Talk: Leech

Oxygen Levels & Ancestral Speculation

I found many articles that stated that the oxygen levels during the Cretaceous (and the Mesozoic overall) were on average, much higher than today (around 35% compared to today's 21%). Here is a link to an article from geology.com that was a republished release of information from the United States Geological Survey. Here is the official link from the USGS. The Proceedings of the National Academy of Sciences of the United States of America (PNAS) also had a chart and article discussing oxygen levels over the past 550 million years, which showed higher oxygen levels during the Cretaceous period. Also, high oxygen levels are hypothesized to be a primary cause for the gigantism of dinosaurs and other Mesozoic life. I was also wondering if you could provide the source of the chart you provided, or other sources that support lower oxygen levels during the Cretaceous. I'm not trying to start a heated debate, I'm just hoping that this can be a healthy, scientific discussion, so we can determine what could be canon for the Wiki.

As for the ancestral speculation that I had made, would you be accepting of the idea if it were listed under a new "speculation" section?

Ca'dli 17:12, October 8, 2011 (UTC)

It looks like different studies give different reconstructions of oxygen levels, maybe we should just link to a few in a "speculations" section--the bottom chart on this page has a graph showing two reconstructions (one with lower oxygen in the Cretaceous and one with higher), the first chart in this paper shows three different reconstructions, here is a graph from this 2007 paper which I found on this page (and which is reproduced on this page from a book on oxygen and evolution), and here is a graph (slightly larger version on p. 31 of this book) from a 2009 study by one of the same authors as the 2007 paper which now shows slightly higher oxygen in the Cretaceous. It looks like none of these reconstructions show Cretaceous oxygen as being nearly as high as during the Carboniferous period when dragonflies and centipedes and other arthropods grew to enormous sizes (see here), though, and I don't think there are any fossils of giant invertebrates from the dinosaur age, so that should be mentioned too. As for ancestry, it could be a leech ancestor or it could be an evolutionary "cousin" from a lineage that went extinct, there's no reason to favor either possibility given the onscreen evidence so if you mention one speculation I think you should mention both, put them in if you want but it seems a little pointless to me since these possibilities are true for a lot of extinct creatures (you could say the same thing about the centipede for example). Hypnosifl 18:30, October 8, 2011 (UTC)

Incidentally, if we're going to discuss oxygen levels on some page it might be better to do it on the Centipede page, since I've definitely read the issue of size being limited by oxygen applies to arthropods but I'm not so sure it applies to other invertebrates like leeches--it certainly doesn't seem to apply to worms, because the largest worm alive today is the African giant earthworm or Microchaetus rappi which can grow to up to 22 feet long! And there are actually some pretty giant leeches in the modern world too, look at this video if you have the stomach for it! Hypnosifl 00:17, October 9, 2011 (UTC)

Also, I looked for some more info about the USGS study you linked to involving studying air bubbles in amber, apparently the methods used by that scientist (Landis) are pretty controversial and a lot of other scientists totally discount the idea that they are actually measuring air that's been trapped 67 million years--see this article and this one. The estimate of 35% oxygen levels does seem to be much higher than the estimates of any of the studies I linked to, including the ones that did say oxygen levels were somewhat higher in the Cretaceous (35% is about the level usually estimated for the Carboniferous). And the PNAS study you liked to showed a figure of only slightly over 25% in the early Cretaceous, and declining from there...also that study is from 1999 and is by Robert Berner, who also did the more recent 2007 and 2009 studies I linked to above. Hypnosifl 00:33, October 9, 2011 (UTC)

Firstly, Hypnosifl, I'd like to thank you for providing those articles! They were very informative and interesting! Secondly, I would like to apologize for the late reply, the site had been giving me trouble connecting today (it kept telling me the site was down for maintenance).

I thought that I had looked into this subject more in the past, and in the research in my comments above, but I see now that I clearly had not done enough research. You brought up a number of good points and I have to agree with you now. Although I'm studying geology and paleontology, I hadn't really looked into the climates and atmospheric compositions of the deep past. I knew that the Carboniferous was always really high in oxygen composition, but from previous readings, documentaries, and scientific shows discussing the Mesozoic, I had always remembered hearing that the oxygen levels were suppossed to be much higher than today. I do understand now, especially from the research articles you provided, that the climate and atmospheric composition of the Cretaceous is the subject of heavy debate. As you pointed out, some articles showed estimates for both higher and lower oxygen levels in the Cretaceous, so I do thank you for editing the page, as at the moment, we do not have enough scientific evidence to definitively support one theory or the other (perhaps we should try and make a real Terra Nova to settle the debate once and for all. . .).

On the ancestral speculation, you also make very good points. As you said, if one speculation were to be included, then both should be, but since that could be said for basically any species, it's probably just not worth it. Entertaining video of the enormous modern-day leech; I would like to compare that species to the prop used in the show! From my perspective, the fictional leech looked almost the same length as the one in the video, but the one in Terra Nova seemed to have more depth to it.

On the PNAS study, I must agree, I thought the claim seemed just a bit out there when I read it, but I overlooked common sense because it was supportive of what I had heard (rather a poor decision on my part). And although the estimated 25% is higher than the current 21%, and it was declining, it still would have been higher than today's oxygen levels. However, as you stated, and I must agree to, it seems that there are a number of conflicting reports, and as it seems that there is no clear winner to the debate, the comment that I had made about higher oxygen levels didn't belong in the article. On that note, perhaps, just as with the ancestral speculation, speculating about the oxygen levels should probably just be kept out of the article since it is such a contested issue.

I know I am dragging this reply out, but I did have a final question. As I am studying to become a paleontologist, I was wondering, are you studying a similar/same field, are already in a similar/same field, or was this just something that you had looked up/have an interest in?

Again, I agree with the points you have made, and am glad to know that we were able to have a healthy and informative discussion! Thanks for your time and the information (as well as the articles, I already plan on reading them again sometime in the future, they were quite stimulating) !

Ca'dli 01:07, October 10, 2011 (UTC)

Hey Ca'dli, thanks for the detailed response, and I'm glad you found the articles interesting. That's awesome that you're studying paleontology! I'm studying physics, but there are a few other subjects in science I like to read about on my own, and I've always really been fascinated by evolution and the history of life so I read a lot about that. And I get involved in a lot of science discussions online so I've gotten pretty good at tracking down studies and such with google. Also, I already knew a little about prehistoric oxygen levels because I had read a very interesting book called Out of Thin Air by Peter Ward which talks about theories about how changing oxygen levels have had a big influence on evolution and extinctions. Ward's a fascinating author and I also recommend his books The Life and Death of Planet Earth which is about what can be predicted about the future of evolution on Earth based on geological and climate changes that are expected over the next few billion years, and Rare Earth which is about a theory that says that although single-celled life may be common throughout the universe, the conditions needed to get multicellular organisms might be very rare. Oh and have you ever seen Carl Sagan's Cosmos series? That show had a great mix of a lot of different subjects too--astrophysics, evolution, the history of science, the workings of the brain, etc. If you're in the US I think all the episodes are available on Hulu here. Anyway, good talking to you and I'm glad to have a fellow science geek working on this wiki ;) Hypnosifl 17:49, October 10, 2011 (UTC)

I believe that I have looked at both of Peter Ward's books in stores and online, but I have never had a chance to purchase/read them! And they looked very interesting, especially Out of Thin Air. Today I was looking through some of my books that I have on geology, biology, and paleontology and I started to remember why I became so passionate about paleontology in the first place, and I thank you for this great discussion, as I believe that our discussion was a strong contributing factor for me to look at my books again! That's great you're studying physics, it's such an amazing field! After biology, I feel that I thoroughly enjoyed my physics courses the most in comparison to the other science courses I took. And it's great to hear that you've been active in good scientific discussions, I've been hoping to find sites for such discussions, but haven't had the time or much luck. Hopefully I can find some soon! I didn't watch that much of Carl Sagan's Cosmos when I was younger, but I do watch it occassionally, both on DVD and on television. I do really enjoy watching Cosmos when I get the chance, you're right, it covers a wide variety of subjects. Aside from paleontology, I've become fascinated with astronomy, astrobiology, astrogeology, marine biology, and hominid evolution. I've also developed an obsession with Mars and the thoughts of colonization; I blame Kim Stanley Robinson's Mars Trilogy (it covers a lot of topics, but it's quite an amazing series in my opinion). Did you hear about Neil deGrasse Tyson's new cosmos series coming out in 2013? I've always liked what Tyson has had to say on the shows he appears on, and personally, I can't think of anyone better to host a successor to Sagan's amazing show! And agreed, great talking with another contributor here who shares similar scientific interests! Ca'dli 21:10, October 10, 2011 (UTC)