Steven Wade Veatch, Western Interior Paleontological Society
Beth Simmons, Western Interior Paleontological Society
John Harrington, Colorado Springs Mineralogical Society (Fossil Group)
During the Mississippian Period, between 360 and 320 million years ago, Colorado was under a broad ocean. As the uplift of the Ancestral Rockies began at the end of the Mississippian Period, the ocean began to withdraw in episodic phases. The Glen Eyrie Formation formed during the transitional time between the Mississippian and the Pennsylvanian Period. In this rock formation, consisting of shales, sandstones, and limestones, are fossils of the marine plants and animals that thrived in this shallow, retreating sea. The Fountain Formation, an arkosic (rich in feldspar) mixture of rocks, sands, and shales overlies the Glen Eyrie Formation (Taylor, 1999).
Sea urchins have a hard calcareous outer skeleton shell known as a test. Narrow ambulacral plates lie along the grooves of the shell where the tube feet emerge. Broad interambulacral plates hold spines (Case, 1982). Sea urchins use their spines, like a porcupine uses quills, to discourage predators. The spines are also used for locomotion, camouflage, and for catching drifting algae to eat. An elaborate hydraulic system provides the power for feeding and motion in this group. Seawater is the hydraulic fluid.
|Figure 1. Aerial view of the Glen Eyrie castle in Queens Canyon. General William Palmer, founder of Colorado Springs, built Glen Eyrie an English Tudor–style castle in 1904. Aerial photo by S.W. Veatch|
Recently Echinoderm fragments were found weathering out of a shale bed in the Glen Eyrie Formation just west of Garden of the Gods. These fragments not only included crinoids but also echinoids identified as Archaeocidaris dininnii (Chronic and Williams, 1978). Archaeocidaris was first described in 1841 by Louis Agassiz (Shrimer and Shrock, 1972). Agassiz later formulated the theory of a great Ice Age.
Archaeocidaris usually occur in large groups, and when the first one was found at this fossil site, the search was on in the area for more. Dozens of additional specimens soon emerged from the shales. Because of a favorable local environment that included plenty of food and protection from waves and currents, these animals banded together. Like modern sea urchins, living in groups improves spawning and provided some measure of protection.
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 (Figure 3). Skin and cord-like muscle, covering the test, moved and rotated the spines in almost any direction around the tubercle. The barbed Archaeocidaris spines apparently provided protection from predators and allowed locomotion.
Modern sea urchins have pedicellariae, modified spines with pincers used to prevent small organisms from attacking or settling on the test and to catch food (Parker and Kalvaas, 1992; Kato and Schroeter, 1985). It is probable that Archaeocidaris had pedicellariae, but pedicellariae are fragile and do not ordinarily fossilize.
While their mouth was located on the underside of their body, wastes were excreted through the anus at the top of the animal. The small size of this opening reflected the little amount of excreta produced. A circle of plates called the apical system surrounded the anus, or periproct. The geometry and orientation of plates within the apical system are used by paleontologists in the classification scheme (Orr, pers. comm.). When the periproct is enclosed within the apical system, sea urchins are termed regular. Sea urchins that have a periproct outside the apical system are known as irregular and have a bilateral symmetry.
- Phylum: Echinodermata
- Subphylum: Echinozoa
- Class: Echinoidea
- Order: Cidaroida
- Genus: Archaeocidaris
- Species: dininnii
We thank Dr. William Orr for his helpful and constructive review of the original manuscript. We are grateful for the field studies made possible by the Colorado Springs Mineralogical Society (Fossil Group).
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