Thalassinoides suevicus (Rieth, 1932)

Diagnosis. Predominantly horizontal, more or less regularly branched, essentially cylindrical components forming large burrow systems; dichotomous bifurcations more common than T-shaped branches (from Frey and Howard, 1985).

Description: Straight or slightly winding, horizontal to oblique, flattened cylinders, 7-20 mm wide, showing Y to T-shaped bifurcations in horizontal system, more or less regularly branched, essentially cylindrical large burrow systems; dichotomous bifurcating common. They are filled with homogeneous dark clayey material. The cylinders are at least 30 mm long and join horizontal systems at different levels. Burrow walls distinct and smooth, burrow fills structureless and sharply different from host sediment. Typically bulbous enlargement at points of branching and burrow ends.

Discussion: Thalassinoides Ehrenberg, 1944 is a domichnial and fodinichnial structure produced by crustaceans, mostly decapods (Frey et al., 1984). It occurs in a great variety of marine environments, yet is most typical of the shelf Cruziana ichnofacies. Many authors (e.g., Fürsich, 1973; Ekdale, 1992; and Schlirf, 2000) have discussed this ichnogenus and its ichnotaxonomy.The suevicus is distinguished from other ichnospecies of this ichnogenus by predominantly horizontal, more or less regularly dichotomous.

Diagnosis More or less regularly branched, predominantly horizontal burrow systems; components may be enlarged slightly at points of bifurcation.

The burrows attributed to Thalassinoides suevicus are predominantly horizontal systems, and visible at sedimentary interfaces preserved as convex hyporelief or providing a nodular fabric to bedding sections. The burrows are large and characterized by an irregular width that can exceed 110 mm, and their length can reach more than 1 m bending occasionally. Burrows branch at acute angles, are Y shaped and frequently have swellings in divergence areas (i.e., turning chambers). Transverse sections of the burrow are elliptical, with the major axis parallel to the bedding planes due to sedimentary compaction; this confirms the common compressed preservation of M. rapax carapaces. Burrow walls are smooth and unlined and the burrow fill is passive (Fig. 2H, Fig. 4), differing from the limestone or fine sandstone sedimentary host rock by the more intense carbonate cementation. These features suggest that burrowing was developed in stable soft-to-stiffgrounds. The burrow fill is also and frequently composed of coarse-grained quartz or bioclasts (Fig. 2F, 3B). The burrow diameter is presently at least three times wider than the width of M. rapax that are usually found inside Thalassinoides. However it is common to overestimate Thalassinoides diameter due to the concretionary enlargement of the burrow during early diagenesis (Fürsich, 1974; Fig. 2C).