THE FOSSIL RECORD OF HYDROZOAN POLYPS

Although the Superclass Hydrozoa (phylum Cnidaria) is recently widespread and diverse, comprising over 3,700 species, its fossil record is sparse and clearly very incomplete. Due to the scarcity of material and uncertain affinities of the taxa, no investigation has thoroughly reviewed the provisional reports of polyps, or “hydroids,” amassed over the past century. Criteria for distinguishing hydroids from the fossils of comparable morphological forms (i.e. dendroid graptolites, erect bryozoans, and algae) are presented. Bona fide hydrozoan polyps that are pertinent to evaluations of evolutionary and actuopaleontological hypotheses are identified and reclassified using a modern morphological- and molecular-based phylogenetic framework. Preserved characters are inadequate for definitive classification within most extant clades, and most taxa remain Incertae sedis.

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.