The nasoturbinal ridges in Massetognathus are largely similar to those in other cynodonts. Posteriorly, these ridges show a shallow groove.
The presence of respiratory turbinals in these cynodonts thus provides compelling evidence that their ventilation rates were elevated significantly above "reptilian" rates and may perhaps have approached the "mammalian" level.
Ridges probably associated with respiratory turbinals first appear among advanced therapsids, the therocephalians and cynodonts. These ridges closely resemble the mammalian maxilloturbinal ridges in their anteroventral location, within the respiratory chamber, and in their association to the ostium of the nasolacrimal duct.
However, the maxilloturbinal ridges in therocephalians and cynodonts suggest that ventilation rates in these taxa had increased substantially.
Cynodonts were closely related to therocephalians, which they superseded during the Early Triassic, but the two groups apparently diverged early in their phylogenetic history (figs.
Throughout their evolution, cynodonts progressively acquired additional mammalian characters, and their morphology was gradually transformed from generally reptilelike to very mammallike.
Ultimately, cynodonts gave rise to the first mammals by the end of the Triassic Period (approximately 215 million yr ago).
Thus, aerobic capacities and stamina of therocephalians and cynodonts could have undergone their apparently gradual expansion without the adaptive "valleys" associated with the "thermoregulatory" scenario.
The postcranial skeleton of African cynodonts. Bulletin of the Peabody Museum of Natural History 36:1-216.