COMPARING ENCRUSTATION PATTERNS ON BRACHIOPOD HOSTS FROM THE DEVONIAN OF THE NORTHWEST TERRITORIES (CANADA) AND IOWA (USA)
Brachiopod specimens were collected from the Givetian and Frasnian (Dev.) Hay River and Twin Falls Formations (NWT, Canada). Six hundred fifty brachiopods from 8 genera were examined for encrustation. Hosts were divided into four groups (large atrypides, small atrypides, strophomenides, spiriferides) to examine the effects of host size and shape on encrustation. The NWT assemblage was compared to one from the Givetian Solon Member (Little Cedar Fm., Iowa) (Webb & Schneider 2004). Both communities were from tropical, non-reefal carbonates in which atrypides were the most abundant brachiopods; species within the Desquamatia complex comprised 37 – 45% of the datasets.
Encrusted brachiopods were larger than unencrusted brachiopods (t-test, p << 0.01); this result holds within all four groups of brachiopods (t-tests, all p <<0.01). Hosts with more than one encrusting taxon were larger than hosts with only one taxon (t-test, p<<0.01). Encrusting organisms were also found to encrust the most abundant brachiopod, Desquamatia, (chi-square test, p =0.004) more often than other hosts. The most abundant encrusters were trepostome bryozoans, Microconchus, Hederella, and Cornulites. All NWT patterns are consistent with those observed in the Iowa data.
Larger hosts present a larger surface area for encrusters, and as they have lived longer, have had more time to be encrusted. Also, larger brachiopods are elevated higher above the substrate which reduces the risk of burial of the encruster and places it more favourably in a current. Encrusters parasitizing a host’s feeding current would benefit from a larger host with a stronger current. The most encrusted host, Desquamatia, was also generally the largest. These results suggest that host communities strongly influence encrustation patterns.