Callocystitids: Ambulacra Advancement and Rhomb Reduction

The Upper Silurian callocystitid Staurocystis quadrifasciata, from Museum Victoria.

The Palaeozoic echinoderms included many distinctive groups that have no close relatives among the modern fauna: blastoids, cornutes, solutes, ctenocystoids... to name just a few. From the Ordovician to the Devonian, this diverse fauna also included a hodge-podge assemblage known as cystoids. Cystoids are a grouping of mostly stalked echinoderms in which certain plates in the theca are perforated by regular arrangements of pores that probably functioned in respiration. Cystoids were not always regularly pentamerous like other echinoderms, and some were notably asymmetrical. The ambulacra were recumbent on the theca, and the feeding appendages were brachioles rather than arms (for the difference between brachioles found in many fossil echinoderms and arms found in crinoids, see the post on blastoids). Cystoids would have been filter-feeders and were probably largely sedentary. Cystoids include some very disparate forms, and many researchers have suggested that they may represent a polyphyletic assemblage. Various authors have suggested cystoid ancestry for other echinoderm groups, such as blastoids or crinoids, but this remains controversial.

The Upper Silurian Schizocystis armata, from Kesling (1967). The two pore rhombs of this species are visible just above the center and at the lower right of the theca.

The Callocystitidae were a family of cystoids that persisted over most of the total cystoid time range. Callocystitids belonged to the major cystoid subgroup called the Rhombifera, in which the diagnostic pore groups were arranged as paired assemblies, commonly called pore rhombs, that spanned the border between two thecal plates (as opposed to the remaining cystoids, the Diploporita, in which pore assemblies each occupied a single plate). Broadhead & Strimple (1978) diagnosed the Callocystitidae based on the arrangement and position of the pore rhombs, together with their possession of a relatively small periproct (the circle of plates that indicates the position of the anus) and the number of radial plates in the theca. All callocystitids possessed a stalk, often divided into a flexible proximal section and a more rigid distal section. Broadhead & Strimple (1978) recognised four subfamilies of callosystitids, but one of these, the Apiocystitinae was explicitly suggested to be paraphyletic to the Callocystitinae and Staurocystinae. This was supported by the numerical phylogenetic analysis of Sumrall & Brett (2002), who furthermore suggested that the Callocystitinae was polyphyletic.

Theca of the Upper Silurian apiocystitine Lovenicystis angelini, from Kesling (1967).

The fourth of Broadhead & Strimple's subfamilies, the Scoliocystinae, was suggested to lie outside the clade formed by the other three; Sumrall & Brett's analysis only included Scoliocystis, but does not contradict this. Scoliocystines have the ambulacra relatively short, restricted to the summit of the theca, and would have had only a small number of brachioles. The most extreme example was the Lower Silurian Osculocystis, which had only a single extremely long brachiole (Paul & Donovan 2011). Another scoliocystine, Schizocystis, had one side of the theca relatively flat and the pore rhombs reduced in number and restricted to the other side, and may have lain on its side in life rather than standing upright.

Reconstruction of Pseudocrinites together with a number of individuals of the discosorid Phragmoceras by Alison Carey.

The remaining three subfamilies had more extensive ambulacra, extending right down to the base of the theca in some species. Apiocystitines and callocystitines had four or five ambulacra, usually branched in callocystitines and unbranched in apiocystitines, that did not strongly protrude above the surface of the theca and had widely spaced brachioles. The more distinctive Staurocystinae had two to four stongly protruding ambulacra that carried tightly packed brachioles. In the staurocystine Pseudocrinites, the theca was discus-shaped with its two ambulacra running around the outer rim of the disc (Kesling 1967).

REFERENCES

Broadhead, T. W., & H. L. Strimple. 1978. Systematics and distribution of the Callocystitidae (Echinodermata, Rhombifera). Journal of Paleontology 52 (1): 164-177.

Kesling, R. V. 1967. Cystoids. In Treatise on Invertebrate Paleontology pt. S. Echinodermata 1. General characters. Homalozoa-Crinozoa (except Crinoidea) (R. C. Moore, ed.) vol. 1 pp. S85-S267. The Geological Society of America, Inc., and The University of Kansas: Lawrence (Kansas).

Paul, C. R. C., & S. K. Donovan. 2011. A review of the British Silurian cystoids. Geological Journal 46: 434-450.

Sumrall, C. D., & C. E. Brett. 2002. A revision of Novacystis hawkesi Paul and Bolton 1991 (Middle Silurian: Glyptocystitida, Echinodermata) and the phylogeny of early callocystitids. Journal of Paleontology 76 (4): 733-740.

The Gagrella Problem Cranked Up to Eleven

A Leiobunum eating a cricket, photographed in North Carolina by Jeffrey Pippen.

In previous posts on this site, I have referred to the problem of Gagrella: a large genus of Asian harvestmen diagnosed on the basis of characters long since recognised as unreliable that remains unrevised because no-one has yet been in a position to take on the amount of work required. Well, a paper has come out in the last few weeks that indicates that the Gagrella issue is just part of a larger problem, one of truly demonic proportions.

The British Nelima gothica, photographed by Gordon.

Gagrella belongs to a family of harvestmen called the Sclerosomatidae. Sclerosomatids are distinguished by having a more heavily sclerotised dorsum than most other long-legged harvestmen, and by the morphology of the male genitalia, with many (but not all) species having lateral extensions (like wings) on the shaft just behind the glans. Within the Sclerosomatidae, most authors have recognised four subfamilies: Sclerosomatinae, Gagrellinae, Leiobuninae and Gyinae. The boundaries between the subfamilies have long been realised to be a bit fuzzy, particularly between the Leiobuninae and Gagrellinae. A few molecular studies (for instance, Giribet et al. 2010) that have included more than one representative of more than one subfamily have failed to resolve them as separate. Nevertheless, the subfamilies have served as a convenient way to divide what is a quite large family of over 1200 described species, at least until a more extensive analysis can be conducted.

Enter Hedin et al. (2012) who take molecular data for six genes from seventy-odd sclerosomatids, mostly of the Holarctic genera Leiobunum and Nelima (both Leiobuninae). One suspects that the primary focus of the authors was originally just the phylogeny of these two genera, with the smattering of other sclerosomatids in the mix primarily intended as outgroups. But then they got a result that looks like this (click on the image for a higher resolution):

Yikes. That is a mess. The two main genera have not only failed to resolve as monophyletic, they have completely exploded. Gyas (the type species of the Gyinae) has apparently been so terrified by this show of opilionid pyrotechnics that it has scurried off to join the Phalangiidae. The Gagrellinae have also undergone something of a breakdown, appearing in four separate places on the tree (two of those including species supposedly of a single genus). But this is not simply a matter of poor resolution: many of the incongruent clades are reasonably supported. It is also worth noting that the results are not inconsistent with those recovered by Giribet et al. (2010) who included seven sclerosomatid species in their analysis.

As noted by the authors, the distribution of taxa in the results is not entirely random. There is a certain amount of biogeographic patterning, with the European and Japanese taxa included forming distinct clades, while the New World taxa form three clades. Within the Japanese taxa (which are some of the best studied sclerosomatids), there is some correlation with species groups recognised on morphological grounds, such as monophyly of the Leiobunum curvipalpe group. This latter example is interesting, as it represents a group of species that exhibit conservative features of external morphology despite showing broad variation in genitalic characters (Tsurusaki 1985).

The sclerosomatine Astrobunus laevipes, photographed by Ch. Komposch.

Draining the morass of sclerosomatid taxonomy and phylogeny will doubtless be an arduous process, with all the large genera currently recognised likely to be polyphyletic. The support by Hedin et al. for previously recognised groups such as the Leiobunum curvipalpe complex suggest that morphological features will not be irrelevant in revising the family, but they will have to be analysed in their proper context. For instance, Hedin et al. suggest that the recovered polyphyly of Nelima may indicate that this genus, primarily defined by the absence of ornamentation found in other genera, may represent convergence through paedomorphosis (the acquisition of sexual maturity in a juvenile stage of development). In particular, this could explain how Giribet et al. (2009) found Nelima silvatica nested within two species of Sclerosomatinae, a primarily Mediterranean group of flattened, particularly heavily sclerotised species that might have been thought better supported than the other sclerosomatid subfamilies (unfortunately, Hedin et al.'s analysis includes only a single sclerosomatine). The view of the road ahead might be daunting, but it promises to be a memorable journey.

REFERENCES

Giribet, G., L. Vogt, A. Pérez González, P. Sharma & A. B. Kury. 2010. A multilocus approach to harvestman (Arachnida: Opiliones) phylogeny with emphasis on biogeography and the systematics of Laniatores. Cladistics 25: 1-30.

Hedin, M., N. Tsurusaki, R. Macías-Ordóñez & J. W. Shultz. 2012. Molecular systematics of sclerosomatid harvestmen (Opiliones, Phalangioidea, Sclerosomatidae): geography is better than taxonomy in predicting phylogeny. Molecular Phylogenetics and Evolution 62: 224-236.

Tsurusaki, N. 1985. Taxonomic revision of the Leiobunum curvipalpe-group (Arachnida, Opiliones, Phalangiidae). I. hikocola-, hiasai-, kohyai-, and platypenis-subgroups. Journal of the Faculty of Science of the Hokkaido University VI, Zoology 24: 1-42.

Marginal Limpets

Shells of Emarginula solidula, photographed by Jan Delsing.

For today's subject taxon, I've drawn the Emarginulini. This is a tribe within the gastropod family Fissurellidae, members of which are commonly known as keyhole or slit limpets. They get these names because of openings in their shells: keyhole limpets have a distinct hole at the apex of their shell, while slit limpets have a longitudinal slit running back from the front of their shell. In both groups, the slit or 'keyhole' functions in excretion. Gastropods undergo a process early in development known as torsion: the viscera of the embryo twists around so that it reverses its original direction (you can see a basic diagram of the process here). The reasons why this happens remain somewhat uncertain (one early suggestion was that it is what allowed the larval gastropod to retract into its shell and close the shell opening with an operculum) but a potentially negative side-effect of the process is that the anus comes to open directly above the mouth. Unless you want to go through your life with a bit of a funny taste in your mouth, this is not ideal. Therefore, many torted gastropods develop some sort of sinus or recess in their shell so the anal opening can be moved rearwards, away from the mouth.

Live individual of Tugali parmophoroidea, from here. Tugali has an expanded mantle, but is still able to retract it body underneath the shell for protection.

The members of the Emarginulini as recognised by Bouchet et al. (2005) are slit limpets rather than keyhole limpets, though in some species the slit has become much reduced and may only be visible on the underside of the shell (i.e there is a ventral groove rather than a full slit). Bouchet et al. (2005) divided the Fissurellidae between the Fissurellinae and Emarginulinae, a classification based on the structure of the radula and shell muscles (Aktipis et al. 2011). The Fissurellinae are all keyhole limpets, but Bouchet et al.'s Emarginulinae included (in addition to the Emarginulini) the tribe Diodorini, whose members are keyhole limpets like the Fissurellinae but have emarginuline internal anatomy. Other authors have recognised this group as a third intermediate subfamily. Two further tribes of Emarginulinae, the Scutini and Fissurellideini, include species with expanded mantles and reduced shells that may be entirely concealed within the soft body of the slug-like animal.

An elephant snail or shield limpet Scutus sp., from here. This genus has a greatly enlarged mantle, which usually folds over to conceal the reduced shell.

However, this classification of the fissurellids was challenged by the molecular analysis of Aktipis et al. (2011). The results of these authors indicated that the Emarginulini of Bouchet et al. (2005) is para- or polyphyletic. A clade of Fissurellinae with Diodorini indicates a single origin of keyhole limpets, with the emarginuline radula and muscle structure being ancestral for fissurellids as a whole. This result is also consistent with the fossil record: 'emarginulines' are known as early as the Triassic, but fissurellines and diodorines have not been found earlier than the Caenozoic. Aktipis et al. (2011) therefore recognised a more restricted monophyletic Emarginulinae containing the genera Emarginula, Montfortula, Tugali and Scutus, while more basal forms (sister to all other fissurellids) were separated as the Hemitominae (Aktipis et al. did not analyse the position of the Fissurellideini). Of Aktipis et al.'s Emarginulinae proper, only Emarginula has a well-developed slit (the others have ventral shell grooves; Scutus has a quite reduced shell), but this genus was also not monophyletic. Instead of aligning by morphology, the species analysed formed clusters corresponding more to their biogeography: a Mediterranean Emarginula clade, a Pacific clade of Emarginula and Montfortula species, and an Australian clade of Scutus and Tugali. The relationships between these three clades varied by analysis method. The Australian Scutus clade was not necessarily sister to the remaining Emarginulinae, so it may not be worthwhile at this point in time distinguishing the tribes Emarginulini and Scutini.

REFERENCES

Aktipis, S. W., E. Boehm & G. Giribet. 2010. Another step towards understanding the slit-limpets (Fissurellidae, Fissurelloidea, Vetigastropoda, Gastropoda): a combined five-gene molecular phylogeny. Zoologica Scripta 40 (3): 238-259.

Bouchet, P., J.-P. Rocroi, J. Frýda, B. Hausdorf, W. Ponder, Á. Valdés & A. Warén. 2005. Classification and nomenclator of gastropod families. Malacologia 47 (1-2): 1-397.

Ormyrids: Attacking the Gall

Female of Ormyrus nitidulus, photographed by Penny Metal.

Everyone knows about God's supposed inordinate fondness for beetles, but it is my opinion that the true poster children for insect diversity should be the wasps. Wasps, admittedly, do not have as many described species as beetles (there are some who suspect that the actual number of species of wasp may eventually be higher, but that remains in the realm of the hypothetical). However, many species of beetle are very difficult to distinguish except by skilled specialists, being otherwise small, brown, and conservative. Wasps, on the other hand, come in a kaleidoscopic array of colours and shapes, such that even a novice may look at an array of wasps (see the top of this post, for instance) and be immediately struck by the disparity.

An unnamed species of Ormyrus, photographed by Simon van Noort.

The Chalcidoidea, commonly referred to as chalcids, are one of the largest subgroups of wasps, a clade of mostly small (often minute), mostly parasitoid wasps (some have larvae that feed on plants). Members of the Ormyridae, one of the commonly recognised families of chalcids, are generally about two to three millimetres long. Ormyrids are distinguished from other chalcids by their robust body form, with a strongly sclerotised gaster* (ormyrids and perilampids tend to look like steroid-abusing pteromalids). The segments of the gaster are usually ornamented by rows of coarse foveae (pits) that give it a distinctive rough appearance, though in some species these foveae are less obvious or are replaced by longitudinal ribs (Bouček 1988). Ormyrids are often recorded in association with plant galls, but are not gall-formers themselves: rather, they are parasites of the insect larvae that formed the galls (usually flies or other wasps). Some ormyrids are associated with figs and parasites of fig wasps.

*Wasp researchers generally refer to the sections of the body behind the head by terms such as 'mesosoma' and 'gaster' (or metasoma), rather than 'thorax' and 'abdomen'. This is because the section of the body that is the first segment of the abdomen in other insects has become the last segment of the mesosoma in Hymenoptera.

A female of Ormyrus on a knopper gall (a type of gall that develops when a developing acorn of the pedunculate oak Quercus robur is parasitised by the cynipid wasp Andricus quercuscalicis), photographed by Tristram Brelstaff.

There are about 125 known species of ormyrid (making this a quite small family by chalcid standards) according to the Universal Chalcidoidea Database (an absolutely wonderful resource). However, there isn't yet a really good classification system within the family. Ormyrids vary to a fair degree, particularly in the form of the antennae or the ornamentation of the gaster, but most authors have placed almost all species within the single genus Ormyrus. Attempts to subdivide this diverse group (for instance, that of Doğanlar, 1991, who recognised four genera of ormyrids with three subgenera within Cyrtosoma) have suffered from not considering the full range of ormyrid diversity. Some of the Australian forms referred to by Bouček (1988), for instance, may not be placeable in Doğanlar's system. Until an appropriately large-scale review is conducted, most authors will probably continue to recognise an all-purpose Ormyrus.

REFERENCES

Bouček, Z. 1988. Australasian Chalcidoidea (Hymenoptera): A biosystematic revision of genera of fourteen families, with a reclassification of species. CAB International: Wallingford (UK).

Doğanlar, M. 1991. Systematic positions of some taxa in Ormyridae and descriptions of a new species of Ormyrus from Turkey and a new genus in the family (Hymenoptera, Chalcidoidea). Türkiye Entomoloji Dergisi 15 (1): 1-13.

An Introduction to Malaconothrus

Specimen of Malaconothrus monodactylus, from the Biodiversity Institute of Ontario (M. mollisetosus was listed as a synonym of M. monodactylus by Subías 2004).

Malaconothrus is a genus of about sixty species of oribatid mites found almost worldwide. The only continent from which Malaconothrus species have not yet been recorded is Antarctica, though M. translamellatus is known from Île Amsterdam in the subantarctic Indian Ocean (Subías 2004). Malaconothrus species specialise in damp habitats, often found among moss or in marshes. They are small yellowish mites, often covered with an ornamented cerotegument (a thick waxy cuticle) (Luxton 1987). They are also parthenogenetic, with females laying unfertilised eggs that hatch into more females.

Schematic drawing of Malaconothrus monodactylus (minus legs) from Luxton (1987).

Malaconothrus belongs to a group of oribatids called the Crotonioidea (often also referred to as nothroids). Because crotonioids are long-lived, slow-breeding and poor dispersers, they have received a certain amount of attention as potential indicators of environment health. In the context of the post linked to above, crotonioids are part of the Desmonomata, so outside the large oribatid clade of the Circumdehiscentiae or Brachypylina*. They have broad genital and anal plates that take up the greater part of the underside behind the legs (Balogh & Balogh 1992). Malaconothrus and its most closely related genus, Trimalaconothrus, differ from other crotonioids in having a band of soft cuticle across the underside between the levels of the second and third legs, i. e. they are dichoid rather than holoid (Norton 2001). They also lack bothridia, specialised enlarged sensory setae that are present at the rear of the prodorsum in the majority of oribatids. Malaconothrus and Trimalaconothrus are distinguished from each other by Malaconothrus having one claw at the end of each leg, while Trimalaconothrus has three. Subías (2004) divided Malaconothrus between two subgenera: in Cristonothrus, the dorsum is divided by a pair of longitudinal ridges, but in Malaconothrus sensu stricto there are no dorsal ridges.

*For some reason, oribatids seem to suffer something of an embarrassment of higher taxon names.

Dorsal and ventral view of Malaconothrus rohri from Balogh (1997). Note the pattern of ridges on the dorsum characteristic of Cristonothrus.

Malaconothrus has suffered a certain degree of confusion about its type status (Luxton 1987). When he first established Malaconothrus in 1904 (as a subgenus of Lohmannia), Berlese only listed one name in explicit combination, Lohmannia (Malaconothrus) egregia. However, in his discussion of this species, Berlese compared it to the pre-existing Nothrus monodactylus in a manner that implied the latter should also be included in his new subgenus. Subsequent authors have disagreed over whether L. egregia or N. monodactylus should be regarded as the type species of Malaconothrus, though more recent authors have settled on the latter.

REFERENCES

Balogh, J. & P. Balogh. 1992. The Oribatid Mites Genera of the World vol. 1. Hungarian Natural History Museum: Budapest.

Balogh, P. 1997. New species of oribatids (Acari) from the neotropical region. Opusc. Zool. Budapest 29-30: 21-30.

Luxton, M. 1987. Mites of the genus Malaconothrus (Acari: Cryptostigmata) from the British Isles. Journal of Natural History 21 (1): 199-206.

Subías, L. S. 2004. Listado sistemático, sinonímico y biogeográfico de los ácaros oribátidos (Acariformes, Oribatida) del mundo (1758-2002). Graellsia 60 (número extraordinario): 3-305.

The State of Peridinium

As I've said on many an occasion before, dinoflagellates are complicated. Obscenely complicated. So when my search for a random post topic brought up the dinoflagellate genus Peridinium, I approached it with a certain amount of dread. If you're not familiar with dinoflagellates, the diagram at the top of this post will explain a lot of the terminology I'm about to use.

Specimen of Peridinium cf. cinctum, photographed by Kate Howell. Peridinium cinctum is the type species of Peridinium.

Peridinium is a genus that has been used in the past to cover a wide range of freshwater and marine dinoflagellates. For a long time, the standard diagnosis of Peridinium was that it contained species with four apical plates (the ring of plates at the front of the cell when it is moving), seven precingular plates (the ring of plates in front of the cingulum), five postcingular plates and two antapical plates (Carty 2008). However, the genus has been divided by differences in the shape and arrangements of the plates making up the theca into a number of species groups, and more recent studies have concurred that these species groups are not all closely related to each other. While support remains low in most phylogenetic studies of dinoflagellates, and many species remain to be analysed, indications are that all of the marine species and many of the freshwater species are not true Peridinium (Horiguchi & Takano 2006; Logares et al. 2007). As it currently stands, the probably monophyletic Peridinium sensu stricto includes two species groups, the P. cinctum and P. willei groups, and is exclusively freshwater. As well as the characters mentioned above, true Peridinium species have three apical intercalary plates between the apical and precingular plates, five cingular plates, and ridges on all the plates forming an areolate pattern. They are also united by a distinct combination of which plates in the front section of the organism break off when the theca is shed during cell division (Craveiro et al. 2009). The two species groups differ in the exact arrangement of the plates anterior to the cingulum: in the P. willei group they are symmetrical relative to the dorsal-ventral axis, vs asymmetrical in the P. cinctum group. Slightly surprisingly, though the presence or absence of an apical pore was one of the first characters used to subdivide the genus Peridinium, Peridinium sensu stricto includes both species with (such as P. bipes) and without (such as P. cinctum and P. willei).

SEM image of Peridinium gatunense, by Pawel Owsiany.

Peridinium species are photosynthetic, with a much-lobed chloroplast that ramifies through the cell. One species, identified by Hickel & Pollingher (1988) as P. gatunense, has been intensely studied as the creator of annual blooms in Lake Kinneret in Israel.

REFERENCES

Carty, S. 2008. Parvodinium gen. nov. for the Umbonatum Group of Peridinium (Dinophyceae). Ohio Journal of Science 108 (5): 103-107.

Craveiro, S. C., A. J. Calado, N. Daugbjerg & Ø. Moestrup. 2009. Ultrastructure and LSU rDNA-based revision of Peridinium group Palatinum (Dinophyceae) with the description of Palatinus gen. nov. Journal of Phycology 45: 1175-1194.

Hickel, B., & U. Pollingher. 1988 Identification of the bloom-forming Peridinium from Lake Kinneret (Israel) as P. gatunense (Dinophyceae). British Phycological Journal 23 (2): 115-119.

Horiguchi, T., & Y. Takano. 2006. Serial replacement of a diatom endosymbiont in the marine dinoflagellate Peridinium quinquecorne (Peridiniales, Dinophyceae). Phycological Research 54: 193-200.

Logares, R., K. Shalchian-Tabrizi, A. Boltovskoy & K. Rengefors. 2007. Extensive dinoflagellate phylogenies indicate infrequent marine–freshwater transitions. Molecular Phylogenetics and Evolution 45 (3): 887-903.

Mosses Have a Place for Reproduction

A Rhizogonium photographed in the Philippines by Leonardo L. Co.

The Rhizogoniaceae are a family of mosses found in tropical and subtropical parts of the world, with a concentration of diversity in the Southern Hemisphere. Many species in the family are epiphytic; in particular, many show a preference for growing on the trunks of tree ferns (O'Brien 2007). The family has been defined by features such as sharply toothed, usually bistratose (i.e. with two cell layers) leaves and sporophytes located in the basal half of the erect stems, but molecular studies have indicated that the Rhizogoniaceae in the broad sense are para- or polyphyletic, and for this post I'll be using Rhizogoniaceae in a more restricted sense, corresponding to the 'clade C' of O'Brien (2007), including genera such as Rhizogonium, Cryptopodium, Calomnium, Goniobryum and Pyrrhobryum. One member of the Rhizogoniaceae, Pyrrhobryum dozyanum, is often used in moss gardens (it appears that there may also be a moss doing the rounds under this name in the European aquarium trade, though I haven't found anything to confirm whether this species, also being referred to as "Mayaca fern" or "Indonesiae bogoriensis", is actually P. dozyanum. Many bryophytes and other such plants in the aquarium trade have been misidentified, sometimes dramatically so).

View under microscope of leaf of Pyrrhobryum dozyanum, showing the toothed margins characteristic of Rhizogoniaceae. Image from here.

Most attention on Rhizogoniaceae from an evolutionary point of view has focused on what they might say about the relationship between acrocarpy and pleurocarpy. To explain what these terms mean, we'll start with the following diagram (from here):

Like other plants, mosses go through an alternation of generations, with both haploid and diploid multicellular stages. The haploid stage of the life cycle, the gametophyte, is the leafy green part of the moss. The gametophyte produces perichaetia, whorls of modified leaves within which the gamete-producing organs are contained. When a female gamete is fertilised, the resulting diploid zygote grows into the sporophyte, the brown thread-like structure you will often see growing out of a moss. The sporophyte produces haploid spores that will be dispersed to grow into new leafy gametophytes.

The diagram above shows an acrocarpous moss, in which the perichaetium is produced at the end of a growing branch of the gametophyte. Other mosses, however, are pleurocarpous, with perichaetia produced on the side of a branch. Whether a moss is acrocarpous or pleurocarpous is one of the first things a botanist will look at when attempting to identify it. However, many Rhizogoniaceae do not easily fall on either side of the acrocarpous/pleurocarpous distinction. They are what is called cladocarpous: the perichaetia are produced at the ends of small side-branches. However, lest any moss enthusiasts accuse me of overly simplifying things, I must point out that a great deal has been written on the exact distinctions between acrocarpous vs cladocarpous vs pleurocarpous. Like so many distinctions in nature, there are examples that blur the distinction between these states. As the perichaetia-bearing side-branches in a cladocarpous moss get progressively shorter, they become less and less distinguishable from pleurocarpy. In light of this, recent authors have suggested that the distinction between cladocarpy vs pleurocarpy should be defined by whether or not the side-branch bearing a perichaetium also bears normal vegetative leaves. If it only bears perichaetial leaves, then it is pleurocarpous: by this definition, some Rhizogoniaceae (including the genus Rhizogonium) are truly pleurocarpous (Bell & Newton 2007).

Goniobryum subbasilare, photographed by David Tng.

The vast majority of pleurocarpous mosses belong to a clade called the Hypnanae, which is massively speciose (probably about half of living mosses are hypnanaens). Because the hypnanaen mosses are so successful, there is a lot of interest in their relationships with other mosses. And as it turns out, the Rhizogoniaceae (with their combination of cladocarpous and pleurocarpous members) are closely related to the Hypnanae. Indeed, the Hypnanae are nested within the older, paraphyletic grade referred to the Rhizogoniaceae (O'Brien 2007). The acrocarpous state is the plesiomorphic one for mosses, with cladocarpy evolving in numerous lineages. Pleurocarpous mosses, it seems likely, have then evolved from cladocarpous ancestors, though either a number of times or with a number of reversals.

REFERENCES

Bell, N. E., & A. E. Newton. 2007. Pleurocarpy in the rhizogoniaceous grade. In: Newton, A. E., & R. S. Tangney (eds) Pleurocarpous Mosses: systematics and evolution pp. 41-64. CRC Press.

O'Brien, T. J. 2007. The phylogenetic distribution of pleurocarpous mosses: evidence from cpDNA sequences. In: Newton, A. E., & R. S. Tangney (eds) Pleurocarpous Mosses: systematics and evolution pp. 19-40. CRC Press.