Paper No. 11
Presentation Time: 4:05 PM
THREE-DIMENSIONAL TREPTICHNUS FROM THE EARLY JURASSIC OF MASSACHUSETTS: IMPLICATIONS FOR BURROW MORPHOLOGY, PRESERVATION, AND TRACEMAKER IDENTITY
GETTY, Patrick R., Center for Integrative Geosciences, University of Connecticut, 354 Mansfield Road U-1045, Storrs, CT 06269, HSIEH, Shannon, Department of Ecology and Evolutionary Biology, University of Connecticut, 75 North Eagleville Road, U-3043, Storrs, CT 06269, MCCARTHY, Thomas D., Center for Integrative Geosciences, University of Connecticut, 354 Mansfield Road, U-2045, Storrs, CT 06269-2045 and BUSH, Andrew M., Ecology and Evolutionary Biology and Center for Integrative Geosciences, University of Connecticut, 75 N. Eagleville Road, Unit 3043, Storrs, CT 06269, patrick.getty@uconn.edu
Treptichnus bifurcus is the type species of an ichnogenus of burrows composed of segments commonly arranged in a zigzag pattern in bedding plane parallel view. The tracemaker typically added new segments just before the end of the preceding segment, producing a bifurcated appearance with each segment projecting beyond the junction of those that follow. The projections may widen toward their ends and are interpreted as (sub)vertical shafts directed to the sediment surface. We examined type and topotype specimens of
T. bifurcus, as well as a suite of three-dimensionally preserved specimens from an Early Jurassic dinosaur tracksite in the East Berlin Formation of Massachusetts, in part to understand why projections expand. We have also examined some similar modern burrows. Our observations suggest that the ichnogenus requires some revision, and support previous hypotheses for tracemaker assignment.
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.
