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Origin of Triassic Lystrosaurus bonebeds in Karoo, South Africa

From: Ben Creisler

A new online paper:

Pia A. Viglietti, Roger M.H. Smith & John S. Compton (2013)
Origin and palaeoenvironmental significance of Lystrosaurus bonebeds
in the earliest Triassic Karoo Basin, South Africa.
Palaeogeography, Palaeoclimatology, Palaeoecology (advance online publication)
doi: http://dx.doi.org/10.1016/j.palaeo.2013.08.015

Earth experienced its most devastating extinction event at the end of
the Permian period 251 million years ago (Ma). Despite an estimated 75
to 90% loss of species globally in both marine and terrestrial realms
across the Permian–Triassic Boundary (PTB), around 20% of the
terrestrial tetrapod genera in southwestern Gondwana survived and were
immediately joined by a number of immigrant taxa to occupy most of the
vacant niches of the earliest Triassic. Preserved in the Karoo Basin
of South Africa is an almost continuous stratigraphic record of
terrestrial sedimentation through the PTB that hosts a fossil record
of ecosystem collapse, survivorship and recovery. The adaptation of
the mammal like reptiles (therapsids) of the Lystrosaurus Assemblage
Zone to a highly seasonal, potentially drought prone semi-arid
earliest Triassic Karoo Basin is associated with changes in modes of
fossilisation. Isolated dicynodont skulls and postcranial elements are
commonly found in the latest Permian. However, in the earliest
Triassic the dicynodonts occur as articulated, curled-up skeletons and
multi-individual monotaxic bonebeds. Lack of epiphyses and relatively
small skull length confirm that the bonebeds comprise several subadult
Lystrosaurus declivis (L. declivis) carcasses. No significant evidence
for hydraulic bone concentration along with clusters of ribs in life
position point to complete carcasses being present at the site of
death, and suggest that animals behaviourally congregated before
perishing together. The bonebeds are hosted by an 8 m thick horizon of
floodplain mudrocks in the lower Katberg Formation named the
Lystrosaurus abundant zone. The bonebed horizon is overlain by
sand-filled mud cracks capped by coarse sediments indicative of rapid
deposition during waning floods. Stable isotope analyses of pedogenic
and early diagenetic calcite nodules in association with the bonebeds
yield average δ13C values ranging from − 9.5 to − 5‰ and δ18O values
of 13.5 to 16‰, respectively. These isotope values support a
seasonally cold, semi-arid climate at high latitude (~ 55ºS). The
presence of vertebrate burrow casts on bonebed horizons and evidence
of shelter sharing suggests tetrapods were attempting to escape
extreme climatic conditions. Aggregation behaviour of subadult
Lystrosaurus during unusual cold snaps may best explain the origin of
bonebed assemblages.