Physiological implications of the abnormal absence of the parietal foramen in a late Permian cynodont (Therapsida)

Abstract

The third eye (pineal eye), an organ responsible for regulating exposure to sunlight in extant ectotherms, is located in an opening on the dorsal surface of the skull, the parietal foramen. The parietal foramen is absent in extant mammals but often observed in basal therapsids, the stem-group to true mammals. Here, we report the absence of the parietal foramen in a specimen of Cynosaurus suppostus, a Late Permian cynodont from South Africa (SA). Comparison with Procynosuchus delaharpeae, a contemporaneous non-mammalian cynodont from SA, demonstrates that the absence of this foramen is an abnormal condition for such a basal species. Because seasonality was marked during the Late Permian in SA, it is proposed that the third eye was functionally redundant in Cynosaurus, possibly due to the acquisition of better thermoregulation or the evolution of specialized cells in the lateral eyes to compensate for the role of the third eye.

Virtual reconstruction of cranial endocasts of traversodontid cynodonts (Eucynodontia: Gomphodontia) from the upper Triassic of Southern Brazil

“…Except for the Probainognathia, in which the parietal foramen is consistently absent, there is variation in the presence/absence of this structure among non‐mammaliaform cynodonts. The parietal foramen and the pineal tube are present in basal epicynodonts such as Procynosuchus , Thrinaxodon , Platycraniellus , and Galesaurus (Abdala, ; Benoit, Abdala, Van Den Brandt, & Manger, ; Liu & Olsen, ; Pusch et al, ). In the Eucynodontia, the parietal foramen, and probably the whole pineal complex, disappeared in some lineages (Benoit, Abdala, Manger, & Rubidge, ; Rodrigues et al, ).…”

“…However, several eucynodont taxa still possess the parietal foramen, such as the cynognathians Cynognathus and Diademodon , and the trirachodontids Langbergia and Cricodon (Liu & Olsen, ; Sidor & Hopson, ). The presence of a parietal foramen is variable in adult individuals of the basal non‐mammaliaform cynodont Cynosaurus and the trirachodontid Trirachodon (Abdala, Neveling, & Welman, ; Benoit et al, , ). In the trirachodontids Beishanodon and Sinognathus , the parietal foramen is absent (Gao, Fox, Zhou, & Li, ; Liu & Abdala, ).…”

Oxygen isotopes suggest elevated thermometabolism within multiple Permo-Triassic therapsid clades

“…According to our results, the closest sampled relative of Eucynodontia, the late Permian basal cynodont (SAM-PK-K05339), was probably ectothermic. Therefore the origin of mammal endothermy could have taken place among ‘intermediate’ groups belonging to the Epicynodontia (such as Cynosaurus from the latest Permian or Thrinaxodon from the Early Triassic of South Africa) that have in the past been considered to have been endothermic, based on anatomical features (Hillenius and Ruben, 2004; Benoit et al, 2015, 2016a). …”

A phenology of the evolution of endothermy in birds and mammals

“…The parietal foramen was lost in the Probainognathia, the lineage of the most derived Late Triassic cynodonts from which the mammaliaforms evolved (Benoit et al, 2015). The parietal foramen is a hole in the roof of the skull housing the 'third eye' in basal synapsids and sauropsids.…”

“…It provides photic input to the pineal gland for T b control and seasonal and reproductive synchrony (Gundy, Ralph & Wurst, 1975). Reliance upon this extra-retinal photic input is presumed to have become redundant in nocturnal mammals following the evolution of endothermy (Benoit et al, 2015).…”