Mortality dynamics and fossilisation pathways of a new metoposaurid-dominated multitaxic bonebed from India: a window into the Late Triassic vertebrate palaeoecosystem

ABSTRACT

A new bonebed recovered from the Late Triassic of India contains various macro- and microfossils, unionid bivalves and coprolites. The macrofossils include more than 700 identifiable skeletal specimens of 27 metoposaurid, four phytosaur and two rhynchosaur individuals whereas the microfossils comprise about 500 isolated teeth of varied chondrichthyans, dipnoans, actinopterygians, archosauriforms, and several non-mammalian cynodonts. The bonebed is a monodominant, multitaxic, parautochthonous, time-averaged, attritional assemblage. The Tiki floodplain was populated by highly diverse animal communities occupying terrestrial, semi-aquatic and aquatic realms, where scavenging and/or predation were common. Two distinct biostratinomic modes deduced are – (i), natural death, decomposition, and differential pre-burial modifications of the large terrestrial and semi-aquatic animals, and (ii) breaching of banks during flooding events, mass death, and high energy deposition of sediment load including various aquatic fishes and unionids. Similar REE patterns of a representative sample set of bone specimens and their encasing matrices suggest same diagenetic condition and a single burial event. The MREE enrichment, cerium and uranium content indicate reducing burial microenvironment for the studied fossil specimens. The Late Triassic scenario comprised highly dynamic animal communities that had different mortality dynamics and fossilisation pathways depending on the habitats and palaeobehaviour of the animals.

KEYWORDS:

• Attritional assemblage
• bonebed
• Late Triassic
• metoposaurid
• taphonomy

Acknowledgments

We are grateful to P. M. Datta, former Director, Geological Survey of India, D. Mukherjee, Durgapur Govt. College, Durgapur, India, M. S. Bhat, Department of Biological Sciences, University of Cape Town, South Africa, D. Datta and S. Taral and D. Upadhyay of the Department of Geology and Geophysics, Indian Institute of Technology (IIT), Kharagpur for excavation, fossil collection, active participation in the field, and advices on sedimentology and geochemical analysis. We thank G. Dyke, M. S. Bhat, D. Mukherjee, and an anonymous reviewer for giving constructive suggestions and advices. We thank A. Krupavathi and personnel of LA-MC-ICPMS Laboratory of IIT Kharagpur for help in taphonomic data collection and geochemical sample analysis, respectively. We gratefully acknowledge the funding and infrastructural facilities provided by the Scientific and Engineering Research Board (SERB), India (SERB/CRG/2019/000388), and the Indian Institute of Technology, Kharagpur, India, respectively.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The work was supported by the Scientific and Engineering Research Board (SERB), India under Grant (SERB/CRG/2019/000388)