New insights into the respiration and metabolic physiology of Lystrosaurus Michael Laaß, 1 Oliver Hampe, 2 Michael Schudack, 3 Corinna Hoff, 4 Nikolay Kardjilov 5 and Andre ´ Hilger 5 1 Institut fu ¨ r Geowissenschaften, Martin- Luther-Universita ¨t Halle-Wittenberg, 06099 Halle (Saale), Germany; 2 Museum fu ¨ r Naturkunde, Leibniz-Institut fu ¨ r Evolu- tions- und Biodiversita ¨tsforschung an der Humboldt-Universita ¨t zu Berlin, Invalid- enstraße 43, 10115 Berlin, Germany; 3 Insti- tut fu ¨ r Geologische Wissenschaften, Freie Universita ¨t Berlin, Malteserstr. 74-100, 12249 Berlin, Germany; 4 Gonometrics, Mu ¨ hlweg 41, 06114 Halle (Saale), Germany; 5 Helmholtz-Zentrum Berlin fu ¨r Materialien und Energie, Glienicker Str. 100, 14109 Berlin, Germany Keywords: Lystrosaurus, Anomodontia, Respiration, Lower Triassic, Karoo basin Accepted for publication: 20 August 2010 Abstract Laaß, M., Hampe, O., Schudack, M., Hoff, C., Kardjilov, N. and Hilger, A. 2011. New insights into the respiration and metabolic physiology of Lystrosaurus. —Acta Zoologica (Stockholm) 92: 363–371. The first examination by neutron tomography of a skull of Lystrosaurus declivis (Therapsida, Anomodontia) from the Lower Triassic of South Africa showed complexly constructed cartilaginous maxilloturbinals in the nasal cavity. They were situated directly in the respiratory air flow and fill out most of the ventral part of the nasal chamber. Because maxilloturbinals in extant mammals and birds serve as a countercurrent exchange system for thermoregulation and humidification, their presence in the anomodont Lystrosaurus suggests strongly that Lystrosaurus was already endothermic. The endothermic metabolism allowed Lystrosaurus to tolerate high ambient temperature fluctuations. The complexly constructed maxilloturbinals could have reduced respiratory water loss because of higher ventilation rates in drought conditions in the Karoo basin. Michael Laaß, Institut fu ¨ r Geowissenschaften, Martin-Luther-Universita ¨t Halle-Wittenberg, 06099 Halle (Saale), Germany. E-mail: michael.laass@ gmx.de Introduction The evolution of endothermy in the ancestors of mammals is a much discussed problem because the status of metabolism is one of the most fundamental features distinguishing extant mammals from reptiles (Ruben et al. 1997; Kemp 2005). Stratigraphic, palaeogeographic and palaeoecologic data, in combination with the analysis of character evolution in early therapsids, indicate that the evolutionary success of therapsids was because of a higher level of metabolic activity and of homeostatic regulation of the body (Kemp 2006a). Because therapsids were adapted to seasonally arid, savanna-like biomes with high ambient temperature fluctua- tions in contrast to pelycosaur-grade synapsids, searches have been undertaken for anatomical evidence in the fossil record for a higher metabolic activity level of therapsids. Endothermy is a feature almost completely related to soft tissue anatomy, i.e., hair, non-nucleated red blood cells, lung structure, blood oxygen carrying capacity, diaphragm, and sweat glands (Ruben 1995). Accordingly, there is usually no preservation of any endothermy indicators in the fossil record. Therefore, several non-soft tissue features were taken to be related to endothermy, i.e. posture (Bakker 1971), brain size (Hopson 1980), bone structure (de Ricqle ´s 1974, 1976), fos- sil bone oxygen isotope ratios (Barrick and Showers 1994), foramina for sensory vibrissae at maxilla and premaxilla – the latter being understood as fur indicator (Brink 1955; Kemp 1982) – the reduction of lumbar ribs as an indication for the presence of muscular diaphragm and elevated lung ventilation (Brink 1955; Kemp 1982), and the presence or absence of a parietal foramen that is associated with an epiphysial complex as an detector for the environmental temperature (Roth and Roth 1980). Even such criteria as predator–prey ratios, fossil- ized trackways and biogeographical distributions were used as endothermy indicators (Bakker 1972, 1980, 1986). Studies in extant reptiles, mammals and birds have shown that all these features are highly speculative or have no clear functional cor- relation to endothermy (Bennett and Ruben 1986; Farlow 1990; Farlow et al. 1995; Kolodny et al. 1996; Chinsamy- Turan 2005). Moreover, a secondary palate allowing permanent ventila- tion during mastication was supposed to be related with higher metabolic rates and endothermy (Brink 1955; Kemp 1982). However, a secondary palate, being present in Acta Zoologica (Stockholm) 92: 363–371 (October 2011) doi: 10.1111/j.1463-6395.2010.00467.x Ó 2010 The Authors Acta Zoologica Ó 2010 The Royal Swedish Academy of Sciences 363