Radiolarians of the Globe

Radiolarians are one of the primary groups of micro-organisms to be found among the marine plankton. These unicellular greeblies are justly famed for their intricate mineralised skeletons, leading to their comparison to living works of art. Today's post is covering one particular group of radiolarians, the Spumellaria.

Haeckel's (1899–1904) figure of Hexancistra quadricuspis from Kunstformen der Natur.

Spumellaria are one of the major subdivisions of radiolarians, containing species characterised by a generally spherical skeletal form. Many authors have also included the colonial radiolarians, which often lack a coherent skeleton and may form colonies up to several metres long, in the Spumellaria but these have more recently been treated as a distinct group. The skeleton of radiolarians is entirely enclosed by cytoplasm in life, though in those species in which the skeleton bears radiating spines, those spines may extend beyond the main body of the cell and be covered by only a thin cytoplasmic layer distally. In Spumellaria and anothre major radiolarian group, the Nassellaria, the skeleton is composed of opal, making these living jewels in more ways than one (another radiolarian group, the Acantharea, composes its skeleton of a mineral by the somewhat ethereal-sounding name of celestite). The cytoplasm of radiolarians is internally divided by a fibrous capsule into two structurally distinct sections, the internal endoplasm and external ectoplasm. The denser endoplasm contains most of the cell's primary organelles, such as the nucleus and large mitochondria. Linear microtubular structures called axonemes extend outwards from the endoplasm, passing through pores in the internal capsule and through the ectoplasm. The ectoplasm is often frothy in texture, containing an extensive assemblage of cellular vacuoles. In many of these radiolarians, some of these ectoplasmic vacuoles will house symbiotic algae that contribute much of the radiolarian's nutrition. Otherwise, radiolarians may feed on other small organisms that are captured on axopodia supported by the axonemes, which in spumellarians radiate outwards from the cell body in all directions. Extension and contraction of the axopodia may also help maintain the radiolarian's position in the water column (Cachon et al. 1990).

Schematic diagram of organisation of Didymocyrtis tetrathalamus from Sugiyama & Anderson (1998).

In many spumellarians, the basic skeletal architecture is one of nested spheres and/or globules. Sugiyama & Anderson's (1998) description of Didymocyrtis tetrathalamus stands as a fairly typical example. The central part of the skeleton is a double sphere well within the cytoplasmic capsule with the lobate nucleus contained in the spaces between the spheres. Radiating axes connect the inner shell with an outer shell mostly just outside the capsule (the capsular wall crosses the skeleton at some points). In Didymocyrtis, this outer shell is not spherical but a sort of peanut shape. At each end of the 'peanut', a further cap is added beyond the main shell. In many spumellarians, the outer shell appears spongy in texture, being constructed of densely criss-crossing fine opal fibres. There may be further extensions of the outer shell such as polar spines or funnels.

Not surprisingly, spumellarian classification has most commonly been based on skeletal architecture. Some attempts have been made to construct alternative classifications incorporating cytoplasmic features such as the relationship between the axopods and the nucleus (Cachon et al. 1990) but, as these systems require access to live specimens to place taxa, they have been less popular (especially as most people studying radiolarians are primarily working with fossil material). A phylogenetic study of recent spumellarians by Ishitani et al. (2012) found evidence for two main lineages within the class that differ in ecology. One, including the families Pyloniidae and Sponguridae, contained species found in temperate and cold waters. The other, including the families Astrosphaeridae, Hexalonchidae and Coccodiscidae, was found in tropical waters. Species assigned to the family Spongodiscidae were divided between both lineages, suggesting the need for some further tinkering with the morphological classification.

REFERENCES

Cachon, J., M. Cachon & K. W. Estep. 1990. Phylum Actinopoda. Classes Polycystina (=Radiolaria) and Phaeodaria. 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. 334–346. Jones & Bartlett Publishers: Boston.

Ishitani, Y., Y. Ujiié, C. de Vargas, F. Not & K. Takahashi. 2012. Two distinct lineages in the radiolarian order Spumellaria having different ecological preferences. Deep-Sea Research II 61–64: 172–178.

Sugiyama, K., & O. R. Anderson. 1998. Cytoplasmic organization and symbiotic associations of Didymocyrtis tetrathalamus (Haeckel) (Spumellaria, Radiolaria). Micropaleontology 44 (3): 277–289.

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.

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.

The Life and Times of Dissodinium

I've referred before to the position of the minute crustaceans known as copepods as one of the major groups of animals making up the marine zooplankton. Copepods form a significant part of the diet for a wide range of other marine animals: fish, molluscs, jellyfish, you name it. They are also targeted by other organisms coming in at a different scale.

Dissodinium pseudolunula: dinospores waiting to be released from the shell of a secondary cyst, copyright Gabriela Hannach.

Dissodinium is a genus of dinoflagellates, another group of organisms that has appeared on this site in the past. Most dinoflagellates are primarily photosynthetic but not Dissodinium: it's a parasite. Specifically, it's a parasite of copepod eggs. Copepods produce relatively large eggs compared to their body size that are full of tasty lipids and other nutrients so it's hardly surprising that they would attract attention. The free-swimming dinospore of Dissodinium initially looks much like a typical dinoflagellate but once they attach to a copepod egg they produce a sucker-like organelle through which they slurp up the egg's contents, swelling to a globular blob. When feeding is finished, this blob detaches from the remains of the egg to begin the process of reproduction.

There are two species of Dissodinium whose asexual life cycles were described by Elbrächter & Drebes (1978). I haven't found any reference to a known sexual reproduction cycle for Dissodinium. In both species, the replete individual forms a spherical primary cyst that floats free within the plankton. The contents of the primary cyst divide within the cyst wall to form the next stage, the secondary cysts. In the most commonly seen species, Dissodinium pseudolunula*, these secondary cysts are distinctively crescent-shaped. Following their release from the original primary cyst wall, the cytoplasm within the secondary cysts further subdivides to form the actively swimming dinospores. These dinospores presumably function as the infective stage for another round of the cycle but it should be noted that Gómez et al. (2009) were unable to induce infection when they incubated newly released dinospores together with copepod eggs. Instead, the dinospores encysted themselves in a hyaline membrane and Gómez et al. suggested that some sort of maturation period may be necessary before infection can take place. The second species of Dissodinium, D. pseudocalani, differs in that the secondary cysts are not crescent-shaped, and divide to release the dinospores while still themselves contained within the original primary cyst wall so the breakdown of the latter releases dinospores directly into the environment. This compression of the life cycle has also sometimes been observed with D. pseudolunula.

*This species has often masqueraded in the past under the name of Dissodinium lunula. The name 'Gymnodinium lunula' was originally used for crescent-shaped cysts by Schütt in 1895. Unfortunately, Schütt's figured examples of this 'species' included representatives of two quite different dinoflagellates, now classified as Dissodinium and another genus Pyrocystis that is not parasitic. The name lunula has become properly attached to the latter species, requiring a different name for the Dissodinium.

Stages in the life cycle of Dissodinium pseudolunula, from Elbrächter & Drebes (1978), running from a freshly released primary cyst at top left to a newly attached parasitic dinospore at bottom right.

Elbrächter & Drebes (1978) included Dissodinium in the Blastodiniales, a morphologically diverse group of parasitic dinoflagellates. The advent of molecular analyses would later demonstrate this grouping to be polyphyletic with parasitic dinoflagellates evolving on numerous occasions from free-living ancestors. Instead, Dissodinium and another parasite of copepod eggs, Chytriodinium, form a clade that is closely related to the major free-living genus Gymnodinium (Gómez et al. 2009). Gómez et al. also found that D. pseudolunula retains some elements of its free-living ancestry: it still retains chlorophyll (chlorophyll is absent in D. pseudocalani and Chytriodinium). Just how functional this chlorophyll remains is an open question: it appears less concentrated within the cell than in a typical photosynthetic dinoflagellate, and Gómez et al. were unable to maintain a culture of D. pseudolunula under conditions that would support a free-living species. Nevertheless, they suggested that a low level of photosynthesis might supplement the dinoflagellate's nutrient requirements while it waited out the aforementioned incubation period before finding itself a host.

REFERENCES

Elbrächter, M., & G. Drebes. 1978. Life cycles, phylogeny and taxonomy of Dissodinium and Pyrocystis (Dinophyta). Helgoländer wiss. Meeresunters. 31: 347–366.

Gómez, F., D. Moreira & P. López-García. 2009. Life cycle and molecular phylogeny of the dinoflagellates Chytriodinium and Dissodinium, ectoparasites of copepod eggs. European Journal of Protistology 45: 260–270.

Tinned Psammon

Psammonobiotus communis, copyright Hugh MacIsaac.

In several previous posts on this site, I have discussed representatives of the remarkable group of organisms that are the Foraminifera. However, forams are not the only group of unicellular amoeboids to encase themselves in a shell. Today, I want to consider another such group, the Psammonobiotidae.

Psammonobiotids are a group of testate amoeboids forming part of (as their name suggests) the psammon, the community of organisms inhabiting the interstitial spaces between sand grains along the edge of the sea. Until the 1960s and '70s, most authors who encountered amoebae tests in marine samples assumed that they were the remains of freshwater organisms washed downstream (Golemansky 2008). Eventually, though, it was realised that there is quite a diversity of amoeboids that not only tolerate salty conditions, they prefer it. The Psammonobiotidae was recognised in the 1970s for a number such organisms. They produce a proteinaceous test without regular scales, the test structure being amorphous or composed of irregular plates. The test is generally more or less flattend to help the organism fit into the narrow spaces between grains. An aperture at one end of the test allows the organism access to the outside world; in many cases, this aperture may be bent to one side to allow the test to lie close to its substrate.

Campascus minutus, from Microworld.

Many psammonobiotids inhabit the supralittoral zone, just above the high tide mark. Groundwater in this region forms the contact zone between fresh water flowing out from under the land and salt water coming in from the sea. As a result, psammonobiotids and other inhabitants of this region need to be able to handle constantly shifting salinity levels. Many interstitial amoeboids can handle variations from 2% salinity in merely brackish waters to 37% in warm tropical seas (Golemansky 2008). Some normally marine psammonobiotids have even been recorded from entirely freshwater streams (Golemansky & Todorov 2007) though I personally suspect misidentifications may be involved.

The relationships of psammonobiotids to other testate amoeboids requires research (Adl et al. 2012). They possess filose rather than lobose pseudopodia, indicating relationships with other testate amoeboid groups in the Cercozoa. A leading possibility is a relation to the Euglyphida, which resemble psammonobiotids in many features but have tests with distinct scales. I haven't found any references to any psammonobiotids being covered by molecular analyses which may reveal where they really come from.

REFERENCES

Adl, S. M., A. G. B. Simpson, C. E. Lane, J. Lukeš, D. Bass, S. S. Bowser, M. W. Brown, F. Burki, M. Dunthorn, V. Hampl, A. Heiss, M. Hoppenrath, E. Lara, E. Le Gall, D. H. Lynn, H. McManus, E. A. D. Mitchell, S. E. Mozley-Stanridge, L. W. Parfrey, J. Pawlowski, S. Rueckert, L. Shadwick, C. L. Schoch, A. Smirnov & F. W. Spiegel. 2012. The revised classification of eukaryotes. Journal of Eukaryotic Microbiology 59 (5): 429-493.

Golemansky, V. 2008. Origin, phylogenetic relations, and adaptations of the marine interstitial testate amoebae (Rhizopoda: Lobosea, Filosea, and Granuloreticulsea). In: Makarov, S. E., & R. N. Dimitrijević. Advances in Arachnology and Developmental Biology. Papers dedicated to Prof. Dr. Božidar Ćurčić pp. 87–100. Inst. Zool, Belgrade; BAS, Sofia; Fac. Life Sci., Vienna; SASA, Belgrade & UNESCO MAB Serbia.

Golemansky, V., & M. Todorov. 2007. Taxonomic review of the genus Centropyxiella (Rhizopoda: Filosea) with data on its biology and geographical distribution. Acta Zoologica Bulgarica 59 (3): 227–240.

Holding Forams Together

Nouria polymorphinoides, from Foraminifera.eu.

In past posts relating to the Foraminifera, I've made reference to the changes in classification undergone by this group over the years. Forams are unusual among unicellular organisms in producing a hard, often complex test that means they have both left an extensive fossil record and provided a number of characters on which to base a classification. However, there has been much disagreement over the relative attention due to particular features of the test. The classification used for forams in the Treatise on Invertebrate Paleontology by Loeblich & Tappan (1964), one of most influential sources in recent decades, made its primary divisions on the basis of the structure and chemistry of the test itself. Forams that produce a test by gluing together (agglutinating) sand particles and other foreign objects were treated as fundamentally distinct from those that secreted calcareous tests. Because the foram cell itself is amoeboid, there was an underlying assumption that the test architecture was too mutable to indicate anything more than low-level relationships.

However, there were some prominent inconsistencies with this assumption (Mikhalevich 2013). One is that the division between agglutinated and calcareous tests is not always perfect. Agglutinated forams might not secrete the bulk of the test themselves but they do secrete the cement used to hold the sand grains together, and there is a definite spectrum in the proportion of sand to cement used by a given foram. In some agglutinated forms, a distinct calcareous layer may underlie the agglutinated section of the test, and it is easy to envision how a progressive reduction in the proportion of agglutinated material could lead to the evolution of an entirely secreted test. This was not in itself fatal to the earlier system as it had generally been assumed that agglutinated forams were likely to represent a paraphyletic group. More problematic was the common appearance of foram species that were extremely similar in test architecture with the only really significant difference being that one was agglutinated and the other calcareous. This lead some authors to argue that whereas a small number of such cases might be accepted as the result of convergence, the abundance of such cases suggested that changes in test composition were more common than previously recognised. Molecular studies of forams are still in their infancy but have offered some support for the significance of test architecture, such as the division between globular and tubular forams (Pawlowski et al. 2013) that I referred to in an earlier post.

Liebusella goesi, from Foram Barcoding.

One effect of this change in focus is that the Mikhalevich (2013) classification divides the agglutinated forams between a number of groups that are not recognised in alternative systems. One such group is the Nouriida, known from the Cretaceous to the present day. Mikhalevich included the Nouriida in a larger group called the Hormosinana; at least one hormosinanan was placed by Pawlowski et al. (2013) at the base of the globular foram lineage. In contrast, Loeblich & Tappan (1964) included most of the nouriidans in the family Ataxophragmiidae, other members of which belong to the tubular forams. Nouriida and other Hormosinana are united by having the aperture of the test in a terminal position; in some nouriidans, it may be raised on a short neck. Nouriida differ from other hormosinanans in the arrangement of chambers in the test. In early stages they tend to be more or less trochospiral; with maturity, the number of chambers to a whorl decreases and the test may become biserial or uniserial. The two subfamilies recognised within the Nouriida by Mikhalevich differ in the internal structure of their chambers: Nourioidea have internally simple chambers but Liebuselloidea have the lumen of the chambers complexly subdivided.

I haven't found much about their ecological role; at least one modern species, Nouria polymorphinoides, seems to be not uncommon in shallower continental shelf waters worldwide. My general impression (just confirmed by asking a colleague who actually works on forams) is that agglutinated forams receive far less attention than calcareous ones. A big part of this is simply that they're harder to find: it takes a lot of practice to be able to pick out an actual agglutinated foram test from any other conglomeration of sand, and if they break apart during sample prep (which they often do) then there is little sign they were ever there to begin with.

REFERENCES

Loeblich, A. R., Jr, & H. Tappan. 1964. Treatise on Invertebrate Paleontology pt C. Protista 2. Sarcodina: chiefly "thecamoebians" and Foraminiferida vol. 1. The Geological Society of America, and The University of Kansas Press.

Mikhalevich, V. I. 2013. New insight into the systematics and evolution of the Foraminifera. Micropaleontology 59 (6): 493–527.

Pawlowski, J., M. Holzmann & J. Tyszka. 2013. New supraordinal classification of Foraminifera: molecules meet morphology. Marine Micropalaeontology 100: 1–10.

Meandering Forams

Specimen of Meandropsina vidali, showing the patterning on the external surface, from Loeblich & Tappan (1964).

There are some taxonomic names that just instantly bring up a mental image of the sort of organism to which they refer. For my part, I've always felt that Meandropsina is one of those names. The Meandropsinidae are another family of relatively large and complex foraminifera (growing up to a number of millimetres across) that are known only from the Upper Cretaceous. The several genera of the family are predominantly European, with only the genus Fallotia also known from the West Indies.

Cross-section of Meandropsina vidali, from Loeblich & Tappan (1964).

Meandropsinids are (as far as I know) more or less lenticular in shape with chambers enrolled in a flat spiral. The name of the type genus Meandropsina refers to the way that the outer margins of the chambers tend to meander irregularly around the test, giving it something of an ornate appearance. Both molecular and structural evidence indicate that multi-chambered forams arose from ancestors with undivided tests on more than one occasion, and the majority of multi-chambered forams can be assigned to two major lineages (Pawlowski et al. 2013). In one lineage, the Globothalamea (which includes, for instance, the rotaliids), the basic chamber shape is globular with successive chambers in the test being wider than long. In the other lineage, the Tubothalamea (including the miliolids and spirillinids), the basic chamber shape is tubular, and the test may grow through a number of spirals before it even starts to be divided into chambers (if at all). Members of the two lineages with calcareous tests may also be distinguished by their test structure: in calcareous globothalameans, the crystals making up the test are arranged regularly so the overall appearance of the test is hyaline (glass-like). In contrast, tubothalameans have the crystals of the test arranged irregularly so the appearance of the test is porcelaneous (like porcelain). Meandropsinids are unmistakeably tubothalameans in both regards.

Like other large forams of the Mesozoic, meandropsinids did not make it past the end of the Cretaceous. Early Palaeocene taxa that have been included in the families represent distinct lineages that evolved to take their place, occupying the ecological spaces opened up by the mass extinction ending the era.

REFERENCES

Loeblich, A. R., Jr, & H. Tappan. 1964. Treatise on Invertebrate Paleontology pt C. Protista 2. Sarcodina: chiefly "thecamoebians" and Foraminiferida vol. 1. The Geological Society of America, and The University of Kansas Press.

Pawlowski, J., M. Holzmann & J. Tyszka. 2013. New supraordinal classification of Foraminifera: molecules meet morphology. Marine Micropalaeontology 100: 1–10.

The Forams that Bind

Cross-section of Fabiania cassis, from BouDagher-Fadel (2008).

Here we see an example of Fabiania. Fabiania is a genus of foraminiferan known from the Eocene epoch that could reach a relatively large size as forams go, up to several millimetres across (nowhere near as large as some that I've covered on this site, maybe, but still respectable). It had a conical test with a rounded apex and a deeply excavated centre; depending on growing conditions, individual Fabiania might be a regular or a flattened cone. In its early stage, Fabiania had two globose thick-walled and perforate chambers; later chambers were cyclical and divided by horizontal and vertical partitions. The aperture of the test was a single row of pores opening into the large umbilicus. The wall of the test was thick and calcareous, and covered with coarse perforations on the upper side of the cone (BouDagher-Fadel 2008; Loeblich & Tappan 1964).

Fabiania lived in association with coral reefs, often preferring the undersides of corals and other sheltered locations. It was primarily found around the mid-depths, not too close to the water's surface but also not too deep (Bosellini & Papazzoni 2003). I've referred in an earlier post to another group of coral-encrusting forams, the acervulinids. Because reef forams tend to be cryptic (in more exposed parts of the reef they tend to get out-competed by coralline algae), and are often variable in morphology making them taxonomically difficult, they tend to be less studied than the reef's more prominent components. However, forams may play a not so insignificant role in developing the reef's structure, helping to bind the reef in place.

REFERENCES

Bosellini, F. R., & C. A. Papazzoni. 2003. Palaeoecological significance of coral-encrusting foraminiferan associations: a case-study from the Upper Eocene of northern Italy. Acta Palaeontologica Polonica 48 (2): 279–292.

BouDagher-Fadel, M. K. 2008. The Cenozoic larger benthic foraminifera: the Palaeogene. Developments in Palaeontology and Stratigraphy 21: 297–418.

Loeblich, A. R., Jr & H. Tappan. 1964. Treatise on Invertebrate Paleontology pt C. Protista 2. Sarcodina, chiefly "thecamoebians" and Foraminiferida vol. 1. The Geological Society of America and The University of Kansas Press.

Fusulinoids: Complex Forams of the Late Palaeozoic

Among the most characteristic fossils of the latter part of the Palaeozoic are the group of Foraminifera known as the fusulinoids. These forams, known from around the middle of the Carboniferous to the end of the Permian, can be extremely abundant. Indeed, I get the impression that some fossil deposits are pretty much made of fusulinoids. Fusulinoids did not merely thrive in their environment; they were the environment.

Limestone block dominated by fusulinids, copyright James St John. Field of view is about 3.9 cm across.

Fusulinoids are distinguished from other forams by their test composition, built from minute granules of calcite, and complex internal structure. Externally, fusulinoids (defined here to exclude their forerunners, the endothyroids) were fairly conservative, with a planispiral, usually involute test (that is, each successive whorl covers the last). The last whorl ended on a transverse wall without a defined aperture; instead, the only connection between the interior and exterior of the test was by a series of pores in said wall. Early forms were disc-shaped; later species could be more globular or fusiform. Some of the later fusulinoids also reached gigantic sizes by single-celled organism standards: whereas the earliest fusulinoids were only a fraction of a millimetre across, the late Permian Polydiexodina could be up to six centimetres along their longest axis (Loeblich & Tappan 1964). Internally, fusulinoids had an incredibly complicated and varied structure which I'm not going to go into too much detail about here, primarily because I barely understand a word of it myself. Any description of fusulinoid morphology quickly devolves into madly throwing about terms like chomata, parachomata, spirotheca, tectorium, and the like, and your humble narrator feeling the need to go look at something else.

Cutaway diagram of a fusulinid, showing an example of internal structure, from here.

I have to go into some detail, though, because some features of the fusulinoid wall structure may explain their success. The ancestral state for the fusulinoid test wall involved a thin layer of solid calcite, the tectum. In most species, the inside of the tectum was coated with a thicker, less dense layer. As the test wall becomes more derived, this inner layer becomes more or less translucent, or pierced by tubular alveoli to produce a honeycomb-like appearance. It has been suggested that these modifications may have been adaptations to accomodating symbiotic microalgae, striking a balance between maintaining the protective test and allowing optimal transmission of light. Microalgal associations with fusulinoids may be corroborated by the discovery of minute fossils of probable planktonic relationships such as Ovummuridae preserved within fusulinoid tests (Vachard et al. 2004).

Ecologically, fusulinoids were restricted to off-shore marine habitats, being mostly found preserved in limestones and calcareous shales. They are absent from deposits that would have been formed in brackish water, and while they may be found in sandstones it is debatable whether such occurrences represent life associations or post-mortem transport (Loeblich & Tappan 1964). Fusulinoids would therefore have been ecologically similar to the inhabitants of modern-day photic zone coral reefs, another reflection of their probable co-dependence with photosynthetic microalgae. However, as successful as the advanced fusulinoids were in their time, they did not make it past the massive extinction event at the end of the Permian. This was not the end of giant and complex forams entirely—indeed, some later forms such as the alveolinids would evolve morphologies very similar to those of fusulinoids—but it was the end of these particular giant forams.

REFERENCES

Loeblich, A. R., Jr & H. Tappan. 1964. Treatise on Invertebrate Paleontology pt C. Protista 2. Sarcodina, chiefly "thecamoebians" and Foraminiferida vol. 1. The Geological Society of America and The University of Kansas Press.

Vachard, D., A. Munnecke & T. Servais. 2004. New SEM observations of keriothecal walls: implications for the evolution of Fusulinida. Journal of Foraminiferal Research 34 (3): 232–242.

A Western Rockweed

Rockweed Silvetia compressa, from here.

Silvetia compressa is a species of brown alga found on shorelines on the western coast of North America, from British Columbia to Baja California. It is a member of the wrack family Fucaceae that I covered in an earlier post. Silvetia compressa is found in midtidal habitats, generally higher up on the shoreline than other large seaweeds. Individual thalli can reach a maximum length of about three feet (90 cm) but are often smaller. This is a slow-growing species, so patches of Silvetia are slow to recover from damage due to trampling and other disturbance. Please try to avoid walking on the rockweed!

Thalli of Silvetia compressa are composed of thin strands a few millinetres in width with irregular, dichotomous branching. Strands of the thalli lack a midrib (distinguishing them from some other Fucaceae species found in the same area). The width of the strands and regularity of the branching varies with environmental conditions: for instance, individuals growing in locations with stronger wave action have more robust strands that branch more frequently. As with other Fucaceae, the reproductive structures a produced on swollen branch tips called receptacles, but these receptacles do not become inflated with gases and buoyant like those of other species. The exact size and shape of the receptacles is, again, variable.

In many older references, Silvetia compressa may be referred to as Pelvetia fastigiata. The supposed species 'Fucodium compressum' and 'F. fastigiatum' were originally distinguished on the basis that the latter was smaller than the former with more fastigiate branches (that is, the branches remained subparallel). As indicated above, these characters represent the effects of environmental conditions, not fixed differences (Silva 1996). They were eventually included in the genus Pelvetia, together with the Atlantic species P. canaliculata, on the basis of the thalli without a midrib, and the production of just two eggs from each oogonium in the receptacles. However, later analyses supported the separation of the Atlantic and Pacific species of Pelvetia. Not only did they not form a clade in molecular analyses, the eggs in the oogonia were separated by a horizontal division in the Atlantic species but a longitudinal or oblique division in the Pacific species (Silva et al. 2004). As such, the Pacific Pelvetia were transferred into a new genus Silvetia.

Further taxonomic complications involved subspecific variation in Silvetia compressa. A distinctive form of 'Pelvetia fastigiata' found at Pebble Beach in California's Monterey Bay, with smaller, finer thalli and more abundant, regular branching, was labelled as a separate forma gracilis. Similar individuals were also found on the islands off California's coast. However, when Silva (1996) examined the original type specimen of P. fastigiata, he discovered that it was an individual of this 'gracilis' form, not the more typical larger form. Later, Silva et al. (2004) examined genetic variation within the Silvetia compressa of California and Baja California. They found that the individuals of the offshore islands were indeed genetically distinct from continental individuals. As well as the differences in growth habit, there was also some difference in receptacle shape: the continental form had receptacles that tended to be linear and pointed whereas those of the island form were ellipsoidal and blunt. However, the island form still could not be labelled with either of the 'fastigiata' or 'gracilis' monikers, as individuals from the type locality of Pebble Beach did not align genetically with insular individuals but with other continental forms. As such, yet another name had to be coined for the insular form which now goes by the name of Silvetia compressa ssp. deliquescens. Let's see if it sticks this time.

REFERENCES

Silva, P. C. 1996. California seaweeds collected by the Malaspina expedition, especially Pelvetia (Fucales, Phaeophyceae). Madroño 43 (3): 345–354.

Silva, P. C., F. F. Pedroche, M. E. Chacana, R. Aguilar-Rosa, L. E. Aguilar-Rosa & J. Raum. 2004. Geographic correlation of morphological and molecular variation in Silvetia compressa (Fucaceae, Fucales, Phaeophyceae). Phycologia 43 (2): 204–214.

Forams with Teeth

Time for another foram post. The above image (copyright Robert P. Speijer, scale bar = 100 µm) shows Turrilina brevispira, a typical Eocene representative of the foram subfamily Turrilininae.

The Turrilininae are a group of calcareous forams that first appeared in Middle Jurassic (Loeblich & Tappan 1964). In most species, the test is what is called a 'high trochospiral' form: that is, it coils in a similar manner to, and overall looks rather like, a high-shelled snail. Each of these whorls is divided into at least three successive chambers, sometimes more. At the end of the test is a loop-shaped aperture. At least one species of turrilinine, Floresina amphiphaga, is a predator/parasite of other forams, drilling into their test to extract their protoplasm.

The turrilinines are most commonly classified in a broader foram superfamily known as the Buliminoidea or Bulimnacea. Other buliminoids commonly resemble turrilinines in their overall form. The group has commonly been defined, however, on the basis of what is called a 'tooth-plate'. This is an outgrowth of the internal wall of the test that runs between the apertures of each chamber. The exact appearance of the tooth-plate differs between taxa; in Turrilina, for instance, it is a trough-shaped pillar that is usually serrated along one end (Revets 1987). I have no idea what the function of the tooth-plate is, if indeed any is known, whether it provides an anchor for some cytoplasmic structure or anything else. However, in more recent decades a number of authors have questioned whether the tooth-plate is as significant a taxonomic feature as previously thought. For instance, Tosaia is a Recent genus of foram whose overall morphology and chamber arrangement is fairly typical for the Turrilininae but which lacks any sign of a tooth-plate (Nomura 1985). Excluding Tosaia from the buliminoids on this basis alone would imply a remarkably strong evolutionary convergence of every other feature of this genus.

REFERENCES

Loeblich, A. R., Jr & H. Tappan. 1964. Treatise on Invertebrate Paleontology pt C. Protista 2. Sarcodina, chiefly "thecamoebians" and Foraminiferida vol. 2. The Geological Society of America and the University of Kansas Press.

Nomura, R. 1985. On the genus Tosaia (Foraminiferida) and its suprageneric classification. Journal of Paleontology 59 (1): 222–225.

Revets, S. A. 1987. A revision of the genus Turrilina Andreae, 1884. Journal of Foraminiferal Research 17 (4): 321–332.