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

Benoit Julien1,2,*, Abdala Fernando1, Van den Brandt Marc1, Manger Paul R2, Rubidge Bruce1,3

1Evolutionary Studies Institute (ESI), University of the Witwatersrand, Braamfontein, 2050, Johannesburg, South Africa

2School of Anatomical Sciences, University of the Witwatersrand, 7 York Road, Parktown, 2193, Johannesburg, South Africa.

3School for Geosciences, University of the Witwatersrand, Braamfontein, 2050, Johannesburg, South Africa

* Corresponding Author: email: ; phone: 0797896503

Electronic Supplementary Material 1

Material and Methods

Scans: Three Cynosaurus skulls (we follow Hopson and Kitching [1972] who consider Cynosaurussuppostus to be the only valid species) from the Dicynodon Assemblage Zone (AZ) of the Beaufort Group of South Africa were CT scanned to determine the status of the parietal foramen and tube that accommodates the pineal complex inside the braincase. All specimens were scanned at the ESI scanning facility using Nikon Metrology XTH 225/320 LC:BPI/1/3926 (Skull length (SL): 115; Voxel size (Vs): 0,0708mm), BPI/1/4469 (SL: 56; Vs: 0,0342mm), and BPI/1/1563 (SL: 50; Vs: 0,0291mm).

Measurements: Seventeen skulls of Procynosuchus delaharpeae from the Cistecephalus and Dicynodon AZs were also studied. The average diameter of the parietal foramen was calculated by summing the maximum and minimum diameters of this foramen and dividing this by half. Given the wide differencein skull length of the sampled specimen (55 to 142mm, see ONLINE RESOURCE 2), comparisons relative to body size were necessary. We did not use skull length itself as a proxy for body size because only eleven skulls were not broken or weathered at the extremity of the snout. Accordingly we preferred to use the average diameter of the foramen magnum because it is preserved in most specimens. Moreover, it has been shown that the size (area) of the foramen magnum closely correlates with the cross–sectional area of the spinal cord that directly depends on the size of the postcranial body and body mass in mammals (Radinsky 1967, 1976). We then performed a bivariate analysis between the average diameter of the parietal foramen and the average diameter of the foramen magnum, in order to understand the variation of the parietal foramen and body size.

References

Hopson JA, Kitching JW (1972) A Revised Classification of the Cynodonts (Reptilia; Therapsida). Paleontol Afr 14:17–885.

Radinsky LB (1967) Relative brain size: a new measure. Science 155(3764):836–8.

Radinsky LB (1976) The brain of Mesonyx, a Middle Eocene mesonychid condylarth. Fieldiana Geology 33(18):323–337.