Rhabdopleurid epibionts from the Ordovician Fezouata Shale biota and the longevity of cross-phylum interactions

Abstract

Evidence of interspecific interactions in the fossil record is rare but offers valuable insights into ancient ecologies. Exceptional fossiliferous sites can preserve complex ecological interactions involving non-biomineralized organisms, but most of these examples are restricted to Cambrian Lagerstätten. Here we report an exceptionally preserved cross-phylum interspecific interaction from the Tremadocian-aged Lower Fezouata Shale Formation of Morocco, which consists of the phragmocone of an orthocone cephalopod that has been extensively populated post-mortem by tubicolous epibionts. Well-preserved transverse bands in a zig-zag pattern and crenulations along the margin of the unbranched tubes indicate that they correspond to pterobranch hemichordates, with a close morphological similarity to rhabdopleurids based on the bush-like growth of the dense tubarium. The discovery of rhabdopleurid epibionts in the Fezouata Shale highlights the paucity of benthic graptolites, which also includes the rooted dendroids Didymograptus and Dictyonema, relative to the substantially more diverse and abundant planktic forms known from this biota. We propose that the rarity of Paleozoic rhabdopleurid epibionts is likely a consequence of their ecological requirement for hard substrates for initial settlement and growth. The Fezouata rhabdopleurid also reveals a 480-million-year-old association of pterobranchs as epibionts of molluscs that persist to the present day.

Colonization of the ocean floor by jawless vertebrates across three mass extinctions

“…Since the biota of continental slope and abyssal oceanic ecosystems (> 200 m depth) was first documented, these complex habitats have been regarded as oases for very old lineages that have since become depauperate or wholly extinct in shallow-water environments. Classic examples of ancient deep-sea lineages include clades as varied as coelacanths ( Latimeria ) [ 2 ], the Vampire Squid Vampyroteuthis infernalis [ 3 , 4 ], crinoids and brittlestars in Echinodermata [ 5 – 7 ], black corals [ 8 ], the living graptolite genus Rhabdopleura and other colonial hemichordates [ 9 – 12 ], and potentially several lineages of deep-sea isopods [ 13 – 15 ], all of which last share common ancestry with related shallow-water and terrestrial forms well over 200 million years ago. At the same time, recent work has shown that these ancient components of the deep sea biota do not represent a simple accumulation, or sink, of remnant biodiversity over geological time [ 16 ].…”