Echinoderm bonanza

Smith et al. (2013) has been sitting on my desktop waiting to be read for the last month or so. Man, am I glad that I finally opened the thing. I’m quite fond of echinoderms, and this paper is full of them. Of course. It’s about echinoderms. Specifically, it’s about the diverse menagerie of them that existed, it seems, a little bit earlier than thought.

The brief little paper introduces new echinoderm finds from two Mid-Cambrian formations in Morocco, which at the time was part of the great continent of Gondwana. As far as I’m concerned, it was worth reading just for this lineup of Cambrian echinoderms. I mean, echinoderms are so amazingly weird in such a variety of ways. They’re a delight.

Smith_etal2013-fig3_decorated

(The drawings themselves are from Fig. 3. of the paper; I rearranged them to fit into my post width, and the boxes are my additions. Dark box = new groups/species from Morocco, light grey box = known groups/species whose first appearance was pushed back in time by the Moroccan finds.)

Although none of the creatures above belong to the living classes of echinoderms, they display a wide range of body plans. You could say their body plans are more diverse* than those of living echinoderms. (And if you said that, the ghost of Stephen Jay Gould would nod approvingly.) For example, modern echinoderms tend to have either (usually five-part) radial symmetry (any old starfish) or bilateral symmetry that clearly comes from radial symmetry (heart urchins).

In these Early- to Mid-Cambrian varieties, you can see some five-rayed creatures, some that are more or less bilateral without any obvious connection to the prototypical five-point star, animals that are just kind of asymmetric, and those strange spindle-shaped helicoplacoids that look like someone took an animal with radial symmetry and wrung it out. And then there are all the various arrangements of arms and stalks and armour plates that I tend to gloss over when reading about the beasts. (Yeah. I have no attention span.)

The Morroccan finds have some very interesting highlights. The second creature in the lineup above is one of them. Its top half looks like a helicoplacoid such as Helicoplacus itself (first drawing). It’s got that characteristic spiral arrangement of plates and a mouth at the top end. However, unlike previously known helicoplacoids, it sits on a stalk that resembles the radially-symmetric eocrinoids (like the creature on its right). It’s a transitional form all right, though we’ll have to wait for future publications and perhaps future discoveries to see which way evolution actually went. It’ll already help palaeontologists make sense of helicoplacoids themselves, though, which I gather is a big thing in itself. The authors promise to publish a proper description of the creature, which is really exciting.

The other exciting thing about the Moroccan echinoderms is their age. As I already hinted at with my grey boxes, the new fossils push back the known time range of many echinoderm body plans by millions of years. This means that the wide variety of body plans we saw above was already present as little as 10-15 million years after the first appearance of scattered bits of echinoderm skeleton in the fossil record.

Smith et al. argue that this is a fairly solid conclusion based on the mineralogy of echinoderm skeletons. Organisms with calcium carbonate hard parts have a tendency to adopt the “easiest” mineralogy at the time they first evolve skeletons. Seawater composition changes over geological time; most importantly, the ratio of calcium to magnesium fluctuates. Calcium carbonate can adopt several different crystal forms, and the Ca/Mg ratio influences which of them are easier to make. So when there’s a lot of Mg in the sea, aragonite is the “natural” choice, whereas low Mg levels favour calcite.

The first appearance of echinoderms around 525 million years ago coincides with a shift in ocean chemistry from “aragonite seas” to “calcite seas”. Echinoderms and a bunch of other groups that first show up around that time have skeletons that are calcite in their structure but incorporate a lot of Mg. Since the ocean before was favourable to aragonite, it’s unlikely that echinoderm skeletons appeared much earlier than this date. In other words, echinoderm evolution during this geologically short period was truly worthy of the name “Cambrian explosion”.

That is, of course, if the appearance of echinoderm skeletons precedes the appearance of echinoderm body plans. The oldest of our Cambrian treasure troves of soft-bodied fossils, such as the rocks that yielded the Chengjiang biota of China, are roughly the same age as the first echinoderm skeletons. However, they don’t contain undisputed echinoderms as far as I can tell (Clausen et al., 2010). Proposed “echinoderms” from before the Cambrian are even less accepted. Of course, the unique structure of echinoderm skeletons is easy to recognise, but how do you identify an echinoderm ancestor without such a skeleton? (Is all that bodyplan diversity even possible without hard skeletal support?)

Caveats aside, this Moroccan stuff is awesome. And also, if my caveat proves overly cautious, echinoderms did some serious evolving in their first few million years on earth. A supersonic ride with Macroevolution Airlines?

***

*OK, if I want to be absolutely pedantic, and I do, then body plans are disparate rather than diverse. “Disparity” in palaeontological/evo-devo parlance refers to how different two or more creatures are. Diversity means how many different creatures there are. Maybe I should do a post on that, actually.

***

References:

Clausen S et al. (2010) The absence of echinoderms from the Lower Cambrian Chengjiang fauna of China: Palaeoecological and palaeogeographical implications. Palaeogeography, Palaeoclimatology, Palaeoecology 294:133-141

Smith AB et al. (2013) The oldest echinoderm faunas from Gondwana show that echinoderm body plan diversification was rapid. Nature Communications 4:1385

Oh, look, an argument!

It seems like forever since I posted about the very old putative bilaterian burrows Pecoits et al. (2012) reported in Science. I read the paper, thought about the implications, wrote the post and then filed the whole thing away in the giant messy cabinet at the back of my mind.

But a big claim like the one Pecoits et al. made – burrows from bilateran animals that appear before the first Ediacaran fossils! – is unlikely to go unchallenged by the scientific community. Now the argument has broken out. Gaucher et al. (2013) wrote a comment in Science criticising the reasoning that put such an old date on the formation where the burrows were found. Pecoits et al. (2013) responded. The plot is thickening!

The main bone of contention seems to be whether the huge body of granite that gave the actual radiometric date of 585 million years lies below the burrow-bearing formation (in which case it must be older than the fossils) or cuts through it (in which case it’s younger). The other question is whether the fossils and the rocks they’re found in actually belong to another nearby formation that is thought to be Permian in age. Burrows in Permian rocks would be no surprise at all . By that time reptiles and the ancestors of mammals walked the earth, insects of all kinds flew over it, and armadas of worms had been boring through soft sediments for hundreds of millions of years. Burrows that far into the Precambrian, on the other hand…

The argument is all very geological, and as I repeatedly said, I’m not much of a geologist. Looking at the figures wouldn’t help me decide who to believe at all. I’m rather amused by some of the snark that gets into the text, though. I have this feeling that Pecoits et al. are annoyed. Watch this, for example:

In this case, Gaucher et al. (1) take no notice of the outcrop-scale relationships and instead prefer to show five photographs from just one hand sample that they assigned to fossil site C to discredit the intrusive nature of the granite [figure 1, B to F, in (1)]. We do not want to speculate on the origin of this sample, but we see no evidence that it comes from fossil site C; it is not the ferruginized basal sandstone we previously documented [figure S3C in (2)].

Oh, yeah. “We do not want to speculate,” but we think something’s fishy with your evidence, only we’re too polite to say it in so many words!

Tee-hee. Academia’s version of an online flame war.

***

References:

Gaucher C et al. (2013) Comment on “Bilaterian burrows and grazing behavior at >585 million years ago”. Science 339:906

Pecoits E et al. (2012) Bilaterian burrows and grazing behavior at >585 million years ago. Science 336:1693-1696

Pecoits E et al. (2013) Response to comment on “Bilaterian burrows and grazing behavior at >585 million years ago”. Science 339:906

Score one for punk eek

Speaking of macroevolution…

I think it’s fair to say that the concept of punctuated equilibria is one of the most famous and most misunderstood ideas in 20th century evolutionary biology. PE, or “punk eek” was proposed by palaeontologists Niles Eldredge and Stephen Jay Gould  (Eldredge and Gould, 1972) as a reconciliation of the Modern Evolutionary Synthesis and the fossil record. Its core idea is that most (visible) evolutionary change happens during the formation of new species, and that this process is usually quick compared to the lifetime of a species. (An excellent layperson-friendly explanation of punk eek is available here.)

Of course, punk eek is not a law of nature – it’s only one way evolution might proceed, and it’s a decent explanation of the dearth of low-level (species to species) transitions in the fossil record. But there’s nothing to say that this is how evolution always proceeds, and consequently, exactly how often it does so is a valid (and still actively debated) question in evolutionary biology.

A related question is how often new species arise by the wholesale transformation of the ancestral species (anagenesis) or by the splitting of the ancestor into two or more descendants (cladogenesis). Since punk eek posits that most new species come from small isolated populations of the ancestor, under punk eek scenarios you’d expect most speciation to occur by cladogenesis.

However, assessing the exact contribution of each requires an exceptionally good fossil record where ancestor-descendant relationships and precise times of appearance and disappearance can be determined. This makes the investigation difficult to impossible in most groups. In the latest issue of PNAS, Strotz and Allen (2013) went to one of the few groups with a good enough record to answer such questions and analysed the living shit out of them.

Foraminiferans of forams for short are single-celled creatures that build hard shells to live in. They are very abundant, widely distributed in the world’s oceans, and because of their shells they make excellent (if tiny) fossils. Their relationships have also been studied with molecular methods, so we have a pretty good understanding of who’s related to whom and how well morphology meshes with genetics.

Therefore, as Strotz and Allen point out, we can say with a fair amount of confidence that what we’ve identified as species in the fossil record are likely to actually be species, not just varieties. (It doesn’t always work the opposite way – some “species” that look exactly the same on the outside are known hide several genetically distinct lineages.) The genetic data also help sort out who begat whom.

Armed with this knowledge of genetics and the detailed fossil record of planktonic forams in the last 65 million years, the pair formulated criteria for identifying cladogenetic events:

  • If morphologically distinct ancestor and descendant(s) overlap in time (factoring in dating and classification error), the descendant must have arisen by cladogenesis.
  • Likewise, cladogenesis must have occurred if the two species occur together in the same sample even if their morphologies overlap at that point.
  • Third, if an ancestor gave rise to a series of descendants, all but the last of those must have formed by cladogenesis – the ancestral form has to continue existing for it to sprout more descendants!

Thus, the possibility of anagenesis only remains for ancestor-descendant pairs that didn’t get caught on any of the above filters. And the number of those turns out to be very low.  Depending on how you estimate the errors associated with identifying fossils, only around 43-64 out of 337 speciation events (less than a fifth of the total) in the last 65 million years shows no evidence against anagenesis. The numbers are even lower, dipping below one-tenth of all events, if you only consider the last 23 million years, for which more precise dating information is available. In conclusion, for planktonic forams since the death of the dinosaurs, splitting an old species has been by far the more common way of forming new species.

It’s important to talk about the things this paper doesn’t say. It doesn’t, for example, say that its findings apply to all organisms. Speciation need not work the same way for all groups, and a subset of forams need not be representative of anything. It also doesn’t say – and the authors are quite explicit about this – that morphological evolution only occurs when species split. Instead, they argue, their findings support a modified view of punk eek in which species do change throughout their lifetimes – but the changes are fluctuations due to short-term influences, and they only persist if populations get isolated.

(Myself, I just think the simple fact that we have a fossil record where such ideas can be tested is pretty amazing. You can complain about the patchiness of the record all you like, but in the meantime it’s worth stopping and appreciating what we do have!)

***

References:

Eldredge N & Gould SJ (1972) Punctuated equilibria: an alternative to phyletic gradualism. In Schopf TJM (ed) Models in Paleobiology. Freeman, Cooper & Co., pp. 82-115

Strotz LC & Allen AP (2013) Assessing the role of cladogenesis in macroevolution by integrating fossil and molecular evidence. PNAS 110:2904-2909

Is Ediacara really stranded?

Heh, when I wrote a confused post about a paper by Greg Retallack that argues that classic Ediacaran fossils like Dickinsonia come from a terrestrial rather than an underwater environment, I said there’s sure to be responses. And I completely managed to miss the responses in the very same issue of Nature, apparently published online on the same day. *shameface* (I don’t think I got the commentary piece by RSS???)

One of them was actually quite nice to Retallack. L. Paul Knauth’s name doesn’t ring a bell, I suspect he’s the “geologist” out of the “palaeontologist and a geologist” the intro mentions. Of Retallack’s analysis itself, all he has to say is that Precambrian sediments can be very difficult to interpret, and one will need genuine expertise in fossilised soils ‘n’ stuff to evaluate Retallack’s claims. However, Knauth rejoices over the mere fact that there are unorthodox opinions like Retallack’s out in the open. In which he is certainly right – science wouldn’t go anywhere without disagreements.

The other commenter, Shuhai Xiao, is not so kind. (Him I’ve actually heard of; he’s published some seriously interesting stuff about Ediacaran fossils.) His commentary is kind of a polite way of saying “what a load of nonsense”. Like Knauth, he considers the evidence for the terrestrial origin of these rocks ambiguous, but he also emphasises features of the rocks that fairly unambiguously point to a marine environment. Funnily enough, he brings up geology that isn’t totally impenetrable to me as evidence, like a neat photo of Dickinsonia specimens on a slab of rock covered in nice symmetrical-looking ripples (the kind that forms under quiet waves). There’s also the fact that I forgot about when I wrote the other post: Dickinsonia itself is sometimes associated with crawling traces. Whatever that thing was and wherever it lived, it ain’t no lichen.

That’s reassuring in terms of not standing my worldview on its head, but I really wish Xiao had been less vague about some of his points. For instance, “the isotope signatures of carbonate nodules in the Ediacara Member can be accounted for by post-depositional alterations that do not involve pedogenic processes,” he says, with no further explanation and no citations. I’m thus far on Xiao’s side, but that doesn’t turn the above into a good argument…

Oh well. Let the debate rage on 🙂

(As of yet, no citations of Retallack’s paper on Google Scholar. We’ll definitely check back later. If I remember…)

The return of the giant lichens?

Gosh, can someone tell me if this is bullshit or if he has a point? O.o

It’s rather annoying when a paper comes out that basically threatens to turn what you think you know on its head, and you’re simply not equipped to evaluate its claims. This is the case with Retallack (2012). I’m fascinated by early animals, and endlessly bewildered by the strange fossils of the late Precambrian. While I’m aware that Ediacaran fossils have been interpreted as everything from microbial mats through animals to giant protists, I had the impression that the non-animal interpretations of iconic fossils like Dickinsonia, Spriggina, Parvancorina or Charniodiscus have slowly retreated to the fringe in the decades since their discovery.

And now this guy, whose name I’ve heard enough times to pay attention, gets into Nature arguing that the namesake formation of the Ediacaran period actually originated on dry land, and the iconic fossils are preserved in a manner more like plants, fungi or lichens than animals.

The paltry one semester of introductory geoscience I did years ago is nowhere near enough to comment on all the stuff he says about soils and microbial mats and preservation. I feel completely out of my depth, rocking precariously at the mercy of the waves…

Obviously, this assessment of the original Ediacara site doesn’t affect every fossil site from the period. The latest Precambrian reefs of the Nama Group remain marine reefs containing the remains of unknown animals that grew some of the first mineralised skeletons.

My big question at the moment is how Retallack would interpret the preservation of the White Sea assemblage. This contains similar kinds of fossils to the sites he’s reinterpreted as terrestrial. There’s Dickinsonia and several others like it, there’s Parvancorina, there’s Cyclomedusa*. And this is where hundreds of specimens of my Platonic love Kimberella come from, often associated with crawling and feeding traces. That guy moved around and grazed – plants and lichens seldom do such things! So was Kimberella a land animal? That would be the biggest palaeontological sensation of the decade if not the century. Or did dickinsoniids etc. occur both on land and underwater? Or did the White Sea fossils span a wide variety of environments? (I’m not sure about the distribution of the various White Sea fossils relative to each other…)

Oh my. I wonder what will come out of this. Publication in Nature makes it dead certain that any expert who’d vehemently disagree will find the article. Let’s pull out the pop corn and watch…

***

*It’s slightly odd that he seemingly treats Cyclomedusa and other “medusoid” fossils as though most people considered them jellyfish. That may have been their original interpretation, but I thought it was widely discredited now.

***

Reference:Retallack GJ (2012) Ediacaran life on land. Nature advance online publication available 12/12/12, doi:10.1038/nature11777

A small planet hidden in plain sight, and a must-read book

Random squeey post!

Squee number one: Dumusque et al. (2012) found a probably earth-sized planet literally next door to the solar system. The newly announced rockball orbits α Centauri B, one of the three(?)* stars making up the star system closest to the sun. Alas, it’s unlikely to harbour life of any kind given its very close orbit – it goes around a star not much cooler than our sun in slightly over 3 days. However, the authors point out that small planets are most likely to be found in multiplanet systems, and given the difficulty of finding this one, the star may well have even harder to detect, more distant companions. I think this is a nice reminder of how much more we need to learn about other solar systems – this little guy has been circling there, a mere four point something light years from us, and we only found it now. Keep it up, planet hunters, you’re doing amazing stuff!

Reference: Dumusque X et al. (2012) An Earth-mass planet orbiting α Centauri B. Nature advance online publication available online 17/10/2012, doi: 10.1038/nature11572

*I’ll let the real astronomers argue whether Proxima Centauri is part of the α Centauri system…

***

Squee number two: I totally must read this book. It’s the ultimate evo-devo book: it’s about fossilised development, basically. It seems overwhelmingly vertebrate-centred based on the review in Developmental Dynamics, but hey, there is an invertebrate chapter and I take what I can get 😀

And wow, it’s not even that expensive as far as university-published hardcover sciencey books go! Hmm. Shall we give in to temptation right now or save it for a Christmas present?

 

So… much… STUFF!

Gods, this is what I’m faced with all the time. Someone needs to tell me how proper science bloggers pick articles to discuss, because I just get my RSS alerts, start squeeing, and end up not writing about anything because damn, I WANT TO WRITE ABOUT EVERYTHING!

I give up. I’ll just dump all the cool stuff that’s accumulated on my desktop and bookmark bar here and return to lengthy meandering whenever I don’t feel like I’ve been caught in a bloody tornado 😉

So, here is some Cool Stuff…

(1) A group measured the rate of DNA decay in 158 moa bones of known age from three sites. Really cool stuff, to go out and directly measure how ancient DNA disappears from dead things under more or less identical conditions. The unsurprising result is that DNA decays exponentially, a bit like radioactive material. This suggests that the main cause of the decay is random breaking of the strands. The surprising bit is that this happens much more slowly than previously estimated, suggesting that in ideal (read: frozen) conditions, it might be worth looking for preserved DNA in samples as old as a million years.

(On a side note, if you ever get a chance to see a talk by Eske Willerslev, one of the authors and a leading expert on ancient DNA, don’t miss it. The man is absolutely hilarious.)

– Allentoft ME et al. (2012) The half-life of DNA in bone: measuring decay kinetics in 158 dated fossils. Proceedings of the Royal Society B FirstCite article, available online 10/10/2012, doi: 10.1098/rspb.2012.1745

(2) The beaks of the finches, or mixing and matching developmental recipes. This study examines the genetic basis of beak shape in three little birds closely related to Darwin’s famous finches. The three finches, just like Darwin’s, share the same basic beak shape, only bigger or smaller. However, there seem to be two distinct developmental programs at work, using different genes and parts of the skeleton to orchestrate beak development. One of the three newly investigated species (the one most closely related to Darwin’s finches) apparently uses the same developmental program as its more famous relatives, even though its beak is shaped more like the other two birds studied here. I told you – genetics, development and homology are complicated 😉

– Mallarino R et al. (2012) Closely related bird species demonstrate flexibility between beak morphology and underlying developmental programs. PNAS 109:16222–16227

(3) Armoured fossil links worm-like molluscs to chitons. There’s a little-known group (or groups) of molluscs called aplacophorans that have only a coat of tiny spicules instead of shells and look more like worms than “proper” molluscs. Exactly where they fit into our picture of mollusc evolution has been controversial to say the least – they could represent an old lineage separate from other molluscs, they could be related to cephalopods, they could be related to chitons, they could be one group or they could be two lineages in completely different places on the tree… Well, a new fossil named Kulindroplax seems to argue for the chiton connection: the animal has the characteristic armour plates of a chiton on an aplacophoran-like body. Similar creatures have been discovered before, but this guy with its detailed 3D preservation provides the clearest evidence of the link so far.

– Sutton MD et al. (2012) A Silurian armoured aplacophoran and implications for molluscan phylogeny. Nature 490:94-97

(4) More cool fossils – this time straight from my beloved Cambrian. Nereocaris, a newly described Burgess Shale arthropod, suggests to its discoverers that the earliest arthropods weren’t predators prowling the seafloor, but swimmers who might have been filter feeders and certainly weren’t predators. The animal has a bivalved shell around its front end, similar to many other Cambrian swimming arthropods, and a long abdomen with paddles at the end. It bears the arthropod hallmark of a hardened and jointed exoskeleton, but it lacks specialised limbs such as antennae or mouthparts. In a cladistic analysis of arthropods and their nearest relatives, the new species comes out on the first branch within true arthropods, and the next few branches as we move towards living arthropods all contain similar shelled, swimming creatures. Since the non-arthropods closest to the real thing (i.e. anomalocaridids) were also fin-tailed swimmers, this arrangement makes the transition between them and true arthropods smoother than previously thought. It also suggests that the hard exoskeleton so characteristic of arthropods originally functioned in swimming – perhaps as an anchor for swimming muscles.

– Legg DA et al. (2012) Cambrian bivalved arthropod reveals origin of arthrodization. Proceedings of the Royal Society B FirstCite article, available online 10/10/2012, doi: 10.1098/rspb.2012.1958

*

And … there was also

… but it’s almost bedtime, and if I wanted to summarise every one of those, I’d be here all weekend 😦

See, this is why being a science nerd today is both amazing and frustrating. There’s just so. Much. Stuff.

Two for Team Mollusc

Yay, more involved invertebrate anatomy I don’t really understand but can’t resist writing about!

In other words, Martin Smith, one of the guys who likes to argue that a number of Cambrian weirdos are actually molluscs, published new stuff about, guess what, Cambrian weirdos he considers molluscs.

Above: the delightfully odd Wiwaxia by Nobu Tamura, from Wikipedia.

I don’t think he and JB Caron had much luck with the Nectocaris-is-a-cephalopod idea, but maybe he’s on to something with Odontogriphus and Wiwaxia. I mean, one of the big complaints about Nectocaris being a proto-squid was that none of the 90-odd specimens preserve any mouthparts at all, and mouthparts are normally among the toughest and most recognisable features of cephalopods. This time in the Proceedings of the Royal Society (Smith, 2012), there are mouthparts galore. The paper is about mouthparts.

Above image from the paper features an electron micrograph, closeup and drawing of a specimen belonging to Odontogriphus, showing two kinky rows of hooked teeth that were probably all embedded in one chunk of tissue in life (they tend to stay together in the fossils). They look kinda freaky, but more importantly, Smith argues, they look remarkably like a simple version of a radula. Which is a very mollusc thing to have – nothing else alive today has one. Most living molluscs sport more complicated radulae than this, with many more than the two or three tooth rows found in the Cambrian creatures in question, but hey, no one said modern radulae suddenly popped into existence fully formed. If they are molluscs, the strange Odontogriphus and Wiwaxia are probably among the early branches within the phylum, which may well mean that their ancestors said bye to the rest of the molluscs before the first hopeful monster with more than three rows of teeth hatched from a slimy clutch of proto-mollusc eggs.

Another major interpretation of these creatures, especially Wiwaxia, is as something close to annelid worms (ragworms, earthworms etc.). Their mouthparts have previously been claimed to resemble dorvilleid annelid jaws, a lovely example of which is shown on the right of this image. (Warning: closeups of polychaete faces are not for the faint-hearted ;)) However, Smith argues that the structure of Odontogriphus and Wiwaxia tooth rows is nothing like that of worm jaws. For example, they consist of teeth that can rotate relative to one another whereas the worm jaws in question have teeth sitting in fixed rows, and the teeth are apparently shed and replaced in completely different ways. Not to mention that the living worms have paired jaws, whereas each tooth row of the Cambrian critters appears connected in the middle. Of course, Smith says they aren’t connected in the same way that some other annelid jaws they’ve also been compared to are.

So, Team Mollusc has delivered some toothy punches, but I’m sure this is not the final word on something as old and weird as Odontogriphus and Wiwaxia. Holding my breath for the responses of Team Worm and Team Grandaddy of Molluscs AND Worms…

(BTW, I recall that Wiwaxia, in addition to its mollusc mouth, also has scaly armour that looks suspiciously like a bunch of fused annelid bristles. Dunno how accurate that information is today, since IIRC I read it in a pretty old book that also had an agenda, but that just goes to show that so soon after all the Common Ancestors lived, family resemblances might get a bit muddled… And crazy thought, what if the common ancestor of annelids and molluscs – they are pretty close relatives, after all – had a radula? Annelids today don’t, but you never know. It wouldn’t be the first time an animal lost something complex in the history of evolution.)

***

Reference

Smith MR (2012) Mouthparts of the Burgess Shale fossils Odontogriphus and Wiwaxia: implications for the ancestral molluscan radula. Proceedings of the Royal Society B, advance online publication, doi: 10.1098/rspb.2012.1577

Ediacaran Underground

As you may have guessed from my blog title, I’m fascinated by the early history of animals. Part of the problem with this early history is that its fossil record is extremely sketchy, extremely difficult to interpret, or both. One of the things that marks the diversification of animals late in the Ediacaran (the last period of the Precambrian) and even more during the Cambrian explosion, is the appearance of burrows and other trace fossils. Marks on the seafloor don’t have to be made by what we usually think of as “complex” animals, though. Burrowing sea anemones are well known, and even single-celled creatures can plough a track in the sediment. That makes things rather complicated when you are looking for the first signs of complex bilaterian animals in the rocks.

Pecoits et al. (2012) are convinced that the trace fossils they found in Uruguay are such a sign, perhaps indeed the oldest. The gently meandering little burrows they report in Science are shaped much like the burrows made by some modern molluscs and annelid worms, possessing features that are hard to explain with a rolling protist or a simple animal. Among other things, they have minute indentations in their walls that may indicate the places where leg-like body parts pushed against the sediment as the animal pulled itself along.

The mysterious burrowers’ path was relatively simple, meandering in broad sine waves that may preserve the unknown creature’s search for food. Burrows often cross, suggesting that their makers made no effort to avoid each other. However, they sometimes disappear and reappear a few millimetres later, as if the animal made a detour upwards or downwards. The few abrupt turns, combined with the width of the burrows, indicate that the creature who left these traces was small, less than a centimetre long.

These burrows – if they are indeed that – are somewhere between 585-600 million years old, at least 30 million years older than the oldest uncontested bilaterians. My question, though, is are they? Bilaterian burrows, I mean. It’s a good thing I double-checked about trace-making protists, since it turns out that the protists in question leave tracks that have supposedly bilaterian characteristics, like consisting of two ruts on either side of a central ridge (Matz et al., 2008). This is one of the similarities the new paper draws between modern bilaterian burrows and their Precambrian precursors! (The weirdest thing is they cite Matz et al. but kind of dismiss the eerily bilaterian character of protist traces…)

Pecoits et al. (2012) do take some time to argue against a protist or non-bilaterian origin of the burrows. There are the little indentations that may have come from something like a worm’s parapodia but are perhaps more difficult to reconcile with a simple rolling ball of cytoplasm. There are also the disappearances and reappearances that indicate the creature moving up and down levels. The authors also argue that the burrows are definitely burrows – i.e. actually under the sediment surface -, though I’m not sure this follows from their evidence.

Then again, they’re the experts here, I’m just a geologically challenged biologist looking at pictures of grooves in rocks… The disappearance-reappearance thing does seem to imply that whatever left these fossils could burrow. (Unless it suddenly hopped off the bottom and landed nearby? And why can’t protists burrow anyway? We didn’t even know they made fake animal traces until a few years ago… I think I’m beginning to sound silly…)

As far as arguments for a bilaterian trace-maker go, a lot is made of the infillings in the burrows, and of the parts that look like the roof collapsed after the animal moved on, but honestly I can’t see what they are talking about in the photos, so I’ll stay on the safe side and reserve judgement there 🙂

And here I meander off track (terrible pun fully intended)

Aaaaanyway, let’s assume they are right, and there’s a reasonably complex worm behind these burrows.

That would be awesome.

However, it poses some questions. In fact, it poses the same questions my friend Kimberella does. I’ve been ruminating about this since I wound up explaining what we know of the bilaterian ancestor to some random guy on the internetz. Let me pour out the contents of my brain here, and let’s hope they make sense >_>

Kimberella. This creature is at the very least a bilaterian, but probably not too far off from molluscs. Its amazing fossil record includes incontrovertible evidence that it could move around and graze on whatever it ate (microbes, probably) by scraping it off the seafloor. It was covered in a knobby armour – fairly flexible and probably not made of a single peace, but definitely a shell of sorts. That’s one of the things that suggest ties to molluscs. Either way, something that we can identify as a member of a subgroup of bilaterians must postdate the bilaterian common ancestor by a fair bit – all those lineages needed time to split and evolve their recognisable body plans.

Palaeontologists and developmental biologists (Budd and Jensen, 2000; Erwin and Davidson, 2002) made a good case in arguing that the last common ancestor of bilaterians must have been small and simple. We know this creature must have predated Kimberella by some time – and this is where we run into problems. One of the strongest arguments for a small and simple bilaterian ancestor is the paucity of Precambrian trace fossils. Large and complex bilaterians make a big mess of seafloor sediment. They dig into it, they walk over it, they churn it up and eat it and spit it out. Nonetheless, the Precambrian is full of virtually undisturbed microbial mats, an unexploited bonanza for any bilaterian with the means to graze. So the conclusion is that large, complex bilaterians must have been rare or non-existent. But, Kimberella? Burrows of a large-and-complex bilaterian* that predate Kimberella by 30 million years?

Something odd is going on here.

I suppose the question could be put as: if these creatures were around in the late Precambrian, why weren’t there more of them? (And where are the bodies?) Why didn’t they swarm out and eat all the microbial mats that made the preservation of many Ediacaran fossils possible (Narbonne, 2005)? Kimberella was doing its best to graze them off the face of the earth, yet this same type of preservation is common even in the formations everyone’s favourite proto-mollusc comes from.

Too little oxygen? But Kimberella is large, relatively compact and partly covered in armour. It’s the sort of creature that we’d expect to have a specialised respiratory system even in an oxygen-rich modern sea, so at least one animal clearly solved this problem…

I suppose it all comes back to what caused the Cambrian explosion, which is a tough question. (Marshall [2006] is a pretty nice review if memory serves.) If we figure out why molluscs and worms and other bilaterians didn’t take over the oceans long before the Cambrian, we’ll have figured out why they did so in the Cambrian.

I’m not sure that did make sense in the end, but I’m glad I could get it off my chest 😀

***

*A centimetre may not sound very large, but a pretty big percentage of the animal kingdom comes nowhere near it in size.

***

References:

Budd GE & Jensen S (2000) A critical reappraisal of the fossil record of the bilaterian phyla. Biological Reviews of the Cambridge Philosophical Society 75:253-295

Erwin DH & Davidson EH (2002) The last common bilaterian ancestor. Development 129:3021-3032

Marshall CR (2006) Explaining the Cambrian “explosion” of animals. Annual Review of Earth and Planetary Sciences 34:355-384

Matz MV et al. (2008) Giant deep-sea protist produces bilaterian-like traces. Current Biology 18:1849-1854

Narbonne GM (2005) The Ediacara biota: Neoproterozoic origin of animals and their ecosystems. Annual Review of  Earth and Planetary Sciences  33:421-442

Pecoits E et al. (2012) Bilaterian burrows and grazing behavior at >585 million years ago. Science 336:1693-1696

Before they became weird

Echinoderms are weird. They are supposed to be bilaterian animals, but they have abandoned bilateral (mirror image) symmetry for looking like fleshy stars, spiny boobs, strange flowers or funky sausages. When they first appear in the fossil record during the Cambrian period*, they show up with an even weirder menagerie of body plans ranging from almost bilateral through asymmetric to all sorts of variations and twists on the standard five-rayed body plan that we know and love. (Below: a selection of weird and wonderful Cambrian echinoderms from Zamora et al. [2012])

(We only know that some of these creatures were echinoderms or very close relatives thereof because they have skeletons with a unique spongy microstructure (stereom) only seen in echinoderms.)

I don’t know nearly enough about echinoderms to properly discuss the latest addition to the march of the weirdos, but damn me if I don’t at least give a proper fangirlish SQUEEE! to a new Cambrian echinoderm – with bilateral symmetry! Zamora et al. (2012) actually describe two fossil finds, but one of them is new specimens of a previously known animal. However, the other is brand new, and what a pretty thing, too! Behold Ctenoimbricata spinosa, straight out of science fiction – or a nightmare :-P! (OK, don’t start having Ctenoimbricata nightmares just yet. The whole animal was less than an inch long.)

The creature was reconstructed from fossils found in Middle Cambrian (about 510 million years old) rocks in northern Spain. The shape of its body and the arrangement of its many armour plates most closely resemble an obscure group of ancient echinoderms called ctenocystoids (represented by fossil A in the first picture). Typical ctenocystoids have slight asymmetries manifested as different arrangements of armour plates on their left and right sides. However, some are well-behaved bilaterians. That’s the other point of the paper: new fossils belonging to a previously known ctenocystoid demonstrate its symmetry. The authors think that the similarly symmetrical Ctenoimbricata was an even more primitive relative of ctenocystoids. In their view, echinoderms started out with mirror image symmetry, then became asymmetric, and only then did they evolve the radial symmetry starfish exemplify.

Ctenoimbricata, according to Zamora et al., is the most primitive echinoderm ever found. The fact that it doesn’t have a stalk or arms suggests that it wasn’t a filter feeder. Instead, it probably operated flat on the seafloor, gulping sediment and sifting out the edible bits. Since ctenocystoids are also stalk- and armless, this might mean that the last common ancestor of all echinoderms lived in a similar way, which has apparently been a matter of some debate. Yay!

Incidentally, I had no idea that Europe had such awesome Cambrian fossils. I thought the best sites were all at a minimum of a half-day plane ride away. So: double squee for our tiny spiny sandmower!

***

*People have argued that a Precambrian fossil called Arkarua may be an echinoderm ancestor, but I wouldn’t bet on that. Just about the only thing those tiny imprints can be shown to share with echinoderms is the five-part symmetry, and it’s not like unusual body symmetries were… unusual for Precambrian animals.

***

Reference:

Zamora S et al. (2012) Plated Cambrian bilaterians reveal the earliest stages of echinoderm evolution. PLoS ONE 7: e38296