AFFINITIES OF THE ENIGMATIC HEDERELLOIDS: PHORONID WORMS RATHER THAN BRYOZOANS OR CORALS?

Hederelloids have branching colonies of calcitic zooids that encrust a variety of marine hard substrates. Our recent survey of this suborder shows 109 described species ranging from the Silurian into the Permian, peaking in the Devonian, with possible single occurrences in the Ordovician and the Triassic. They can be common components of Middle Paleozoic sclerobiont communities, typically forming dense networks of interwoven tubes on the shells of brachiopods, corals and bryozoans. Hederelloids have been most often classified as cyclostome bryozoans, but have also been affiliated with phylactolaemate bryozoans and auloporid corals. Evidence from scanning electron microscopy, along with observations of budding patterns and tube sizes, supports a new interpretation of hederelloid affinities. The microstructure of hederelloid tube walls consists of microprismatic calcite, not lamellar calcite as found in Paleozoic bryozoans and auloporids. Colonies originate from a protozooid with a bulbous proximal end resembling a bryozoan protoecium but lacking the distinct zone of astogenetic change found in bryozoans. Unlike auloporid corals, branching is not dichotomous. Hederelloid zooids range in diameter through an order of magnitude, from 0.15 to 1.5 mm, exceeding the maximum size found among true bryozoans. We hypothesize that hederelloids were colonial, tube-dwelling phoronid worms. Recent species of this lophophorate phylum do not secrete mineralized skeletons, but some develop pseudocolonies with similar budding patterns and tube diameters (0.2 to 5.0 mm) as hederelloids. Phoronids also have a bulbous structure (ampulla) at their proximal end which resembles the proximal part of the hederelloid protozooid. The stratigraphic range of hederelloids fits within that of phoronids (?Cambrian to Recent), with branching phoronid borings (Talpina) being common in marine hard substrates from the Devonian onwards. Hederelloids appear to have been ecological and morphological analogues of later cyclostome bryozoans. They were part of a Middle Paleozoic peak in the abundance and diversity of worm-like lophophorates.