tag:blogger.com,1999:blog-37148773.post4036715011088030562..comments2024-03-27T14:50:47.345-04:00Comments on <center>Sandwalk</center>: Pikaia is most primitive vertebrate knownLarry Moranhttp://www.blogger.com/profile/05756598746605455848noreply@blogger.comBlogger10125tag:blogger.com,1999:blog-37148773.post-45472859751407693902012-03-09T12:55:19.856-05:002012-03-09T12:55:19.856-05:00I use the word "primitive" to describe a...<i>I use the word "primitive" to describe ancient organisms. In that sense, it's the opposite of "living" or "extant." Thus, Pikaia is a primitive chordate and Myllokunmingia is a primitive vertebrate. </i><br /><br />You can do that, but be aware that the biologists who deal with ancient organisms (e.g. systematists and paleontologists) don't. "Primitive" is just the inverse of "derived". That is, it refers to conditions rather than age. <br /><br />You have a point that it's probably bad to talk about whole organisms as primitive. Every organism has derived features *unless* we are talking about the actual ancestor of some other organism, which must have the primitive state for every character by which it differs from the descendant. Since we can't detect ancestors, that's moot. However, it's not extremely bad to say that organisms displaying a great many primitive traits are themselves primitive.<br /><br />Watch out for that word "advanced". It has connotations of inevitable progress, and so is avoided these days in preference to "derived", which doesn't. "Primitive" isn't pejorative either.<br /><br />Pikaia, though younger than any of the Chengjiang chordates, seems (according to Conway Morris's paper) to have the primitive state for a great many observable characters for which the Chengijiang animals have derived states. It makes some sense to label it as primitive, its presumed autapomorphies notwithstanding. As for the genomes, we have no access to them, but would it help to say "morphologically primitive"? And what you say is in fact the same thing that Conway Morris said, if you understand the usage in paleontology.john harshmannoreply@blogger.comtag:blogger.com,1999:blog-37148773.post-53313547870156653132012-03-09T09:58:54.583-05:002012-03-09T09:58:54.583-05:00I use the word "primitive" to describe a...I use the word "primitive" to describe ancient organisms. In that sense, it's the opposite of "living" or "extant." Thus, <i>Pikaia</i> is a primitive chordate and <i>Myllokunmingia</i> is a primitive vertebrate. <br /><br />I have no problem with identifying a fossil as the oldest primitive vertebrate or the oldest primitive chordate but that relies on dating fossils and not on some arbitrary choice of evolved characteristics. <br /><br />I have a problem identifying one extant species as being more "primitive" than another. I shudder, for example, when people suggest that chimps are more primitive than humans or salmon are more primitive than leopard frogs. All extant species have evolved for the same length of time and just because you can identify that some external features look more like the ancient common ancestor does not mean you can label the entire organism as more "primitive." It could be the case that the enzymes of the citric acid cycle are much more "advanced" in the so-called "primitive" organism.<br /><br />The <i>Pikaia</i> of the Burgess Shale lived millions of years after vertebrates diverged from the other chordates and so there are plenty of fossil species that are older than <i>Pikaia</i>. If we could examine their genomes we would find that those other species are a lot closer to the common ancestor than <i>Pikaia</i>. It does not make sense to label <i>Pikaia</i> as the most primitive chordate because we know for certain that this isn't true when you take into account all the evolutionary changes that must have occurred in the lineage leading from the real oldest chordate to <i>Pikaia</i>.<br /><br />It would be acceptable to say that certain features of <i>Pikaia</i> probably resemble the oldest chordate more than the same features in <i>Myllokunmingia</i> but that's not the same thing as what Conway Morris said.Larry Moranhttps://www.blogger.com/profile/05756598746605455848noreply@blogger.comtag:blogger.com,1999:blog-37148773.post-84724239047721970472012-03-08T19:29:05.410-05:002012-03-08T19:29:05.410-05:00...the most primitive known vertebrate and therefo...<i>...the most primitive known vertebrate and therefore the ancestor of all descendant vertebrates...</i><br /><br />That's a weird conclusion...Hawkshttps://www.blogger.com/profile/11246883471860150444noreply@blogger.comtag:blogger.com,1999:blog-37148773.post-75563743286263709402012-03-08T17:08:41.857-05:002012-03-08T17:08:41.857-05:00Strictly speaking, "primitive" applies t...Strictly speaking, "primitive" applies to character states, not organisms or species. It is however reasonable to say that an organism that displays many primitive character states is itself primitive, especially if it displays very few derived states, as would for example be the situation in an ancestor, if we could identify ancestors, which we can't. There is no particular reason to suppose that the fossil displaying the greatest number of primitive states should be the oldest, unless you think the fossil record is very good or that we are likely to find and identify ancestors.<br /><br />But Conway Morris's use is hardly unusual or odd. It would be very odd indeed if he had said "primitive" and meant "old". When you say "Pikaia is younger than several primitive vertebrate", what exactly do *you* mean by "primitive"? I don't believe you're thinking this through.john harshmannoreply@blogger.comtag:blogger.com,1999:blog-37148773.post-66957060094314090822012-03-08T12:05:14.809-05:002012-03-08T12:05:14.809-05:00"The press release made the mistake of equati..."The press release made the mistake of equating "primitive" with "old". I hope you aren't making that same mistake."<br /><br />I did make that mistake. Conway Morris is using a strange definition of primitive if that's what he meant. <i>Pikaia</i> is younger than several primitive vertebrate so by saying that it is "the most primitive chordate" is like saying that hammerhead sharks are more primitive than elephants or that lemurs are the most primitive primates.Larry Moranhttps://www.blogger.com/profile/05756598746605455848noreply@blogger.comtag:blogger.com,1999:blog-37148773.post-993280293320390742012-03-08T05:11:29.286-05:002012-03-08T05:11:29.286-05:00Larry, you are correct. The body of the press rel...Larry, you are correct. The body of the press release is also wrong. <i>Haikouichthys</i> has vertebrate morphology and if not a vertebrate via the cladistic definition, it is at least more closely related to living vertebrates. And it is older than <i>Pikaia</i>. Thus, it is impossible for <i>Pikaia</i> to be our direct ancestor.<br /><br />John W.<br />@anaxyrusAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-37148773.post-20295361434123863782012-03-07T20:28:16.729-05:002012-03-07T20:28:16.729-05:00There are some interesting things about this new d...There are some interesting things about this new description of Pikaia.<br /><br />Firstly, it is clearly not a cephalochordate. It has features more primitive than Amphioxus, including potentially external gills and a terminal anus (rather than a postanal tail).<br /><br />Secondly, the myomeres appear to be sigmoidal (W-shaped) leading to the possible conclusion that the V-shaped myomeres of Amphioxus are a derived, rather than primitive, condition.<br /><br />The new discovery that tunicates are closer to vertebrates than is Amphioxus has freed us from the constraint to look for sessile chordate ancestors. The new interpretation of Pikaia gives us some better insights as to the likely primitive chordate condition, and makes us more aware that Amphioxus is not a "transitional form" but an extant animal with potentially many of its own derived features that don't represent the primitive chordate condition.<br /><br />Christine Janis (aka paleobarbie) --- apologies about posting as anonymous but my university google system is blocking me from signing in any other way.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-37148773.post-4374227073123673592012-03-07T20:23:57.773-05:002012-03-07T20:23:57.773-05:00From what I can get from the abstract (I don't...From what I can get from the abstract (I don't have full-text access at the moment), SC-M is using the word "primitive" to mean plesiomorphic, not chronologically old. He said <i>Pikaia</i> could have the most stem-ward position (closest to the origin of the clade) among chordates. That is before the cephalochordate-urochordate split. It's possible that the resemblance between <i>Pikaia</i> and cephalochordates is mostly symplesiomorphic.Geoxushttps://www.blogger.com/profile/00480560335679211508noreply@blogger.comtag:blogger.com,1999:blog-37148773.post-44070287530266182222012-03-07T19:03:49.780-05:002012-03-07T19:03:49.780-05:00Conway Morris doesn't say that Pikaia is the o...Conway Morris doesn't say that <i>Pikaia</i> is the oldest chordate. He says it's the most primitive known chordate, and he is apparently using a branch-based definition of "chordate": any deuterostome more closely related to a vertebrate than to a starfish. Or, to put it another way, the phylogenetic analysis in the new <i>Biological Reviews</i> paper puts <i>Pikaia</i> as the sister group of all other chordates.<br /><br />The press release made the mistake of equating "primitive" with "old". I hope you aren't making that same mistake. Later we can discuss the mistake of equating "the less diverse of two sister groups" with "primitive".John Harshmannoreply@blogger.comtag:blogger.com,1999:blog-37148773.post-34738031188621089452012-03-07T14:52:32.904-05:002012-03-07T14:52:32.904-05:00I read Wonderful Life too and so when I heard abou...I read Wonderful Life too and so when I heard about this press release I assumed that the paleontologists had been working to improve the identification of Pikaia as a chordate; even I could tell from reading Wonderful Life that the idea was, how should I say, not well supported at that time. And it looks like that was exactly what happened. So "Old news" is a bit harsh, I think.<br /><br />As for the rest: from my reading of the press release, Conway Morris and Caron et al. have only provided better evidence that Pikaia was a chordate; I don't see either of them claiming it was the "earliest". That looks like another embellishment by the writer of the press release.Paul Claphamhttps://www.blogger.com/profile/02446087684049798691noreply@blogger.com