Priapulid Origin of Treptichnids: Evidence from Experimental Ichnology  

The major evolutionary events that characterize the Precambrian-Cambrian transition are accompanied by profound ecological changes in the composition of benthic communities, the nature of the substrate, and the occupation of marine ecospace.  The increased animal activity on and within the substrate is attested by numerous trace fossils, such as the cosmopolitan Treptichnus pedum, whose first appearance is used as the GSSP to mark the base of the Cambrian.   In spite of its stratigraphical importance, the trace makers of Treptichnus, and more generally the treptichnids, have long remained an enigma.  Our experimental ichnology with Recent worms solves this puzzle.  Well-preserved treptichnids from the Cambrian of Poland, Sweden, Greenland, China and other regions closely resemble horizontal traces produced by Recent priapulid worms in laboratory conditions.  This suggests that priapulids created treptichnids, and allows the construction process of the trace to be reconstructed in three dimensions. 

Treptichnids are sub-horizontal burrow systems, produced in the sub-surface and generally preserved as positive hyporeliefs.  They typically consist of a series of subcylindrical segments that join each other at angles varying between 20° and 90°. Our data clearly show the segmented pattern of treptichnids to arise from the successive retraction and reorientation of the worm’s proboscis.  They also indicate that the striae present along the segments of treptichnids were probably imprinted in sediment by the external ornament of the worm (e.g. scalid rows on proboscis), used as anchoring features.  The size range and overall morphology of treptichnids are consistent with those of priapulid worm body-fossils in Cambrian lagerstätten (e.g. Maotianshan Shale, China). The segments of numerous treptichnids are directed upwards, and are interpreted as the probing branches of the burrow system towards the water-sediment interface.  Although the exact function of these probings is unclear, they may have been part of the worms’ feeding strategy, and would suggest random predation upon small epibenthic invertebrates.  The gut contents of Cambrian priapulid worms, which sometimes contain diverse epibenthic organisms such as hyoliths and tiny arthropods, strongly support this interpretation.  The antiquity of treptichnids, with possible occurrences below the Precambrian-Cambrian boundary, would designate priapulids as one of the oldest infaunal inhabitants of the substrate.