Hillichnus lobosensis: a behavioural revolution in Bathonian Tellinacean bivalves

Hillichnus lobosensis, a multi-morphological preserved trace fossil represents the combined locomotion and feeding behavioral activities of the tellinacean bivalves. Several specimens of this complex well preserved, endogenic structures are found in the mixed siliciclastic-carbonate rocks of the Middle Jurassic (Bathonian-Callovian) of the Island Belt Zone of Kachchh, India. The structures show different morphological levels of Hillichnus from Level E to Level A. The earliest paleontological evidence of the deposit-feeding tellinacean bivalves is known from the Early Cretaceous. Here we interpret the locomotory and deposit-feeding activity of tellinacean bivalves during the Bathonian-Callovian age based on ichnological evidence. The preserved dual behavioural and functional activities suggest a pronounced change in siphonal function which has switched over partly/completely from infaunal suspension-feeding mode to deposit-feeding mode marking the revolution in tellinacean bivalves coinciding with the global transgression.