Asymmetry in an Ordovician conulariid cnidarian
Article first published online: 23 JAN 2012
© 2012 The Authors, Lethaia © 2012 The Lethaia Foundation
Volume 45, Issue 3, pages 423–431, July 2012
How to Cite
SENDINO, C., ZÁGORŠEK, K. and TAYLOR, P. D. (2012), Asymmetry in an Ordovician conulariid cnidarian. Lethaia, 45: 423–431. doi: 10.1111/j.1502-3931.2011.00302.x
- Issue published online: 15 JUN 2012
- Article first published online: 23 JAN 2012
Sendino, C., Zágoršek, K. & Taylor, P.D. 2012: Asymmetry in an Ordovician conulariid cnidarian. Lethaia, Vol. 45, pp. 423–431.
Conulariids are fossils of the presumed polyps of an extinct scyphozoan cnidarian group. Their cone-shaped skeletons normally show perfect tetraradial symmetry. However, in the Ordovician species Metaconularia anomala (Barrande 1867) from Drabov (Czech Republic), tetraradial symmetry is compromised in three ways: (1) the skeleton often shows torsion; (2) the four sides may vary in width at the same level within one individual; and (3) one side may be deleted to give a triradial skeleton. Almost 2000 specimens were studied in museum collections. About 56% of analysed specimens are twisted in an anticlockwise direction (sinistral) when viewed from the apex towards the aperture, 28% show no torsion, 1% exhibit clockwise torsion (dextral) and the remaining 15% cannot be classified. Maximum measured torsion rate was 1.5°/mm. A significant negative correlation between torsion rate and length suggests that more highly torted individuals may have survived less well. Almost 5% of individuals show loss of one side for at least part of their lengths. Although many individuals have four sides of equal width, in a significant proportion the sides are of unequal width, up to a maximum/minimum side width ratio of 2 (i.e. widest face twice the width of the narrowest). In the absence of a satisfactory taphonomic model to explain the asymmetries, they are regarded as mirroring asymmetries in the living conulariids, with the strong preference for sinistral torsion interpreted as an example of a fixed asymmetry that was genetically controlled and heritable. It is speculated that the signalling protein Nodal as well as Hox-like genes were involved in controlling the asymmetries described in M. anomala.