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A complete reconstruction of the hyolithid skeleton

Published online by Cambridge University Press:  14 July 2015

Mónica Martí Mus
Affiliation:
Área de Paleontología, Universidad de Extremadura, Avenida de Elvas s/n, Badajoz 06006 Spain,
Lennart Jeppsson
Affiliation:
Department of Geology, Lund University, Sölvegatan 12, 223 62 Lund, Sweden,
John M. Malinky
Affiliation:
Physical Science Department, San Diego City College, 1313 Park Boulevard, San Diego, CA 92101, USA,

Abstract

Hyolithids are a group of Paleozoic lophotrochozoans with a four-pieced skeleton consisting of a conch, an operculum, and a pair of lateral ‘spines' named helens. Both the conch and operculum are relatively well known and, to a certain extent, have modern analogues in other lophotrochozoan groups. The helens, on the other hand, are less well known and do not have clear modern analogues. This has hindered the knowledge of the complete morphology of the hyolithid skeleton, as well as other aspects of hyolithid biology, such as the organization of soft parts, and their ability to move. The material studied herein, consisting of disarticulated skeletal elements from the Silurian of Gotland, Sweden, illustrates a complete developmental sequence of a hyolithid species and includes the first complete, three-dimensionally preserved helens. Our material confirms that helens were massive skeletal elements, whose growth started proximally with the deposition of a central, coherent lamella. Further shell accretion took place around this lamella, but followed a particular accretion pattern probably constrained by the presence of marginal muscle attachment sites on the proximal-most portion of the helens. These muscle attachment sites were ideally located to allow a wide range of movements for the helens, suggesting that hyolithids may have been relatively mobile organisms.

Type
Research Article
Copyright
Copyright © The Paleontological Society 

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