Solving a cold case: New occurrences reinforce juvenile callianassids as the Ophiomorpha puerilis tracemakers

Abstract

The ichnotaxon Ophiomorpha puerilis was erected based on material from late Pleistocene shallow-marine, siliciclastic deposits of southeastern Brazil. These tiny burrows with a distinctive lining of cylindrical pellets having rounded ends were originally interpreted as burrows of juvenile callianassid shrimp, with polychaetes also considered as a possible tracemaker. Recent discoveries of O. puerilis in late Pleistocene shallow-marine carbonates from two localities in the Bahamas and from the Miami Limestone of south Florida, described herein, have expanded significantly the geographic range of the ichnotaxon and provided numerous, well preserved specimens for detailed structural analysis of its burrow form and pellets. The new information strengthens the interpretation of juvenile callianassids as the tracemakers, resolving the “cold case” of the burrow's tracemaker. Microscopic analyses revealed that the pellets of O. puerilis are constructional, not fecal. Furthermore, larger forms of Ophiomorpha are common and occur in close association with O. puerilis at each locality. These new data allowed close comparison of O. puerilis pellets with those of Sergio mirim, a modern callianassid from southeastern Brazilian beaches, Ophiomorpha nodosa from Brazilian Pleistocene deposits, and fossil polychaete burrows filled with fecal pellets as described in the literature. The combined evidence allows us to exclude polychaetes as potential O. puerilis tracemakers. Rather, O. puerilis records the settling and initial burrowing behavior of callianassid decapodids. The characteristics of an Ophiomorpha ichnofabric with the addition of O. puerilis also is described and explored. An emended diagnosis for the O. puerilis ichnotaxon is provided, based on the new structural data for this burrow.