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Upper Triassic vertebrate bonebed from Tiki Formation, India



From: Ben Creisler
bcreisler@gmail.com

A new online paper:



Debarati Mukherje & Sanghamitra Ray (2012)
Taphonomy of an Upper Triassic vertebrate bonebed: A new rhynchosaur
(Reptilia; Archosauromorpha) accumulation from India.
Palaeogeography, Palaeoclimatology, Palaeoecology (advance online publication)
http://dx.doi.org/10.1016/j.palaeo.2012.03.010
http://www.sciencedirect.com/science/article/pii/S0031018212001502?v=s5


The Upper Triassic Tiki Formation of India is a rich storehouse of
varied vertebrate fossil assemblages. So far, there is no information
on taphonomic signatures of the Tiki vertebrate assemblages in
comparison to that of other fossiliferous horizons of India. We report
a new, low diversity, mono-dominant, multitaxic vertebrate
accumulation where rhynchosaurs constitute the dominant component. The
formation of the rhynchosaur bonebed is attributed to biological
aggregation with a hydraulic overprint resulting in a mixed-origin
concentration. Other taxa include rauisuchid, phytosaur, small
indeterminate archosaur and cynodonts. Taphonomic study based on 617
skeletal specimens of rhynchosaurs collected from nine sites within an
area of about 250x217 sq m shows that most of the specimens are
disarticulated and disassociated but in close spatial proximity to one
another; some are associated specimens and few are articulated. About
13 to 20 individuals of rhynchosaur at different ontogenetic stages
are estimated from the specimens collected, suggesting gregarious
behaviour, possibly herding. These specimens show varying degree of
weathering, breakage, encrustation, abrasion and deformation. The
bonebed is preserved within the Tiki red mudstone unit and is found in
association with paleosol profiles, suggesting prolonged subaerial
exposure. Spatial distribution and relative bone frequencies show
differential susceptibility of the skeletal specimens to fluvial
transport. 55.4% of the collected skeletal specimens belonged to
Voorhies Group I, whereas 12.4% and 24% constituted Voorhies Group II
and III respectively, and 8.2% of the collected specimens belonged to
the intermediate group I and II. It appears that the animals
concentrated in the vicinity of the water sources during prolonged
period of aridity and died possibly during high seasonal rainfall that
resulted in a major flood event. Subsequently, the soft tissues
decomposed, and the skeletons suffered prolonged subaerial exposure
when the water receded leading to disarticulation and fragmentation
followed by minor dispersion by low velocity water currents. This
resulted in segregation of skeletal specimens, which were gradually
covered by mud deposited during later flooding events. Based on the
known flora and fauna, the Tiki Upper Triassic ecosystem is
reconstructed for the first time. In the aquatic ecosystem, the
metoposaurid labyrinthodonts occupied the top of the food pyramid
together with the semi-aquatic parasuchids, which occupied an
ecological niche similar to that of the present day crocodilians. The
abundance of herbivorous rhynchosaurs at the base with few large and
carnivorous rauisuchids and parasuchids at the top suggest a trophic
structure similar to that of a modern day terrestrial ecosystem.