Taxon of the Week #1: Gonostomatidae

I had intended for this to appear earlier in the week, but it's been a fairly busy one for me. Here's how this is going to work, barring accidents - every week I'll do an introduction to a (somewhat) randomly-chosen taxon. As this is my first entry, you can be sure I've selected something exciting, cutting-edge, mind-blowing, insert your choice of superlative adjective here - the Gonostomatidae or bristlemouths.

If the names not ringing any immediate bells, then shame on you. Bristlemouths are possibly the most abundant vertebrates in the world (or so every source I've looked at [e.g. Bond (1996), Craddock & Hartel (2002)] says, though I've yet to find an actual figure). They are mesopelagic or bathypelagic fish (or 'feesh' as they say here in Western Australia), mostly quite small (according to Craddock & Hartel [2002], some species mature at less than 20 mm) and, like oh so many mesopelagic fish, not overly attractive (see images at wikipedia). Gonostomatidae have an elongate body form with relatively big mouths (according to the afore-linked wikipedia page, the name 'bristlemouth' refers to the evenly-sized bristle-like teeth, a description that appears more true for some genera [Cycothone] than others [Gonostoma]). There are one or more rows of photophores along the length of the body - some like Triplophos have multiple rows almost covering the animal, while others such as Bonapartia have only a single row on the lower edge of the body, plus a few scattered over the head (Harold, 1999). They are micropredators of small crustaceans and such (Craddock & Hartel, 2002).

The Gonostomatidae belong to the order Stomiiformes, sometimes known as dragonfishes (some of the other members of the order reach a reasonable size, and you may have seen illustrations of them before - they're the elongate deep-sea fish with the mouths full of enormous teeth). As an aside, the intro for Stomiiformes in Collette & Klein-MacPhee (2002) refers to it as a 'very large group'. Only vertebrate workers would consider a few hundred species a 'very large group'. The Gonostomatidae itself includes seven or eight genera in two subfamilies, the Gonostomatinae (Gonostoma, Sigmops, Margrethia, Bonapartia, Cyclothone) and Diplophinae (Diplophos, Manducus, Triplophos).

Unlike many mesopelagic fish, bristlemouths do not appear to engage in daily vertical migrations (McClain et al., 2001). Their small size and deep-water habitat mean that, despite their abundance, they are rarely seen except by researchers.

REFERENCES

Bond, C. E. 1996. Biology of Fishes, 2nd ed. Saunders College Publishing.

Collette, B. B., & G. Klein-MacPhee (eds.) 2002. Bigelow and Schroeder's Fishes of the Gulf of Maine, 3rd ed. Smithsonian Institution Press: Washington.

Craddock, J. E., & K. E. Hartel. 2002. Bristlemouths. Family Gonostomatidae. In Bigelow and Schroeder's Fishes of the Gulf of Maine, 3rd ed (B. B. Collette & G. Klein-MacPhee, eds.) pp. 181-184. Smithsonian Institution Press: Washington.

Harold, A. S. 1999. Gonostomatidae: Bristlemouths. In Western Central Pacific Identification Guide for Fishery Purposes (K. Carpenter & V. H. Niem, eds.) pp. 1896-1899. FAO Species Identification and Data Programme vol. 3.

McClain, C. R., M. F. Fougerolle, M. A. Rex & J. Welch. 2001. MOCNESS estimates of the size and abundance of a pelagic gonostomatid fish Cyclothone pallida off the Bahamas. Journal of the Marine Biology Association of the United Kingdom 81: 869-871.

Another word on arachnid phylogeny

Shultz (2007) has just published a new paper on arachnid phylogenetics, based on morphology. As you can see if you scan my profile blurb, I'm currently working on arachnids - specifically on harvestmen (Opiliones), so I'm always happy to see work on them. Shultz (2007) sits in couterpoint to the most recent other publication on arachnid high-level phylogeny, Giribet et al. (2002), which used combined molecular and morphological data.

I do feel the need to make a few comments on character coding. One of the issues with high-level morphological phylogenetics is that it becomes increasingly difficult to code characters without interpretative bias. One example of this that I can spot in Shultz (2007) is his coding for the tracheal system (character 126). In the past, presence or absence of a tracheal system has generally been treated as a single character. Shultz argues on the basis of differences in the layout of the tracheal system that it has probably evolved independently a number of times (an idea that does gain some support from the definitely independent evolution of a tracheal system in some spiders), and codes the different tracheal systems as different character states. While Shultz is likely to be correct that the different tracheal systems have evolved independently, his coding of the systems separately a priori excludes the possibility of their homology. Also, in his comments on presence of a penis (character 160), a synapomorphy of Opiliones, Shultz notes that 'A clearly homologous structure is present in Cyphophthalmi (Opiliones) and apparently functions in depositing a spermatophore in the female’s genital chamber', then follows with 'The ‘penis’ in Oribatida is really a spermatopositor; it functions in construction of a spermatophore'. Is it really justifiable without prior phylogenetic expectations to code the cyphophthalmid structure as a 'penis', but not the oribatid structure?

Shultz uses a few exemplars from each of the living arachnid orders, as well as a fossil exemplars of a number of them (he includes more fossil taxa than Giribet et al.) plus the fossil Eurypterida, Trigonotarbida and Plesiosiro. An analysis is run without fossil taxa, then one with. At first glance, the inclusion or exclusion of fossil taxa has a significant impact on topology. Without fossils, and using Xiphosura (horseshoe crabs) as an outgroup, the recovered topology is (Palpigradi ((Ricinulei (Anactinotrichida, Actinotrichida)) ((Araneae (Amblypygi, Uropygi)) ((Scorpiones, Opiliones) (Pseudoscorpiones, Solifugae))))). However, through in a few fossil taxa and you get ((Scorpiones, Opiliones) (Palpigradi, (Actinotrichida (Ricinulei, Anactinotrichida), ((Pseudoscorpiones, Solifugae) (Araneae (Amblypygi, Uropygi)))))*.

*I wanted to use actual trees for this section, if only in ASCII format, because that would be one hell of a lot easier to read, but it looks like they won't show up properly in the final page. if anyone knows of a way I can put in trees on this site, I'd be ever so grateful to hear it).

Actually these two topologies are nowhere near as different as they appear - the support values for most supraordinal clades are ghastly. If we collapse all nodes in the final tree with less than 50% support, we get (Scorpiones, Opiliones, Palpigradi, (Ricinulei, Anactinotrichida, Actinotrichida), Solifugae, Pseudoscorpiones, (Trigonotarbida, Araneae (Plesiosiro (Amblypygi, Uropygi)))). To somewhat mitigate the drawbacks of this low support, however, Shultz does test his results against other past theories.

Shultz runs his neontological data set through a number of analyses constrained to recover particular clades. Clades suggested in the past that appear in trees only one step longer than Shultz's most parsimonious tree are Ricinulei + Anactinotrichida, Megoperculata (Palpigradi + Araneae + Amblypygi + Uropygi) and Rostrosomata (all arachnids except Scorpiones and Opiliones). The last one, notably, is what is recovered when palaeontological data are included. Scorpions sister to all other arachnids is only two steps longer, as is Micrura (arachnids except Scoropiones, Opiliones, Pseudoscorpiones and Solifugae). It is a little disappointing that these comparisons are run on the neontological data set alone rather than the complete data set, considering that the fossil taxa are not without influence on the result. Two comparisons are made using the full data set, testing a scorpion + eurypterid clade and a trigonotarbid + ricinuleid clade (the latter possibility was found by Giribet et al., 2002). Both possibilities are noticeably longer than the preferred tree.

A notable absence from Shultz (2007) is the Pycnogonida. Pycnogonids or 'sea spiders' are patently bizarre marine animals of very obscure relationships. Traditionallly they have been regarded as basal chelicerates owing to their possession of chelate pre-oral appendages, and many authors still support this view. Other authors regard pycnogonids as the sister group to all other living arthropods. When pycnogonids were included in the analysis of Giribet et al. (2002), they appeared in a completely unexpected position as sister to Palpigradi, within Arachnida. My impression on reading Giribet et al. is that the authors themselves are extremely sceptical of this result, and seem more inclined to attribute it to the high level of autapomorphy in pycnogonids. While it would have been interesting to see Shultz test the position of pycnogonids, it is possible that said degree of autapomorphy may have simply blown Shultz's analysis out of the water.

REFERENCES

Giribet, G., G. D. Edgecombe, W. C. Wheeler & C. Babbitt. 2002. Phylogeny and systematic position of Opiliones: a combined analysis of chelicerate relationships using morphological and molecular data. Cladistics 18: 5-70.

Shultz, J. W. 2007. A phylogenetic analysis of the arachnid orders based on morphological characters. Zoological Journal of the Linnean Society 150 (2): 221-265.

Filling in the gaps

I'm going to continue on with the algal theme here, because I keep getting reminded lately of neat examples. However, I'm going to take a great leap sideways and deal with a different group from rhodophytes. I'm moving towards the brown algae (sort of...)

It is universally accepted these days that the algae are a polyphyletic grouping, at least from the viewpoint of nuclear and cytoplasmic ancestry. Chlorophyll originally developed within the blue-green algae, actually a clade of bacteria (Cyanobacteria). Chloroplasts in eukaryotes then arose through endosymbiosis between a non-photosynthetic protist and a cyanobacterium. However, many authors now agree that there was probably only one such primary endosymbiosis event that led to the majority of modern chloroplasts (there is a lonely cercozoan*, Paulinella, that appears to have derived its chloroplast independently). The direct descendants of this lucky protist today are the green plants and algae (Viridiplantae), the red algae (Rhodophyta) and a small group of unicells called the blue-green algae (Glaucophyta). Red and blue-green algae both have only a single chlorophyll type, chlorophyll a, while green algae possess a second form as well, chlorophyll b (interestingly, a small handful of cyanobacteria also possess chlorophyll b, and molecular phylogenies show that these oddjobs are not closely related within the cyanobacteria). The remaining algae are derived from secondary symbioses, where a eukaryotic alga has become an endosymbiont of another eukaryote followed by loss of the endosymbiont's independence and genetic material. This is rather spectacularly demonstrated by two secondary algal groups, the chlorarachneans and cryptophytes, whose chloroplasts retain a highly-reduced eukaryotic nucleus between the membranes surrounding the chloroplast. Two groups, the amoeboid chlorarachneans and flagellate euglenoids, have chloroplasts derived from green algae as shown by their possession of chlorophyll b. Four groups, the dinoflagellates, cryptophytes, haptophytes and ochrophytes, have chloroplasts seemingly derived from red algae. These four groups also share a third chlorophyll type, chlorophyll c, as well as the ancestral chlorophyll a, and on this basis it has been suggested that they all derive from a single endosymbiotic ancestor (though this seems likely, the case is not airtight as there are a number of non-photosynthetic protists without chloroplasts that seem to be closely related to one or another of the chlorophyll c groups). Some dinoflagellates have replaced their ancestral chloroplasts with chloroplasts derived from haptophytes in a tertiary endosymbiosis.

*Originally I had identified Paulinella in this post as an amoebozoan, but it belongs to a different amoeboid group, the Cercozoa.

Multicellularity has evolved a number of times within algae. Viridiplantae became multicellular multiple times, while red algae probably evolved multicellularity once at the base of the Bangiophyceae + Florideophyceae clade (see the previous post). Ochrophytes include two groups of multicellular algae, the brown algae (Phaeophyceae) and some members of the yellow-brown algae (Xanthophyceae).

Ochrophytes are the clade of photosynthetic heterokonts. Heterokonts are a well-supported clade of protists distinguished in most members by, among other features, a shorter posterior flagellum and a longer anterior flagellum with numerous side bristles (mastigonemes). At cell division, the anterior flagellum moves backwards and loses the mastigonemes to become the posterior flagellum, while a new anterior flagellum is generated (Andersen, 2004). As well as the two above-mentioned classes, ochrophytes include diatoms and a whole bunch of unicellular algae previously united as the golden algae (chrysophytes). The chrysophytes have proven to be paraphyletic with regard to the other ochrophytes, and have been divided into a whole host of smaller classes.

Now we've gotten through all that, I'll finally introduce the star of today's post, Schizocladia ischiensis Henry, Okuda & Kawai in Kawai, Maeba et al., 2003. The position of the brown algae in relation to other ochrophytes has been obscured by the absence of clear connecting features between the multicellular brown algae and the various unicellular golden algae. The significance of Schizocladia is that it goes some way towards filling that gap. Schizocladia is a small marine ochrophyte that grows as filaments of cells in single file. Like phaeophytes, Schizocladia has cell walls impregnated with alginates. Unlike brown algae, Schizocladia lacks cellulose or plasmodesmata (cytoplasmic connections between cells). Propagation in Schizocladia was via zooids produced in individual compartments in swollen cells at the end of the filaments.

The molecular phylogenies presented in the original description of Schizocladia agreed in positioning it as the sister group of Phaeophyceae. They also agreed with the result found by other studies that there is a clade composed of Phaeophyceae (+ Schizocladia), Xanthophyceae and the unicellular Phaeothamniophyceae (the unicellular Chrysomeridales may also belong to this clade, but do not appear to have been investigated molecularly). While the Xanthophyceae do include some multicellular members, it also includes unicellular forms, and multicellularity was probably evolved independently of Phaeophyceae. Xanthophyceae do possess cellulose in the cell walls, and the presence of alginates has also been demonstrated in some species.

Due to the absence of some supposed key phaeophyte characters, Schizocladia was not included in Phaeophyceae but placed in its own independent class Schizocladiophyceae. Nevertheless, its simple morphology provides a nice connection between the unicellular ochrophytes and multicellular phaeophytes.

REFERENCES

Andersen, R. A. 2004. Biology and systematics of heterokont and haptophyte algae. American Journal of Botany 91 (10): 1508-1522.

Kawai, H., S. Maeba, H. Sasaki, K. Okuda & E. C. Henry. 2003. Schizocladia ischiensis: a new filamentous marine chromophyte belonging to a new class, Schizocladiophyceae. Protist 154: 211-228.

A Parasite in the Family

In the previous post, I wrote about an epiphytic red alga, and mentioned in passing the interesting phenomenon of adelphoparasitism, where a parasite is very closely related phylogenetically to its host. Since then, I've been wondering how such a situation arose, and specifically whether there was a connection between red algal adelphoparasitism and the complexities of red algal life cycles (see here for a summary).

Red algae fall into three to seven classes - Rhodellophyceae (which Yoon et al., 2006, divide into five, but as I haven't yet seen the paper I'll let it slide), Bangiophyceae and Florideophyceae. Rhodellophyceae are unicellular, and I confess I don't know the details of their life cycles. Bangiophyceae (which include Porphyra, the nori used in making sushi) alternate between distinct haploid and diploid generations. Florideophyceae include the vast majority of red algae, and verge on the completely insane in life style complexity. The basic florideophycean life cycle (which, as shown in the previous post, not all members of the class go through) involves no less than three alternating generations (I linked to the diagram here in the previous post, but I'll do it again because it's a good'un). Starting with the diploid tetrasporophyte, the tetrasporophyte releases haploid spores that settle and grow into gametophytes. Male gametophytes release spermatia (aflagellate sperm) that are captured by the female gametophytes and fertilise the carpogonia. The carpogonium (and this is the interesting part for this post) then grows into a carposporophyte, which remains attached to the parent gametophyte, releasing diploid spores that grow into new tetrasporophytes. So in effect, parasitism is already part of the florideophycean life cycle. Is it somehow possible that this parasitism is behind the rise of adelphoparasitism?

It's worth noting here that similar patterns to "adelphoparasitism" are not unique to red algae. They have also been recorded among social Hymenoptera as well as mistletoes. Red algal parasites have traditionally been divided between adelphoparasites (which are closely related to their hosts) and alloparasites (not so closely related). The two classes are also supposedly distinguished by the mode of parasitism. In both, after the parasite rhizoid invades the host it adheres to and fuses with the host cells, injecting parasite nuclei and mitochondria. In adelphoparasites, the parasite nuclei then multiply within the host cell, hijacking it and causing the formation of growths which release spores of the parasite species (Goff et al., 1997), In alloparasites, the parasite nuclei do not divide in the host cytoplasm, though they do alter its physiology to facilitate the transfer of nutrients from host to parasite, and (I assume) the parasite reproductive bodies grow from the parasite rhizoid itself. Goff et al. (1997) demonstrated that one 'genus' of adelphoparasites had actually arisen polyphyletically from the host 'genus'. Zuccarello et al. (2004) demonstrated the same thing for a 'family' of alloparasites. The latter authors therefore suggested that the terms 'adelphoparasite' and 'alloparasite' were not useful. However, this does still leave the question of the different cytoplasmic interactions (Zuccarello et al. implied that this might be due to the taxa studied belonging to different orders).

Goff et al. (1997) give two possible scenarios for the origin of parasitic red algae. In one, the parasites are ancestrally epiphytic, later becoming endophytic and eventually parasitic. In the second, the parasites derive directly from spores that lose the ability to survive independently of the parent. The existence of the carposporophyte, in my opinion, gives a lot of support to this option. One possibility is that adelphoparasites arose by the second method while alloparasites arose by the first.

Goff et al. also examined the main complaint towards the second origin - even if some mutant parasitic individual does arise, what is to stop it backcrossing to the parent population? How does the parasite become established as a new species? At present, there is no really satisfying answer to this question. Goff et al. do point out that parasitic taxa have life cycles taking a fraction of the time of the host species. At any given time, only a small percentage of the individuals in a population of algae are reproductive - perhaps the difference in timing of life cycles simply meant that the chance of backcrossing between parasite and non-parasite was too low to prevent speciation?

REFERENCES

Goff, L. J., J. Ashen & D. Moon. 1997. The evolution of parasites from their hosts: a case study in the parasitic red algae. Evolution 51 (4): 1068-1078.

Zuccarello, G. C., D. Moon & L. J. Goff. 2004. A phylogenetic study of parasitic genera placed in the family Choreocolacaceae (Rhodophyta). Journal of Phycology 40: 937-945.

Little Discs of Doom

Okay, total hyperbole in the title, but I wanted to get your attention. Today I'll be looking at Pihiella liagoraciphila, a very distinctive member of the red algae that was only described a few years ago (Huisman et al., 2003).

Pihiella is an endo/epiphyte found on members of the red algal family Liagoraceae, but not a parasite as far as I can tell (red algae are notable for the range of associations between different taxa, most interestingly the occurrence of what is call 'adelphoparasitism', where parasitic species are closely related to their hosts - I'll have to write on that some day). It has a quite simple disc-shaped or subspherical morphology with rhizoids to attach it to the host and long hairs and trichogynes (hair-like appendages of the female carpogonia that catch the male gametes). Mature discs are very small, up to 400 μm in diametre and 150 μm thick, though the hairs can be up to 800 μm long. Specimens were first observed as long ago as 1858, but were interpreted as buds of the host plant. Authors thereafter disagreed as to whether the so-called 'monosporangial discs' were asexual reproductive organs of the host or an independent organism. All authors agreed that the discs were asexually reproductive.

Sexually reproductive organs on the discs weren't recorded until 2003, when Huisman et al. established that the discs were indeed a separate organism from the host. Pihiella seems to lack the obscenely complicated triphasic life cycles of other red algae (see here for an example). As already mentioned, the carpogonia (sexual organs) possess a long hair-like trichogyne, and Huisman et al. did observe examples with spermatia (the aflagellate male sex cells) attached. Nevertheless, Huisman et al. were unable to conclude whether the mature sporangia observed were asexually produced monosporangia, sexually produced zygotosporangia, or both (I feel the last option seems most likely, but what do I know?). No carposporophytes or tetrasporangia were observed (see the link above to find out what these are).

The morphology of Pihiella was too distinct from any other red alga to be phylogenetically informative, but Huisman et al. were able to assess the phylogeny molecularly. Pihiella turned out to be quite isolated from other red algae, enough that Huisman et al. established a new monotypic order for it. Interestingly, the trees recovered Pihiella as sister taxon to Ahnfeltia, another phylogenetically isolated taxon, with a high level of support. Morphologically, Ahnfeltia is very distinct from Pihiella, being a large cartilaginous plant with a triphasic life cycle found in cool waters (the host family of Pihiella, Liagoraceae, is a mostly warm-water group). Though Ahnfeltia and Pihiella are each other's closest relatives, the relationship is not close. Liagoraceae, in contrast, was in a quite distant part of the tree.

REFERENCES

Huisman, J. M., A. R. Sherwood & I. A. Abbott. 2003. Morphology, reproduction, and the 18S rRNA gene sequence of Pihiella liagoraciphila gen. et sp. nov. (Rhodophyta), the so-called 'monosporangial discs' associated with members of the Liagoraceae (Rhodophyta), and proposal of the Pihiellales ord. nov. Journal of Phycology 39: 978-987.

TAFKAMI

I was at a bit of a loss as to what to post on next, so I asked my partner to "name an organism, any organism". He suggested "amoeba", so I'm following his instructions. Besides, the subject of today's post was bound to raise its pseudopodia somewhere along the line. I've decided to write on what I'll informally dub TAFKAMI - The Amoeba Formally Known As Mastigamoeba invertens. TAFKAMI is an anaerobic amoeboflagellate with a cilium shorter than the body.

A little backgroud, first. Protist phylogeny has always been a contentious, uncertain world - compared to multicellular organisms, unicells have relatively few obvious characters to unite various groups of taxa. However, the availability of better and better electron microscopy and the continued improvements in molecular phylogenies mean that in recent years, a growing consensus has developed that the majority of eukaryotes fall into a few large "supergroups" (Simpson & Roger, 2004) - the opisthokonts (including fungi and animals), amoebozoans, excavates (mostly flagellates), rhizarians (including radiolarians and foraminiferans), chromalveolates (including ciliates, brown algae and dinoflagellates) and plants (including green and red algae). There are still a few random taxa that don't necessarily fall into any of these groups.

The supergroup Amoebozoa includes the majority of amoebae with lobose pseudopodia, as well as most slime moulds and the Archamoebae, a group of amitochondriate anaerobic amoebae (take a moment to appreciate the assonance). The Archaemoebae include Mastigamoeba proper (as well as Entamoeba, the causative organism of amoebic dysentery).

According to Walker et al. (2006), TAFKAMI was first isolated back in 1992. Since then, it has been what is technically referred to as a right pain in the khyber. Comparisons using both microscope and molecular techniques between TAFKAMI and other supposedly related organisms increasingly indicated that it was not Mastigamoeba, or any other known amoeba. In molecular studies (such as Cavalier-Smith & Chao, 2003) TAFKAMI leapt about madly, sometimes with amoebozoans, sometimes with apusomonads (another small group that doesn't fit into any of the supergroups), sometimes entirely elsewhere.

Walker et al. (2006) recently established TAFKAMI as a new taxon, Breviata anathema (the original description of Mastigamoeba invertens from 1892 does not allow reliable identification of what that species is). They also presented a detailed comparison of Breviata with the main contenders for close relationship.

Cavalier-Smith et al. (2004) felt that TAFKAMI was the basalmost member of the Amoebozoa, placing it in its own class Breviatea. The main reason for doing so was that TAFKAMI was supposed to possess a single ciliary basal body (the organelle that the flagellum emerges from). However, Walker et al. found that Breviata actually had double basal bodies. There is reasonably good evidence (Cavalier-Smith, 2002; Simpson & Rogers, 2004) that eukaryotes with double basal bodies form a single über-clade, the bikonts (including the excavates, rhizarians, chromalveolates and plants). It seems quite believable that Breviata is a member of this clade. Breviata also lacks the molecular markers of Amoebozoa proper (Cavalier-Smith et al., 2004).

Under certain parameters, molecular phylogenies supported an association of Breviata with the afore-mentioned apusomonads, which are also bikonts. Breviata also has similar pseudopodia to apusomonads. However, I would be just as sceptical of a direct apusomonad-Breviata connection. At the present, I don't feel that Breviata can be placed as anything more that "basal bikont". (It is worth noting, too, that one amoebozoan group, the Myxogastrea, has independently evolved double basal bodies.)

Perhaps most interestingly, Walker et al. identified a large organelle overlying the nucleus that they suggested as a possible hydrogenosome. Hydrogenosomes are respiratory organelles that are generally regarded as having been independently derived from mitochondria in a number of anaerobic groups. As such, Breviata potentially joins the growing list of supposedly amitochondriate taxa retaining mitochondrial remnants.

REFERENCES

Cavalier-Smith, T., & E. E.-Y. Chao. 2003. Molecular phylogeny of centrohelid Heliozoa, a novel lineage of bikont eukaryotes that arose by ciliary loss. Journal of Molecular Evolution 56 (4): 387-396.

Cavalier-Smith, T., E. E.-Y. Chao & B. Oates. 2004. Molecular phylogeny of Amoebozoa and the evolutionary significance of the unikont Phalansterium. European Journal of Protistology 40: 21-48.

Simpson, A. G. B., & A. J. Roger. 2004. The real ‘kingdoms’ of eukaryotes. Current Biology 14 (17): R693-R696.

Walker, G., J. B. Dacks & T. M. Embley. 2006. Ultrastructural description of Breviata anathema, n. gen., n. sp., the organism previously studied as ‘‘Mastigamoeba invertens’’. Journal of Eukaryotic Microbiology 53 (2): 65-78.

A Frustrating Giant Bird

Darren Naish in a recent post on his most excellent Tetrapod Zoology blog on a completely different subject mentioned the giant fossil bird Eremopezus, which inspired me to look it up (I was nearly inspired to change subject by watching a bagmoth in the lab here sealing itself into its bag in preparation for pupating, but another time, perhaps...)

Eremopezus is known from leg bones from the upper Eocene of the Fayum of Egypt. The most recent review is by Rasmussen et al. (2001), but it was first described in 1904. Lambrecht later divided the then-available material into two genera, Eremopezus Andrews 1904 and Stromeria Lambrecht 1929, but there is little significant difference between material assigned to the two and they are now regarded as synonymous.

Being a giant landbird, Eremopezus was originally thought to be related to modern giant landbirds, the ratites. Ratites are a group of flightless birds distributed between the southern continents - the ostrich (Africa), rheas (South America), emu, cassowaries (Australia), moa and kiwis (New Zealand). Arguments have run back and forth about whether the ratites are monophyletic, or have arisen independently from different ancestors. Recent molecular phylogenies have been pretty much unanimous that the ratites are indeed monophyletic, and together with the flighted tinamous (Tinamidae) are the sister group to the remaining modern birds. On the basis of a prominent ridge on the tarsometatarsus, Lambrecht (1933) suggested that Eremopezus was related to the elephant birds (Aepyornithidae) of Madagascar.

The problem is that this is simply not very significant evidence, as pointed out by Rasmussen et al. (2001). Large flightless birds show a great deal of similarity in the form of the leg bones, due to similar functional requirements. The flightless carnivorous bird Diatryma has hindlimb bones indistinguishable from those of ratites, despite being more closely related to the modern Anseriformes (ducks and geese). Rasmussen et al. concluded that Eremopezus could not be reliably associated with any other known group of birds.

To add to this, Eremopezus showed a number of distinct features all of its own. It appears to have been a fairly lightly-built bird, but slightly larger than a cassowary or rhea. The distal end of the tarsometatarsus is markedly flattened dorsoventrally, and the trochleae (and hence the toes) are quite widely splayed (an attachment scar indicating the presence of a hallux - the rear-pointing toe - is present, but this was probably small as an adaptation for terrestriality). The trochleae on either side have relatively light grooves, suggesting that the toes were quite mobile. The modern birds with the most similar morphologies are Sagittarius serpentarius (secretarybird) and Balaeniceps rex (shoebill). Both these birds use their feet for manipulation - Sagittarius is a ground predator that catches prey with its feet, while Balaeniceps uses its feet to grasp floating vegetation in swampy habitats. The Fayum of the Eocene also appears to have been a quite swampy habitat, but the appeal of a gigantic secretarybird is not to be denied. In the meantime, we simply have to wait on further remains to turn up before we can say more on the subject.