The Karoo LIP (Marsh et al. 1997, 2004) is bounded by the Drakensberg Volcanics on the south, the Lebombo Mountains of Swaziland and Mozambique to the east, the Etjo Formation of northeast Namibia on the west, and the Batoka, Lupata, and Chikwawa basalts of Zimbabwe, Mozambique, and Malawi to the north. Fitch and Miller (1984) reviewed radiometric dates from southern Africa and concluded that lavas were progressively younger from south to north, and that the distribution of dates showed episodic centers of volcanic activity. A range of dates from Middle Jurassic to Cretaceous was indicated. There are indeed Cretaceous volcanic rocks in Malawi, particularly ring structures and carbonatites (Dixey et al. 1955; Garson 1965). However, Duncan et al. (1997); Marsh et al. (1997); and Jones et al. (2001) demonstrate with new data that Karoo LIP basalts were erupted in a narrow range of time, as were similar basalts in Antarctica, at 184-179 Ma. This narrow clustering of dates characterizes the flood basalt eruptions associated with the rifting of eastern Gondwana. As such, the Karoo LIP is a component of the larger Karoo-Farrar Magmatic Province, which represents one of the largest known continental flood basalt events, extending from Africa, across Antarctica, and into Tasmania and Australia (Duncan et al. 1997). At its western geographic extreme, Karoo LIP basalt extends to 18° E in Namibia, in close proximity to the Early Cretaceous African and South American Parana-Etendeka Flood Basalt Province. That Cretaceous volcanic province is associated with rifting of western Gondwana and the opening of the South Atlantic.
In addition to heralding the tectonic reshaping of continental landmasses, flood basalt events coincide with some extinction events. The vast flood basalt eruptions of the Karoo-Farrar Magmatic Province have been implicated in the moderate Toarcian-Aalenian extinctions affecting marine invertebrates (Duncan et al. 1997). Nevertheless, even accepting an affect on marine invertebrates, the effects of the volcanic event that produced the Karoo-Farrar Magmatic Province on therapsids is unclear.
Of the fossils from the Chiweta Beds, our focus on dicynodonts was to refine the age of the Chiweta Beds; in studying Mal 290 it has been to evaluate its position among the taxa included by Sidor and Welman (2003) in their analysis of the burnetiamorph Lemurosaurus. In the interim, Sidor et al. (2004) described a new basal burnetiamorph with a systematic analysis including the Russian taxon Niuksenitia (see also Battail and Surkov 2000) and an unnamed South African taxon. Including Mal 290, this brings to half a dozen the number of taxa (named or unnamed) that can be included in the Burnetiidae, with two more added in the more inclusive Burnetiamorpha. None of the taxa is well known and their stratigraphic distribution suggests numerous long ghost lineages. However, another possibility is that as characters are discerned and refined, phylogenetic analyses may yield trees with different topologies.
The nasal boss of Bullacephalus is more circular in cross section than that of Burnetia, which has likewise been scored as having a transversely expanded nasal boss. The presence of squamosal horns in Bullacephalus is also in question. In addition, the position of the pineal foramen on the dorsal surface of the skull reflects the orientation of the parietals, which are less inclined than in Burnetia and Mal 290, assuming no significant taphonomic distortion. The skull bones of Bullacephalus are clearly pachyostotic, and the genus is derived in that feature, but given its other features, and as new burnetiamorphs come to light, a re-evaluation of its status may become warranted.
The therapsids of the Chiweta Beds are interesting paleogeographically when considered in the context of the northern Malawi Karoo. Smith et al. (1993) interpreted the climate during the deposition of the Beaufort Group in South Africa as semi-arid with highly seasonal rainfall. In Malawi, Yemane et al. (1989) hypothesized a string of large, interconnected lakes of such a geographic extent that they ameliorated the rigors of climate at paleolaltitudes between 45° and 60° S (Yemane 1993). Based on oxygen isotopic studies of carbonate concretions found within the lacustrine sediments underlying the Chiweta Beds, Yemane and Kelts (1996), suggested mean annual surface temperatures possibly as high as 10°C, comparable to those of continental temperate climates, such as that of Switzerland today.
There is no evidence as to what the therapsid fauna was like at the time the region was covered by large lacustrine systems, but by the time of deposition of the Chiweta Beds, the lakes were gone from that area of Malawi, as far as can be determined from the rocks that remain. The Chiweta therapsids seem to have existed in a fluvially dominated ecosystem similar to that existing to the south. Their range of tolerances is unknown, but the presence of Oudenodon in the lower Cistecephalous Assemblage Zone of Malawi may indicate rather wide tolerances and an ability to maintain its niche as suitable habitat became available. On the other hand, Mal 290, as representative of a new burnetiid taxon, might indicate that the invasion of new territory following environmental change was an important factor in the diversification of burnetiamorphs.