PZ Myers. 2004 Jun 07. A few more tidbits about Vernanimalcula. <http://pharyngula.org/index/weblog/a_few_more_tidbits_about_vernanimalcula/>. Accessed 2008 Dec 04.

Posted on M00o93H7pQ09L8X1t49cHY01Z5j4TT91fGfr on Monday, June 07, 2004

A few more tidbits about Vernanimalcula

Schematic of different possible times of protostomedeuterostome divergence (orange circles), in relation to other events in the late Neoprotoerozoic. In scenario1 there is little incongruence between the fossil record and the time of divergence, which occurs following the second or Marinoan glacial interval. In scenario2, the divergence occurs between the two glaciations, and in scenario3, before the Sturtian glaciation. Both scenarios 2 and 3 imply a much longer missing interval of post-PDA history. A, Middle Cambrian Burgess Shale fauna; B, Lower Cambrian Chengjiang fauna; C, Diverse Ediacaran fauna; D, Doushantuo phosphorite assemblage (with poor age constraints shown by uncertainty in position).

• The name Vernanimalcula does not imply that there is a companion animal of species ernest, genus Worrelli. It means "Small Spring Animal", referring to its appearance after the great thaw of Snowball Earth, after the Marinoan glaciations.
  
• The Snowball Earth hypothesis is a rather controversial idea, supported by geological evidence, that the planet experienced extremely severe climate changes between 600 and 800 million years ago, and that there were periods of extensive glaciation lasting millions of years during that period.
  The diagram to the right illustrates the timing. Vernanimalcula is from the period labeled "D", when the Doushantuo phosphorite assemblage was laid down. It will also help you see where these fossils fall in relation to the Burgess shale (A), the lower Cambrian (B), and the Ediacaran (C).
  This diagram is from a paper by Erwin and Davidson (2002), in which they present models for the divergence of protostomes (P) and deuterostomes (D). In the Chen et al. paper, they clearly come down on the side of lineage (1), in which the protostomes and deuterostomes diverge shortly after the end of the last major glaciation before the Cambrian. They are ambiguous (and rightly so!) about whether Vernanimalcula belongs to the stem group for both protostomes or deuterostomes, or is an ancestor to just one or the other clade.
• Vernanimalcula is very, very rare. They found 10 specimens, but each specimen required examining 5-10 thousand other microfossils. Somebody was working hard, and I hope there are other interesting fossils in the 100,000 other specimens that were examined, that will be described later.
• We're lucky to have these specimens. They, like the embryo fossils that have been found in that region, were preserved by phosphatization, or direct deposition of phosphates on the structures of the original embryo. No one knows how this process works, or under what conditions it operates. Since the pre-Cambrian fauna seem to have all been small, delicate, and soft-bodied, unusual conditions are required to preserve them, and it is not surprising that they are scarce.

Aww, we need more pictures of this nifty animal. Here is a set of closeups of the fossils.

Close-up images of prominent anatomical features of Vernanimalcula guizhouena discussed in text. The scale bar represents 18 µm in A, 32 µm in B, 24 µm in C, and 28 µm in D. SO, Sensory Organ, i.e., external pit; LU, Lumen; PH, Pharynx; MO, Mouth; CO, Coelomic lumen; CW, mesodermal Coelomic Wall; GU, Gut. (A) Detail of collared mouth, multilayered pharynx, and one anterior surface pit. In this image, which is from the holotype specimen (Fig. 1 A), the floor of the pit can be seen to be composed of a specialized concave layer. Note the coelomic wall, which here as elsewhere in these specimens has a thickness of about 5-6 μm. (B) Mouth of a fourth specimen, Q3105, displaying collared mouth and pharynx, ventral view. (C) Lumen of pharynx from a fifth specimen, X10419, secondarily encrusted, but revealing morphology of opening of pharynx into gut similar to that seen in all specimens shown in Fig. 1. (D) Close-up of spaced external pits, interpreted as possible sensory organs, from the same specimen as shown in Fig. 1 B.

Note that at this scale, each of the blocky bricks that we're seeing that make up the structures is almost certainly a single cell. That's how small the whole animal is,and also gives you an idea of the amazing level of detail preserved in the specimens.

1. Great stuff, PZ.
   
   But this raises once more a question that came up a few weeks back: which came first, radial or bilateral symmetry?
   
   Radial seems like it would be "easier" to evolve, at the very earliest stages of multicellularity. And coelomate embryos show basic radial symmetry up to the point of gastrulation -- they're all little hollow cups, before they start growing heads and tails.
   
   Yet radial symmetry is found only in two major phyla, and appears to have evolved independently; and even the very ancient Cnidaria have bilateral members. And now you tell us that the oldest well-preserved metazoan fossil is bilateral.
   
   What should we think?
   
   
   Doug M.
   #: Posted by  on  06/08  at  01:41 AM
2. I think the problem here is that you are regarding radial symmetry as an absence of polarity. It isn't. Radially symmetry of the kind we see in echinoderms, for instance, is a highly derived characteristic. Another way to think of it is which is easier to evolve: an embryo with one or two specified axes, like this Vernanimalcula, or one with 5, like a modern starfish?
   
   I suspect that evolving responsiveness to a symmetry-breaking event is really easy to do, since it has occurred independently many times. Evolving a coordinated network of interactions that generates multiple axes of mirror symmetry is harder. When you get organisms more complex than a Volvox, that have to make a decision to generate a specialized cell type here but not there, you've got to generate an asymmetry, and bilaterality is fairly simple in that sense.
   
   And don't forget spiral cleavage in molluscs—they exhibit a chiral asymmetry right from the first division.
   #: Posted by PZ Myers  on  06/08  at  08:25 AM
3. PZ,
   
   I have no problem seeing radial symmetry in echinoderms as being highly derived and "difficult". It seems to be a rediscovery of radial symmetry, right? The best guess just now is that the ur-echinoderm was bilateral. And the underlying genetic mechanisms may be very different from the ones in cnidarians.
   
   But the cnidarians and the ctenophores seem to be much more primitive altogether. And they're mostly radial. Also, some sponges show a crude radial symmetry -- though of course, this may have evolved independently.
   
   Chiral asymmetry... seems like a bit of a red herring. It doesn't obviously lead to either radial or bilateral symmetry. We don't have any idea whether spiral cleavage is more primitive than the alternatives, right?
   
   Then there are the funny little Placozoa. Actually, in size and (inferred) habits, the Vernanimalcula sound a lot like placozoans. Except that placozoans aren't symmetrical at all... and they seem to have /derived/ asymmetry, since the molecular data shows that they make a clade together with the Bilateria.
   
   In sum: I'm still confused. My old HS biology textbook had a nice, simple, tree-of-life style progression -- asymmetry (sponges) to radial symmetry (cnidaria) to bilateral symmetry, ain't progress wonderful, pay no attention to the echinoderm behind the curtain. I realize now it's nowhere near that simple, but still: which sort of symmetry came first? Are we even close to knowing?
   
   
   Doug M.
   #: Posted by  on  06/08  at  02:27 PM
4. Just a question - can you point me to some information about "genus Worrelli" like what it is and how it got my name?
   
   Thanks, Dave Worrell
   #: Posted by  on  06/08  at  05:34 PM
5. Sorry, Dave, it was a joke: Ernest P. Worrell? The guy who was always saying, "Hey, Vern"?
   #: Posted by PZ Myers  on  06/08  at  08:04 PM
6. Don't worry, PZ, *I* got your reference.
   
   "Ernest Goes To The Vendian", anyone?
   #: Posted by  on  06/08  at  10:35 PM