Protists, not animals

I’ve written about the spectacular phospatized embryos of the Doushantuo formation before. It’s a collection of exceptionally well preserved small multicellular organisms, so well preserved that we can even look at cellular organelles. And they’re pre-Cambrian, as much as 630 million years old.

They’ve been interpreted as fossilized embryos for which we have no known adult forms. They certainly look like embryos, but one thing has always bothered me — they all look like blastula-stage embryos at various points in their early divisions, and the absence of later stages was peculiar: how did gastrulae and neurulae and other stages avoid getting preserved?

One explanation was that we weren’t seeing metazoan fossils at all — they were colonies of large bacteria. That’s disappointing if you have an animal bias, but still cool — as I pointed out then, it just highlights the fact that the transition from single-celled to multi-celled life isn’t that remarkable.

Now we have another alternative explanation that seems even better to me: they aren’t animals, and they aren’t bacteria, they’re protists. Some of the Doushantuo specimens are rather peanut-shaped, and others are vermiform, odd for an animal embryo, but entirely compatible with the idea that these are encysted stages of propagating protists.

Here are some of these oddly shaped Doushantuo specimens.

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Tianzhushania from the Ediacaran Doushantuo Formation, Datang Quarry, Weng’an, Guizhou Province, China. (A) Regular and (B to J) irregular forms, the latter interpreted to be in the germinating stage: MESIG 10022 [(A) SEM micrograph]; MESIG 10023 [(B) SEM micrograph (19)]; MESIG 10024 [(C) SEM micrograph (19)]; MESIG 10021 [(D) SEM micrograph]; SMNH X 4447 [(E) to (G) srXTM renderings]; SMNH X 4448 [(H) to (J) srXTM renderings]. (A) Surface of regular globular specimen shows envelope structure, to be compared with the similar envelope structure in (B) to (D). [(B) and (C)] Germinating specimens show protruding tubes and envelope structure. (D) Peanut-shaped specimen shows envelope structure. (E) Isosurface rendering of peanut-shaped specimen. (F) Orthoslice through (E). (G) Detail of approximate level in (F), showing cellular units. (H) Isosurface rendering of peanut-shaped specimen. (I) Orthoslice through (H). (J) Detail of approximate level in (I), showing cellular units. There is a progressive individuality of cellular units toward the periphery, including detachment of single- and oligocellular units (arrows).
i-c2ed0d62b380987fc5477a57e4735e06-lifecycle.jpeg
Proposed life cycle of Tianzhushania through hypertrophic growth of mother cell, encystment in multilayered wall, palintomic cleavage resulting in a tightly packed mass of pre-propagules, germination by opening of outer cyst wall, and release of prop- agules by degradation of inner cyst wall. Shown is the role of the outer and inner cyst walls in forming the peanut-shaped germination stages (see also modern mesomycetozoean examples in fig. S7). The outer cyst wall (seldom preserved) is indicated in black; the inner cyst wall dark is indicated in gray.

Their proposed explanation convinces me. These were protists that were single-celled in their free-living stage which would periodically grow hypertrophically and encyst, forming a capsule containing the dividing cells. These cells would replicate at differnt rates, forming zones of maturation; eventually, the cyst would rupture, released a cloud of propagules, or spores, and the life cycle would begin again.

That would explain a lot about the distribution of forms in these phosphatized specimens — we don’t find any gastrulating embryos because there never were any. These weren’t animals, period!

They belong outside crown-group Metazoa, within total-group Holozoa (the sister clade to Fungi that includes Metazoa, Choanoflagellata, and Mesomycetozoea) or perhaps on even more distant branches in the eukaryote tree. They represent an evolutionary grade in which palintomic cleavage served the function of producing propagules for dispersion.

That’s still very interesting, and again, it reminds us that the transition to multicellularity had many antecedents and could have been reached by many different paths.


Huldtgren T, Cunningham JA, Yin C, Stampanoni M, Marone F, Donoghue PC, Bengtson S (2011) Fossilized nuclei and germination structures identify Ediacaran “animal embryos” as encysting protists. Science 334(6063):1696-9.

(Also on FtB)

The eyes of Anomalocaris

Look with your puny camera eyes! Some new specimens of Anomalocaris, the spectacular Cambrian predator, have been discovered in South Australia. These fossils exhibit well-preserved eyes, allowing us to see that the bulbous stalked balls on their heads were actually fairly typical compound eyes, like those of modern insects.

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Anomalocaris eyes from the Emu Bay Shale. a-d, Eye pair, SAM P45920a, level 10.4 m. a, b, Overview and camera lucida drawing. Scale bars, 5 mm. Grey fill in b represents visual surface, the proximal part in the upper eye extrapolated from the lower eye. c, Detail of ommatidial lenses located by horizontal white box in a. Scale bar, 1 mm. d, More complete eye, showing transition between visual surface and eye stalk (white arrows). Scale bar, 2 mm. e, Detail of ommatidial lenses in counterpart SAM P45920b. Scale bar, 0.3 mm. es, eye stalk; I.c., Isoxys communis; us, undetermined structure; vs, visual surface. Tilted white box in a represents area analysed using SEM-EDS.

The cool part of this discovery: the investigators were able to count the density of lenses and estimate how many were present in the intact eye. The number is 16,000 ommatidia in each eye, which is more than a little impressive: to put it in context, Drosophila has about 800. The emphasis on high-resolution vision suggests that Anomalocaris was diurnal predator in shallow water.

Oh, and just in case you’re one of those strange beings who isn’t instantly familiar with what the anomalocarids looked like, here’s a video to remind you.


Paterson JR, García-Bellido DC, Lee MS, Brock GA, Jago JB, Edgecombe GD (2011) Acute vision in the giant Cambrian predator Anomalocaris and the origin of compound eyes. Nature 480(7376):237-40.

(Also on FtB)

Creationist abuse of cuttlefish chitin

A few weeks ago, PLoS One published a paper on the observation of preserved chitin in 34 million year old cuttlebones. Now the Institute for Creation Research has twisted the science to support their belief that the earth is less than ten thousand years old. It was all so predictable. It’s a game they play, the same game they played with the soft tissue preserved in T. rex bones. Here’s how it works.

Compare the two approaches, science vs. creationism. The creationists basically insert one falsehood, generate a ludicrous conflict, and choose the dumbest of the two alternatives.

The Scientific Approach

find traces of organic material in ancient fossils

Cool! We have evidence of ancient biochemistry!

Science!

The Creationist Approach

declare it impossible for organic material to be ancient

steal other people’s discovery of organic material in ancient fossils

Cool! Declare that organic material must not be ancient, because of step 1, which we invented

Throw out geology, chemistry, and physics because they say the material is old

Profit! Souls for my Lord Arioch!

You see, the scientists are aware of the fact that organic materials degrade over time, but recognize that we don’t always know the rate of decay under all possible conditions. When we find stuff that hasn’t rotted away or been fully replaced by minerals, we’re happy because we’ve got new information about ancient organisms, and we may also be able to figure out what mechanisms promoted the preservation of the material.

The creationists start with dogma — in this case, a false statement. They declare

Chitin is a biological material found in the cuttlebones, or internal shells, of cuttlefish. It has a maximum shelf life of thousands of years…

and

Because of observed bacterial and biochemical degradation rates, researchers shouldn’t expect to find any original chitin (or any other biomolecule) in a sample that is dozens of millions of years old–and it therefore should be utterly absent from samples deposited hundreds of millions of years ago. Thus, the chitin found in these fossils refutes their millions-of-years evolutionary interpretation, just as other fossil biomolecules already have done.

But wait. How do they know that? The paper they are citing says nothing of the kind; to the contrary, it argues that while rare, other examples of preserved chitin have been described.

Detection of chitin in fossils is not frequent. There are reports of fossil chitin in pogonophora, and in insect wings from amber. Chitin has also been reported from beetles preserved in an Oligocene lacustrine deposit of Enspel, Germany and chitin-protein signatures have been found in cuticles of Pennsylvanian scorpions and Silurian eurypterids.

So the paper is actually saying that the “maximum shelf life” of chitin is several tens of millions of years. And then they go on to describe…chitin found in Oligocene cuttlefish, several tens of millions of years old. The creationists are busily setting up an imaginary conflict in the evidence, a conflict that does not exist and is fully addressed in the paper.

The creationists do try to back up their claims, inappropriately. They cite a couple of papers on crustacean taphonomy where dead lobsters were sealed up in anoxic, water- and mud-filled jars; they decayed. Then they announce that there’s only one way for these cuttlebones to be preserved, and that was by complete mineralization, and the cuttlebones in the PLoS One paper were not mineralized.

…mineralization–where tissues are replaced by minerals–is required for tissue impressions to last millions of years. And the PLoS ONE researchers verified that their cuttlebone chitin was not mineralized.

Funny, that. You can read the paper yourself. I counted 14 uses of the words “mineralized” and “demineralized”. They state over and over that they had to specifically demineralize the specimens in hydrochloric acid to expose the imbedded chitin. And of course the chitin itself hadn’t been mineralized, or it wouldn’t be chitin anymore! Did the creationists lie, or did they just not understand the paper?

The scientists also do not claim that the chitin has not been degraded over time. They actually document some specific, general properties of decay in the specimens.

β-chitin is characterized by parallel chains of chitin molecules held together with inter-chain hydrogen bonding. The OH stretching absorbance, at about 3445 cm−1 in extant chitin, is diminished in the fossil and shifted to lower wavenumbers, showing that the specimen is losing OH by an as yet undetermined mechanism. The N-H asymmetric stretching vibration is shifted to slightly lower wavenumbers, showing that it is no longer hydrogen bonding exactly as in extant specimens. Changes in the region 2800-3600 cm−1 indicate that biomolecules have been degraded via disruption of interchain hydrogen bonds.

So, yes, the creationists seem to have rather misrepresented what the paper said. Here’s another blatant example of lying about the contents of the paper.

The question they did not answer, however, is why the original organic chitin had not completely fallen apart, which it would have if the fossils with it were 34 million years old…

Actually, they did. A substantial chunk of the discussion was specifically about that question, a consideration of the factors that contributed to the preservation. It was a combination of an anoxic environment, the presence of molecules that interfered with the enzymes that break down chitin, and the structure of cuttlebone, which interleaves layers containing chitin with layers containing pre-mineralized aragonite.

In vivo inorganic-organic structure of the cuttlebone, in combination with physical and geochemical conditions within the depositional environment and favorable taphonomic factors likely contributed to preservation of organics in M. mississippiensis. Available clays within the Yazoo Clay in conjunction with suboxic depositional environment may have facilitated preservation of original organics by forming a physical and geochemical barrier to degradation. One key to the preservation of organic tissues, in particular chitin and chitosan, is cessation of bacterial degradation within environments of deposition. Bacterial breakdown of polymeric molecules is accomplished through activities of both free extracellular enzymes (those in the water column) and ektoenzymes (those on the surface of the microbial cell) such as chitinases. Chitinases function either by cleaving glycosidic bonds that bind repeating N-acetyl-D-glucosamine units within chitin molecules or by cleaving terminal N-acetyl-D-glucosamine groups. These enzymes adsorb to the surface of clay particles, which inactivates them. Strong ions in solution like iron may act in the same manner. Once bound to functional groups within these polymeric molecules, Fe2+ ions prevent specific bond configuration on the active-site cleft of specific bacterial chitinases and prevents hydrolysis, thus contributing to preservation.

Organic layers within cuttlebones are protected by mineralized layers, similar to collagen in bones, and this mineral-organic interaction may also have played a role in their preservation. Specimens of M. mississipiensis show preserved original aragonite as well as apparent original organics. These organics appear to be endogenous and not a function of exogenous fungal or microbial activity. Fungi contain the γ form of chitin not the β allomorph found in our samples. Also, SEM analyses shows there is no evidence of tunneling, microbes, or wide-spread recrystallization of the aragonite. Therefore the chitin-like molecules detected in fossil sample are most likely endogenous. Similar to collagen in bone, perhaps, organics could not be attacked by enzymes or other molecules until some inorganic matrix had been removed.

Like I always say, never ever trust a creationists’ interpretation of a science paper: they don’t understand it, and they are always filtering it through a distorting lens of biblical nonsense. They make such egregious errors of understanding that you’re always left wondering whether they are actually that stupid, or that sleazily dishonest. Or both.

But imagine if the creationists hadn’t screwed up royally in reading the paper, if they had actually found an instance of scientists being genuinely baffled by a discovery that should not be. What if there was actually good reason to believe that chitin could not last more than ten thousand years?

Then the only sensible interpretation of this observation of 34 million year old chitin would be that the prior estimation of the shelf-life of chitin was wrong, and that it could actually last tens of millions of years. What the creationists want to do is claim that that minor hypothetical is actually correct, and that instead the entirety of nuclear physics, geology, radiochemistry, and modern cosmology is wrong. On the one hand, uncertain details about the decay of one organic molecule; in the other, entire vast fields of science, already verified, and with complex modern technologies built on their operation…and which hand would the creationists reject? The trivial one, of course.

I’m leaning towards “stupid” as the explanation for their bad arguments.


Oh, after I started this dissection, I discovered someone had already beaten me to it: here’s another analysis of the creationist misinterpretations.

(Also on FtB)

Traces of a Triassic Kraken?

At first I thought this discovery was really cool, because I love the idea of ancient giant cephalopods creating art and us finding the works now. But then, reality sinks in: that’s a genuinely, flamboyantly extravagant claim, and the evidence better be really, really solid. And it’s not. It’s actually rather pathetic.

It consists of the discovery of ichthyosaur vertebrae lying in a flattened array. They look like this.

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Photo shows shonisaur vertebral disks arranged in curious linear patters with almost geometric regularity. The arranged vertebrae resemble the pattern of sucker discs on a cephalopod tentacle, with each vertebra strongly resembling a coleoid sucker.

Wait, what? That’s it?

This work was presented at a meeting of the GSA under the title “Triassic kraken: the Berlin ichthyosaur death assemblage interpreted as a giant cephalopod midden “, with this argument:

“It became very clear that something very odd was going on there,” said McMenamin. “It was a very odd configuration of bones.”

First of all, the different degrees of etching on the bones suggested that the shonisaurs were not all killed and buried at the same time. It also looked like the bones had been purposefully rearranged. That it got him thinking about a particular modern predator that is known for just this sort of intelligent manipulation of bones.

“Modern octopus will do this,” McMenamin said. What if there was an ancient, very large sort of octopus, like the kraken of mythology. “I think that these things were captured by the kraken and taken to the midden and the cephalopod would take them apart.”

In the fossil bed, some of the shonisaur vertebral disks are arranged in curious linear patterns with almost geometric regularity, McMenamin explained.The proposed Triassic kraken, which could have been the most intelligent invertebrate ever, arranged the vertebral discs in double line patterns, with individual pieces nesting in a fitted fashion as if they were part of a puzzle.

Even more creepy: The arranged vertebrae resemble the pattern of sucker discs on a cephalopod tentacle, with each vertebra strongly resembling a coleoid sucker. In other words, the vertebral disc “pavement” seen at the state park may represent the earliest known self portrait.

Let me explain something here. This “Triassic kraken” has not been found; no fossils, no remains at all, no evidence of its existence. It is postulated to have been large enough to hunt and kill ichthyosaurs, which is remarkable—comparison to modern giant squid is invalid, since they are prey, not predator. This fossil bed is being over-interpreted as a trace fossil, with the bones arranged by intent, by an intelligent cephalopod, which they have not seen. Furthermore, a line of discs is being seen as a picture of a cephalopod tentacle, classic pareidolia. This is trivial: dump a pile of Necco wafers on a table, and I’ll see a picture of squid suckers. This is a whole series of tenuous and unlikely speculations stacked together to make an ultimately ridiculous hypothesis.

After I read the abstract and realization settled in that this was nonsense, something else was nagging me. That name, McMenamin — I’d heard it somewhere before. A little search, and there it was: I’ve encountered him tangentially before. He’s the geologist who so effulgently endorsed the imaginative pattern-spotting of Stuart Pivar. He also claims “that mariners of ancient Carthage made it to America long before Eriksson and Columbus, some time around 350 BC.”

I think I would concur with the idea that Mark McMenamin is exceptionally imaginative.

(Also on FtB)

Not like a worm?

Ann Coulter is back to whining about evolution again, and this week she focuses on fossils. It’s boring predictable stuff: there are no transitional fossils, she says.

We also ought to find a colossal number of transitional organisms in the fossil record – for example, a squirrel on its way to becoming a bat, or a bear becoming a whale. (Those are actual Darwinian claims.)

Darwin postulated that whales could have evolved from bears, but he was wrong…as we now know because we found a lot of transitional fossils in whale evolution. Carl Zimmer has a summary of recent discoveries, and I wrote up a bit about the molecular genetics of whale evolution. Whales have become one of the best examples of macroevolutionary transitions in the fossil record, all in roughly the last 30 years — which gives us a minimal estimate of how out of date Ann Coulter’s sources are.

But then she writes this, which is not only wrong, but self-refuting.

To explain away the explosion of plants and animals during the Cambrian Period more than 500 million years ago, Darwiniacs asserted – without evidence – that there must have been soft-bodied creatures evolving like mad before then, but left no fossil record because of their squishy little microscopic bodies.

Then in 1984, “the dog ate our fossils” excuse collapsed, too. In a discovery the New York Times called “among the most spectacular in this century,” Chinese paleontologists discovered fossils just preceding the Cambrian era.

Despite being soft-bodied microscopic creatures – precisely the sort of animal the evolution cult claimed wouldn’t fossilize and therefore deprived them of crucial evidence – it turned out fossilization was not merely possible in the pre-Cambrian era, but positively ideal.

And yet the only thing paleontologists found there were a few worms. For 3 billion years, nothing but bacteria and worms, and then suddenly nearly all the phyla of animal life appeared within a narrow band of 5 million to 10 million years.

It’s so weird to read that: yes, people have been predicting that the precursors to the Cambrian fauna would have been small and soft-bodied (what else would you expect), and that they would be difficult to fossilize…but not impossible, and further, scientists have been out finding these fossils. Somehow this is a refutation of evolution? What we’re seeing is exactly what evolution predicted!

What we have is a good record of small shelly fossils and trace fossils from the pre-Cambrian — before there were fully armored trilobites, there were arthropod-like creatures with partial armor that decayed into scattered small fragments of shell after death, and before that there were entirely soft-bodied, unarmored creatures that left only trackways and burrows. Even in this period Coulter wants to call abrupt, we find evidence of gradual transitions in animal forms.

And then to claim that there is an absence of transitional forms because all that was found were worms! Um, if you take an animal with an armored exoskeleton or bones, and you catch it before the hard skeleton had evolved, exactly what do you think it would look like? Like a worm.

As evolution predicted. As the evidence shows.

I can’t even guess what Ann Coulter was expecting a pre-Cambrian animal to look like. Not like a worm, apparently…but like what?

(Also on FtB)

Bacterial fossil doubts resolved

I raised a few questions about those 3.4 billion year old bacterial fossils, primarily that I was bugged by the large size and that they cited a discredited source to say that they were in the appropriate range of diameters for bacteria. Now my questions have been answered by Chris Nedin, and I’m satisfied. In particular, he shows data from 0.8 and 1.9 billion year old fossils in which the bacterial sizes are in the same range. It’s also a good review of the other evidence used to infer that they actually are bacterial microfossils.

(Also on FtB)

A questions about those ancient bacterial fossils…

Both Jerry Coyne and Larry Moran have good write-ups on the recent discovery of what are purportedly the oldest fossil cells, at 3.4 billion years old. I just have to add one little comment: a small, niggling doubt and something that bugs me about them. All the smart guys are impressed with this paper, but this one little thing gives me pause.

I’m a microscopist — I look at micrographs all the time, and one of the things I always mentally do is place the size of things in context. And I was looking at the micrographs of these fossils, and what jumped out at me is how large they are. They’re not impossibly large, they’re just well out of the range I expect for prokaryotes.

Most prokaryotes have diameters in the range of 1-10µm, while typical eukaryotes are about 10 times that size. There are exceptions: Thiomargarita gets up to 500µm across, so like I say, there’s nothing impossible about these cells, it’s just that the micrographs show lots of cells with 10-30µm diameters. And the authors come right out and report that:

The size range is also typical of such assemblages, with small spheres and ellipsoids 5-25 µm in diameter, rare examples (<10) of larger cellular envelopes up to 80 µm in diameter, and tubes 7-20 µm across (see ref. 24).

How odd. When I poke into the nervous system of an embryonic insect or fish, those are the sizes of cells I often see (well, except there aren’t many tubes of that size!). When I poke into a culture or embryo contaminated with bacteria, they’re much, much smaller. So maybe paleoarchaean bacteria tended to be larger? And they do cite a source for that size range of prokaryotes…

Then here’s a new problem: the source cited, ref. 24, is the Schopf paper, the older paper that claimed to have found ancient bacterial fossils, a claim that has since been discredited! Uh-oh. What they’re calling “typical of such assemblages” is a data set that’s widely considered artifactual now. Furthermore, that’s a simplified version of what Schopf said — he actually broke the sizes down into categories, and the range was more like 1-30 µm.

  • Very small solitary, paired or clustered rods (ca 0.75 µm broad, ca 1.5 µm long), inferred to be prokaryotic (bacterial) unicells: one unit (ca 2600 Myr old), one morphotype.
  • Small, solitary, paired or clustered coccoids (average diameter ca 3 µm, range ca 2-5 µm), inferred to be prokaryotic (bacterial, perhaps cyanobacterial) unicells: three units (range 3320-2600 Myr old), three morphotypes.
  • Large solitary or colonial coccoids (average diameter ca 13 µm, range ca 5-23 µm), inferred to be prokaryotic (bacterial, perhaps cyanobacterial) unicells: three units (range 3388-2560 Myr old), four morphotypes.
  • Narrow unbranched sinuous filaments (average diameter ca 1.25 µm, range ca 0.2-3 µm), with or without discernable septations, inferred to be prokaryotic (bacterial, perhaps cyanobacterial) cellular trichomes and/or trichome-encompassing sheaths: 10 units (range 3496-2560 Myr old), 17 morphotypes.
  • Broad unbranched septate filaments (average diameter ca 8 µm, range ca 2-19.5 µm), inferred to be prokaryotic (perhaps cyanobacterial) cellular trichomes: four units (range 3496-2723 Myr old), 10 morphotypes.
  • Broad unbranched tubular or partially flattened cylinders (average diameter ca 13 µm, range ca 3-28 µm), inferred to be prokaryotic (perhaps cyanobacterial) trichome-encompassing sheaths: five units (range 3496-2516 Myr old), five morphotypes (e.g. figures 3a-e and 4l).

So Schopf was reporting larger cells in his older samples, and now Wacey et al. are describing what look like very large cells to me in their 3.4 billion year old rocks. I’m not a microbiologist so I could be way off on this, but…isn’t this just a little bit strange? Maybe there are some micro people out there who can reassure me that this isn’t a surprising result.


Wacey D, Kilburn MR, Saudners M, Cliff J, and Brasier MD (2011) Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia. Nature Geoscience Published online Aug. 21, 20110 [doi:10.1038/ngeo1238]

(Also on FtB)

Magnificent momma

This is one beautiful plesiosaur, Polycotylus latippinus.

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(Click for larger image)

(A) Photograph and (B) interpretive drawing of LACM 129639, as mounted. Adult elements are light brown, embryonic material is dark brown, and reconstructed bones are white. lc indicates left coracoid; lf, left femur; lh, left humerus; li, left ischium; lp, left pubis; rc, right coracoid; rf, right femur; rh, right humerus; ri, right ischium; and rp, right pubis.

The unique aspect of this specimen is that it’s the only pregnant plesiosaur found; the fore and hind limbs bracket a jumble of bones from a juvenile or embryonic Polycotylus. It’s thought to actually be a fetal plesiosaur, rather than an overstuffed cannibal plesiosaur, because 1) the smaller skeleton is still partially articulated, and it’s large enough that it is unlikely it could have been swallowed whole, 2) the two sets are of the same distinctive species, 3) the juvenile is incompletely ossified and doesn’t resemble a post-partum animal, 4) the bones aren’t chewed, etched by acids, or accompanied by gastroliths. I think we can now confidently say that plesiosaurs were viviparous, which is what everyone expected.

There are other surprising details. The fetus is huge relative to the parent, and there’s only one — so plesiosaurs had small brood sizes and invested heavily in their offspring.

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(Click for larger image)

Reconstructions of female P. latippinus and newborn young. Gastralia were present in both animals but have been omitted for clarity.

The authors speculate beyond this a bit, but it’s all reasonable speculation. That degree of parental investment in fetal development makes it likely that there would have been extended maternal care after birth, and rather more tenuously, that they may also have lived in larger social groups. The authors suggest that their lifestyle may have resembled that of modern social marine mammals — picture a pod of dolphins, only long-necked and lizardy.


O’Keefe FR, Chiappe LM (2011) Viviparity and K-Selected Life History in a Mesozoic Marine Plesiosaur (Reptilia, Sauropterygia) Science 333 (6044): 870-873.

(Also on FtB)

Xiaotingia zhengi

A lovely new dinosaur fossil from China is described in Nature today: it’s named Xiaotingia zhengi, and it was a small chicken-sized, feathered, Archaeopteryx-like beast that lived about 155 million years ago. It shares some features with Archaeopteryx, and also with some other feathered dinosaurs.

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(Click for larger image)

a, b, Photograph (a) and line drawing (b). Integumentary structures in b are coloured grey. cav, caudal vertebra; cv, cervical vertebra; dv, dorsal vertebra; fu, furcula; lc, left coracoid; lfe, left femur; lh, left humerus; li, left ilium; lis, left ischium; lm, left manus; lp, left pes; lpu, left pubis; lr, left radius; ls, left scapula; lu, left ulna; md, mandible; rfe, right femur; rfi, right fibula; rh, right humerus; ri, right ilium; rm, right manus; rr, right radius; rt, right tibiotarsus; ru, right ulna; sk, skull; ss, synsacrum.

Now here’s why this particular fossil has some paleontologists in a dither. Systematics uses a set of objective, computer-based tools to objectively build phylogenetic trees: you plug a set of character parameters for a set of organisms into it, and it analyzes them and determines the most likely or most parsimonious tree to describe their relationships. Plugging in data from modern birds, Archaeopteryx, and dromeosaurs, for instance, generates trees in which Archaeopteryx clusters with the birds, and not the dromeosaurs. Archaeopteryx was not a direct ancestor of modern birds, but was thought to be related to the basal avians — so it was a kind of close cousin.

When Xiaotingia‘s data is tossed into the calculation, though, the results change. Xiaotingia doesn’t cluster so tightly with birds; it’s a more distant relative. However, Archaeopteryx shares enough significant features with Xiaotingia that they now cluster together, pulling Archaeopteryx out of the basal Aves and into a new classification. It says that Archaeopteryx is now a kind of second cousin, a little less closely related to the birds than previously thought.

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(Click for larger image)

Archaeopteryx has historically been regarded as the most basal bird (avialan), but the discovery of the closely related Xiaotingia led Xu et al.1 to pull these archaeopterygids out of avialans (birds) and into deinonychosaurs along with dromaeosaurids and troodontids. This new grouping better accounts for the evolution of feeding strategies among bird-like dinosaurs. Previous research suggested that herbivory was common among this group, as reflected in the tall, boxy skulls of oviraptorosaurs and basal avialans such as Epidexipteryx. The triangular, sharp-toothed skull of Archaeopteryx was incongruous among basal avialans, but fits better among the carnivorous dromaeosaurids and troodontids.

I have to say that I think it’s extremely cool that we have a new fossil from down around the roots of the bird family tree, and it does sharpen our knowledge of what was going on down there in the middle and late Jurassic. There was a whole assortment of delicate-boned, feathered, bipedal dinosaurs that were flourishing and diversifying in that window of time, and we’ve now got enough data that we can distinguish details in the family tree, which is absolutely fabulous.

However, a lot of the fuss over the specimen as somehow radically changing the importance of Archaeopteryx is a bit overblown. The relative status of Archaeopteryx and Xiaotingia is a bit of taxonomic detail — important details in working out the specific history of life — but it’s the equivalent of deciding that a fossil belongs in one pigeonhole rather than the pigeonhole next to it. Its shift in status means that there’s a bigger gap in the early history of the true birds than we thought, and it also means that there was a greater diversity of bird-like forms than we expected in the Jurassic. One other suggestion is that removing the carnivorous Archaeopteryx from the base of the bird family tree opens up the possibility that modern birds might have descended from the vegetarian side of the family — if the last common ancestor of birds was an herbivore, that has interesting implications for the paths evolution took.

But don’t worry, Archaeopteryx still represents a beautiful example of a transitional form. This new fossil is just another transitional form discovered. Creationists cannot take any consolation from it: Archaeopteryx isn’t suddenly gone, it’s become a part of a richer picture of bird evolution.


Xu X,
You H,
Du K
Han F (2011) An Archaeopteryx-like theropod from China and the origin of Avialae. Nature 475, 465-470.