Siphusauctum lloydguntheri: An enigmatic filter-feeding animal from the Middle Cambrian Spence Shale of Utah.

Fossil Lagerstätte such as the Burgess Shale and Chengjiang Biota provide information on the earliest animal assemblages on Earth, including the oldest known members of most animal phyla. These assemblages preserve a number of filter-feeding animals, some of which can be assigned to modern groups, while others remain enigmatic. One of these is Siphusauctum gregarium, which is known only from the Tulip Beds of the Cliff Shale Member of the Burgess Shale Formation, where it forms large colonies.

In a paper published in the Journal of Paleontology on 2 June 2017, Julien Kimmig of the Biodiversity Institute at the University of Kansas, and Luke Strotz and Bruce Lieberman, of the Biodiversity Institute and Department of Ecology & Evolutionary Biology at the University of Kansas, describe a second species of Siphusauctum from the Spence Shale of Utah.

The Middle Cambrian Spence Shale of northern Utah, which is slightly older than the Burgess Shale and has produced a wide variety of Algae, Sponges, Brachiopods, Eldoniids (soft-bodies animals of uncertain affinities), stem-Molluscs (animals of apparent Moluscan affinities, but which lived or split off from the other Molluscs before the common ancestor of all living groups), Cycloneuralians (the group that includes Kinorynches, Priapulids and Nematodes), Deuterostomes (the group that includes Vertebrates and Echinoderms), Lobopodians (the probable Cambrian ancestors of the Modern Velvet Worms), and a variety of Arthropods, including Trilobites, Carapace-bearing Arthropods (which may be related to the later Crustaceans), Megacheirans (an extinct group of probably predatory Arthropods), Xenopods (an extinct group of soft-bodied Arthropods) and enigmatic forms such as Meristosoma paradoxum and Utahcaris orion.

The new species is named  Siphusauctum lloydguntheri, in honour of palaeontologist and Trilobite-expert Lloyd Gunther. The species is described from a single specimen collected by Gunther in 1976 and donated to the Biodiversity Institute at the University of Kansas. The specimen is a stalked animal with an obovate (roughly oval, but wider at the top than at the base) upper part measuring 35.1 by 29.9 mm and a long stalk measuring 45.6 by 5.7 mm.

Holotype of Siphusauctum lloydguntheri from the Spence Shale, middle Cambrian, Antimony Canyon, Utah, in lateral view: (1) part; (2) counterpart. Scale bar represents 10 mm. Kimmig et al. (2017).

The stem of Siphusauctum lloydguntheri is double-layered, with an inner and outer part; the outer part does not reach all the way to the end of the stem, but this is thought to be a preservational artifact, suggesting that the inner part of the stem was more durable that the outer. At the tip of the stem is a holdfast 5.7 mm wide and 0.9 mm high, and apparently made of the same material as the inner stem.

Close-up of the stem of Siphusauctum lloydguntheri from the Spence Shale, middle Cambrian, Antimony Canyon, Utah: (1) part; (2) explanatory drawing of the part, dashed line indicates uncertain outline. Scale bar represents 5 mm. Abbreviations: H, Holdfast, IS, Inner Stem, OS, Outer Stem. Kimmig et al. (2017).

The upper part of the animal appears to have been covered by a thick upper sheaf. Half way up this sheaf are a series of small protrusions which may have been openings. Two dark carbonaceous structures are preserved inside this outer structure, which may have been part of the lower digestive tract. There are two grows close to the top of this structure which may represent the location of the location of the anus. A series of striations may have formerly been the attachment for a filter-feeding apparatus.

Close-up of the calyx of Siphusauctum lloydguntheri from the Spence Shale, middle Cambrian, Antimony Canyon, Utah: (1) part; (2) explanatory drawing of the part, dashed line indicates uncertain outline. Scale bar represents 5 mm. Abbreviations: A, Anus, ES, External Sheath, LDT, Lower digestive tract, G, Gut. Kimmig et al. (2017).

The taxonomic affinities of Siphusauctum gregarium have to date been impossible to decipher, however Siphusauctum lloydguntheri shows a number of similarities to Dinomischus isolatus, a Burgess Shale animal interpreted as an early Entoproct. Such an affinity had been ruled out for Siphusauctum gregarium, due to the presence of feeding structures unlike those of Entoprocts, but these are not present in the new species, which implies that either they were lost between he death and preservation of the specimen, or that these were novel structures which arose in Siphusauctum gregarium and were not present in even close relatives.In addition both Siphusauctum and Dinomischus appear to have had a six-fold radial symmetry, something also found in Entoprocts, but otherwise somewhat rare (it is also sometimes seen in Cnidarians and Echinoderms, but Siphusauctum seems unlikely to have been closely related to either of these groups.

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The first Entoproct?

The Lophotrochozoa are a diverse group of Invertebrate animals indicated to have a common ancestry by genetic analysis. The group includes the Annelida, Mollusca, Bryozoa, Cycliophora, Brachiopoda, Entoprocta and Phoronida. Within this group several groups are united by the presence of a crown of tentacles (the Lophophore) surrounding the mouth, which continuously opens and shuts while feeding, snatching planktonic food items which are then consumed. This group, the Superphylum Lophophorata, comprises the Bryozoa, Cycliophora, Brachiopoda and Entoprocta. Of these to Phyla, the Brozoa and Entoprocta, form colonies made up of large numbers of connected individuals, called zooids (ecologically but not morphologically similar to Corals). While the two groups are superficially similar, they are considered to have been separate since early in their evolutionary history, due to the different arrangements of their digestive tracts, with the anus of the Brozoans located outside the ring of the lophophore, but that of the entoprocts located within.

The Lophophorate Phyla are believed to have differentiated early in the Cambrian (the shelled Brachiopods becoming a distinctive part of the marine fauna early in the fossil record) and the colonial Bryozoans appear by the early Ordovician. Like many soft bodied groups the Entoprocts lack an extensive fossil record, with the oldest known fossils to date coming from the Jurassic of the UK, so while their relationship to the other Lophoporate and Lophotrichozoan groups is supported by the genetic data, the origin of the group and nature of its earliest members remains obscure.

In a paper published in the journal Nature on 17 January 2013, a group of scientists led by Zhifei Zhang of the Early Life Institute at Northwest University and the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences describe previously described animal from the Early Cambrian Chengjiang Lagerstätte of Yunnan Province as a possible early Entoproct.

The animal, Cotyledion tylodes, does not obviously resemble the modern Entoprocts, which are small, typically colonial animals (though some solitary forms are known); it has a cup shaped body with a long stalk by which it is attached to the substrate (typically the shell of another animal), and is covered in sclerites (shell elements), whereas Entoprocts are entirely lacking in mineralized tissue. It is also considerably larger than any known Entoproct, and had previously been considered to be a Carpoid Echinoderm (more closely related to Vertebrates than to Lophotrochozoans).

Sclerites on the calyx and stem of the putative Entoproct Cotyledion tylodes from the Cambrian Chengjiang Fauna of Yunnan, China. (a) Entire specimen, dashed boxes indicate positions of (b) and (d), note the seemingly increased arrays of sclerites; (b), details of (a) as indicated; note the elongate sclerites marked by an arrow; (d), Enlargement of (a) as indicated, showing the merged two sclerites indicated by two arrows. Zhang et al. (2013).

However like the modern Entoprocts Cotyledion tylodes has a U-shaped gut, with both the mouth and anus inside the ring of the lophophore, a trait which is considered to be unique to, and indicative of, the Entoprocts.

(c) A laterally compressed specimen of Cotyledion tylodes attached to the gena of a trilobite, showing U-shaped gut with 3-D buccal cavity and enlongate anal tube enclosed inside an apertural cavity; (d), interpretative drawing of the same. The hemispheric buccal cavity with basal mouth (anterior) and elongate anal papillae (posterior) marked with solid arrows and hollow arrows, respectively. Zhang et al. (2013).

While the presence of a shelly exoskeleton made up of numerous sclerites would not be predicted in a fossil relative of the modern Entoprocts by examination of the animals alone, this is a trait thought to have been present in early members of several other Lophotrochozoan groups, notably the Molluscs, Anelids, Brachiopods and Phoronids, so to find such a covering in an Entoproct is not a complete surprise. Cotyledion tylodes also has a larger body and more complex body-plan than any modern Entoproct, but again this is not completely unexpected. Evolution proceeds by simplification as often as by increased complexity, and many Early Cambrian fossils show unexpected features not seen in their modern relatives.

Reconstruction of Cotyledion tylodes in life position. Zhang et al. (2013).

See also A mysterious Glass Sponge from the Early Cambrian of south-east China, Punctatus emeiensis, not a Cnidarian after all? Soft tissue preservation in Linguloid Brachiopods from the Early Ordovician Fenxiang Formation of Hubei Province, China, The first Arthropod? and Are the Vetulicolians Deuterostomes?

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