Ancient Sea Urchins of Colorado Springs: Incredible Porcupines of the Sea

Just west of Garden of the Gods in Colorado Springs—about 3 kilometers from the beginning of Rampart Range Road—are the remains of fossil sea urchins found weathering out of the Glen Eyrie Formation. These fossil sea urchins are Archaeocidaris dininnii. These ancient animals reveal a span of time when Colorado Springs was under a sea and home to a large number of marine creatures.

Archaeocidaris occured in large groups since the environment included plenty of food and protection from waves and currents. Like modern sea urchins, living in groups improves spawning and provided protection. Once the first sea urchin was found at this fossil site the search was on for more. Dozens of additional specimens were collected.

Archaeocidaris had a spherical, calcareous skeleton or test made of moderately thick plates that were arranged radially in two types of double columns. The first double column, termed the ambulacrum (plural-ambulacra), had two pores in each plate for the projection of tube feet. Hydraulically powered tube feet aid in locomotion, anchoring, feeding, sensing the environment, and respiration.


The second double column, the interambulacrum, alternates with the ambulacra. Archaeocidaris had a distinctive arrangement of four columns of plates in each interambulacrum. Moveable spines were joined onto a single large tubercle on each interambulacral plate.



Polygonal interambulacral plates that form part
of the Archaeocidaris test. Spines fit on the large knobs
or tubercles in the center of the plates.Spines are
 rarely preserved as fossils.

Skin and cord-like muscle, covering the test, moved and rotated the spines in almost any direction around the tubercle. The barbed spines of Archaeocidaris provided protection from predators and allowed locomotion. 

The interambulacral plates have conspicuous bumps in
the center where the spines were once connected. 

 When a sea urchin dies, the tissue that holds the plates together decays, and the plates disassemble and scatter on the seafloor. of the Archaeocidaris dininnii fossils found at the Rampart Range Road site are represented by separate plates and spines.

Because sea urchins are generally one of the first marine organisms to show signs of stress if something is wrong with the water, the Environmental Protection Agency uses them as an indicator organism for water quality near shores and in bays. When conditions are poor, sea urchins will stop moving, their spines will droop, and they will die.