THREE-DIMENSIONAL TREPTICHNUS FROM THE EARLY JURASSIC OF MASSACHUSETTS: IMPLICATIONS FOR BURROW MORPHOLOGY, PRESERVATION, AND TRACEMAKER IDENTITY
The Jurassic burrows were preserved as shallow, bedding-parallel structures within thin mud laminae separating cross-bedded sandstone layers. Shafts, where present, are subvertical, short, and open to the surface, suggesting an origin for the infilling sand. Instead of shafts, some burrows exhibit bulbous, 3D expanded terminations also open to the surface. It has been proposed that such bulbous terminations reflect compaction of vertical shafts, but numerous lines of evidence contradict this hypothesis, indicating that the projections were chambers produced by the tracemaker. We suggest that expanded projections in the type material were produced similarly, and note that slightly different 3D morphologies are present in T. bifurcus.
When viewed from below, some burrow segments are bisected longitudinally by a thin groove, and some also exhibit two series of punctate marks arranged into pairs, indicating a bilaterian, possibly arthropod, tracemaker. Most importantly, some of the Jurassic burrows are nearly identical to those produced by dipteran fly larvae today.