A detailed ichnological analysis focused on Thalassinoides structures recorded just below the Cretaceous-Paleogene (K-P) boundary layer of the Agost section (southeast Spain) aimed to evaluate macrobenthic colonization during the K-P boundary event. Stereomicroscopy and field emission scanning electron microscopy analyses conducted on material filling in Thalassinoides trace fossils reveal the presence of dark infill material that includes spherule-like elements, similar to those previously differentiated in the K-P boundary layer. Energy-dispersive X-ray spectrometry indicates a ferruginous composition (goethite spherules). This represents the first record of spherules related to the K-P boundary event within trace-fossil infilling material. The passive filling of this material into the Thalassinoides bioturbation allows interpretation of the gravitational deposition just after Thalassinoides trace-maker colonization. The absence of erosive sedimentary structures or other material deposited between the Thalassinoides trace-maker colonization and the spherule deposition is interpreted as evidence of rapid colonization during the K-P boundary event, nearly contemporary with the spherule layer deposition.

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