The origin and paleoecologic significance of the trace fossil Asteriacitesin the Pennsylvanian of Kansas and Missouri

The trace fossil Asteriacites, recorded in Cambrian to Recent shallow- and deep-marine facies, is traditionally interpreted as the resting trace of as terozoans. Well-preserved specimens of A. lumbricalis are abundant in Penns ylvanian (Upper Carboniferous) shallow- and marginal-marine siliciclastic d eposits of eastern Kansas and western Missouri. Detailed morphologic analys is of these specimens suggests that they record the activities of mobile ep ifaunal ophiuroids. Evidence of a brittle star (ophiuroid) producer rather than sea star (asteroid) is provided by (1) trace-fossil morphologic featur es reflecting the anatomy of the producer (e.g., well-differentiated centra l structure, slender vermiform arms) and ophiuroid burrowing technique (e.g ., proximal arm expansion, arm branching), and (2) mode of occurrence (e.g. , gregarious behavior, horizontal and vertical repetition). Vertical and ho rizontal repetition produces complex aggregates of A. lumbricalis that are interpreted either as escape structures (fugichnia) or as feeding structure s, respectively. Ophiura texturata is proposed as a modem analogue for the A. lumbricalis producer, based on inferred life habit and feeding behavior. Asteriacites lumbricalis is present in two different intertidal trace-foss il assemblages. The first assemblage is characterized by high diversity and records tidal flats developed outside of embayments under normal marine co nditions. The second assemblage consists of A. lumbricalis together with a few other ichnotaxa and represents a depauperate association that developed in restricted tidal flats within an embayment or estuarine setting. This c hallenges the conventional view of Asteriacites as a normal-marine salinity indicator. Some echinoderms, and particularly asterozoans, penetrate and i nhabit modern environments of depressed salinity. The presence of Asteriaci tes in Pennsylvanian marginal-marine facies of Kansas and Missouri provides evidence that ophiuroids had adapted to brackish-water conditions by the l ate Paleozoic.