Fossil Friday: A Scientific Controversy About Warm-Blooded Animals
This Fossil Friday features the exceptionally well-preserved fossil bird Nahmavis grandei from the Eocene Green River Formation of Wyoming, as an example for a fossil representative of warm-blooded animals. Nowadays, every high-school biology class teaches that mammals and birds, even though both are warm-blooded tetrapods, are not closely related and were derived from different reptilian-like ancestors. Their similar physiology is attributed to so-called convergent evolution, thus is claimed to have had an independent evolutionary origin. However, if generally anatomical, physiological, genetic, and behavioral similarities are mostly explained by common descent, why are all warm-blooded animals not grouped together as descendants of a common warm-blooded ancestor? Indeed, based on much earlier observations of John Ray (1693), Charles Darwin’s famous opponent at the British Museum for Natural History, the paleontologist Richard Owen (1866), who had coined the word dinosaur, had first suggested to group birds and mammals together in a taxon Haematothermia, based on their similar warm-blooded physiology.
This was mostly ignored by other biologists until about 120 years later, when the two maverick biologists Søren Løvtrup (1977, 1985) and Brian Gardiner (1982, 1993) took up the idea and again suggested that all warm-blooded vertebrates form a clade Haemothermia, thus birds and mammals would be most closely related sister groups. They both suggested that the complex physiological similarities are unlikely to be convergences and rather represent deep evolutionary homologies. Nobody less than the famous French vertebrate paleontologist Philippe Janvier (1984) even published a reconstruction drawing how a hypothetical ancestor of Haemothermia might have looked like (reproduced by Sivgin 2020). This went against a growing consensus among evolutionary biologists that mammals were derived from synapsid “mammal-like reptiles” like the Permian pelycosaurs, while birds were diapsids more closely related to dinosaurs and crocodiles as well as other living reptiles. Consequently, their suggestion was immediately met with harsh criticism (Benton 1985, 1991, Kemp 1988, Gauthier et al. 1988a, 1988b) and their “radical hypothesis” (Peters 2014) was ultimately rejected as absurd (Kuhn-Schnyder 2009). The reason were the numerous other similarities from skeleton to genomics (e.g., Janke & Arnason 1997) that rather supported the mainstream view.
Pterosaurs and Dinosaurs
However, it must be noted that Gardiner (1993) explicitly agreed that pterosaurs and dinosaurs are close relatives of birds, and “mammal-like reptiles” were relatives and ancestors of mammals. He simply included those reptile-like groups in Haematothermia, and indeed there has been considerable evidence accumulated in the past decades that they also were warm-blooded. Here is a quote from Gardiner’s (1993) abstract:
An exhaustive parsimony analysis of amniote phylogeny using 97 characters has substantiated the hypothesis that mammals and birds are sister groups. This deduction is further supported by parasitological and molecular evidence. The presumed importance of “synapsid” fossils in amniote phylogeny is questioned and it is concluded that they represent a transformation series which, when broken down into constituent monophyletic groups, does not support the separation of the Mammalia from the remainder of the amniotes. Fossil members of the Haematothermia include pterosaurs and “dinosaurs” (both stem-group birds) and Dinocephalia, Dicynodontia, Gorgonopsida and Therocephalia (all stem-group mammals). The Dromaeosauridae are the most crownward stem-group birds and the Morganucodontidae the most crownward stem-group mammals.
Thus, Gardiner (1993) rather suggested that Synapsida and Archosauria are sister groups, which is a hypothesis that is still endorsed by the highly controversial fringe paleontologist David Peters (2024) in his large reptile tree based on 2323 taxa and 236 characters.
Gardiner is said to still have embraced the Haematothermia hypothesis until later in his life (Naish 2008, 2012). Nevertheless, the idea of such a clade of warm-blooded animals was quickly buried and forgotten by the scientific community again, so that the work of Løvtrup and Gardiner is not even mentioned anymore in modern treatises on the origin of endothermy in vertebrates (e.g., Koteja 2004, Nespolo et al. 2011, Benton 2021, Grigg et al. 2022, Faure-Brac et al. 2024). After all, isn’t it really silly to just look at a superficial similarity like warm-bloodedness and ignore all the conflicting evidence. Yes, that might have been silly indeed, but it was not at all what Løvtrup and Gardiner did. Indeed they assembled substantial evidence for their Haematothermia hypothesis that went far beyond only a superficial similarity in physiology, but included a cladistic analysis of 28 specific similarities, of which even the most ardent critics recognized at least 8 as valid (Kemp 1988).
Also, other authors often admitted that birds and mammals share many similarities of the “cardiovascular, renal, gastrointestinal, endocrine and nervous systems” (Carvalho & Gonçalves 2011). Even more recently, a paper on the supposed convergences between birds and mammals published by Wu & Wang (2019) in the Proceedings of the Royal Society, confirmed these similarities and suggesting even more:
Extant birds and mammals share a number of highly similar characteristics, including but not limited to, enhanced hearing, vocal communication, endothermy, insulation, shivering, respiratory turbinates, high basal metabolism, grinding, sustained activity, four-chambered heart, high blood pressure, and intensive parental care.
A Very Incomplete List
Here is a very incomplete list of a dozen complex derived similarities shared by birds and mammals, which I stumbled upon during a quick survey of the recent scientific literature I made for this article:
Visceral endothermy or warm-bloodedness means that birds and mammals share the ability to maintain a stable internal body temperature, a characteristic crucial for active living in a wide range of environmental conditions (Nespolo et al. 2011). If this endothermy would be homologous in birds and mammals we should expect that they acquired this trait at the same time, which is exactly what we find (Benton 2021), allegedly based on a shared adaptation to nocturnality in their early evolution (Wu & Wang 2019). Of course, the warm-bloodedness correlates with high metabolic rates in birds and mammals, compared to most reptiles, supporting their increased energetic demand for sustained activity and thermoregulation.
Mammals and birds possess a four-chambered heart that efficiently separates oxygenated and deoxygenated blood, facilitating high metabolic rates required for endothermy. However it must be noted that a four-chambered heart is also present in crocodylians, who may have secondarily reverted to ectothermy (Grigg et al. 2022). With this knowledge I suppose that Gardiner would have decided to include crocs in his Haematothermia clade of synapsids and archosaurs.
Both birds and mammals posses integumental structures (feathers in birds and fur in mammals), made from keratin and originating from placodes that are homologous to reptilian scales, as specialized body coverings for insulation to reduce heat loss, which is of course a crucial adaptation for endothermy (Chernova 2005, Dhouailly 2009, Di-Poï & Milinkovitch 2016).
Even though very different in organisation, birds and mammals have the most complex lungs among vertebrates and a highly efficient respiratory systems that support their high metabolic demands (Meyer et al. 1981, Powell & Hopkins 2004, West et al. 2007).
Both birds and mammals exhibit relatively large brains compared to body size, particularly in regions associated with higher cognitive function, such as learning, problem-solving, and social behaviors. More specifically, only mammals and birds possess a well-developed neocortex, called dorsal ventricular ridge (DVR) in birds (UChicago Medicine 2012, Kebschull 2020, Stacho et al. 2020, Ball & Balthazart 2021). Apart from the increased relative brain size and highly laminar telencephalic areas, birds and mammals also share a complex cerebellar folding, enhancing motor control and coordination, as well as advanced auditory circuits capable of processing complex sounds (Striedter & Northcutt 2019).
Only mammals and birds have episodic-like memory (Rattenborg & Martinez-Gonzalez 2011, 2013).
Even though sleep was for decades considered to be exclusive to mammals and birds, it is meanwhile shown to be a widespread phenomenon in the animal kingdom. However, “REM sleep, the major source of dreams, and slow wave sleep are unique to mammals and birds” (AAAS 2015, Hayashi et al. 2015).
Birds and mammals have specialized hearing mechanisms, including middle ear ossicles (ossicular chain) that transmit sound vibrations effectively, allowing for acute auditory sensitivity (Köppl 2011, Anthwal et al. 2012: fig. 1).
Both groups possess complex endocrine hormonal systems that regulate growth, metabolism, and reproduction. Actually, “birds produce homologues of the vast majority of mammalian hormones. These can have similar roles in birds and mammals.” (Scanes 2015). For instance, the thyroid and adrenal glands play essential roles in metabolic rate control, and prolactin controls seasonality in birds and mammals (Stewart & Marshall 2022).
Both birds and mammals engage in complex social behaviours, including cooperation, communication, intricate mating rituals and significant parental care, including prolonged juvenile periods and provisioning of food, which enhances offspring survival in challenging environments. Play behaviour was long considered to be unique to mammals and birds (Dinets 2023), but it has been meanwhile recorded from a few ectothermic animal species as well, but it is still only widely occurring and well-developed in birds and mammals.
Both groups possess complex endocrine hormonal systems that regulate growth, metabolism, and reproduction. Actually, “birds produce homologues of the vast majority of mammalian hormones. These can have similar roles in birds and mammals.” (Scanes 2015). For instance, the thyroid and adrenal glands play essential roles in metabolic rate control, and prolactin controls seasonality in birds and mammals (Stewart & Marshall 2022).
Both birds and mammals engage in complex social behaviours, including cooperation, communication, intricate mating rituals and significant parental care, including prolonged juvenile periods and provisioning of food, which enhances offspring survival in challenging environments. Play behaviour was long considered to be unique to mammals and birds (Dinets 2023), but it has been meanwhile recorded from a few ectothermic animal species as well, but it is still only widely occurring and well-developed in birds and mammals.
A cladistic study of 16 ultrastructure characters of spermatozoa (Jamieson & Healy 1992) strongly supported the bird + mammal clade (Haemothermia) with three uniquely derived similarities, and did not substantiate the traditional grouping. This is significant because it is a totally independent source of evidence unrelated to warm-blooded physiology.
In spite of differential rates of transposable element accumulation “the genome size in mammals and birds shows remarkably little interspecific variation compared with other taxa.” The results of a study by Kapusta et al. (2017) imply that “DNA removal in both mammals and birds has proceeded mostly through large segmental deletions”, which has been called an “accordion model” of genome size evolution.
All these striking similarities would have to be considered as convergent adaptations, which were the result of similar selective pressures in birds and mammals that have led to the independent origin of these complex traits. So, both alternatives, the mainstream view and the Haematothermia hypothesis, imply a lot of convergences, so that many similarities cannot be readily explained with common descent. Ultimately, a bureaucratic counting of which alternative is supported by a few more similarities (see Kemp 1988) decides for evolutionary biologists, which common descent hypothesis is embraced and which is rejected as absurd. If you look at many of the publication dates of the references in my list above, you see that a lot more characters that would support Haematothermia have been discovered since the time of Løvtrup and Gardiner.
To be clear, I do not suggest that the Haematothermia hypothesis is a better alternative to the mainstream view, but rather suggest that the strongly conflicting data point to a deeper problem. In the view of us critics of neo-Darwinian evolution, the large amount of incongruent and conflicting evidence rather questions all alternatives and the very paradigm of common descent itself. Even though common descent may well still be true, either on a universal level or at least for more restricted groups, it cannot be convincingly demonstrated by just pointing to shared similarities. Those similarities would have to be overwhelmingly congruent and mostly point to the same nested hierarchy, if the story of a single tree of life would be true. But they don’t. Incongruent evidence is found abundantly in all groups and all levels of the taxonomic hierarchy. The theory of Darwinism made a prediction of similarities that neatly fall into a nested hierarchy without significant incongruence, but it failed the test by empirical data miserably. Other processes than common descent with modification have to account for the similarities and differences between organisms, and intelligent design definitely is a premier candidate.
Ignoring the Evidence
How do popularizers of Darwinism such as Richard Dawkins react? Unsurprisingly, they just ignore the evidence and boldly tell their gullible fanboys (and girls) that evolution is a proven fact because all data unambiguously suggest a single true tree of life. Is this mere ignorance or deliberate deception? The materialist-naturalist world view critically depends of Darwinian evolution and must defend it at all cost, even if it means that the facts have to be tweaked, fudged, and denied to fit the theory. And all critics must be silenced as dangerous science-deniers and peddlers of pseudoscience and evil religious superstition. More and more people no longer fall for this crude propaganda and rather follow the evidence wherever it leads. Isn’t this what science is all about, or at least should be?
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