Thalassinoides Ehrenberg, 1944

Diagnosis. Large burrow-systems consisting of smooth-walled, essentially cylindrical components. Branches Y- to T-shaped and typically enlarged at bifurcation points. Burrow dimensions variable within a given system (from Frey and Howard, 1985).

Burrows and systems of burrows consisting of vertical or inclined cylindrical shafts and horizontal tunnels, solitary, straight, or forming branched sizevariable interlacements (labyrinths,nets with polygonal meshes). External surfaces of burrow casts are usually smooth, although frequently bearing longitudinal irregular to, less commonly, regular ridges and grooves representing scratching traces left by limbs of animals. The tubular wall lining is usually missing or not preserved; rare specimens have a very thin smooth envelope. Cross sections of shafts and tunnels are characterized by sizevariable diameters ranging from a few millimeters to 22 cm; locally, burrows bifurcate in the Y or T manners with their diameters increasing at bifurcation sites to form bulbs and irregular inflations. In outcrops, burrows are mostly represented by casts; fragments of shafts are usually characterized by circular cross sections, while tunnels exhibit oval or ellipsoidal sections depending on host rock lithology. Some tunnels and shafts demonstrate local inflations. In some specimens, shafts grade at their distal ends into complex polygonal horizontal systems with regular or irregular meshes. Locally, short dead-end chambers diverge in the fanshaped manner from lower ends of shafts. The burrows are mostly confined to coarsegrained rocksprimarily representing incoherent sediment: littoral and upper littoral sandstones and, less commonly, to bioclastic–ooid pack to grainstones. The burrows were passively backfilled by gravitational processes.