Abstract
Lanthanostegus is an unusual dicynodont known from only two partial skulls from a single locality near Jansenville in the Eastern Cape Province of South Africa. Although these specimens can be constrained to near the base of the late middle Permian (Guadalupian) Abrahamskraal Formation, their precise age is uncertain as a result of diachroneity of the base of the Formation and the absence, in the Jansenville area, of index taxa to correlate this horizon with the biostratigraphy established in the Western Cape Province. Here, we describe a third skull that we identify as Lanthanostegus, which we recently discovered from a locality north of Laingsburg, on the western side of the main Karoo Basin. This skull reveals morphological details of the palate, occiput, and lower jaw that are not preserved in the described specimens of Lanthanostegus mohoii and will advance understanding of this poorly known dicynodont. This discovery provides the first direct correlation between the lower Abrahamskraal Formation at Jansenville on the eastern side of the basin and the southwestern part of the basin, and suggests that Lanthanostegus occurs in the lowest Tapinocephalus Assemblage Zone (AZ), or possibly to a new assemblage transitional between the Eodicynodon and Tapinocephalus AZs. This supports earlier work proposing that the Eodicynodon AZ is present only on the western side of the Karoo Basin and that the transition from a marine to continental depositional environment occurred later toward the East.
Highlights
Systematic fossil collecting from the rocks of the middle Permian Abrahamskraal Formation of the lower Beaufort Group of South Africa over four decades has revealed more than ten new basal therapsid genera, with most of them being dicynodonts (Day and Rubidge, 2020; Rubidge and Day, 2020)
These fossils, together with other middle Permian taxa, have enabled subdivision of the Abrahamskraal Formation into three dicynodont-based biozones defined by the ranges of Eodicynodon, Eosimops, and Diictodon (Day and Rubidge, 2020; Rubidge and Day, 2020)
We found no fossils of Eodicynodon, which would be expected if the horizon was within the Eodicynodon Assemblage Zone (AZ) given the preponderance of this taxon there
Summary
Systematic fossil collecting from the rocks of the middle Permian Abrahamskraal Formation of the lower Beaufort Group of South Africa over four decades has revealed more than ten new basal therapsid genera, with most of them being dicynodonts (Day and Rubidge, 2020; Rubidge and Day, 2020). Targeted collecting efforts to the east and the north of the known extent of the Eodicynodon AZ have resulted in the discovery of several new anomodont species that are different from the species described from the Eodicynodon AZ. These include Anomocephalus africanus, Colobodectes cluveri, and Prosictodon dubei. The most bizarre species, which has anteriorly oriented orbits, is Lanthanostegus mohoii, and is known from only two fragmentary specimens from the farm Mandalay, north of Klipplaat in Jansenville district of the Eastern Cape Province (Modesto et al, 2002; Modesto and Rubidge, 2003)
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