Sphaerexochus: A Possibly Predatory Trilobite

Sphaerexochus brittanicus, from Museum Victoria.

The fossil in the image above belongs to a genus of trilobites that lived from the Mid-Ordovician to the end of the Silurian. Distinguishing characters of Sphaerexochus include the massive inflation of the glabella, the central section of the trilobite head, which became almost spherical. The cheeks on either side of the glabella, in contrast, remained fairly small. The inflated glabella was marked on each side in the posterior part by a deep furrow that run in a curve from the side to the posterior margin. In the photo above, this furrow marks out the circular section that looks a bit like a large eye; the actual eye can just be made out in the photo (I think) as a crescent-shaped structure on the cheek below the glabella. The rear outer corners of the cheeks were more or less blunt in adult individuals, generally lacking the prominent cheek-spines of many other trilobites (small spines were present in juveniles). The spines on the trilobite's rear end (the pygidium) were also blunt and stout.

Přibyl et al. (1985) recognised four subgenera within Sphaerexochus, and this classification was largely supported by Congreve & Lieberman (2011) in a phylogenetic analysis (one of the subgenera, Onukia, is not known from well-preserved material and hence could not be analysed by Congreve & Lieberman). One of the subgenera, Korolevium, retained short cheek-spines as adults; they were absent in other subgenera. The remaining two subgenera, Sphaerexochus and Parvixochus, were distinguished by features of the hypostome, a plate on the underside of the head that covered the mouth in life. Korolevium and Parvixochus were both extinct by the end of the Middle Ordovician, but the Sphaerexochus subgenus sailed on and passed through the mass extinction event at the end of the Ordovician apparently unscathed (Congreve & Lieberman 2011).

Another view of Sphaerexochus brittanicus, from the Carnegie Institution.

Sphaerexochus was restricted to warmer waters in the Palaeozoic ocean (fortunately for Sphaerexochus, during the Silurian this was just about everywhere). Fortey & Owens (1999) interpreted its large glabellar furrows as providing an attachment site for the muscles of a correspondingly well-developed pair of limbs. Features of the hypostome indicate that Sphaerexochus had a relatively large oral cavity, and the combination of large anterior limbs and a big mouth lead Fortey & Owens to suggest that it may have been a raptorial predator. Přibyl et al. (1985) noted that Sphaerexochus specimens have often been found with the main body and head bent at an angle from each other, and suggested that they may have spent a lot of time with the body buried in the sediment with only the head above the surface. These observations add together to suggest Sphaerexochus living as an ambush predator, lurking in wait whilst largely hidden in the sand for any smaller animal unwary enough to get too close.

REFERENCES

Congreve, C. R., & B. S. Lieberman. 2011. Phylogenetic and biogeographic analysis of sphaerexochine trilobites. PLoS ONE 6(6): e21304. doi:10.1371/journal.pone.0021304.

Fortey, R. A., & R. M. Owens. 1999. Feeding habits in trilobites. Palaeontology 42 (3): 429-465.

Přibyl, A., J. Vaněk & I. Pek. 1985. Phylogeny and taxonomy of family Cheiruridae (Trilobita). Acta Universitatis Palackianae Olomucensis Facultas Rerum Naturalium Geographica-Geologica XXIV 83: 107-193.

Sociable Spider-Hawks

The wasp in the photo above (taken by Henrik Gyurkovics) is Telostegus inermis, a member of the family Pompilidae. I've always known pompilids by the vernacular name of spider-hawks; other names I've heard include spider-hunters or tarantula hawks. They get these names because the females capture spiders that they paralyse with their sting. The helpless spider is then dragged into a burrow, where the spider-hawk lays an egg on it. When the egg hatches, the spider will become food for the developing larva.

Spider-hawks of the genus Telostegus are known from the greater part of the Old World. Normally, at this point, I would say something about their distinguishing characters, but I'm afraid that you've got me there. A morning spent trying to dig up descriptions has largely failed, with the necessary references being scattered and inaccessible to yours truly. Though spider-hawks are among the more visible of wasp families, they have not been that extensively studied. Indeed, one of the first things I came across in my search was this dis-heartening exchange discussing how there was (as of 2009) only one researcher in Europe with the experience to reliably identify pompilids, who is difficult to contact due to failing health. Sadly, this is a scenario all too familiar in the world of taxonomy.

The nesting behaviour of three Australian Telostegus species (one under the since-synonymised genus Elaphrosyron) was described by Evans & Matthews (1973). Each built nests in which a single entrance lead through branching tunnels to multiple cells, each containing a single spider. In at least one species, Telostegus socius, the soil from the burrow was piled in a mound in front of the entrance. Evans & Matthews also noted that another pompilid species, Ceropales ligea, would sometimes lay its own eggs on the Telostegus' spider as the female of the latter was in the process of transporting it, making C. ligea a cuckoo pompilid. The name of Telostegus socius refers to another characteristic of its nests: a large number of females would build their nests in close proximity. Such gregarious behaviour is also known from wasps in other families (such as the sand wasps of the crabronid genus Bembix). It does not represent true social behaviour like that of ants or vespid wasps, as each female is still constructing and stocking her own nest. Nevertheless, any would-be predators may now be faced with a whole group of defending wasps instead of just one, and it is tempting to see such gregarious nesting as an early step towards true social behaviour.

REFERENCE

Evans, H. E., & R. W. Matthews. 1973. Behavioural observations on some Australian spider wasps (Hymenoptera: Pompilidae). Transactions of the Royal Entomological Society of London 125 (1): 45-55.

The Zealot Spiders

Female Zelotes longipes, photographed by Jørgen Lissner.

The spider in the photo above is a fairly typical representative of the genus Zelotes. As it currently stands, this is a large genus found worldwide, with around 400 species described so far and new ones continuing to debut at a fair rate of knots. I have no idea why Johannes Gistel, when he named this genus back in 1848, thought it to be especially zealous. My guess would be that there was probably no particular significance to the name; Gistel may have simply chose it in the well-established tradition of the time of providing organisms with classical names.

Members of the Gnaphosidae, the family of spiders to which Zelotes belongs, do not construct a permanent web but are ground-running active hunters. 'Running' being the operative word: part of the reason why new gnaphosid species continue to be described even from well-populated parts of the world is that, if you want to describe them, first you have to catch them. Zelotes species seem to be generalist in their habitats, with members of a single species found in a wide range of environments. Gnaphosids are something of a notoriously difficult group of spiders to identify, and Zelotes is no exception. Distinguishing features of Zelotes include the presence of a ventrodistal comb of stiff hairs on the metatarsi of the third and fourth pairs of legs, used in preening; the posterior median pair of eyes being roughly similar in size to the outer posterior eyes (gnaphosids have eight eyes in two rows of four); and the presence of an extra sclerite in the male genitalia (Ubick 2005). The genital sclerite was recognised as an important characteristic of the genus by Platnick & Shadab (1983), but their review was mostly restricted to North American species. Many species in other parts of the world remain unrevised, and future studies may affect their placement in Zelotes.

That said, most Zelotes species are fairly uniform in overall appearance (but then, so are gnaphosids in general). The species pictured at the top of this post is European. Compare it with a typical East Asian species:

Female Zelotes iriomotensis, photographed by Akio Tanikawa.

Or a North American species:

Female Zelotes fratris, photographed by Kyron Basu.

In general, the myriad Zelotes species can only be distinguished by examination of their genitalia. Most of us would be doing well to even identify it as a Zelotes.

REFERENCES

Platnick, N. I., & M. U. Shadab. 1983. A revision of the American spiders of the genus Zelotes (Araneae, Gnaphosidae). Bulletin of the American Museum of Natural History 174 (2): 97-192.

Ubick, D. 2005. Gnaphosidae. In: Ubick, D., P. Paquin, P. E. Cushing & V. Roth (eds) Spiders of North America: an identification manual, pp. 69-74. American Arachnological Society.

This Post Contains Smut

And here's some of that smut, right in your face:

Specimens of the grass Briza minor infected with the smut Jamesdicksonia brizae, from Roger Shivas.

Smuts are a form of plant-parasite fungus that produce large numbers of dusty spores from sori that rupture from the host-plant tissue. The majority of smuts belong to the basidiomycetes clade Ustilaginomycotina (making them distant relatives of the familiar mushroom). The focus of today's post will be the smut order Georgefischeriales, the vast majority of which are parasites of the vegetative tissue of grasses (Bauer et al. 2001). The exceptions include species of the genus Georgefischeria that parasitise species of the plant family Convolvulaceae (the morning-glories) and a small number of species that parasitise sedges. The species Gjaerumia ossifragi parasitises the bog-asphodel Narthecium ossifragum (Bauer et al. 2005). Members of the Georgefischeriales are distinguished from other smuts in that the septa dividing cells within mature sori lack pores. Members of one georgefischerialean family, the Gjaerumiaceae, have septal pores in young hyphae, but by the time they reach maturity the pores have closed over (Bauer et al. 2005). Like other smuts, Georgefischeriales have a life-cycle that alternates between a parasitic and a non-parasitic, saprobic stage, as summarised in this diagram from Bauer et al. (2001):

If a number of the terms on that diagram don't mean much to you: don't worry, you are not alone. The first thing to understand is that, compared to us, fungi kind of do sexual reproduction in reverse. For most of our life-cycle, we are diploid, with two complementary sets of chromosomes in each cell. When we produce sex cells (eggs and sperm), the number of chromosome sets in the cell is reduced to one, making the cell haploid, so that when egg and sperm fuse the resulting offspring is back to being diploid. Fungi resemble us in having two haploid cells fuse into a diploid offspring, but differ in that they then separate the chromosome sets again to produce separate haploid cells. Fungi are therefore haploid rather than diploid for most of their life-cycle. In the diagram above, it is the non-parasitic stage that is the haploid stage.

The parasitic stage of a smut's life-cycle comes about through the fusion of two haploid non-parasitic cells. However, even though the cells fuse, the nuclei in those cells don't. Instead, they form what is called a dikaryon, a chimaeric organism in which each cell contains two separate genomes in separate nuclei. Many fungi have lifestyles featuring dikarya: mushrooms, for instance, are also dikaryotic. The separate nuclei don't actually fuse until they form the reproductive structures, the basidia. In smuts, these basidia become thick-walled and separate from the parent sorus to produce the dispersal structures, the teliospores. The haploid basidiospores that come from the teliospores are what grow into the next non-parasitic generation.

Sori of Gjaerumia ossifragi on Narthecium, from here.

In three of the four families of Georgefischeriales (Georgefischeriaceae, Tilletiariaceae and Gjaerumiaceae), the saprobic stage of the life cycle grows into filamentous hyphae, but the fourth family, the Eballistraceae (containing the single genus Eballistra), produces a budding yeast as its non-parasitic stage instead. Eballistra also differs from other Georgefischeriales in that it does not produce ballistic spores that are flung from the parent sorus, relying instead on more passive dispersal. Eballistra does resemble Georgefischeriaceae and Gjaerumia in producing holobasidia, basidia that are not subdivided by septa. The Tilletariaceae are distinct in that they produce internally-divided phragmobasidia. Tilletiariaceae include three genera: Tilletiaria have spiny spores while those of Phragmotaenium and Tolysporella are smooth. It is worth noting that Tilletiaria is currently known only from laboratory culture and has not been identified in the wild, but it is expected to be a grass parasite like other Tilletiariaceae. Tolysporella produces teliospores that are clustered into spore-balls; those of Phragmotaenium are released individually. In the Georgefischeriaceae, two genera are distinguished by host range and appearance: Georgefischeria produces light-coloured sori on Convolvulaceae while Jamesdicksonia produces dark sori on grasses.

REFERENCES

Bauer, R., D. Begerow, F. Oberwinkler, M. Piepenbring & M. L. Berbee. 2001. Ustilaginomycetes. In: McLaughlin, D. J., E. G. McLaughlin & P. A. Lemke (eds) The Mycota vol. 7. Systematics and Evolution, part B, pp. 57-83. Springer-Verlag: Berlin.

Bauer, R., M. Lutz & F. Oberwinkler. 2005. Gjaerumia, a new genus in the Georgefischeriales (Ustilaginomycetes). Mycological Research 109 (11): 1250-1258.

Dighloti

Just a brief one today. The above picture, from Assam Plants.com, shows leaves of the dighloti, Litsea salicifolia, a tree of the laurel family Lauraceae found in sparse valley forests of southern Asia from India to Vietnam (Li et al. 2008). Litsea salicifolia is fairly variable in appearance, but general characters include long elliptic, alternate, leathery and evergreen leaves, glabrous branchlets and petioles, 4-6-flowered umbels and small oblong fruit.

Dighloti is cultivated in Assam, with two main uses. It is supposed to be a mosquito repellent, and this has been corroborated by laboratory studies (Phukan & Kalita 2005). It is also used as a secondary food source for the muga silkworm Antheraea assamensis (Bindroo et al. 2006). Muga silk, one of the so-called 'wild silks' produced from caterpillars other than the domesticated silkworm Bombyx mori, is raised primarily on two other Lauraceae species, the som Persea bombycina and the soalu Litsea polyantha. However, dighloti bushes are also grown in som plantations as a supplementary food for young or under-developed silkworms. Muga silk has a naturally golden-brown coloration (photo below from here).

REFERENCES

Bindroo, B. B., N. T. Singh, A. K. Sahu & R. Chakravorty. 2006. Muga silkworm host plants. Indian Silk, April 2006: 13-17.

Li X., Li J., Huang P., Wei F., Cui H. & H. van der Werff. 2008. Lauraceae. In: Flora of China Editorial Committee (eds) Flora of China vol. 7. Menispermaceae through Capparaceae. Science Press: Beijing, and Missouri Botanical Garden Press: St. Louis.

Phukan, S., & M. C. Kalita. 2005. Phytopesticidal and repellent efficacy of Litsea salicifolia (Lauraceae) against Aedes aegypti and Culex quinquefasciatus. Indian Journal of Experimental Biology 43: 472-474.