On the origin of hyolith helens

Christian B. Skovsted*, Mónica Martí Mus, Zhiliang Zhang, Bing Pan, Luoyang Li, Fan Liu, Guoxiang Li, Zhifei Zhang

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

2 Citations (Scopus)
3 Downloads (Pure)

Abstract

Helens, the curved lateral spines inserted between the conch and operculum of some hyoliths, are a unique morphological adaptation characterizing the order Hyolithida. These structures are paired, movable and had a mechanical function, probably related to orienting the hyolith conch and lifting its aperture above the sea floor. We show that helens are intimately associated with the hyolith opercula and are structurally comparable to the rod like units that constitute the clavicles, internal wall-like structures of the hyolithid operculum that probably evolved to secure the operculum from lateral displacement in the conch aperture. In some early Cambrian hyolith taxa that lack helens, such as Paramicrocornus, new clavicle rods are added in the gap separating the clavicles from the cardinal processes, the same position where helens are inserted in later hyolithids. We also show that the size of incipient helens at the earliest ontogenetic stage matches the size of the clavicles in associated opercula. We propose that helens are modified clavicle rods that were detached from the operculum and developed into lateral spines through allometric growth during early ontogeny. Further, we suggest a four-step model for the evolution of hyolithid hyoliths from orthothecid ancestors: 1, Externally fitting operculum; 2, Stabilizing, radially arranged structures on the inside of the operculum; 3, Ligula and folded operculum; 4, Detachment of clavicle rods and origin of helens.

Original languageEnglish
Article number109848
Pages (from-to)1-11
Number of pages11
JournalPalaeogeography, Palaeoclimatology, Palaeoecology
Volume555
DOIs
Publication statusPublished - 1 Oct 2020

Bibliographical note

Copyright the Author(s) 2020. Version archived for private and non-commercial use with the permission of the author/s and according to publisher conditions. For further rights please contact the publisher.

Keywords

  • Hyolithida
  • Orthothecida
  • Evolution
  • Clavicles
  • Operculum
  • Cambrian

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