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 mar ine organisms, most importantly decapod crustaceans such as thalassini d shrimp, in modern. intertidal and shallow subtidal environments. Low er Paleozoic examples of Thalassinoides are poorly documented despite the abundance of Ordovician ''burrow mottled'' shelf limestone that is generally thought to represent dense populations of Thalassinoides bu rrows. The paleontological record of decapods begins only in. the Devo nian, and thus, a considerable time gap exists between the early occur rences of Thalassinoides burrows and the oldest fossils of organisms m ost commonly associated with their construction, particularly in post- Paleozoic forms. Specimens of a new ichnospecies of Thalassinoides fro m the lower Paleozoic Manitou and Peerless formations of Colorado, her ein defined as Thalassinoides horizontalis, consist of bedding-paralle l polygonal networks of smooth-walled, unlined, horizontally branching burrows. Burrows form both Y- and T-junctions but contain. no swellin gs at junctions or elsewhere. They consist of a narrow diameter tube o r inner core (<4 mm) and a 3-4 mm outer wall, the latter representing a diagenetic halo formed around a mucus-impregnated burrow. Five style s of preservation relate to the presence or absence of burrow-fill and patterns of mineral replacement and weathering. The extremely small b urrow diameters, lack of some features such as scratchings and swellin gs, absence of vertical shafts, and the common regularity of branching separates this new ichnospecies from other Thalassinoides ichnospecie s in either Paleozoic or post-Paleozoic strata. Thalassinoides horizon talis burrows were not made by large decapod crustaceans like those th at made post-Paleozoic Thalassinoides, but by small crustaceans or, mo re 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 fee ders to colonize infaunal habitats.