Taphonomy and paleoenvironments of Middle Triassic bone accumulations in the Lifua Member of the Manda Beds, Songea Group (Ruhuhu Basin), Tanzania

ABSTRACT

We report new data on the climate, paleoenvironments, and burial history of tetrapod fossils in the Middle Triassic Lifua Member of the Manda Beds (Songea Group) of southern Tanzania. Two bone-bearing intervals have been identified, both hosted by rubified floodplain mudrocks deposited alongside rivers that flowed from the Ruhuhu rift scarps into a series of subsiding basins under a warm, seasonally wet climate. The lower occurrence is a bonebed containing fossils of a large dicynodont (Dolichuranus), large cynodonts (Cynognathus), temnospondyls, small reptiles, and at least two archosauromorph reptiles. A chaotic melange of semiarticulated, disarticulated, and reworked bones associated with pedogenically mottled sandy siltstone is interpreted as having accumulated in a distal crevasse splay complex. The middle to upper Lifua bone accumulations are associated with floodplain pond and sheetwash deposits. Outcropping as isolated patches of strongly calcified rubified mudstones with lenses of reworked glaebule conglomerate, these accumulations contain partially articulated archosaur (Asilisaurus, Nundasuchus) and cynodont (Scalenodon) skeletons along with vertebrate coprolites and nonmarine bivalves (‘Unio’). Changes in floodplain facies, faunal assemblage, and taphonomic style between lower and middle to upper Lifua strata are similar to those recorded between the middle and upper Burgersdorp Formation (subzones B to C of the Cynognathus Assemblage Zone) of the main Karoo Basin of South Africa. We propose that an increase in mean annual temperature and rainfall in southwestern Gondwana during Early to Middle Triassic times resulted in vegetated, semipermanent water bodies in the floodplain depressions that supported a relatively diverse assemblage of herbivorous dicynodont, cynodont, and early archosaur populations.

Citation for this article: Smith, R. M. H., C. A. Sidor, K. D. Angielczyk, S. J. Nesbitt, and N. J. Tabor. 2018. Taphonomy and paleoenvironments of Middle Triassic bone accumulations in the Lifua Member of the Manda Beds, Songea Group (Ruhuhu Basin), Tanzania; pp. 65–79 in C. A. Sidor and S. J. Nesbitt (eds.), Vertebrate and Climatic Evolution in the Triassic Rift Basins of Tanzania and Zambia. Society of Vertebrate Paleontology Memoir 17. Journal of Vertebrate Paleontology 37(6,Supplement).

ACKNOWLEDGMENTS

We acknowledge the following funding bodies for their ongoing support of the TZAM project: Tanzania 2007: NGS 7787–05 (to C.A.S.) and NSF DBI-0306158 (to K.D.A.); Tanzania 2008: The Grainger Foundation (to K.D.A.); Tanzania 2012: NGS 8962–11 (to C.A.S.); Tanzania 2015, 2017: NGS 9606–14 (to S.J.N.), NSF EAR-1337569 (to C.A.S.), and NSF EAR-1337291 (to K.D.A. and S.J.N.). We thank C. Saanane (University of Dar es Salaam), and A. Tibaijuka, L. Nampunju, and M. Salehe (Department of Antiquities, Ministry of Natural Resources and Tourism) for assistance in arranging and carrying out field work in the Ruhuhu Basin. M. Abdalla, W. Simpson, J.-S. Steyer, M. Stocker, and L. Tsuji also made numerous contributions to the field work and subsequent research. Editorial assistance from R. Irmis and corrections and improvements suggested by R. Rogers and an anonymous reviewer greatly improved the manuscript.

Additional information

Funding

National Geographic Society 7787-05,,8962-11,9606-14; National Science Foundation NSF EAR-1337291,NSF EAR-1337569