Both remaining groups of therapsids, the therocephalians and the cynodonts, became increasingly mammallike in appearance during their evolution.
Cynodonts supplanted the gorgonopsians and therocephalians as the dominant therapsids during the Early Triassic Period (Kemp 1982; Carroll 1988) and persisted throughout the Triassic and into the Jurassic.
Longitudinal turbinal ridges have long been noted on the inner surfaces of the nasal, frontal, and prefrontal bones of many cynodonts [Procynosuchus, Kemp 1979; Thrinaxodon (including "Nythosaurus"), Watson 1913; Fourie 1974; Diademodon, Watson 1913; Brink 1955, 1956; Exaeretodon, Bonaparte 1966; Luangwa, Kemp 1980; Oligokyphus, Kuhne 1956].
However, in several cynodonts, ridges occur in the anterolateral portion of the nasal cavity, which have not been described previously.
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Cynodonts were closely related to therocephalians, which they superseded during the Early Triassic, but the two groups apparently diverged early in their phylogenetic history (figs.
Although two different groups of very mammal-like non-mammalian cynodonts are known from the McCoy Brook Formation, no diagnostic skeletal remains of mammaliaforms have yet been recorded from this unit.
(2011) argued for strong provinciality in the moist equatorial Pangaean region; there, traversodontid cynodonts are abundant, whereas remains of these therapsids are rare or absent and procolophonid reptiles are abundant in the Northern Hemisphere subtropics (as, for example, in the Evangeline Member of the Wolfville Formation).
Late Triassic traversodont cynodonts from Nova Scotia and southern Africa.