known lugworm, Arenicola. Its castings characterize especially sandy tidal 

 flats during ebb tide. Newer investigations proved that their rather deep- 

 reaching burrows are not always in the shape of a U but sometimes a J. These 

 burrows often show a fine annulation directly under the funnel. Such impress- 

 ions of the body are also to be observed on some fossil burrows from which we 

 are able to conclude that they were made by annelids, and not be arthropods 

 which also produce similar burrows in sand and mud. The burrow of Arenicola 

 is a typical "dwelling burrow" (in German "Wohn-Bau" according to a proposal 

 of A. Seilacher). Its upper end has often been somewhat consolidated by slime 

 and even by ferruginous material. Such burrows can be preserved during the 

 diagenesis of the sediments. From the German middle Triassic Muschelkalk 

 of Bavaria we know trace -fossils represented by funnels and small flat cone- 

 like elevations sometimes connected by a U-shaped burrow. They resemble 

 closely the recent Arenicola funnels and castings. The bed with these fossils 

 is to be found through a distance of about 25 kilometers and they can serve as 

 guide -fossils for a special horizon. The name Arenicola has been used (some- 

 times also erroneously) to designate different fossil burrows believed to be 

 made by similar worms: Arenicolites, Arenicoloides and Archarenicola . 

 Such burrows are by no means indicators of fossil tidal regions as supposed by 

 some authors: Arenicola funnels and castings may be seen on the shore of the 

 Baltic Sea on occasions when western storms blow back the water; this area is 

 nearly free of tides. 



Another polychaete worm, Nereis , often produces trails resembling 

 the form of antlers. Its locomotion trails are often branched or show small 

 lateral branches, while the dwelling burrow of Nereis is represented by a 

 rather complicated system of irregularly dug burrows one connected with an- 

 other. The crawling trail is sometimes used several times by the same ani- 

 mal and it can be observed to turn back in the same trail. Such branching of 

 trails was formerly believed impossible for fossil worm trails. Before 

 Nathorst's observations of branched recent worm trails branching was one of 

 the chief "proofs" for the plant origin of most fossils called "Problematica". 



On the coast of the Frisian Islands and on the coast of the Baltic Sea, 

 meandering trails in form of a rather regular spiral were detected; some years 

 later it was found that they are produced by the worm Paraonis . Only few ani- 

 mals live in the sandy region of the surf. A little polychaete worm ( Scoleco- 

 lepis) was here observed by Seilacher and its "Lebensspuren" are circular 

 markings made by its tentacles while fishing with them for nutrient particles. 

 Very similar markings are also known from Cambrian and Jurassic strata. 



Other worms living in the tidal flats or in somewhat deeper water be- 

 long to the well-known Terebellids which build tubes ( La nice , Lagis , Sabellaria ), 

 There are many fossil counterparts in rocks ranging in age from the Paleozoic 

 to the present. 



The little annelid Polydora boring a U-shaped burrow in the shell of 

 pelecypods, gastropods etc., gives a good example for that which we call in 

 German "Spreiten-Bau". By the continuously prolonged deepening of the U 



1Z7 



