Abstract
Thalassinoides burrows, commonly interpreted as dwelling structures, are widespread in marine and coastal environments. This study presents the first documented occurrence of a significant assemblage of Thalassinoides ichnofossils within the Lower Cretaceous Qamchuqa Formation in the Kurdistan Region of northeastern Iraq. These trace fossils are preserved in coarse-grained detrital limestones, terrigenous carbonates, and mixed sediments, and are associated with ichnofacies indicative of coastal to shallow-marine settings, particularly the Skolithos and Cruziana ichnofacies.
Numerous extensive burrow networks were observed. During the Mesozoic–Cenozoic, the likely producers of these burrows were decapod crustaceans, supported by the presence of the crayfish Hoploparia within the structures. Notably, crab trackways and gastropod bulldozing trails reaching lengths of up to 0.5 meters represent some of the longest known invertebrate cursorial traces in the fossil record.
The burrow systems consist of branching horizontal tubes connected to the sediment surface by vertical or inclined shafts. Swellings, typically eccentric, occur at branching points, and the burrows exhibit backfilled sediment indicative of active construction by the tracemakers. The tubes range from 5 to 20 mm in diameter and have smooth walls. Evidence of lateral movement suggests the potential emergence of crab-like forms and sideways locomotion during the Cretaceous. This study represents the first recorded occurrence of Thalassinoides in Iraq.
Keywords:
Ichnofossils, Thalassinoides, Lower Cretaceous, Qamchuqa Formation, Ichnofacies, NE Iraq
