Dream-fish, Coelacanths and Super-Predators: The Sarcopterygians

For the subject of today's post, I drew the Sarcopterygii, the 'lobe-finned fishes'. Though something of a poor relation to their considerably more diverse sister-group, the ray-finned fishes of the Actinopterygii, this is a group most of my readers will have probably encountered in some capacity. As their names both formal and vernacular indicate, the Sarcopterygii were originally characterised by the development of the fins as fleshy lobes, with at least some fins possessing an internal skeleton of serial bones. Living sarcopterygians belong to three major groups, the coelacanths, lungfishes and tetrapods (in which, of course, the ancestral fins have been modified into walking limbs). The majority of recent studies have placed the coelacanths as the most divergent of these groups, with lungfishes and tetrapods as sister taxa. As the tetrapods are a particularly tedious group of organisms, with little to interest the casual observer, I'll put them aside for this post (you can go to Tetrapod Zoology if you must). The lungfishes, too, warrant a more detailed look at another time.

The oldest known sarcopterygian (and, indeed, the oldest known crown-group bony fish) is the Guiyu oneiros (shown above in a reconstruction by Brian Choo for Zhu et al. 2009), whose species name suggests the vernacular name of 'dream fish'. The dream-fish is known from the late Silurian of China, with a number of other stem-sarcopterygians such as Psarolepis and Meemannia known from the early Devonian of the same region. These taxa retained a number of ancestral features such as heavy ganoid scales (a type of scale also found in basal actinopterygians), and strong spines in front of the fins. However, crown-group sarcopterygians had also evolved and diverged by the early Devonian, as shown by the presence of the stem-lungfish Youngolepis.

Congregation of West Indian Ocean coelacanths Latimeria chalumnae, photographed by Hans Fricke.

The coelacanths are, of course, best known to most people for the discovery of the living Latimeria chalumnae in 1938 in South Africa, after the lineage had been thought to have become extinct in the Cretaceous. The subsequent media frenzy must have been interesting to fishermen in the area who had long known the coelacanth primarily as an infernal nuisance. Though only captured occasionally as bycatch, a landed coelacanth represents two metres or more of snap-jawed bad temper, while the oily flesh is inedible. More recently, a second species of living coelacanth, Latimeria menadoensis has been described from near Sulawesi in Indonesia.

Because of the circumstances of its discovery, Latimeria became a textbook example of a 'living fossil'. However, all fossil coelacanths were not mere duplicates of Latimeria. To begin with, Latimeria is quite a bit larger than the majority of its fossil relatives (Casane & Laurenti 2013). These included such distinctive forms as the fork-tailed speedster Rebellatrix and the eel-like Holopterygius. And then there was Allenypterus montanus, a Carboniferous taxon that... well, just look at the thing (photo from here):

Though Latimeria may lord it over its immediate relatives, it is far from the largest sarcopterygian (even excluding the tetrapods). The tetrapod stem-group also included a number of large predators, including the famous Eusthenopteron (how many other fossil fish have been referred to by name in an episode of Doraemon?). Particularly dramatic were the Rhizodontida, freshwater ambush predators of the Devonian and Carboniferous. Though probably very low on the tetrapod stem (and hence not directly related to limbed tetrapods), rhizodontids developed enlarged pectoral fins that articulated with the body in a not dissimilar manner to tetrapod forelegs. Like tetrapods, rhizodontids probably used their pectoral fins to push against the substrate and provide explosive propulsion (Davis et al. 2004). The jaw of rhizodontids contained enlarged tusks interspersed among smaller teeth that would have hooked into struggling prey. The largest rhizodontids have been estimated to be about seven metres in length, and were the sort of predator that the term 'apex' was invented for.

Reconstruction of Rhizodus by Mike Coates.

REFERENCES

Casane, D., & P. Laurenti. 2013. Why coelacanths are not 'living fossils'. BioEssays 35: 332-338.

Davis, M. C., N. Shubin & E. B. Daeschler. 2004. A new specimen of Sauripterus taylori (Sarcopterygii, Osteichthyes) from the Famennian Catskill Formation of North America. Journal of Vertebrate Paleontology 24 (1): 26-40.

Zhu, M., W. Zhao, L. Jia, J. Lu, T. Qiao & Q. Qu. 2009. The oldest articulated osteichthyan reveals mosaic gnathostome characters. Nature 458: 469-474.

The Perils of Lamellorthoceras in the Land of Taphonomy

Exfoliated specimen of Lamellorthoceras gracile, from Sweet (1964). The outer shell has been lost.

The title of today's post, offhand, is a hideously contrived allusion to something that I suspect many (most?) of you will not recognise. Those of you that do recognise it, possibly wish that you didn't. Nevertheless, I'll leave it to each of you to decide for yourself whether or not this post would have been improved by the inclusion of kabuki-inspired haute couture, or chariots pulled by topless busty Amazons in lieu of horses.

Lamellorthoceras, to introduce the star of today's post, is a genus of straight-shelled cephalopods from the Lower and Middle Devonian of northern Africa. It was not a large cephalopod. Like most straight-shelled Palaeozoic cephalopods, fossils of Lamellorthoceras represent pieces of the original shell rather than the entire thing (making judging its size when alive a bit tricky), but even with a generous estimate I don't think we're talking about anything more than a few centimetres long. Lamellorthoceras forms the core of a small, mostly Devonian family, the Lamellorthoceratidae, distinguished by a very interesting feature. Like other cephalopods, the shell of lamellorthoceratids was divided into a series of chambers, with a fleshy siphuncle presumably running the length of the shell. In most other cephalopods, the chambers around the siphuncle were more or less hollow, filled with gas to give the shell buoyancy. In fossils of lamellorthoceratids, however, the chambers are filled with thin lamellae arranged in a radial pattern between the shell and the siphuncle. This is so unusual compared to other cephalopods that the lamellorthoceratid Arthrophyllum was initially described as a type of coral! Genera of lamellorthoceratids have been distinguished based on the overall shape of the shell, and by the structure of the lamellae. Arthrophyllum, for instance, has simple straight lamellae in transverse section, while the lamellae of Lamellorthoceras are wavy and/or bifurcating.

Cross-section of Lamellorthoceras vermiculare, from Sweet (1964), showing the radiating lamellae.

In a previous post on this site, I discussed some of the implications of such structures, called cameral deposits, for the soft anatomy of fossil cephalopods. If we were to assume that all fossil cephalopods had much the same anatomy as our only real living model, the pearly nautiluses of the Nautilidae, then cameral deposits present us with a real problem. In Nautilus, the siphuncle is sealed away from each chamber by a structure called the connecting ring, and the walls of the chambers are devoid of living tissue. The siphuncle serves to control the buoyancy of the shell by controlling the ratio of fluid to gas in the chambers, but this fluid is only secreted or absorbed via pores in the connecting rings. The only part of the nautilus shell where mineral deposits are being actively laid down is in the anterior body chamber where the living animal is housed. For fossil cephalopods to have been laying down mineral deposits within the chambers behind the body chamber, there would have had to have been outgrowths of the mantle still present in the chambers. The siphuncle could not have been an isolated unit the way it is in Nautilus. Unfortunately, the connecting rings of nautilids are delicate structures that do not preserve easily as fossils, so seeing whether they were present in lamellorthoceratids is not as simple as just looking for them. Nevertheless, Kolebaba (1999) claimed after close examination of the Upper Silurian Nucleoceras that the connecting rings of lamellorthoceratids were at least open dorsally.

However, some researchers (e.g. Mutvei 2002) hold a quite different interpretation of what the cameral deposits meant for the living animal: absolutely nothing. Perhaps they were not a feature of the living cephalopod at all, but represent sediment build-up in empty shells after the animal's death. This would have interesting implications for the lamellorthoceratids, if their primary claim to fame was a taphonomic illusion! Evidence for the inorganic origin of the cameral deposits cited by Mutvei (2002) include their different chemical make-up from the main shell, often more similar to the surrounding matrix, and specimens preserved flattened in shales with no sign of cameral deposits. However, cameral deposits are not laid down haphazardly within a shell as one might expect if they were post-mortem artefacts, but more or less consistently between specimens. Deposits growing out from opposing chamber walls and septa do not merge seemlessly, but remain separated by breaks in the deposits ('pseudosepta') that may represent tissue membranes. Flattened shale specimens may indicate an original absence of cameral deposits, or they may represent preferential dissolution of the cameral deposits under those preservation conditions. It is also possible that cameral deposits present during life may have provided nuclei for further sediment deposition after death.

Reconstruction of a sectioned chamber of the lamellorthoceratid Esopoceras sinuosum, showing the internal arrangement of lamellae, from Stanley & Teichert (1976). Esopoceras had more strongly sinuous lamellae than Lamellorthoceras.

Needless to say, our views on the presence in life of cameral deposits could also have strong implications for our understanding of these animal's lifestyles. If the intra-cameral lamellae of Lamellorthoceras were present in life, the shell would have held little, if any, space for buoyant gas. As such, it probably would not have had the swimming abilities of modern cephalopods; instead, it may have had a more benthic lifestyle.

REFERENCES

Kolebaba, I. 1999. Sipho-cameral structures in some silurian cephalopods from the Barrandian area (Bohemia). Acta Musei Nationalis Pragae, Series B, Historia Naturalis 55 (1-2): 1-16.

Mutvei, H. 2002. Connecting ring structure and its significance for classification of the orthoceratid cephalopods. Acta Palaeontologica Polonica 47 (1): 157–168.

Stanley, G. D., Jr & C. Teichert. 1976. Lamellorthoceratids (Cephalopoda, Orthoceratoidea) from the Lower Devonian of New York. The University of Kansas Paleontological Contributions 86: 1-14, 2 pls.

Sweet, W. C. 1964. Nautiloidea – Orthocerida. In Treatise on Invertebrate Paleontology pt. K. Mollusca 3. Cephalopoda – General Features – Endoceratoidea – Actinoceratoidea – Nautiloidea – Bactritoidea (R. C. Moore, ed.) pp. K216-K261. The Geological Society of America and the University of Kansas Press.

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.