Linnaeus was a Botanist

The latest edition of Berry Go Round, the monthly plant blog carnival, has appeared at 10,000 Birds. Birds eat plants, you know.

It's also nearly Linnaeus' Legacy time, and the next edition will be at Life Photo Meme. Get your submissions in to me, Kate (doridoidae at gmail.com), or use the submission form at Blog Carnival.

When is a Cephalopod Like a Snake?

The tarphycerid Lituites lituus. Photo from here.

When its a snakestone.

Just a quick post on tarphycerids today - haven't much time. The Tarphycerida were the earliest group of cephalopods to develop a coiled shell, back in the lower Ordovician. Sweet et al. (1964) regarded tarphycerids as sharing a common ancestor in the lower Ordovician Bassleroceratidae with the Oncocerida, the lineage ancestral to modern nautilids. Though tarphycerids are therefore more closely related to nautilids than to coleoids (and therefore, unlike the other Palaeozoic cephalopods I've covered so far, have a case to actually be regarded as nautiloids), the fact that the earliest oncocerids were not coiled (Sweet, 1964a) indicates that the coiled form was independently derived in tarphycerids from nautilids. A few tarphycerid lineages later became more loosely coiled, or uncoiled.


Campbelloceras, a more typical tarphycerid. Note the significant size of the body chamber compared to earlier chambers. From Palaeos.com.

Most tarphycerids were more or less evolute. Closely coiled cephalopod shells can be described as evolute, involute or convolute, though these aren't distinct forms so much as lines on a spectrum. In involute forms, the successive whorls of the shell overlap and cover the earlier whorls to some degree. This is taken to the extreme in convolute forms such as modern Nautilus in which the later whorls entirely conceal the older whorls. In evolute forms, however, the successive whorls lie alongside each other and remain clearly visible. There seems to have been a repeated tendency for evolute forms to be replaced by involute forms - I'm guessing because the involute arrangement was more sturdy and robust. Evolute shells are described as 'serpenticone', which refers to an old English belief that such shells were the petrified remains of coiled snakes. Indeed, it was not uncommon for coiled cephalopod fossils to be sold as 'snakestones' (supposedly protecting against snakebite) with carved snake heads attached to them.


A specimen of the ammonite Dactylioceras commune, modified into a snakestone. Photo from here.

One tarphycerid genus, Lituites, was one of the earliest fossil cephalopods recognised, and though officially published by Bertrand in 1763, Bertrand was merely validating a name that pre-dated Linnaeus (Furnish & Glenister, 1964). Lituitidae started life as coiled shells, but soon changed their growth pattern and grew in a straight line, with the straight segment of shell far larger than the coiled section - lituitids might be up to a foot in length, with the coiled section less than an inch across. While most tarphycerid families were widespread (albeit uncommon), lituitids are only known from northern Europe (and mostly from erratic boulders, which are an absolute bugger to place stratigraphically).

Many tarphycerids showed changes in growth habit through their life. The siphuncle often changed in position - all tarphycerids had ventral siphuncles when young, but for many species they became dorsal with age. Juvenile tarphycerids had standard round apertures, but in many species the aperture changed significantly in form at maturity, becoming contracted by ingrowing lobes of the shell with deep hyponomic and ocular sinuses. Whether and how this change in aperture form was reflected by any change in soft-body morphology can, of course, only be speculated upon. It is worth noting, as well, that the mature body chamber was often spectacularly long - in the family Ophidioceratidae, it may occupy more than an entire whorl of the shell. Again, in the absence of soft-body fossils there is no way of knowing whether the adult body was similarly long, or whether the animal was shorter and able to retreat deep into its shell.

Tarphycerids survived into the mid-Devonian - Sweet (1964b) separated the lineage including all later forms as the order Barrandeocerida, but all more recent authors seem to include the barrandeocerids in the Tarphycerida* (e.g. Turek, 2008). A number of barranderocerids became non-planar coilers (torticones), adopting a more gastropod-like form. Because non-planar coils would not have had such a centred buoyancy distribution compared to planar forms, these species would have probably been benthic in their lifestyle. The tarphycerids disappeared at about the same time as the ecologically similar nautilids and ammonoids were diversifying.

*The concept of paraphyletic taxa in a phylogeny-based classification is a bit like the concept of God in a secular society. People may not actually have anything against the idea, they may argue vociferously for the retention of the idea, but as time goes by they just refer to the idea less and less, and eventually the realisation dawns that there's just no real practical point in hanging on to it.

REFERENCES

Furnish, W. M., & B. F. Glenister. 1964. Nautiloidea - Tarphycerida. In Treatise on Invertebrate Paleontology pt K. Mollusca 3 (R. C. Moore, ed.) pp. K343-K368. The Geological Society of America and The University of Kansas Press.

Sweet, W. C. 1964a. Nautiloidea - Oncocerida. In Treatise on Invertebrate Paleontology pt K. Mollusca 3 (R. C. Moore, ed.) pp. K277-K319. The Geological Society of America and The University of Kansas Press.

Sweet, W. C. 1964b. Nautiloidea - Barrandeocerida. In Treatise on Invertebrate Paleontology pt K. Mollusca 3 (R. C. Moore, ed.) pp. K368-K382. The Geological Society of America and The University of Kansas Press.

Sweet, W. C., C. Teichert & B. Kummel. 1964. Phylogeny and evolution. In Treatise on Invertebrate Paleontology pt K. Mollusca 3 (R. C. Moore, ed.) pp. K106-K114. The Geological Society of America and The University of Kansas Press.

Turek, V. 2008. Boionautilus gen. nov. from the Silurian of Europe and North Africa (Nautiloidea, Tarphycerida). Bulletin of Geosciences 83 (2): 141-152.

Open Query – What are Cephalopod Shells For?

Cross-sectioned reconstruction of the Late Cambrian cephalopod Plectronoceras, from Palaeos.com.

I'm continuing with the "nautiloid" theme here (previous posts on Palaeozoic cephalopods can be found here and here). The primary reason why Palaeozoic cephalopods have been occupying my mind lately is that I am currently working my way through the Treatise of Invertebrate Paleontology volume on "nautiloids" (as explained in an earlier post, the lumping of all non-ammonoid, non-coleoid cephalopods under the name of "nautiloid" is a really unfortunate example of paraphyletic lumping, which is why I'm insisting on the quotation marks). For those unfamiliar with them, the Treatise of Invertebrate Paleontology (or simply the Treatise to its friends) is a series of volumes cataloguing the known genera of fossil invertebrates, and each volume also includes a series of introductory essays describing the anatomy, palaeoecology, etc. of the group it covers. I've commented before that there is something incredibly purgatorial about a Treatise volume. It's a long, painful, torturous slog that hammers you both mentally and physically*, but when you finally come out the other end there can be no doubt that you've done something really worth achieving.

*Anyone who doubts that a book is able to challenge you physically has not had to carry a bag holding the entire 1000 pages plus in three volumes of the Treatise section on crinoids, nor been assualted by a falling copy of the 1100 pages of Roewer's Die Weberknechte der Erde.

The question that is currently residing in my mind also, in a roundabout sort of way, relates to the recent publication of the fossil avialian Epidexipteryx ('Les'). Typically for a Nature article, the revolutionary nature of that fossil has been more than a little exaggerated, but it has publicised the growing consensus among palaeontologists that feathers were not originally used for flight when they first evolved. It is more likely that they were originally used for insulation, and only later became used for other functions such as display and flight*. Gould & Vrba (1982) actually coined a term for this phenomenon, 'exaptation', which was meant to refer to cases where a feature that originally evolved for one function was co-opted for use in another function, as opposed to 'adaptation', when a feature originally evolved directly for its current function. The term 'exaptation' in itself never really caught on, because almost all 'adaptations' are in some way 'exaptations'. However, the verb form 'exapted' remains useful when describing examples of such a process.

*I'm going to speculate a little and suggest that the use of feathers for display may have even been a necessary prerequisite for their use for flight, because the planar surface required for flight may have been less likely to be selected for insulation than for maintaining a stereotyped form for display. This may be why flightless maniraptorans such as Caudipteryx possessed planar tail and arm fans.


Cross-section of the Late Cambrian cephalopod Yanheceras, showing the closely-spaced septa in this 12 mm shell. Image from Palaeos.com again.

What is the connection between cephalopods and the origin of feathers? As note before, fossil cephalopods can be distinguished from all other molluscs by their unique shell structure, with the shell divided into a series of internal chambers. In most shelled cephalopods, the chambers would have been/are mostly hollow and filled with gas whose volume can be adjusted to control shell buoyancy. But how and why did this unique structure evolve in the first place? Contrary to first impressions, the chambers were probably not used as floats when they first appeared. The really early cephalopods, such as plectronoceratids and ellesmeroceratids, were small subconical shells, generally only a few centimetres in length. Despite their small size, their shells were divided into relatively large numbers of chambers, with the dividing septa packed close to each other. Even if the chambers were filled with gas (which they may not have been - it is currently unknown just when in cephalopod evolution the chambers became gas-filled), it is unlikely that the volume of the chambers would have been enough to lift the weight of the shell. Plectronoceratids, etc., were almost certainly benthic (Furnish & Glenister, 1964). Because these early forms are mostly outside the cephalopod crown group, there is currently no way of knowing whether they shared any of the soft-body features associated with modern cephalopods, such as tentacles and the siphon, or whether they still had a more primitive, superficially snail-like (though untorted) morphology.

Holland (1987), if I understand correctly, suggests that the septate shell, either by increasing relative buoyancy (even if it did not make the animal actually buoyant) or by changing the distribution of the shell's weight, may have made cephalopod locomotion more energy-efficient, allowing greater mobility. Does this seem like a likely explanation? Can any of you think of an alternative? And how would you suggest we test any likely explanations?

REFERENCES

Furnish, W. M., & B. F. Glenister. 1964. Paleoecology. In Treatise on Invertebrate Paleontology pt K. Mollusca 3 (R. C. Moore, ed.) pp. K114-K124. The Geological Society of America and The University of Kansas Press.

Gould, S. J., & E. S. Vrba. 1982. Exaptation; a missing term in the science of form. Paleobiology 8 (1): 4-15.

Holland, C. H. 1987. The nautiloid cephalopods: a strange success. Journal of the Geological Society of London 144 (1): 1-15.

Moore, R. C. (ed.) 1964. Treatise on Invertebrate Paleontology pt K. Mollusca 3. The Geological Society of America and The University of Kansas Press.

Getting Crabs

The purple shore crab Leptograpsus variegatus of the southern subtropical Indo-Pacific ocean. Photo by Benjamint444.

When I was but an ickle lad, and my family would camp over Christmas at the beach by the estuary beneath the house of my great-grandparents, I would spend many hours turning over rocks and catching the crabs that I found underneath them. The most common variety I would find was the tiny grey-brown mud crab (Helice crassa), which could be handled easily, but if I managed to turn over one of the really big rocks then I would be able to find the larger purple shore crabs (Leptograpsus variegatus), which required a more careful approach lest they inflict great pain. One thing I didn't know at the time about either animal, however, was that they were both members of the superfamily Grapsoidea.

Grapsoidea is a grouping of crabs including at least seven families. The classification of Grapsoidea is currently undergoing something of a revision, and has shifted about a little in recent years. While most grapsoids were once included in the single family Grapsidae, the recognition of the latter as paraphyletic to the Gecarcinidae has lead to the elevation of the various prior subfamilies of Grapsidae to separate families. The family Glyptograpsidae was only established in 2002 (Schubart et al., 2002), while the genus Xenograpsus was moved into its own family within the past year (Ng et al., 2007). Other families in the group are Sesarmidae, Varunidae and Plagusiidae. The majority of grapsoids are found on the shoreline, but some (such as the Chinese mitten crab Eriocheir sinensis) move into fresh water. At least one genus, Planes (Grapsidae), is pelagic, while Xenograpsus has been found to depths of 270 m (McLay, 2007). Xenograpsus is found in association with hydrothermal vents, and populations of X. testudinatus living on sulphur vents near Taiwan make their living by feeding on the rain of dead zooplankton killed by toxic discharges from the vents (Ng et al., 2007).


Gecarcoidea natalis, Christmas Island red crab migration. Photo from here.

Some members of the Gecarcinidae live their adult lives terrestrially as adults on tropical islands. Nevertheless, all grapsoids (as far as I can tell) retain the ancestral state of marine planktonic larvae, so all terrestrial gecarcinids must return to the coast to spawn. The Christmas Island red crab, Gecarcoidea natalis has become renowned for the vast numbers that can be seen in its mass migrations, as the entire island's population of crabs (more than 40 million when estimated in 1995 - Adamczewska & Morris, 2001) moves down to the coast over the course of a week or so. Tragically, recent years have seen a population explosion on Christmas Island of the introduced yellow crazy ant* (Anoplolepis gracilipes), which was estimated to have killed off some 15 million-plus crabs by 2003 (O'Dowd et al., 2003), and has essentially eliminated crab populations wherever it has established colonies. Foraging crabs are attacked in large numbers by crazy ants defending their nests, and poisoned with large amounts of formic acid. Crazy ants will also occupy crab burrows, removing their former inhabitants with extreme prejudice. Not only are resident crabs killed, but crabs migrating from elsewhere have been destroyed as they crossed crazy ant-infested locations on their way to the coast. Where red crabs have been eliminated, the forest vegetation structure has begun to change significantly, as seedlings that would have once been grazed by crabs are able to establish a dense undergrowth.

*So called because of the seemingly random way in which they wander about when foraging.


Xenograpsus testudinatus at the base of a sulphur vent. Photo from here.

The Grapsoidea are closely related to another shore-crab family, the Ocypodoidea, and apparently species included in these two superfamilies were once united (back in the 1800s) under the taxon name Catometopa (Schubart et al., 2006), a name that I think deserves resurrection (just try saying it a couple of times - "Catometopa!"). While it seems to be universally accepted that these two superfamilies form a clade, the molecular phylogenetic analysis of Schubart et al. (2006) indicated that each of the "superfamilies" was polyphyletic within this clade, and recommended that they not be recognised as distinct. So far, I haven't been able to find what are the characters that are supposed to separate the two groups. Davie & Ng (2007) stated that morphological data maintained the monophyly of Grapsoidea, but omitted to cite any details in support of this statement.

REFERENCES

Adamczewska, A. M., & S. Morris. 2001. Ecology and behavior of Gecarcoidea natalis, the Christmas Island red crab, during the annual breeding migration. Biological Bulletin 200: 305-320.

Davie, P. J. F., & N. K. Ng. 2007. Two new subfamilies of Varunidae (Crustacea: Brachyura), with description of two new genera. Raffles Bulletin of Zoology Supplement 16: 257-272.

McLay, C. 2007. New crabs from hydrothermal vents of the Kermadec Ridge submarine volcanoes, New Zealand: Gandalfus gen. nov. (Bythograeidae) and Xenograpsus (Varunidae) (Decapoda: Brachyura). Zootaxa 1524: 1-22.

Ng, N. K., P. J. F. Davie, C. D. Schubart & P. K. L. Ng. 2007. Xenograpsidae, a new family of grapsoid crabs (Crustacea: Brachyura) associated with shallow water hydrothermal vents. Raffles Bulletin of Zoology Supplement 16: 233-256.

O'Dowd, D. J., P. T. Green & P. S. Lake. 2003. Invasional 'meltdown' on an oceanic island. Ecology Letters 6 (9): 812-817.

Schubart, C. D., S. Cannicci, M. Vannini & S. Fratini. 2006. Molecular phylogeny of grapsoid crabs (Decapoda, Brachyura) and allies based on two mitochondrial genes and a proposal for refraining from current superfamily classification. Journal of Zoological Systematics and Evolutionary Research 44 (3): 193-199.

Schubart, C. D., J. A. Cuesta & D. L. Felder. 2002. Glyptograpsidae, a new brachyuran family from Central America: larval and adult morphology, and a molecular phylogeny of the Grapsoidea. Journal of Crustacean Biology 22(1): 28-44.

The Floating Egg

Reconstruction of a mature ascocerid with the positions of the chambers drawn as visible, from Holland (1999).

Have you ever come across an example of something that looks like an incredibly good idea, but for some unknown reason it just never catches on? In the oceans of the Silurian, the Ascocerida must have seemed to be the ultimate cephalopod. Furnish & Glenister (1964b) described the ascocerids as "almost perfectly adapted for an active nektonic [swimming] mode of life", which, in the normally dry and dusty context of a Treatise on Invertebrate Paleontology volume, comes across as high praise indeed. Despite this, ascocerids never attained a high degree of diversity or wide distribution, and by the end of the Silurian they had disappeared forever.

In some ways, the Ordovician and Silurian were the high points of cephalopod diversity. While the total number of cephalopod species may not have been as high as in later periods, this was more than made up for by the diversity of body forms. Cephalopod shells (internally shelled or shell-less cephalopods were not to appear until much later) ran the gamut from coiled forms similar to later nautilids and ammonoids (also both not yet on the scene) through to completely straight orthocones and everything in between. In older classifications, this diversity is lumped under the heading of "nautiloids", a particularly unfortunate imposition of obscurity that hides just how different the early "nautiloids" were from the modern Nautilus.

One way in which Palaeozoic cephalopods probably did resemble modern taxa is that the majority of cephalopods have probably all been active predators. In actively-swimming modern cephalopods, the main means of propulsion is through the expulsion of water through the hyponome, a muscular tube close to the mouth and tentacles, which shoots the animal through the water. For shelled cephalopods, jet propulsion adds a particular challenge, as the shell must be buoyant to allow the animal to swim freely, and finely balanced to allow the animal to move horizontally. To achieve and control bouyancy, the cephalopod shell is divided into a series of hollow chambers, with the bulk of the animal occupying the anterior body chamber and only a long cord-like extension, the siphuncle, extending into the posterior chambers. The only two cephalopod lineages with external shells to survive into the Jurassic, ammonoids and nautilids, had both independently developed a tightly-coiled form as the most effective way to maintain balance. Straighter-shelled forms appear to have developed complex arrangements of valves, diaphragms and mineral deposits to keep the shell effectively counter-balanced.

Ascocerids, on the other hand, took a different approach. Ascocerids were small for cephalopods - the largest known examples were about fifteen centimetres in length, but some species reached maturity at less than two centimetres. Their shells were lightweight marvels - even the largest examples rarely had shells more than one millimetre thick, while the paper-thin septa dividing chambers within the shell were generally around 0.1 millimetre thick. Ascocerids started their life as fairly ordinary cyrtocones - that is, rather than being perfectly straight their shells were curved like a crescent, but not so curved as to be coiled - with fairly ordinary evenly-spaced concave septa. As an ascocerid reached maturity, though, the mode of growth changed significantly. The shell become inflated and bulbous. The septae became tightly-spaced posteriorly, and sigmoid and extended forward dorsally, so that the gas chambers contained by the septa lay above rather than behind the body chamber. The posterior juvenile part of the shell was then shed, leaving the flask-shaped mature shell only. It is quite likely that there was a concurrent change in lifestyle from more benthic juvenile to fully nektic adult. With their lightweight construction and well-placed dorsal chambers, ascocerids would have been among the most mobile of Palaeozoic cephalopods, probably rivalling modern-day cuttlefish.


Reconstruction of various stages in the life-cycle of the ascocerid Billingsites noquettensis from juvenile cyrtocones (a) to ovoid adult (g), from Kesling (1961).

It might be thought that such marvels of construction would be well-placed to take over the world, but ascocerid fossils are spectacularly rare. Though specimens have been found in a number of localities in Europe and North America, they have generally only been found in very small numbers, and only in localities that have been intensely collected. Holland (1999) noted that he had seen many thousands of cephalopods from the Silurian of Britain, but only eight ascocerids. What is more, most of those specimens that are known from these localities are in very poor condition. The only two localities from which ascocerids have been found in any sort of numbers are in Gotland in Sweden and Bohemia. Gotland is also the only locality from which juvenile ascocerids have been recovered. Modern chambered Nautilus shells are capable of significant floating dispersal after the death of the animal inhabiting them - for instance, specimens have been washed up on the coast of New Zealand roughly a thousand miles south of the limits of their living distribution. Furnish & Glenister (1964a) suggested that was not impossible that the Gotland locality was in fact the only locality that had been inhabited by living ascocerids, and that all other specimens had floated to their eventual point of deposition post mortem.

I am completely at a loss as to why ascocerids remained as restricted as they did. All I can do for now is just write it off as another one of life's little mysteries.

REFERENCES

Furnish, W. M., & B. F. Glenister. 1964a. Paleoecology. In Treatise on Invertebrate Paleontology pt K. Mollusca 3 (R. C. Moore, ed.) pp. K114-K124. The Geological Society of America and The University of Kansas Press.

Furnish, W. M., & B. F. Glenister. 1964b. Nautiloidea – Ascocerida. In Treatise on Invertebrate Paleontology pt K. Mollusca 3 (R. C. Moore, ed.) pp. K261-K277. The Geological Society of America and The University of Kansas Press.

Holland, C. H. 1999. The nautiloid cephalopod order Ascocerida in the British Silurian. Palaeontology 42 (4): 683-689.

Kesling, R. V. 1961. A new species of Billingsites, an ascoceratid cephalopod, from the Upper Ordovician Ogontz formation of Michigan. Contributions from the Museum of Paleontology, the University of Michigan 17 (3): 77-121.

Les is More

The avialan theropod previously referred to here as "Les" has now made its official, honest-to-goodness debut in the latest issue of Nature, and I can now reveal the proper name for Les - Epidexipteryx hui (the genus name means "display feather" and refers to the fact that the feathers preserved for Epidexipteryx appear to have been used for display rather than flight). It's a pretty fossil, but Nature has pulled its usual frustrating trick of giving us just enough information to whet the appetite, and leaving us howling frustratedly for more information...

Reconstruction of Epidexipteryx hui above taken from The Loom.

More Tales of the Crunchy

Male of the Sri Lankan cyphophthalmid Pettalus. Photo from Gonzalo Giribet.

This week's Taxon of the Week post had to be delayed a couple of days because I thought the Monday slot was better devoted to the proposed new electronic publication rules for the ICZN. It now being Wednesday, I am perfectly seated to introduce the delayed post. Today's highlight taxon is something not too far from my own research stomping grounds - the harvestman (Opiliones) superfamily Sironoidea.

Sironoidea was one of the three superfamilies of Cyphophthalmi or mite-like harvestmen recognised by Shear (1980). Largely thanks to the work of Gonzalo Giribet and associates, cyphophthalmids have in recent years become the most thoroughly studied of the major groups of harvestmen. I have previously posted on them here, and I'd recommend reading through that post before this one. As originally defined by Shear, Sironoidea included two families, the Holarctic Sironidae and the temperate Gondwanan Pettalidae (the subject of the previous post). Later, Shear (1993) added the New Caledonian Troglosiro to the superfamily. Sironoidea were united by Shear (1980) on the basis of their having a single series of dorsal setae (three groups of setae in other cyphophthalmids), coxae II free from coxae III (vs. fused), and the presence of anal glands and a modified anal region. Sironoidea was the only superfamily in the infraorder Temperophthalmi, the temperate cyphophthalmids, while the other two superfamilies, Stylocelloidea (containing the single south-east Asian family Stylocellidae) and Ogoveoidea (Ogoveidae and Neogoveidae of tropical America and west Africa) in the infraorder Tropicophthalmi, both had tropical distributions. One tropical species, the Kenyan Marwe coarctata, has since been supported by Bivort & Giribet (2004) as forming a clade in the Sironidae with the genera Paramiopsalis and Iberosiro.

More recent studies have broken down Shear's classification. Both of Shear's infraorders are likely to be polyphyletic, but relationships between the various families are difficult to pin down. The molecular analysis of Boyer et al. (2007) identified two possible arrangements - one with Pettalidae sister to the remaining cyphophthalmids in the arrangement Sironidae + (Stylocellidae + (Troglosiro + Neogoveidae)), while the other had Stylocellidae as the basalmost branch and Pettalidae sister to the clade formed by Sironidae and (Troglosiro + Neogoveidae). In contrast, the morphological analysis of Bivort & Giribet (2004) supported a basal position for Stylocellidae with the Ogoveoidea forming a paraphyletic series to a monophyletic Sironoidea. Until recently, the belief that Stylocellidae were the only family of cyphophthalmids to retain eyes was thought to support a basal position for them, but eyes were identified in a species of Pettalidae for the first time by Sharma & Giribet (2006), and have since been shown to be widespread in the family (Boyer & Giribet, 2007). Cyphophthalmid eyes (in those species that have them) are minute, integrated into the ozophore, of doubtful functionality and lack lenses in many Pettalidae, which is how it was possible for them to have gone unnoticed.


Siro carpaticus, a cyphophthalmid species from Poland. Not that while originally described as more closely related to species now included in Siro than to species now included in Cyphophthalmus, this species has not been properly reanalysed since Boyer et al. (2005) divided the two genera, and its generic assignment is not certain. Image from here.

Even the family Sironidae, established by Shear (1980) mainly on the basis of fused anal sclerites, is not necessarily monophyletic. As with Pettalidae (see the earlier post), each of the genera recognised within Sironidae shows a distinct geographical distribution, demonstrating that vicariance has been a significant factor - in fact, the primary factor - in the diversification of these spectacularly poorly-dispersing soil animals. Interestingly, there appears to be a closer evolutionary connection for cyphophthalmids between western Europe and North America (both inhabited by members of the genus Siro) than between western Europe and eastern Europe (the latter inhabited in the Balkan peninsula by the genus Cyphophthalmus - Boyer et al., 2005), which might be regarded as a case of life imitating politics. The Iberian peninsula has been the site of a notable mini-radiation of sironids, being the only home of the monotypic genera Odontosiro lusitanicus, Paramiopsalis ramulosus and Iberosiro distylos while the genus Parasiro is found in Spain, Corsica and Italy. Other sironid genera are the Bulgarian Tranteeva paradoxa and the Japanese Suzukielus sauteri. Boyer et al. (2007) supported the monophyly of a core clade for Sironidae including Siro, Cyphophthalmus and Paramiopsalis, but Suzukielus and Parasiro were not closely associated with this clade - Suzukielus only joined the core Sironidae in the Stylocellidae-rooted tree, while Parasiro never did.

It's worth my adding a bit more about cyphophthalmid eyes. One things arachnids are known for by the general public is their possession of multiple pairs of eyes. Harvestmen, however, stand out in this regard by never having more than a single pair of eyes (the one supposed exception, the "cyphophthalmid" Gibocellum sudeticum, seems never to have existed outside the overactive imagination of its author - Sørensen, 1906). In the Phalangida (the sister clade to Cyphophthalmi containing the majority of harvestmen), the eyes are usually placed on either side of a raised structure called the ocularium or eyemound positioned on the midline of the prosoma (visible in the photo here), but there are a few examples in which the ocularium has disassociated and the eyes are positioned closer to either side of the animal (some examples are visible among the images on this page). The eyes of cyphophthalmids may be homologous to the median eyes of Phalangida. Alternatively, they could correspond to the lateral eyes of other arachnids. Shultz (2007) identified the sister group of Opiliones as scorpions, which have both median and lateral eyes, so no help there. A paper in the most recent Journal of Arachnology (Alberti et al., 2008) apparently identifies the ultrastructure of cyphophthalmid eyes as more like the median eyes of other arachnids, but this paper is so new that I haven't even seen the issue in question as yet.

REFERENCES

Alberti, G., E. Lipke & G. Giribet. 2008. On the ultrastructure and identity of the eyes of Cyphophthalmi based on a study of Stylocellus sp. (Opiliones, Stylocellidae). Journal of Arachnology 36 (2): 379-387.

Bivort, B. L. de, & G. Giribet. 2004. A new genus of cyphophthalmid from the Iberian Peninsula with a phylogenetic analysis of the Sironidae (Arachnida: Opiliones: Cyphophthalmi) and a SEM database of external morphology. Invertebrate Systematics 18: 7-52.

Boyer, S. L., R. M. Clouse, L. R. Benavides, P. Sharma, P. J. Schwendinger, I. Karunarathna & G. Giribet. 2007. Biogeography of the world: a case study from cyphophthalmid Opiliones, a globally distributed group of arachnids. Journal of Biogeography 34 (12): 2070-2085.

Boyer, S. L., & G. Giribet. 2007. A new model Gondwanan taxon: systematics and biogeography of the harvestman family Pettalidae (Arachnida, Opiliones, Cyphophthalmi), with a taxonomic revision of genera from Australia and New Zealand. Cladistics 23: 337-361.

Boyer, S. L., I. Karaman & G. Giribet. 2005. The genus Cyphophthalmus (Arachnida, Opiliones, Cyphophthalmi) in Europe: a phylogenetic approach to Balkan Peninsula biogeography. Molecular Phylogenetics and Evolution 36 (3): 554-567.

Sharma, P., & G. Giribet. 2006. A new Pettalus species (Opiliones, Cyphophthalmi, Pettalidae) from Sri Lanka with a discussion on the evolution of eyes in Cyphophthalmi. Journal of Arachnology 34 (2): 331-341.

Shear, W. A. 1980. A review of the Cyphophthalmi of the United States and Mexico, with a proposed reclassification of the suborder (Arachnida, Opiliones). American Museum novitates 2705: 1-34.

Shear, W. A. 1993. The genus Troglosiro and the new family Troglosironidae (Opiliones, Cyphophthalmi). Journal of Arachnology 21: 81-90.

Shultz, J. W. 2007. A phylogenetic analysis of the arachnid orders based on morphological characters. Zoological Journal of the Linnean Society 150 (2): 221-265.

Sørensen, W. 1906. Un animal fabuleux des temps modernes. Analyse critique. Oversigt over det Konigelige Danske Videnskabernes Selskabs Forhandlinger 4: 197-232.