest in order to induce the invading male to accept their offspring as his own. Production of young in all colobines often involves their being "shared around" between members of a troop, and females will often "borrow" and nurse the young of other females.

Brandon-Jones, D. 1984. Colobus and leaf monkeys. In All the World’s Animals: Primates (D. Macdonald, ed.) pp. 102-113. Torstar Books: New York.

Groves, C. P. 2001. Primate Taxonomy. Smithsonian Institution Press: Washington.

Osterholz, M., L. Walter & C. Roos. 2008. Phylogenetic position of the langur genera Semnopithecus and Trachypithecus among Asian colobines, and genus affiliations of their species groups. BMC Evolutionary Biology 8: 58.

Viruses upon viruses

Mimivirus. From Mimiviridae.

So, naturalists observe, a flea
Has smaller fleas that on him prey;
And these have smaller still to bite ’em;
And so proceed ad infinitum.

--Jonathan Swift, 1733, On Poetry, a Rhapsody

Linnaeus' Legacy #10: The Warbler Has Landed

In Which I Reveal Just How Much of a Freak I Am

Life Before it had Facial Features

Fossil cyanobacteria of the form taxon Myxococcoides minor from the Bitter Springs Formation of Australia. Photo from UCMP.

On a Monday morning when I am feeling every little nuance of the fact that it's a Monday morning, it seems appropriate to discuss a section of organismal diversity whose study seems pretty severely crippled before it has even begun. I speak of the study of fossil bacteria, and the subject of today's Taxon of the Week post is the Proterozoic fossil taxon Myxococcoides.

Myxococcoides is a small (1-35 µm) spherical to ellipsoidal fossil without distinctive ornamentation or other visible features found either singly or in loose colonies without an enclosing sheath or other distinct colony shape. It is an oft-repeated, but perhaps little appreciated, fact that bacteria were around and about long before a few of them considered getting together and making a eukaryote. I mean really long before. The earliest evidence of bacteria in the fossil record dates back nearly four billion years, while the earliest unequivocal evidence for eukaryotes is only about 850 million years old* (Cavalier-Smith, 2002). In other words, fully three-quarters of the history of life on this planet is represented only by prokaryotes. Only members of a species with severely anthropocentric delusions of grandeur would imagine that biodiversity did nothing in all that time except twiddle its thumbs and wait for the nucleus to develop, but there are some serious hurdles to understanding what was happening for the first three billion years of life.

*It will probably come as no surprise that the earliest date for eukaryotes is rather debatable - Cavalier-Smith (2002) gives a brief, if somewhat partisan, review. The 850 Mya date represents the earliest appearance of protist fossils of eukaryote cell size and complex cell morphology that implies the existence of a well-developed microfilament skeleton to hold it all in place. Certain fossils dating back as far as 1200 Mya or even 2100 Mya have been identified as eukaryotes, such as the putative "red alga" Bangiomorpha. However, these taxa have fairly simple cell morphologies and their identification as eukaryotes rather than prokaryotes rests on relatively few characters such as cell size. As argued by Hofmann (1976), supposed 'nuclei' in fossil cells may represent degradational artefacts where cytoplasm has become detached from the surrounding cell wall. While prokaryote cells are generally much smaller than eukaryote cells, bacteria can occassionally reach considerable sizes - the largest known bacterium, the sulphur-oxidizing Thiomargarita namibiensis, has cells almost a millimetre in diameter, a size that, as pointed out by Schütt (2003), is more than twice that of the smallest known spiders, which is a great piece of information to bring up at parties (technically, some actinobacteria such as Streptomyces are arguably even larger, but have a fungus-like filamentous hyphal morphology). It is therefore a perilous activity to label Proterozoic fossils as eukaryotes on the basis of size alone, especially as it is not unlikely that bacteria may have occupied a number of niches prior to the appearance of eukaryotes from which they were later excluded.

Lacking as they do the well-developed eukaryote cytoskeleton, the morphology of most prokaryotes is decidedly simple, with the majority of taxa conforming to the basic rod or sphere. For instance, Thermoproteus and Mycobacterium are both rod-shaped prokaryotes with colonies formed through snapping division that may be morphologically almost indistinguishable despite one being a archaebacterium and the other a Gram-positive eubacterium. Instead, bacterial taxa are generally distinguished by features of their genetics, biochemistry and physiology - all features that, of course, are generally completely unavailable when studying fossilised remains. As a result, taxa based on fossilised bacteria are doomed to be form taxa or morphotaxa - labels to indicate a particular morphology without necessarily indicating the actual relationships of the fossils involved. To complicate matters further, a single living morphology may potentially give rise to multiple fossil 'taxa' due to the level of degradation prior to preservation, as shown in the figure below from Hofmann (1976) of various stages of degradation from a Myxococcoides-like morphology.

Needless to say, the relationships of forms such as Myxococcoides to modern taxa is difficult if not impossible to establish. Most Precambrian fossil bacteria have been found in association with stromatolites and interpreted as cyanobacteria. They have then been assigned to modern orders on the basis of colony morphology, so forms without defined colony structures such as Myxococcoides have been assigned to the Chroococcales. However, phylogenetic analysis of recent taxa has shown that the Chroococcales (not surprisingly seeing as it was defined solely on negative characters) is a strongly paraphyletic assemblage from which filamentous forms have arisen polyphyletically (Litvaitis, 2002).

So why, some of you may be asking yourselves at this point, should we study fossil bacteria at all? Well, the simple fact is that, murky as it is, the bacterial fossil record remains our main window into three billion years of evolution. Some distinctive probable cyanobacterial groups, such as the family Aphralysiaceae (Vachard et al., 2001), have been identified solely from fossils, while others, such as the stromatolite-forming Entophysalidaceae, held far more ecological significance in the past than presently. If, as alluded to above, forms such as Grypania and Bangiomorpha represent prokaryotes convergent on eukaryotes that were later replaced by actual eukaryotes, then such diversity would have remained unknown except through the fossil record. Three billion years is a long time to miss out on.

Hofmann, H. J. 1976. Precambrian microflora, Belcher Islands, Canada: significance and systematics. Journal of Paleontology 50 (6): 1040-1073.

Litvaitis, M. K. 2002. A molecular test of cyanobacterial phylogeny: Inferences from constraint analyses. Hydrobiologia 468: 135-145.

Schütt, K. 2003. Phylogeny of Symphytognathidae s.l. (Araneae, Araneoidea). Zoologica Scripta 32 (2): 129-151.

Vachard, D., M. Hauser, R. Martini, L. Zaninetti, A. Matter & T. Peters. 2001. New algae and problematica of algal affinity from the Permian of the Aseelah Unit of the Batain Plain (East Oman). Geobios 34 (4): 375-404.

Linnaeus' Birding

Inevitable Moles in a Lonely Universe

In 1989, the book Wonderful Life by Stephen Jay Gould made its triumphant appearance - of all the many writings by that prolific author, it was to become perhaps the most famous of all*. In this exploration of (then-)recent advances in our understanding of the animals making up the Cambrian explosion, the relatively rapid appearance of the distant ancestors of most living major animal groups, Gould argued extensively for the role of contingency and chance in evolution. If we were somehow able to turn time back to the Cambrian then let it run all over again, claimed Gould, then we would not see a repeat of the same evolutionary history. Major groups of organisms in our modern environment might fail to appear, while other groups that are currently minor and marginal might diversify to take their place. In particular, humans or intelligent life in general might never come to be.

*Something I find quite surprising - I personally find Wonderful Life rather drekky and overblown, and its arguments ultimately rather weak. To quote Moe Szyslak in the angel episode of The Simpsons (in which Gould actually made a guest appearance, "How about you stop telling us what it ain't, and start telling us what it am?"

Central to this argument of Gould's was his interpretation of the studies on animals from the Canadian Burgess Shale by Simon Conway Morris. Gould argued that very few of the animals present in the Burgess Shale could be definitely associated with taxa that survived the Cambrian, while most of the Burgess animals represented isolated lines that would eventually go extinct. Were one to look at the Burgess fauna in the absence of knowledge about future events, there would be no way of distinguishing which taxa were to survive and which would not. As it turned out, Gould's interpretation of Conway Morris' work was to rankle quite significantly with Conway Morris himself, who has since written two books that essentially counter Gould's book. In the 1998 The Crucible of Creation, Conway Morris attacked Gould's characterisation of the Burgess taxa as phylogenetically isolated oddballs, arguing instead that most were identifiable as stem-taxa showing connections to modern animals. While far more scientifically rigorous than Gould's Wonderful Life, The Crucible of Creation does suffer significantly from the constant shadow of Conway Morris' evident ire at Gould's 'misappropriation' of Conway Morris' work - Richard Fortey was later to comment in his 2000 book Trilobite! that he had "never encountered such spleen in a book by a professional".

I haven't yet read Conway Morris' second book of reply, the 2003 Life's Solution: Inevitable Humans in a Lonely Universe, but I would like to comment on its basic premise. In Life's Solution, Conway Morris addressed Gould's contention that the course of evolution was contingent on past history and essentially unpredictable. Instead, Conway Morris uses the prevalence of convergent evolution, the independent evolution of similar characters in unrelated organisms occupying similar habitats, to argue that selection pressures result in a relatively small number of potentially viable forms, and that even if Earth's history were rerun then a roughly similar assemblage of organisms would result. There might not be humans in exactly the form we now them, but intelligent, self-aware organisms of some kind would eventually appear.

Of course, both Gould's and Conway Morris' propositions are not directly empirically testable - there is no way of actually rerunning the course of evolution. However, convergent evolution is a widely prevalent phenomenon that we can examine and stimate its effect on the form of organisms. Convergent evolution seems to be the result of strong selective pressures - when a certain habitat or lifestyle strongly favours a certain morphology.

Moles, for instance, are definitely inevitable, at least among mammals. At least three different groups of living mammals have independently adopted a burrowing lifestyle - the Holarctic true moles, the African golden moles and the Australian marsupial moles. All three groups have developed a very similar morphology - reduced or lost eyes, dense fur, compact body with short limbs and spade-like forelimbs. Among fossil mammals, the marsupial (or at least metatherian) Necrolestes and the erinaceid Proterix have also been interpreted as burrowers with a very mole-like morphology. Even at least one group of burrowing insects, the mole crickets, went through a similar development of shortened limbs and spade-like forelimbs.

Cacti are fairly inevitable. A number of groups of plants inhabiting arid habitats - most notably various members of the Euphorbiaceae as well as the cacti proper - have evolved thickened water-storing stems with the loss or reduction of functional leaves to reduce water loss. Interestingly, the alteration of leaves to spines has occurred on numerous occassions, and it has been suggested that as well as the obvious benefit of protection, spines may help promote the condensation of moisture onto the plant as dew.

Lice, I'm sorry to say, seem to be inescapable. As well as the lice proper, a number of other insect groups have become ectoparasites living among the fur or feathers of other animals, such as a number of families of flies (including the sheep ked and bat flies) and the earwig Hemimerus. All such groups show a loss of wings, reduction of sensory organs such as eyes and antennae, and development of a flattened morphology that is probably less able to be squashed or scratched off by the host.

On the other hand, antelope are not inevitable. While the primary cursorial grazers of Eurasia and Africa are slender-legged quadrupeds, in Australia their niche is occupied by the bipedal kangaroos. I am not sure why kangaroos became jumping bipeds rather than running quadrupeds, but my suspicion is that their arboreal ancestry (phylogenetic analyses suggest that kangaroos descend from a possum-like ancestor) meant that the ancestral kangaroos started out with a difference in length between hind- and forelimbs. The point is that in this case two very different morphologies have developed to cope with very similar niches.

And humans? Well, it is my personal opinion that we have no reason to regard humans as inevitable. Invoking convergent evolution to claim the inevitability of humans runs up against the major stumbling block that we have no other examples of convergence on the human form. Questioning whether intelligence and self-awareness were destined to arrive as a result of selective pressure demands that we answer the insanely difficult yet crucial questions of how we define "intelligence" and "self-awareness", and how we would recognise them once defined. So difficult are these questions that yours truly is going to be a complete weasel and avoid them (I've spent enough time on this post already), but I will rather weakly point out that most behaviours cited as evidence for self-awareness in humans, such as figurative language and the production of art, are as yet unknown in other organisms when not encouraged by direct human intervention (but refer back to the recognition problem above). How is one to claim convergent evolution to support the existence of something for which no convergences are known?

A coda. In Kurt Vonnegut's 1985 novel Galapagos, a small boatload of people are shipwrecked on the Galapagos islands at about the same time as a plague wipes out humanity in the rest of the world. Over the course of the following million years, the descendants of this small group of shipwrecked survivors lose many of the features that have generally been regarded as the keys to what make us human but which are no longer selectively advantageous in their new environment, such as manual dexterity and large brains. Instead, humans become seal-like animals, covered in dense water-repellent fur with flippers for swimming and catching fish. It doesn't really matter what you're talking about - whether or not it's a good thing really depends on circumstance.