. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. A NEW DICYNODONT ANCESTOR FROM THE UPPER ECCA 125 the preparietal would seem to be the one in which the preparietal is situated anterior to the pineal foramen. The increased participation of the bone in the formation of the borders of the pineal foramen in some forms and its absence in others can be considered to be stages in the later development of the skull roof. Judging by its relationships with surrounding elements it would seem cer- tain that the pineal foramen retains a fairly constant positio
. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. A NEW DICYNODONT ANCESTOR FROM THE UPPER ECCA 125 the preparietal would seem to be the one in which the preparietal is situated anterior to the pineal foramen. The increased participation of the bone in the formation of the borders of the pineal foramen in some forms and its absence in others can be considered to be stages in the later development of the skull roof. Judging by its relationships with surrounding elements it would seem cer- tain that the pineal foramen retains a fairly constant position in the skull roof and that it is the preparietal which is subject to backward displacement. In forms such as Dicynodon woodwardi (Broom 1921), Aulacephalodon peavoti (Broom 1921) and Aulacephalodon haughtoni (Broom 1921), all from the Cistecephalus zone, this backward movement would seem to be accompanied by an increase in the relative size of the preparietal. The postparietal(s?) occupies the dorsomedial area of the slanting occipital plate (Fig. 13). The name postparietal is here preferred to that of interparietal, usually used, as the bone is not situated between the parietals in this form. It also differs significantly from the usual dicynodont condition in that a medial postero-ventrally directed longitudinal ridge divides the bone into bilateral concave elements. The longitudinal ridge shows a pronounced midline groove or cleft extending the length of the bone. It is not possible to see whether this represents an entire or partial separation of the postparietal into two elements, but it would seem to reflect the ancestral paired condition of this element as found in Captorhinus and other cotylosaurs. It is interesting to note that the postparietal is single in most pelycosaurs (Romer & Price 1940: 50). Although a well-developed ridge is absent, a longitudinal mound in the midline separates the concave areas on its sides. In most known Gorgonopsia the bone is also si
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectnaturalhistory, booky
