To dump a chunk of trunk

The Mammal has deemed that Hox genes and good old-fashioned feel-good evo-devo are a good way to blink back to life*. Also, tardigrades. Tardigrades are awesome. Here is one viewed from above, from the Goldstein lab via Encyclopedia of Life:

hypsibius_dujardini_eol

Tardigrades or water bears are also a bit unusual. Their closest living relatives are velvet worms (Onychophora) and arthropods. Exactly who’s closest to whom in that trio of phyla collectively known as the Panarthropoda is not clear, and I don’t have the energy to wade into the debate – besides, it’s not really important for the purposes of this post. What Smith et al. (2016) concluded about these adorably indestructible little creatures holds irrespective of their precise phylogenetic position.

Anyway. I said tardigrades were unusual, and I don’t mean their uncanny ability to survive the apocalypse¬†and pick up random genes in the process (Boothby et al., 2015). (ETA: so apparently there may not be nearly as much foreign gene hoarding as the genome paper suggests – see Sujai Kumar’s comment below! Doesn’t change the fact that tardigrades are tough little buggers, though ūüôā ) The oddity we’re interested in today¬†lies in the fact that all known species are built to the exact same compact body plan.¬†Onychophorans and many arthropods are elongated animals with lots of segments, lots of legs, and often lots of variation in the number and type of such body parts. Tardigrades? A wee head, four chubby pairs of legs, and that’s it.

How does a tardigrade body relate to that of a velvet worm, or a centipede, or a spider? Based solely on anatomy, that’s a hell of a question to answer; even the homology¬†of body parts between different kinds of arthropods can be difficult to determine. I have so far remained stubbornly uneducated on the minutiae of (pan)arthropod segment homologies, although I do see papers purporting to match brain parts, appendages¬†and suchlike between different kinds of creepy-crawlies¬†on a fairly regular basis. Shame on me for not being able to care about the details, I guess – but¬†the frequency with which the subject comes up suggests that the debate is far from over.

Now, when I was first drawn¬†to the evo-devo field, one of the biggest attractions was the notion that the expression of genes as a body part forms¬†can tell us what that body part really is even when anatomical clues are less than clear. That, of course, is too good to be simply true, but sometimes the lure of genes and neat¬†homology stories is just too hard to resist. Smith et al.‘s investigation of¬†tardigrade Hox genes is definitely that kind of story.

Hox genes are generally a good place to look if you’re trying to decipher¬†body regions, since their more or less neat, orderly expression patterns are remarkably conserved between very distantly related animals (they are¬†probably as old as the Bilateria, to be precise). A polychaete worm, a vertebrate and an arthropod show the same general pattern – there is no active Hox gene at the very front of the embryo, then Hoxes 1, 2, 3 and so on appear in roughly that order,¬†all the way to the rear end. There are variations in the pattern – e.g. the expression of a gene¬†can have sharp boundaries or fade in and out gradually; different genes can overlap to different extents, the order isn’t always perfect, etc. – but staggered Hox gene expression domains, with the same genes starting up in the same general area along the main body axis, can be found all across the Bilateria.

Tardigrades are no exception, in a sense – but they are also quite exceptional. First, their complement of Hox genes is a bit of a mess. At long last, we have a tardigrade genome to hand, in which Smith et al. (2016) found good honest¬†Hox genes. What they didn’t find was a Hox cluster, an orderly series of Hox genes sitting like beads on a DNA string. Instead, the Hox genes in Hypsibius dujardini, the sequenced species,¬†are all over the genome, associating with all kinds of dubious fellows¬†who aren’t Hoxes.

What Smith et al.¬†also didn’t find was half of the Hox genes¬†they expected. A typical arthropod has ten or so Hox genes, a pretty standard ballpark for an animal that isn’t a vertebrate. H. dujardini has only seven, three of which are triplicates of Abdominal-B, a gene that normally exists in a single copy in arthropods. So basically, only five kinds of Hox gene – number two and most of the “middle” ones are missing. What’s more, two more tardigrades that aren’t closely related to H. dujardini also appear to have the same five Hox gene types (though¬†only one Abd-B each), so this massive loss is probably a common feature of Tardigrada. (No word on whether the scattering of the Hox ¬†cluster¬†is also shared by the¬†other two species.)

We know that the genes are scattered and decimated, but are their expression patterns similarly disrupted? You don’t actually need an intact Hox cluster for orderly Hox expression, and indeed, tardigrade Hox genes are activated in a perfectly neat and perfectly usual pattern that resembles what you see in their panarthropod cousins. Except for the bit where half the pattern is missing!

Here’s¬†part of Figure 4 from the paper, a schematic comparison of tardigrade Hox expression to that of other panarthropods – a generic arachnid, a millipede and a velvet worm. (otd is a “head” gene that¬†lives in the Hox-free anterior region;¬†lab is the arthropod equivalent of¬†Hox1, Dfd is Hox4, and I’m not sure which of Hox6-8 ftz is currently supposed to be.) The interesting thing about this is that according to Hox genes, the entire body of the tardigrade corresponds to just the front end of arthropods and velvet worms.

Smith_etal2016-hox_tardigrade_fig4A

In addition, one thing that is not shown on this diagram is that Abdominal-B, which normally marks the butt¬†end of the animal, is still active in the tardigrade, predictably in the last segment (L4, that is). So if you take the Hox data at face value, a tardigrade is the arse end of an arthropod tacked straight onto its head. Weird. It’s like evolution took a perfectly ordinary velvet worm-like creature and chopped out most of its trunk.

The tardigrade data suggest¬†that the original panarthropod was probably more like arthropods and velvet worms than tardigrades – an elongated animal with many segments.¬†The strange tardigrade situation can’t be the ancestral one, since the Hox genes that tardigrades lack long predate the panarthropod ancestor. Now, it¬†might be¬†possible to lose half your Hox genes while keeping your¬†ancestral body plan, but an unusual body plan and an unusual set of Hox genes is a bit of a big coincidence, innit?

Smith et al. point out that the loss of the Hox genes was unlikely to be the cause of the loss of the trunk region¬†– Hox genes only specify what grows on a segment, they don’t have much say in how many segments develop in the first place. Instead, the authors reason, the loss of the trunk in the tardigrade ancestor probably made the relevant¬†Hox genes dispensable.

Damn, this story makes me want to see the Hox genes of all those oddball lobopodians from the Cambrian. Some of them are bound to be tardigrade relatives, right?

***

References:

Boothby TC et al. (2015) Evidence for extensive horizontal gene transfer from the draft genome of a tardigrade. PNAS 112:15976-15981

Smith FW et al. (2016) The compact body plan of tardigrades evolved by the loss of a large body region. Current Biology 26:224-229

***

*The Mammal has been pretty depressed lately. As in mired up to her head in weird energy-sucking flu. Unfortunately, writing is one of those things that the damn brain monster has eaten most of the fun out of. Also, I have a shitty normal person job at the moment, and shitty job taking up time + barely enough motivation to crawl out of bed and pretend to be human¬†means I have, at best, one afternoon per week that I actually spend on catching up with science. That is just enough to scroll through my feeds and file away the interesting stuff, but woefully insufficient for the writing of posts, not to mention that my ability to concentrate is, to be terribly technical, absolutely fucked. It’s not an ideal state of affairs by any stretch, and I’m pretty sure that if I made more of an effort to read and write about cool things, it would pay off in the mental health department, but‚Ķ well. That sort of reasonable advice is hard to hear with the oozing fog-grey suckers of that thing clamped onto my brain.

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In which a “living fossil’s” genome delights me

I promised myself I wouldn’t go on for thousands and thousands of words about the Lingula genome paper (I’ve got things to do, and there is a LOT of stuff in there), but I¬†had¬†to indulge myself a little bit. Four or five years ago when I was a final year undergrad trying to figure out things about Hox gene evolution, I would have killed for a complete brachiopod genome. Or even a complete brachiopod Hox cluster. A year or two ago, when I was trying to sweat out something resembling a PhD thesis, I would have killed for some information about the genetics of brachiopod shells that amounted to more than tables of amino acid abundances. Too late for my poor dissertations, but a brachiopod genome is finally sequenced! The paper is right here, completely free (Luo et al., 2015). Yay for labs who can afford open-access publishing!

In case you’re not familiar with Lingula, it’s this guy (image from Wikipedia):

In a classic case of looks being deceiving, it’s not a mollusc, although it does look a bit like one except for the weird white stalk sticking out of the back of its shell. Brachiopods, the phylum to which Lingula belongs, are one of those strange groups no one really knows where to place, although nowadays¬†we are pretty sure they are somewhere in the general vicinity of molluscs, annelid worms and their ilk.¬†Unlike bivalve molluscs, whose shell valves are on the left and right sides of the animal, the shells of brachiopods like Lingula have top and bottom¬†valves. Lingula‘s shell is also made of different materials: while bivalve¬†shells contain¬†calcium carbonate deposited into a mesh of chitin and silk-like proteins,* the subgroup of brachiopods Lingula belongs to uses calcium phosphate, the same mineral that dominates our bones, and a lot of collagen (again like bone). But we’ll come back to that in a moment…

One of the reasons the Lingula genome is particularly interesting is that Lingula is a classic “living fossil”. In the Paleobiology Database, there’s even an entry for a Cambrian fossil classified as Lingula, and there are plenty of entries from the next geological period. If the database is to be believed, the genus¬†Lingula has existed for something like 500 million years, which must be some kind of record for an animal.** Is its genome similarly conservative? Or did the DNA hiding under a deceptively conservative shell design evolve as quickly as anyone’s?

In a heroic feat of self-control, I’m not spending all night¬†poring over the paper, but I did give a couple of interesting sections a look. Naturally, the first thing I dug out was the Hox cluster hiding in the rather large supplement. This was the first clue that Lingula‘s genome¬†is definitely “living”¬†and not at all a fossil in any sense of the word. If it were, we’d expect one neat string of Hox genes, all in the order we’re used to from other animals. Instead, what we find is two missing genes, one plucked¬†from the middle of the cluster and tacked onto its “front” end, and two genes totally detached from the rest. It’s not too bad as Hox cluster disintegration goes – six out of nine genes are still neatly ordered – but it certainly doesn’t look like something left over from the dawn of animals.

The bigger clue that caught my eye, though, was this little family tree in Figure 2:

Luo_etal2015-fig2

The red numbers on each branch indicate the number of gene families that expanded or first appeared in that lineage, and the green numbers are the families shrunk or lost. Note that our “living fossil” takes the lead in both. What I find funny is that it’s miles ahead of not only the animals generally considered “conservative” in terms of genome evolution, like the limpet Lottia and the lancelet Branchiostoma, but also the sea squirt (Ciona). Squirts are notorious for having incredibly fast-evolving genomes; then again, most of that notoriety was based on the crazily divergent sequences and often wildly scrambled order of its genes. A genome can be conservative in some ways and highly innovative in others. In fact, many of the genes involved in basic cellular functions are very slow-evolving in Lingula. (Note also: humans are pretty slow-evolving as far as gene content goes. This is not the first study to find that.)

So, Lingula, living fossil? Not so much.

The last bit I looked at was the section about shell genetics. Although it’s generally foolish to expect the shell-forming gene sets of two animals from different phyla to be similar (see my first footnote), if there are similarities, they could potentially go at least two different ways. First, brachiopods might be quite close to molluscs, which is the hypothesis Luo et al.‘s own treebuilding efforts support. Like molluscs, brachiopods also have a specialised mantle that secretes shell material, though having the same name doesn’t mean¬†the two “mantles” actually share a common origin. So who knows, some molluscan shell proteins, or shell regulatory genes, might show up in Lingula, too.

On the other hand, the composition of Lingula’s shell is more similar to our skeletons’. So, since they have to capture the same mineral, could the brachiopods share some of our skeletal proteins? The answer to both questions seems to be “mostly no”.

Molluscan shell matrix proteins, those that are actually built into the structure of the shell, are quite variable even within Mollusca. It’s probably not surprising, then, that most of the relevant genes¬†that are even present in Lingula are not specific to the mantle, and those that are are the kinds of genes that are generally involved in the handling of calcium or the building of the stuff around cells¬†in all kinds of contexts. Some of the regulatory mechanisms might be shared – Luo et al. report that BMP signalling seems to be going on around the edge of the mantle in baby Lingula, and this cellular signalling system is also involved in molluscan shell formation. Then again, a handful of similar signalling systems “are involved” in bloody everything in animal development, so how much we can deduce from this similarity is anyone’s guess.

As for “bone genes” – the ones that are most characteristically¬†tied to bone are missing (disappointingly or reassuringly, take your pick). The SCPP protein¬†family is so far known only from vertebrates, and its various members are involved in the mineralisation of bones and teeth. SCPPs originate from an ancient protein called SPARC, which seems to be generally present wherever collagen is (IIRC, it’s thought¬†to help collagen fibres arrange themselves correctly). Lingula has a gene for SPARC all right, but nothing remotely resembling an SCPP gene.

I mentioned that the shell of Lingula is built largely on collagen, but it turns out that it¬†isn’t “our” kind of collagen. “Collagen”¬†is just a protein¬†with a particular kind of repetitive sequence. Three amino acids (glycine-proline-something else, in case you’re interested) are¬†repeated¬†ad nauseam in the collagen chain,¬†and these repetitive regions let¬†the protein twist into characteristic rope-like fibres¬†that make collagen such a wonderfully tough basis for connective tissue. Aside from the repeats they all share, collagens are¬†a large and diverse bunch. The ones that form most of the organic matrix in bone¬†contain a non-repetitive and rather easily recognised domain at one end, but when Luo et al. analysed the genome and the proteins extracted from the Lingula shell, they found that none of the shell collagens possessed this domain. Instead, most of them had EGF domains, which are pretty widespread in all kinds of extracellular proteins. Based on the genome sequence, Lingula has a whole little cluster of these collagens-with-EGF-domains that probably originated from¬†brachiopod-specific gene duplications.

So, to recap: Lingula is not as conservative as its looks would suggest (never judge a living fossil by its cover, right?) We also finally¬†have actual sequences for lots of its shell proteins, which reveal that when it comes to building shells, Lingula does its own thing. Not much of a surprise, but still, knowing is a damn sight better than thinkin’ it’s probably so. We are scientists here, or what.

I am Very Pleased with this genome. (I just wish it was published five years ago ūüėõ )

***

Notes:

*This, interestingly, doesn’t seem to be the general case for all molluscs. Jackson et al. (2010) compared the genes building the pearly layer of snail (abalone, to be precise) and bivalve (pearl oyster) shells, and found that the snail¬†showed no sign of the chitin-making enzymes and silk type proteins that were so abundant in its bivalved cousins. It appears that even within molluscs, different groups have found different ways to make often very similar shell structures. However, all molluscs shells regardless of the underlying genetics are predominantly composed of calcium carbonate.

**You often hear about sharks, or crocodiles, or coelacanths, existing “unchanged” for 100 or 200 or whatever million years, but in reality, 200-million-year-old crocodiles aren’t even classified in the same families, let alone the same genera, as any of the living species. Again, the living coelacanth is distinct enough from its relatives in the Cretaceous, when they were last seen, to warrant its own genus in the eyes of taxonomists. I’ve no time to check up on¬†sharks, but I’m willing to bet the situation is similar. Whether Lingula‘s jaw-dropping 500-million-year tenure on earth is a result of taxonomic lumping or the shells genuinely looking that similar, I don’t know.¬†Anyway, rant over.

***

References:

Jackson DJ et al. (2010) Parallel evolution of nacre building gene sets in molluscs. Molecular Biology and Evolution 27:591-608

Luo Y-J et al. (2015) The Lingula genome provides insights into brachiopod evolution and the origin of phosphate biomineralization. Nature Communications 6:8301

Wherein scientists DON’T spill blood over a Precambrian animal

Having gone through much of my backlog, I was going to post about pretty blue limpet shells, then I saw that people have been arguing over Haootia. You remember Haootia? It’s that Precambrian fossil with probable muscle impressions that looks kind of like a modern-day staurozoan jellyfish (living staurozoan Haliclystus californiensis by Allen Collins, Encyclopedia of Life; Haootia quadriformis reconstruction from Liu et al., 2014):

Liu_etal2014-haootia_recon

It’s pretty much a law of Precambrian palaeontology that no interpretation of a fossil can ever remain uncontested, and Haootia is no exception. Nonetheless, this might be the tamest debate anyone ever had about a Precambrian fossil, and it gives me all kinds of warm feels.

Good news: Miranda et al. (2015) don’t dispute that the fossils show muscle impressions. They don’t even dispute that they belong to a cnidarian-grade creature. However, they question some of the details of the muscular arrangement, which could have implications for what this creature was and how it functioned.

They don’t have much of an issue with the muscles that run along the stalk and arms. The main point of contention, as far as I can tell, is that the muscles that run around the body (called coronal muscles in modern jellies) are not that big in living staurozoans. Those are the muscles that regular jellyfish use to contract their bells while swimming, but staurozoans don’t swim and therefore don’t need huge coronal muscles.

By Liu et al.‘s (2014) reconstruction (see above), Haootia has pretty massive coronal muscles. Miranda et al. (2015) wonder whether this was really the case, or the deformation of the fossils combined with the subconscious influence of regular jellyfish misled the original authors. They offer an alternative reconstruction, in which most of the body musculature runs up and down rather than around the body wall:

Miranda_etal2015-haootia_alt_recon

However, they also entertain the possibility that Liu et al.‘s reconstruction is correct – in which case, they note, Haootia must have done something with those muscles. Did jellyfish-like pulsations somehow form part of its feeding method? Could this even be a precursor to the jellyfish way of swimming? Who knows!

Liu et al. (2015) gave the most amazing response – much of their short reply to Miranda et al.‘s comments is basically thanking them for all the extra information and insight. They seem really pleased that biologists who study living cnidarians are taking an interest in their fossils, and enthusiastic about fruitful discussions in the future. (I concur. Biologists and palaeontologists need to talk to each other!)

They did take another, closer look at Haootia and maintain that they still see a large amount of musculature running around the body. So perhaps this peculiar Precambrian animal was doing something peculiarly Precambrian that has few or no parallels in modern seas. “We must keep in mind,” they write,¬† “that some, or maybe most, Ediacaran body plans and feeding strategies may have been specifically adapted to Ediacaran conditions.”

Either way, the whole exchange makes me very warm and fuzzy – I love to see scientists having constructive debates and learning from each other. (I also love that Miranda et al. thank Alex Liu in their acknowledgements; they were so obviously not out to tear one another down.) Plus both teams agree that we DO have a cnidarian-type creature from the Precambrian, and we DO have lovely lovely muscle impressions. Here’s to nice people, and to the slowly sizzling fuse of the Cambrian explosion! ūüôā

***

References:

Liu AG et al. (2014) Haootia quadriformis n. gen., n. sp., interpreted as a muscular cnidarian impression from the Late Ediacaran period (approx. 560 Ma). Proceedings of the Royal Society B 281:20141202

Liu AG et al. (2015) The arrangement of possible muscle fibres in the Ediacaran taxon Haootia  quadriformis. Proceedings of the Royal Society B 282:20142949

Miranda LS et al. (2015) Is Haootia quadriformis related to extant Staurozoa (Cnidaria)? Evidence from the muscular system reconsidered. Proceedings of the Royal Society B 282:20142396

Hi, real world, again!

The Mammal has emerged from a thesis-induced supermassive black hole and a Christmas-induced food coma, only to find that in the month or so that she spent barely functional and buried in chapters covered in the supervisor’s dreaded Red Pen, things actually happened in the world outside. This, naturally, manifested in thousands of items feeling thoroughly neglected in RSS readers and email inboxes. (Jesus. How many times have I vowed never to neglect my RSS feed again? Oh well, it’s not like unemployment is such a busy occupation that I can’t deal with a measly two and a half thousand articles ūüėõ )

… earlier tonight, the paragraph here said I wasn’t doing a proper post yet, “just pointing out” a couple of the cooler things I’ve missed. Then somehow this thing morphed into a 1000+ word post that goes way beyond “pointing things out”. It’s almost like I’ve been itching to write something that isn’t my thesis. >_>

So the first cool thing I wanted to “point out” is the genome paper of the centipede Strigamia maritima, which is a rather nondescript little beast hiding under rocks on the coasts of Northwest Europe. This is the first sequenced genome of a myriapod – the last great class of arthropods to remain untouched by the genome sequencing craze after many genomes from insects, crustaceans and chelicerates (spiders, mites and co.).¬† The genome sequence itself has been available for years (yay!), but its “official” paper (Chipman et al., 2014) is just recently out.

Part of the appeal of Strigamia – and myriapods in general – is that they are considered evolutionarily conservative for an arthropod. In some respects, the genome analysis confirms this. Compared to its inferred common ancestor with us, Strigamia has lost fewer genes than insects, for example. Quite a lot of its genes are also linked together similarly to their equivalents in distantly related animals, indicating relatively little rearrangement in the last 600 million years or so. But this otherwise conservative genome also has at least one really unique feature.

Specifically, this centipede – which is blind – has not only lost every bit of DNA coding for known light-sensing proteins, but also all known genes specific to the circadian clock. In other animals, genes like clock and period mutually regulate one another in a way that makes the abundance of each gene product oscillate in a regular manner (this is about the simplest graphical representation I could find…). The clock runs on a roughly daily cycle all by itself, but it’s also connected to external light via the aforementioned light-sensing proteins, so we can constantly adjust our internal rhythms according to real day-night cycles.

There are many blind animals, and many that live underground or otherwise find day and night kind of irrelevant, but even these are often found to have a functioning circadian clock or keep some photoreceptor genes around. However, based on the genome data, our favourite centipede may be the first to have completely lost both. The authors of the genome paper hypothesise that this may be related to the length of evolutionary time the animals have spent without light. Things like mole rats are relatively recent “inventions”. However, the geophilomorph order of centipedes, to which Strigamia belongs, is quite old (its most likely sister group is known from the Carboniferous, so they’re probably at least that ancient). Living geophilomorphs are all blind, so chances are they’ve been that way for the last 300+ million years.

Nonetheless, the authors also note that geophilomorphs are still known to avoid light – the question now is how the hell they do it… And, of course, whether Strigamia has a clock is not known – only that it doesn’t have the clock we’re used to. We also have no idea at this point how old the gene losses actually are, since all the authors know is that one other centipede from a different group has perfectly good clock genes and opsins.

In comparison with fruit flies and other insects, the Strigamia genome also reveals some of the ways in which evolutionary cats can be skinned in multiple ways. There is an immune-related gene family we share with arthropods and other animals, called Dscam. The product of this gene is involved in pathogen recognition among other things, and in flies, Dscam genes are divided into roughly 100 chunks or exons, most of which are are found in clusters of variant copies. When the gene is transcribed, only one of these copies is used from each such cluster, so in practical terms the handful of fruit fly Dscam genes can encode tens of thousands of different proteins, enough to adapt to a lot of different pathogens.

A similar arrangement is seen in the closely related crustaceans, although with fewer potential alternative products. In other groups – the paper uses vertebrates, echinoderms, nematodes and molluscs for comparison – the Dscam family is pretty boring with at most one or two members and none of these duplicated exons and alternative splicing business. However, it looks like insects+crustaceans are not the only arthropods to come up with a lot of DSCAM proteins. Strigamia might also make lots of different ones¬†(“only” hundreds in this case), but it achieved this by having dozens of copies of the whole gene instead of performing crazy editing feats on a small number of genes. Convergent evolution FTW!

Before I paraphrase the entire paper in my squeeful enthusiasm (no, seriously, I’ve not even mentioned the Hox genes, and the convergent evolution of chemoreceptors, and I think it’s best if I shut up now), let’s get to something else that I can’t not “point out” at length: a shiny new vetulicolian, and they say it’s related to sea squirts!

Vetulicolians really deserve a proper discussion, but in lieu of a spare week to read up on their messiness, for now, it’s enough to say that these early Cambrian animals have baffled palaeontologists since day one. Reconstructions of various types look like… a balloon with a fin? Inflated grubs without faces? I don’t know. Drawings below (Stanton F. Fink, Wikipedia) show an assortment of the beasts, plus Yunnanozoon, which may or may not have something to do with them. Here are some photos of their fossils, in case you wondered.

Vetulicolians from Wiki

They’re certainly difficult creatures to make sense of. Since their discovery, they’ve been called both arthropods and chordates, and you can’t get much farther than that with bilaterian animals (they’re kind of like the Nectocaris of old, come to think of it…).

The latest one was dug up from the Emu Bay Shale of Australia, the same place that yielded our first good look at anomalocaridid eyes. Its newest treasure has been named Nesonektris aldridgei by its taxonomic parents (Garc√≠a-Bellido et al., 2014), and it looks something like this (Diego Garc√≠a-Bellido’s reconstruction from the paper):

Garcia-Bellido_etal2014-nesonektris_recon

In other words, pretty typical vetulicolian “life but not as we know it”, at first glance. Its main interest lies in the bit labelled “nc” in the specimens shown below (from the same figure):

García-Bellido_etal2014-nesonektris_notochords

This chunky structure in the animal’s… tail or whatever is a notochord, the authors contend. Now, only one kind of animal has a notochord: a chordate. (Suspicious annelid muscle bundles notwithstanding. Oh yeah, I also wanted to post on Lauri et al. 2014. Oops?) So if this thing in the middle of Nesonektris’s tail is a notochord, then at the very least it is more closely related to chordates than anything else.

Why do they think it is one? Well, there are several long paragraphs devoted to just that, so here goes a summary:

1. It’s probably not the gut. A gut would be the other obvious ID, but it doesn’t fit very well in this case. Structures interpreted as guts in other vetulicolians – which sometimes contain stuff that may be half-digested food – (a) start in the front half of the body, where the mouth is, (b) constrict and expand and coil and generally look much floppier than this, (c) don’t look segmented, (d) sometimes occur alongside these tail rod-like thingies, so probably aren’t the same structure.

2. It positively resembles modern half-decayed notochords. The notochords of living chordates are long stacks of (muscular or fluid-filled) discs, which fall apart into big blocks as the animal decomposes after death. Here’s what remains of the notochord of a lamprey after two months for comparison (from Sansom et al. (2013)):

Sansom_etal2013-adult_lamprey_notochord_d63

This one isn’t as regular as the blockiness in the fossils, I think, but that could just be the vetulicolians not being quite as rotten.

There is, of course, a but(t). To be precise, there are also long paragraphs discussing why the structure might not be a notochord after all. It’s much thicker than anything currently interpreted as such in reasonably clear Cambrian chordates, for one thing. Moreover, it ends right where the animal does, in a little notch that looks like a good old-fashioned arsehole. By the way, the paper notes, vetulicolian tails in general don’t go beyond their anuses by any reasonable interpretation of the anus, and a tail behind the anus is kind of a defining feature of chordates, though this study cites a book from the 1970s claiming that sea squirt larvae have a vestigial bit of proto-gut going all the way to the tip of the tail. (I suspect that claim could use the application of some modern cell labelling techniques, but I’ve not actually seen the book…)

… and there is a phylogenetic analysis, in which, if you interpret vetulicolians as deuterostomes (which impacts how you score their various features), they come out specifically as squirt relatives whether or not you count the notochord. I’m never sure how much stock to put in a phylogenetic analysis based on a few bits of anatomy gleaned from highly contentious fossils, but at least we can say that there are other things – like a hefty cuticle – beyond that notochord-or-not linking vetulicolians to a specific group of chordates.

Having reached the end, I don’t feel like this paper solved anything. Nice fossils either way ūüôā

And with that, I’m off. Maybe next time I’ll write something that manages to be about the same thing throughout. I’ve been thinking that I should try to do more posts about broader topics rather than one or two papers (like the ones I wrote about ocean acidification or homology versus developmental genetics), but I’ve yet to see whether I’ll have the willpower to handle the necessary reading. I’m remarkably lazy for someone who wants to know everything ūüėÄ

(Aside: holy crap, did I ALSO miss a fucking Nature paper about calcisponges’ honest to god ParaHox genes? Oh my god, oh my GOD!!! *sigh* This is also a piece of incredibly exciting information I’ve known for years, and I miss it when it actually comes out in a journal bloody everyone reads. You can tell I’ve been off-planet!)

References:

Chipman AD et al. (2014) The first myriapod genome sequence reveals conservative arthropod gene content and genome organisation in the centipede Strigamia maritima. PLoS Biology 12:e1002005

García-Bellido DC et al. (2014) A new vetulicolian from Australia and its bearing on the chordate affinities of an enigmatic Cambrian group. BMC Evolutionary Biology 14:214

Lauri A et al. (2014) Development of the annelid axochord: insights into notochord evolution. Science 345:1365-1368

Sansom RS et al. (2013) Atlas of vertebrate decay: a visual and taphonomic guide to fossil interpretation. Palaeontology 56:457-474