In contrast, the ridge of the maxilloturbinal is located in the respiratory passage, anterior to the transverse lamina.
The maxilloturbinal ridge is also situated in close proximity to the opening of the nasolacrireal canal, by which the nasolacrimal duct enters the nasal cavity.
The course of the nasolacrimal duct is not described in Lambdopsalis, but in Morganucodon a groove on the ascending process of the maxillary suggests that it lies at the base of the maxilloturbinal, as in modern mammals (Kermack et al.
However, the authors indicated the presence of several additional ridges on the ascending process of the maxillary, supporting a complex series of maxilloturbinal elements (Lillegraven and Krusat 1991, p.
In extant mammals, the only structures associated with ridges in the lateral portion of the nasal cavity are the respiratory maxilloturbinal and the nasolacrimal duct.
These ridges correspond in location to the maxilloturbinal ridges of extant mammals and are similar to the ridge for the respiratory turbinal of Glanosuchus, described above.
As a result, these ridges more closely match the typical position of the base of the mammalian maxilloturbinal.
The ridges in the anterolateral portion of the nasal cavities of Massetognathus, Trirachodon, and Thrinaxodon closely resemble the basal ridge of the mammalian maxilloturbinal.
There appears to be a functional relationship between the maxilloturbinal and this duct, which provides some of the moisture required to operate the turbinal complex.
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.
These structures are well situated to modify bulk respired air and probably have a function similar to the mammalian maxilloturbinals, discussed below.