Marine Sclerobiofacies: Encrusting and Endolithic Communities on Shells Through Time and Space

Abstract

The concept of sclerobiofacies is defined herein as suites of sclerobiont encrusters and endiont borers (collectively sclerobionts) preserved on skeletons that characterize particular facies/environments. Skeletal components provide biologically standardized substrates; when possible, comparison of encrusting assemblages on fossil shells of the same or closely related eurytopic species provides a degree of substrate control comparable to modern experimentally deployed shells. Taxonomic composition of sclerobiont suites varies rather predictably among marine environments (e.g., based upon depth) but is primarily useful for comparisons of environments within local areas and limited time frames. Parameters that may be used to compare sclerobiofacies across broader spatial and temporal dimensions include: per shell and cumulative species richness (diversity), frequency of encrustation, areal coverage, and guild structure of encrusting taxa. Herein, we summarize characteristic sclerobiofacies in a series of Recent and ancient examples. Modern subtropical marine encrusters, documented on experimentally deployed molluscan shells at sites ranging from 15 to over 200 m, show high biont richness in shallow subtidal areas. Maximal areal coverages in Bahamian samples occur at about 20–30 m, whereas species richness increases downward to the deeper euphotic zone (∼75–80 m). Below this level, rapid decline in both richness and percent coverage results in deeper Dysphotic–Aphotic zone samples yielding only a few species with coverage rarely exceeding 5%. Burial is also a key factor such that rapidly buried shells in the Shallow Euphotic zone have species coverages, richnesses, and taxonomic compositions resembling long-exposed shells in deeper areas below the euphotic zone. Shelly substrates from the Cambrian to Early Ordovician exhibit only minor encrustation by solitary attached taxa, especially echinoderms; however, by the Late Ordovician various solitary (e.g., cornulitids, craniid brachiopods) and colonial forms (e.g., trepostome and tubuliporate bryozoans) form distinctive sclerobiofacies. Photic zone-related environments, judged independently on the basis of microendoliths, show overall lower taxonomic richness than modern counterparts. However, they also show common patterns, including a general decrease of richness and percent encrustation from Shallow Euphotic to Dysphotic/Aphotic zones. Comparable trends are seen in Middle Devonian exemplars from New York State. Not only were there consistent trends toward lowered diversity/coverage into deep-water settings but also an additional factor related to turbidity and/or sedimentation rate was identified from assemblages at comparable depths arrayed along a distal to proximal gradient with respect to siliciclastic input sources. Carboniferous sclerobiont suites from varied sites in North America show many of the same traits as their Devonian counterparts, although detailed depth zonations are not documented at present. The Permo-Triassic extinctions appear to have had a strong impact on the taxonomic composition of marine sclerobiofacies, although a paucity of studies obscures details of Mesozoic and Cenozoic sclerobiofacies. In general, they appear to have taxonomic compositions and patterns similar to those observed in the Recent. The concept of sclerobiofacies provides another tool for paleoenvironmental analysis. Together with litho-, ichno-, bio-, and taphofacies, the properties of shell encrusting assemblages will yield detailed further insights into ancient environmental gradients.