Field of Science

Getting Your Diatoms in a Row

Diatoms are one of the world's primary groups of aquatic unicellular algae. Perhaps only the cyanobacteria rival them for ecological significance. They play a crucial role in the production and fixation of nutrients on which other organisms depend.

Colony of Melosira moniliformis attached to some sort of weed, copyright Frank Fox. The last individual seems to have suffered some unfortunate bisection.


Diatoms live protected in a siliceous test or, to put it another way, they really do live in glass houses. The test is composed of a pair of opposed valves; as noted by Round & Crawford (1990), the arrangement of valves is commonly compared to that of a Petri dish. The valves themselves do not overlap directly in the manner of a Petri dish, but a series of girdle bands around the edge of each valve does overlap. Diatoms come in a range of shapes and structures (artistically minded microscopists [or microscopically minded artists, however you wish to phrase it] have been known to create kaleidoscopic patterns through the careful arrangement of diatoms on a slide) and have commonly been divided between two major groups on the basis of the main symmetry of the valves. Centric diatoms have valves that are radial in appearance when viewed from above whereas pennate diatoms have elongated, more bilateral valves.

Melosira is a widespread genus of centric diatoms found in both fresh and salt water. It might be considered the classic centric: the test is circular in dorsal view and rectangular in side view so the overall shape is that of a hat box. Individual cells remain united by pads of mucilage following division, resulting in the formation of long chains. Species of the genus differ in their preferred habitats. One freshwater species, Melosira varians, is commonly found in polluted or poor quality waters. Conversely, a marine species M. arctica is the most abundant algal species known from the Arctic Ocean, responsible for nearly half the Arctic's primary production. Diatoms lack flagella for most of their life cycle (only their gametes are ever flagellate) so they are not active swimmers. In life, they are either found attached to a substrate or, if floating as planktonic, suspended in the water column by turbulence. One species, M. italica, is known to survive in sediment during quiescent periods of the year and resume growth when winter turbulence returns it to the light (Round & Crawford 1990).

Auxospores of Melosira varians, copyright Kristian Peters.


When diatom cells divide, each daughter cell receives one of the parent's original test valves and secretes a new valve to match it. As noted above, the marginal girdles of the valves overlap, and the new valve is always secreted as the inner partner of this overlap. As a result, and because the glass valves cannot change in size once secreted, successive generations of diatom cells become inexorably smaller over time. Obviously, this process cannot continue indefinitely least the cells dwindle to extinction, so sexual reproduction plays a vital role in resetting the process of diatom development. Centric diatoms like Melosira produce distinct gamete types, motile spermatozoids and immobile eggs (in contrast, many pennate diatoms produce only a single gamete type with no such distinction). Zygotes produced from the fusion of these gametes grow into a cell called an auxospore that differs from normal diatom cells in possessing a organic cell covering instead of solid glass valves. This organic covering may be reinforced with individual siliceous scales, but some Melosira auxospores remain contained and protected within the valves of their parent and lack scales of their own (Medlin & Kaczmarska 2004). The auxospore will not produce a uniform glass test until it has reached full mature size; in Melosira this initial test differs from the standard in being globular rather than pillbox-shaped. The auxospore will then begin dividing into daughter cells in the usual well which will themselves produce test valves of the standard shape. But each of the auxospore's daughters, of course, will receive one of it's initial valves, so as the Melosira chain develops it will remain hemispherical at each end.

REFERENCES

Medlin, L. K., & I. Kaczmarska. 2004. Evolution of the diatoms: V. Morphological and cytological support for the major clades and a taxonomic revision. Phycologia 43 (3): 245-270.

Round, F. E., & R. M. Crawford. 1990. Phylum Bacillariophyta. In: Margulis, L., J. O. Corliss, M. Melkonian & D. J. Chapman (eds) Handbook of Protoctista. The structure, cultivation, habitats and life histories of the eukaryotic microorganisms and their descendants exclusive of animals, plants and fungi. A guide to the algae, ciliates, foraminifera, sporozoa, water molds, slime molds and the other protoctists pp. 574–596. Jones & Bartlett Publishers: Boston.

How the Worm Turns (Into a Worm)

Those of you who have suffered through some of my posts on turrids may recall me discussing the subject of how differences in the mode of development of marine organisms relate to their classification. Features that were once considered of high significance are affected by whether the animal develops as a free-swimming larva or is nourished by a yolk supply provided in the egg, and may change more readily than previously thought. And indeed, it turns out that there are some cases where both developmental modes can be found in a single species.

Boccardia polybranchia, from here.


Boccardia is a genus of twenty-odd species of marine worm belonging to the family Spionidae. These are sedentary worms, living in tubes that they construct for themselves out of sediment bound together by mucus, or that they bore into substrates such as mollusc shells or coral. Boccardia and other spionids have a pair of long palps extending from the head that they use for feeding, sweeping them around to gather up detritus and such. Boccardia differs from other genera in the Spionidae in having branchiae (vascularised appendages that function as gills) starting on the second segment of the body, and two differentiated spine rows on the fifth segment with falcate spines in the upper row and bristle-tipped spines in the lower row (Williams 2001).

One of the best-studied Boccardia species is B. proboscidea, a species about one or two centimetres in length found around various parts of the Pacific, including along the western coast of North America. Boccardia proboscidea is very catholic in its habitat preferences: it can be found in the intertidal or shallow subtidal zones, and anywhere from mudflats to rubble to reefs to burrowed into the shells used by hermit crabs (Gibson et al. 1999). It also shows the aforementioned variation in larval development: some individuals hatch as small larvae and live and feed as plankton, others feed on the yolks from nurse eggs and don't hatch until they reach a more advanced stage of development. Whichever way the individual develops, the resulting adult seems to be more or less the same.

Nevertheless, it would be fair to wonder if this variation is as it appears. Combine the variation in development with the variation in habits, and you might wonder whether two or more morphologically similar species are being confused. However, not only are the adults of each larval type completely interfertile, but differently developing individuals may even come from a single egg case. Gibson et al. (1999) compared individuals of this species from two widely separated populations both morphologically and genetically, and found that while there were some differences between the populations, there was little or no difference between developmentally distinct individuals within each population. How and why this developmental variation is maintained seems to be an open question but there is some evidence that other spionids may show the same plasticity. After all, it doesn't matter how you get there, so long as you get there.

REFERENCES

Gibson, G., I. G. Paterson, H. Taylor & B. Woolridge. 1999. Molecular and morphological evidence of a single species, Boccardia proboscidea (Polychaeta: Spionidae), with multiple development modes. Marine Biology 134: 743–751.

Williams, J. D. 2001. Polydora and related genera associated with hermit crabs from the Indo-West Pacific (Polychaeta: Spionidae), with descriptions of two new species and a second polydorid egg predator of hermit crabs. Pacific Science 55 (4): 429-465.

Sweat Bees

For many people, the common domestic honey bee may be the only bee species that they are aware of. In fact, bees are incredibly diverse, with well over 17,000 species known worldwide (and counting). Not all bees live in social hives like honey bees: the majority are solitary, with individual females each constructing their own nest and stocking it with food stores for their young. One particularly diverse group of bees is the Halictinae.

Foraging Lasioglossum, copyright Beatriz Moisset.


Halictines are mostly small bees, sometimes referred to as 'sweat bees' owing to the predilection of many species for lapping up sweat from the skin of hot humans and other animals (a habit that, while generally harmless, can be rather annoying). They can be distinguished from other bees by a distinctive curve at the base of the basal vein in the forewing. Michener (2007) recognised two tribes within the Halictinae, the cosmopolitan Halictini and the strictly Western Hemisphere Augochlorini. Augochlorins are often bright metallic in coloration; Halictini are less commonly so. Even among bee specialists, halictines can be notorious for the difficulties involved in trying to make sense of them. For instance, the cosmopolitan genus Lasioglossum alone comprises over 1300 known species, and having spent my own time attempting to identify bee specimens back in Australia I can confirm that there are times when it feels like all Lasioglossum, all the time. The majority of halictines construct their nests in burrows in soil; some species build in rotting wood.

Female Augochlora pura mosieri, copyright Bob Peterson.


The Halictinae are a particularly interesting group for studies of bee evolution because they include both solitary and social species. Indeed, some species may be either depending on circumstances. The most common nest type in Halictinae involves a long central tunnel with radiating side branches leading to globular brood cells. In most Augochlorini and species of the genus Halictus, however, the cells are arranged in a single cluster that is suspended within an underground cavity, held in place by earthen struts or by the rootlets of plants. The cells are lined with a protective waxy membrane rich in lactones, secreted by the builder from a gland near the base of the sting. Some species may be communal, with more than one female sharing a single burrow but each building and laying in its own cells (such communality is not necessarily a step on the road towards true sociality but may be a response to a shortage of good nesting opportunities). In social species, the queen is commonly not that different in appearance from associated workers, and if the queen dies the workers may begin producing eggs of their own (if, indeed, they were not already doing so while the queen was alive). Some species, though, may exhibit development of a distinct soldier or major class among the workers with massively enlarged heads and mandibles. In the Australian species Lasioglossum hemichalceum, there may be similarly large-headed males. These big-headed males also have reduced wings, rendering them flightless and bound to the nest. No more than one major male may be present in a colony; if another such male is present, the two will fight to the death. Unlike honey bees, halictine colonies do not often live for more than one season; instead, males and reproductive females usually mate near the end of the growing season, followed by the death of the males. The females hibernate over winter before beginning construction of their own nests the following spring.

Sphecodes albilabris, copyright Fritz Geller-Grimm.


In contrast, a number of halictine species, such as members of the genus Sphecodes, do not construct their own nests but instead lay their eggs in the nests of other bees. This behaviour, known as kleptoparasitism, has arisen in many bee lineages and is usually associated with a recurring set of evolutionary trends. Many kleptoparasites are closely related to their hosts: most kleptoparasitic halictines attack the nests of other halictines though some Sphecodes species mooch off bees in more distant subfamilies and families. Kleptoparasitic bees are commonly less hairy than their self-sufficient relatives, as they have little or no need of the pollen-carrying hairs used by other bees. Many kleptoparasites are more heavily armoured than other bees, to protect them against host resistance. Female Sphecodes have blunt spines on the outside of the hind tibia that may help them push into a host nest. Females of most kleptoparasitic halictines destroy the host egg in a nest cell before laying their own egg; in contrast, bees of other kleptoparasitic lineages usually leave the host egg undisturbed and it is the parasitic larva that executes the host. In most cases, the kleptoparasitic female abandons the nest once she has laid there, but in some species parasitising social hosts, the kleptoparasite may remain in the nest and inveigle herself into society there, continuing to enjoy the fruit's of her hosts' labours.

REFERENCE

Michener, C. D. 2007. The Bees of the World 2nd ed. John Hopkins University Press: Baltimore.

The Tiny Lurking Fear

The world of micro-organisms can be a cut-throat one. Minute grazers are under constant threat from minute predators. It can be an existence red in tooth and claw or, in the case of today's subjects, haemolymph-covered in chelicera and grasping seta.

The domestic cheyletid Cheyletus eruditus, from here.


The Cheyletini are one of fifteen tribes recognised by Bochkov & Fain (2001) in the mite family Cheyletidae. Cheyletids are small mites, generally less than half a millimetre in length, that are close relatives of the follicle mites seen on this site previously. Many cheyletids (including most Cheyletini) are, nonetheless, voracious predators of other mites. Other members of the family live as parasites on birds or mammals. In the past, such parasitic forms were recognised as a distinct family Cheyletiellidae but it is now recognised that they are descended from predatory ancestors, possibly on more than one occasion.

Predatory cheyletids are not to be sniffed at: the Hemicheyletia wellsina nymph on the left has managed to bring down another much larger predatory mite Metaseiulus occidentalis. Copyright Haleigh Ray.


The Cheyletini can be considered representative of this ancestral form; indeed, as members of the tribe are distinguished from others in the family solely by their retention of features likely to be primitive, it is likely to be non-monophyletic (Bochkov & Fain 2001). Cheyletini have more or less oval or oblong bodies with moderate-length legs, shorter than the length of the body, all tipped by a claw. The gnathosoma (the front section of the body bearing the chelicerae and palps) is well developed and generally makes up a full third of the body length. The palps are the real business end of a cheyletin, though. In many groups of prostigmatic mites, the last segment of the palp (the tarsus) is offset from the main line of the appendage and opposed to a large claw at the end of the tibia, the two of them together functioning to grab whatever the mite wishes to grab. Predatory cheyletids have the tibial claw and offset tarsus but the tarsus also bears a number of intimidating enlarged, claw-like setae that add to the mite's grabbing power. In the Cheyletini, there are four such setae at the end of the tarsus, a pair of comb-like setae dorsally and a pair of sickle-shaped setae ventrally. The mite will generally sit in place, motionless, with its palps held open. Should a potential prey animal come close enough to the predator, the palps will swing together and the prey will be caught.

Cheyletini are diverse in habitat. Many genera are free-ranging hunters on trees but others show preferences for more constrained locales. In particular, a group of genera centred around the type genus Cheyletus includes species living in the nests and burrows of mammals and birds. Most of these species benefit their hosts by hunting down potential parasites and the like or cleaning up organic residue. One genus, Cheletophyes, is found in the nests of carpenter bees Xylocopa and can actually be transported between nests by the host bee in special pockets on the thorax called acarinaria. However, it is not that big a step to take from feeding on shed organic particles in the host's nest to feeding more directly on the host itself and this is presumably how some cheyletids made the switch to parasitism. One member of the Cheyletini, Pavlovskicheyla platydemae, is an ectoparasite of tenebrionid beetles, attaching to them in spots concealed beneath the host elytra (Walter et al. 2009).

Female Hemicheyletia wellsina patrolling near her batch of eggs (in the upper left, under a protective silk covering she has woven for them), copyright Haleigh Ray.


Other Cheyletus species are known from human-associated habitats such as in houses or grain stores where, again, they are usually considered a net benefit due to their controlling effect on pests such as dust mites or flour mites. Indeed, the common species Cheyletus eruditus has been commercially marketed for control of stored product pests under the name Cheyletin. Females of this species in domestic habitats lay their eggs in crevices or other such concealed spaces and remain to guard their brood, driving away other animals that may pose a threat. However, hatching offspring need to disperse quickly, as if they hang around the nesting site too long they may be eaten by the mother herself (Walter & Krantz 2009).

REFERENCES

Bochkov, A. V., & A. Fain. 2001. Phylogeny and system of the Cheyletidae (Acari: Prostigmata) with special reference to their host-parasite associations. Bulletin de l’Institut Royal des Sciences Naturelles de Belgique, Entomologie 71: 5–36.

Walter, D. E., & G. W. Krantz. 2009. Oviposition and life stages. In: Krantz, G. W., & D. E. Walter (eds) A Manual of Acarology 3rd ed. pp. 57–63. Texas Tech University Press.

Walter, D. E., E. E. Lindquist, I. M. Smith, D. R. Cook & G. W. Krantz. 2009. Order Trombidiformes. In: Krantz, G. W., & D. E. Walter (eds) A Manual of Acarology 3rd ed. pp. 233–420. Texas Tech University Press.

Meet the Tityrids

South America may be the most biodiverse continent in the modern world. More species are known from the northern half of South America than from any comparable region of the planet. And yet, for whatever reason(s), many notable groups of South American animals remain distinctly under-represented in pop-culture depictions of biology.Take, for instance, the group of birds known as the New World suboscines. With something in the area of 1200 known species, this is an incredibly diverse group, but many popular bird books will devote far less attention to them than warranted in comparison to the more recognisable songbirds.

Masked tityra Tityra semifasciata costaricensis, copyright Nick Athanas.


Indeed, there are entire families of New World suboscines that barely raise a blip in the popular recognition stakes. Once such group is the Tityridae, a family of small to medium-sized insectivorous and fruit-eating birds found in tropical and subtropical regions of North and South America. Granted, part of this lack of representation may be due to tityrids not being recognised as a group until the late 1990s. Previously, the 30-odd species now placed in this family were divided between three larger related families: the Tyrannidae (tyrant flycatchers), Cotingidae (cotingas) and Pipridae (manakins). Nevertheless, it had long been recognised that each tityrid species was a poor fit in its original family, and in 1989 a group including most of the current tityrids (excluding only the genus Tityra) was proposed based on features of the syringeal anatomy (Barber & Rice 2007). Molecular data would later add Tityra into the mix and eventual inspire recognition of the family in its current form.

Brown-winged mourner or brown-winged schiffornis Schiffornis turdina wallacii, copyright Nick Athanas.


As recognised by Ohlson et al. (2013), the Tityridae includes seven genera divided between two subfamilies. The mourners of the genera Schiffornis, Laniisoma and Laniocera make up the subfamily Schiffornithinae (which has sometimes been labelled by the junior name Laniisominae). The Tityrinae includes the tityras Tityra, the purpletufts Iodopleura and the becards of the genera Xenopsaris and Pachyramphus (an earlier recognised becard genus Platypsaris is now generally synonymised with Pachyramphus). Some authors have also included the sharpbill Oxyruncus cristatus and three flycatcher genera Onychorhynchus, Myiobius and Terenotriccus in the Tityridae. A clade uniting these latter four genera with the tityrids was supported by Ohlson et al. (2013) though they chose to separate the latter taxa into distinct families, making this largely another taxon calibration question. It should be noted, however, that the name Oxyruncidae has priority over Tityridae so should properly be the name used if the broader clade is recognised as one family. Most recent authors who have united the two have insisted on ignoring this priority but their arguments for doing so seem generally handwavy and weak, based on the equally handwavy and weak concept of a 'traditional classification'.

Buff-throated purpletuft Iodopleura pipra, copyright Rick Elis Simpson.


For the most part, the tityrids are not brightly coloured birds. Perhaps the most dramatically coloured members are the tityras which are patterned in black and white and/or pale grey. The mourners are mostly more or less olive green; a couple of species are cinnamon brown and the Laniisoma species have yellow underparts and black caps. The dumpy little purpletufts get their name from bright patches of violet feathers on the shoulders of males but these are often concealed when the wings are closed. Becards, the most speciose subgroup with nearly twenty species in the genus Pachyramphus, come in a range of patterns from uniformly dark grey or cinnamon brown to grey or green and white to green and yellow. Males of a couple of species have a red bib on the throat. In those becard species previously placed in the genus Platypsaris, the males have patches of bright white feathers on the shoulders that are normally held concealed, only being revealed when the male is displaying to a female during courtship (Miller et al. 2015).

Green-backed becard Pachyramphus viridis viridis, copyright Cláudio Dias Timm.


The two subfamilies of tityrids differ from one another in their breeding behaviour (Barber & Rice 2007). Where breeding has been observed, the Schiffornithinae are polygamous with males not taking any part in nesting and rearing the chicks. The Tityrinae, in contrast, are generally monogamous with both parents doing their bit to feed their offspring. In Iodopleura species, parents may even be further assisted by offspring from previous clutches that have not yet begun breeding themselves. The ancestral nest type for Tityridae, as found in Schiffornithinae, Iodopleura and Xenopsaris, seems to have been a cup shape. Cup nests in Schiffornithinae are bulky and constructed from leaves; those of Iodopleura and Xenopsaris are more compact and woven from materials such as fungus, plant fibres and spider webs. In Tityra, the nest is cup-shaped but loose and concealed within a cavity in a tree. Finally, Pachyramphus species build globular nests with entrances at the side and below, and they may place their nest alongside a beehive just for that little bit of extra protection. The two groups also differ in their preferred habitats: schiffornithines are mostly found deep in forest interiors whereas Tityrinae tend to prefer more open habitats (Ohlson et al. 2013).

REFERENCES

Barber, B. R., & N. H. Rice. 2007. Systematics and evolution in the Tityrinae (Passeriformes: Tyrannoidea). Auk 124 (4): 1317–1329.

Miller, E. T., S. K. Wagner, J. Klavins, T. Brush & H. F. Greeney. 2015. Striking courtship displays in the becard clade Platypsaris. Wilson Journal of Ornithology 127 (1): 123–126.

Ohlson, J. I., M. Irestedt, P. G. P. Ericson & J. Fjeldså. 2013. Phylogeny and classification of the New World suboscines (Aves, Passeriformes). Zootaxa 3613 (1): 1-35.

With Fronds Like These

I'm sure pretty much anyone who's spent time looking into rock pools along the coast will be familiar with sea anemones. These sessile animals with their squidgy bodies and crown of tentacles can be seen almost anywhere there's a rock for them to stand on and a tide to cover them. As a kid, I used to amuse myself by poking them with a finger, noting the slight velcro-ish feel as the harassed anemone would vainly attempt to sting its attaker as it withdrew for protection. In hindsight, I was perhaps just fortunate that New Zealand anemones lacked the strength of venom to affect a human.

Waratah anemones Actinia tenebrosa, copyright John Turnbull.


Many of the anemones I was encountering as a child probably belong to a particular clade known as the Actinioidea. As recognised by Rodríguez et al. (2014), familiar members of this group include the beadlet anemone Actinia equina* from the Atlantic coasts of Europe and Africa, the red sea anemone Actinia tenebrosa of eastern Australia and New Zealand, and the aggregating anemone Anthopleura elegantissima and giant green anemone Anthopleura xanthogrammica of the Pacific coast of North America. Wikipedia informs me that another actinioid, the snakelocks anemone Anemonia viridis, is eaten after being marinated in vinegar and fried in parts of the Mediterranean. Rodríguez et al. recognised their Actinioidea primarily on the basis of molecular phylogenetic analysis but most members of this group had previously been recognised as relatives due to their possession of a sphincter muscle around the edge of the gastric cavity near the top of the column. This muscle allows the body cavity to be pulled tightly closed, providing protection and, for intertidal species, holding water inside the body to protect against desiccation.

*Actinia equina, offhand, was given its species name by Carl Linnaeus who described it under the name Priapus equinus. 'Equinus' means 'of a horse' whereas 'priapus' means... exactly what you think it means. Yes, the name of this species literally means 'hung like a donkey'.

Pompom anemone Liponema brevicornis, copyright Ocean Networks Australia.


Other common features of actinioids include well-developed muscles around the base of the column and an adhesive basal disc for clinging to rocks. However, both the upper sphincter muscle and the basal muscles have been lost in various subgroups of the actinioids, often at the same time. Anemones lacking these muscles, such as the ghost anemones Haloclava, are generally deeper water forms that do not cling to rocks but instead live burrowed into sand with their tentacles extended above the surface. One such anemone, the twelve-tentacled parasitic anemone Peachia qinquecapitata, develops as a larva as a parasite on the hydrozoan medusa Clytia gregaria. The larvae gain entry to their host by being eaten as food particles but proceed to themselves feed on the contents of the host's gastric cavity and eventually on the host itself. Another group of deep-sea actinioids, including such species as the deeplet anemone Bolocera tuediae and the pompom anemone Liponema brevicornis, are able to shed their tentacles as a defence thanks to small sphincter muscles at the base of each tentacle. Bolocera tuediae, found in the North Sea, is a particularly large anemone reaching up to a foot in diameter.

Aggregating anemones Anthopleura elegantissima fighting over space, copyright Brocken Inaglory. The white 'tentacles' the anemones are extending towards each other are inflated acrorhagi (see below).


Many actinioids form symbiotic associations with microscopic algae such as zooxanthellae, containing them within their body and supplementing their own nutrition through the algae's photosynthesis. A number of species reproduce by brooding larvae within the body cavity, only releasing them when they are more developed and better equipped to survive the outside world. Finally, many species of actinioid have the column ornamented by various protuberances such as vesicles or verrucae. These structures may serve environmental protective functions, such as increasing desiccation resistance or functioning in camouflage. Members of Anthopleura and related genera often have specialised bulbous protuberances called acrorhagi around the distal part of the column (Daly et al. 2017). These acrorhagi are packed with stinging cells and are used not so much to protect against predators as against other sea anemones. The acrorhagi-equipped anemone flails its column about, pressing the acrorhagi against any competitor that gets too close and stinging it until it is forced to back off. Its a tough world out there and any anemone worth its salt has got to be willing to defend its position.

REFERENCES

Daly, M., L. M. Crowley, P. Larson, E. Rodríguez, E. H. Saucier & D. G. Fautin. 2017. Anthopleura and the phylogeny of Actinioidea (Cnidaria: Anthozoa: Actiniaria). Organisms, Diversity & Evolution 17: 545–564.

Rodríguez, E., M. S. Barbeitos, M. R. Brugler, L. M. Crowley, A. Grajales, L. Gusmão, V. Häussermann, A. Reft & M. Daly. 2014. Hidden among sea anemones: the first comprehensive phylogenetic reconstruction of the order Actiniaria (Cnidaria, Anthozoa, Hexacorallia) reveals a novel group of hexacorals. PLoS One 9 (5): e96998.

Fusulinellidae, -inae, summat like that...

In an earlier post, I introduced you all to the fusulinids, a group of complex foraminiferans that were abundant during the later Palaeozoic. In that post, I alluded to the complex array of terminology that can be used when describing fusulinids but said that I would rather not cover it at that time. Well, this time I'm going to be dredging some of it up because I've drawn the Fusulinellidae as the topic for today's post.

Sectioned reconstruction of Fusulinella, from here. Labels: нк = primary chamber, са = septal folds, с = septa, сб = septal furrows, х = chomata, у = septal aperture, т = tunnel.


The Fusulinellidae as recognised by Vachard et al. (2013) are a family of fusulinids with fusiform or oblong tests known from the Middle to Late Pennsylvanian (during the later part of the Carboniferous). One genus, Pseudofusulinella, persists into the early Permian (Ross 1999). They are a part of the larger superfamily Fusulinoidea, a group of fusulinids characterised by what is known as a diaphanotheca. This is a thick, more or less translucent layer in the test wall. As noted in my earlier post, such a test structure may have functioned to allow light through to symbiotic microalgae (or possibly captured chloroplasts from algal prey) sheltered within. Fusulinellids are distinguished from other fusulinoids by the structure of the septa dividing chambers within the test, which are mostly flat except for some folding near the poles of the test (in the Fusulinidae, in contrast, the septal walls were folded throughout). As the test developed, sections of the septa were resorbed to form tunnels connecting adjacent chabers (and presumably allowing the transmission of materials between chambers in life). The course of the tunnels is commonly delimited within the chambers by chomata, discrete ridges of shell material. In other species, the chomata are absent but axial fillings of calcite were formed in the chambers instead.

How fusulinids are more commonly seen: sections of fusulinellid Dagmarella iowensis from Vachard et al. (2013). Image on left = subaxial section (scale bar = 0.1 mm); image on right, larger individual = tangential section (scale = 0.5 mm). The smaller individual on the right is a juvenile Profusulinella cf. fittsi, which depending on the author may or may not be considered a fusulinellid.


Being so widespread and abundant when they lived, fusulinellids are commonly used as index fossils for identifying when a deposit was formed. However, this process is complicated somewhat by ongoing debates about fusulinid systematics. Rauzer-Chernousova et al. (1996) proposed a classification of fusulinids that represented an extensive modification from previous systems. Part of this was simply a question of ranking, with Rauzer-Chernousova et al. recognising many groups at higher ranks than previously (so, for instance, recognising the separate family Fusulinellidae as opposed to its previous recognition as a subfamily of Fusulinidae). Nevertheless, some subsequent authors have felt that Rauzer-Chernousova et al. and their followers attribute too much significance to relatively minor variations. For instance, Kobayashi (2011) synonymised several genera under Profusulinella that Rauzer-Chernousova et al. regarded as belonging to distinct families (and Vachard et al. 2013 even placed in separate superfamilies). Some of the features regarded by Rauzer-Chernousova et al. as indicating separate genera were regarded by Kobayashi as representing variation within a single species. Indeed, there have even been arguments that some 'significant' features may represent post-mortem preservation artefacts (I've come across the term 'taphotaxa' used to refer to taxa based on such features). At present, my impression is that there is something of a geographical divide in preferred systems with eastern European authors following the lead of Rauzer-Chernousova et al. whereas authors from elsewhere may keep to a more conservative arrangement. The Berlin Wall may be down but the Fusulinid Cold War continues.

REFERENCES

Kobayashi, F. 2011. Two species of Profusulinella (P. aljutovica and P. ovata), early Moscovian (Pennsylvanian) fusulines from southern Turkey and subdivision of primitive groups of the family Fusulinidae. Rivista Italiana di Paleontologia e Stratigrafia 117 (1): 29–37.

Rauzer-Chernousova, D. M., F. R. Bensh, M. V. Vdovenko, N. B. Gibshman, E. Y. Leven, O. A. Lipina, E. A. Reitlinger, M. N. Solovieva & I. O. Chedija. 1996. Spravočnik po Sistematike Foraminifer Paleozoâ (Èndotiroidy, Fuzulinoidy). Rossijskaâ Akademiâ Nauk, Geologičeskij Institut, Moskva "Nauka".

Ross, C. A. 1999. Classification of the Upper Paleozoic superorders Endothyroida and Fusulinoida as part of the class Foraminifera. Journal of Foraminiferal Research 29 (3): 291–305.

Vachard, D., K. Krainer & S. G. Lucas. 2013. Pennsylvanian (Late Carboniferous) calcareous microfossils from Cedro Peak (New Mexico, USA). Part 2: smaller foraminifers and fusulinids. Annales de Paléontologie 99: 1–42.