The nasoturbinal ridges in Massetognathus are largely similar to those in other cynodonts.
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.
However, the maxilloturbinal ridges in therocephalians and cynodonts suggest that ventilation rates in these taxa had increased substantially.
If early therocephalians possessed only moderately elevated ventilation rates, the same was likely also true for the first cynodonts.
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.
Cynodont postcranial anatomy and the "prototherian" level of mammalian organization.