Fossil Phoronida and their inferred ichnotaxa

Abstract

Various ichnotaxa found in hard substrates are interpreted as "phoronid" trace fossils. Their records are briefly reviewed. An interpretation of Diorygma found in the shells of brachiopods is not compatible with phoronid morphology and anatomy. Criteria for the discrimination of phoronid burrows and borings from those of similar organisms from others are difficult to establish even when the evidence and conclusions made therefrom are sound.Key Words: Phoronida; borings; Diorygma; Talpina; Skolithos; ichnotaxa. Découverte perforante dans des coquilles de brachiopodes, Diorygma montre une structure incompatible avec la morphologie et de l'anatomie des phoronidiens. Les critères de discrimination entre les tubes et les perforations des phoronidiens avec ceux d'autres organismes semblables sont très difficiles, voire impossibles, à établir, notamment en l'absence de preuve démontrant qu'il s'agit bien d'une action liée à la présence de phoronidiens.

Non‐palimpsested crowded Skolithos ichnofabrics in a Carboniferous tidal rhythmite: Disentangling ecological signatures from the spatio‐temporal bias of outcrop

“…Skolithos is a facies‐crossing trace fossil with a Phanerozoic range, and the ichnogenus most likely exemplifies behavioural convergence amongst a wide range of invertebrate tracemakers (Curran & Frey, 1977; Knaust, 2017; Knaust et al ., 2018). However, some dense, monospecific assemblages of Skolithos have previously been ascribed to the activity of communities of phoronids (‘horseshoe worms’) (Sundberg, 1983; Emig, 2010; Knaust et al ., 2018). Phoronids are one of the smallest phyla of invertebrates, closely related to brachiopods, and have an evolutionary history extending back from the present until at least the Devonian (Emig, 1982; Santagata, 2015).…”

“…Irrespective of the actual tracemaker, phoronids offer an analogy for the type of viable ontogenies that would have emerged amongst a multitude of unknown (and unknowable) tracemakers in geological history and demonstrate that unexpectedly expeditious registration of ichnological signatures can happen. Planktonic phoronid larvae swim in open seawater for approximately seven days, before sinking to the seabed and then undergoing a catastrophic 15 to 30 min metamorphosis in which they transform into juveniles with an elongate vermiform trunk (Emig, 2010; Santagata, 2015). Once a phoronid has developed this form, it can burrow downwards into soft‐sediment from its posterior by utilizing hydrostatic pressure changes to coelomic fluid and metasomal muscle contractions, achieving full burial (apart from the lophophore) in as little as eight hours (Emig, 1982).…”

Phoronida—A small clade with a big role in understanding the evolution of lophophorates

“…For obvious reasons, the fossil record is biased toward organisms with hard structures, and therefore the fossil record of phoronids is very poor when compared to that of brachiopods or bryozoans (Taylor et al, 2010). There are some fossilized structures interpreted as preserved borrowings of phoronid‐like animals (Emig, 2010; Taylor et al, 2010), however, they seem to be identical to the borrowings of modern‐day phoronids (Emig, 2010) and do not tell us anything about the steps leading to the establishment of the phoronid body plan. Instead, the crucial fossil groups that can be used to infer the evolution of phoronid lineage are tommoitiids.…”