Fossil skulls alone cannot predict if animal was warm-blooded
The ability of most mammals to maintain a relatively constant and high body temperature is considered a key adaptation, enabling them to successfully colonize new habitats and harsh environments. Eager to determine how this ability evolved, some scientists proposed that a particular region of the mammal skull – the anterior nasal cavity, which houses structures known as the maxilloturbinals – plays a pivotal role in body temperature maintenance.
But a new study in Nature Communications cautions against using the presence and relative size of these skull structures to determine if an animal, whether living or extinct, is capable of maintaining heat and moisture for survival. The work provides new insights into the nose of mammals and its relation to their warm-bloodedness. Ricardo Araújo, a palaeontologist at IPFN, is a co-author of the study, together with colleagues from institutions in the Czech Republic, France, Germany, the UK and the US.
The evolution of endothermy in vertebrates is a major research topic in recent decades that has been tackled by a myriad of research disciplines including palaeontology, anatomy, physiology, evolutionary and developmental biology. The ability of most mammals to maintain a relatively constant and high body temperature is considered a key adaptation, enabling them to successfully colonize new habitats and harsh environments.
It has been proposed that in mammals the anterior nasal cavity, which houses the maxilloturbinal, plays a pivotal role in body temperature maintenance, via a bony system supporting an epithelium involved in heat and moisture conservation. The presence and the relative size of the maxilloturbinal have been proposed to reflect the endothermic conditions and basal metabolic rate in extinct vertebrates.
However, prior to this study, such hypotheses were based on a limited dataset and lacked strong evidence. To properly test these hypotheses, the international team CT-scanned the heads of over 300 mammals from international museum collections.
The international team's findings are based on an analysis of CT scans of the heads of more than 300 mammals from international museum collections. Using this technique, they were able to visualize the maxilloturbinal in 3D and quantify it. They demonstrate that neither corrected basal metabolic rate nor body temperature significantly correlate with the relative surface area of the maxilloturbinal.
Contrary to previous hypotheses, the researchers concluded that there is no evidence to support a relationship between the origin of endothermy and the development of the maxilloturbinal. Instead, they identified important variations in the relative surface area, morpho-anatomy, and complexity of the maxilloturbinal across the mammalian phylogeny and species ecology.'
Our respiratory turbinals do help humans and other mammals warm the air we inhale, as well as conserve water from the air we exhale,' says co-author Stan Braude. 'The size of the underlying bony structures (the maxilloturbinals) does not correlate with metabolic rate or body temperature. This is likely because mammals live in such diverse environments and have various other adaptations to those conditions.'
'The dogma that maxilloturbinals in fossil species indicate their ability to maintain body temperature – i.e. homeothermy – is oversimplified and unjustified,' Braude said.
To know more:
Q. Martinez, J. Okrouhlík, R. Šumbera, M. Wright, R. Araújo, S. Braude, T. B. Hildebrandt, S. Holtze, I. Ruf, P. H. Fabre, Mammalian maxilloturbinal evolution does not reflect thermal biology. Nature Communications 14, 4425 (2023).
