Salinella - what the crap was it?

In 1991, Haszprunar et al. published a brief book chapter in which they listed a collection of modern animals of exceedingly uncertain relationships. Any one of those organisms would make for an interesting blog - some of them have now been placed with reasonable confidence in the animal family tree. The worm-like Xenoturbella masqueraded as a mollusc for a while, but now appears to be a basal deuterostome (Bourlat et al., 2003 - see Palaeos.com for a section I wrote on this affair a couple of years ago). The also-worm-like Buddenbrockia has, rather spectacularly, been shown to be a very basal member of the parasitic Myxozoa, the least animal-like of all animals (Okamura et al., 2002). But today, I'm going to touch on perhaps the most mysterious of all Haszprunar et al.'s subjects - Salinella salve Frenzel 1892.

Salinella has only ever been found once, in a saline culture derived from salt beds in Argentina (Brusca & Brusca, 2003 - see Answers.com for further details). It was described as having a unique body plan, with a single layer of cells surrounding a hollow sac, open at both ends. All cells were densely covered by cilia both inside and out, and there were longer cilia around the openings (which were the mouth and anus). Other than that, there appears to have been no distinction into organs, tissues, whatever. Salinella moved by ciliary gliding, and reproduced asexually by fission.

If this description was accurate, then the affinities of Salinella become very difficult indeed. The presence of a through-gut of sorts gives Salinella a superficially bilaterian appearance, but there is no way it could be a bilaterian. All known bilaterians are triploblastic (i.e. possess three basic cell layers, with the possible exception of mesozoans, if they are reduced bilaterians), and even the outgroup of bilaterians, whether cnidarians or ctenophores, has at least a diploblastic organisation (two cell layers). Even sponges are essentially diploblastic. If there was a monoblastic stage in the evolution of animals, as has been suggested by some authors, then it would have been very early in their history. It is possible that Salinella might represent this stage, which has inferred from the blastula stage in embryonic development (Clark, 1922).

Salinella has been compared in the past to the simple animals Trichoplax and the Mesozoa. Trichoplax (generally placed in its own phylum, Placozoa) is a flattened organism with only four cell types, and has been referred to as the simplest-organised animal known (Syed & Schierwater, 2002). It basically comprises an upper epithelium, a lower epithelium and a central mass of cells. Digestion occurs through the formation of a hollow underneath the lower epithelium into which the animal exudes digestive juices and absorbs nutrients. Epithelial cells are ciliated, but externally only.

The Mesozoa are two groups of internal parasitic animals, the Orthonectida and Dicyemida (it is now thought quite likely that these two groups are not closely related to each other). Both have a basic structure of an outer layer of ciliated cells surrounding a central mass. In dicyemids, the central sector is a single long tube cell. In Orthonectida, the central area contains gametes. Before the formation of gametes, orthonectids are a multinucleate plasmodium without distinct cells.

Beyond the superficial similarity of undifferentiated cell layers, however, Salinella as little in common with these animals. It has long been suggested that, rather than being primitively simple organisms, mesozoans represent derived animals that have become secondarily simplified as a result of their parasitic lifestyles. For both groups, there is genetic evidence to support this (Hanelt et al., 1996; Kobayashi et al., 1999). Trichoplax has a better claim to be genuinely primitive. However, Salinella lacks Trichoplax's central cellular layer, and Trichoplax does not have Salinella's cilia on both sides of the cell. And certainly there is no similarity between Trichoplax's external digestion and Salinella's through gut.

Which brings us to the final possibility, the one that many zoologists have suspected - Salinella never actually existed. It is possible that Frenzel was mistaken in his description of Salinella's structure (I've never seen Frenzel's original description, and I'd be interested in seeing how likely this is). Could Frenzel have actually been looking at Trichoplax-like organism? Unfortunately, unless more specimens of Salinella are recovered, we are unlikely to ever know. And if Frenzel was severely mistaken, then even if the organism he was looking at is recovered, it may never be recognised as such.

REFERENCES

Bourlat, S. J., C. Nielsen, A. E. Lockyer, D. T. J. Littlewood & M. J. Telford. 2003. Xenoturbella is a deuterostome that eats molluscs. Nature 424: 925-928.

Brusca, R. C., & G. J. Brusca. 2003. Invertebrates, 2nd ed. Sinauer Associates, Inc., Publishers: Sunderland (Massachusetts).

Clark, A. H. 1922. Animal evolution. Proceedings of the National Academy of Sciences of the USA 8: 219-225.

Hanelt, B., D. Van Schyndel, C. M. Adema, L. A. Lewis & E. S. Loker. 1996. The phylogenetic position of Rhopalura ophiocomae (Orthonectida) based on 18S ribosomal DNA sequence analysis. Molecular Biology and Evolution 13 (9): 1187-1191.

Haszprunar, G., R. M. Rieger & P. Schuchert. 1991. Extant "problematica" within or near the Metazoa. In The Early Evolution of Metazoa and the Significance of Problematic Taxa (A. M. Simonetta & S. Conway Morris, eds.) pp. 99-105. Cambridge University Press.

Kobayashi, M., H. Furuya & P. W. H. Holland. 1999. Dicyemids are higher animals. Nature 401: 762.

Okamura, B., A. Curry, T. S. Wood & E. U. Canning. 2002. Ultrastructure of Buddenbrockia identifies it as a myxozoan and verifies the bilaterian origin of the Myxozoa. Parasitology 124: 215-223.

Syed, T., & B. Schierwater. 2002. Trichoplax adhaerens: discovered as a missing link, forgotten as a hydrozoan, re-discovered as a key to metazoan evolution. Vie Milieu 52 (4): 177-187.

Taxon of the Week #3: Rana

The taxon that has been chosen to receive the coveted Taxon of the Week spot today is the frog genus Rana. Rana is a large primarily Holarctic genus of frogs, and probably the inspiration for most depictions of frogs in the world (see the page on Wikipedia and linked pages for images). Well-known species are the edible frog (Rana × esculenta - actually not a true species but a hybrid) and the European common frog (Rana temporaria).

I thought I'd look up the info on Rana over lunchtime. Pretty soon, my head was swimming. The genus Rana has been hit with two major investigations in recent years, both of which have received some frosty responses. Frost et al. (2006) in their investigation of the 'Amphibian Tree of Life' divided Rana between more than fifteen smaller genera to remove its previous paraphyly (for instance, the above-mentioned Rana esculenta would become Pelophylax esculentus). As happens with any wholesale name change, there has been quite a bit of outcry at the idea of having to update the filing systems. Also, a number of authors have felt that the number of taxa sampled by Frost et al. was not enough to inspire confidence in their results. The review by Wiens (2007) was particularly vitriolic - the scientific equivalent of attempting to hold the subject down and kick them repeatedly in the teeth. Smith and Chiszar (2006) have suggested the more mollifying approach of treating Frost et al.'s various genera as subgenera, though unless one was willing to accept a paraphyletic genus this would also require sinking some well-established genera such as Staurois within Rana. Division of the genus Rana was also supported by Che et al. (2007).

The other cause of debate was perpetrated by Hillis & Wilcox (2005), who investigated the phylogeny of New World species of 'Rana' (most of which would belong to Lithobates in the Frost et al. system). The problem came when Hillis & Wilcox suggested a whole series of subgeneric taxa for nested groups of species that they defined according to the rules of the PhyloCode, but also allowed for use under the ICZN as subgenera. Debate promptly exploded about whether Hillis & Wilcox's names were validly published and usable (Dubois, 2006, 2007; Hillis, 2007). Compared to this argument, Frost et al.'s division appears quite simple. I may return to this in a later post, if my brain doesn't implode first. Check out the Dubois (2006) paper in particular - Dubois thinks that the answer to our problems is to make the ICZN more complicated. No. Thank. You.

REFERENCES

Che, J., J. Pang, H. Zhao, G.-F. Wu, E.-M. Zhao & Y.-P. Zhang. 2007. Phylogeny of Raninae (Anura: Ranidae) inferred from mitochondrial and nuclear sequences. Molecular Phylogenetics and Evolution 43 (1): 1-13.

Dubois, A. 2006. New proposals for naming lower-ranked taxa within the frame of the International Code of Zoological Nomenclature. Comptes Rendus Biologies 329 (10): 823-840.

Dubois, A. 2007. Naming taxa from cladograms: A cautionary tale. Molecular Phylogenetics and Evolution 42 (2): 317-330.

Frost, D. R., T. Grant, J. Faivovich, R. H. Bain, A. Haas, C. F. B. Haddad, R. O. de Sá, A. Channing, M. Wilkinson, S. C. Donnellan, C. J. Raxworthy, J. A. Campbell, B. L. Blotto, P. Moler, R. C. Drewes, R. A. Nussbaum, J. D. Lynch, D. M. Green & W. C. Wheeler. 2007. The amphibian tree of life. Bulletin of the American Museum of Natural History 297: 1-370.

Hillis, D. M. 2007. Constraints in naming parts of the Tree of Life. Molecular Phylogenetics and Evolution 42 (2): 331-338.

Hillis, D. M., & T. P. Wilcox. 2005. Phylogeny of the New World true frogs (Rana). Molecular Phylogenetics and Evolution 34 (2): 299-314.

Smith, H. M., & D. Chiszar. 2006. Dilemma of name-recognition: why and when to use new combinations of scientific names. Herpetological Conservation and Biology 1 (1): 6-8.

Wiens, J. J. 2007. Review: The Amphibian Tree of Life. Quarterly Review of Biology 82: 55-56.

Big Shout-Outs to my Homies

Last week, my blog was linked to by a couple of others, and it seems to me that the least I could do would be to return the favour.

First off, John Wilkins at Evolving Thoughts. John writes on scientific philosophy (or philosophy of science, whichever it is). I'd especially like to highlight his recent series of posts on the early chapters of Genesis, of which this is the most recent entry.

Second was Coturnix's A Blog Around the Clock. Coturnix's specialty is biological rhythms, but he blogs on other subjects as well.

I don't spend a great deal of time scanning other websites, I'm afraid. Time is a frustrating thing - there never seems to be enough of it. Mein Herr's explanation in Lewis Carroll's Sylvie and Bruno of how the people of his home country save up time from when they don't need it to be re-used later when they did never fails to fill me with envy. Of course, I'm not entirely sure when I'd take the excess time from, though - when I'm sleeping, perhaps. I'm sure a lot more would get done if I could just dispense with sleeping.

That said, I'd like to mention a couple of sites. Darren Naish's Tetrapod Zoology is one of the best, and was actually the inspiration for my starting this blog (I found his accounts endlessly fascinating, but was slightly frustrated by his self-imposed restriction to tetrapods when there are so many amazing inverts, fungi, bacteria, what-have-you out there). Darren's enhusiasm for his subjects is infectious, and his frequent distractions, tangents and subject-changes express just why we all love this complicated, never-ending subject.

It's not science, but I was somewhat saddened recently by the completion of David Plotz's Blogging the Bible. I was always entertained by David's sometimes bemused, sometimes confused but always respectful commentary on the Big Book. Especially Job, which was one that always confused me too.

Back at Scienceblogs, I've always been impressed the few times I've taken a look at Jason Rosenhouse's EvolutionBlog. Jason never fails to look at his subject in detail, and seemingly with endless patience. Creationism is probably not a topic I'm likely to cover here at the Catalogue (except that I wouldn't mind writing on the supposed creationism of influential figures such as Linnaeus and Owen) - there's other topics I'd rather cover, and there are people such as Jason to cover that topic far better than I ever could, for which I thank them.

Bouncing bristletails!

Last post I briefly mentioned my recent encounter with an archaeognathan (specifically, I found it mixed in a specimen vial with a bunch of harvestmen). Archaeognatha are one of the few living orders of wingless insects. In fact, under the current most-commonly-used definition for Insecta (which excludes the entognathous hexapods such as Collembola), Archaeognatha are the basalmost living order. As such, I was pretty excited to finally see one, even if only in the corpse. The Tree of Life page for this order has an absolutely fantastic photo of a live specimen. Archaeognathans are also referred to as bristletails in reference to the long cerci extending behing the abdomen.

The first feature that grabs the attention is the distinct hump that the back makes. This hump contains muscles for the archaeognathan to rapidly bend the abdomen downwards, pushing itself into the air and jumping up to 10cm high. Archaeognathans also have very large forward-facing compound eyes that actually meet in the middle. The maxillary palps are very large and could almost be mistaken for an extra pair of legs coming off the head.

If you want to see what makes archaeognathans really cool, though, you'll have to look a little closer. As befits the basalmost insect order, they retain a few uber-primitive features that have disappeared from other modern insects. They are the only modern insects with monocondylic mandibles - i.e. the mandibles have only one condyle (the socket where they attach and articulate with the head). All other insects have two. And if you were to look underneath the abdomen, you would see that each segment bears a pair of small pointed styles. These styles are moveable by muscles, and are thought to represent reduced legs. Let me repeat that in italics for emphasis - reduced legs. Archaeognathans are the only insects sensu stricto with abdominal styles, though they are also present in diplurans and proturans, two entognathous hexapod orders (I have not been able to find any indication that the styles in these orders are mobile, however).

While they have a fairly wide distribution worldwide, archaeognathans do not appear to be abundant and are fairly localised. I have heard that California is fairly well-blessed with them (big ones, too), but here in Australia they are most abundant in the eastern states.

Insects Never Fail to Amaze

Recently I saw my first ever specimen of Archaeognatha. I was going to write on that, so I picked up the lab's faithful copy of The Insects of Australia (CSIRO, 1991) to look up information. Before I found the Archaeognatha chapter, however, I came across something else that just blew me away so much that I had to share it with you all. Meet the freaky little marine midge Pontomyia (the only image I could find online was a rather blurry one here. Sorry).

There are very few marine insects, and only a single genus, the waterstrider Halobates, has species that are actually found on the open ocean (van der Hage, 1996). Other marine insects are restricted to inshore habitats. Pontomyia appears to be an inhabitant of tide pools and lagoons in the West Pacific. It belongs to the large family Chironomidae, mosquito-like (but non-parasitic) midges with aquatic larvae

Individuals of Pontomyia spend most of their lives as benthic larvae. After they emerge as non-feeding adults, they only live for a couple of hours (Soong et al., 1999). In this brief time, they must find a mate and produce eggs.

Pontomyia adults emerge at dusk or after sunset. At least one species, Pontomyia oceana, only emerges around the new and full moons (in combination with the specific emergence time, this probably ensures that the females end up laying eggs at low tide). Pupae swim to the surface and emerge as adults. The females are vermiform and structurally degenerate, with seemingly little activity as far as I can tell.

The males are the freaky ones. They skim the water surface film on the tips of the stout second and stilt-like, trailing third pairs of legs. The first pair of legs is immensely long and curve out on either side of the body as a pair of 'outriggers', barely skimming the surface and maintaining the animal's balance. The paddle-like wings propel the midge by flicking the air just above the water surface (Norris, 1991).

Females do not complete emergence from the pupa unless males are nearby (Soong et al., 1999). Males generally emerge up to an hour before females, and have been observed stripping the pupal skin from females to help them emerge. Once a male has found a female, he picks her up with the second legs and the base of the third legs and carries her while mating. Males are apparently quick movers, and I was especially taken by this sentence in Soong et al. (1999): 'They did not appear to slow down after catching females, sometimes climbing the vertical substrate up to 15 cm above the water level while dragging a female along'. One can't help wondering what Germaine Greer would make of the verbs in there.

After mating, the male drops the female. She lays her eggs on bits of dead coral or the like sticking up about the water surface in long interconnected strings. And that, as they say, is that.

REFERENCES

Norris, K. R. 1991. General biology. In The Insects of Australia (CSIRO, eds.), 2n ed., vol. I pp. 68-108. Melbourne University Press.

Soong, K., G.-F. Chen & J.-R. Cao. 1999. Life history studies of the flightless marine midges Pontomyia spp. (Diptera: Chironomidae). Zoological Studies 38 (4): 466-473. (Pdf here)

van der Hage, J. C. H. 1996. Why are there no insects and so few higher plants, in the sea? New thoughts on an old problem. Functional Ecology 10: 546-547.

Top Ten Follow-up

Paul W. complained that ten was too few for the previous post, so I'll add the runners-up to bring the total to twenty-four. They were:

Brachytrachelopan mesai: The short-necked sauropod of South America - a smashing little stunner that surprised us all. You've got to love the name, too.

Carnotaurus sastrei: Another real oddball. Combine the boxy horned skull with the ridiculously tiny forearms (it makes Tyrannosaurus look like a gorilla) and it's a mystery what this creature was doing for a living.

Diplodocus carnegii: Another classic sauropod. I think my scoring system favoured sauropods a little - they pretty much all scored highly in the 'impressiveness' box, but how are you going to call these giants anything else?

Microraptor gui: The 'four-winged' miniature marvel that inspired much passionate debate on its flying abilities (also, the first non-avian dinosaur to be claimed as a flier). Though there is still no agreement about whether the long feathers preserved on the legs formed functional wings, the evolution of flight in dinosaurs will never be viewed so simplistically again.

Opisthocoelicaudia skarzynskii: My favourite sauropod as a kid. Let's have that name again - Opisthocoelicaudia. Just kind of rolls off the tongue, doesn't it? Even better than Parasaurolophus.

Plateosaurus longiceps: Prosauropods are kind of the poor relative in the dinosaur family - never given much time, and about the only major group to not even have a cameo on a Jurassic Park movie. That said, Plateosaurus is the archetypal prosauropod (by definition, as it happens).

Stegosaurus ungulatus: Everyone knows this critter. As David Marjanovic has pointed out, the original thagomiser.

Turiasaurus riodevensis: Europe's largest known dinosaur, and type of a previously unknown group of sauropods. There's something vaguely funny about a previously unseen sauropod.

Centrosaurus apertus: It was either this one or Styracosaurus albertensis. Like Triceratops, but funkier.

Coelophysis bauri: When one is used to thinking of dinosaur finds as a few disarticulated remains, the Ghost Ranch deposit with more individuals of Coelophysis than can be counted are just stunning.

Herrerasaurus ischigualastensis: The prototype of later theropods. Herrerasaurus may no longer be the basalmost known dinosaur - recent workers favour a position very basal saurischian, plus there's always Eoraptor to trump it - but I still have a soft spot for the big lunk.

Pachyrhinosaurus canadensis: The ceratopsian that ditched the ceras, favouring a big ol' ugly boss instead. Still not to be messed with.

Psittacosaurus mongoliensis: I used this species as stand-in for the whole seemingly endless run of Psittacosaurus species. Every time I turn around there seems to be another one. They've been found huddled together in nests, they've been found with apparent long quills on the tail, they've been found halfway down a mammalian gullet. You can't escape them.

The Top Ten Dinosaurs - Triceratops beats Tyrannosaurus

Time it took me to do something hopelessly populist in order to try and draw more attention to the Catalogue of Organisms: 24 days. What can I do that's guaranteed to work up some steam?

In that light, I here present a list of my top ten dinosaurs, inspired by Don Robertson's Top 50 Birds (actually top eleven - some tied). Candidates for the list were rated completely subjectively on five factors: (1) Impressiveness - if I were to come across one of these critters, would the appropriate response be 'wow' or 'meh'? (2) Knowledge - how extensively the species has been studied and how well it is known. (3) History and Significance - if the discovery of this species had much significance, especially at the time it was discovered. This is also the category that was influenced by how much attention this species has received from the general public over time. (4) Controversy - has the species has inspired much debate over the years? (5) Special Factors - like Don Robertson did with his bird list, I also scored for an entirely subjective character of any special significance the species has that isn't really covered by the previous four categories. This category also reflected my own personal feelings about the animal in question (and whether or not I wanted it to win).

The list:

6th Equal:
Deinonychus antirrhopus: The little evil-looking buggers with the massive sickle claws on the feet. The description of Deinonychus has been directly credited with inspiring the renaissance in views on dinosaur metabolism. Older reconstructions of sluggish, low energy dinosaurs just made no sense when applied to this obvious speedster.

Euoplocephalus tutus: Everyone's favourite living tank. What more can you say about a creature so heavily armoured that even its eyelids would have clanged when it blinked? Not to mention the thagomiser at the end of the tail.*

*I'm not sure if the term thagomiser has ever been used formally, but it has a reasonable amount of informal currency as a term for an offensive structure at the end of a tail (like the ankylosaurid club, or the stegosaurid spike array). I believe it derives from a Far Side cartoon about cavemen, where it is named after the late Thag.

Falcarius utahensis: Apparently known from more specimens than you can shake a thagomiser at, this discovery of a couple of years ago represents the basalmost member of the therizinosaurs, gigantic (probably) herbivorous theropods with ridiculously oversized claws. Falcarius was a nice find because it perfectly slotted into the gap between derived therizinosaurs and their supposed relatives.

Iguanodon bernissartensis: The classic European dinosaur, one of the earliest discovered and possibly the earliest known from significant remains (this is the best known of the multiple Iguanodon species - while Iguanodon was one of the original three dinosaurs, the specific species involved there was Iguanodon anglicus, which is no longer regarded as identifiable).

Mononykus olecranus: Arguably the wierdest of all dinosaurs, with still no real idea about its lifestyle. A small bird-like theropod, Mononykus has greatly shortened yet very stout single-clawed forelimbs. The structure of the forelimbs appears suited for digging, yet the light cursorial form of the the rest of the body doesn't appear suitable for this.

Tyrannosaurus rex: Undoubtedly the best known of all dinosaurs. To be honest, I was kind of hoping old Tyrannosaurus would fall off the list - I think she's hogged the limelight for long enough. If you want to know more, a quick Google search will tell you more than you ever wanted to know - just pay no attention to the bit about coconuts.

2nd Equal:
Archaeopteryx lithographica: The Urvogel, the Missing Link (though obviously it's not missing anymore, is it?). Archaeopteryx was the original inspiration in recognising the connection between Cretaceous dinosaurs and modern birds. While later discoveries mean that Archie (as he is known to his friends) is now but one of many fossils demonstrating this link, he still retains one of the best represented by specimens, and his historical significance will never be lost.

Oviraptor philoceratops: The name means 'egg thief and lover of ceratopsians' - when the original specimen was found it was lying on a nest of eggs that were then believed to belong to Protoceratops, and it was thought to have died while attempting to predate them. It has since been found that the eggs belonged to Oviraptor itself, and not only was it not eating them, but it would have been sitting astride them bird-style to keep them warm - history's ugliest broody chicken. The true diet of the strange-looking, beaky Oviraptor is a cause of great debate.

Triceratops horridus: Another classic. There are few people who would not recognise this beast with its broad frill and intimidating spiky bits. You know you love him, just don't stick your hand near his mouth if you want to keep it.

And my choice for the Greatest Dinosaur Ever:

Brachiosaurus brancai (or Giraffatitan brancai, depending on whom you ask): What can one say when faced with a giant sauropod except WOW! There may be bigger sauropods than Giraffatitan, there may be prettier, but this is still the classic giant and one of the best-known. Besides, the difference between unbelievably mind-blowingly HUGE and stupidly unbelieveably mind-blowingly HUGE is not that great when you consider that both can reduce you to a small greasy puddle underfoot and barely even break their stride.