THE SIGNIFICANCE OF INTERNAL SUPPORTS AND PLATE INTERLOCKS FOR THE TEST INTEGRITY AND THE PRESERVATION POTENTIAL OF CLYPEASTEROID ECHINOIDS

The differential preservation potentials of sea urchin skeletons depend on numerous factors including the architecture of the test, mode of death, ambient ecological factors and so on. Clypeasteroid echinoids are known to have a high preservation potential due to the presence of internal supports in most taxa that bridge the oral and aboral side of the test as well as stereom plate interlocks that strengthen test integrity. In this study, scanning electron microscopy and x-ray micro-computed tomography are used to comparatively analyze the morphological structures of three clypeasteroids: Clypeaster rosaceus, Echinocyamus pusillus, and Leodia sexiesperforata.

Micrographs have revealed that Clypeaster rosaceus and Echinocyamus pusillus feature similar plate interlocks with stereom protrusions reaching into the pores of the stereom of adjoining plates thus securely interlocking neighboring plates. Leodia sexiesperforata, along with the stereom interlocks also shows needle-like joints that penetrate from one plate to another. The internal supports vary between the three species; Clypeaster rosaceus features free-standing, in part fused wall-like pillars. Echinocyamus pusillus features prominent interambulacral buttresses reaching from the basicoronal plates on the oral side to the outer extent of the petalodium on the aboral side of the test. Leodia sexiesperforata, with a discoidal and highly flattened form, features internal supports that form delicate pillars in the more proximal parts and a meshwork of thin walls in the more distal part of the test close to the ambitus. These differential morphologies of connecting plate types and internal supports are correlated to overall shape of the skeleton as well as observed breakage patterns of the test.