Tetrapod Zoology

Tetrapod Zoology

Amphibians, reptiles, birds and mammals - living and extinct

The Haematothermia hypothesis


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.

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

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