THE FOSSIL RECORD OF HYDROZOAN POLYPS
Hydroid preservation potential is restricted by their delicate, chitinous exoskeletons, dependence on hard substrates for propagation, and limited post-mortem dispersal range. Thecate hydroids (Subclass Leptomedusae) are most commonly preserved and reported, although other hydrozoan clades may have long geological histories. Hydroids occur either solitarily or en masse in diverse facies, most frequently lacking bioturbation; carbonaceous microfossils are found in limestone whereas carbonaceous compressions and impressions are associated with environments below the storm wave base. Molds produced by bioimmuration are known from the originally calcareous shells of symbiotic hosts. Early Paleozoic materials that corroborate actuopaleontological hypotheses imply that hydroids underwent few ecological changes in the Phanerozoic. By recognizing possible sources of material, new and better preserved fossils may be uncovered that further elucidate hydrozoan evolutionary history.