Articles | Volume 20, issue 2
Foss. Rec., 20, 105–127, 2017
Foss. Rec., 20, 105–127, 2017

Research article 02 Mar 2017

Research article | 02 Mar 2017

Modeling the physiology of the aquatic temnospondyl Archegosaurus decheni from the early Permian of Germany

Florian Witzmann1 and Elizabeth Brainerd2 Florian Witzmann and Elizabeth Brainerd
  • 1Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Invalidenstraße 43, 10115 Berlin, Germany
  • 2Department of Ecology and Evolutionary Biology, Brown University, Providence, RI 02912, USA

Abstract. Physiological aspects like heat balance, gas exchange, osmoregulation, and digestion of the early Permian aquatic temnospondyl Archegosaurus decheni, which lived in a tropical freshwater lake, are assessed based on osteological correlates of physiologically relevant soft-tissue organs and by physiological estimations analogous to air-breathing fishes. Body mass (M) of an adult Archegosaurus with an overall body length of more than 1 m is estimated as 7 kg using graphic double integration. Standard metabolic rate (SMR) at 20 °C (12 kJ h−1) and active metabolic rate (AMR) at 25 °C (47 kJ h−1) were estimated according to the interspecific allometry of metabolic rate (measured as oxygen consumption) of all fish (VO2 = 4. 8 M0. 88) and form the basis for most of the subsequent estimations. Archegosaurus is interpreted as a facultative air breather that got O2 from the internal gills at rest in well-aerated water but relied on its lungs for O2 uptake in times of activity and hypoxia. The bulk of CO2 was always eliminated via the gills. Our estimations suggest that if Archegosaurus did not have gills and released 100 % CO2 from its lungs, it would have to breathe much more frequently to release enough CO2 relative to the lung ventilation required for just O2 uptake. Estimations of absorption and assimilation in the digestive tract of Archegosaurus suggest that an adult had to eat about six middle-sized specimens of the acanthodian fish Acanthodes (ca. 8 cm body length) per day to meet its energy demands. Archegosaurus is regarded as an ammonotelic animal that excreted ammonia (NH3) directly to the water through the gills and the skin, and these diffusional routes dominated nitrogen excretion by the kidneys as urine. Osmotic influx of water through the gills had to be compensated for by production of dilute, hypoosmotic urine by the kidneys. Whereas Archegosaurus has long been regarded as a salamander-like animal, there is evidence that its physiology was more fish- than tetrapod-like in many respects.

Short summary
Based on the estimation of its body mass and metabolic rate, we calculated heat balance, gas exchange, osmoregulation, and digestion of the 290-million-year-old aquatic amphibian Archegosaurus. Our results suggest that its physiology was more fish- than tetrapod-like in many respects. We conducted this study, which is mainly based on theoretical calculations and comparison with extant analogs, to shed light on the physiology and lifestyle of early tetrapods.