The Usili Formation is a Late Permian geologic formation in Tanzania. It preserves fossils of many terrestrial vertebrates from the Permian, including temnospondyls, pareiasaurs, therapsids, and the archosauromorph Aenigmastropheus.
History of study
One of the first to study rocks of the Usili Formation was British geologist G. M. Stockley. In 1932, Stockley explored the geology of the Ruhuhu Basin in Tanzania. He called a series of layers dating from the Late Carboniferous to the Middle Triassic of the Songea Series and divided it into eight units labelled K1-K8. Stockley was also the first to describe fossils from these rocks, naming an older layer the "Lower Bone Bed" and a younger layer the "Upper Bone Bed".
In 1957, paleontologist Alan J. Charig described many more fossils from the upper bone beds in his Ph.D. thesis for the University of Cambridge.Charig, A. J. (1957). New Triassic archosaurs from Tanganyika, including Mandasuchus and Teleocrater: Dissertation Abstracts. Cambridge University. Subsequently, Stockley's units were renamed, Charig (1963) calling unit K6 the Kawinga Formation, K7 the Kingori Sandstones, and K8 the Manda Formation. Fossils were identified in many strata, invalidating Stockley's division into two distinct bone beds. Since Charig's description, the Kawinga Formation has been renamed the Usili Formation, the Kingori Sandstones have become the Kingori Sandstone Member of the Manda Formation, and Charig's original Manda Formation has become a subunit of the formation called the Lifua Member. Six formations and one informal unit are currently recognized in the Songea Group (Ruhuhu basin) rocks range in age from Pennsylvanian to Anisian, including the Idusi (K1), Mchuchuma (K2), Mbuyura (K3), Mhukuru (K4), Ruhuhu (K5), and Usili (K6) formations and the informal Manda Beds, which include the Kingori Sandstone (K7) and Lifua Member (K8).
Recent studies have described the Usili Formation as a thick fluvio-lacustrine succession made up of a lowermost conglomeratic interval that is approximately 5 meters thick, grading up into a trough cross-bedded, coarse-grained, sandstone-dominated interval that is thick, overlain by massive nodular siltstone and laminated mudstone beds with minor ribbon sandstones forming the bulk of the succession. Since Parrington (1956), the Usili Formation became widely recognized as a Late Permian formation that correlates with the Teekloof and Balfour formations of South Africa, as well as with the Zambian Upper Madumabisa Mudstone (Cistecephalus AZ). Comparison of Usili tetrapods with those of the lower Beaufort Group has suggested a broad biostratigraphic correlation with the Cistecephalus, Dicynodon, and Tropidostoma assemblage zones. Sidor et al. (2010) recognized only one undivided tetrapod faunal assemblage in the Usili Formation, which includes Aenigmastropheus, temnospondyls, pareiasaurs, gorgonopsians, therocephalians, cynodonts, and dicynodonts, whose remains were collected from various localities. This suggests that several therapsid genera have unequal stratigraphic ranges and temporal durations in the Ruhuhu and Karoo basins. The Kingori Locality is regarded as being equivalent in time to the Lower Daptocephalus Assemblage zone.
Sidor et al. (2010) and Sidor et al. (2013) noted that it is probable that the Chiweta Beds of Malawi and the Usili Formation of Tanzania represent the same rock unit, separated only by political boundaries and geologic faulting (being located on either side of Lake Nyasa). Except for the burnetiid MAL 240, which is unique to the Chiweta Beds, the Usili Formation hosts identical genera, including Aelurognathus, Dicynodontoides, Rhachiocephalus, Endothiodon cf. E. bathystoma, Oudenodon baini, Gorgonops? dixeyi and an indeterminate tusked dicynodont (SAM-PK-7862, SAM-PK-7863).
Paleobiota
Tetrapods
Temnospondyls Taxon Species Locality Material Notes Images Peltobatrachus P. pustulatus A stereospondyl, endemic to this formation.
Parareptiles Taxon Species Locality Material Notes ImagesAnthodonA. serrarius B18 A specimen A pareiasaur. Originally named Anthodon minisculus, it was considered a junior synonym of A. serrarius by Lee (1997). PareiasaurusP. serridens A pareiasaur. Unidentified Parareptilia Indeterminate GPIT K72, six or seven dorsal vertebrae with articulated osteoderms A pareiasaur originally described by von Huene (1944), endemic to this formation.
Eureptiles Taxon Species Locality Material Notes Images AenigmastropheusA. parringtoni B35 UMZC T836, a partial postcranial skeleton including five posterior cervical and anterior dorsal vertebrae, the distal half of the right humerus, a fragment of probable left humeral shaft, the proximal end of the right ulna, and three indeterminate fragments of bone, one of which may represent a partial radius. A protorosaurid archosauromorph, endemic to this formation.
Therapsids
Anomodonts Taxon Species Locality Material Notes Images CompsodonC. helmoediL35, Z04A highly damaged mandible as well as articulated craniodental remains (skull and mandible)First preservation of a mandible for this taxon.Daptocephalus D. huenei B2 (Kingori) SAM-PK-10630, fragmentary skull and postcrania; 7 additional skulls A dicynodontoid dicynodont, the holotype was formerly assigned as a species of Dicynodon. D. leoniceps A dicynodontoid dicynodont, previously considered to be a species of Dicynodon, known only from South Africa. DicynodonD. angielczyki UMZC T1089, a complete skull; 6 additional partial to complete skulls and some possible postcrania A species named for specimens of Dicynodon formerly assigned to Daptocephalus huenei. D. lacerticeps A dicynodontoid dicynodont, known only from South Africa. D. robertsi Originally considered to be a species of Dicynodon, but it is a junior synonym of Oudenodon bainii. D. tealei B32 SAM-PK-10631, fragmentary skull roof An indeterminate dicynodont, a nomen dubium. DicynodontoidesD. kubwa A kingoriid dicynodont, previously considered to be a species of DicynodonD. nowacki Kingori GPIT/RE/7174, a nearly complete skull; other skulls and skeletons Endothiodon E. cf. bathystoma Basal dicynodont, endothiodontid. E. sp. nov. A new, yet unnamed species. E. uniseries Basal dicynodont, endothiodontid. Originally placed in its own genus Esoterodon. Euptychognathus E. bathyrhynchus Kingori GPIT/RE/7104 (UT K 19), well preserved partial skull Basal lystrosaurid dicynodont, previously considered to be a species of Dicynodon. Geikia G. locusticeps GPIT K87/UT von HUENE 1942 Abb.3, juvenile partial skull and dentary; GPIT K114, skull and dentary A geikiine cryptodont, previously considered to be a species of Dicynodon. Katumbia K. parringtoni B19 (Kingori); B4 (Katumbi); Usili-Berges GPIT K120/UT Huene 1942 S.155, fragmentary skull; UMZC T761, UMZC T791, incomplete skulls; A dentary Basal dicynodontoid, endemic to this formation. Kawingasaurus K. fossilis Kingori UT K 52, skull and dentary; UT K 56, skull; UT K 55, 5 skulls and postcrania A cistecephalid dicynodont, endemic to this formation. MdomowabataM. trilobops Many partial to complete skulls, a partial skeleton A cryptodont dicynodont, endemic to this formation. Oudenodon O. bainii Many skulls Basal oudenodontid cryptodont. Pachytegos P. stockleyi B32 SAM 10639, SAM 10642, fragmentary skull elements Basal dicynodont, endothiodontid, endemic to this formation. Pristerodon P. mackayi NMT RB38 Basal dicynodont, eumantellid. Previous reports by King (1988, 1992), King and Rubidge (1993), and Gay and Cruickshank (1999) were based on the holotype specimen of Katumbia parringtoni. The first diagnostic specimen of Pristerodon mackayi from this formation, NMT RB38, was discovered in 2008. Rhachiocephalus R. behemoth GPIT K 15, nearly complete skull; GPIT K 15B, fragmentary skull A rhachiocephalid cryptodont. R. magnus Many specimens A rhachiocephalid cryptodont. Unidentified cryptodont Indeterminate NMT RB22, a partial maxilla An unidentified dicynodont from the base of the formation, originally thought to be a specimen of what is now Mdomowabata.
Biarmosuchians Taxon Locality Material Notes Images Burnetiidae indet. NMT RB4, partial isolated skull roof; NMT RB36, fragmentary right dorsal margin of the orbit A burnetiid most closely related to Burnetia mirabilis from the Dicynodon AZ of South Africa. Too fragmentary to be assigned to a new taxon, its morphology is unique among Cistecephalus AZ taxa.
Cynodonts Taxon Species Locality Material Notes Images ProcynosuchusP. delaharpeae IGP K 92, fragmentary skull A procynosuchid
Therocephalians Taxon Species Locality Material Notes Images SilphictidoidesS. ruhuhuensis Kingori K 70, nearly complete skull and dentary; K 125, nearly complete skull, dentary and right humerus A baurioid, endemic to this formation. Theriognathus T. microps Kingori K 107 and K 84, two fragmentary skulls A whaitsiid
Gorgonopsians Taxon Species Locality Material Notes Images Cyonosaurus C. broomianus A gorgonopsian Dinogorgon D. rubidgei Kingori IGP K 16, nearly complete skull and dentary A gorgonopsid, endemic to this formation. Gorgonops G. sp. A gorgonopsid Inostrancevia I. africana NMT RB380, an isolated left premaxilla A gorgonopsid LycaenopsL. sp. A gorgonopsid NjalilaN. nasuta IGP K 52, nearly complete skull; IGP K 96, fragmentary skull; 6 more skulls A gorgonopsid, endemic to this formation. RubidgeaR. atrox IGP K 46, fragmentary skull A gorgonopsid, formally named Broomicephalus, endemic to this formation. Ruhuhucerberus R. haughtoni Katumbi, B4 MZC T891, nearly complete skull A gorgonopsid, endemic to this formation. SauroctonusS. parringtoni Usili-Berges IGP U 28, complete skull and dentary and fragmentary skeleton A gorgonopsid, endemic to this formation. ScylacopsS. capensis Kingori, B19 MZC T885/FRP 93, skull and cervical vertebrae A gorgonopsid Sycosaurus S. nowaki Kingori IGP K 47, nearly complete skull A gorgonopsid, endemic to this formation.
References