TAPHONOMY AND PALEOENVIRONMENTS OF MIDDLE TRIASSIC BONE ACCUMULATIONS IN THE LIFUA MEMBER OF THE MANDA BEDS, SONGEA GROUP (RUHUHU BASIN), TANZANIA Author(s): ROGER M. H. SMITH, CHRISTIAN A. SIDOR, KENNETH D. ANGIELCZYK, STERLING J. NESBITT and NEIL J. TABOR Source: Memoir (Society of Vertebrate Paleontology), Vol. 17, Vertebrate and Climatic Evolution in the Triassic Rift Basins of Tanzania and Zambia (November 2017), pp. 65-79 Published by: Taylor & Francis, Ltd. on behalf of The Society of Vertebrate Paleontology Stable URL: https://www.jstor.org/stable/44878131 Accessed: 08-07-2021 09:41 UTC

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This content downloaded from 86.59.13.237 on Thu, 08 Jul 2021 09:41:05 UTC All use subject to https://about.jstor.org/terms Society of Vertebrate Paleontology Memoir 17 Journal of Vertebrate Paleontology Volume 37, Supplement to Number 6:65-79 © 2017 by the Society of Vertebrate Paleontology

TAPHONOMY AND PALEOENVIRONMENTS OF MIDDLE TRIASSIC BONE ACCUMULATIONS IN THE LIFUA MEMBER OF THE MANDA BEDS, SONGEA GROUP (RUHUHU BASIN), TANZANIA

ROGER M. H. SMITH,1'2 * CHRISTIAN A. SIDOR,3 KENNETH D. ANGIELCZYK,4 STERLING J. NESBITT, © 5 and NEIL J. TABOR6 Evolutionary Studies Institute, University of the Witwatersrand, P.O. Wits 2050, Johannesburg, South Africa, [email protected]; 2Iziko South African Museum, P.O. Box 61, Cape Town, 8000, South Africa, [email protected]; 3Burke Museum and Department of Biology, University of Washington, Seattle, Washington 98195, U.S.A., casidor@u. washington.edu; integrative Research Center, Field Museum of Natural History, 1400 South Lake Shore Drive, Chicago, Illinois 60605, U.S.A., [email protected]; department of Geosciences, Virginia Tech, Blacksburg, Virginia 24060, U.S.A., [email protected]; 6Roy A. Huffington Department of Earth Sciences, Southern Methodist University, 3225 Daniel Avenue, Dallas, Texas 75275, U.S.A., [email protected]

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, faunai 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).

INTRODUCTION museums in Cambridge, London, Cape Town, and Tübingen (see Sidor and Nesbitt, 2018). In 2007, our team carried out the first paleontology-focused Tetrapod fossils were initially found in the Songea region of research expedition to the Ruhuhu Basin since the 1963 expedi- southern Tanzania in 1930 by British geologist G. M. Stockley tion. This was followed by return visits in 2008, 2012, 2015, and while doing field work for the first geological map of the Ruhuhu 2017. Our investigations focused on three main research ques- Basin (Stockley and Oates, 1931; Haughton, 1932; Stockley, tions: (1) has the fauna been completely documented or are new 1932). In the following years, from 1933 to 1936, F. R. Parrington taxa yet to be found?; (2) is the currently accepted biostrati- from the University of Cambridge and E. Nowack from the Uni- graphic and chronostratigraphic correlation correct?; and (3) versity of Tübingen made larger collections, which established a what are the depositional sedimentary environments and tapho- close similarity between the Ruhuhu tetrapods and those of the nomic processes that led to preservation of the Permian and Tri- main Karoo Basin of South Africa (Nowack, 1937; Huene, 1942, assic faunas? Most of the published research on these fossils has 1944, 1950) and dated the assemblage as Middle Triassic, Anisian concentrated on their taxonomy and biostratigraphic correla- (Charig, 1956, 1963). Serious collecting efforts were suspended tions, with few details of their preservation, modes of occurrence, for the next 25 years (but see Boons tra, 1953) until 1963 when and interpreted taphonomic pathways. This report is the first the 'British Museum (Natural History)-University of London attempt at filling this information gap by presenting the results Joint Paleontological Expedition' focused for 2 months on the of our recent field investigations into the depositional history of Triassic Manda Beds (Attridge et al., 1964), mainly in the east- the various types of bone accumulations in the Middle Triassic ern part of the basin, south of the Rutikura River. Today, the Manda Beds, including a recently discovered bonebed (Z183) in numerous fossils from these early collecting efforts reside in the lower portion of the Lifua Member. The results presented are preliminary and observational rather than quantitative. This is because much of the excavated material awaits full prepara- * Corresponding author. tion, and the logistics of conducting field work in these areas lim- Color versions of one or more of the figures in the itsarticle the can amount be found of time available to complete quarry-type online at www.tandfonline.com/ujvp. excavations. However, in May and June 2017, we did manage to

This content downloaded from 86.59.13.237 on Thu, 08 Jul 2021 09:41:05 UTC All use subject to https://about.jstor.org/terms 66 Smith et al. - Triassic bone accumulations in Tanzania completely uncover the Z183 bonebed; system" thus, under a more"hyperhumid, taphonomic warm climate which changes to data have been recovered for this occurrence than for those slightly more arid and hotter conditions in the upper Lower Tri- higher in the succession. assic." He favored a tectonic control over the approximately Institutional Abbreviations - NMT, National Museum 100-m-thickof Tan- megacycles and noted a general west to east paleo- zania, Dar es Salaam, Tanzania; UMZC, University Museum current of direction that is reflected in a steady decrease in channel Zoology, Cambridge, U.K. sandstone grain size, a decrease in the degree of sorting and rounding, and a corresponding increase in volume of mudrocks. Previous Paleoenvironmental Interpretations In the search for exploitable hydrocarbons, Markwort (1991) made an exhaustive geological appraisal of the reservoir poten- Stockley (1932) published the first geological map tial of of thethe Lifua Member, concluding that it was a fluviatile depo- Ruhuhu Basin that showed the distribution of the lower sitional (K7) system with point bar, crevasse splay, and floodplain and upper (K8) units of the Manda Beds. These were depositslater for- represented by discrete lithofacies. These were depos- merly named the Kingori and Lifua members, respectively, ited under by warm, semiarid climatic conditions evidenced by ped- Wopfner (2002), although he still referred to them collectively ogenic calcretesas and desiccation cracks. Wopfner's (2002) review Manda Beds. Stockley (1932:616) summed up the depositional of the tectonic and climatic events controlling the Tanzanian environment of the Manda Beds as "an alternation of Karooperiods basins of refined Markwort's (1991) interpreted depositional rapid sedimentation under shallow water conditions with environment periods for the Lifua Member to that of meandering rivers of comparatively quiet, deeper water conditions." andStockley floodplains in a semiarid fluviodeltaic environment. He also (1932) also recognized two bone-bearing intervals - a assignedPermian palynomorphs from lower Lifua to the late Scythian one below the Kingori Member (the Lower Bone Bed, and or theK6, temnospondyl ' Parotosuchus' from the mid-Lifua to the now referred to the Usili Formation) and a Triassic oneAnisian. above In this memoir, Wynd et al. (2018) further discuss the (the Upper Bone Bed, or K8, now referred to as the Lifua age ofMem- the Lifua Member. ber). Interestingly, that study noted that the Upper Bone Bed had a basinwide distribution and was confined to the middle part GEOLOGICAL SETTING of an estimated 440 m of strata overlying the arenaceous Kingori Member. Soon after, Nowack (1937) published the results The of Ruhuhuhis Basin was part of a chain of intracratonic exten- field work throughout the basin, including many more sional fossil basins in the western Gondwanan tectonic province that localities. He, followed by Bishop (1968), agreed with Stockley'sbegan rifting soon after formation of Pangea during the Permo- interpretation of the depositional environment and noted Carboniferous that and remained active until the beginning of the most productive vertebrate fossil localities were inbreakup a 50-70- in the Late Triassic (Fig. 1). These rift basins opened m-thick interval in the middle to upper part of the Manda along Beds. a southwesterly structural trend that propagated from the Kreuser (1990:6) gave formation status to the Manda Tethyan Beds, margin, as a release for extensional and transtensional which he interpreted as "fluvial quartzarenites changing forces into brought about by a simultaneous thermally induced cyclical - fining-upward sequences of a meandering crustal stream updoming (Wopfner, 1990, 1994; Veevers and Powell,

FIGURE 1. Location and general geology of the Ruhuhu Basin. A, satellite image of the study area with 1:50,000 topographic map references; B, simplified geology of the study area showing main stratigraphie and struc- tural elements and location of the Triassic fossil localities described in this paper.

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1994; Bordy and Catuneanu, 2002) scaleas well trough as right-lateral and planar transi- cross-bedding commonly with over- tion slip between Gondwana and turned Laurasia friction-drag (Veevers, and et convolute al., de watering structures (Wopf- 1994). Despite the strong tectonic ner,control 2002). of sedimentPaleocurrents accumula- are directed mainly east-southeast, tion, it has become apparent that andclimatic the memberfluctuations is interpreted also left a as the remnants of a low-angle mark on the stratigraphie and paleontological distributive braidplain record in that these prograded across a gently subsiding, basins. These climatic overprints are easterly proving dipping to be useful paleoslope chrono- into a large lake (Markwort, 1991). stratigraphic markers to correlate This the paleoslope numerous is rift-bounda possible indication that crustal thinning had Karoo-aged sequences in south central already Africa begun with prior the to mainthe detachment of the Indo-Madagascan foreland sequence to the south plate(Catuneanu (Veevers et et al., al., 2005). 1994). As a general trend, the climate changed The from Lifua cold Member and semiarid conformably dur- overlies the Kingori Member ing the Late Carboniferous-earliest and Permian makes upinterval, the upper to halfwarmer of the fifth and final first-order cycle and eventually hot with fluctuating (megasequence precipitation duringof Markwort, the Tri- 1991) of the Songea Group. The assic, giving way to arid erg deserts top of of the the Early Lifua Jurassic Member (Smith is not preserved, because it is exposed et al., 1993a). Karoo successions of as the an rifteroded basins land broadly surface com- across the basin. Although its original prise five tectonically controlled thickness upward-fining is unknown, depositional the low degree of compaction and diagen- sequences, each commencing with esis a coarse-grained of the Lifua sedimentsor conglom- suggests that there were no major eratic facies near the base and terminating accumulations in a ofmudrock-domi- younger strata in this part of the basin. Kaaya nated interval (Fig. 2; Wopfner, 2002). (1992) recognized that Cenozoic-aged block faulting was clearly Although our Ruhuhu Basin project responsible includes forthe thePermian separate and eastern and western exposures of Triassic (fourth and fifth sequences) the strata Manda of Beds the today,Songea but Group, he noted that during Lifua deposition this report is confined to the fifth these depositional two parts sequenceof the basin - the had different source areas and thus Olenekian- to Middle Triassic-aged he regarded Manda themBeds as(see separate below), sub-basins. Today, the Lifua Mem- with an emphasis on the last unit ber of comprises the sequence, at least the six Lifua 150-200-m-thick upward-fining second- Member. Triassic deposition commences order cycleswith the (Fig. Kingori 2) totaling Sand- approximately 1300 m (Wopfner, stone Member, which rests with 2002).a low-angle These haveunconformity been interpreted on as allocyclic fluviodeltaic rocks of the upper Permian Usili Formation.cycles controlled The base either is marked by axial lake-level fluctuations under a in places by a conglomerate of cobble semiarid and climaticboulder-sized regime rip-up (Markwort, 1991) or by climatically clasts overlain by a monotonous sequenceinduced ofupward-desiccating medium- to coarse- cyclothems (Wopfner et al., 1991). grained, clean quartz sandstone, laid Numerous down in smaller-scalethick sets of sandstone/mudrocklarge- third-order cycles

FIGURE 2. Stratigraphy of the Carboniferous-to-Triassic Songea Group with a summary lithological log of the Manda Beds and stratigraphie posi- tions of detailed measured sections through tetrapod bone accumulations in the lower (Z183) and middle (Z29, 34, 40, 81, and 26) portions of the Lifua Member. Black crosses represent isolated tetrapod fossils.

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(estimated to be ca. 50 in number bycynodont Nowack, concentrations 1937), with calcareouseach on encrustations the and sug- order of 15-20 m thick, are evident gested in thatthe these outcrops carcasses accumulated (see detailed in 'shallow mud basins.' sections in Figs. 3-6), with the more Our recentresistant field work gently failed to locate dipping any identifiable speci- lower sandstone units forming parallel mens within ridges the main cliff-formingtraceable portion along of the Kingori Mem- strike for up to 3 km. Cox (1991) mappedber. However, some we did discoverof these a relatively ridges diverse multitaxa that were associated with the 1963 bonebedexpedition's close to the fossil base of thelocalities overlying Lifuain Member. the western part of the basin. The We ridges found no signare of formeda hiatus in this by rapid feld- change from low-sinu- spathic channel sandstone at the base, osity channel-dominatedoverlain by fluvial green plain of and the upper red portion of the mottled micaceous siltstones with minor mudstone lenses. These Kingori Member to medium- to high-sinuosity floodplain-domi- smaller upward-fining cycles are typical of distributive fluvial nated sys-facies association of the lower portion of the Lifua Mem- tems in rift settings subjected to autocyclic switching of berchannel (Wopfner, 1990; Markwort, 1991). Thus, the age of the belts in response to differential gradients created by aggradation Kingori Member has the potential to be better constrained by (Hajek et al., 2010), as well as to base-level changes induced biostratigraphic by correlation of the new lower Lifua faunai assem- the short-term oscillations in water level of the axial lake blage (Blair (Nesbitt et al., 2017; Wynd et al., 2018). Below are descrip- and Bilodeau, 1988; Holz et al., 2014). tions of the sedimentology, taphonomy, and interpreted palaeoenvironments of the most productive of the Lifua fossil SEDIMENTOLOGY AND TAPHONOMY OF LIFUA accumulations. BONE ACCUMULATIONS

The precise age of the Kingori Member is still Z183: uncertain, Lowermost because Lifua Member - Crevasse Channel and Splay no radiometric dates are available and biostratigraphically useful fos- Near the village of Uwanja-wa-Ndege, in an area overlooking sils are scarce. Nowack (1937) reported a few bone fragments from the Rutikura river valley to the north, stream-head erosion is the Kingori Member sandstones, but none of these are identifiable. actively cutting back into the bedrock, revealing a number of gul- Cruickshank (1965) described a juvenile specimen of Kannemeyeria ley-type cliff exposures up to 10 m high (Fig. 3A). Locality Z183 simocephalus (see Cruickshank, 1970; Renaut, 2000, for taxonomie is one of these gulleys. details) collected by Parrington about a mile south of Stockley's Sedimentology and Taphonomy - The Z183 gulley is floored locality B17 (F. R. Parrington, unpublished field notes). Stockley by medium-grained, 'dirty' trough-cross-bedded sandstone with (1932) mapped this locality in the Kingori Member, and Parrington's scattered, well-rounded, extrabasinal quartz pebbles up to 2 cm unpublished field notes show that he agreed with this assessment. in diameter. It is generally pinkish gray with bedding planes pref- Our recent prospecting in that area yielded no fossils, but some 6 km erentially stained by dark red hematite. The upper contact of this south of B17 we collected an as yet unidentified medium-sized distal interpreted channel fill shows sloping wedge-shaped stringers of femur from a pedogenic nodule conglomerate. The stratigraphie medium-grained sandstone interdigitating with the overlying position of this locality is estimated to be some 50 m above the top dark reddish-brown/olive-gray mottled sandy siltstone, a charac- of the Kingori Member in the lowermost Lifua Member. The pres- teristic feature of inner-bank point-bar deposits (Diaz-Molina, ence oí' Kannemeyeria has led to the Kingori Member being consid- 1993). ered to be slightly older than the overlying Lifua The main Member bone-on-bone (e.g., multitaxa bonebed occurs 1.5 m Cruickshank, 1986 a, 1986b; Cox, 1991; Lucas, 1998, 2010; Gay and above the upper point bar facies in a narrow 45-cm- thick interval Cruickshank, 1999; Rubidge, 2005; Hancox et al., 2013) and equiva- of alternating tabular sandstone/siltstone beds with minor string- lent to the Cynognathus 'B' subzone of the South African Karoo ers of dark brown claystone (4.25 m on log in Fig. 3). Here, Basin, with the Lifua Member being equivalent to the Cynognathus numerous large and small partially articulated, disarticulated, 'C' subzone. The Cynognathus 'B' subzone has generally been con- and fragmented bones of dicynodonts, cynodonts, and archosaur- sidered to date to the early Anisian (e.g., Rubidge, 2005), but recent radiometric dates from South American strata correlated with the omorphs (including Teleocrater rhadinus; Nesbitt et al., 2017) occur in two discrete taphonomic modes. The larger bones are Cynognathus Assemblage Zone have raised the possibility of an age mostly of the dicynodont Dolichuranus (e.g., NMT RB554; C. F. as young as Carnian (Ottone et al., 2014; Marsicano et al., 2016). Kammerer, pers. comm.) (Fig. 3B) and the cynodont Cynogna- thus (Fig. 3C; Wynd et al., 2018), and they are commonly semiar- Previous Taphonomic Observations of Lifua Fossils ticulated and closely associated, suggesting that the skeletons Stockley (1932) made the general observation that archosaur, were buried at, or very close to, the site of death with minimal rhynchosaur, and dicynodont remains found in the Lifua Mem- transportation. They are hosted by a bed of white/pinkish-gray ber tended to be more completely articulated than those of cyno- mottled fine-grained silty sandstone with dark brown mudstone donts, but he made no detailed record of their modes of chips and pebbles that immediately overlies a basal lag of occurrence. Nowack (1937) was the first to publish taphonomic smaller, more fragmented, and taxonomically diverse bones of studies of the Ruhuhu tetrapod fossils, including sketches of archosauromorphs in (e.g., Teleocrater rhadinus , NMT RB498, and situ bone scatters. He noted that most complete reptile speci- an allokotosaurian, NMT RB550), temnospondyls (NMT mens (e.g., rhynchosaurs) were confined to elongate floodplain RB551), and small reptiles (NMT RB552) (Fig 3D). The depressions that were only affected by gentle currents. Others bonebed he succession of upward-coarsening, thin, tabular traction- attributed to the 'bloat and float' taphonomic pathway of currentgas- sandstone sheets, bounded by pedogenically modified filled carcasses transported by floodwaters and beached on mudrocks,mud is interpreted as a distal crevasse splay (Smith, 1993). banks, and he also suggested that some skeletons had been disar- In such a setting, the bone accumulation mechanism was likely ticulated by scavenger activity. However, he presented no direct shallow sheetwash events that intermittently swept the flood- evidence of bone surface modification to support these interpre- plain and dumped the bedload into temporary standing water tations. Years later, Cruickshank (1965, 1967, 1986a) described bodies to be subsequently buried by the prograding distal cre- tooth scrapes and indentations (e.g., Mandaodonites coxi' vasse splay complex (Smith et al., 1989). The larger associated UMCZ T1225; also see Angielczyk et al., 2018) on dicynodont carcasses were transported only once, whereas the underlying limb and skull bones from the mid-Lifua Member, which he carpet of fragmented and abraded bones have been mobilized attributed to the scavenging activity of carnivorous tetrapods, several times by successive flood events before final burial. possibly of archosaurs ( = 'rauisuchid thecodonts' of Cruick- Before significant compaction had taken place, the dicynodont shank, 1986a:421). Bishop (1968:24) confirmed the association remains of in the more arenaceous matrix had become thickly

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FIGURE 3. Sedimentology and taphonomy of a lower Lifua Member bonebed (Z183). A, panorama of cliff exposure containing the Z183 bonebed at the level of the white bags. B, exhumed skull minus lower jaw of the dicynodont Dolichuranus lying left side up with snout dipping slightly down- ward into the upper levels of the bonebed. Note the rough-textured calcareous crust enveloping the skull. C, limb bones from an associated skeleton of Cynognathus from the middle of the bonebed (NMT RB459). D, planimetrie view of the basal lag of the Lower Lifua bonebed that contains disar- ticulated and broken elements of at least five small archosauromorph skeletons as well as isolated temnospondyl bones. Scale bar equals 5 cm (C).

encrusted with calcareous nodular crushing material and(Fig. in 3B). situ In bone con- breakage especially of the reptile skull trast, most of the lag deposit bones bones, within scapulae, finer-grained neural matrix spines of vertebrae, and ilia. The juxta- are free of surface mineralization. positionIn the 2017 of fresh field andseason, rolled the bone fragments in a bonebed filling lower bone-on-bone lag accumulation an elongate was completely depression exhumed in the floodplain surface suggests multi- to reveal an elongate carpet of bone ple roughlytransport 6 mevents. long However,by 2 m the bulk of the bonebed is made wide with thickness ranging from up5 to of 25 unabraded cm, with brokenthe thicker bones that appear to have been derived sections filling irregularly shaped fromdepressions five carcasses in the scourthat disarticulated sur- in the immediate vicinity. face. From field observations, the A bonebed similar appearsmix of totaphonomic be com- pathways was recorded in lower posed of the disarticulated and fragmented Permian bonesvertebrate of at leastbonebeds five of the El Cobre Canyon Forma- individuals of two different archosauromorph tion of New taxa Mexico ( Teleocrater (Lucas et al., 2005). The latter were inter- rhadinus and an allokotosaur; Nesbitt preted et al., as 2017), having a widebeen range preserved in the distal reaches of of ontogenetic stages of a dicynodont anastomosed ( Dolichuranus crevasse ), and splay rare channels at the transition between fragments of a large temnospondyl. semiaridPreburial floodplainbone-weathering and lake is shore (Eberth and Miall, 1991). A negligible; however, there is evidence similar of preburial setting isabrasion interpreted and for the Z183 bonebed, with the

This content downloaded from 86.59.13.237 on Thu, 08 Jul 2021 09:41:05 UTC All use subject to https://about.jstor.org/terms 70 Smith et al. - Triassic bone accumulations in Tanzania lower deposit likely accumulated in andthe temnospondyl crevasse amphibians channel (unnumbered) thalweg were recovered. after a period of incision and reworking However, none of of thethese taxa alluvium occur in the Asilisaurus and bonebedthe overlying fill having been rapidly deposited itself. This archosaur in accumulation low-density lies within a scour sedi- trough in ment-laden flash flood events. the middle of a 10-m-thick interval of overbank mudrocks. It was found by following a trail of loose bone fragments upslope to just below the apex of a low mudrock interfluve between two gul- Z34: Middle to Upper Lifua Member - Distal Floodplain Pond lies (Fig. 4A). Some of the loose bones are matrix-free (Fig. 4B); Margin however, most are completely encrusted with a rough-textured Locality Z34 is the type locality for the silesaurid dinosauri- calcareous nodular layer that completely masks the shape of form, Asilisaurus kongwe (Nesbitt et al., 2010). It occurs in mid- smaller elements (Fig. 4D). Lifua Member strata near the town of Litumba Kuhamba, close The underlying sandstone is a medium-grained, well-sorted pale to the southern margin of the western part of the Ruhuhu Basin olive-gray feldspathic arenite, trough cross-bedded at the base and (the Ketewaka-Mchuchuma sub-basin of Kaaya, 1992; Fig. 1). ripple cross-laminated towards the top. The upward transition into Sedimentology and Taphonomy - Considering only macrover- the overlying dark reddish-brown/olive-gray mottled mudstone is a tebrates, the Z34 locality may be regarded as a monotaxic series of alternating thin flat-bedded sandstones and purple bonebed dominated by the remains of at least 14 Asilisaurus mudrocks with numerous distinctive vertical and horizontal Plano- kongwe individuals (based on the number of second sacral verte- lites- type invertebrate burrows. This arenaceous unit is interpreted brae discovered; Fig. 4C). From gullies in the surrounding area, as a fluvial channel fill of a low-sinuosity distributary stream in the isolated postcrania of cynodonts {Aleo don brachyrhamphus and low-lying distal parts of a fluvial fan, close to the shoreline of an Scalenodon angustifrons ), some associated elements (including a axial rift valley lake. Similar channel/overbank cyclothems were complete skull, NMT RB42) of the dicynodont Sangusaurus described by Mack and Leeder (1999) in the Plio-Pleistocene (Angielczyk et al., 2018), and more fragmentary remains of strata filling the Rio Grande rift of Mexico. Angonisaurus (NMT RB155; Hancox et al., 2013), archosauro- The overlying floodplain deposits are pedogenically modified morphs ( Stenaulorhynchus and pseudosuchians, NMT RB28), with general rubification and reduction mottles along root

FIGURE 4. Sedimentology and taphonomy of mid-Lifua vertebrate locality Z34. See Figure 3 for legend. A, type locality of Asilisaurus kongwe. B, slightly abraded vertebra with no encrustation displays fresh unweathered periosteal bone. C, reconstruction of Asilisaurus kongwe (modified from Nesbitt et al., 2010) from this locality. D, close-up of in situ tibia with encrustation of calcareous nodular material. Scale bars equal 1 cm (B), 2 cm (D), and 10 cm (C).

This content downloaded from 86.59.13.237 on Thu, 08 Jul 2021 09:41:05 UTC All use subject to https://about.jstor.org/terms Smith et al. - Triassic bone accumulations in Tanzania 71 channels. Incipient BCa horizonation other, is evidenced although four by or fivea few may becalci- closely associated within an fied skew planes and horizons of small area of irregularly 10 m . At least 25 skulls shaped were documented calcar- in the loose eous concretions. From 7.5 to 9.25 bone m debris (Fig. emanating 4), the from mudrocks one of these occurrences (Z95; become sandier and contain stacked scour surfaces each overlain Fig. 5B). Two small articulated skeletons of a very rare procolo- by thinly bedded silts tone/fine sandstone with clay s tone phonid veneers parareptile (Tsuji et al., 2013; Tsuji, 2018) had micrite and mudstone chips. The Asilisaurus bonebed occurs atform the within top the internal bone cavities soon after burial and of this unit and appears to have accumulated in a shallowbefore complete permineralization had taken place (Fig. 5C; scoured depression that has been filled with a chaotic melange Holtz and of Schultz, 1998). locally derived elastics, including small reworked pedogenic The glae- fifth bonebed, dominated by abundance of the dinosauri- bules, coprolites, and articulated nonmarine bivalves (identified form Asilisaurus kongwe (Z90 of Fig. 5), occurs as a densely as ' Unio ' karooensis ; Cox, 1932; Yates et al., 2012). These packed are allconcentration within a unit of dark olive-gray mudstone heavily coated and cemented together with dark brown with weather- reddish-brown mottling and is characterized by abundant ing calcareous nodular material. The bonebed is overlain disarticulated by a postcranial elements from dozens of small Asili- massive olive mudstone, with pedogenic gley features thatsaurus sug- skeletons. Scattered in and around this pocket of isolated gest that it was seasonally drier and more vegetated than bones lower are reworked carbonate nodules, bone-bearing coprolites, down. Brown mottles along vertical root channels, calcareous and nonmarine bivalves. The fragile and largely hollow Asilisau- nodules, fenestrated calcite laminae, rhizocretions, and rus calcified bones are exceptionally well preserved, without having been skew planes all indicate stronger seasonality fluctuations subjected in to the permineralization, Assuring, and compression groundwater level. A 50-m-wide by 2-m-thick muddy sandstonethat affected the cynodont bonebeds. Screen washing of a 45-kg- channel fill caps the exposure at this locality, displaying bulkthe low- sample from this site yielded tens of nearly complete verte- sinuosity 'shoestring' geometry of a distal distributary brae, channel fragments of limb bones, phalanges, isolated teeth, temno- (Mack and Leeder, 1999). spondyl vertebrae, and hundreds of small bone fragments. The The association of the bone accumulation within a scour limb bones are uniformly incomplete, and any fragment of bone depression, reworked floodplain soil debris, and articulated more than non- 3 cm long is rare in this sample. Because of its loca- marine bivalves is interpreted as further evidence for tion the at thepres- top of the exposure, just beneath the modern soil pro- ence of more persistent wet patches on the seasonally file, it isdry possible that the bones may have been secondarily floodplains. Such conditions have been interpreted asconcentrated the accu- into the dense 'pocket' by recent down-wasting of mulating agents of Jurassic dinosaur bonebeds in Utah the land (Rogers, surface. 1990; Gates, 2005) and are evident today in Kenya and Multiple Ethiopia cynodont skeletons with a similar areal distribution around the numerous pans on the rift valley floor. but During with athe greater degree of disarticulation and disassociation dry season, these seasonal alkaline ponds and lakes arehave the been focus documented in the Middle Triassic uppermost Santa of intense megafaunal activity to the extent that Mariatrampling Formation of Brazil (Holz and Schultz, 1998; Bertoni- and bioerosion accelerates denudation of the surrounding Machado andsoils Holz, 2006; Temp-Muller et al., 2015). Here, both (Conybeare and Haynes, 1984). herbivorous and carnivorous cynodonts make up the bone assemblage, which is characterized by isolated but complete ele- ments of a range of sizes preserved in similarly pedogenically Z81: Middle to Upper Lifua Member - Distal Floodplain Pond modified floodplain mudrocks. They interpret the carcasses as and Sheetwash Deposits having accumulated over several seasons on the margin of a Bone accumulations at the Z81 locality were discovered playa lake near and propose selective prédation, particularly by pro- the village of Lutimba Kuhamba in gulley exposures terosuchians, of mid- as the main agent of concentration. Comparable Lifua mudrocks that were created by the headward Middle erosion Triassic of a cynodont accumulations also occur in the lower northerly flowing tributary of the Ruhuhu River. The part gulley of the pen- Upper Omingonde Formation of Namibia (Smith etrates some 10 m into gently southward-dipping strata, and Swart,exposing 2002) where multiple semiarticulated skeletons of a near-continuous 40-m-thick stratigraphie interval the (Fig. cynodont 5A) Luangwa (Abdala and Smith, 2009) were col- over a distance of approximately 500 m. lected from a massive pale red rooted siltstone bed interpreted Sedimentology and Taphonomy - After 6 days' prospecting as wind-transported at loess deposited on a semiarid floodplain. this locality, we located five stratigraphically separate As in thefossil Brazilian assemblage, the preponderance of isolated accumulations within the pedogenically modified lower floodplain jaws in the Upper Omingonde record is particularly mudrock exposures. Four of the concentrations (see notable. sections in Fig. 5; Z81 at 11 m, Z91 at 17 m, Z94 at 20 m, and The Z 198concentrations at of carcasses in the Lifua floodplain 28 m) have the same host lithologies and taphonomic mudrocks style. are considered to be the combination of drought- They are characterized by numerous heavily calcified, induced articu- death assemblages around floodplain ponds or seeps lated and semiarticulated skeletons of the cynodont (Rogers, Scalenodon 1990) and intense monsoonal-type thunderstorms angustifrons , along with some associated rhyncosaurs resulting and a in few sheet run-off, gullying, and erosion of the floodplain isolated archosaur and dicynodont elements. The fifth surface concen- followed by dumping of the eroded material into low- tration (at 41.5 m) is a dense pocket of small disarticulated lying areas. As such, climatic seasonality influenced behavioral postcrania of the archosaur Asilisaurus and other smalland physical verte- concentration mechanisms, leading to the repeated brates. All three cynodont horizons are lying on low-relief concentration scour and burial of cynodont carcasses in and around surfaces within reddish-brown mudstone and are associated floodplain ponds. with thin (<20 cm thick) lenses of calcareous mud-pebble The herbivorousand cynodont Scalenodon , with transversely pedogenic glaebule conglomerates. The conglomerates expanded are postcanine teeth adapted to grind fibrous vegetation more resistant to erosion and have distinctive very darksuch brownas ferns and equisetalians (Crompton, 1955), is the most weathered surfaces. Each cynodont horizon comprises common many taxon found in mid-Lifua vertebrate localities. Why juvenile and subadult individuals (up to 25 based on a Scalenodoncollection was particularly susceptible to dying in and around of ex situ lower jaws) of the herbivorous cynodont theS. angusti- Lifua floodplain ponds is not known. However, it is possible frons. The skeletons are preserved mainly dorsal-up withthat likelimbs Trirachodon , a closely related neighbor in the main spread on either side, and with the lower jaw in articulation. Karoo Basin, they sheltered underground in communal mounds The individual skeletons are rarely in direct contact (Groenewald with each et al., 2001), and as a social group they were bound

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FIGURE 5. Sedimentology and taphonomy of mid-Lifua vertebrate locality Z81. See Figure 3 for legend. A, overview of erosion gulley with expo- sures of mid-Lifua mudrocks at the Z81 locality near Lutimba Kuhamba (see Fig. 1). Mountains in the distance mark the horst of basement rocks sep- arating the Ruhuhu Basin into western and eastern blocks. B, surface collection of numerous loose skulls of the traversodont cynodont Scalenodon from a small mudrock mound. Several semiarticulated specimens of herbivorous cynodonts were also excavated from this accumulation. C, procolo- phonoid skull (Tsuji et al., 2013; Tsuji, 2018) as found, lying dorsal-up with some articulated anterior postcrania (NMT RB167). Note the thin encrus- tation of rough- textured micrite. Scale bars equal 1 cm (C) and 5 cm (B).

to their home territory, even when Z128:fodder Middle became to Upper Lifua scarce. Member Rogers- Pond Margin Wetlands (1990) reviewed several possible bone accumulation mechanisms for the dinosaur bonebeds in the Upper Locality Z128Cretaceous is another locality Two exposing Medi- middle to upper cine Formation of Montana and concluded Lifua strata inthat the Lutimba drought Ndyosi District was that the contains a diver- most plausible causal agent. He highlighted sity of archosauromorph the reptiles effect and cynodont that skeletons with water dependency has on the behavior contrasting preservationof modern styles within savannah a 4-m-thick succession of mammals during the three phases pedogenicallyof drought modified (after floodplain Shipman, mudrocks (Fig. 6). 1995) and noted that juveniles and aged Sedimentology individuals and Taphonomy of gregarious - The reddish-gray massive herbivores were more susceptible tomudstone death has distinctivethrough vertically starvation elongated and downward- during extreme drought events. Not branching only light is gray there mottles asuggestive behavioral of the former presence of concentrating mechanism that binds decaying the root weakened systems. More direct animals signs of seasonalto wetting and the waterhole, but also after death their drying carcassesare evidenced bytended evenly spaced to accu-vertical fissures up to mulate on the devegetated floodplain 1.5 m inlong theand 5 cmvicinity wide at the oftop andwater tapering downwards. holes (Conybeare and Haynes, 1984; These Haynes,interpreted shrinkage 1988) cracks as are well preferentially as cemented along ephemeral floodplain channels with micrite (Behrensmeyer laminae containing mudrock et chipsal., that show that 2012). We propose that the mid-Lifua calcification Scalenodon and fissure concentrationsformation were contemporaneous and have the taphonomic signature of drought incremental. Suchaccumulations large-scale desiccation around polygons are common a shrinking pond. This is supported inby distal the floodplains numerous surrounding semiarticu- a standing water body where lated skeletons (and coprolites) lying the waterin dorsal-uptable is close to the attitude surface. A 0.25-m- in thickan horizon- interpreted desiccation pose, encrusted tally bedded with and pondplanar-cross-bedded margin light car- gray sandstone with a bonate concretionary material and 'lag'associated of reworked withcalcareous nonmarine nodules and rare angular quartz bivalves. clasts immediately overlies the desiccated interval (Fig. 6A).

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FIGURE 6. Sedimentology and taphonomy of mid-Lifua vertebrate locality Z128. See Figure 3 for legend. A, horizontal and pla- nar-cross-bedded crevasse channel at 3 m on the measured section, overlain by the fossilif- erous massive siltstone unit that contains dis- articulated yet closely associated archosaur skeletons. Geological pick length equals 30 cm. B, overview of distal floodplain pond facies exposed at an archosaur (NMT RBI 87) excavation. C, in situ dentary and osteoderm of Parringtonia gracilis (NMT RB426) without a trace of permineralization. D, in situ archosaur femur (NMT RB187) with thin calcitic encrustation. Scale bars equal 1 cm (C) and in cm (D).

The sharp top contact displays sinuous-crested, small current structures rip- are interpreted as those of a low-sinuosity tributary to ple forms. This sandstone unit, interpreted as a crevasse the main splay river that flowed along the rift axis. Three micrite- deposit, is overlain by massive dark reddish-brown silty encrusted mud- semiarticulated rhynchosaur skeletons were recovered stone with very few nodules, showing no desiccation from or pedo-mudrocks immediately overlying the channel fill (Fig. 7B, genic modification (Fig. 6B). This is interpreted as proximalC), possibly representing animals that became trapped between floodplain alluvium deposited close to the contributing the bankschannel. of the incised channel. The cynodont and archosauromorph scattered skeletal Theelements scattered skeleton of Nundasuchus songeaensis (Nesbitt recovered immediately below the calcified interval are et devoidal., 2014) of was collected from reddish-brown silty mudstone any perimineralization and have the appearance of iron-stained with distinctive pale olive reduction haloes around downward- porcelain (Fig. 6C); those within the desiccated interval branching are root channels. Isolated irregularly bulbous-shaped cal- heavily coated with rough-surfaced micrite. In contract, careous the scat- nodules (ca. 5 cm diameter) and calcite-lined subvertical tered skeletal remains of multiple individuals of an arcuate armored surfaces ('skew planes' of Brewer, 1964) in the massive pseudosuchian ( Parringtonia gracilis , NMT RB426; mudrockssee Nesbitt attest to vertisol-type shrink/swell movements in the et al., 2018) recovered from the noncalcified silty mudstones ancient soil profile similar to those commonly seen in modern above the sandstone are only thinly coated with a calcite clay-rich crust alluvium in semiarid climates (Ahmad, 1983). (Fig. 6D). The degree of disarticulation likely reflects theFurther resi- evidence for a warm, seasonally dry climate may be dence time that these carcasses spent on the floodplain inferred surface from the taphonomy of several articulated rhynchosaur before complete burial, and the amount of calcite periminerali- (Stenaulorhynchus stockleyi) skeletons, one of which was pre- zation was controlled by time spent within the subsoil served BCa hori-dorsal-up with its gastralia still in life position (Fig. 7E). zon, both of which were ultimately controlled by floodplainThis posture is interpreted as evidence for preburial mummifica- aggradation rates (Bown and Kraus, 1981). tion of the carcasses and their subsequent burial with skin intact. The complete fibrous calcite crust (Fig. 7C) around each bone is similar to the Microcodium structure of algal- or fungal-medi- Z40: Middle to Upper Lifua Member - Proximal Floodplain Paleosol ated micrite precipitation around living roots (Klappa, 1978) and is here taken as evidence of carbonate precipitation in the reduc- Mid-Lifua strata exposed by downcutting tributaries tion of halo the created by bacterial decomposition of collagen and Ndatira River in Litumba Ndyosi District (Fig. 7A) have connective been tissue around fresh bone (Bao et al., 1998). Early dia- prospected by a number of expeditions over the past genesis,80 years, and the lack of vertical compression in these bones, sug- including four separate visits (2007, 2008, 2012, 2017) gests by thatour the groundwater table and aggradation rates were team. These have yielded an abundant and diverse fossil relatively verte- high on the proximal floodplains alongside the main brate assemblage, including a scattered but associated tributary and near- channels. complete skeleton of the recently described archosaur Nundasu-At the top of the exposed section, a 'pocket' of isolated cyno- chus songeaensis (Nesbitt et al., 2014; NMT RB48; Z41 dontof Fig. bones 7) belonging to Cricodon metabolus (NMT RB227; and numerous rhynchosaur and cynodont remains. Sidor and Hopson, 2018) and a possible probainognathian Sedimentology and Taphonomy - Most of the bones (NMT are RB390)pre- yielded well-preserved postcanine teeth with no served within a 3.5-m-thick interval of rubified, rooted, micrite and encrustations (Fig. 7D). The taphonomic processes lead- pedogenically calcified floodplain mudrocks overlying ing ato 1 accumulation-in- of these clean bone pockets are not well thick bed of trough-cross-bedded channel sandstone. understood.The light However, the fact that they lie immediately gray sandstone is poorly cemented micaceous wacke beneathwith red the modern soil profile might suggest a concentrating mudrock-pebble intraclasts and numerous vertical Skolithos- mechanism linked to recent landscape denudation and soil type burrow casts on the top surface. The dimensions erosion. and infill

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FIGURE 7. Sedimentology and taphonomy of mid-Lifua vertebrate locality Z40. See Figure 3 for legend. A, panoramic overview of the head of an erosion gulley with cliff exposures of mid-Lifua mudrocks containing numerous disarticulated and semiarticulated rhynchosaurs, archosaurs (including the type of Nundasuchus songeaensis , NMT RB48), and cynodont skeletons. B, outcrop view of the fossil-rich rough-textured paleocalcrete horizon at 4.6 m on the measured section with an in situ rhynchosaur premaxilla in the foreground. C, cross-section of a rhynchosaur limb bone illustrating the thickness and fabric of the sparry calcite crust with irregular outer surface. D, postcanine teeth of Cricodon metabolus (NMT RB227) that were found free of encrustation. These secondary concentrations of more durable elements are found near the top of the cliff sections and are likely generated by recent denudation of the modern land surface. E, an articulated skeleton of the rhynchosaur Stenaulorhynchus stockleyi (NMT RB806) lying dorsal- up in olive-gray mottled mudstone. This specimen still has its ventral gastralia in full articulation, which suggests rapid burial with skin still intact; geo- logical pick length equals 33 cm. Scale bars equal 5 cm (B) and 1 cm (C, D).

Z29: Middle to Upper Lifua Member degree - ofDistal disarticulation, Floodplain and a close association Pond with limestone Deposits sheets. The fossiliferous interval (Fig. 8) is interpreted as having accumulated around the margins of a standing water body within Locality Z29 is a small exposure of mid-Lifua mudrocks in the the distal floodplain depression between meanderbelts. The chan- Litumba Kuhamba District (Fig. 1) that has yielded a scattered nel sandstones below and above have different lithologies and archosaur skeleton and an articulated partial skeleton of Par- geometries. The lower is filled with laterally accreted trough-cross- ringtonia gracilis (NMT RB28; Nesbitt et al., 2018) as well as bedded fine sand (Fig. 8E), the upper with vertically accreted with numerous skulls, isolated lower jaws, and individual postcranial tabular units of medium-grained sandstone and reworked pedo- elements of cynodonts ( Scaledodon and Aleodon ; Fig. 8A). genic nodule conglomerate (Fig. 8D). Sinuous unlined meniscate Sedimentology and Taphonomy - Most of the cynodonts burrows (cf. Taenidium) preserved on, and just beneath, the upper emanate from a thin but laterally extensive sheet of rough-sur- surface of the lower sandstone are attributed to sediment-ingesting faced calcareous nodular material capping a 1.25-m-thick bed of deposit feeders such as annelid worms (D' Alessando and reddish-brown silty mudstone (4.5 m on section in Fig. 8). Many Bromley, 1987). It appears that the invertebrate burrowers isolated bones and clusters of nonmarine bivalves are fully encased exploited the layer of organic particles deposited on the upper sur- in the micritic material (Fig. 8B). The mudrocks immediately face of a point bar during waning flood, while the sediments were below the calcareous sheet contained a scattered Parńngtonia skel- wet but not saturated. This type of colonization is commonly eton (NMT RB28) and numerous tetrapod coprolites (Fig. 8C). encountered in rivers with fluctuating hydrodynamic regimes The taphonomic signature of the numerous cynodont skeletons where bar surfaces are subaerially exposed soon after a deposi- at this site mimics that of Z128 and Z34, with similar host facies, tional event (Smith et al., 1993b). The upper conglomeratic

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FIGURE 8. Sedimentology and taphonomy of mid-Lifua vertebrate locality Z29. See Figure 3 for legend. A, cynodont locality in rubified mid-Lifua calcified siltstone (5.5-6.5m on the sedimentological log) in the Lutimba Kuhamba District. Inset dorsal view of small cynodont skull (NMT RB52). B, archosaur limb bone with patchy encrustations of iron-rich calcite. C, collection of vertebrate coprolites from the surface of the small outcrop shown in A. D, intraformational pedogenic glaebule conglomerate from the base of a wadi-type channel fill at 9.5 m on measured section. E, view of upper part of the ephemeral channel sandstone at 10.6 m on sedimentological section with meniscate Taenidium burrows arrowed. F, close-up of horizontal and vertical unlined meniscate invertebrate burrows assigned to Taenidium ichnogenus. Scale bars equal 1 cm (A, D, F), 2 cm (C), and 3 cm (B).

sandstone body is interpreted as a probablywide, tenuous shallow given thelow-sinuosity different tectonic settings. wadi-type ephemeral stream channel Despite(Piatt, our 1989),best efforts, incised our team into has not the discovered any floodplain alluvium to drain away run-off identifiable fossilsfrom from the the Kingorisurrounding Member that are biostra- floodplain during the rainy season. This tigraphically interpretation informative; the isolatedis based bone fragments on occa- its poorly sorted gravelly sandstone fill sionally and found vertical in the Kingori accretion Member cannot units be referred to overlying conglomerate-lined internal specific scour clades. However,surfaces correlating (9-11 the m lower on Lifua faunai graphic log in Fig. 8). assemblage of Z183 with those of the South African Karoo Basin as well as the Namibian Huab, and Brazilian Santa DISCUSSION Maria basins, does help to constrain the age of the upper Kingori Member to early Anisian. Thus, at this stage, the Litho- and Biostratigraphic Correlation with the Main Karoo Basin Z183 fauna confirms that the juvenile Kannemeyeria skull reported by Cruickshank (1965) is most likely from the King- Our geological and paleontological field work supports Now-ori Member, and that contrary to Wopfner 's (2002) proposal, ack's (1937) initial observation that the Kingori and Lifua mem-the Kingori Member and the Katberg Formation should no bers are part of a depositional continuum, and even though longertheir be considered entirely contemporaneous. contact represents a rapid transition from channel-dominated In the main Karoo Basin, the facies transitions between the braidplains to floodplain-dominated alluvial plains, it is never- Katberg and overlying Burgersdorp formations have a biostrati- theless very likely to be diachronous (Fig. 9). graphic signature that has been demonstrated to be mildly dia- Wopfner (2002) suggested a lithological and chronostrati- chronous (Hancox et al., 1995; Neveling et al., 2005). We are graphic correlation between the Kingori Member and confidentthe that the recently discovered vertebrate collection from Katberg Formation of the main Karoo Basin, but this theis Kingori/Lifua transition zone (Z183), which contains the

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such that it can be considered a basinwide phenomenon. By contrast, the lower Lifua bonebed concentrations have so far only been recorded in the eastern section. Evaluating possible biogenic versus physical factors (after Rogers and Kidwell, 2007) leading to the Z183 bone accumulation within a crevasse splay sandstone, we propose that the crevasse channel itself provided a physical trap site for animals, espe- cially the larger, more ungainly dicynodonts. Over several seasons, their disarticulated skeletons were winnowed, rolled, and reorientated by successive through-bank flood events, forming a secondary hydraulic concentration downstream from the death site. Regarding the middle to upper Lifua bone accumulations, the association with laminar carbonate precipitation, large-scale polygonal shrink/swell fissure fills, and gleying in these upward- fining second-order cycles (Fig. 2) suggest that ponds and springs were a much more common occurrence on the Ruhuhu Basin floodplains at this time than before or after. Such fluctuations in groundwater table in an active half-graben setting are likely to have been tectonically controlled, but in the short term (decades to centuries), the amount of surface water varies with the topog- raphy of the floodplains and prevailing climate. The sedimento- logical and taphonomic evidence from this study favors an increase in mean annual temperature and onset of a higher sea- sonal rainfall regime during middle to upper Lifua times. We propose that the increase in number and size of ponds, springs, and seeps in middle to upper Lifua times (Fig. 9) promoted perennial vegetation that became diversity hotspots for the resi- dent tetrapods, especially the herbivorous cynodonts and small FIGURE 9. Paleoenvironmental reconstructions of the Middle Triassic archosaurs. This led to a basinwide increase in the size of tetra- Manda Beds with generalized stratigraphie sequences that document pod populations the around ponds, springs, and seeps that naturally changing fluvial style and pedogenesis in the active rift basin. Theresulted Kingori in increased bone accumulation in these depositional Sandstone Member accumulated on a braidplain with wide, shallow facies. chan- Thus, we favor climatic rather than tectonic control over nels and midchannel bars filling the rift valley floor. The mid-Lifua the distribution land- of bone accumulations in the middle to upper scape was dominated by floodplains dotted with seeps and standing Lifua. water A similar scenario was proposed by Hancox (1998) in bodies that became the preferred habitat of small herbivorous interpretingcynodonts the changing depositional environment of the Cyn- and archosaurs and provided the conditions for maximizing bone burial. ognathus subzone B to subzone C in the main Karoo Basin of South Africa. It is therefore interesting to note that the faunai affinities of the lower Lifua bonebed are closest to subzone B dicynodont Dolichuranus , is time-equivalent to the Upper and thoseOmin- of the middle to upper Lifua bone accumulations are gonde Formation of Namibia and Cynognathus subzone closer B of to the those of subzone C, suggesting that not only are they main Karoo Basin (Abdala et al., 2012). It follows contemporaneous,that the but also that the ecosystems in the different underlying Kingori Member sandstones are likely to bebasins a time- were responding to the same short-term climate change. and facies-equivalent of the upper Katberg Formation. The In conclusion,ques- the distinctive change in faunai assemblage and tion now remains as to whether the same allocyclic control taphonomic mech- style between lower and middle to upper Lifua anism caused the main drainage systems to amalgamate strata into suggests increasingly warmer, wetter climatic conditions in braidplains in both basins at the same time, and if so, this was part the of Pangea during early to mid-Triassic times, similar to cause tectonic or climatic or both? the transition from Cynognathus subzone B to subzone C in the The Cynognathus Assemblage Zone is generally considered main to Karoo Basin of South Africa. We therefore propose that be early Anisian (Hancox et al., 1995; Rubidge, 2005). However, increased mean annual rainfall raised lowland water tables in recent high-resolution radiometric dating has shifted the age western for Gondwana during this time, resulting in well-vegetated tetrapod-bearing intervals of the Argentinian Chañares Forma-semipermanent water bodies in floodplain depressions, which tion (Marsicano et al., 2016) and the Puesto Viejo Group became the preferred habitat of small-bodied cynodont and early (Ottone et al., 2014) from late Anisian to early Carnian. archosaur Both populations and provided favorable conditions for these faunas, as well as the lower Lifua Z183 assemblage, bone have burial. taxa in common with the Cynognathus C subzone, and it follows that they may well be 5-10 Ma younger than previously thought. ACKNOWLEDGMENTS

We acknowledge the following funding bodies for their ongo- Vertebrate Taphonomy and Paleoenvironments of the Lifua Member ing support of the TZAM project: Tanzania 2007: NGS 7787-05 (to C.A.S.) and NSF DBI-0306158 (to K.D.A.); Tanzania 2008: We have identified two distinct taphonomic styles within The Grainger Foundation (to K.D.A.); Tanzania 2012: NGS the vertebrate fossil occurrences in the Lifua Member: cre- 8962-11 (to C.A.S. ); Tanzania 2015, 2017: NGS 9606-14 (to S.J. vasse channel-hosted bonebeds in the Kingori/Lifua transitionN.), NSF EAR-1337569 (to C.A.S.), and NSF EAR-1337291 (to beds and, some 300 m higher in the succession, the K.D.A. distal and S.J.N.). We thank C. Saanane (University of Dar es floodplain/pond margin concentrations of the middle Salaam),to upper and A. Tibaijuka, L. Nampunju, and M. Salehe Lifua. The middle to upper Lifua taphonomic style has (Department been of Antiquities, Ministry of Natural Resources and identified in both the western and eastern parts of theTourism) basin for assistance in arranging and carrying out field work

This content downloaded from 86.59.13.237 on Thu, 08 Jul 2021 09:41:05 UTC All use subject to https://about.jstor.org/terms Smith et al. - Triassic bone accumulations in Tanzania 77 in the Ruhuhu Basin. M. Abdalla, W. Cox, C. Simpson,B. 1991. The Pangaea dicynodontJ.-S. Rechnisaurus Steyer, and the M.compara- Stocker, and L. Tsuji also made numerous tive biostratigraphy contributions of Triassic dicynodont faunas. to Palaeontologythe field work and subsequent research. Editorial 34:767-784. assistance from R. Irmis and corrections and improvements Cox, L. R. 1932. suggested Lamellibranchia fromby the R. Karroo Rogers Beds of the Ruhuhu coalfields, Tanganyika Territory. Quarterly Journal of the Geologi- and an anonymous reviewer greatly improved the manuscript. cal Society of London 88:623-633. Crompton, A. W. 1955. On some Triassic cynodonts from Tanganyika. Proceedings of the Zoological Society, London 125:617-669. ORCID Cruickshank, A. R. I. 1965. 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Wynd, B. M., C. A. Sidor, M. R. Whitney, Yates, A. M.,and F. H. B.Neumann, R. Peecook.and P. J. Hancox. 2018. 2012. The earliest post- The first occurrence of Cynognathus Paleozoic in Tanzania freshwater bivalves and preserved Zambia, in coprolites from the with biostratigraphic implications for Karoo the Basin, age South of Africa. Triassic PLoS ONE 7:e30228.strata doi: 10.1371/jour- in southern Pangea; pp. 228-239 in C. nal.pone.0030228.A. Sidor and S. J. Nesbitt (eds.), Vertebrate and Climatic Evolution in the Triassic Rift Basins of Tanzania and Zambia. Society of Vertebrate Submitted September 1, 2016; revisions received November 21, 2017; Paleontology Memoir 17. Journal of Vertebrate Paleontology 37 accepted November 24, 2017. (6, Supplement). Memoirs editor: Randall Irmis.

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