Thalassinoides and the enigma of early Paleozoic open-framework burrow systems

The trace fossil Thalassinoides, a common constituent of the Zoophycos and Glossifungites ichnofacies, is abundant in post-Paleozoic rocks. Modern Thalassinoides-like burrows are constructed by a variety of marine organisms, most importantly decapod crustaceans such as thalassinid shrimp, in modern intertidal and shallow subtidal environments. Lower Paleozoic examples of Thalassinoides are poorly documented despite the abundance of Ordovician mottled shelf limestone that is generally thought to represent dense populations of Thalassinoides burrows. The paleontological record of decapods begins only in the Devonian, and thus, a considerable time gap exists between the early occurrences of Thalassinoides burrows and the oldest fossils of organisms most commonly associated with their construction, particularly in post-Paleozoic forms. Specimens of a new ichnospecies of Thalassinoides from the lower Paleozoic Manitou and Peerless formations of Colorado, herein defined as Thalassinoides horizontalis, consist of bedding-parallel polygonal networks of smoothwalled, unlined, horizontally branching burrows. Burrows form both Yand T-junctions but contain no swellings at junctions or elsewhere. They consist of a narrow diameter tube or inner core (<4 mm) and a 3-4 mm outer wall, the latter representing a diagenetic halo formed around a mucus-impregnated burrow. Five styles of preservation relate to the presence or absence of burrow-fill and patterns of mineral replacement and weathering. The extremely small burrow diameters, lack of some features such as scratchings and swellings, absence of vertical shafts, and the common regularity of branching separates this new ichnospecies from other Thalassinoides ichnospecies in either Paleozoic or post-Paleozoic strata. Thalassinoides horizontalis burrows were not made by large decapod crustaceans like those that made postPaleozoic Thalassinoides, but by small crustaceans or, more likely, soft-bodied organisms that made semi-permanent to permanent tunnels within firmground sediment. The open-framework burrow systems may record the activities of some of the oldest vagile suspension feeders to colonize infaunal habitats.