AM'DA'MA: JURASSIC SEA URCHIN

This lovely little biscuit is a Holectypus sea urchin from 120 million-year-old deposits from the Lagniro Formation of Madagascar.

The specimen you see here is in the collections of my beautiful friend Ileana. She and I were blessed to meet in China many years ago and formed an unbreakable bond that happens so few times in one's life. 

Holectypus are a genus of extinct echinoids related to modern sea urchins and sand dollars. They were abundant from the Jurassic to the Cretaceous (between 200 million and 65.5 million years ago).

This specimen is typical of Holectypus with his delicate five-star pattern adorning a slightly rounded test and flattened bottom. The specimen has been polished and was harvested both for its scientific and aesthetic value. 

I have many wonderful memories of collecting their modern cousins that live on the north end of Vancouver Island and along the beaches of Balaklava Island. In the Kwak̓wala language of the Kwakiutl or Kwakwaka'wakw, speakers of Kwak'wala, of the Pacific Northwest, sea urchins are known as a̱m'da̱'ma and it is this name that I hear in my head when I think of them.

In echinoids, the skeleton is almost always made up of tightly interlocking plates that form a rigid structure or test — in contrast with the more flexible skeletal arrangements of starfish, brittle stars, and sea cucumbers. Test shapes range from nearly globular, as in some sea urchins, to highly flattened, as in sand dollars. 

Sea Urchin Detail

Living echinoids are covered with spines, which are movable and anchored in sockets in the test. These spines may be long and prominent, as in typical sea urchins and most have lovely raised patterns on their surface. 

In sand dollars and heart urchins, however, the spines are very short and form an almost felt-like covering. The mouth of most echinoids is provided with five hard teeth arranged in a circle, forming an apparatus known as Aristotle’s lantern.

Echinoids are classified by the symmetry of the test, the number and arrangement of plate rows making up the test, and the number and arrangement of respiratory pore rows called petals. Echinoids are divided into two subgroups: regular echinoids, with nearly perfect pentameral (five-part) symmetry; and irregular echinoids with altered symmetry.

Because most echinoids have rigid tests, their ability to fossilize is greater than that of more delicate echinoderms such as starfish, and they are common fossils in many deposits. The oldest echinoids belong to an extinct regular taxon called the Echinocystitoidea. 

They first appeared in the fossil record in the Late Ordovician. Cidaroids or pencil urchins appear in the Mississippian (Early Carboniferous) and were the only echinoids to survive the mass extinction at the Permo-Triassic boundary. Echinoids did not become particularly diverse until well after the Permo-Triassic mass extinction event, evolving the diverse forms we find them in today. 

True sea urchins first appear in the Late Triassic, cassiduloids in the Jurassic, and spatangoids or heart urchins in the Cretaceous. Sand dollars, a common and diverse group today, do not make an appearance in the fossil record until the Paleocene. They remain one of my favourite echinoderms and stand tall amongst the most pleasing of the invertebrates.