Cynosaurus

Cynosaurus is an extinct genus of cynodonts. Remains have been found from the Dicynodon Assemblage Zone in South Africa.^[1] Cynosaurus was first described by Richard Owen in 1876 as Cynosuchus suppostus. Cynosaurus has been found in the late Permian period. Cyno- is derived from the Greek word kyon for dog and –sauros in Greek meaning lizard.

Fossils of Cynosaurus have been found in the Cistecephalus and Daptocephalus Assemblage Zones, in the Balfour Formation of the Beaufort Group, pertaining to the Karoo Supergroup of South Africa.^[2] In the Karoo Basin of South Africa riverbanks would be over flooded creating floodplains that could hold all that water to start soil accumulation.^[3] In the lower Balfour Formation, the soil deposits suggest a lacustrine environment with abundant leaf impressions (Viglietti et al., 2018). This suggests that there was coastal marshes and swamps.^[3] There was also trace fossils found in the formation from aquatic organisms.^[3]

Cynosaurus was first described by Richard Owen in 1876. Owen wrote and journal titled “Descriptive and illustrated catalog of the fossil reptilia of South Africa in the collection of the British Museum” in 1876. Owen named the fossil Cynosuchus suppostus Owen, 1876 which later gets renamed as Cynosaurus by K. Schmidt in 1927.^[4] Owen described Cynosuchus suppostus as similar to Cynochampea in where the incisors and canines are located. The difference is that Cynosuchus suppostus had smaller and more upward location of nostril. The external nostril of Cynosuchus suppostus along with the forends of the upper and lower jaws were close in location with the nostril nearly horizontal. Owen described the molar teeth as relatively larger in size. Owen also noted the constriction of the upper jaw as it recedes and is combined with large molar teeth that shows Cynosuchus suppostus to have a broader and shorter skull. The nasal bones are broad and thick and overlapped by the maxillaries (Owen, 1876).

Derived traits for Cynosaurus are: subvertical mentum on anterior lower jaw, robust mandible with relative high horizontal ramus, broad snout up to 32% of skull length and adult Cynosaurus lacking pineal foramen (Van den Brandt et al., 2018). In early Cynodonts the parietal bone extends ventrally to the sidewall of the braincase (Rubidge et al., 2001). The epipterygoid is also expanded to make new contact with the frontal as well as the parietal crest is elongated to incorporate the pineal foramen (Rubidge et al., 2001).

The septomaxilla is the flat bridge that divides the nasal into upper and lower (Van den Brandt et al., 2018). The nasal is broader posteriorly than anteriorly (Van den Brandt et al., 2018). On the surface of the maxilla there are many small nutritive foramina forming two horizontal parallel lines (Van den Brandt et al., 2018). For the premaxilla there is a gap along the midline between the premaxilla and the palatal processes (Van den Brandt et al., 2018). The vomer is unpaired and tapers and reaches a point sharp (Van den Brandt et al., 2018). The vomer also doesn't reach the pterygoid posteriorly (Van den Brandt et al., 2018). Micro-CT scans allows internal structures of fossil skulls to be observed (Benoit et al., 2017). From micro-CT scans, a pair of ossificationorbitosphenoid were observed in four specimens of Cynosaurus (Benoit et al., 2017). In orbitosphenoid consisted of two thin plate-like structures appear to articulate ventromedially and in cross section, it appears to be in an U-shape (Benoit et al., 2017).

The rapid evolution of the masseter insertion area is able to show early diversification of early Cynodonts (Botha et al., 2007). In Procynosuchus and Dvinia the location of masseteric fossa high on the coronoid process is seen as an initial stage of differentiation of masseter (Botha et al., 2007). In Cynosaurus and Nanictosaurus the extension of masseteric fossa is to the base of the dentary (Botha et al., 2007).

Formations in South Africa

Parietal foramen (labeled pf in image) of Labidosaurus, an early reptile