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.
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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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.
See also...
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