Phoronida: Fossils

updated on 19 November 2015    
 

 

The origin of the Phoronida, certainly monophyletic with the other Lophophorata, goes back to approximately 800-1000 MA. Though as fossils, the phoronids are known since the Devonian (the brachiopods from the Cambrian and the bryozoans from the Ordovician): they have preserved the ancestral form and life way, nearly to those of the predecessor protolophophorate (Emig, 1984).

Several authors suggested that tubes or tubicolous burrows in fossil records, described under various ichnogenera, i.e. Skolithos, Talpina, Diorygma..., belong to Phoronida (Fenton & Fenton, 1934; Avnimelech, 1955; Josey, 1959; Voigt, 1972; Mackinnon and Biernat, 1970; Sokoloski, 2005). Recently, Voigt (1975) has proved the identity of Phoronis species with the ichnogenus Talpina von Hagenow, 1840, confirmed in 1978. He also demonstrated (Voigt, 1972) that Conchotrema Teichert 1945 is a junior synonym of Talpina, as was confirmed by Plewes (1996).

Talpina is abundant today and is well known from the Cretaceous (Voigt, 1972) and Jurassic (Fürsich et al., 1994). The ichnogenus ranges back to the Late Devonian (Thomas, 1911; Rodriguez & Gutschick, 1970; see Bromley, 2004). The fossil phoronid burrows seem to have been present since Devonian times (see Emig, 1982).

The Talpina burrowed in such diverse calcareous substrata as calcareous algae, echinids, mollusc shells and rostra of Belemnites. Voigt gives criteria used for the discrimination of the phoronid burrows from other similar ones such as those of Thallophytes, sponges, Bryozoa or "worms". The frequent presence of agglutinating Foraminifera surrounding the opening the tube of a worm-like fossil animal provisionally determined as Phoronopsis and suggesting commensalisms between both fossil organisms (Voigt, 1970) in Upper Maastrichtian, has never been confirmed in recent observations on Phoronida. Tubes of the ichnogenus Talpina ramosa which occur frequently within the guards of Belemnella and Belemnitella (cf. Voigt, 1972) are described within the cavities probably originating from diagenetically destroyed aragonitic corals of the Maastrichtian chalk-tuff (Voigt, 1978).

Towe (1978) and Larsson (1979) raised the possibility that Tentaculites were related to the brachiopods or perhaps more closely to the phoronids. However, the later author noted that the lophophorate feeding system of phoronids is incompatible with the planktonic mode of life proposed for some tentaculitids. Vinn (2005) and Vinn & Mutvei (2005) hypothesized that phoronids were perhaps the closest living relatives of Cornulites and Tentaculites, , but also the brachiopods or the bryozoans (Vinn & Isakar, 2007). Nevertheless, to reconstruct cornulitids as 'skeletal phoronids' is entirely speculative. The only two groups with which Cornulites can be compared directly are bryozoans, and cnidarians with which comparable morphologies and shell structures are found (Herringshaw et al., 2007).

Iotuba chengjiangensis, a form known from three speciemens from the Lower Cambrian Chengjiang fauna of China, has been interpreted by Chen & Zhou (1997) as a phoronid, having a U-shaped gut and is tentaculated. In the same fauna, another soft-bodied fossil has been claimed to be phoronid, Eophoronis, by Chen (2004). But the affinities of these fossils have been questioned (Cohen & Weydmann, 2005), and according to Conway Morris (2006) and Huang (2006), they may be priapulids.

There is a tendency in the recent work to attribute ichno-tubes to fossil phoronid (see recent references) without any comparison the real structure and chemical composition of the characteristic tube of the Phoronida. Moreover, it seems curious to propose a phyletic tree base upon ichno-tubes hypothetically attributed to Phoronida (see Skovsted et al., 2008). A short review has been proposed by Emig (2010), see also "Trace fossils refered by phoronids."

Fig. - Molecular divergence estimates for brachiopods and phoronids (from Sperling et al., 2011, modified).

According to these authors, the mean age for the divergence along the Brachiopoda between the Linguliformea and Rhynchonel-liformea is 547 Ma, while that between brachiopods and phoronids is at 578 Ma.
Personal comments: on the basis of the morpho-anatomical characters, we have estimated this latter divergence much earlier (Emig, 2008). The tree on the figure confirms the presence of three main phoronid groups as stated and described by Emig (1985). P: possible divergence between Phoronis (probably P. psammophila) and Phoronospsis (ptobably Phoronopsis albomaculata); G: possible divergence between Lingula and Glottidia; era colors according to the International Chronostratigraphic Chart (2013).

The tube-worms  generally identified as Spirorbis range from Ordovician to Recent, often profusely encrusting shells and other substrates. Whereas Recent Spirorbis is a polychaete annelid, Taylor & Vinn (2006) consider that details of tube structure in pre-Cretaceous ‘Spirorbis’ are related to Microconchida. Nervertheless, this later extinct order is a highly hypothetical lophophorate group with a spirorbiform microchonchid tube. Such a tube can be related to Phoronida or to a phoronid-like… on the basis of the specific phoronid tube structure and rectilign shape (see below; and References).
Spirorbis’ worm tubes, described from the geologic record spanning the Silurian through the Middle Jurassic, have been assigned freshwater to brackish to marine affinities: they have now be interpreted as phoronid tubes of microconchid origin by Gierlowski-Kordesch & Cassle (2015). Nevertheless the phoronid tubes are always straight because related to the tubicolous life: the longitudinal muscle system allowing a fast retraction in the tube needs a straight tube. The tubes are always embedded in soft substrates or in burrows, only exceptionally encrusting and always in large masses. The tubes figured by Vinn (2006), Taylor & Vinn (2006), Gierlowski-Kordesch & Cassle (2015) cannot be interpreted as phoronid tubes, mostly because of the cellular secretions used in the tube formation and of its structure and rigid morphology (Pourreau, 1979a, 1979b).
Furthermore Phoronida are not only restricted to true marine environments as pointed out by Gierlowski-Kordesch & Cassle (2015); they are also able to live in brackish waters as described by Emig, according to salinity fluctuations and to time-depending in such conditions.

Reminder: the phoronid tube is so characteristic that it cannot be confused with any other tube. But this means one must have already seen and touched such tubes, not on a picture. About its fossilization, the question remains under debate, because taphonomy seems only possible under special conditions - see the case of lingulide valves (Brachiopoda) (see Emig, 1990).


More...



References on fossil phoronids