Paramesochra acutata

Copepod taxonomy, it seems, is largely about counting setae. In his review of relationships within the interstitial harpacticoid family Paramesochridae, Huys (1987) recognised four species groups within the genus Paramesochra (which previously got a look-in at this site here). One of these groups, labelled the P. acutata-group, was characterised by reductions in numbers of setae, having lost the inner setae on the first segments of the endopods on the third and fourth legs.

Paramesochra taeana, a close relative of P. acutata, from Back & Lee (2010).

The group takes its name from the species Paramesochra acutata, described by Klie in 1935 from samples taken from coastal groundwater near the town of Schilksee on the northeastern coast of Germany, in the state of Schleswig-Holstein. Other notable features of P. acutata include the presence of four setae on the antennary exopod, well-developed narrow, triangular endopodal lobes on the modified fifth legs of the females, and conical caudal rami produced into spinose processes (Back & Lee 2013). I haven't been able to find whether P. acutata has been collected much beyond its initial locality but other members of its species group have been found around the world. One of these, P. hawaiensis, from (nach) Hawaii, is similar enough that it was until recently treated as a subspecies of P. acutata.

Appendages of female Paramesochra acutata, from Klie (1935).

So what, if anything, does all this mean? That, I'm afraid, is getting a bit beyond me. The fifth legs are used in spermatophore transfer and differences between species might presumably function in recognising suitable mates. Regarding the details of setation and ramus appearance, one wonders if there could be any relation to preferred micro-habitat. Are harpacticoids with fewer setae and more robust rami adapted for crawling among coarser sand grains? Honestly, I have no idea. Anyone care to find out?

REFERENCES

Back, J., & W. Lee. 2010. A new species of the genus Paramesochra (Copepoda: Harpacticoida) from Korean waters. Proceedings of the Biological Society of Washington 123 (1): 47–61.

Back, J., & W. Lee. 2013. Three new species of the genus Paramesochra T. Scott, 1892 (Copepoda: Harpacticoida: Paramesochridae) from Yellow Sea, Korea with a redescription of Paramesochra similis Kunz, 1936. Journal of Natural History 47 (5–12): 769–803.

Huys, R. 1987. Paramesochra T. Scott, 1892 (Copepoda, Harpacticoida): a revised key, including a new species from the SW Dutch coast and some remarks on the phylogeny of the Paramesochridae. Hydrobiologia 144: 193–210.

Klie, W. 1935. Die Harpacticoiden des Küstengrundwassers bei Schilksee (Kieler Förde). Schriften des Naturwissenschaftlichen Vereins für Schleswig-Holstein 20 (2): 409–421.

Kirkby's Small Ostracods (or Small Kirkby's Ostracods)

I do not envy those who find themselves working with ostracods. These minute crustaceans, typically less than a millimetre in length, seem altogether too fiddly to handle. Nevertheless, the long history of ostracods, together with their diversity and the high fossilisation potential of their calcified carapace valves, have made them a common focus for studying biostratigraphy and historical environments. The classification of modern ostracods is commonly informed by features of the legs and other appendages but such characters are not commonly preserved in fossil representatives. As a result, there are many groups of ostracods known from the Palaeozoic whose relationships remain uncertain.

Left valve of Kirkbyella delicata, from Hoare & Merrill (2004).

One such group is classified by Liebau (2005) as the superfamily Kirkbyelloidea. Members of this group are small ostracods with reticulate valves. The dorsal and ventral margins of the valves tend to be more or less straight. They are commonly impressed with a single dorsal sulcus, extending downwards from the dorsal margin about halfway along the valve's length. Below this sulcus is a protruding horizontal lobe ending in members of the family Kirkbyellidae in a small spine. Evidence of sexual dimorphism, a not-uncommon feature of Palaeozoic ostracods, is not known from kirkbyelloids.

Definite kirkbyelloids are known from the Devonian to the Permian. If the earlier family Ordovizonidae is included, their record extends all the way back to the Ordovician. As noted above, it is unclear where kirkbyelloids sit in the ostracod family tree. Becker (1994) suggested a relationship via Ordovizona to the Ordovician Monotiopleuridae which resemble kirkbyelloids in the outline of the carapace valves and features of the adductor muscle scars. Though long-lived, kirkbyelloids don't seem to have ever been massively diverse, and they can probably be counted among the many lineages of organisms that never made it past the end of the Palaeozoic.

REFERENCES

Becker, G. 1994. A remarkable Ordovician ostracod fauna from Orphan Knoll, Labrador Sea. Scripta Geologica 107: 1–25.

Hoare, R. D., & G. K. Merrill. 2004. A Pennsylvanian (Morrowan) ostracode fauna from Texas. Journal of Paleontology 78 (1): 185–204.

Liebau, A. 2005. A revised classification of the higher taxa of the Ostracoda (Crustacea). Hydrobiologia 538: 115–137.

The Diosaccinae: Worldwide Sediment Dwellers

The harpacticoid copepods have been featured on this site a reasonable number of times now. These tiny crustaceans are among the most numerous animals in the world, both in terms of numbers of individuals and (in certain habitats) numbers of species. And among the most widespread representatives of the harpacticoids are members of the subfamily Diosaccinae.

Diosaccus tenuicornis, from Sars (1906).

The Diosaccinae are currently recognised as members of the family Miraciidae; earlier sources will usually refer to a family Diosaccidae but the recognition of the pelagic Miraciinae as derived members of this group (Willen 2000) requires use of the older name. Distinctive features of the Miraciidae compared to other harpacticoids include the presence of a relatively large, mobile rostrum and a number of distinctive arrangements of setae, including the inner seta on the basal endopodal segment of the first peraeopod (trunk leg) arising distally (Nicholls 1941, Willen 2000). Miraciids are also unusual in that females carry paired egg-sacs laterally; most other harpacticoid families carry only a single median egg-sac. Miraciids are divided between three subfamilies of which the Diosaccinae are the most diverse. Diosaccines are most readily distinguished by their retention of a number of plesiomorphic features such as crawling legs and relatively short caudal rami (Nicholls 1941; this author divided the current diosaccines between two subfamilies, the Diosaccinae sensu stricto and Amphiascinae, based on the presence or absence, respectively, of a clear distinction in breadth between the metasome and urosome, or 'trunk' and 'abdomen', but this division does not appear to have been recognised at this level by any subsequent authors). The great majority of diosaccines are marine, free-living and benthic. A handful of species have been described as associates of lobsters, whether commensals or semi-parasites. A small radiation of species of the genus Schizopera is known from Lake Tanganyika, and Karanovic & Reddy (2004) described a species Neomiscegenus indicus from subterranean fresh water in India. Marine diosaccines are found at all depths from the intertidal zone to the deep abyss. I don't know for sure but, though they are sediment dwellers, I don't get the impression (I could be wrong) that they are strictly meiofaunal. As noted earlier, many do not have the vermiform body shape characteristic of interstitial copepods. Many species also are around the half-millimetre size range, which I think may be relatively large for meiofauna?

Four species of Schizopera collected from Korea, from Karanovic & Cho (2016). Left to right: S. yeonghaensis, S. daejinensis, S. gangneungensis, S. sindoensis.

The other two subfamilies of Miraciidae are the aforementioned Miraciinae and the Stenheliinae, which have the endopod of the first peraeopod adapted for swimming rather than grasping and longer caudal rami. Though potential synapomorphies of the Diosaccinae were identified by Willen (2000), they're a bit weaksauce. There is a distinct possibility that further studies may identify the diosaccines as paraphyletic to the other two subfamilies. In particular, some diosaccines say a very unusual form of nauplius larva with the Stenheliinae, in which the body is strongly foreshortened and crab-like (Dahms et al. 2005). These nauplii also move sideways in a crab-like fashion and do not swim in the water column like the nauplii of other species. Practical considerations have lead most investigators of crustacean phylogeny to emphasis adult over larval morphology but the larval morphology of diosaccines raises some interesting questions.

REFERENCES

Karanovic, T., & Y. R. Reddy. 2004. A new genus and species of the family Diosaccidae (Copepoda: Harpacticoida) from the groundwaters of India. Journal of Crustacean Biology 24 (2): 246–260.

Nicholls, A. G. 1941. A revision of the families Diosaccidae Sars, 1906 and Laophontidae T. Scott, 1905 (Copepoda, Harpacticoida). Records of the South Australian Museum 7 (1): 65–110.

Willen, E. 2000. Phylogeny of the Thalestridimorpha Lang, 1944 (Crustacea, Copepoda). Cuvillier Verlag: Göttingen.

Sea Spiders

With arthropods being such a massively diverse sector of the global biota (and even that feels like an understatement; describing arthropods as 'very diverse' seems a bit like describing the Andromeda Galaxy as 'very far away'), it is only to be expected that it contains some very weird corners. And definitely among the weirder of those corners are the Pycnogonida, commonly known as the 'sea spiders'.

Anoplodactylus evansi, copyright Mick Harris & Claudia Arango.

Pycnogonids are a group of marine arthropods found around the world (not actual spiders, of course, though honest-to-goodness marine spiders are a thing that does exist). Their relationships to other arthropods have long been in dispute but the majority view is that they are distant relatives of the terrestrial arachnids. Pycnogonids are not uncommon in both coastal and deep-sea habitats but tend to go unnoticed: they feed on rock-encrusting colonial animals such as hydrozoans and are often coloured to disguise themselves against their prey. If one ever does see a sea spider, the first thing to stand out about them is how they are made of legs. The central body is often remarkably small compared to its limbs, to the extent that the dubbing of pycnogonids as 'no-bodies' by an early 20^th Century author has become something of a cliché. Certain major organs, such as the gonads and parts of the digestive system, have even been diverted into the legs to make up for the lack of space in the body. Most pycnogonids possess four pairs of walking legs though there are species with more. At the front of the body on the underside of the head is a large proboscis that is used for sucking the juices out of prey, flanked by pairs of pincer-bearing chelifores and/or palps used for tearing it open. Near the first pair of walking legs there is often a pair of slender leg-like appendages known as the ovigers, used for carrying bundles of eggs until they hatch. The greater part of the body behind the head is taken up by the leg-bearing thorax; the legless abdomen is reduced to the merest nub like the docked tail of a dog.

Close-up on preserved male Anoplodactylus lentus, from Florida Museum of Natural History.

One of the largest recognised genera of pycnogonids is Anoplodactylus, with over 130 species worldwide and many continuing to be described (Lucena et al. 2015). This genus can be distinguished by the possession of chelifores with functional chelae (pincers) but palps are absent or reduced to buds. Both the chelifores and the proboscis are relatively short (Child 1998). Ovigers are five- or six-segmented and present in males only (male care of eggs is the standard pattern among pycnogonids). Species vary from 0.6 to 6 millimetres in body length. The majority of species of Anoplodactylus are found in shallow waters in temperate and tropical regions with a smaller number of species found in polar and deep waters. Alvarez & Ojeda (2018) record finding a single specimen of the species A. batangensis among vegetation on the surface of an anchialine pool in the Yucatan Peninsula of Mexico. Though the surface of these pools is more or less fresh water, deeper sections are saline owing to subterranean connections to the sea. The collection of a pycnogonid near the surface of this pool suggests an ability to adjust to very low salinity though one questions whether it would be able to survive indefinitely.

Larvae of Anoplodactylus are very small compared to those of other pycnogonids and have what has been termed an 'encysting' development (Burris 2011). As bizarre as the appearance of adult pycnogonids is, their larvae are arguably even weirder, being essentially nothing more than a head bearing chelifores, proboscis, and two pairs of undifferentiated appendages. The remaining segments of the body are added over the course of development. In Anoplodactylus, the larvae develop as parasites, forming a cyst in the gastrocoel (the stomach cavity) of cnidarians (having presumably been placed there somehow by their fathers, though I haven't found if we know how). They become free-living upon reaching the first juvenile stage, emerging from their host to pursue their predatory lives.

REFERENCES

Alvarez, F., & M. Ojeda. 2018. First record of a sea spider (Pycnogonida) from an anchialine habitat. Latin American Journal of Aquatic Research 46 (1): 219–224.

Burris, Z. P. 2011. Larval morphologies and potential developmental modes of eight sea spider species (Arthropoda: Pycnogonida) from the southern Oregon coast. Journal of the Marine Biological Association of the United Kingdom 91 (4): 845–855.

Child, C. A. 1998. The Marine Fauna of New Zealand: Pycnogonida (Sea Spiders). National Institute of Water and Atmospheric Research (NIWA).

Lucena, R. A., J. F. de Araújo & M. L. Christoffersen. 2015. A new species of Anoplodactylus (Pycnogonida: Phoxichilidiidae) from Brazil, with a case of gynandromorphism in Anoplodactylus eroticus Stock, 1968. Zootaxa 4000 (4): 428–444.

The Concilitergans: Sitting Next to Trilobites

The last few decades have seen a vast increase in our understanding of life during the early Cambrian. Long one of the most famous groups of invertebrates of the Palaeozoic, the trilobites are now known to have shared their early environment with a number of related lineages that bore some resemblance in overall appearance but lacked their mineralisation of the exoskeleton. One such group was labelled by Hou & Bergström (1997) as the Conciliterga.

Reconstruction of the concilitergan Kuamaia lata from Hou & Bergström (1997). Note that the reconstructed appearance of the eyes is probably erroneous, as explained below.

Concilitergans are a group of flattened marine arthropods known from the early and middle Cambrian of a number of parts of the world, including North America, China and Australia. Most species were ovoid in shape (like a typical trilobite), tapering somewhat towards the rear and often ending in a point. An Australian species, Australimicola spriggi, was more elongate in form and ended in a pair of terminal spines. Some were quite sizable; one species, Tegopelte gigas, reached nearly a foot in length and was one of the largest known animals of its time. Concilitergans also resembled trilobites in possessing a more or less semi-circular head shield followed by a series of regular segments and often a final larger pygidial segment. Towards the front of the body, the segment boundaries were anteriorly reflexed (Paterson et al. 2012). In a number of species, the body segmentation was more prominent medially than laterally with the tergites overlapping slightly down the mid-line but not along the edges. A pair of antennae arose from the underside of the head near the front. In most species, with the exception of Australimicola, a pair of prominent teardrop-shaped bulges was also present dorsally near the front of the head. These bulges were interpreted as a pair of dorsal eyes by Hou & Bergström (1997) but re-interpreted by Edgecombe & Ramsköld (1999) as raised areas of the exoskeleton that provided accomodation for the actual eyes located on the underside of the head.

Reconstruction of Tegopelte gigas, copyright Marianne Collins.

Phylogenetic analyses have confirmed a close relationship between concilitergans and trilobites (Edgecombe & Ramsköld 1999) and the two groups probably resembled each other in life-style. With their ovoid shape, flattened body and down-cast eyes, concilitergans were also not dissimilar in overall conformation to modern cockroaches and a comparison is tempting. Study of trackways attributed to Tegopelte, owing to their size and structure, indicated that it mostly walked with a slow, low gait but was also capable of adopting a higher, faster gait for quickly skimming across the sediment surface (Minter et al. 2012). It should be noted that while news reports on the latter study (like this one) repeatedly refer to Tegopelte as a predator, the original paper consistently describes it as a "predator or scavenger". One can imagine concilitergans crawling along the sea-bed, picking up fragments of organic matter and scavenging on the remains of the less fortunate. Eventually, though, their lack of armament compared to their longer-surviving allies might have been their downfall as they were less prepared to deal with the diversification of active predators as the Cambrian progressed.

REFERENCES

Edgecombe, G. D., & L. Ramsköld. 1999. Relationships of Cambrian Arachnata and the systematic position of Trilobita. Journal of Paleontology 73 (2): 263–287.

Hou X. & J. Bergström. 1997. Arthropods of the Lower Cambrian Chengjiang fauna, southwest China. Fossils and Strata 45: 1–116.

Minter, N. J., M. G. Mángano & J.-B. Caron. 2012. Skimming the surface with Burgess Shale arthropod locomotion. Proceedings of the Royal Society of London Series B—Biological Sciences 279: 1613–1620.

Paterson, J. R., D. C. García-Bellido & G. D. Edgecombe. 2012. New artiopodan arthropods from the Early Cambrian Emu Bay Shale Konservat-Lagerstätte of South Australia. Journal of Paleontology 86 (2): 340–357.

The Splanchnotrophidae: Comfy inside a Sea Slug

In previous posts, I've referred to the great significance of the minute crustaceans known as copepods to aquatic ecosystems. At the time, I was referring to free-living members of this group but the copepods also include a wide range of parasitic forms. Some of these parasitic copepods have evolved into forms so derived and bizarre that they are barely recognisable as crustaceans. One example of this is the family Splanchnotrophidae.

Sea slug Janolus fuscus with protruding egg sacs of a splanchnotrophid copepod, probably Ismaila belciki, copyright Michael D. Miller.

Splanchnotrophids are a group of copepods endoparasitic on two orders of shell-less marine gastropods (sea slugs), the Nudibranchia and Sacoglossa. They are characterised by reduced mouthparts and appendages though they retain a distinct pair of claw-like antennae. These antennae seem to be used to hold the copepod in place in their preferred location within the body cavity of their host. Though the exact means of feeding by splanchnotrophids is not certain, their rudimentary mouthparts, combined with a rarity of observations of actual tissue damage in parasitised hosts, indicate that they probably suck nutriment from their host's haemolymph. Females and males live in association within the host, the minute (and slightly more recognisably copepod-y) males holding close to their comparatively gigantic mates. As well as their size, female splanchnotrophids differ from males in the possession of elongate, tubular dorsal outgrowths of the thorax. These are most commonly presumed to function to provide more space for the female's enlarged ovaries, though some have suggested additional functions such as maintaining position within the host, respiration or absorbing nutrients (Anton & Schrödl 2013). The female's tubular egg-sacs extend through an opening in the host's body wall to release eggs into the water column. Usually, these egg-sacs will emerge close to some outgrowth of the host's own body, such as gills or papillae, and may be coiled if relatively long; these measures presumably help protect the egg-sacs from external damage. How the released larvae find and colonise new hosts remains unknown but it is possible the antennules (the smaller second pair of antennae possessed by most crustaceans) are used to locate hosts chemically, with their reduced condition in adults the result of a halt to development once their purpose has been fulfilled.

Female (left) and male Ismaila aliena dissected out from host, from Anton & Schrödl (2013).

Relatively few splanchnotrophids have been recognised to date, maybe about a dozen species divided between five genera. A few other species that had earlier been included in the family on little more grounds than that they were endoparasites of gastropods were excluded by Huys (2001)*. A sixth genus and species Chondrocarpus reticulosus is of uncertain relationships. If correctly associated with the splanchnotrophids, it is of interest in parasitising a different group of sea slugs (the pleurobranchids) and in its massive size (growing to twelve millimetres vs only a few millimetres for females of the other genera), but the only available description is inadequate for its proper characterisation. In some localities, splanchnotrophids have proven to be surprisingly abundant. A once-off survey of potential host species in Oregon found no less than 62% of individuals of one species to be infected (25 other potential host species were completely free of parasites), whereas a longer-term survey off the coast of Chile found an overall infection rate of 13% with some particular host species approaching 100% infection (Schrödl 2002). Host specificity seems to vary within the family: a study by Anton et al. (2018) found that species of the genus Ismaila tended to restrict themselves to a single host species, whereas species of Splanchnotrophus are more catholic and undiscriminating. Nevertheless, a lack of correlation between relationships of splanchnotrophid species and those of their host species suggests that, even in the more discriminating Ismaila, host changes may not have been uncommon.

*As a concise indication of just how sloppy some of the earlier work on 'splanchnotrophids' had been, one misattributed species was re-identified by Huys (2001) as having been based on the detached head of a pelagic amphipod.

The broader relationships of splanchnotrophids within copepods also remain poorly understood. A phylogenetic study by Anton & Schrödl (2013) suggested that Splanchnotrophidae may form a clade with another genus of copepods endoparasitic in gastropods, Briarella, with this clade being in turn derived from ectoparasitic ancestors. However, by the authors' own admission, this study was heavily biased in both taxon and character coverage to the Splanchnotrophidae, and may have been affected by insufficient scrutiny of non-splanchnotrophid taxa. Though derivation of the endoparasitic splanchnotrophids from ectoparasitic ancestors has a definite intuitive appeal, further study is required before we can feel confident about it.

REFERENCES

Anton, R. F., D. Schories, N. G. Wilson, M. Wolf, M. Abad & M. Schrödl. 2018. Host specificity versus plasticity: testing the morphology-based taxonomy of the endoparasitic copepod family Splanchnotrophidae with COI barcoding. Journal of the Marine Biological Association of the United Kingdom 98 (2): 231–243.

Anton, R. F., & M. Schrödl. 2013. The gastropod-crustacean connection: towards the phylogeny and evolution of the parasitic copepod family Splanchnotrophidae. Zoological Journal of the Linnean Society 167: 501–530.

Huys, R. 2001. Splanchnotropid systematics: a case of polyphyly and taxonomic myopia. Journal of Crustacean Biology 21 (1): 106–156.

Schrödl, M. 2002. Heavy infestation by endoparasitic copepod crustaceans (Poecilostomatoida: Splanchnotrophidae) in Chilean opisthobranch gastropods, with aspects of splanchnotrophid evolution. Organisms, Diversity & Evolution 2: 19–26.

The Origin of Hexapods

Insects have been described as the most evolutionarily successful group of animals in the modern world, and with good reason. Something like two-thirds of the currently known animal species are insects, and they are near-ubiquitous in the terrestrial and freshwater environments (for whatever reasons, they've never made that much of a go of it marine-wise). Nevertheless, the questions of how and when insects first came to be remains very much an open one.

The long-necked fungus beetle Diatelium wallacei, one of the countless weird oddballs in the insect world. Copyright Artour Anker.

Insects are usually recognised as including three main subgroups: the winged insects, silverfish and bristletails. They are readily united into a group known as the hexapods with a few less speciose assemblages: the springtails, the proturans and the diplurans. All living hexapods have the body divided into a head, thorax and abdomen, with three pairs of walking legs on the thorax and none on the abdomen. Though monophyly of the hexapods has been questioned in the past (which is why the springtails and the like are usually excluded from our concept of 'insect' these days despite having been included previously), the majority view is now firmly in favour of regarding them as a single, coherent lineage. How hexapods are related to other arthropods has been more vigorously debated. Earlier authors commonly associated them with the myriapods, the lineage including centipedes and millipedes. In more recent years, an increasing number of studies have instead associated insects with crustaceans. This realignment has primarily been pushed by molecular studies but there are also a number of interesting morphological features such as eye and brain structure that are more crustacean- than myriapod-like in insects. Indeed, it seems not unlikely that insects are not merely related to but are nested within crustaceans: for instance, a few recent studies have supported a relationship between hexapods and a rare group of crustaceans known as remipedes (Schwentner et al. 2017). The features previously seen as shared between insects and myriapods, such as tracheae and uniramous (unbranched) limbs, are then held to probably be convergent adaptations to a terrestrial lifestyle.

Whatever its relationships, it seems most likely that the immediate ancestor of the living hexapods was indeed terrestrial. Of the six basal hexapod lineages referred to above, five (all except winged insects) are almost exclusively terrestrial and were probably ancestrally so. The winged insects include a number of basal subgroups (such as mayflies and dragonflies) that are aquatic for at least the early part of their life cycle, but a terrestrial origin for winged insects as a whole remains credible.

Head of Rhyniella praecursor, from Dunlop & Garwood (2017).

From the perspective of the fossil record, the evidence related to hexapod origins is incredibly slight. The earliest fossil species that have been directly proposed as hexapod relatives are known from the Early Devonian and less than half a dozen such species have been mooted as such in recent years. The only named Devonian fossil whose status as a hexapod seems unimpeachable is Rhyniella praecursor, a springtail from the Rhynie chert of Scotland (Dunlop & Garwood 2017). The same deposit provided Rhyniognatha hirsti, a fragmentary fossil comprising a pair of mandibles and surrounding parts of the head capsule. Rhyniognatha has long been thought to be an insect, possibly even an early member of the winged insect lineage, but Haug & Haug (2017) recently argued that it could just as easily be the head of a centipede (a group already known from other fossils in the Rhynie chert).

Rhyniognatha hirsti, from the University of Aberdeen. Scale bar = 200 µm; m = mandible.

The Windyfield chert, a deposit of similar age and location to the Rhynie chert, has provided Leverhulmia mariae, originally described as a myriapod but reinterpreted as a hexapod relative by Fayers & Trewin (2005). Leverhulmia is a difficult beast to know what and how much to make of it. The original specimen is, speaking charitably, a bit of a mess: a flattened smear looking a bit like a sausage burst open after cooking for too long on the pan. The front and back ends of the animal both appear to be missing and the only features really distinguishable are a series of small jointed legs. Other specimens associated with this species by Fayers & Trewin (2005) are simply more legs detached from their original body. These legs, though, do preserve a reasonable amount of detail, including the presence of paired lateral claws at the ends of the tarsi like those of most insects (Leverhulmia also possesses a smaller median claw between the lateral claws, a feature not found in winged insects but present in silverfish and bristletails). In contrast, the legs of myriapods (as well as those of springtails and proturans) end in a single terminal claw.

Holotype specimen of Leverhulmia mariae, from Dunlop & Garwood (2017); the size of the scale bar was not specified but the entire specimen is about 12 mm long.

The overall appearance of Leverhulmia's legs might therefore be seen a suggestive of a relationship specifically to insects and not just to hexapods in general, but their number provides something of a barrier to accepting Leverhulmia as a bona fide insect. The train-wreck nature of Leverhulmia's preservation means we can't state confidently how many legs it had but there were at least five pairs: a couple more than the hexapods' standard-issue three. A number of structures on the abdomens of some living hexapods are potentially derived from modified legs, such as the springing furca of springtails and the ventral styli in hexapods other than springtails and winged insects, so some parallelism in appendage reduction is not out of the question. Nevertheless, unjointed styli are one thing; fully-jointed, functional walking legs are another. Supposed early members of the bristletail and silverfish lineages with jointed abdominal legs have been described from the Carboniferous by Kukalová-Peck (1987) but (as I've noted before) many of the more outlandish reconstructions of early insects by Kukalová-Peck have failed to stand up to subsequent scrutiny.

Similar interpretative difficulties surround Strudiella devonica, described as an early relative of the winged insects from the Late Devonian of Belgium. Though I was not unfavourable to this specimen when it was first described, Hörnschemeyer et al. (2013) would later argue against recognising it as an insect. The latter authors professed to be simply unable to discern many of the features cited by its original describers as evidence of insect affinity, and saw Strudiella as closer to a Rorschach blot than a dragonfly. Strudiella's status was defended by its original authors (Garrouste et al. 2013) but a number of subsequent authors seem to have taken Hörnschemeyer et al.'s caution to heart.

Close-up of the head of Strudiella devonica from Hörnschemeyer et al. (2013); the asterisk marks the base of a structure originally interpreted as an antenna.

The final candidate for stem-hexapod status worthy of consideration here is Wingertshellicus backesi from the Lower Devonian Hunsrück Slate of Germany. This marine fossil was interpreted as a stem-hexapod under the name Devonohexapodus bocksbergensis, with a thorax bearing three pairs of legs and an elongate abdomen with uniramous appendages. However, it was reinterpreted by Kühl & Rust (2009) who synonymised Devonohexapodus with the previously described Wingertshellicus, regarded the previously described 'thoracic legs' as appendages of the head, and did not accept the presence of differentiated thorax and abdomen. The appendages of the trunk (previously seen as the abdomen) were biramous rather than uniramous with a small endopod and a large flap-like exopod adapted for swimming, and the end of the body bore a pair of fluke-like appendages (comparable to the tail of a crayfish). Wingertshellicus thus lacked any resemblance to a hexapod, and Kühl & Rust doubted that it even belonged to the crown group of arthropods.

Laterally preserved specimen of Wingertshellicus backesi, from Kühl & Rust (2009); scale bar = 10 mm.

An attempt to estimate the age of divergence of hexapods from other arthropods using a molecular clock analysis by Schwentner et al. (2017) suggested that hexapods and remipedes went their separate ways in the late Cambrian or early Ordovician. This is up to 100 million years earlier than the fossils described above but we should be careful how much to read into this discrepancy. If most of the features associated with hexapods are related to adoption of a terrestrial lifestyle, then it might be difficult to recognise any early marine relatives if found. Conversely, while it is uncertain how much if any terrestrial vegetation was present prior to the Devonian, the only potential cover would have been low lichens, non-vascular plants or micro-algae. If stem-hexapods emerged onto land during this time, the environment would not be conducive to their preservation in the fossil record. Finally, not only are hexapods other than winged insects not found in the fossil record before the Devonian, they are barely found after it: after Rhyniella, none are known until the appearance of amber-producing trees during the Cretaceous. So if we can't find any sign of them for some 300 milion years that we know that they are around, then we obviously can't say too much about not finding them over the previous hundred million years. The stem-hexapods may have been around in this time but they remain in hiding.

REFERENCES

Dunlop, J. A., & R. J. Garwood. 2017. Terrestrial invertebrates in the Rhynie chert ecosystem. Philosophical Transactions of the Royal Society of London Series B—Biological Sciences 373: 20160493.

Fayers, S. R., & N. H. Trewin. 2005. A hexapod from the Early Devonian Windyfield Chert, Rhynie, Scotland. Palaeontology 48 (5): 1117-1130.

Garrouste, R., G. Clément, P. Nel, M. S. Engel, P. Grandcolas, C. D'Haese, L. Lagebro, J. Denayer, P. Gueriau, P. Lafaite, S. Olive, C. Prestianni & A. Nel. 2013. Is Strudiella a Devonian insect? Garrouste et al. reply. Nature 494: E4–E5.

Haug, C., & J. T. Haug. 2017. The presumed oldest flying insect: more likely a myriapod? PeerJ 5: e3402.

Hörnschemeyer, T., J. T. Haug, O. Bethoux, R. G. Beutel, S. Charbonnier, T. A. Hegna, M. Koch, J. Rust, S. Wedmann, S. Bradler & R. Willmann. 2013. Is Strudiella a Devonian insect? Nature 494: E3–E4.

Kühl, G., & J. Rust. 2009. Devonohexapodus bocksbergensis is a synonym of Wingertshellicus backesi (Euarthropoda)—no evidence for marine hexapods living in the Devonian Hunsrück Sea. Organisms, Diversity & Evolution 9: 215–231.

Schwentner, M., D. J. Combosch, J. P. Nelson & G. Giribet. 2017. A phylogenomic solution to the origin of insects by resolving crustacean-hexapod relationships. Current Biology 27: 1818–1824.

Leptocaris: Living on the Edge

Some of the most remarkable faunal diversity in the marine environment is to be found in the interstitial spaces between grains of sand. Grazers, predators and scavengers can be found creating entire food webs at scales of less than one millimetre. The minute crustaceans known as copepods are among the more abundant inhabitants of the interstitial, and today's subject, Leptocaris, is among those interstitial copepods.

Dorsal habitus of female (left) and male Leptocaris ryukyuensis, from Song et al. (2012).

Leptocaris contains more than twenty-five species of extremely slender, cylindrical harpacticoid copepods growing to a bit over half a millimetre in length. Characteristic features of the genus include having the maxillipeds (one of the pairs of appendages making up the mouthparts) reduced or lost, and the proximal part of the endopod of the first swimming leg bearing a special anteriorly directed seta with a terminal comb (Song et al. 2012). Representatives of this genus have been collected from localities around the world though mostly in the Northern Hemisphere. Nevertheless, one can't help wondering how much of the genus' apparent rarity in the Southern Hemisphere is an artefact of low collection effort. This possibility should also be kept in mind when considering differences in the ranges of individual species: whereas many have only been collected from single localities (Song et al. 2012), the species L. trisetosus has been found from Finland to the Bahamas to South Africa, as well as in Korea with the last population being treated as a distinct subspecies (Lee & Chang 2008).

The majority of collections of Leptocaris have been from among sand but the genus has also been found in other microhabitats. In general, they are found in sediments with a high organic content. They are found in euryhaline and eurythermal habitats: that is, locations subject to wide variations in salinity and temperature. These may include beaches and brackish pools. They have been found among decomposing leaves in mangrove swamps (offhand, I haven't found if the diet of Leptocaris has been firmly established but I suspect they are probably detritivores). One species, L. kunzi, was described from an estuarine lake in Louisiana; another, L. stromatolicolus, is known from among stromatolites in Mexico. Two species, L. brevicornis and L. sibiricus, have even been found in continental fresh waters in Europe as well as in coastal brackish waters (Song et al. 2012). Overall, Leptocaris species seem to be most abundant in marginal habitats that may be too harsh and unstable for other copepods, making them fronteir harpacticoids.

REFERENCES

Lee, J. M., & C. Y. Chang. 2008. Copepods of the genus Leptocaris (Harpacticoida: Darcythompsoniidae) from salt marshes in South Korea. Korean Journal of Systematic Zoology 24 (1): 89–98.

Song, S. J., H.-U. Dahms & J. S. Khim. 2012. A review of Leptocaris including a description of L. ryukyuensis sp. nov. (Copepoda: Harpacticoida: Darcythompsoniidae). Journal of the Marine Biological Association of the United Kingdom 92 (5): 1073–1081.

Ferreting up a Bird's Nose

Mites, as I may have commented before, seem to have an almost fractal level of diversity: the closer you look, the more there is of it. This is nowhere more apparent than when it comes to parasitic mites which infest almost any host in any way that you can imagine. For the subject of this post, I drew one such mite: the honeyeater nasal mite Ptilonyssus myzanthae.

Venter (left) and dorsum of female Ptilonyssus myzanthae, from Domrow (1964). The scale bar equals 500 µm.

Bird nasal mites of the family Rhinonyssidae are, as their name indicates, inhabitants of the nasal passages of birds. General adaptations of the family for their parasitic lifestyle include tendencies towards reduction of the body sclerotisation and reduction in the length and number of setae. They use the claws on their front legs to tear openings in the host's mucous membranes and then feed on its blood. Transmission of nasal mites seems to happen during bill-to-bill contact such as when parents are feeding their young or during mating activities, or indirectly through water or on the surface of perches or the like. Rhinonyssid nasal mites are not known to transmit any actual diseases between hosts but they can cause the formation of lesions or inflammation or the like. All in all, probably not very pleasant for the bird (see here for some more details).

Whole-body illustration of a different rhinonyssid species, from Greg Spicer.

Nevertheless, infection rates in bird populations can be very high and most (if not all) bird species will be host to some nasal mite species. Most species of nasal mite are very host specific, known on only one or a few bird species (it must be noted, though, that the question of just how many researchers choose to look up a bird's schnozz in search of mites may not be irrelevant here). Ptilonyssus myzanthae was described by Domrow (1964) from two species of honeyeater in Queensland, Australia: the noisy miner Manorina melanocephala and the little wattlebird Anthochaera chrysoptera. Distinctive features of this species compared to others in the genus include a subhexagonal anterior dorsal shield on the body, a narrow genital shield, and a divided pygidial shield (the small pair of shields near the rear of the dorsum). Both of the known hosts are widespread and common in eastern Australia and it is likely that this mite is similarly ubiquitous. Studies of honeyeater phylogeny tend to place the genera Manorina and Anthochaera as close relatives, so it is possible that P. myzanthae has been infesting them since before their lineages diverged. It would be worth looking for the species in other related honeyeaters to see if we find any further clues.

REFERENCE

Domrow, R. 1964. Fourteen species of Ptilonyssus from Australian birds (Acarina, Laelapidae). Acarologia 6 (4): 595–623.

Parastenocaris

Parastenocaris brevipes, copyright A. Hobaek.

It's time for another consideration of the overwhelming diversity of stygofaunal copepods. Parastenocaris is a genus of copepods found on almost all the landmasses of the world except, presumably, Antarctica (New Zealand also stands out as an intriguing void in the genus' distribution). The majority of species in this genus are insterstitial, mostly found in soils saturated with fresh water; a small number of species are found in brackish habitats such as estuaries. A few species have also been found above ground, particularly in the tropics (Galasi & Laurentiis 2004). The type species of the genus, P. brevipes, has been found in sphagnum bogs (Karanovic 2005).

As commonly recognised, Parastenocaris is a pretty huge genus, with well over 200 species having been assigned to it over the years. However, the genus has been poorly defined and many authors have questioned its integrity. Galasi & Laurentiis (2004) suggested that Parastenocaris should be restricted to those species most closely related to the type species, P. brevipes. Such a group would still be pretty cosmopolitan; indeed, P. brevipes itself has a Holarctic distribution and is known from both Europe and North America (this stands in pretty stark contrast to the super-short ranges of some stygofaunal copepods). Distinctive features of this restricted P. brevipes group include a characteristic endopodal complex on leg 4 of the male, with the endopod hyaline and with one or two large claws. In contrast, the leg IV endopod in females is long and distally serrate.

Parastenocaris lacustris, from here

Members of the Parastenocaris brevipes groups are found closer to the soil surface than many other members of their family (Karanovic 2005). They are also relatively large, reaching the absolutely monstrous size (I'm sure) of half a millimetre or more. Karanovic (2005) suggested that this larger size could reflect the larger size of the sand grains they live among closer to the surface, or it could simply reflect their access to more reliable food sources that are available to their more deeply buried relatives.

REFERENCES

Galasi, D. M. P., & P. de Laurentiis. 2004. Towards a revision of the genus Parastenocaris Kessler, 1913: establishment of Simplicaris gen. nov. from groundwaters in central Italy and review of the P. brevipes-group (Copepoda, Harpacticoida, Parastenocarididae). Zoological Journal of the Linnean Society 140: 417–436.

Karanovic, T. 2005. Two new subterranean Parastenocarididae (Crustacea, Copepoda, Harpacticoida) from Western Australia. Records of the Western Australian Museum 22: 353–374.

Salpidobolus

The photo above (copyright Dmitry Telnov) shows a millipede of the genus Salpidobolus, photographed in West Papua. Salpidobolus is a genus of the family Rhinocricidae (in the order Spirobolida) that is found over a range from the Philippines, Sulawesi and Lombok in the west to Fiji in the east and Queensland in the south. There are also a handful of species that have been described from northern South America as part of Polyconoceras, a genus now regarded as synonymous with Salpidobolus, but Hoffman (1974) expressed the expectation on biogeographical grounds that future revision will show these species to be misplaced. Salpidobolus species are scavengers of vegetable matter and most active at night. When threatened, they can release a caustic spray from glands on the body segments that can cause irritation if it contacts mucous membranes such as around the eyes (Hudson & Parsons 1997). There are also reports (albeit unconfirmed) of production of bioluminescence by Salpidobolus (see here); observations on other millipedes suggest such bioluminescence could be related to the aforementioned caustic spray.

As has been mentioned in an earlier post, most millipedes tend not to be extravagant in their external variation, and spirobolidan millipedes look about as millipede-y as you can get. Notable features of the spirobolids as a whole include the presence of only a single pair of legs on each of the first five body rings, and modification of the eight and ninth pairs of legs into the gonopods (Milli-PEET). The Rhinocricidae are characterised by a broad collum (the first segment behind the head) with a rounded ventrolateral margin, and the anterior gonopods forming a single, more or less triangular, transverse plate. Sensory pits called scobinae are often present on the dorsal segments (Marek et al. 2003). Below the family level, as with other millipedes, it all comes down to genitalia. In Salpidobolus, the distal section of the posterior gonopods is flagellate and divided into two branches, one branch carrying the seminal channel (Hoffman 1974).

Gonopods of Salpidobolus meyeri, from Hoffman (1974).

The status of Salpidobolus was most recently reviewed by Hoffman (1974). The majority of species now included in the genus had previously been placed in the separate genera Dinematocricus or Polyconoceras. Salpidobolus was initially restricted to the type species, S. meyeri from Sulawesi, which differs from other species in the presence on the first three pairs of legs of distinct processes on some of the leg segments. Dinematocricus and Polyconoceras were supposed to differ on the basis of the number of sensilla at the end of each antenna: four in Dinematocricus, more than four in Polyconoceras. Hoffman felt that none of these differences warranted generic separation in light of the consistency of gonopod structure between the three 'genera', and united them all under the oldest available name.

REFERENCES

Hoffman, R. L. 1974. Studies on spiroboloid millipeds. X. Commentary on the status of Salpidobolus and some related rhinocricid genera. Revue Suisse de Zoologie 81(1): 189–203.

Hudson, B. J., & G. A. Parsons. 1997. Giant millipede ‘burns’ and the eye. Transactions of the Royal Society of Tropical Medicine and Hygiene 91: 183–185.

Marek, P. E., J. E. Bond & P. Sierwald. 2003. Rhinocricidae systematics II: a species catalog of the Rhinocricidae (Diplopoda: Spirobolida) with synonymies. Zootaxa 308: 1–108.

Hadromeros: A Trilobite Survivor

Reconstruction of Hadromeros subulatus, from Kielan-Jaworowska et al. (1991).

Trilobites of the genus Hadromeros were widespread in the Late Ordovician and Early Silurian of Eurasia and North America. They are classified in the Cheiruridae, the same family that includes another trilobite genus that has been featured on this site, Sphaerexochus. However, Hadromeros differs from Sphaerexochus in that its glabella (its 'nose') is not as large. Whereas in my earlier post I suggested that Sphaerexochus may have been a predator, Hadromeros was probably a less aggressive feeder. It was possibly a detritivore, picking bits of nutritious material out of the sand and mud. This interpretation is supported by the known leg morphology of a closely related genus, Ceraurus, in which the legs are fairly generalised and show little adaptation for food processing (Bergström 1973).

Hadromeros and Ceraurus are placed in the Cheirurinae, a distinct subfamily from the Sphaerexochinae that includes Sphaerexochus. One characteristic feature of many members of the Cheirurinae is that the bases of the pleura, the plates that run down each side of the thorax, are swollen in dorsal view. The purpose of these swellings remains unknown. You might expect that, trilobites being as abundant in the fossil record as they are, we would know a great deal about them, and in many respects that is quite true. However, in other respects our knowledge is also frustratingly incomplete. Trilobites have an extensive fossil record because their dorsal exoskeleton was mineralised, and it is this that is usually preserved. The ventral section of the body, on the other hand, was not mineralised, and is only preserved under exceptional circumstances. This includes such significant features as the legs and mouthparts (as indicated above, we have some knowledge of the leg morphology of Cerarurus, but no direct evidence for Hadromeros). It is possible that the cavities underneath the cheirurine pleural bases housed some modification of the gills, if the gills in trilobites were comparable to those of living crustaceans. But how or to what purpose the gills were modified can only be speculated upon.

Morphologically, Hadromeros was a fairly unremarkable trilobite, but it stands out from the other genera of the Cheirurinae in one important respect (that has, indeed, already been alluded to in this post). The end of the Ordovician saw a mass extinction in marine life, by some measures the second largest to have ever occurred. Few groups of animals made it through unscathed, and the cheirurids were no exception. Of the eight subfamilies of Cheiruridae recognised by Přibyl et al. (1985), only three made it through to the Silurian: the Cheirurinae, Sphaerexochinae and Deiphoninae. Within the Cheirurinae, Hadromeros is the only genus currently known from both sides of the Ordovician-Silurian boundary, and may have been ancestral to all other post-Ordovician cheirurines. While other genera were whisked away, Hadromeros became the Trilobite that Lived.

REFERENCES

Bergström, J. 1983. Palaeoecologic aspects of an Ordovician Tretaspis fauna. Acta Geologica Polonica 23 (2): 179-206.

Kielan-Jaworowska, Z., J. Bergström & P. Ahlberg. 1991. Cheirurina (Trilobita) from the Upper Ordovician of Västergötland and other regions of Sweden. Geologiska Föreningen i Stockholm Förhandlingar 113 (2-3): 219-244.

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.

Horny-Arsed Trilobites

Reconstruction of Ceratopyge, from here.

Just a short post for today. The Ceratopygidae are a family of trilobites known from the Late Cambrian and Early Ordovician. The name of the type genus, Ceratopyge, means 'horned rump', and one of the features that has classically defined the family is the presence of one or two pairs of spines on either side of the pygidium, the plate the makes up that hind end of a trilobite. These spines appear to be derived from lateral extensions of one of the anterior segments incorporated into the pygidium. However, there are also some genera without pygidial spines that share other features with the family (such as a narrow rim to the cheeks) and so have also been recognised as ceratopygids. Ceratopygids also possessed narrow spines extending back from the posterior corners of the head. The number of segments between head and pygidium varied between genera: early genera have nine segments, but some later genera have only six (Fortey & Chatterton 1988) (offhand, the drawing above looks to have one too many segments).

Proceratopyge gamaesilensis, from here.

Otherwise, ceratopygids seem to have been fairly generalised trilobites. The eyes were present but not large, and there don't appear to be any features suggesting they were swimmers. The features of the underside of the head are poorly known in ceratopygids overally, but where known, the hypostome (the plate on the underside of the head that would have sat in front of the mouth) is firmly attached to the anterior margin of the head. Trilobites with this arrangement are believed to have been scavengers or predators on small invertebrates (Fortey & Owens 1999). In some later genera, such as Ceratopyge, the glabella in the midline of the cephalon expanded forward, with a corresponding reduction in the width of the anterior margin. As the glabella would have contained the trilobite's stomach, its enlargement may indicate that these later ceratopygids were taking larger prey.

REFERENCES

Fortey, R. A., & B. D. E. Chatterton. 1988. Classification of the trilobite suborder Asaphina. Palaeontology 31 (1): 165-222.

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

Stunning Central American Millipedes

Blue cloud forest millipede Pararhachistes potosinus, copyright Luis Stevens.

For my semi-random selection of taxon to write about this week, I drew the millipede family Rhachodesmidae. Rhachodesmids are members of the millipede group called the Polydesmida, characterised by the presence of lateral keels on each segment of the body. The presence of these keels had lead to platydesmidans sometimes being referred to as 'flat-backed millipedes' though depending on how strong the keels are, not all species are necessarily 'flat-backed'.

In an earlier post on millipedes, I stressed the importance of genitalia in characterising millipedes, and the Rhachodesmidae are no exception. In polydesmidans, it is the front pair of legs on the seventh segment that is modified into the gonopods in males (with one notable exception that I may refer to later). Gonopods of rhachodesmids lack the solenite or coxal spur found in many other polydesmidans, and the inner side of the gonopod has a distinct elongate or oval concavity that is densely setose. Other noteworthy features of rhachodesmids are that they are often relatively large, with a conical terminal segment and more or less thickened rims to the lateral keels (Loomis 1964).

An unidentified rhachodesmid, copyright Sergio Niebla.

Beyond that, rhachodesmids become a little more difficult to characterise. Though they are not a widespread group, being restricted to Mexico and Central America, they are very diverse in appearance. Loomis & Hoffman (1962) commented that, "Rhachodesmoids collectively are members of a group notable for great variability and the development of bizarre features. Among their ranks we find millipeds which are bright blue, green, orange, and even pure white as adults; here the gonopod structure ranges from the normal polydesmoid appearance down to monoarticular fused remnants. Body form varies from a slender juliform shape to broad, flat, limaciform contour. Within the limits of this so-called single family occurs more variation than in all of the remaining polydesmoids." They also noted that the group was in need of review, something that apparently remains undone to this day (though there is someone working on it). If the photographs I've commandeered in this post are any indication, this is definitely a group that deserves more love.

Paratype of Tridontomus procerus, from Loomis & Hoffman (1962).

Loomis & Hoffman (1962) made their comments in comparing the Rhachodesmidae to another Central American polydesmidan family they were then describing as new, the Tridontomidae, and if I'm referring to the rhachodesmids then I should probably give a shout-out to these remarkable beasts as well. So far as I've found, this family is still only known from two species, Tridontomus procerus and Aenigmopus alatus, from Guatemala. Not only is the appearance of tridontomids striking, with long spinose processes on either side of the body, but the genital morphology of one species, A. alatus, is especially bizarre: it doesn't have any where it should. Where males of other polydesmidans have the legs of the seventh segment modified into gonopods, those of A. alatus have a perfectly ordinary pair of walking legs. In normal polydesmidans, the gonopods are used to transfer sperm from seminal processes on the coxae of the second pair of legs to the female's genital opening (more details are available here), but obviously Aenigmopus must do things differently. The seminal processes are still present, and the second legs themselves are thickened compared to other millipedes; it is possible that they are somehow used to transfer sperm directly from process to female without the use of gonopods. However it does it, there is no question that Aenigmopus is unique in the world of polydesmidans.

REFERENCES

Loomis, H. F. 1964. The millipeds of Panama (Diplopoda). Fieldiana: Zoology 47 (1): 1-136.

Loomis, H. F., & R. L. Hoffman. 1962. A remarkable new family of spined polydesmoid Diplopoda, including a species lacking gonopods in the male sex. Proceedings of the Biological Society of Washington 75: 145-158.

Bobble-Nosed Trilobites

Cranidium and part of thorax of Onchonotellus sp., from Bao & Jago (2000).

While most fossil invertebrates manage to completely fly under the radar when it comes to popular culture (it's not as if we're drowning under cartoon depictions of euthycarcinoids or eldoniids), one group that will often get a passing nod is the trilobites. Any depiction of early animal life worth its salt is going to feature a couple of these crunchy bugs scurrying about. Nevertheless, the range of varieties of trilobite shown will generally be low, and will usually be something similar to Olenellus or Elrathia. Seeing as trilobites persisted for hundreds of millions of years, it should be no surprise that their actual diversity was much higher.

The fossil shown at the top of this post is a representative of the trilobite genus Onchonotellus. Remains have been assigned to this genus from the late Cambrian and the early Ordovician, though Adrain (2013) expressed some reserve about the genus' monophyly. It has been assigned to the Catillicephalidae, a mostly Cambrian group of trilobites, but again the coherence of this total group is uncertain.

Most known fossils of Onchonotellus are represented by isolated cranidia, the plates that in life covered the trilobites' head. Onchonotellus and other catillicephalids are characterised by an inflation of the glabella, the middle lobe of the cranidium. Generally, the glabella of Onchonotellus is barrel-shaped. In some Onchonotellus specimens, the glabella may almost look spherical in side-view, making this trilobite look like a Bubble O'Bill. In many trilobites, the glabella will bear a series of furrows along the sides, but in Onchonotellus these disappear so that the glabella surface is smooth. In other catillicephalids, the glabella extends right to the front margin of the head, but Onchonotellus does retain a distinct rim around its front. The cheeks on either side of the glabella are relatively broad (Öpik 1967; Shergold 1980). In their time, members of the genus were found around the world, and some species have been highlighted as index fossils, useful in determining the age of rock strata.

So what was the significance of the large glabella? Most researchers have suggested that it probably held some sort of expansion of the digestive system, such as a crop for storing food. Some trilobites in which the glabella became large enough that it actually overhung the front margin have been suggested to be predatory (Fortey & Owens 1999), with the glabella containing a large oesophagus that allowed the trilobite to swallow larger food items. Onchonotellus probably didn't take things that far: not only was its glabella just that little bit smaller, but I haven't found any indication of it possessing the enlarged eyes also found in the predatory forms. The furrows on the glabella of most trilobites may have marked the attachment positions for muscles associated with the oesophagus/crop/whatever, so did the reduction of these furrows in Onchonotellus indicate a correspondingly less muscular pharynx? Perhaps Onchonotellus was a detritus feeder, with an expanded crop allowing it to take in mouthfuls of sediment from which to sieve out tasty organic morsels. Or perhaps it was a scavenger, breaking off lumps from the carcasses of other animals. Whatever it was doing, it was something that involved a big nose that was in actuality a big mouth.

REFERENCES

Adrain, J. M. 2013. A synopsis of Ordovician trilobite distribution and diversity. Geological Society, London, Memoirs 38: 297-336.

Bao, J.-S., & J. B. Jago. 2000. Late Late Cambrian trilobites from near Birch Inlet, south-western Tasmania. Palaeontology 43 (5): 881-917.

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

Öpik, A. A. 1967. The Mindyallan fauna of northwestern Queensland. Commonwealth of Australia, Department of National Development, Bureau of Mineral Resources, Geology and Geophysics—Bulletin 74, vol. 1: 404 pp., vol. 2: 166 pp., 67 pls.

Shergold, J. H. 1980. Late Cambrian trilobites from the Chatsworth Limestone, western Queensland. Bureau of Mineral Resources, Geology and Geophysics—Bulletin 186: 1-111.

Giant Centipedes (That Aren't All Giants)

Scolopendra morsitans, copyright Jiri Lochman/Lochman Transparencies.

It was a dark night, but not stormy (nights tend to be dark, as a rule). We were out collecting for our regular survey when we encountered a large centipede (the same species as in the photo above) crossing the road, and decided to add it to our collection. Taking out the large 20-cm forceps that we had on hand for dealing with venomous animals, one of us used them to grab the centipede.

The response was electric. Rather than trying to escape its attacker, the centipede instantly whipped back and lashed itself around the forceps, doing its best to bite into them. Had the actual wielder of the forceps been within its reach, they would have been in for a world of pain. When dealing with scolopendrid centipedes, you should always remember three things: they are big, they are fast, and they are mean.

Scolopendridae are unmistakeable. They include the giants of the centipede world, with the largest species (the South American Scolopendra gigantea) reaching up to a foot in length. Even the smaller species are relatively robust compared to other centipedes. Like all centipedes, the first pair of legs is modified into a robust pair of 'fangs' used for delivering venom (when I referred above to a centipede 'biting', this is what I was properly referring to). Large scolopendrids have the most dangerous centipede stings, potentially causing intense pain, though fatalities are very rare (Bush et al., 2001, noted that no centipede fatalities were known from the US, though they did refer to a single known child fatality in the Philippines). Their hunting prowess is amply demonstrated in this video of a large scolopendrid hunting bats by quite literally snatching them out of the air:


But lest you think that scolopendrids are all venom and viciousness, let me point out that they also have their endearing qualities. Female scolopendrids make devoted mothers, coiling around their egg clutches and regularly cleaning them to prevent fungal attack. Even after the eggs hatch, the female continues to coddle and groom her young. Brunhuber (1970) recorded that females of Cormocephalus anceps spent at least three months (from late September to late December) caring for young before they struck out on their own. Even after becoming independent, the young do not reach sexual maturity until they are at least two years old. Other scolopendrids may mature more quickly, at about one year (Lewis 1972). Individual centipedes may live for several years.

Female Scolopendra morsitans cleaning her eggs, copyright H. J. B..

The Scolopendridae belong to a larger centipede group called the Scolopendromorpha. Most scolopendromorphs have bodies with 21 or 23 leg-bearing segments, except for one remarkable scolopendrid species from central Brazil that has 39 or 43 leg-bearing segments (Chagas-Junior et al. 2008). Non-scolopendrid species are often much smaller than the Scolopendridae, with some being only about 10 mm in length. These smaller scolopendromorphs also differ in eye morphology: Scolopendridae have a patch of four ocelli on either side of the head, but other scolopendromorphs are mostly blind and lack ocelli. In the past the blind scolopendromorphs have been treated as a single family Cryptopidae, but recent authors have mostly recognised three separate families Cryptopidae, Scolopocryptopidae and Plutoniumidae in light of uncertainty about the monophyly of a broader Cryptopidae. Nevertheless, a recent phylogenetic analysis by Vahtera et al. (2012) combining both morphological and molecular data did support a single blind clade. Unfortunately, no phylogenetic analysis to date has been able to include Mimops orientalis, an odd scolopendromorph known from a single specimen collected in China in 1903 and placed by Lewis (2006) in its own distinct family. Mimops possesses but a single ocellus on either side of the head, potentially making it very intriguing for the question of whether blindness has evolved in scolopendromorphs more than once.

The blind scolopendromorph Scolopocryptops sexspinosus, copyright Troy Bartlett.

REFERENCES

Brunhuber, B. S. 1970. Egg laying, maternal care and development of young in the scolopendromorph centipede, Cormocephalus anceps anceps Porat. Zoological Journal of the Linnean Society 49 (3): 225-234.

Bush, S. P., B. O. King, R. L. Norris & S. A. Stockwell. 2001. Centipede envenomation. Wilderness and Environmental Medicine 12 (2): 93-99.

Chagas-Junior, A., G. D. Edgecombe & A. Minelli. 2008. Variability in trunk segmentation in the centipede order Scolopendromorpha: a remarkable new species of Scolopendropsis Brandt (Chilopoda: Scolopendridae) from Brazil. Zootaxa 1888: 36-46.

Lewis, J. G. E. 1972. The life histories and distribution of the centipedes Rhysida nuda togoensis and Ethmostigmus trigonopodus (Scolopendromorpha: Scolopendridae) in Nigeria. Journal of Zoology 167 (4): 399-414.

Lewis, J. G. E. 2006. On the scolopendromorph centipede genus Mimops Kraepelin, 1903, with a description of a new family (Chilopoda: Scolopendromorpha). Journal of Natural History 40 (19-20): 1231-1239.

Vahtera, V., G. D. Edgecombe & G. Giribet. 2012. Evolution of blindness in scolopendromorph centipedes (Chilopoda: Scolopendromorpha): insight from an expanded sampling of molecular data. Cladistics 28: 4-20.

The Araeolaimida: We Barely Know Ye

Axonolaimus sera, from here.

Overall, the nematodes cannot be considered one of the best-known groups of animals. This is not because they are at all uncommon: there is the oft-cited factoid that nematodes are so abundant in every corner of the world that, if everything other than them was somehow instantaneously removed, the ghostly shadow of the planet Earth would supposedly still be visible as a cloud of microscopic worms. Nematodes are even found in places other animals are not: they have been found further beneath the Earth's surface than any other multicellular organism. There are some nematode species that attract attention, such as those that cause diseases, or are notable crop or animal pests. The nematode Caenorhabditis elegans has been a workhorse of developmental biology for many a year. But these well-studied taxa represent only a small proportion of the full nematode diversity out there.

Being very small and soft-bodied, nematodes do not usually present taxonomists with a great variety of clearly defined morphological features. As a result, dividing nematodes into well-supported groupings has not been an easy task (there are some notable exceptions: try looking up the Desmoscolecida one of these days). Take, as an example, the group known as the Araeolaimida. This name spent many years as a bit of a wastebasket for various non-parasitic nematode families. Eventually, it was restricted by Ley & Blaxter (2002) to just four families: the Axonolaimidae, Comesomatidae, Diplopeltidae and Coninckiidae, with many taxa previously treated as Araeolaimida included in a separate order Plectida. Fonseca & Bezerra (2012) include a fifth family, the Bodonematidae, that was not mentioned by Ley & Blaxter. Even in this restricted sense, the Araeolaimida may not represent a coherent group. There is no single feature shared by all araeolaimidans that is not found in other nematodes, and a molecular phylogenetic study of nematodes by van Megen et al. (2009) did not recover a monophyletic araeolaimidan clade. Nevertheless, Araeolaimida normally have the ovaries outstretched within the bodies of females (in many other nematode taxa, they are folded back on themselves), and the amphids (sensory grooves on the sides of the head) are usually spiral or looped in shape. The majority of araeolaimidans are marine, with freshwater and terrestrial environments being home to two genera of Diplopeltidae, and a few species of Axonolaimidae (Fonseca & Bezerra 2012). We don't know much about their diet, but they are probably grazers on micro-algae or bacteria. About 400 species of Araeolaimida have been described, but it would be very surprising if there weren't more out there.

Head end of the freshwater diplopeltid Cylindrolaimus, photographed by Peter Mullin. Note the dark circle near the end: this is the amphid.

The separate families are a bit easier to define (Fonseca & Bezerra 2012). The single known species of Bodonematidae, Bodonema vossi, stands out by having a pharynx with the mid-part differentiated into a series of muscular bulbs, as opposed to the fairly simple pharynxes of other Araeolaimida. Coninckia, the only genus of Coninckiidae, has the amphids sitting on differentiated plaques that are not present in other taxa. The Comesomatidae have spiral amphids, while the Axonolaimidae and Diplopeltidae have simpler looped amphids. The last two families are distinguished by the shape of the buccal cavity, which is larger and more strongly sclerotised in the Axonolaimidae.

One detail which caught my eye when researching this post is that males of some axonolaimids produce two different forms of spermatozoa (Riemann 1986). The sperm cells produced in the anterior testis of Nicascolaimus punctatus are more than three times the size of those produced in the posterior testis. In another axonolaimid species, Axonolaimus helgolandicus, it is the posterior testis that produces the larger cells. Both types of sperm were shown in N. punctatus to be transferred to females, but the reason for the two different sperm types is unknown. Pomponema, a genus belonging to a separate group of nematodes from the Araeolaimida, produces dimorphic sperm in which the larger cells seem to break down before they are transferred to the female, and it is possible that only one sperm type functions to fertilise the female in axonolaimids as well. Perhaps the other sperm type represent some sort of nuptial gift? Or could they somehow interfere with fertilisation by other males? We await the nematode enthusiast who will find out.

REFERENCES

Fonseca, G., & T. N. Bezerra. 2012. Order Araeolaimida De Coninck, 1965. Zoology Online. Berlin, Boston: De Gruyter. Retrieved 3 Jun. 2014, from http://www.degruyter.com/view/Zoology/HBZ-2011-000076.

Ley, P. de, & M. Blaxter. 2002. Systematic position and phylogeny. In: Lee, D. L. (ed.) The Biology of Nematodes, pp. 1-30. Taylor & Francis: Florence (Kentucky).

Megen, H. van, S. van den Elsen, M. Holterman, G. Karssen, P. Mooyman, T. Bongers, O. Holovachov, J. Bakker & J. Helder. 2009. A phylogenetic tree of nematodes based on about 1200 full-length small subunit ribosomal DNA sequences. Nematology 11 (6): 927-950.

Riemann, F. 1986. Nicascolaimus punctatus gen. et sp.n. (Nematoda, Axonolaimoidea), with notes on sperm dimorphism in free-living marine nematodes. Zoologica Scripta 15 (2): 119-124.