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 250x217m^2 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 behavior, 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 Groups II and III respectively,
and 8.2% of the collected specimens belonged to the intermediate Groups 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.
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