Study of Fossils : Dickinsonia

DICKINSONIA FOSSIL

Dickinsonia fossil  photographed by O. Louis Mazzatenta

Systematic Position: (Sprigg, 1947)

Kingdom: Animalia

   Subkingdom: Eumetazoa

       Phylum: Proarticulata

          Class: Dipleurozoa

            Genus: Dickinsonia

Origin:

During Ediacaran Period.

 It is a geological period that spans 94 million years from the end of the Cryogenian Period 635 million years ago to the beginning of the Cambrian Period .

 It marks the end of the Proterozoic Eon, & the beginning of the Phanerozoic Eon.

Fossils are known only in the form of imprints & casts in sandstone beds. The specimens found range from a few milimeters to about 1.4 meters (4ft 7 in ) in length & from a fraction of a milimeters to a few milimeters thick.

Ontogeny of Dickinsonia costata following the glide reflection interpretation Photo:Aleksey Nagovitsyn (Alnagov)

Characters:

 Discovery of Cholesterol molecules in fossils of Dickinsonia lends support to the idea that Dickinsonia was an animal.

 Bilaterally symmetric, segmented, round or oval in outline, slightly expanded to one end .

 The rib-like segments are radially inclined towards the wide and narrow ends, and the width and length of the segments increases towards the wide end of the fossil.

The segments are separated by a thin ridge or groove along the axis of symmetry into right & left halves.

Segments are organized in an alternating pattern according to glide reflection symmetry rather than bilateral symmetry; thus, these 'segments' are isomers.

Evolutionary Significance:

 One of the earliest known animal fossil.

 Bilateral symmetry, distinct anterior/posterior and likely dorsal/ventral differentiation in addition to modules that met precisely at the midline, along with the presence of paleo cholesterol inclusions indicating definite animal like growth pattern.

 Amongst the animals, morphological analyses have suggested possible shared common ancestry with living marine parasitic worms, Cnidaria, Ctenophora, Platyhelminthes and Placozoa but the precise position of Dickinsonia within the animal tree remains uncertain at present. Taphonomic variants of Dickinsonia reveal that it was structurally resilient for a soft-bodied organism, highly extensible compared with most modern biopolymers, capable of elastic & plastic deformation,& composed of relatively thick, differentiated tissue.

 Recent fossil finding of D.tenuis in Bhimbetka shows India and Australia was evolutionary neighbours.

 Many of these characters are consistent with bilaterians today this fossil probably represents an extinct clade within the Eumetazoa.