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The Haematothermia hypothesis

The views expressed are those of the author and are not necessarily those of Scientific American.


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Here’s a slightly modified version of a Tet Zoo classic (from ver 2, first published March 2008)…

From Janvier (1984). From the cover of Mundo Cientifico, no less.

Molecular, morphological and behavioural data convincingly demonstrates that birds are deeply nested within the amniote clade that also includes crocodilians, squamates and turtles – the clade most typically termed Reptilia. Birds are thus substantially removed from mammals, and indeed the bird and mammal lineages diverged as long ago as the Carboniferous (more than 320 million years ago). This is non-controversial and has been considered ‘mainstream’ for many decades now (it’s depicted in the simplified phylogeny shown below). However, the endothermic physiology present in both groups and their generally similar four-chambered hearts and epidermal, insulative structures (hairs and feathers, respectively) led pioneering Victorian anatomist Richard Owen to propose that both groups might be especially close relatives (Owen 1866). Imagining that both might have shared a common ancestor*, he used the name Haematothermia for his hypothesised bird-mammal clade.

* As is well known, Owen did not agree with Darwin’s concept of evolution. However, he definitely thought that a transmutation of sorts had occurred through time, and that organisms were allied through descent from shared ancestors.

Simplified tetrapod cladogram, showing the consensus topology supported by a substantial amount of morphological, molecular and behavioural data. Birds are part of Dinosauria.

So, according to this haematotherm model, birds and mammals are sister-taxa, united by their endothermy, fully divided heart, respiratory turbinates, nerve and vascular characters, and so on. The best known proponent of this concept has been Brian Gardiner; he published a few reasonably lengthy papers on the subject in high-impact journals, the best known of which is Gardiner (1982). Unfortunately, Gardiner has since become best known for this above all else, whereas his writings on vertebrate phylogeny in general, Piltdown, and on Darwin’s correspondence should be better known. I met him a few years ago (2002 I think) when he acted as external examiner for Alberto Vasconcellos’s Phd thesis at the University of Portsmouth, and he was still very keen on the idea then, arguing that if I was happy with the ‘traditional’ version of amniote relationships then that was up to me (i.e., I was stupid).

The whole thing. The original is in colour, but I don't own a copy of that.

Within post-Victorian times, Gardiner isn’t, however, responsible for resurrecting the whole haematotherm concept. Danish embryologist Søren Løvtrup published on the hypothesis a few years earlier (Løvtrup 1977), and later published a paper further supporting the proposal (Løvtrup 1985)*. Both Løvtrup and Gardiner cited and discussed observations made by John Ray in 1693 and Owen in 1866, both of whom supported the idea of a bird-mammal group that did not include other tetrapods (yes, I said 1693 and 1866). Neither Løvtrup nor Gardiner used Owen’s term Haematothermia; instead, they went with the alternative spelling Haemothermia.

* I have only recently become aware of the fact that Løvtrup is best known as a staunch critic of evolutionary theory; he has argued that evolution does not proceed as proposed by Darwin, instead occurring via substantial saltational events known as macro-mutations.

As was later discussed by a whole string of authors (e.g., Gauthier et al. 1988a, b, Kemp 1988, Benton 1985, 1991), one can only conclude that birds and mammals are especially close relatives within Tetrapoda by ignoring and excluding a vast amount of contradictory data. Løvtrup and Gardiner both ignored fossils, relied predominantly on soft tissue characters, and included only a handful of characters (literally, three or four) that contradicted their favoured topology and supported the traditional one: neither author included or discussed the huge number of bony and soft tissue characters that unite crocodilians and birds, for example. Furthermore, nearly all of the haematotherm ‘synapomorphies’ could be shown to be more widely distributed than proposed, non-homologous, or just plain wrong (e.g., Benton 1985, pp. 103-106).

Hypothetical 'stem-haematotherm', with roach in hand, from Janvier (1984).

I admit to being a huge fan of what are politely termed non-standard hypotheses. I’m also a big fan of speculative zoology. If you combine an interest in these two areas, one obvious question stands out: what would an ancestral haematotherm look like? In a parallel universe where mammals and birds share a direct ancestor, we need to imagine a creature that somehow combines the traits of both. Herein we find the origin of the weird creature shown in the accompanying illustrations: this is the ancestral haematotherm, as illustrated for an article on the hypothesis published by Janvier (1984). I regret that I don’t know the name of the artist (I will add this when I find out).

I originally posted pictures of this animal on Tet Zoo with the invitation that people try and guess what it is. Suggestions included a squabrat from The Dark Crystal (if there is such a thing), Romer’s hellasaur, an old picture of a colugo, a proto-bat, proto-pterosaur, arboreal theropod, antiquated archaeopterygid, treeshrew, climbing duck-possum, arboreal gorgonopsian, proto-ropen, or one of Dougal Dixon’s arbrosaurs. Great stuff; the Tet Zoo readership did me proud.

‘Alternative’ proposals like the haematotherm concept can be a good thing because they force workers to tighten up ‘traditional’ models and to marshal a more convincing supporting data set, but they can also cause people to waste a lot of time when they could be doing something far more useful.

For previous Tet Zoo articles on ‘non-standard hypotheses’, see…

Refs – -

Benton, M. J. 1985. Classification and phylogeny of the diapsid reptiles. Zoological Journal of the Linnean Society 84, 97-164.

- . 1991. Amniote phylogeny. In Schultze, H.-P. & Trueb, L. (eds) Origins of the Higher Groups of Tetrapods: Controversy and Consensus. Cornel University Press (Ithaca, London), pp. 317-330.

Gardiner, B. G. 1982. Tetrapod classification. Zoological Journal of the Linnean Society 74, 207-32.

Gauthier, J. A., Kluge, A. G. & Rowe, T. 1988a. Amniote phylogeny and the importance of fossils. Cladistics 4, 105-209.

- ., Kluge, A. G. & Rowe, T. 1988b. The early evolution of the Amniota. In Benton, M. J. (ed) The Phylogeny and Classification of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds. Clarendon Press (Oxford), pp. 103-155

Janvier, P. 1984. El divorcio del ave y del cocodrilo. Mundo Cientifico 32, 14-16.

Kemp, T. S. 1988. Haemothermia or Archosauria? The interrelationships of mammals, birds, and crocodiles. Zoological Journal of the Linnean Society 92, 67-104.

Løvtrup, S. 1977. The Phylogeny of Vertebrata. John Wiley, London.

- . 1985. On the classification of the taxon Tetrapoda. Systematic Zoology 34, 463-470.

Owen, R. 1866. On the Anatomy of Vertebrates, Volume 2. Longmans Green and Co., London.

Darren Naish About the Author: Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com! Follow on Twitter @TetZoo.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. vdinets 6:34 am 10/3/2012

    At least the fact that such theories still get published shows that natural sciences are not ruled by evil establishment with a fixed paradigm.

    Link to this
  2. 2. John Harshman 10:14 am 10/3/2012

    Darren,

    1) I see you left turtles out of your tree as a clever avoidance tactic.

    2) Have you ever read Løvtrup’s book, Darwinism: The refutation of a myth? It’s actually rather interesting, in part. About half of it consists of historical invective proving that Darwin wasn’t a very nice guy, and the other half is a critique of the explanatory power of natural selection together with an attempt at a real theory explaining evolution. I don’t actually remember any saltationism, just some laws relating ecology to phylogeny. I actually think he had a little bit of a point (about explanatory power, not Darwin), and that we should really be wondering if there are any principles or patterns to be noticed. But I don’t think Løvtrup’s principles actually make any sense. Still, he was groping (unsuccessfully) after something we should be thinking about.

    Link to this
  3. 3. naishd 10:38 am 10/3/2012

    Yes – turtles left out to make my life easier (smiley).

    And I had Darwinism: The refutation of a myth? specifically in mind when writing the biographical comments about Løvtrup above. There is definitely stuff in there about how – in Løvtrup’s opinion – gradual, continual change is insufficient to explain the patterns seen in evolution, and how macro-mutation is the way to go. I’m reminded of this sort of thing.

    Darren

    Link to this
  4. 4. Lars Dietz 10:50 am 10/3/2012

    I’ve seen Løvtrup’s book “The Phylogeny of Vertebrata”. Haven’t looked into his other book that John Harshman mentioned, but this one also starts with formulating laws, in this case about reconstruction of phylogenies. One of them stated that fossils are irrelevant for reconstructing the phylogeny of extant taxa, and another one stated that physiological characters are more important than morphological ones. In his book he actually supported the traditional phylogenetic positions of birds and mammals, but stated that without the fossil evidence, the data would support Haemothermia. In a review it was pointed out that this was inconsistent with his principles of phylogeny, and in the 1985 paper he acknowledged this and supported Haemothermia in his revised phylogeny. Some of his other ideas on vertebrate phylogeny were also unorthodox, such as paraphyly of lepidosaurs or coelacanths as sister group of Chondrichtyes.

    Link to this
  5. 5. Therizinosaurus 4:28 pm 10/3/2012

    But why would a stem-haematotherm have derived bird features lacking in mammals like a tridactyl foot (though V is oddly the small digit while I is absent), and derived mammal features lacking in birds like ear pinnae?

    Link to this
  6. 6. Mythusmage 4:45 pm 10/3/2012

    Excuse the intrusion, but wasn’t the seymouria a synapsid amphibian? Indeed, didn’t the defenestration of the tetrapod (synapsida and diapsida) skull predate the appearance of the shelled egg?

    Just wondering.

    Link to this
  7. 7. Therizinosaurus 7:32 pm 10/3/2012

    Mythusmage- Your issue seems to be confusing descriptive terms with phylogenetic/classification terms. While Seymouria had an amphibious lifestyle, it was more closely related to us than to frogs and salamanders, so is not a member of Amphibia. Seymouria is not synapsid in the sense of having only lower temporal fenestrae, as it has no temporal fenestrae, only a large otic notch. And it is certainly not a member of Synapsida, the group of animals more closely related to mammals than to reptiles.

    As for the order of appearence of shelled eggs and temporal fenestrae, based on Pineiro et al. (2012), we don’t know if the first amniote had lower temporal fenestrae. And I have no idea how far back before Amniota shelled eggs go (which is another case where the trait described is different than the group of animals named after it).

    Link to this
  8. 8. Dartian 3:52 am 10/4/2012

    “Darwinism: The refutation of a myth?”

    Why would any self-respecting anti-darwinist add a question mark to the title of such a book? Fake humility?

    Link to this
  9. 9. naishd 5:11 am 10/4/2012

    In response to Mickey’s (= Therizinosaurus) comment # 5… yeah, I have no idea how much thought went into (or didn’t go into) the creation of that hypothetical creature. I wonder if the artist just tried to combine bird-ish and mammal-ish traits without thinking about a genuine likely ancestral condition. After all, neither four toes nor pinnae can plausibly be considered primitive for anything ‘ancestral’ to birds and mammals.

    Darren

    Link to this
  10. 10. David Marjanović 5:24 am 10/4/2012

    Darren, is your Ichthyostega putting its hindfeet on the ground?

    I actually think he had a little bit of a point (about explanatory power, not Darwin), and that we should really be wondering if there are any principles or patterns to be noticed. But I don’t think Løvtrup’s principles actually make any sense.

    That’s all a little confusing. What are his principles, and what do you think we should watch out for instead?

    For the record, I’m not aware of any reason to think that “macroevolution” is anything other than accumulated “microevolution”.

    But why would a stem-haematotherm have derived bird features lacking in mammals like a tridactyl foot (though V is oddly the small digit while I is absent), and derived mammal features lacking in birds like ear pinnae?

    QFT.

    While Seymouria had an amphibious lifestyle,

    Nah, terrestrial. Both species are AFAIK only known from completely terrestrial environments. Other seymouriamorphs are known from aquatic juveniles and neotenes, though, complete with external gills in some cases.

    it was more closely related to us than to frogs and salamanders, so is not a member of Amphibia.

    Alternatively, frogs, salamanders and caecilians are more closely related to us than to Seymouria; either way, Seymouria isn’t an amphibian.

    And I have no idea how far back before Amniota shelled eggs go

    Unknown for diadectomorphs; it’s either just Amniota or Amniota + Diadectomorpha, then.

    Diadectomorphs also lack skull fenestration, so if Bob the Basal Amniote had fenestrae, they can’t have evolved much earlier.

    Link to this
  11. 11. naishd 5:27 am 10/4/2012

    Yes, for shame, the Ichthyostega is an old, semi-Jarvikian-style one (though with polydactyly). That’s because I’ve been recycling a picture I did some time in the 1990s and have yet to replace it…

    Darren

    Link to this
  12. 12. John Harshman 10:06 am 10/4/2012

    David,

    That’s all a little confusing. What are his principles, and what do you think we should watch out for instead?

    For the record, I’m not aware of any reason to think that “macroevolution” is anything other than accumulated “microevolution”.

    Well, it’s been 20 years since I read Løvtrup’s book. Perhaps Darren’s experience is more recent. But if I recall his critique of natural selection wasn’t that there is some other basic process operating but that saying “natural selection” doesn’t explain anything; for that we need to get into the details of what sort of variation existed and what the environment favored. He thought there were predictable rules, especially for interactions among closely related species, that would show up as patterns on trees. I swear I don’t remember any saltation.

    Again, I don’t think his rules would pan out if you actually tried to test them, but I think it’s interesting to wonder if some kind of structure might exist. Not a replacement or alternative to selection, but a look behind selection.

    By the way, I do think there are arguments that there are macroevolutionary mechanisms separate from natural selection, but that’s another topic, as is whether those mechanisms are very important.

    Link to this
  13. 13. Therizinosaurus 2:08 pm 10/4/2012

    “Nah, terrestrial. Both species are AFAIK only known from completely terrestrial environments. Other seymouriamorphs are known from aquatic juveniles and neotenes, though, complete with external gills in some cases.”

    Well, the latter was what I meant when calling Seymouria amphibious. Modern amphibians were called amphibians because of their aquatic larvae, right?

    Link to this
  14. 14. Andreas Johansson 3:05 pm 10/4/2012

    Modern amphibians were called amphibians because of their aquatic larvae, right?

    Presumably not – Linnaeus’ Amphibia included things like turtles and snakes.

    Link to this
  15. 15. Heteromeles 8:41 pm 10/4/2012

    That haematotherm looks like it belongs in an independent comic book.

    Link to this
  16. 16. THoltz 9:37 am 10/5/2012

    To be fair, “macroevolution” is a useful word for the patterns observed at above-species level scale, even if they are ultimately derived from microevolutionary sorting events.

    Link to this
  17. 17. David Marjanović 11:42 am 10/5/2012

    Oops, should have specified that both (very different) species of Seymouria are only known from late ontogenetic stages.

    Linnaeus’ Amphibia included things like turtles and snakes

    And sturgeons and other assorted wonders at some point.

    Link to this
  18. 18. JoseD 11:58 am 10/5/2012

    “As was later discussed by a whole string of authors (e.g., Gauthier et al. 1988a, b, Kemp 1988, Benton 1985, 1991), one can only conclude that birds and mammals are especially close relatives within Tetrapoda by ignoring and excluding a vast amount of contradictory data.”

    & yet Gardiner implied that you’re stupid? Now that’s irony for ya!

    BTW, does anyone remember the “Dinos In The Air” episode of Paleoworld ( http://www.youtube.com/watch?v=lrK7CTggLsM&feature=plcp )? That’s where I 1st heard about haemothermia.

    Link to this

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