PZ Myers. 2004 Jul 22. Spectacular echinoderms from the Lower Cambrian. <http://pharyngula.org/index/weblog/spectacular_echinoderms_from_the_lower_cambrian/>. Accessed 2008 Dec 04.

Posted on M00o93H7pQ09L8X1t49cHY01Z5j4TT91fGfr on Thursday, July 22, 2004

Spectacular echinoderms from the Lower Cambrian

Pretty! and a neat discussion of deuterostome phylogeny, too.

This week's Nature includes a wonderful description of an exotic group of deuterostomes from the lower Cambrian, 520 million years ago. Deuterostomes are animals characterized by their embryology: when they gastrulate, the site of closure of the migrating tissues, the blastopore, becomes the anus of the animal. This is in contrast to the protostomes, in which the blastopore becomes the mouth. We chordates are deuterostomes, as are echinoderms, some marine worms called hemichordates, and the urochordates, or sea squirts.

Shu et al. have identified some animals called vetulocystids, and they are most closely related to modern echinoderms. Echinoderm evolution is confusing and complicated, largely because the modern forms are so highly derived and distinct from the ancestral forms, and because the echinoderm lineage has been spectacularly diverse morphologically. The authors jump from some somewhat speculative interpretations of the fossil anatomy (the specimens, as you can see below, are peculiar and the structures difficult to identify) to an idea that clarifies the organization of the deuterostome lineage.

Vetulocystis was a strange little creature, looking like a baggy blob on a stalk. The bagginess is probably an artifact of fossilization, and what it suggests is that the body was not calcified, unlike modern echinoderms, and was instead soft and easily deformed. The stalk was a tail-like derivative, and although this animal was probably a semi-sessile filter feeder, the tail may have allowed it to move slowly. The tail also bears indications of segmentation. Shu et al. also think a thin strand through the tail is the intestine, and even see signs of fecal material in it. (Think about that. How many weblogs do you read that have pictures of half-billion-year-old echinoderm poop?)

The authors are trying to puzzle out the evolutionary tree of the deuterostomes. We have molecular data, and now fossil evidence, that they are using to sort out these questions.

One of the principal unsolved questions in metazoan phylogeny concerns the origin and earliest evolution of the echinoderms. Molecular data indicate echinoderms to be the sister group of hemichordates, but the two phyla are so disparate that attempts to envisage a common ancestor are highly conjectural. In large part this is because the echinoderm body plan has undergone radical reorganization. This includes a pervasive pentamery, a unique water vascular system and calcitic stereom (mesodermal skeleton), loss of pharyngeal gill slits, and major redeployment of developmental genes. Whereas the Cambrian record of echinoderms is otherwise fairly extensive, many of the early forms have a bizarre appearance, and are the subject of continuing phylogenetic controversy. In part this is because of unresolved differences of opinion concerning possible similarities (for example, gill slits) to other deuterostomes.

So where do vetulocystids, modern echinoderms, and we vertebrates fit into the picture? Here is a tentative diagram of our family tree. (There's lots of detail here that just isn't resolvable at the scale of images here; click on it to get a bigger, readable picture.)

The vetulocystids are a branch off the echinoderm family tree between modern forms, with their highly derived traits of radially symmetric body plans, and the tailed, wormlike hemichordates. The diagram shows that the things we see as so characteristic of echinoderms, such as the calcareous skeleton, lack of segmentation, pentameric organization, and absence of gill slits are all radical innovations within this one branch of the tree. They are arguing that Vetulocystis represents an intermediate form between the primitive tailed, gilled worm and the derived squat starfish/sea urchin. It's not a definitive answer yet, though, and the authors admit that there are a few matters that still need to be clarified. In particular, they need better material to confirm that the strand they observe is the intestine, and to see if the tail is actually segmented.

In this phylogeny the vetulocystids mark a key stage in the early evolution of the Ambulacraria, specifically marking the transformation from an active vetulicolian-like organism to a semi-sessile bipartite animal which pre-figures the homalozoan echinoderms. With respect to the vetulicolians, important changes include the reduction of the respiratory organ (assumed to be equivalent to the pharyngeal opening of other deuterostomes) to a single structure adjacent to the tail, and restriction of the gut to the theca with concomitant development of oral and anal cones. On the assumption that the vetulocystids provide a key link between the vetulicolians and the homalozoans, two questions depend on presently equivocal evidence. The first revolves around the interpretation of the posterior strand as a possible intestine. If correct, this suggests that the restriction of the alimentary canal to the anterior theca took place either in vetulocystids more advanced than V. catenata or in the earliest homalozoans. The second question concerns the evidence for posterior segmentation in some vetulocystids, and the obvious differences to the complex and variable tail structures seen in cinctans, solutes and stylophorans.

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Shu D-G, Conway Morris S, Han J, Zhang Z-F, Liu J-N (2004) Ancestral echinoderms from the Chengjiang deposits of China. Nature 430:422-428.

1. "The authors are trying to puzzle out the evolutionary tree of the deuterostomes." That must be because there is no evolutionary tree for these - they were put there by a mythical trickster deity a couple of hundred years ago so that fundamentalists could get all self-righteous and stupid.
   
   ONLY KIDDING
   
   I love this blog, it really expands my mind. Thanks.
   #: Posted by Andrew  on  07/22  at  03:57 PM
2. And we've got half-billion-year-old echinoderm poop! You won't see that from Atrios or Instapundit or any of the other bigtime bloggers, that's for sure.
   #: Posted by PZ Myers  on  07/22  at  04:00 PM
3. Oh, sorry for the emoticon btw. Typed it without thinking about hour hieroglyphophobia and I apologize for polluting your site with cartoons.
   
   Man, I so want to end that sentence with a smiley! Must .... resist...
   #: Posted by Andrew  on  07/22  at  04:07 PM
4. My own software betrays me there...I tried deleting the smiley file, but then it ends up punctuating people's comments with the "missing image" icon. I'll survive it.
   #: Posted by PZ Myers  on  07/22  at  04:12 PM
5. As we say here, old poos is good poos
   
   
   
   [Let's see what sort of emoticon that generates]
   #: Posted by  on  07/22  at  09:32 PM