Carpoid Stylophora Evolution
Evolution
 
Evolutionary Placement of the Enigmatic Carpoids


 

 


Cambrian PhyllocaridCarpoids (Stylophorans) are enigmatic fossils often found in Cambrian strata, and prevalent in the fossil record from about 500 to 300 million years ago. Debate has persisted for decades as to the group’s evolutionary origins and placement. The three competing hypotheses of Stylophora origins have been:

1) They are very primitive echinoderms with a mobile stalk or single arm filled with muscle; Lower Devonian Carpoid from Hunsruck Slate, Bundenbach Germany
2) Stylophorans are highly derived echinoderms related to crinoids possessing an ambulacrum with tube-feet and an oral tegmen with pharynx; and,
3) They are neither primitive or advanced echinoderms, but were more primitive chordates that retained a calcite exoskeleton from an older common ancestor of echinoderms and chordates, with the stalk containing muscle, notochord and brain.

Carpoid Castericystis from Marjum Formation of UtahIn a recent publication in Nature (Nature 438, 351-354; 2005), Clausen and Smith report evidence that support the first hypothesis and reject the last two based on study of a ceratocystid stylophoran from the Middle Cambrian of Morocco. The weight of evidence now holds that the enigmatic carpoids are, indeed, primitive echinoderms and a stem group from the crinoids.

 

 

Also see:
Clausen, S., Smith, A.B., Palaeoanatomy and biological affinities of a Cambrian deuterostome (Stylophora). Nature 2005 Nov 17;438(7066):351-354.
Ruta, M., Brief review of the stylophoran debate. Evolution and Development. 1999 Sep-Oct;1(2):123-35