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.

See also...

Follow Sciency Thoughts on Facebook.

Utahcaris orion and the origin of the Chelicerates.

Burgess Shale-type faunas are fossil Lagerstätten which provide unique insights into life in the Early-to-Middle Cambrian. The best known examples of these are the highly productive Middle Cambrian Burgess Shale deposits of British Colombia and the Early Cambrian Chengjiang biota of South China, though other less productive sites are also known, such as 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.

In a paper published in the Geological Magazine on 31 August 2016, DavidLegg of the Oxford University Museum of Natural History and Steve Pates of the Department of Zoology at the University of Oxford, redescribe the original specimen of Utahcaris orion plus a new specimen and discuss the relationship of this species to other Arthropods, in particular the highly successful Chelicerates (Arachnids, Horseshoe Crabs, their most recent common ancestor and everything descended from it).

The original specimen of Utahcaris orion was discovered by Benjamin Dattilo in Antimony Canyon, and was described by Simon Conway Morris and Richard Robinson in a paper published in TheUniversity of Kansas Paleontological Contributions in 1988. It is preserved as a compression fossil in dorsal view, 87 mm in length. This specimen has a large, subtriangular cephalic shield, plus eleven trunk and two abdominal segments. Remarkably this specimen also shows a section of preserved, phosphatized gut containing the fragmentary remains of a number of Trilobites.

 Utahcaris orion, Antimony Canyon specimen. (a) Part, photographed using polarized lighting, and (b) accompanying camera lucida drawing. Gut content is coloured grey. Abbreviations: P? – putative podomere; and T1–T11 – trunk tergites 1–11. Legg & Pates (2016).

The new specimen was discovered in Miners Hollow by Robert and Nancy Meyers, and is described for the first time, providing new insights into the species and its biology. This specimen is preserved in lateral (side) view, and shows a number of features not visible in the original specimen, notable a compound eye beneath the cephalon and several pairs of elongate flattened limbs.

 Additional specimen of Utahcaris orion from Miners Canyon. (a) Part, photographed using polarized lighting, and (b) accompanying camera lucida drawing. Gut content and eyes are coloured grey. Abbreviations: Ap – appendages; E – eye; and T1–T10 – trunk tergites 1–10. Legg & Pates (2016).

The original study of Utahcaris orion suggested that it might be related to Sanctacaris uncata, a fossil from the Burgess Shale which has been suggested to be the earliest known Chelicerate, as well as to the Leanchoiliid Megacheirans Actaeus, Alalcomenaeus, and Leanchoilia. The new specimen of Utahcaris orion reveals no further similarities to the Leanchoiliid Megacheirans, indicating that the similarities between Utahcaris and this group were a product of the limited nature of the material available rather than any true close biological relationship, however the relationship to Sanctacaris uncata is more strongly supported, and Legg and Pates suggest that these two species, along with Wisangocaris barbarahardyae (a species from the Ema Shale of South Australia to be described in a paper published on 13 May 2016 in the journal Palaeontology by James Jago, Diego Garcia-Bellído and James Gehling) be united as the Family Sanctacarididae, the earliest known Chelicerate group, and that as the Spence Shale is slightly older than the Burgess Shale, Utahcaris orion is therefore the oldest currently know Chelicerate.

The mouthparts of Utahcaris orion are unknown, however the closely related Sanctacaris uncata is known to have robust gnathobases (chewing organs), and the presence of fragmentary Trilobite remains in the gut of the first specimen of Utahcaris orion suggests that it may have had similar structures. This, combined with the presence of limbs apparently modified for swimming, strongly implies that the Chelicerates may have been active predatory animals from the outset, a lifestyle still seen in the majority of living species today.

See also...

Oesia disjuncta: Enigmatic Cambrian fossil re-interpreted as a tub-dwelling vermiform Hemichordate.                                                The Hemichordates are Deuterostome Animals. members of the group which also...

 Scathascolex minor: A Palaeoscolecid Worm from the Middle Cambrian Burgess Shale of British Colombia.                            Palaeoscolecids are a group of vermiform animals (worms) known from Cambrian to Silurian deposits. They had rings similar to...

Eokinorhynchus rarus: A Kinorhynch from the Early Cambrian of Sichuan Province, China.                                                 Kinorhynches are tiny (at most 3 mm) worm like animals found in marine sediments, with segmented tube- or barrel-shaped bodies, separate head and neck regions and evertable pharynxes. They

Follow Sciency Thoughts on Facebook.