Symbiotic association of a pyrgomatid barnacle with a coral from a volcanic middle Miocene shoreline (Porto Santo, Madeira Archipelago, Portugal)

Abstract

The bioclaustration of the pyrgomatid barnacle Ceratoconcha aff. costata within the carbonate skeleton of a colonial hermatypical coral (Tarbellastrea reussiana) is described from a middle Miocene basaltic rocky palaeoshore on a small north-eastern Atlantic islet of Porto Santo (Madeira Archipelago, Portugal). The resulting structure is named as a new trace fossil Imbutichnus igen. nov., characterized by a small, funnel-like cavity, a circular to oval crosssection, with a nearly cylindrical shape in the upper part and a conical shape towards the base. Imbutichnus costatum isp. nov. is defined as a bioclaustration structure produced by the overgrowth of a pyrgomatid barnacle by a coral. From an ethologic point of view, Imbutichnus is attributed to the Impedichnia category. In terms of palaeoecology, it is interpreted as the result of a parasitic relationship. This is also the oldest record of pyrgomatid barnacles in the eastern Atlantic and clearly demonstrates that the Miocene palaeogeographic range of Ceratoconcha was much wider than previously assumed.

Fossil Crustaceans as Parasites and Hosts

“…Members of this genus are known from the Oligocene to the present (Ross and Newman, 2002), although the Oligocene occurrence was based on a personal communication only. Santos et al (2012) indicated that pyrgomatids were associated with corals at least since the Miocene, while they may have appeared in the early Cenozoic, based on molecular analyses (Tsang et al, 2014). Savignium, Pyrgoma, Cantellius and Nobia may represent an intermediate step from filter feeding to full parasitism.…”

“…Negative effects on the host included inhibition of skeletal growth, modification of coral wall geometry and the secretion of additional material in response to stress. Santos et al (2012) indicated that these barnacles do not feed on coral tissue, but instead use their thoracic cirri for filter feeding purposes. Probably for this reason, Malay and Michonneau (2014 , Table 1) classified extant species of Ceratoconcha as planktotrophic rather than parasitic.…”

“…Bertling et al (2006) were of the opinion that bioclaustrations do not represent trace fossils, but many others disagree. Bioclaustrations were called trace fossils by Palmer and Wilson (1988) and, more recently, by Tapanila and Ekdale (2007), Bromley et al (2008), Tapanila (2008b), McKinney (2009), Knaust (2012), Santos et al (2012) and C onsole-Gonella and Marquillas (2014). The swellings in decapods were classified ichnotaxonomically because as Bertling et al (2006, p. 268) noted with regard to hybrid structures resulting from borings and growth, 'for convenience and in order to maintain stability of names with a high ecological meaning, we advocate nomenclaturally (not taxonomically) treating them as if they were ichnotaxa'.…”

“…Therefore, it is not considered further here. Santos et al (2012) recently described evidence for parasitism in the middle Miocene coral Tarbellastrea reussiana by the barnacle Ceratoconcha aff. C. costata (Figure 9).…”

Differentiating Parasitism and Other Interactions in Fossilized Colonial Organisms

“…While pyrgomatids are normally preserved as body fossils, loss of the plates can leave a deep funnel-shaped or cylindrical hole in the coral host. This was given the trace fossil name Imbutichnus by Santos et al (2012) and compared to other bioclaustrations (see above).…”