Abstract
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.
Highlights
Quantitative modeling of an extinct animal’s physiology may lead to a better understanding of its mode of life, including activity, breathing, feeding, or habitat preferences
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 airbreathing fishes
The garfish Lepisosteus osseus, which breathes water at low temperatures via its gills and becomes an obligate air breather as metabolic rate and ambient water temperature increase, has to come to the surface only every 4 to 9 min to gulp air at 25 ◦C (Rahn et al, 1971). These values refer to O2 uptake of Lepisosteus (CO2 is always eliminated via the gills), they show that the values estimated for the Archegosaurus model without gills would imply that it had to break through the water surface unusually often for an aquatic animal
Summary
Quantitative modeling of an extinct animal’s physiology may lead to a better understanding of its mode of life, including activity, breathing, feeding, or habitat preferences. The long, deep-swimming tail, the presence of lateral line sulci on the skull, the poorly ossified and comparatively weak fore- and hind limbs, and the retention of branchial teeth on the gill arches indicate that the large adults were primarily aquatic animals and were capable of only short sojourns on land (Witzmann, 2006a; Witzmann and Schoch, 2006). Adults breathed via their lungs, whose presence is indicated by the extant phylogenetic bracket (Schoch and Witzmann, 2011). Archegosaurus is regarded here as a primarily aquatic tetrapod that breathed via internal gills and lungs as an adult and possessed external gills as a larva, analogous to extant lepidosirenid lungfishes and polypterid actinopterygians (Graham, 1997)
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