Bivalves Born Free (Taxon of the Week: Pectinoidea)

Spondylus imperialis, one of the more extravagant of the secondarily sessile scallops of the Spondylidae. Photo from here.

The ancestral lifestyle for the bivalves was one burrowed into the sediment, hidden beneath the mud or sand with only a pair of siphons protruding to draw in food particles from the surrounding environment. It was a perfectly successful lifestyle, and the majority of bivalves live in this manner to this very day. However, one clade of bivalves was determined to be different. The Pteriomorphia hauled themselves out of the mud and pursued a new lifestyle on the surface, where they diversified into such familiar bivalves as mussels, oysters and scallops. It is the scallops that I'm considering today - the pteriomorph clade that did more than merely rise to the surface. With a twitch of their shells, scallops became the only group of bivalves to fly.

The Pectinoidea include four living families - the Propeamussiidae, Entoliidae, Spondylidae and Pectinidae (Waller, 2006), as well as a few fossil families that I'm going to leave for another day. Most of you are probably familiar with the basic scallop shape - an ovoid shell, about as wide as long, round at one end, pointed at the other, with a pair of wings on either side of the point. Many species also have unequal valves to some degree, taken to an extreme in the genus Pecten which has the right valve deeply convex and the left valve entirely flat or slightly concave. Waller (2006) assigned taxa to the Pectinoidea going back to the Devonian (in the paraphyletic family Pernopectinidae), but the pectinoid crown group dates back to the early Triassic.


The pectinid scallop Chlamys hastata. Note the dark spots just inside the shell, which are the eyes. Photo from Dave Cowles.

The main defining feature of the Pectinoidea is the unique pectinoid resilium. The resilium is the ligament that holds the two valves together at the hinge, and that of the Pectinoidea has a rubbery central core that lacks the aragonitic fibres found in other bivalve resilia. This rubbery resilium can withstand a lot more rapid compression than those of other bivalves, allowing for the unique pectinoid mode of swimming, where the rapid opening and closing of the valves pumps water to move the animal by jet propulsion. Perhaps not the most graceful forms of movement, but it does the job. Amusium pleuronectes, one of the fastest of the scallops, has been clocked travelling at 73 cm/s. And even the slower species still have the protection of their shells to fall back on if necessary. Most pectinoids start out life attached to the substrate by a byssus (beard) like mussels, but many abandon their byssus over the course of their growth and become entirely free-living. Even those forms that remain byssus-attached retain their swimming abilities. Nevertheless, a few taxa, notably the pectinid Hinnites and the Spondylidae, have abandoned the ability to swim and have become permanently cemented to the substrate.

The Entoliidae are represented in the modern fauna by the single relictual genus Pectinella, a smallish (about a centimetre across) smooth thin-shelled genus with roughly symmetrical right and left valves found in the Caribbean and tropical Pacific. The Propeamussiidae are a small-sized (often less than a centimetre), often translucent, and often strongly-ribbed family of about 200 species. The dentition at the hinge is reduced or even absent, the gill structure is simplified, and many species lack eyes or guard tenticles around the edge of the mantle.



The Pectinidae and Spondylidae both include forms much larger in size than the other families. A number of anatomical features are shared between these two families - most notably their unique eye structure*, shown above in a figure from Speiser & Johnsen (2008). Pectinid and spondylid scallops have the best-developed eyes of any bivalve, dotted around the outside of the mantle. While most bivalve eyes are little more than light detectors, serving merely to give warning to close the valves against approaching danger, pectinid eyes are developed enough to produce a rough image and can be used to direct the animal when swimming (Speiser & Johnsen, 2008). Pectinid eyes have a lens in front of not one but two retinas, one behind the other, with a concave mirror in the back of the eye. As established by Land (1965), the lens does not throw an image directly on the retina as in our eyes, rather light is reflected off the mirror at the back of the eye and an image is formed using that light by the distal retina (the retina closer to the lens). The function of the lens is to correct for distortion produced by the concave mirror. The proximal retina does not produce an image; intead, it probably functions as a light-dark receptor as in other bivalves. In the sessile Spondylus, the function of the distal retina is reduced, so it does not have the image-forming capabilities of free-swimming pectinids (Speiser & Johnsen, 2008) - without the habit of swimming, it has no need for such things. Nevertheless, the presence of the seemingly unnecessarily complex pectinid eyes in spondylids supports the idea that the sessile spondylids are derived from a free-swimming pectinid-like ancestor.

*Well, potentially unique. Pectinella does have eyes (Waller, 2006), but I haven't found a description of their structure.

REFERENCES

Land, M. F. 1965. Image formation by a concave reflector in the eye of the scallop, Pecten maximus. J. Physiol. 179: 138-153.

Speiser, D. I., & S. Johnsen. 2008. Comparative morphology of the concave mirror eyes of scallops (Pectinoidea). Amer. Malac. Bull. 26; 27-33.

Waller, T. R. 2006. Phylogeny of families in the Pectinoidea (Mollusca: Bivalvia): importance of the fossil record. Zoological Journal of the Linnean Society 148 (3): 313-342.

Won't Somebody Please Think of the Lesbians?

I'm probably going to have to apologise in advance for this post, as I'm quite possibly going to make a twit of myself while writing it. Suffice to say, it's not in my usual line of subject matter, it's not really something I'm qualified to write about, it's just something I've been thinking about for a little while and just feel the need to make a statement about.

As a homosexual myself, it should hardly come as a surprise that I have a passing interest in GLBTI rights. On the other hand, I have something of a complaint about the term "GLBTI". The problem is that the term explicitly lumps gay men, lesbians, transexuals, etc. together as if we are all facing the same issues, and the problems for one are the problems for another. They are not. These are all different bundles of issues, each with their own distinct nuances. And the solution for one set of issues will not necessarily be the solution for another.

There is another, deeper problem with the term "GLBTI", which relates to how people approach GLBTI issues - GLBTI issues become gay issues become gay men's issues. I think it's a reasonable null hypothesis that about half the world's homosexual community are women - so where are they? We are presented with the concept of Queer Eye for the Straight Guy*, but where is the Dyke Eye for the Straight Woman? Pick up a gay rights magazine or newspaper, one supposedly written for both men and women, and take a good look at just how much press time is given to lesbians. If they're lucky, they'll be given a column, somewhat condescendingly buried somewhere around page seven. Take a look at the cover - in the eternal parade of male cover models, how often does the cover feature a woman? Gay men, I am sorry to say, are just as guilty of discrimination towards gay women as are heterosexuals.

*A programme for which I also found the concept offensive, but that's an issue for another day. Suffice to say that I found it offensive in much the same way that a woman finds being told that she has nice tits offensive.

If you were to compile a history of moral and legal attitudes towards homosexuality, I think you'd find much more negative reaction to homosexual men than homosexual women (such the famous occasion [though I'm not certain of its truthfulness] when Queen Victoria supposedly signed off on a law making sodomy a crime, but refused to permit a similar law on sapphism). Does this mean that lesbians suffered less from persecution? Not in the least. Lesbians were not made the specific subject of discrimination, not because their behaviour was regarded as acceptable, but because they were women. They had already been put in their place, thank you very much. Where was the need to legislate on a woman's choice of sexual partners, when she had no rights regarding her choice in the first place? Lesbian rights are a different set of issues from gay men's rights because lesbian rights are much more part and parcel of the whole wider issue that is women's rights in general. Gay women are doubly discriminated against. And if you happen to be an overweight black gay woman confined to a wheelchair, then I quite understand if you start randomly punching people.

I have to make a confession - I am sexist. I am racist. And I probably always will be. We all are. Discrimination seems to be a hardwired part of human mentality - we are driven to notice differences and then generalise from them. Burning crosses and gaybashings are only part of the problem, and quite frankly, they're the easy part to deal with. It is the underlying attitudes - the attitudes that we all have, and the attitudes that we don't even realise we have - that are the hard part. We sneer at hardcore feminists' complaints about the "patriarchy" - but you take a look around, and really think about things, and start to come to the uncomfortable realisation that holy shit, they have a point. So the next time you start thinking to yourself that you are not discriminatory, stop. I'm sorry, but you are. Because we all are. And it's not until we recognise our own faults, and strive to correct them, that there's any hope of progress.

Thank you for your time - normal service will be resumed shortly. And in the meantime, may I direct your attention to this post of Zuska's on the question of why there are so few women working in science, and whether the supposed need for women to choose between career and family, often presented as an explanation, is a distraction from other, deeper issues. The main feeling I find coming through Zuska's post is frustration - almost like (if she'll excuse the clumsy metaphor) someone whose car isn't running, so she opens the bonnet, cleans all the connections, makes sure everything is where it should be, primed and ready to go, hops back into the driver's seat, turns the key - and it still won't bloody start...

More Crunchy Scleritome Goodness

Yep, it's time for another installment on my favourite assemblage of polyphyletic problematica. Two significant new additions have been made to the repertoire of articulated scleritomes:


An assortment of articulated Lepidocoleus (each about an inch long). Take especial note of figure d! From Högström et al. (in press).

Firstly, does anyone remember machaeridians? The animals for which I labelled the discovery of a specimen preserving soft tissue as "the greatest announcement of 2008"? (And now that 2008 has been and gone, I wholeheartedly support that designation.) Well, there's more. Högström et al. (in press) have described a collection of articulated machaeridians from the Devonian Hunsrück Slate in Germany, and among them is a second specimen with soft-tissue remains!

There are a few reasons why this is a very satisfying discovery. Firstly, the specimen supports the annelid affinities proposed for machaeridians by Vinther et al. (2008) when the first soft-tissue specimen was described from the Ordovician Fezouata Formation of Morocco. Secondly, the Hunsrück specimens represent a different family (Lepidocoleidae) from the Fezouata specimen (Plumulitidae), which confirms that the machaeridians do represent a monophyletic grouping, and are not unrelated taxa that have convergently developed similar sclerite morphologies (always a possibility with animals only known from disarticulated sclerites). [I should point out that articulated lepidocoleid scleritomes have been found before, but not preserving any soft tissue.] Whereas plumulitids appear to have had rather loosely articulated sclerites, giving them an ornamental spined appearance, lepidocoleids had a much more tightly-woven, armour-plated scleritome (see the earlier post for a comparative picture).

And thirdly, just as a kind of cherry on the top, one of the other Hunsrück specimens, while it may not have soft-tissue remains, has something else to commend it that some palaeontologists would probably find even more exciting. It's sitting neatly positioned at the end of a well-preserved trail. Trace fossils are often the best evidence you can get for working out the behaviour of extinct animals, but it can be a frustrating exercise because often the best conditions for preserving traces are not very good for preserving the animals that made them, and vice versa. When I was on a palaeontology field trip as an undergrad, I was told that one of the lecturers had a standing offer of a crate of beer for anyone who found a body fossil in association with a trace fossil. Finding a machaeridian in association with a trace fossil, I feel, would have warranted at least two.


Tommotiids. On the left, the articulated Eccentrotheca. On the right, sclerites of Micrina placed to show their suggested life positions. Photo from here.

The second big announcement comes from the Cambrian Arrowie Basin of South Australia - another articulated tommotiid! Last year, I reviewed a new reconstruction of the tommotiid Micrina presented by Holmer et al. (2008). This reconstruction, with two valves on either side of an attached stalk (though do note, it was a reconstruction rather than a description of an articulated specimen, so it's not immune to revision), was intriguing in its resemblance to a basal brachiopod (to which group of Recent animals tommotiids are almost certainly related, sharing a very similar shell microstructure). However, it was in fairly stark contrast to the previously described articulated tommotiid Eccentrotheca, which has its sclerites stacked one above another to form a tubular structure (Skovsted et al., 2008). The new articulated tommotiids described by Skovsted et al. (in press) may just go some way towards bridging the divide.


Paterimicra. On the left, apical (above) and lateral (below) views of the large sclerite S1. On the right, S1 in suggested life position with an S2 sclerite within the triangular notch. Scale bars for this and the next figure = 200 μm. Figures from Skovsted et al. (in press).

Skovsted et al. (in press) have described articulated scleritomes of the tommotiid Paterimitra. Like Eccentrotheca, Paterimitra had more sclerites in its scleritome than the two suggested for Micrina. However, unlike the tubular Eccentrotheca, Paterimitra had the scleritome dominated by a single basal S1 sclerite, shaped a bit like a wonky four-sided pyramid with one side extended out further than the other. On each of these two opposing sides was a deep notch or sinus, with the notch on the steeper side much deeper and with an outwards-pointing flange at the bottom. Inside this deeper notch would sit the smaller, triangular S2 sclerite, which also had an outwards-pointing flange at its bottom end that lined up with the flange of the S1 to form a loose protective tube. There were also a number of smaller, twisted-plate-shaped L sclerites. I have to confess, I'm still trying to work those out to some extent, but as far as I can tell they stacked on one side on the top of the S1 to form some degree of protective covering for the opening of the pyramid.


Lateral view of a partially-articulated Paterimicra specimen with L sclerites fused to the top of the S1 sclerite. It is noteworthy that the available articulated Paterimicra specimens with fused sclerites (this one, which is the only one to retain the L sclerites in place, particularly) show signs of injury or pathology at some point in development. This suggests that sclerite fusion was a pathological response in these individuals, not a normal part of scleritome development, which may partially explain why articulated specimens are so rare. Figure from Skovsted et al. (in press).

Skovsted et al. (in press) suggest a sessile life position for Paterimitra with the S1+S2 pyramid standing point-downwards, attached to the substrate by an organic stalk (like the pedicle of modern sessile brachiopods) passing between the flanges of the sclerites. They suggest Paterimitra may have been derived from an Eccentrotheca-like ancestor by the enlargement of the basal sclerites. Another Eccentrotheca-type lineage may have lost the sclerites entirely to give rise to the modern worm-like phoronids (though note that a few recent authors have suggested, based on soft-body characters shared with linguloids, that phoronids may be derived from within brachiopods). Micrina (in its suggested form) could be derived from a Paterimitra-type animal essentially by the loss of the L-sclerites. After that, it's simply a matter of extending the two remaining sclerites so that the shell is able to fully close (both Micrina and Paterimitra would have been permanently open to some degree, though Holmer et al., 2008, suggested a protective guard of long setae for Micrina), and what you've got is a quite passable basal brachiopod!

REFERENCES

Högström, A. E. S., D. E. G. Briggs & C. Bartels (in press, 2009) A pyritized lepidocoleid machaeridian (Annelida) from the Lower Devonian Hunsrück Slate, Germany. Proceedings of the Royal Society of London Series B - Biological Sciences.

Holmer, L. E., C. B. Skovsted, G. A. Brock, J. L. Valentine & J. R. Paterson. 2008. The Early Cambrian tommotiid Micrina, a sessile bivalved stem group brachiopod. Biology Letters 4 (6): 724-728.

Skovsted, C. B., G. A. Brock, J. R. Paterson, L. E. Holmer & G. E. Budd. 2008. The scleritome of Eccentrotheca from the Lower Cambrian of South Australia: lophophorate affinities and implications for tommotiid phylogeny. Geology 36 (2): 171-174.

Skovsted, C. B., L. E. Holmer, C. M. Larsson, A. E. S. Högström, G. A. Brock, T. P. Topper, U. Balthasar, S. Petterson Stolk & J. R. Paterson. (in press, 2009). The scleritome of Paterimitra: an Early Cambrian stem group brachiopod from South Australia. Proceedings of the Royal Society of London Series B - Biological Sciences.

Vinther, J., P. Van Roy & D. E. G. Briggs. 2008. Machaeridians are Palaeozoic armoured annelids. Nature 451 (7175): 185-188.

Snail Mimics and Marine Symbionts (Taxon of the Week: Pleustidae)

The pleustid amphipod Incisocalliope aestuarius. Photo by Marco Faasse.

The Pleustidae are a family of marine amphipods, distributed around the world. However, despite their seemingly cosmopolitan distribution and abundance, it has been surprisingly difficult to find information about this family online. Pleustids are one of the many families in the largest of the amphipod suborders, the Gammaridea (gammarideans include the sandhoppers and other sandhopper-looking crustaceans that are what most people think of when they think of an amphipod). Gammaridean interrelationships are a great tangled mess, and in many places fall into a state that we scientists technically refer to as "buggered beyond belief". Only slowly are researchers beginning to draw some sense out of things, and they'll probably be at it for a long time yet. For those who wish to track them down (for the most part, I haven't seen them), the main revisions of Pleustidae were published by Bousfield & Hendrycks (1994 and following) in Amphipacifica*. Stock (1986) (who very tentatively assigns three oddball stygobiotic species from Japan to the Pleustidae) gives the defining characters of the family as "the rudimentary condition of the accessory flagellum of the first antenna, the biramous third uropod (rami lanceolate), the elongate telson, the weak and more or less similar gnathopods 1 and 2, and... the bilobed condition of the labrum". Bousfield & Hendrycks (1994, 1995) divided the family into a number of subfamilies.

*The short-lived journal Amphipacifica ended up with three volumes published between 1994 and 2001 (I haven't found any indications that a planned fourth volume ever made an appearance). While I haven't found a complete contents listing, it appears that most (if not all) articles had the chief editor, Edward Bousfield, as author or co-author, and the majority were on amphipod taxonomy. Perhaps unjustly, the journal is not remembered for its contributions to crustacean systematics as much as it is for Bousfield's perhaps unwise foray into vertebrate taxonomy. Yes, this was the journal that saw the publication of the infamous Bousfield & LeBlond (1995), and the name Cadborosaurus wellsi - a paper so controversial that two of the journal's editors promptly handed in their resignations in protest. For more details, see Darren Naish's 2006 review.

Not having the resources on hand to give you a a decent overview of the family (sorry), I'm just going to give you a couple of the highlights that I have been able to locate - (as it says in the title) the snail mimics and the symbiotic taxa.


Pleustes panopla, a close relative of one of the snail-mimicing pleustids. Photo via here.

Snail mimicry has been recorded for two pleustids, Pleustes platypa and unidentified species of Stenopleustes (Field, 1974). Pleustes platypa lives in kelp beds and mimics the marine gastropod Mitrella carinata. Mimetic Stenopleustes (it is not known how many species are involved) live in beds of Zostera (seagrass) and mimic various species of snails of the genus Lacuna. Different Stenopleustes individuals may have different colour patterns, each matching a different Lacuna species. The amphipod clings tightly to the seagrass, moving slowly to match the speed of a snail. Occassional rocking back and forth mimics the rocking movement of the snails. And while amphipods are perfectly adept swimmers, snail-mimicing Stenopleustes would only swim under extreme provocation, preferring to crawl to the other side of the seagrass blade instead when possible. Obviously, starting to swim would quickly give away that the animal was not a snail!

Other intriguing pleustids are the species that live as symbionts (generally commensals) of larger marine invertebrates. Commensipleustes commensalis is a symbiont of crabs, with enlarged spines on the underside of the forelimbs against which the dactylus (claw) can be folded back, allowing the amphipod to hang onto the host's setae. Members of the genus Dactylopleustes, on the other hand, live on sea urchins, and their legs have notched claws that can be placed around the host's spines. Another species, Mesopleustes abyssorum, clings to the legs of sea spiders. But most remarkable of all is the lifestyle that has been inferred for the species Myzotarsa anaxiphilius (Cadien & Martin, 1999). Like Commensipleustes, Myzotarsa is a symbiont of crabs (in this case, king crabs of the genus Paralithodes), but while most crab-symbiotic amphipods live around the densely setose mouthparts, Myzotarsa lives underneath the crab's recurved abdomen. Without setae to cling on to, the claws on the walking legs of Myzotarsa bear special suckers to allow the animal to latch on. What makes Myzotarsa really remarkable is that not just any crab will make a suitable host - instead, the amphipod shows a strong preference for crabs that are parasitised by rhizocephalans (out of 179 specimens of Myzotarsa referred to by Cadien & Martin, 1999, 167 came from parasitised hosts while only six came from a non-parasitised host [the remainder came from hosts whose parasite status was not recorded). It seems that the diet of the little Myzotarsa is the eggs being incubated underneath the abdomen. While healthy crabs will only be carrying eggs if they're female and if it's the right season, the chemically-castrated, feminised (if male) and mind-controlled infected crabs will be carrying externa filled with yummy rhizocephalan eggs all year round...

REFERENCES

Bousfield, E. L., & E. A. Hendrycks. 1994. A revision of the family Pleustidae (Crustacea: Amphipoda: Leucothoidea). Part I. Systematics and biogeography of component subfamilies. Amphipacifica 1: 17-57.

Bousfield, E. L., & E. A. Hendrycks. 1995. The amphipod family Pleustidae on the Pacific coast of North America: Part II. Subfamilies Parapleustinae, Dactylopleustinae, and Pleusirinae. Systematics and distributional ecology. Amphipacifica 2: 65-133.

Bousfield, E. L., & P. H. LeBlond. 1995. An account of Cadborosaurus willsi, new genus, new species, a large aquatic reptile from the Pacific coast of North America. Amphipacifica 1 (Supplement 1): 3-25.

Cadien, D. B., & J. W. Martin. 1999. Myzotarsa anaxiphilius, new genus, new species, an atylopsine amphipod (Gammaridea: Pleustidae) commensal with lithodid crabs in California. Journal of Crustacean Biology 19 (3): 593-611.

Field, L. H. 1974. A description and experimental analysis of Batesian mimicry between a marine gastropod and an amphipod. Pacific Science 28 (4): 439-447.

Stock, J. H. 1986. Amphipoda: Pleustidae. In Stygofauna Mundi: A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabiting Subterranean Waters (including the Marine Interstitial) (L. Botosaneanu, ed.) pp. 560-561. E. J. Brill / Dr. W. Backhuys: Leiden.

Pathogens, or More than Pathogens? (Taxon of the Week: Aeromonas)

Dropsy in a goldfish infected with Aeromonas hydrophila. Keepers of freshwater fish will probably be all too sadly familiar with this sight - dropsy is highly contagious, and almost impossible to eliminate once it becomes established (of course, fish not being great exhibitors of external symptoms, "once it becomes established" is roughly equivalent to "by the time you notice that something's happening"). Image from MicrobeWiki.

Yeah, it's late. Tough. What with moving house over the weekend, having learnt that I had to move on Monday (long story), I've been a little preoccupied. I rather doubt that anyone marks their calendar for these posts anywho.

Anyway, Aeromonas is a genus of rod-shaped facultatively anaerobic gamma proteobacteria that is abundant in aquatic habitats (mostly freshwater). 16S rDNA phylogenetically speaking, it belongs to a clade that also includes the Vibrionaceae, Pasteurellaceae and Enterobacteriaceae, all of them pretty economically significant families because they all contain common pathogens, and Aeromonas follows suit. Of the about fifteen species currently recognised in the genus, at least five have been implicated in human infections, while the others are known primarily as pathogens of aquatic organisms such as fish and frogs. The most common symptoms of Aeromonas infection are gastrointestinal diseases (just to make you wince, a quote from Janda & Abbott, 1998 - "Chronic diarrhea exceeding 1 year's duration due to [Aeromonas] caviae or A. hydrophila has been recorded"), but conditions such as septicaemia, wound infections or even meningitis have been caused by this genus, among others.


Human leg infected with Aeromonas hydrophila. Photo from Microbe Wiki.

So far, Aeromonas species have been almost entirely studied as pathogenic and/or animal-associated organisms. However, they have been isolated in large numbers from freshwater and brackish habitats (particularly nutrient-rich ones) (Kaper et al., 1981; Martin-Carnahan & Joseph, 2005), and, in comparison with other genera such as Streptococcus, Staphylococcus and Vibrio, appears to me to show every sign of being primarily a facultative pathogen. Infection causes a range of pathologies, with no obvious standard symptom arc*, low host specificity, and without a tight correlation between the presence of Aeromonas and whether or not a pathology develops (Aeromonas has been isolated from the gut of patients with no sign of Aeromonas-related pathologies). Compare this to an obligate pathogen such as Treponema pallidum (the cause of syphilis), which causes a more standardised range of symptoms and for which presence of the pathogen almost always leads to presence of the disease**. It is quite possible that Aeromonas' role in the environment as a pathogen is in fact only secondary to whatever else it's doing out there (my first guess would be decomposition). However, this neglect of Aeromonas' environmental ecology is hardly unusual. The truth is that our knowledge of environmental real-time ecology for all bacteria is depressingly low. Not only have pathogens received the greater share of attention, but even studies of non-pathogenic taxa have generally been conducted in highly artificial laboratory environments (for obvious reasons of practicality). Laboratory culture media, for instance, generally offer a uniform abundance of nutrients that is entirely unlike the poorer and more patchy nutrient distributions elsewhere, and it is a very open (and very difficult) question how much and in what way observations made in the laboratory environment really reflect what happens elsewhere.

*Yeah, you can tell I'm not thoroughly briefed in my medical terminology and I'm just making this up as I go along, can't you?

**Anybody out there with actual medical knowledge can feel free to rip into me if I've just made an idiot of myself.

Back to Aeromonas. While (as I said) about fifteen or so species are currently recognised, traditionally these have been divided into two groups - Aeromonas salmonicida (guess what it is known to do) vs. everything else. Aeromonas salmonicida is psychrophilic (it grows best at low temperatures, from 2 to 30°C) and nonmotile, while the other species (the 'Aeromonas hydrophila' group) are mesophilic (growing at temperatures between 10 and 42°C) and motile, usually with a single polar (at one end) flagellum but occasionally with lateral flagella. 16S rDNA phylogeny, however, shows that A. salmonicida is nested deep within the genus, and represents a derived taxon. Reflecting this position, A. salmonicida still possesses the genes for flagella production despite their inactivity. And just to really hammer the point home, an as-yet unnamed Aeromonas strain has been isolated (Martin-Carnahan & Joseph, 2005) that is mesophilic, motile, but genetically identical to A. salmonicida!

REFERENCES

Janda, J. M., & S. L. Abbott. 1998. Evolving concepts regarding the genus Aeromonas: an expanding panorama of species, disease presentations, and unanswered questions. Clinical Infectious Diseases 27: 332-344.

Kaper, J. B., H. Lockman, R. R. Colwell & S. W. Joseph. 1981. Aeromonas hydrophila: ecology and toxigenicity of isolates from an estuary. Journal of Applied Microbiology 50 (2): 359-377.

Martin-Carnahan, A., & S. W. Joseph. 2005. Genus I. Aeromonas Stanier 1943, 213^AL. In G. M. Garrity, D. J. Brenner, N. R. Krieg & J. T. Staley (eds) Bergey's Manual of Systematic Bacteriology, 2nd ed., vol. 2. The Proteobacteria. Part B. The Gammaproteobacteria, pp. 557-578. Springer.

Define "Published"

The other day, Rob Taylor of the DML was asking about the status of electronic and Advance Online publications in the eyes of the ICZN - do they count as officially published? You can read my response to him here (unfortunately, the DML archive has borked Rob's original message, otherwise I'd link directly to that, but the greater part of the original is present in indented form through my reply). I've discussed electronic publication on this site before (see here and here). I think I've stayed fairly neutral in those posts, though my personal attitude towards electronic-only publication is roughly comparable to my attitude to such things as prostitution and recreational drug use. They're probably a bad idea, but they're going to happen anyway, so it's probably more important to structure things so that they can be properly managed than to simply hope they're going to go away.

But in all this discussion of how to establish when online references are validly published - what are the necessary archiving requirements, if a publication comes out online before it appears printed on paper should it be dated from its online or printed appearance, etc. - there is one ugly but widely-known secret that has been mentioned surprisingly little. As difficult as it may be to establish whether an online resource has been validly "published", it can be equally difficult to establish the same for a printed resource.

In the eyes of the ICZN, it is not enough for the printed resource to simply exist, and for fairly obvious reasons. If I set up a printing press in my garage (after all, seeing how I don't own a car, I'd have no shortage of space in a garage) and printed myself a copy of some pamphlet naming a new taxon, that new taxon still wouldn't be valid. Taxonomy, like all other sciences (indeed, in some respects even more than other sciences) is ultimately dependent on communication if it is going to work properly. It's no coincidence that the word "published" is so similar to the word "public". If there's only a single copy of my little pamphlet, how is it going to be communicated? How are my fellow taxonomists going to be able to critique the validity or otherwise of my new taxon? What is the likelihood of that pamphlet being available ten, twenty, two hundred years down the track after I myself am dead and buried*? There's also the credibility factor - say some amazing new taxon is published in a high-level journal (Ubertyrannosaurus ohbuggerme, or something), and I start waving around my little pamphlet, which is dated to earlier than the journal, shouting that I described this new taxon first. If nobody else has seen it before, what evidence do I have that I didn't print out the pamphlet after the journal article came out and slap a fake date on it?

*Though if I am the type of person who churns out secret pamphlets on my own printing press in the garage, that would possibly be buried within a raccoon.



So instead of letting just anything go, the ICZN has the following requirements:

8.1. Criteria to be met. A work must satisfy the following criteria:

8.1.1. it must be issued for the purpose of providing a public and permanent scientific record,

8.1.2. it must be obtainable, when first issued, free of charge or by purchase, and

8.1.3. it must have been produced in an edition containing simultaneously obtainable copies by a method that assures numerous identical and durable copies.

I can still print papers out on my private press at home (alas, some people do) but in order for anything in those papers to be valid, there has to be a decent number of copies, those copies have to be readily available after publication, and there has to be a decent chance that copies are still going to be out there and available thirty years down the track. Still, it's all just a little vague, innit? Just how many copies is a "decent number"? It's one of the great ironies of the ICZN (and it hasn't gone uncommented on) that the Code has strict guidelines for the one form of electronic publication it does currently allow (CD publications must have at least five copies deposited in separate major public libraries), but has no such baseline for the paper publications that are its supposed backbone.

It doesn't take a great deal of imagine to develop hypothetical problem cases for these rules. Say I print out a large number of copies of my private paper, more than enough to satisfy that part of the requirements. But then instead of distributing them right away, I keep them stacked in a box in my basement*. Even though the copies exist, they're still not satisfying the desired purpose of communication. They shouldn't really be counted as "published" until I actually get off my arse and start distributing them to people. The problem is, though, that the date on their cover would be the date I printed them out, not the date I made them available (this is exactly one of the issues involved in the Scansoriepidendrosauropteryx fiasco). There are a number of cases in taxonomy of synonymous names being published at very close times, and working out which of the two (or more, if your luck is really evil) names has priority can be an absolute bitch. The longer ago the papers were published, the harder it can become to establish their relative priorities. What's more, there's always a nearly comical sense of bleak futility about such investigations. In the case of the aforementioned Scansoriopteryx, it's currently widely known that its source, the Dinosaur Museum Journal (volume one, if I recall correctly) didn't become publicly available until a while after its printing, making the date on the book out by at least a month (during which time, the probable synonym Epidendrosaurus appeared in an Advance Online publication). The thing is that while that may be general knowledge now, will it necessarily still be general knowledge in twenty years' time. If the information floats around in informal communications, but no-one actually mentions in print that the date is out**, then that piece of info is going to be forgotten, and the claimed date will become treated as correct by default.

*I seem to have both a garage and a basement - I'm beginning to like this hypothetical house of mine.

**In this specific case they have (Harris, 2004), but let's make allowances for hypotheticals.


The Japanese arachnologist Kyukichi Kishida, publisher and editor of the ill-fated journal Lansania. Photo from here

As a concrete example, I'm going to present you with one of the worse cases out there. The journal Lansania was privately published in Japan by the zoologist Kyukichi Kishida from 1929 to around-about-1941, and its history was recently reviewed by Tennent et al. (2008), whose article makes recommended if somewhat disturbing reading for anyone interested in the subject of this post. For the first couple of years, Lansania seems to have a fairly normal run, with issues appearing as one might expect. After that, though, it all seems to have gone a little pear-shaped (funding for publication may have become an issue). Issues appeared erratically, and not necessarily in order. A number of scheduled issues never seem to have been printed in any form. For others, individual articles exist as offprints, but there is no sign of a complete issue. Issue 58, for instance, is represented in offprints by pages 113-115 and pages 125-128, but there is no sign of 116-127*. What happened to these intervening pages? Did they ever actually exist? It seems that what happened was that Kishida assigned individual articles to planned issues as they were submitted for publication, had them printed as offprints when they had been prepared for publication, then printed the final issue when all the articles for it were ready.

*Ironically, the article represented by pages 113-115 of this issue, which does exist, is supposedly a supplement to an "earlier" Lansania article that seemingly doesn't exist.

Things were not improved by Kishida's somewhat lax attitude towards publication and dating. There is evidence that in many (but not all) cases, the date placed on a published issue or article was the date the article was submitted for publication, not the date it appeared in print (which might not have been until some years afterwards). One publication of Kishida's bore a date of "1938", but gave his address as one which he did not reside in until 1957! When asked by a colleague to explain the discrepancy, Kishida apparently replied that he had begun composing the article in 1938. On a number of occasions, Kishida also seemingly cited as if published articles and names from articles that would have still been "in press", not all of which ever made it into actual print.

Unless they did... You see, Lansania never had a huge circulation, and copies of issues have become few and far between (Tennent et al. give a listing of all the holdings for Lansania they could find - for instance, the latest issue they could locate, no. 127, is only held by two libraries in Japan, while it is possible that only a single copy still exists of each of the issue 58 offprints). Kishida stored a number of unpublished manuscripts and published journals in a shed behind his house that were disposed of after his death in 1968 - could these have included some of the missing issues of Lansania? A lot of Kishida's material had earlier been taken for use as waste paper in times of shortage during the Second World War. Could some of the "unpublished" issues (or even extra copies of the rare published issues) still be held in private libraries? Should the taxa described in the exceedingly rare printed issues or offprints of Lansania even be treated as published at all?

Though extreme, the case of Lansania is not unique. Anyone who has had to invest a fair amount of time in obtaining copies of old publications knows that they can sometimes be exceedingly difficult to find (one of the first pieces of advice I would give to a student starting taxonomic research is to make friends with a really good interloans librarian). They may have satisfied all the requirements for valid publication when they first appeared - but over the years, copies get lost, destroyed... Are there taxa out there in current use for which copies of the original descriptions simply don't exist anywhere anymore? What is the status of these taxa under the ICZN? Can an available name become unavailable?

REFERENCES

Harris, J. D. 2004. 'Published works' in the electronic age: recommended amendments to Articles 8 and 9 of the Code. Bulletin of Zoological Nomenclature 61 (3): 138-148.

Tennent, W. J., M. Yasuda & K. Morimoto. 2008. Lansania Journal of arachnology and zoology – a rare and obscure Japanese natural history journal. Archives of Natural History 35 (2): 252-280.

Get Used to Disappointment, Princess

Permit me a moment's whinging. First, I receive a notice in my e-mail telling me that a seminar is being given here at Curtin University by no less a person than Per Ahlberg on his experiences using synchrotron CT-scanning to study Devonian vertebrates. Excitedly, I read on to find when the seminar is being given...

Thursday morning. Exactly when I'm scheduled to teach a lab on flatworms. Poo.

I hope those students appreciate what I'm giving up for them...

Flowers in the Water (Taxon of the Week: Hydrocharitaceae)

Hydrocharis morsus-ranae, frogbit (and is there anyone out there who can tell me why this plant is called "frogbit"?), an insect-pollinated example of Hydrocharitaceae. Photo by J. R. Crellin.

The Hydrocharitaceae is a small but morphologically diverse family of fully aquatic monocots. About eighty species are included in about fifteen genera, excluding the genus Najas which has occasionally been suggested to belong in this family (Tanaka et al., 1997). Hydrocharitaceae includes both freshwater and marine species, such as seagrasses (Enhalus, Thalassia and Halophila), eelgrasses (Hydrilla, Vallisneria) and oxygen weeds (Egeria, Elodea, Lagarosiphon). Depending on the species, Hydrocharitaceae may live their lives partially or entirely submerged.

A major factor in the diversity of this family derives from the diversity of pollination methods. Hydrocharitaceae may be monoecious (separate male and female flowers, but on the one plant) or dioecious (separate male or female plants). Many of the genera in this family are insect-pollinated (entomophilous), with flowers protruding from the water, and comparison with related families suggests that this is the ancestral condition for the family. However, more that one lineage of Hydrocharitaceae has evolved to take advantage of their home environment by becoming water-pollinated (hydrophilous). Such species may be epihydrophilous, with flowers borne at the water surface, or hypohydrophilous, with flowers completely underwater. The genera Vallisneria, Lagarosiphon, Nemachandra and Enhalus have become epihydrophilous in a way that no other aquatic flowering plant has - rather than releasing pollen into the water like other hydrophilous plants, the entire male flower is released to float on the surface of the water until it reaches a female flower. Remarkably, while this process is unique to Hydrocharitaceae, genera with detaching flowers do not form a single clade - instead, the process has evolved at least three times within the Hydrocharitaceae (Tanaka et al., 1997, who did not test the position of Nemachandra).


Inflorescence of the seagrass Enhalus acoroides, and a floating congregation of the small detached male flowers. Photo from Team Seagrass.

Hydrocharitaceae also have other reproductive options open to them. Vegetative reproduction is common among plants, a factor which gardeners have profited from for generations. The Hydrocharitaceae are vegetative reproducers par excellence, with broken-off fragments all too ready to reroot and establish themselves. In the case of the Florida seagrass Halophila johnsonii, the only marine plant listed as endangered in the United States, vegetative reproduction may be the only thing keeping it going. Only female flowers have ever been recorded for this species, and seed production has never been recorded. While some have suggested that Halophila johnsonii may reproduce apomictically (parthenogenetically for the zoologically-inclined), York et al. (2008) demonstrated that this is probably not the case - ovules and gametes are produced in the same manner as other sexually-reproducing Halophila species. Theoretically, pollination of these ovules should be entirely possible - but somehow, the males have all disappeared, and the Halophila johnsonii females are waiting for a pollinator that will never come.


Elodea canadensis, one of the fully submerged "oxygen weeds" widely used in fish tanks, from whence they escape to take over the world. Photo by Ondřej Zicha.

Unfortunately, this propensity for vegetative propagation is also the dark side of Hydrocharitaceae. Widely propagated as aquatic ornamentals or for aquaria and fish tanks, Hydrocharitaceae have found it all too easy to escape their alloted positions and invade exotic waterways. In New Zealand, three species of oxygen weed (Egeria densa, Lagarosiphon major and Elodea canadensishave become leading invasives, despite the fact that, for all three, individuals of almost invariably only a single sex are present in the wild, rendering sexual reproduction nonexistent (Healy & Edgar, 1980). In 1968, power production at a hydroelectric station on the Waikato River was brought to a halt by dense Lagarosiphon major blocking the station's water intake. Elodea canadensis achieved even greater invasions:

The heavy stand in Lake Rotoroa, Nelson Lakes National Park is noteworthy: between 1965 and 1971 the plant formed a virtually complete marginal weed-bed to a depth of 8.5 m, with stems to 6 m high. Here, in terms of dry matter per m², the amount of weed herbage is significantly higher than that recorded for any other freshwater macrophyte community elsewhere in the world. (Healy & Edgar, 1980)

REFERENCES

Healy, A. J., & E. Edgar. 1980. Flora of New Zealand vol. III. Adventive cyperaceous, petalous and spathaceous monocotyledons. P. D. Hasselberg, Government Printer: Wellington (New Zealand).

Tanaka, N., H. Setoguchi & J. Murata. 1997. Phylogeny of the family Hydrocharitaceae inferred from rbcL and matK gene sequence data. Journal of Plant Research 110 (3): 329-337.

York, R. A., M J. Durako, W. J. Kenworthy & D. W. Freshwater. 2008. Megagametogenesis in Halophila johnsonii, a threatened seagrass with no known seeds, and the seed-producing Halophila decipiens (Hydrocharitaceae). Aquatic Botany 88: 277-282.

More From the "They Don't Write Papers Like They Used To" Files

But before that, a couple of other things:

Recent Carnivals - Berry Go Round is here, Circus of the Spineless is here.

Also recently, the Open Laboratory 2008 collection of some of the year's best online science writing is now available. In the interests of disclosing all interests (naturally), yours truly has a piece featured in this publication - on citrus fruit. Anybody who has heard me commenting on my poor knowledge and outright terror of botany will know that this is a Very Funny Thing.

On to the main point of the post, which will not be anything I'm writing at all...

Recently, I was reading a paper from 1866 by a Dr H. Dohrn with the unprepossessing title of "Synopsis of the birds of Ilha do Principe, with some remarks on their habits and descriptions of new species" (Proceedings of the Zoological Society of London 1866: 324-332). Principe, for those who don't know it, is a part of the country of Sao Thomé and Principe in west-central Africa. This is the second paragraph of that publiction:

It seems to me that the most remarkable feature in the fauna of Ilha do Principe is that not a single bird of prey exists on the island, whilst they are abundant on the two other islands and on the nearest part of the continent. I saw hundreds of Milvus parasitus in San Thomé; Gypohierax angolensis and some other species are not uncommon in Fernando Po; but the whole tribe avoids Principe. The inhabitants of the latter place and of San Thomé assert that there is a deadly hatred between the Grey Parrots (Psittacus erythacus) of Principe and the Kites of San Thomé, and that, if ever a Milvus visits the neighbouring island, hundreds of Parrots fall upon him and kill him, and that the Kites take revenge if perchance a Parrot should venture a trip to their kingdom. There must be some family reason for this strange degree of enmity, for they seem to live in tolerable peace together on the coast.

You will probably not be surprised to hear that I was somewhat taken aback by this - even allowing for the different time period, it seemed a rather credulous account. But as I read further, I realised that Dohrn was employing something that is a little alien to modern academic writing: irony.

To demonstrate my point, here are my other favourite passages from Dohrn:

On a species of drongo:

The native name is "Maria Palu, feiticeira" (translated, Maria Palu, the sorceress). The bird is black, with red eyes; seems very indolent in daytime, and shows a great ability in the imitation of some other birds' voices. Of course there must be some "feiticeira" to it: therefore, sitting on the roof of a house and singing in its melancholy manner, it prophecies the death of one of the inhabitants; and this, of course, takes place, but often a long time after this prophecy.

On the bald ibis:

Soon after my arrival on the island I was informed by some natives that there was a very remarkable bird in the island called "Corvão." One told me that it was a kind of raven with splendid metallic wings; another described the bird "with the head of an owl and the feet of a duck, climbing up and down trees;" and others gave me other extravagant descriptions of it; but all of them agreed that the bird lived in almost inaccessible rocky and wooded localities of the southern district, and that if ever a specimen passed over the town it was a bad omen for the white inhabitants, who in such case were exposed to heavy disease or death. Of course I was very curious to see this species, and settled for a fortnight in a negro's hut in those desert parts of the island*. Whoever has visited those large tropical forests knows the difficulty of proceeding there. I enjoyed the special favour of heaven in arriving there when the rains set in a month before they usually do, and it was very hard work to run after these birds. I saw them daily at great distances, and heard them crowing like a Raven; but as soon as I entered the forests the monkeys made so much noise, barking and howling, as to alarm all the animals in the neighbourhood. Thus I was finally very glad when one of my native hunters appeared with a female specimen of the Corvão, which turned out to be Geronticus olivaceus.

*I think in this case "desert" means "deserted", not "arid".

The Uglier Side of the Family (Taxon of the Week: Ceratomorpha)

Lowland tapir, Tapirus terrestris, in a blatant attempt to exploit the cute factor. Photo by Antonio Pinheiro.

The modern perissodactyls are, sadly, but a shadow of their former glory. Once among the planet's dominant herbivores, the odd-toed hoofed mammals have become reduced to less than twenty living species (the majority of which are critically endangered to boot). Nevertheless, their secure position as charismatic megafauna means that they are familiar animals to most people (at least conceptually). One species in particular, the horse Equus caballus, has developed a close association with humanity and holds a high position in the human psyche (or at least the Eurasian and American psyche). But this post won't be dealing with horses - this is for the other side of the perissodactyls. The Ceratomorpha may not have been blessed with the aesthetic appeal of the horses, but they're not without their charms.


Skull of Tapirus terrestris. Note the severely recessed nasal bones, positioned above the eyes, that indicate the presence of the muscular trunk in life. Tapirs are often thought of as more "primitive" than other perissodactyls, but no other perissodactyl has a skull like that. Photo by Matthew Colbert.

The Ceratomorpha contains two living families, the Tapiridae (tapirs) and Rhinocerotidae (rhinoceroses). Most of the people reading this will, I'm guessing, probably be familiar with the appearance of both, though rhinos do get given a little more press than tapirs*. Rhinos are also marginally more diverse in the modern environment, with five species to the tapirs' four - but considering that at least two of the rhino species are hovering on the brink of extinction, that may yet change. As regards fossil taxa, the limits of Ceratomorpha are a little more hazy, mostly because different authors have applied slightly different concepts for 'Ceratomorpha' vs. the related name 'Tapiromorpha'. In my opinion, the most sensible definitions for both are those proposed by Holbrook (1999), who used 'Ceratomorpha' for the crown clade of tapirs + rhinos, and 'Tapiromorpha' for the total group of anything more closely related to tapirs and rhinos than to horses. These definitions are better than the alternatives proposed by Froehlich (1999) in being agnostic as to whether the extinct chalicotheres are tapiromorphs (as supported by Froehlich, 1999, among others) or not (as indicated by, e.g., Hooker & Dashzeveg, 2004, who placed chalicotheres outside the perissodactyl crown group).

*Completely unrelated aside to everything else - the Japanese name for 'tapir' is 'baku'. Originally, a baku was a trunked mythical creature that was supposed to feed on people's dreams (particularly nightmares), but living tapirs have a somewhat more material diet. This isn't the only example in Japanese of a living exotic animal being granted the name of a pre-'existing' mythical creature: giraffes are known as 'kirin'.

Within the Ceratomorpha, then, the primary division is between the Tapiroidea and the Rhinocerotoidea, each including (obviously) the taxa more closely related to one of the living families than the other*. The Rhinocerotoidea are far better understood that the Tapiroidea, which have a more spotty fossil record. Four families are generally assigned to the Tapiroidea - Helaletidae, Lophialetidae, Deperetellidae and Tapiridae. The mostly Eocene 'Helaletidae', small tapiroids of North America and Eurasia, however, are probably paraphyletic with regard to other tapiroids. Holbrook (1999), for instance, excluded the North American genus Heptodon from the Helaletidae as he found it to be sister to all other tapiroids. At least one 'helaletid' genus, the North American Oligocene Colodon, is of interest because it possessed significantly retracted narial opening, indicating the presence of a trunk as in modern tapirs. However, Colbert (2005) included Colodon within the Tapiridae, closer to modern tapirs than previous authors.

*Be warned, though - older references tend to use the term 'tapiroid' as a grade concept for non-rhinocerotoid tapiromorphs, including a number of taxa that would be regarded as stem-Ceratomorpha in this post.


Hyrachyus, an Eocene rhinocerotoid. Hyrachyus was very similar to the tapiroid 'Helaletidae' of the same time period (which is pretty much what one would expect, really) and represents the general basal morphology for Ceratomorpha. Picture from here, though it has a definite Zdenek Burian look about it.

The Lophialetidae and Deperetellidae were two strictly Asian late Eocene families that suffer from a severe lack of study. Both have been mostly regarded as tapiroids for as long as they have been known, but Holbrook (2001) pointed out that the evidence for doing so is pretty slight. Members of both families (supported as forming a monophyletic clade by Holbrook, 1999) showed a reduction in the number of toes from four to three and a longer, more slender foot than other tapiroids, indicating that they were more cursorial (Radinsky, 1969). The Eocene also saw the appearance of the first Tapiridae, which (with the possible exception of Colodon, depending on its position) seem to have been the only tapiroids to survive into the Oligocene. The earliest tapirid genus, Protapirus, has been described from both North America and Eurasia, but its monophyly is uncertain (Colbert, 2005).


Assortment of Amynodontidae as drawn by Stanton Fink - front to back, they are Cadurcodon, Gigantamynodon and Metamynodon. This isn't the first time I've used one of Stanton's drawings - other ones are here and here. I must confess to having something of a love-hate reaction to Stanton's work - his style isn't entirely to my taste aesthetically, but one thing I do think is great is how often he reconstructs animals that other artists just ignore. Besides, I can't draw to save myself, so I have absolutely no right to criticise.

The other superfamily, Rhinocerotoidea, has had a much greater diversity described, both in terms of number of species and morphological range. Three major families have been recognised - the Amynodontidae, Hyracodontidae and Rhinocerotidae. The middle Eocene to Middle Oligocene Amynodontidae were the really unfortunate members of the superfamily appearance-wise - as the saying goes, they would have not only been hit with the ugly stick, they would have fallen out of the ugly tree hitting every ugly branch on the way down. Amynodontids have mostly been characterised as subaquatic, like modern hippos, but Wall (1998) points out that only a single derived subgroup, the Metamynodontini, shows adaptations for such a lifestyle. The remaining amynodontids would have been terrestrial. One group of amynodontids, the Cadurcodontini, appears to have convergently evolved a short trunk like that of the tapirids.


A rather disgruntled looking Hyracodon, reconstructed by Heinrich Harder.

The Eocene to Oligocene Hyracodontidae are the real taxonomic dog's breakfast of the Rhinocerotoidea. Radinsky's (1966) influential definition of which taxa should be included in Hyracodontidae essentially amounted to "anything which doesn't belong to Amynodontidae or Rhinocerotidae". Characters that have been suggested to support a monophyletic Hyracodontidae, such as an elongate foot and three toes, are also found in other perissodactyls (in fact, if you look upwards you'll notice that I mentioned the exact same characters in connection with Lophialetidae). In general, hyracodontids were more cursorial than other rhinocerotoids, and small genera such as Hyracodon would have been fairly pony-like. Also usually included in the 'Hyracodontidae' where the gigantic Indricotheriinae, of which the central Asian Oligocene genus Paraceratherium is famed as the largest known land mammal*. The phylogenetic analysis of Holbrook (2001) supported treating the smaller Hyracodontinae as a separate family from the larger Indricotheriinae, but failed to support the latter as monophyletic.

*Though sometimes in disguise. Paraceratherium has held a few different names over the years - 'Indricotherium' and 'Baluchitherium' are two of the most commonly used. It remains a rather fraught question as to whether all these represent the same animal, or a number of closely related animals.


A herd of Paraceratherium (would Paraceratherium have really travelled in herds?) passing by scavenging Hyaenodon. Painting by Mauricio Antón.

And finally, the Rhinocerotidae. But I think I've rabbited on and wasted enough of everyone's time for today, so I guess I'll have to leave the hippo-like Teleoceras, or the double-horned Menoceras (two horns side by side, that is), or the gigantic Elasmotherium (with what must have been one of the most terrifying pieces of headgear this side of Arsinoitherium) for another time.

REFERENCES

Colbert, M. W. 2005. The facial skeleton of the Early Oligocene Colodon (Perissodactyla, Tapiroidea). Palaeontologia Electronica 8 (1): 8.1.12A.

Froehlich, D. J. 1999. Phylogenetic systematics of basal perissodactyls. Journal of Vertebrate Paleontology 19 (1): 140-159.

Holbrook, L. T. 1999. The phylogeny and classification of tapiromorph perissodactyls (Mammalia). Cladistics 15 (3): 331-350.

Holbrook, L. T. 2001. Comparative osteology of early Tertiary tapiromorphs (Mammalia, Perissodactyla). Zoological Journal of the Linnean Society 132 (1): 1-54.

Hooker, J. J., & D. Dashzeveg. 2004. The origin of chalicotheres (Perissodactyla, Mammalia). Palaeontology 47 (6): 1363-1386.

Radinsky, L. B. 1966. The families of the Rhinocerotoidea (Mammalia, Perissodactyla). Journal of Mammalogy 47 (4): 631-639.

Radinsky, L. B. 1969. The early evolution of the Perissodactyla. Evolution 23 (2): 308-328.

Wall, W. P. 1998. Amynodontidae. In Evolution of Tertiary Mammals of North America: Terrestrial Carnivores, Ungulates, and Ungulatelike Mammals (L. L. Jacobs & K. M. Scott, eds.) pp. 583-588. Cambridge University Press.