Helical burrow casts of therapsid origin from the Beaufort Group (Permian) of South Africa

Abstract

Large helical burrow casts of vertebrate origin have been discovered at several localities in the lowermost strata of the Teekloof Formation of the Beaufort Group (Permian) in the southwestern Karoo basin, South Africa. They are not only the oldest but also the first vertebrate burrows of helical geometry to be found in rocks of pre-mammalian age. The burrow casts are vertically disposed spiralling tubes filled with fine-grained sandstone or siltstone often with a calcareous cement. Tube diameters gradually increase from 60 mm in the upper entrance decline, through two non-contacting dextral coils, to about 160 mm at the base of the spiral whereupon the tube straightens and widens into a horizontal terminal chamber. Burrow depths range from 0.5-0.75 m and the ramp angle ranges from 10 to 32° although within any single spiral it is constant. Articulated skeletons of the small dicynodont Diictodon have been found within the terminal chambers and spirals of three sand-filled burrows. Linear ridges preserved on the outer walls and floor of some of the terminal chambers resemble scratch marks possibly made by the claws and “beak” of Diictodon during burrow excavation. The sedimentary sequences at two separate localities are interpreted as having been deposited by episodic sheet floods and distributary splay-fans in the proximal floodbasin areas flanking an aggrading meanderbelt rise. Comparison is drawn between helical burrow casts in the Lower Beaufort and other recorded occurrences, in particular, Daimonelix, a beaver burrow from the Miocene of Nebraska. Fundamental similarities far outweigh differences to the extent that the Beaufort helical burrows may be grouped with the “daimonelices”. It is concluded that the excavation of large helical burrows, an activity usually attributed to rodents, was pre-empted by mammal-like reptiles some 50 million years before the first “anatomical mammals” evolved.