Monday 9 June 2014

The nature and development of the mysterious Cambrian fossil Olivooides.

In 1997 Stefan Bengtson of the Department of Palaeozoology at the Swedish Museum of Natural History and Yue Zhao of the  Institute of Geology of the Chinese Academy of Geological Sciences announced the discovery of fossilized animal embryos from basal Cambrian rocks in China in a paper in the journal Science. Since this time these embryos have been studied extensively and a number of development stages leading to an apparent mature animal have been discovered, and a number of different theories as to the precise nature of these fossils, named Olivooides, have been put forward, without any general consensus being reached.

In a paper published in the Proceedings of the Royal Society Series B Biological Sciences on 27 February 2013, a team of scientists led by Xi-Ping Dong of the School of Earth and Space Science at Peking University and the State Key Laboratory of Palaeobiology and Stratigraphy at the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Science, describe a series of Olivooides fossils showing preserved internal anatomical features and providing a more complete set of developmental stages, from which they are able to discuss the nature of Olivooides and draw conclusions about its affinities. 

Embryos and polyps of Olivooides. (a) Cleavage-stage embryo. (b) Stellate embryo with raised apertural rays. (c) Stellate embryo with a raised apertural region. (d) Stellate embryo with an infolded apertural region. (e–f) Stellate embryo showing a characteristic pattern of bulges in (e) lateral, and (f) apertural aspect. (g) Embryo within fertilization envelope that preserves the characteristic pattern of bulges, but not the stellate tissue. (h–j) Early post-embryonic polyp stage exhibiting characteristic stellate integument of embryonic stage, but exhibiting the annulated striated adapertural tissue, and the pentamerally enfolded aperture, in (h) lateral, (i) apertural and (j) oblique aspects. (k) Later polyp-stage specimen in lateral aspect. Scale bars: (a) 69 mm, (b) 116 mm, (c) 139 mm, (d ) 125 mm, (e,f ) 132 mm, (g) 164 mm, (h) 275 mm, (i) 118 mm, ( j) 250 mm, (k) 262 mm. Dong et al. (2013).

Dong et al. considered three main theories on the affinities of Olivooides that have previously been proposed, namely that it is a Priapulid, an Echinoderm or a Cnidarian. 

Olivooides shows some resemblance to the larvae of a Priapulid in overall morphology, and shows pentaradial symmetry, which is rare outside of Echinoderms, but which is found in the mouthparts and pharynx of Priapulids. However Olivooides lacks a through gut, which is seen in both larval and adult Priapulids, and appears to develop directly (gradually) from the embryo stage to the adult, unlike Priapulids which undergo a distinct metamorphosis between their juvenile and adult stages. 

The pentaradial symmetry seen in Olivooides is most common in Echinoderms, and is often considered diagnostic of the group. However Olivooides lacks any of the other features diagnostic of this group, notably a multi-element calcite exoskeleton, a through gut and tube feet. 

Cnidarians lack a true gut, as is the case in Olivooides, and also typically show direct development. Olivooides also shows some affinities with the polyps of extant Coronate Scyphozoans (Jellyfish), as well as with contemporary Cambrian Conulariids. Cnidarians do not typically show pentaradial symmetry, however it is seen in some Cnidarians with fluctuating symmetry, so an exclusively penetaradial species among the earliest members of the group would not be that surprising. 

Finally some specimens of Olivooides show a strange stellate organ, which is hard to interpret as analogous with any organ seen in Priapulids or Echinoderms, but which Dong et al. interprest as budding medusa larvae on the Cnidarian polyp. Medusa larvae are a motile stage produced by many attached Cnidarians, a form of asexual reproduction in which a larvae buds of then swims freely in the water column as a miniature Jellyfish for some time before settling down to develop into a new attached polyp. Such species often alternate sexual reproduction, which produces embryos with two parents, with asexual reproduction, producing medusa larvae which are clones of their single parent, so a form that develops from an embryo to an adult with budding medusa larvae is entirely in keeping with a Cnidarian interpretation of Olivooides.

Two adpressed ephyra-stage Olivooides specimens. (a) SEM image showing the whole specimen in aboral view. (b) SEM image showing a detail of the aboral surface; note the alternately overlying and underlying arms. (c) SRXTM surface rendering showing a detail of the adoral surface including the presumed mouth. Scale bars: (a) 49 mm, (b) 27 mm, (c) 28 mm. Dong et al. (2013).

The life cycle of a modern Scyphozoan Jellyfish. Tree of Life.

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