The things you can tell from a pile of corpses…

I’m really late to this party, but I never claimed to be timely, and the thing about the reproductive habits of Fractofusus is too interesting not to cover.* Rangeomorphs  like Fractofusus are really odd creatures. They lived in that Ediacaran twilight zone between older Precambrian seas devoid of macroscopic animals and younger Cambrian seas teeming with recognisable members of modern groups. Rangeomorphs such as RangeaCharnia and Fractofusus itself have such a unique fractal body plan (Narbonne, 2004) that no one really knows what they are. Although they were probably not photosynthetic like plants or algae (they are abundant in deep sea sediments where there wouldn’t have been enough light), their odd body architectures are equally difficult to compare to any animal that we know.

Mitchell et al. (2015) don’t bring us any closer to the solution of that mystery; they do, however, use the ultimate power of Maths to deduce how the enigmatic creatures might have reproduced. Fractofusus is an oval-shaped thingy that could be anywhere from 1 cm to over 40 cm in length. Unlike some other rangeomorphs, it lay flat on the seafloor with no holdfasts or stalks to be seen. Fractofusus fossils are very common in the Ediacaran deposits of Newfoundland. Since there are so many of them, and there is no evidence that they were capable of movement in life, the researchers figured their spatial distribution might offer some clues as to their reproductive habits. A bit of seafloor covered in Fractofusus might look something like this (drawing from the paper):

clusters within clusters

(The lines between individuals don’t actually come from the fossils, they just represent the putative connection between a parent and its babies.)

Statistical models suggest that the fossils are not randomly distributed but clearly clustered: small specimens around medium-sized ones, which are in turn gathered around the big guys. Two out of three populations examined show these clusters-within-clusters; the third has only one layer of clustering, but it’s still far from random. As the authors note, the real populations they studied involve a lot more specimens than shown in the diagram, but they “rarefied” them a bit for clarity of illustration while keeping their general arrangement.

The study looked not only at the distances between small, medium and large specimens, but also directions – both of where the specimens were and which way they pointed. If young Fractofusus spread by floating on the waves, they’d be influenced by currents in the area. It seems the largest specimens were – they are unevenly distributed in different directions. In contrast, smaller individuals were clustered around the bigger ones without regard to direction. Small and large specimens alike pointed randomly every which way.

What does this tell us about reproduction? The authors conclude that the big specimens probably arrived on the current as waterborne youngsters, hence their arrangement along particular lines . However, once there, they must have colonised their new home in a way that doesn’t involve currents. Mitchell et al. think that way was probably stolons – tendrils that grew out from the parent and sprouted a new individual at the end. This idea is further strengthened by the fact that among thousands of specimens, not a single one shows evidence for other types of clonal reproduction – no fragments, and no budding individuals, are known. (Plus if a completely sessile organism fragments, surely the only way the pieces could spread anywhere would be by riding currents, and that would show up in their distribution.)

Naturally, none of this tells us whether Fractofusus was an animal, a fungus or something else entirely. Sending out runners is not a privilege of a particular group, and while there is evidence that the original founders of the studied populations came from far away on the waves, we have no idea what it was that floated in to take root in those pieces of ancient seafloor. Was it a larva? A spore? A small piece of adult tissue? Damned if we know. Despite what Wikipedia and news headlines would have you believe, there is nothing to suggest that sex was involved. It may have been, but the evidence is silent on that count. (Annoyingly, the news articles themselves acknowledge that. Fuck headlines is all I’m saying…)

While sometimes we gain insights into ancient reproductive habits via spectacular fossils like brooding dinosaurs or pregnant ichthyosaurs, this study is a nice reminder that in some cases, a lot can be deduced even in the absence of such blatant evidence. This was an interesting little piece of Precambrian ecology, and a few remarks in the paper suggest more to come: “Other taxa exhibit an intriguing range of non-random habits,” the penultimate paragraph says, “and our preliminary analyses indicate that Primocandelabrum and Charniodiscus may have also reproduced using stolons.”

An intriguing range of non-random habits? No citations? I wanna know what’s brewing!

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*Also, I’ve got to write something so I can pat myself on the back for actually achieving something beyond getting out of bed. Let’s just say Real Life sucks, depression sucks worse, and leave it at that.

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References:

Mitchell EG et al. (2015) Reconstructing the reproductive mode of an Ediacaran macro-organism. Nature 524:343-346

Narbonne GM (2004) Modular construction of Early Ediacaran complex life forms. Science 305:1141-1144

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OH MY GOD A FILTER-FEEDING ANOMALOCARIDID!!!

ETA: OK, technically it should be “suspension-feeding”, because there’s a good chance that its feeding mechanics involved more than simple filtering (see comments). I hate retconning, so I’ll leave the post as it is aside from this addendum. Thanks for the heads-up, Dave Bapst 🙂

This is when I put everything resembling work aside to squee madly over a fossil.

(Imagine me grinning like crazy and probably bouncing up and down a bit in my seat)

Tamisiocaris is a newly “updated” beast from the Cambrian, and the coolest thing I’ve seen since that helicoplacoid on a stalk (most cool things come from the Cambrian, right?). It is the Cambrian equivalent of a baleen whale.

Anomalocaridids were close relatives of arthropods and are among the most iconic creatures of the Cambrian. Most anomalocaridids we know of were large, swimming predators with large head appendages bearing sturdy spines to grab prey and bring it to that trilobite-crunching pineapple slice mouth. Going with the whale analogy, they were more like the killer whales of their time (although they would be easy snacks for an actual killer whale). In fact, when the putative head appendage of Tamisiocaris was originally described by Daley and Peel (2010), the only odd thing they noted about it was that it was not hardened or obviously segmented the way those of Anomalocaris were.*

Tamisiocaris was already cool back then, because it was the first animal of its kind found at Sirius Passet in Northern Greenland, one of the lesser known treasure troves of fabulous Cambrian fossils. However, since then, more appendages have been found, and it turns out that those long spines had been hiding a fascinating secret.

They were… kind of hairy.

Vinther_etal2014-sf3_crop

Closer examination of the appendages shows that their long, slender spines bore closely spaced bristles, making each spine look like a fine comb (whole appendage and close-up of a spine above from Vinther et al. [2014]). With all the spines next to each other, the bristles would have formed a fine mesh suitable for catching prey smaller than a millimetre. Compared with modern filter-feeding animals, Tamisiocaris fits right in – it would have “fished” in a similar size range as a greater flamingo. Vinther et al. (2014) suggest that Tamisiocaris would have brought its appendages to its mouth (which isn’t among the known fossils) one at a time to suck all the yummies off.

These guys are tremendously interesting for more than one reason, as the new study points out. First, HOLY SHIT FILTER FEEDING ANOMALOCARIDIDS! (Sorry. I’m kind of excited about this.) Second, the mere existence of large**  filter feeders implies a richness of plankton people hadn’t thought existed at the time. Third, there is some remarkable convergent evolution going on here.

Often, really big plankton eaters evolve from really big predators – see baleen whales, basking sharks, and these humongous fish for example. It’s not an already filter-feeding animal growing bigger and bigger, it’s an already big animal taking up filter-feeding. The interrelationships of anomalocaridids suggest the same story played out among them – ferocious hunters begetting “gentle giants” in a group with a totally different body plan from big vertebrates. For all the dazzling variety evolution can produce, sometimes, it really rhymes.

And finally, Vinther et al. did something really cool that tickles my geeky side in a most pleasant way. In their phylogenetic analysis that they did to find out where in anomalocaridid evolution this plankton-eating habit came along, they found that Tamisiocaris was closely related to another anomalocaridid with (on a second look) not dissimilar appendages. They named the group formed by the two the cetiocarids – after an imaginary filter-feeding anomalocaridid created by artist John Meszaros for the awesome All Your Yesterdays project.

Man. That’s definitely worth some squee.

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*Disclaimer: I’m basing this on the abstract only, since palaeontological journals are one of the unfortunate holes in my university library’s otherwise extensive subscriptions.

**For Cambrian values of “large” – based on the size of the appendages, this creature would have been something like two feet long.

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References:

Daley AC & Peel JS (2010) A possible anomalocaridid from the Cambrian Sirius Passet Lagerstätte, North Greenland. Journal of Palaeontology 84:352-355

Vinther J et al. (2014) A suspension-feeding anomalocarid from the Early Cambrian. Nature 507:496-499

Oxygen and predators and Cambrian awesomeness (with worms!)

I didn’t plan to write anything today, but damn, Cambrian explosion. And polychaetes. I can’t not. Plus I’m going on holiday soon, so I might as well get something in before I potentially disappear off the internet. (Below: a Cambrian polychaete, Canadia spinosa, via the Smithsonian’s Burgess Shale pages.)

First, a confession.

I’m a bit of a coward about the Cambrian explosion.

Make no mistake, I love it. It’s fascinated me ever since I came across the heavily Stephen Jay Gould-flavoured account in The Book of Life. It’s an event that made the world into what it is today, with its complex ecosystems full of animals eating, cooperating or competing with each other. And it’s one of the great mysteries of palaeontology. What actually happened? What caused it? Why did it happen when it did? Why didn’t it happen again when animal life was nearly wiped out at the end of the Permian?

The problem is, I love it so much that I’m afraid to have an opinion about it. You have no idea how many times I wanted to discuss the big questions, only to shy away for fear of getting it wrong. Which is really kinda stupid, because no one has the one and only correct answer. Whether I’m qualified to comment on it is a different issue, but it wouldn’t be the first subject I comment on that I don’t fully understand.

So, here I take a deep breath and plunge into Sperling et al. (2013).

The abstract started by scaring me. It begins, “The Proterozoic-Cambrian transition records the appearance of essentially all animal body plans (phyla), yet to date no single hypothesis adequately explains both the timing of the event…” To which my immediate reaction was “why the fuck would you want a single hypothesis to explain it?” But luckily, they don’t. They actually argue for a combination of two hypotheses, which they think are more connected than we thought.

But let’s just briefly establish what the Cambrian explosion is.

I want to make this absolutely clear: it’s not the sudden appearance of modern animals out ot nowhere. It could be more accurately described as the appearance of basic body plans we traditionally classify as phyla, such as echinoderms, molluscs, or arthropods, in a relatively short geological period.

Doug Erwin (2011) trawled databases and literature to draw up a timeline of first appearances for animal phyla, and he found that they increase in number gradually over a period of 80 million years (see Erwin’s plot below).

Erwin2011-cumulativePhyla

Appearance of “phyla” also doesn’t equal appearance of modern animals, as Graham Budd has been known to emphasise (e.g. Budd and Jensen, 2000). For example, I already mentioned how none of the mid-Cambrian echinoderms recently described by Smith et al. (2013) would look familiar today. In fact, the modern classes of echinoderms, which include sea urchins, starfish and sea lilies, didn’t appear until after the Cambrian. Likewise, while there were chordates (our own phylum), and probably even vertebrates, in the Cambrian, such important vertebrate features as jaws or paired appendages were yet to be invented. (If memory serves, both of those inventions date to the Silurian.)

There is also a discussion to be had about the meaning and validity of concepts like a phylum or a body plan, but let’s not complicate things here. I have a paper to get to! 🙂

With that out of the way…

There is no doubt that something significant happened shortly before and during the Cambrian. Before the very latest Precambrian, fossils show little evidence of movement, of predation, or of the diverse hard parts that animals use to protect themselves or eat others today. All of these become commonplace during the Cambrian, establishing essentially modern ecosystems (Dunne et al., 2008).

There are many explanations proposed to account for the revolution. I’ve not the space (or the courage) to discuss them in any detail. If you’re interested, IIRC Marshall (2006) is a very nice and balanced review. (Link leads to a free copy.) However, we can discuss what Sperling et al. have to say about two of them.

The first hypothesis is oxygen, which likely became more abundant in the ocean towards the end of the Precambrian. That  could explain the timing, but maybe not the nature of the explosion. Oxygen levels impose a limit on the maximum size of animals, but what compels larger animals to “invent” more disparate body plans? (Also, on a side note, many Ediacaran organisms weren’t exactly tiny, so I’m not sure how much of a size limit there is in the first place.)

The second one is animal-on-animal predation (Sperling et al. prefer the term “carnivory”), which can lead to predator-prey arms races and therefore encourage the evolution of innovations like shells or burrowing or jaws that give one party an edge. This is a decent enough basis for body plan innovation, but it applies for any time and place with animals. So if carnivory is the explanation, why did the explosion happen when it did?

Because, Sperling et al. argue, carnivory and oxygen are linked.

I’m intrigued by their approach. They’re not looking at fossils in this study at all. (I always like it when palaeontology and the biology of the living join forces!) They are looking at oxygen-poor habitats in modern oceans. Specifically, they asked how low oxygen levels affect polychaete worm communities.

Why polychaetes? The authors give a list of reasons. One, polychaetes are really, really abundant on the seafloor, and particularly so in low-oxygen settings. Two, different species feed in almost every conceivable way from filtering plankton through chewing through sediment to flat out devouring other animals, and their feeding mode can usually be guessed even if you haven’t seen that particular species eat. Three, they are actually quite good at handling oxygen limitation. This is important because back in the Precambrian, all animals would have been well adapted to a low-oxygen environment, so a group that can tolerate the same may be the best comparison. (They do note that  a previous study of a single low-oxygen site that took other animals into account came up with similar results to theirs.)

They worked partly with pre-existing datasets that met a set of criteria designed to get a complete and unbiased view of local polychaete diversity. In total, they analysed data from 68 sites together featuring nearly a thousand species of worms. They also had some of their own data.

They categorised their study sites into four levels of oxygen deprivation, and counted numbers of carnivorous individuals and species at each site. They came to the conclusion that lack of oxygen basically makes carnivores disappear. The lowest-oxygen samples contained fewer carnivores on both the individual and species levels, and they were more likely to be devoid of predators altogether (# species plot from the paper below):

sperling_etal2013-fig2c

There are a couple of different ways in which lack of oxygen could limit predators. For example, the aforementioned size limit is one, because it’s good for a predator to be larger and stronger than its prey. But the biggest factor according to the authors is the energy required for an active predatory lifestyle. While a suspension feeder can sit in one place all day and only move to stuff a food-laden tentacle into its mouth, a predator has to find, subdue and eat its prey, which are all pretty expensive activities. Then it also has to digest a sudden, large meal, whereas the suspension feeder’s digestion works at a low and steady rate. Animals can get energy from a variety of metabolic processes, but by far the most efficient route requires oxygen. And that really sucks when you are a hunter who might need large amounts of energy at short notice.

Hmm…

Although I’m quite intrigued by the study, there are a couple of issues that bother me. For example, as far as I could tell, all of the study sites included in the analyses were low on oxygen. I would have liked to see them compared to “normal” sites, in particular because the trend in predator abundance wasn’t a neat straight upwards line. In fact, the least oxygen-deprived habitats appeared less predator-infested than slightly more oxygen-poor ones. What’s going on there?

In terms of interpretation in relation to the Cambrian, I also would have liked to see a comparison of the oxygen levels at their study sites to what’s estimated for the geological periods in question. I take it they just didn’t have precise enough estimates, because one of the things they discuss in the closing paragraph is the need to measure just how much oxygen went into the oceans during this late Precambrian oxygen increase.

And my semi-silly question is, how does this apply to “predators” who don’t run around chasing after and wrestling with prey? For example, sea anemones might be perfectly happy to eat large creatures. But they don’t really do much. They just sit and wait, and if a poor fish stumbles onto their sticky venomous tentacles, tough luck for it. Or there’s the unknown predator that drilled holes in late Precambrian Cloudina specimens (Bengtson and Zhao, 1992). Cloudina was sessile, the creature didn’t have to chase it… Predators such as these still have to cope with the energy demands of digesting sudden large meals, I suppose, so maybe the energetics idea still applies. And of course, if there’s no oxygen, large prey is less likely to be swimming around bumping into your tentacles.

Is this “the” explanation of the Cambrian explosion? Probably not, says the cynic in me. I highly doubt we’re done with that question. Is it a good explanation? Well, it is certainly evidence-based, and I like it that it tries to take different factors together and in context. What I don’t think it does is explain the uniqueness of the Cambrian. A thousand words or so ago, I mentioned the Permian extinction. That cataclysm very nearly left the earth devoid of animals. Afterwards, there was certainly enough oxygen for predators to thrive in the sea, and indeed they did, from sea urchins to ichthyosaurs. So why didn’t the first 40 million years of the Mesozoic era beget many new phyla the way the first 40 million years of the Palaeozoic did? Is that just an artefact of our classifications or was something really fundamentally different going on?

I ain’t Jon Snow, but when it comes to the Cambrian, I still feel like I know nothing…

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References:

Bengtson S & Zhao Y (1992) Predatorial borings in Late Precambrian mineralized exoskeletons. Science 257:367-369

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

Dunne JA et al. (2008) Compilation and network analyses of cambrian food webs. PLoS Biology 6:e102

Erwin DH (2011) Evolutionary uniformitarianism. Developmental Biology 357:27-34

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

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

Sperling EA et al. (2013) Oxygen, ecology, and the Cambrian radiation of animals. PNAS 110:13446-13451