November guest authors and I write about ancient sea urchins in Colorado Springs

Ancient Sea Urchins of Colorado Springs: The Incredible Porcupines of the Sea
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).

Just west of Garden of the Gods in Colorado Springs the remains of fossil sea urchins were found weathering out of the Glen Eyrie Formation. The fossil site is located about 3 kilometers from the beginning of Rampart Range Road in the Garden of the Gods Park. Sea urchin fossils from this time interval are rare in the fossil record.

Today, beachcombers find all manner of sea urchins washed up on the shore. Sea urchins populate the ocean floor from the beach down to abyssal depths. They belong to the phylum Echinodermata; the spiny skinned invertebrate animals. There are five classes in this phylum: starfish (asteroids), sea lilies (crinoids), brittle stars (ophiuroids), sea urchins (echinoids) and sea cucumbers (holothurians). Fifteen other classes, found in the fossil record, do not exist today. Overall, there are about 6,000 species (Sprinkle and Kier 1987). Echinoderms are one of the few invertebrates that never escaped the oceanic realm.

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.

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.

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

G.I. Finlay designated the type section (original description) of this formation in 1907 at the Glen Eyrie estate, about 5 miles northwest of Colorado Springs (Finlay, 1907). The Glen Eyrie Formation lays under the Fountain Formation and is poorly exposed. The Glen Eyrie Formation consists of 3 meters (10 feet) to 110 meters (360 feet) of gray to black alternating sandstones, coaly shales, and marly limestone (Chronic and Williams, 1978). The alternating sequences, called cyclothems, are repeated sequences of rocks caused by the periodic rise and fall of the sea level.

The Glen Eyrie Formation is rich in marine plants and invertebrates, consistent with organisms found in late Mississippian and early Pennsylvanian strata, suggesting that this formation is transitional between Mississippian and Pennsylvanian times (Chronic and Williams, 1978). This makes the Glen Eyrie Formation somewhat older than previously thought (Early to Middle Pennsylvanian).

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.

Archaeocidaris, a cidaroid, was the ancient ancestor of the modern sea urchin. Small cidaroids first appeared in the Mississippian Period. The cidaroids were the only echinoids that did not become extinct by the close of the Paleozoic. Cidaroids are distinguished from most other echinoids by their simple ambulacral plates, large, knob-like tubercles centered in interambulacral plates, and barbed spines (Figure 2). Modern cidaroids or "pencil slate urchins" are restricted to tropical waters, and in the fossil record cidaroids are regarded as an excellent indicator of very warm and shallow conditions (Orr, pers. comm.).

Figure 2. Polygonal interambulacral plates that form part of the Archaeocidaris test are on the left. Spines (on the right) fit on the large knobs or tubercles in the center of the plates. Spines are rarely preserved as fossils. Specimens from the S.W. Veatch collection. Image by Mike Estlick.

Archaeocidaris had a spherical calcareous skeleton or test made of moderately thick plates 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 (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.

When a sea urchin dies, the tissue that holds the plates together decays, and the plates disassemble. This process, aided by predators and wave action, produces a cover of plates on the seafloor. Whole sea urchin tests are exceptionally rare. All of the Archaeocidaris dininnii fossils found at the Rampart Range Road site are represented by separate plates and spines.

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.

Figure 2. Polygonal interambulacral plates that form part of the Archaeocidaris test are on the left. Spines (on the right) fit on the large knobs or tubercles in the center of the plates. Spines are rarely preserved as fossils. Specimens from the S.W. Veatch collection. Image by Mike Estlick.

A sea urchin's mouth, or peristome, is located in the center on the lower (ventral) surface of test. The opening is large and within it are beak-like jaws called pyramids and five curved calcareous teeth. Combined, the pyramids and teeth form an unusual chewing structure called Aristotle's lantern. New teeth grew to replace worn-down ones. Archaeocidaris, just like modern sea urchins, probably ate seaweed or decaying organic mater.

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.

Classification

• Phylum: Echinodermata
• Subphylum: Echinozoa
• Class: Echinoidea
• Order: Cidaroida
• Genus: Archaeocidaris
• Species: dininnii

There are few outcrops of the Glen Eyrie Formation in the area; the unusual exposure west of the Garden of the Gods yields sea urchin fossils. These ancient animals reveal a very different age—a span of time when Colorado Springs was under a sea and home to a large number of marine creatures.

Acknowledgments:

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).

References Cited:

Case, G.R. 1982. A Pictorial Guide to Fossils. Van Nostrand Reinhold Company, NY, 515 p.

Chronic, J. and Williams, C.A. 1978. The Glen Eyrie Formation (Carboniferous) near Colorado Springs. Rocky Mountain Association of Geologists 1978 Symposium, p. 199 - 206.

Finlay, G.I. 1907. The Gleneyrie Formation and its bearing on the age of the Fountain Formation in the Manitou region, Colorado. Journalof Geology 15: 586-589.

Kato, S. and Schroeter, S.C. (1985) Biology of the red sea urchin, Strongylocentrotus franciscanis, and its fishery in California. Marine Fisheries Review, 47(3):1-20.

Parker, D. and Kalvass, P. 1992. Sea Urchins. in, W. L. Leet, C. M. Dewees, and C. W. Haugen, (eds.) California'sLiving Marine Resources and Their Utilization p. 41-43. Sea Grant Extension Publication UCSGEP-91-12, Sea Grant Extension Program, Wildlife and Fisheries Biology Department, University of California, Davis, CA.

Shimer, H. W. and Shrock, R.R., 1972. Index Fossils of North America. The M.I.T. Press, Cambridge, p 217.

Sprinkle, J. and Kier, P.M. 1987. Phylum Echinodermata In Boardman, R.S., Cheetham, A.H., Rowell, A.J. (eds.) Fossil Invertebrates. Blackwell Scientific Publications, Palo Alto, CA. p 596 -611

Taylor, A.W. 1999. Guide to the Geology of Colorado. Cataract Lode Mining Company, Golden, CO, 222 p.