The Rhipidothyrididae: Brachiopods of the Devonian

Specimen of Rhenorensselaeria, copyright Miguasha National Park.

In the modern world, the brachiopods are an unfamiliar group to most people. To most, they would probably not be readily distinguished from the much more abundant bivalves that they superficially resemble (a resemblance that is literally only skin deep: brachiopods and bivalves are in no way close relatives, and their internal anatomy is fundamentally different). However, this was not always the case. If one was to travel back to some point in the Palaeozoic era, one would find the situation reversed. At this time, it was the brachiopods that dominated the world's seas, while the bivalves were relegated to a minor supporting role. Their respective fortunes changed around the beginning of the Mesozoic, though whether that was because changing conditions favoured the bivalves, or whether the bivalves simply got a head start in recovering from the Rocks Fall, Everyone Dies clusterf*** that was the end-Permian extinction event, I couldn't tell you.

The fossil shown at the top of this post is one of these Palaeozoic brachiopods, a member of the family Rhipidothyrididae. Rhipidothyridids were among the earliest families of the order Terebratulida, which includes the majority of surviving brachiopods but in the Palaeozoic was just one group among many. Half a dozen genera from the Devonian period have been assigned the Rhipidothyrididae (Lee 2006). They often occur in mass assemblages, with a low diversity of other fossils (Boucot & Wilson 2004). That these assemblages represent their habits in life is indicated by the fact that the individual brachiopods in them are usually articulated; because the shells lacked a toothed hinge, the valves would soon become disassociated if transported after death.

The relationships of the rhipidothyridids are somewhat uncertain. A significant feature used in terebratulid classification is the morphology of the loop, a calcified ring at the base of the shell that provides part of the support for the lophophore in life. In some terebratulids, the loop is long and provides most of the lophophore support; in others, the loop is much shorter and lophophore support is partially taken over by free spicules embedded in the lophophore itself. However, because the loop is a quite delicate structure, its study in fossil taxa requires careful sectioning of specimens, with due consideration of the possibility of post-mortem damage. To date, this has not yet been done for the rhipidothyridids, so their loop morphology remains unknown.

REFERENCES

Boucot, A. J. & R. A. Wilson. 1994. Origin and early radiation of terebratuloid brachiopods: thoughts provoked by Prorensselaeria and Nanothyris. Journal of Paleontology 68 (5): 1002–1025.

Lee, D. E. 2006. Stringocephaloidea. In: Kaesler, R. L. (ed.) Treatise on Invertebrate Paleontology pt H. Brachiopoda (Revised) vol. 5. Rhynchonelliformea (part) pp. 1994–2018.

To Make a Willow Weep

Pair of spotted willow leaf beetles Chrysomela vigintipunctata, copyright P. V. Romantsov.

As noted in an earlier post, the leaf beetles of the Chrysomelidae include some very attractive representatives. The two individuals in the photo above belong to the widespread genus Chrysomela, many species of which feed on leaves of members of the tree genera Salix, the willows, and Populus, the poplars. Some species can become numerous enough on their hosts to cause extensive defoliation, and the cottonwood leaf beetle Chrysomela scripta is regarded as a serious pest of trees such as the cottonwood Populus deltoides.

Mating pair of Chrysomela populi, copyright Beentree.

Chrysomela beetles that feed on willows are able to sequester salicin from the willow's leave and use it to secrete a defensive compound of their own, salicylaldehyde. In one European species, Chrysomela lapponica, distinct populations have been identified that feed respectively on willow or birch leaves. Experimental studies have shown that the birch- and willow-feeding populations are largely reproductively isolated from each other: either their inter-fertility is reduced, or hybrid larvae that differ in feeding preference from their mother will be laid on the wrong host tree and be unable to survive. As such, the populations can be recognised as either in the process of diverging into separate species, or as already distinct cryptic species. As birch does not contain salicin, birch-feeding C. lapponica do not produce the salicylaldehyde found in willow-feeding populations, and birch-feeders fed on willow leaves are unable to utilise salicin (Kirsch et al. 2011).

Female Chrysomela lapponica ovipositing on birch leaf, copyright Juergen Gross. As well as the variation in host plant described above, members of this species also vary widely in coloration, from red and black as in the photo to entirely black in some individuals.

As willow is most likely the ancestral food type for C. lapponica, how did some populations make the change to feeding on birch despite losing a significant factor in their own defenses by doing so? One possibility that has been suggested is that the change happened not despite the loss of salicylaldehyde, but because of it (Gross et al. 2004). While the salicylaldehyde acts as an effective defense against generalist predators, some specialist predators and parasitoids of the beetles seem to be directly attracted to it, using it as a marker to track down their target. Pressure from this angle might favour the spread of a population that does not produce the alluring salicylaldehyde.

REFERENCES

Gross, J., N. E. Fatouros, S. Neuvonen & M. Hilker. 2004. The importance of specialist natural enemies for Chrysomela lapponica in pioneering a new host plant. Ecological Entomology 29: 584-593.

Kirsch, R., H. Vogel, A. Muck, K. Reichwald, J. M. Pasteels & W. Boland. 2011. Host plant shifts affect a major defense enzyme in Chrysomela lapponica. Proceedings of the National Academy of Sciences of the USA 108 (12): 4897-4901.

Ant-like Ichneumons

Female Gelis, copyright Krister Hall.

The ichneumons are one of the best-known groups of parasitoid wasps. The most familiar ichneumons are relatively large for parasitoid wasps, and sometimes even for wasps in general. This can make them somewhat intimidating in appearance, especially considering the likelihood of the long ovipositor of a female being mistaken for a sting by those not in the know. However, not all ichneumons are giants. The photo above shows a tiny ichneumon of the genus Gelis, females of which are wingless and bear a distinct superficial resemblance to ants. This resemblance is likely to afford them some protection from potential predators, and at least one Gelis species, G. agilis, has been shown to release a chemical when threatened very similar to the alarm pheromones of the black garden ant Lasius niger (Malcicka et al. 2015). On the other hand, one might be tempted to wonder if this mimicry may sometimes serve a more nefarious purpose: another species, G. apterus, has been recorded as a parasitoid of the ant-eating spider Zodarion styliferum (Korenko et al. 2013). However, G. apterus has not been recorded to use its ant appearance to lure its host; instead, the female ichneumon uses its ovipositor to pierce the igloo-like silken retreat that the spider occupies during the day. Other species of Gelis are known to be parasitoids of moth cocoons rather than spiders (Gauld 1984), so Gelis' status as an ant-mimic and its choice of host may be simple coincidence.

Phygadeuon exiguus, copyright James K. Lindsey.

Gelis belongs to a world-wide tribe of ichneumons known as the Phygadeuontini (sometimes referred to in older sources as the Gelini), a diverse group including well over 100 genera. Most, but not all, phygadeuontins are also among the smaller ichneumons. The range of hosts attacked by the group is equally diverse, including (among others) moths and lacewing pupae, and spider egg sacs, while some are hyperparasitoids on the pupae of other parasitoid wasps (Gauld 1984). Species of the genus Phygadeuon include parasitoids of wood-burrowing beetles that use the enlarged ends of their antennae to tap at wood in search of hollow burrows within. Some phygadeuontins are external parasitoids, while others are endoparasitoids. The larvae of Gelis apterus can even be regarded as true predators, as they attack not the eggs of their host but its newly-hatched spiderlings (Korenko et al. 2013). A common theme between these diverse hosts, though, is the production by most of them of silken cocoons or other protective structures that the female phygadeuontin is able to pierce with her ovipositor.

REFERENCES

Gauld, I. D. 1984. An Introduction to the Ichneumonidae of Australia. British Museum (Natural History).

Korenko, S., S. Schmidt, M. Schwarz, G. A. P. Gibson, & S. Pekár. 2013. Hymenopteran parasitoids of the ant-eating spider Zodarion styliferum (Simon) (Araneae, Zodariidae). Zookeys 262: 1–15.

Malcicka, M., T. M. Bezemer, B. Visser, M. Bloemberg, C. J. P. Snart, I. C. W. Hardy & J. A. Harvey. 2015. Multi-trait mimicry of ants by a parasitoid wasp. Scientific Reports 5: 8043. doi:10.1038/srep08043.

The Running of the Spiders

Nursery-web spider Dolomedes minor, sitting atop its nursery web. Copyright Konstable.

Spiders are one of the most familiar groups of invertebrates out there. There's no denying this: everybody knows what a spider is. But for various reasons, the classification of spiders tended to lag a bit behind that of other terrestrial invertebrates. Being softer-bodied than insects, they tend not to exhibit the wealth of features that made many insect groups instantly discernible. To the modern arachnologist's eye, the earliest classifications of spiders can verge on the humorous. Latreille (1802), in his Histoire Naturelle des Crustacés et des Insectes, classified the entirety of what would now be called the araneomorph spiders into a single genus Aranea, divided into sections labelled not with formal names but with schematic diagrams of the arrangement of eyes found in that section.

A few decades later, in 1829 (translated into English in Cuvier, 1831), Latreille was to present a more detailed classification of the spiders, in which they were divided into groups largely on the basis of their life habits. The araneomorphs were hence divided between the Sedentariae, those spiders which captured their prey in webs or laid in ambush, and the Vagabundae, those spiders that actively hunted down their prey. The Vagabundae were in turn divided between two sections: the Citigradae or runners, and the Saltigradae or jumpers. Latreille's classification was subsequently more or less abandoned, as his behavioural groupings failed to line up directly with morphological clusters. Almost by accident, however, those taxa included by Latreille in his Citigradae have continued to be associated, and in modern classifications are classified within the Lycosoidea (Jocqué & Dippenaar-Schoeman 2007).

The lycosoids are, indeed, mostly active hunters. Their behaviour is reflected in the vernacular names of a number of the constituent families: the wolf spiders of the Lycosidae (previously featured here and here), the lynx spiders of the Oxyopidae, the prowling spiders of the Miturgidae. But the correspondence to Latreille's 'araignées loups' is not perfect: the Zoropsidae, for instance, are lycosoids that spin extensive webs. Nor are they mere rapacious hunters: many are devoted parents, carrying and/or guarding their egg-sacs to protect them from predators, and in the case of the Lycosidae even providing a certain degree of care for the newly hatched spiderlings.

One group of lycosoids has even gotten a name for parental care. The nursery-web spiders of the Pisauridae construct protective webs for their babies, containing them within a tent constructed by wrapping sheets of silk around suitable vegetation. When I was a child in New Zealand, I used to be fascinated by the nursery webs constructed by the species Dolomedes minor. Like many pisaurids, this species is associated with water, diving into it to hunt for fish and other small aquatic animals. The females would often build their nursery webs by tying together the ends of nearby rushes. Though it seems a little cruel to my adult self, the younger me loved to pull these webs apart to see the eruption of tiny spiders come scurrying out.

REFERENCES

Cuvier, G. 1831. The Animal Kingdom arranged in conformity with its organization, vol. 3. The Crustacea, Arachnides and Insecta, by P. A. Latreille, translated from the French with notes and additions, by H. M'Murtrie. G. & C. & H. Carvill: New York.

Jocqué, R., & A. S. Dippenaar-Schoeman. 2007. Spider Families of the World. Royal Museum for Central Africa: Tervuren (Belgium).

Latreille, P. A. 1802. Histoire Naturelle, générale et particulière des Crustacés et des Insectes, vol. 3. F. Dufart: Paris.