Friday 20 January 2012

Mysterious new animal from the Burgess Shale.

The Burgess Shale is a fossil Lagerstätte (literally 'Bonanza'; a rich fossil find), located within the Yoho National Park in British Columbia. It was discovered in 1909 by American Palaeontologist Charles Doolittle Walcott, and is noted for a large number of exceptionally well preserved soft-bodied animals from the Middle Cambrian. Many of these animals have proved hard to classify, being early members of their groups, and not closely related to other forms of which we have records; fossils of soft bodied animals are exceptionally rare, some forms found in the Burgess Shale are more closely related to living animals than to any known fossils. Some fossils from the Burgess Shale have never been placed within a larger group of animals, they remain unique organisms of uncertain affinities, collectively referred to as 'problematica'.

On 18 January 2012 a paper appeared in the journal PLoS ONE by Lorna O'Brien and Jean-Bernard Caron, both of the Department of Ecology and Evolutionary Biology at the University of Toronto and the Department of Natural History at the Royal Ontario Museum, in which they describe an enigmatic new stalked animal from the upper part of the Burgess Shale, named as Siphusauctum gregarium (the gregarious, large glass-shaped animal).

Reconstruction of Siphusauctum gregarium, by illustrator Marianne Collins. From O'Brien and Caron (2012).

S. gregarium had a goblet shaped body, a flexible stem and an apparently retractable holdfast. The main body, or 'calyx' was covered by a flexible sheath, and had six openings at the base, and a larger, single opening at the top. It appears to have been able to pump water in through the holes at the bottom and out through the hole at the top, making this upper hole an anus. It has a differentiated internal gut surrounded by six combs of cilia, which were apparently able to pump water through the body and may have aided in filter feeding.

Diagrammatic reconstruction of the body of S. gregarium. (A) Longitudinal section of the entire body. (B) Cross-section of the caylx. (C) Longitudinal section of the caylx. Abbreviations: (A) Anus (BC) Body Cavity (Con) Conical Structure (CS) Comb Segments (ES) External Sheath (H) Holdfast (IC) Internal Conical structure with central tube (IS) Internal Stem (LD) Lower Digestive tract (MD) Middle Digestive tract (OS) Outer Stem (SP) Sheath Protrusion (TG) Transverse Groove (UD) Upper Digestive Tract. From O'Brien and Caron (2012).

The animals are preserved in large groups, apparently in the position they occupied in life. They seem to have formed large colonies on soft sediments, a variety of other organisms have been found with them, including a number of different worms. For the most part the preservation is excellent, indicating the animals were buried rapidly, although some show signs of partial decomposition, suggesting they had died at the surface, began to decompose, then been buried.

The Holotype of S. gregarium (when scientists describe a new species an individual is named as the Holotype; when future specimens are found they are compared to this Holotype, and if judged to be the same they are ascribed to the same species). Scale bar = 10 mm. Scale bar on inset box = 1 mm. (Con) Conical structure (CS) Comb Segments (ES) External Sheath (H) Holdfast (IS) Inner Stem (LD) Lower Digestive tract (LS) Longitudinal Suture line (OS) Outer Stem (S) Striae (TG) Transverse Groove. From O'Brien and Caron (2012).

O'Brien and Caron considered carefully the possible relationship between S. gregarium and other known animals.

The cilia combs of S. gregarium are similar in form to those of ctenophores (comb jellies), which are known from the Burgess Shale, but not strictly analogous, and not located on the same part of the body or used in the same way. Neither has any known ctenophore ever possessed a stalk or holdfast.

Some Ediacaran fossils (Late Precambrian fossils of uncertain affinities) have stalks and holdfasts, and even goblet-shaped bodies, but they do not seem to be similar in any finer detail.

Crinoids and some other primitive echinoderms have holdfasts, stems and (sometimes) goblet-shaped calyxs, but their internal structure is different, they have a single mouth at the top and a single anus bellow, and their holdfasts appear to have always been fixed structures for attaching to hard sediments.

Tunicates (sea squirts) have a broadly similar form to S. gregarium, but are structurally quite different.

Phlogites longus is an enigmatic animal from the Chengjiang Lagerstätte in China. It has a similar general shape to S. gregarium, but its it has a pentameral symmetry, unlike the hexagonal symmetry of S. gregarium, an anus on the side of the caylx, and five tentacles.

Herpetogaster collinsi is another enigmatic stalked fossil from the Burgess Shale, it appears to have affinities with P. longus but not with S. gregarium.

Dinomischus venustus is another stalked Chengjiang fossil, known from a single, poorly preserved, specimen; it may have relatives in the Burgess Shale and the Kaili Biota of southern China. It has a stalk a calyx surrounded by a ring of bracts, and a tube at the top which may be an anal pore, or possibly a broken piece of stem; if the former it may be a relative of S. gregarium, if not then it is clearly unrelated. It appears to have about 18-20 internal subdivisions, which may-or-may-not be analogous to the cilia combs of S. gregarium.

The extant entoprocts and ectoprocts are both groups of small stalked sessile animals with calyxs, but are anatomically quite different to S. gregarius.

Other stalked animals from the Burgess Shale. (A) Herpogaster collinsi, (B-D) Dinomischus isolatus, (E) Priscansermarinus bametti, (F) Lyracystis radiata. Scale bars A-E = 5 mm. F = 30 mm. Abbreviations: (Br) Bract, (Bt) Branchlet, (Ca) Calyx, (Cp) Chitinous plate, (Ph) Pharynx, (Seg) Segment, (St) Stem, (Stom) Stomach, (Ta) Theca, (Te) Tentacle, (Td) Terminal disc. From O'Brien and Caron (2012).