Are hyoliths Palaeozoic lophophorates?

Area of Science:

• Paleontology
• Evolutionary Biology
• Invertebrate Zoology

Background:

• The evolutionary relationships of hyoliths, an extinct group of marine invertebrates, have been debated.
• Previous interpretations suggested hyoliths might be lophophorates, possibly related to brachiopods, based on fossilized feeding structures and apical extensions.

Purpose of the Study:

• To investigate the phylogenetic position of hyoliths within the animal kingdom.
• To re-evaluate the nature of the feeding apparatus and apical structures in hyoliths based on new fossil evidence.

Main Methods:

• Detailed morphological analysis of soft tissues associated with the feeding apparatus of the orthothecid hyolith *Triplicatella opimus* from the Chengjiang biota.
• Comparative analysis of feeding organs in hyolithids and apical structures in orthothecids.

Main Results:

• The feeding apparatus of *Triplicatella opimus* exhibits a unique tuft-like tentacle arrangement, differing from hyolithids, suggesting direct substrate feeding.
• Reassessment indicates hyolith feeding organs are not true lophophores.
• Apical structures in some orthothecids are identified as crushed shell fragments, not homologous to the brachiopod pedicle.

Conclusions:

• Hyoliths are unlikely to be lophophorates closely related to brachiopods.
• The findings support hyoliths being basal members of the Lophotrochozoa, a major bilaterian clade.
• This study refines our understanding of early animal evolution and hyolith phylogeny.