Towards a Better Understanding of the Origins of Microlens Arrays in Mesozoic Ophiuroids and Asteroids
Echinoderms are characterized by a calcite endoskeleton with a unique microstructure, which is optimized for multiple functions. For instance, some light-sensitive ophiuroids (Ophiuroidea) and asteroids (Asteroidea) possess skeletal plates with multi-lens arrays that are thought to act as photosensory organs. The origins of these lens-like microstructures have long been unclear. It was recently proposed that the complex photosensory systems in certain modern ophiuroids and asteroids could be traced back to at least the Late Cretaceous (ca. 79 Ma). Here, we document similar structures in ophiuroids and asteroids from the Early Cretaceous of Poland (ca. 136 Ma) that are approximately 57 million years older than the oldest asterozoans with lens-like microstructures described thus far. We use scanning electron microscopy, synchrotron tomography, and electron backscatter diffraction combined with focused ion beam microscopy to describe the morphology and crystallography of these structures in exceptional detail. The results indicate that, similar to Recent light-sensitive ophiuroids, putative microlenses in Cretaceous ophiuroids and asteroids exhibit a shape and crystal orientation that would have minimized spherical aberration and birefringence. We suggest that these lens-like microstructures evolved by secondary deposition of calcite on pre-existing porous tubercles that were already present in ancestral Jurassic forms.
KeywordsEchinoderms Photosensitivity Cretaceous Microlenses Calcite Tomography
This work was completed while the first author was a recipient of a grant from the Polish National Science Centre (NCN) Grant number DEC-2011/03/N/ST10/04798 and was performed in part in the NanoFun laboratory co-financed by the European Regional Development Fund within the Innovation Economy Operational Programme POIG.02.02.00-00-025/09. IAR was funded by an 1851 Royal Commission Research Fellowship. SZ was funded by grants RYC-2012-10576 and CGL2013-48877 from the Spanish MINECO. We acknowledge the Paul Scherrer Institut, Villigen, Switzerland for the provision of synchrotron radiation beamtime on the TOMCAT beamline at the Swiss Light Source and thank Professor Charles G Messing (Nova Southeastern University) for providing the extant ophiuroid specimen. We also thank two anonymous reviewers for their supportive comments.
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Conflict of interest
The authors declare that they have no conflict of interest.
Investigations comply with the current laws of the country in which they were performed.
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