Cambrian to Cretaceous changes in hardground communities

Abstract

The changing nature of the communities of boring and encrusting taxa found on upward‐facing hard‐grounds has been studied from the standpoints of (a) diversity, (b) faunal composition, and (c) nature of the niches occupied. After a rapid initial increase in the early Palaeozoic, diversity remained at much the same level from the Middle Ordovician until the late Cretaceous. However, there is a considerable turnover in the identity of the individual taxa between successive sample intervals. The incoming and outgoing of the major groups parallel their fortunes in the marine realm as a whole. Niche analysis suggests that the same feeding levels are occupied for most of the history of hardground communities, but Mesozoic faunas contain a much higher proportion of species with true exoskcletons, or which lived infaunally. The evolution of these forms was probably influenced by the Mesozoic radiation of marine predators and duriphages, but it also resulted in Mesozoic hardground faunas being more resistant than their Palaeozoic counterparts to episodic corrasion. Resulting higher population densities in the Mesozoic were probably one reason why cavity faunas beneath some of these hardground surfaces are more diverse than those beneath Palaeozoic examples. □Hardground, community, evolution.

Encrustation patterns on Late Cretaceous (Turonian) echinoids from southern Poland

“…This suggests that the low diversity of 10 taxa in our case, is the result of substrate homogeneity, as well as of the state of preservation, rather than of the type of substrate or location. Both Palmer's (1982) data and the current study stay in contrast with the data gathered by Ž itt and Nekvasilová (1996) who reported 29 and 23 species from Cenomanian/Turonian nearshore settings nearly contemporaneous with our own. Their data, however, like those of , also come from substrates which experienced longer exposure on the sea floor, are better preserved, and thus could be resolved to lower taxonomic units.…”

“…For example, noted at least 26 epilithozoan Fig. 16 Size histograms of encrusted echinoid tests (n = 55) Facies (2013) Palmer (1982) noted 13 taxa for the most diverse Cretaceous hardground and showed that total species richness at that time was higher than during the Triassic, and lower when compared to the Jurassic. In general terms, Bambach (1977) pointed out that a mean of 7.5 species are expected in Mesozoic high stress communities, while in variable nearshore and open-marine settings, 17 and 25 taxa are predicted, respectively.…”

“…Heterogeneous substratum is more attractive for settlers and thus increases diversity and abundance (e.g., Wilson and Taylor 2001;. Our material comes from Cretaceous deposits, where much fewer encrusting taxa are found, as compared to other intervals, where they are more diverse as (e.g., during the Jurassic; Palmer 1982;Lescinsky 2001).…”

“…The suspension-feeding epibionts primarily lived at the scrub layer (sensu Palmer 1982), and only in a few instances at the field and canopy layers, while the ground layer was unexplored. Our reasoning is supported by the size of the hosts, the small grain size of the surrounding sediment and the fact that echinoids were a principal component of the shelled benthos and thus served as the main substrate for secondary tiering in the investigated environments.…”

Bioerosion and availability of shelter for benthic reef organisms

“…Lastly, many biological processes (includmg predation, bioturbatlon and bioerosion) have been shown to have increased in intensity over geological time, in particular during and since the Mesozoic (Vermeij 1978, Thayer 1979, Palmer 1982. If a correlation between bioerosion and the size and number of cryptic reef organisms can be demonstrated, then ~t follows that the substantial increase in available habitat due to the proliferation may have allowed radical increases in the diversity of this important cryptic reef biota dunng the Mesozoic.…”