Abstract
Several derived sand dollar (Echinoidea: Clypeasteroida) families are characterized by the presence of Gregory’s diverticulum, an accessory organ of the digestive tract. This soft tissue structure is composed of a central tubular cecum that gives off multiple lobes into the periphery of the test. Most notable are the organ’s capacity to selectively store sand grains that the animal has taken up from the surrounding sediment as well as the gradual reduction of Gregory’s diverticulum during ontogeny. Several aspects of the biology of this structure have remained unexplored, including the organ’s precise morphology and structural diversity. In order to provide a concise basis for future histological, physiological, and functional analyses, a comprehensive comparative morphological and phylogenetic study across numerous taxa was undertaken. Taxon sampling comprised over 100 clypeasteroid species, including various fossil taxa. This extensive dataset permits establishing a concise terminology that incorporates all of the organ’s substructures. In addition, three-dimensional models of Gregory’s diverticulum are presented that provide an improved spatial understanding of the organ’s morphology in situ. The combined data from dissection, X-ray imaging, microcomputed tomography, and magnetic resonance imaging reveal a previously unknown variability of the structure, which also yields several phylogenetically informative morphological characters. Among those sand dollar families that possess Gregory’s diverticulum, the organ is present in two distinct shapes, which can be distinguished by the number, shape, and location of substructures. In addition, the data provide unequivocal evidence that Gregory’s diverticulum is absent in the extant taxa Rotulidae and Astriclypeidae, but also in the enigmatic Marginoproctus.
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Abbreviations
- μCT:
-
Microcomputed tomography
- 2D:
-
Two-dimensional
- 3D:
-
Three-dimensional
- Amb:
-
Ambulacrum
- BMNH:
-
British Museum of Natural History, London, UK
- BPBM:
-
Bernice P. Bishop Museum, Honolulu, HI, USA
- CASG:
-
California Academy of Sciences Geology, San Francisco, CA, USA
- CASIZ:
-
California Academy of Sciences Invertebrate Zoology, San Francisco, CA, USA
- CT:
-
Computed tomography
- FDNR:
-
Florida Department of Natural Resources, St. Petersburg, FL, USA
- IAmb:
-
Interambulacrum
- MCZ:
-
Museum of Comparative Zoology, Cambridge, MA, USA
- MMBS:
-
Misaki Marine Biological Station, Misaki, Japan
- MNHN:
-
Muséum national d'Histoire naturelle, Paris, France
- MRI:
-
Magnetic resonance imaging
- Mya:
-
Million years ago
- NMNS:
-
National Museum of Natural Science, Taipei, Taiwan
- SIO:
-
Scripps Institution of Oceanography, San Diego, CA, USA
- UB:
-
Université de Bourgogne, Dijon, France
- UCMP:
-
Museum of Paleontology, University of California, Berkeley, CA, USA
- UNEFM:
-
Museo de la Universidad Nacional Experimental Francisco de Miranda, Falcón, Venezuela
- USNM:
-
United States National Museum, Washington, DC, USA
- UT:
-
University of Texas at Austin, Austin, TX, USA
- UTO:
-
University of Toronto, Ontario, Canada
- WAM:
-
Western Australian Museum, Perth, Western Australia, Australia
- ZMB:
-
Museum für Naturkunde, Berlin, Germany
- ZMH:
-
Zoologisches Museum Hamburg, Hamburg, Germany
- ZMK:
-
Zoologisk Museum København, Copenhagen, Denmark
- ZSM:
-
Zoologische Staatssammlung München, Munich, Germany
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Acknowledgments
We would like to thank the following colleagues for providing access to museum specimens: Nadia Améziane (MNHN), Andrew Cabrinovic (BMNH), Jean DeMouthe (CASG), Mariko Kondo (MMBS), Kwen-Shen Lee (NMNS), Carsten Lüter (ZMB), Kelly Markello (CASIZ), Jørgen Olesen (ZMK), David L. Pawson (USNM), Chrissy Piotrowski (CASIZ), Bernhard Ruthensteiner (ZSM), Andreas Schmidt-Rhaesa (ZMH), and Robert M. Woollacott (MCZ). We are grateful to Felix Beckmann, Johannes Müller (ZMB), Nina Nagelmann, and Malte Ogurreck for their kind help with imaging equipment. Adam Baldinger (MCZ), Tom Schiøtte (ZMK), and Berit M. Ullrich provided valuable assistance with specimen handling and shipment. Constructive comments and suggestions by Andreas Kroh and one anonymous reviewer helped to improve the text. We are furthermore indebted to Gonzalo Giribet (MCZ) and Andreas Ziegler for facilitating access to scanning systems.
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Ziegler, A., Lenihan, J., Zachos, L.G. et al. Comparative morphology and phylogenetic significance of Gregory’s diverticulum in sand dollars (Echinoidea: Clypeasteroida). Org Divers Evol 16, 141–166 (2016). https://doi.org/10.1007/s13127-015-0231-9
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DOI: https://doi.org/10.1007/s13127-015-0231-9