Recent studies have shown that glycerol dialkyl glycerol tetraether (GDGT) lipids, thought be unique for (hyper)thermophilic archaea, are also present in non-thermophilic archaea and environments. In this study, an unusual H-shaped isoprenoid GDGT, in which the two biphytanyl carbon skeletons are covalently bound by a carbon-carbon bond, was unambiguously identified in a number of sediments from marine and lacustrine environments. Its abundance varies from 0.2% to 6% of isoprenoid GDGTs. The H-shaped isoprenoid GDGT was originally identified in hyperthermophilic archaea and its presence in marine and lacustrine sediments suggests it is also synthesized by certain non-thermophilic relatives, possibly Euryarchaeota. Abstract Copyright (2008) Elsevier, B.V.