a true image producing organ or one that could only register light and dark, like that of the modern tuatara. Most labyrinthodonts had special sense organs in the skin, forming a lateral line organ for perception of water flow and pressure, like those found in fish and a number of modern amphibians. This would enable them to pick up the vibration of their prey and other waterborne sounds while hunting in murky, weed filled waters. Early labyrinthodont groups had massive stapes, likely primarily anchoring the brain case to the skull roof. It is a question of some doubt whether early terrestrial labyrinthodonts had the stapes connected to a tympanum covering their otic notch, and if they had an aerial sense of hearing at all. The tympanum in anurans and amniotes appear to have evolved separately, indicating most, if not all, labyrinthodonts were unable to pick up airborne sound. Respiration The early labyrinthodonts possessed well developed internal gills as well as primitive lungs, derived from the swim bladders of their ancestors. They could breathe air, which would have been a great advantage for residents of warm shoals with low oxygen levels in the water. There was no diaphragma and the ribs in many forms were too short or spaced too closely to aid in expanding the lungs. Likely air was inflated into the lungs by contractions of a throat sac against the skull floor like in modern amphibians, which may be the reason for the retention of the very flat skull in later forms. Exhalation with the aid of the ribs probably evolved only in the line leading to amniotes. Many aquatic forms retained their larval gills in adulthood. With the high atmospheric oxygen and carbon dioxide pressure, particularly during the Carboniferous, the primitive throat sac breathing would have been sufficient for obtaining oxygen even for the large forms. Getting rid of carbon dioxide would present a greater problem on land, and the larger labyrinthodonts probably combined a high tolerance for blood carbonic acid with returning to the water to dissipate the carbon dioxide through the skin. The loss of the armour of rhomboid scales of their piscine ancestors allowed for this as well as additional respiration through the skin as in modern amphibians. Hunting and feeding Like their sarcopterygian ancestors, the labyrinthodonts were carnivorous. The rather broad, flat skulls and hence short jaw muscle would however not allow them to