A new arthropod resting trace from the middle Cambrian of Texas

Abstract

A new arthropod resting trace, attributed to a phyllocarid crustacean, is described from the middle Cambrian (upper Miaolingian) Hickory Sandstone Member of the Riley Formation (Moore Hollow Group) at Crabapple Creek in Gillespie County, central Texas, USA. Minterichnus shieldi igen. et isp. nov. is preserved as small, isolated, bilaterally symmetrical hypichnia comprising two regions. The wider anterior region shows imprints of antennae, a rostrum, five thoracopods (walking legs) and at least five pleopods. The narrower posterior region terminates in a bilobed (furca) imprint and terminal (telson) imprint. Three specimens of M. shieldi occur on the base of a sandstone slab, deposited in a tidal channel or tidal flat environment, associated with a juvenile bivalved phyllocarid carapace and worm burrow, interpreted as either the same individual repositioning itself or as seperate animals crowded together, trapped in a tidal pool. These fossils were previously interpreted as the ventral surface of a chasmataspidid (Euchelicerata), representing the oldest record of this clade; the oldest unequivocal euchelicerates are now Ordovician horseshoe crabs.