Cambrian stalked echinoderms show unexpected plasticity of arm construction

Area of Science:

• Paleontology
• Echinoderm Evolution
• Cambrian Biota

Background:

• Feeding arms with coelomic extensions of the theca were previously considered unique to crinoids among stemmed echinoderms.
• The morphology of early Cambrian echinoderm appendages has been poorly understood, limiting insights into their evolutionary history.

Purpose of the Study:

• To investigate the morphology and phylogenetic position of a novel two-armed echinoderm from the earliest Middle Cambrian of Spain.
• To re-evaluate the evolutionary origins of blastozoan appendages and their relationship to other echinoderm clades.

Main Methods:

• X-ray microtomographic analysis was employed to examine the detailed internal and external structure of the fossilized arms.
• Phylogenetic analysis was conducted to determine the evolutionary relationships of the new taxon within the Echinodermata.

Main Results:

• The new echinoderm possesses polyplated proximal arms with a coelomic lumen, transitioning to a biserial structure distally, resembling blastozoan brachiole.
• Phylogenetic analysis places this taxon basally to rhombiferans, as the sister-group to pleurocystitid and glyptocystitid blastozoans, extending their Cambrian origin.
• The study demonstrates significant variability in appendage construction among Cambrian echinoderms.

Conclusions:

• The appendages of at least some blastozoans evolved as direct outgrowths of the body, similar to crinoid arms.
• This discovery expands our understanding of early echinoderm diversity and the evolutionary pathways of appendage development in the Cambrian period.