Milwaukee Public Museum

• Featured
• Anthropology
• Botany
• Geology
• History
• Invertebrates
• Vertebrates
• Library

Sponge Spicules

Tunneling into Milwaukee's Ancient Past:
Sponges from 200 Feet Underground
by Patricia J. Coorough and Rodney Watkins

EDITOR'S NOTE: This article is reprinted from LORE magazine, a benefit of museum membership. MPM, Inc. holds copyright to LORE magazine and to this posting. ©1996 Milwaukee Public Museum, Inc.

The Deep Tunnel project is well known in Milwaukee as a wastewater storage system, but it has also offered geologists a rare look at a time when the Milwaukee area was part of a tropical sea. About 420 million years ago, Wisconsin was covered by shallow waters that teemed with marine life and stretched from New York to Nevada. Geologists refer to this as the Silurian Period. A unique record of Silurian sea life is buried under southeastern Wisconsin in layers of dolomite bedrock. Most of these rocks are covered by thick deposits of Younger sediments, making them difficult to view or collect. The Deep Tunnel Project provided a special chance for the Milwaukee Public Museum (MPM) to sample this Silurian bedrock.

The Deep Tunnel system was excavated in bedrock hundreds of feet below ground, and large amounts of rock were brought to the surface and transported to dump sites. Geologists cannot learn much from rock samples if they do not know exactly where the rock came from, and this requires help from personnel at the construction site. Milwaukee Metropolitan Sewerage District officials and their contracting firm, CH2M Hill, arranged to have the engineers call the Museum, when they started pulling up rock. If we wanted to collect, we needed to come over quickly and look through the pile they set aside. Tunneling and rock removal in a large construction project wait for no one.

We arrived at the site and were shown the pile set aside for us. The rocks consisted of grey dolomite, a kind of limestone, with scattered white chert nodules that represented fossil burrow systems. Looking through large amounts of rock revealed only a few impressions of small fossil shells. This did not look at all promising for making the collection of Silurian sea life that we were hoping for. However, we decided to collect several burlap bags of these rather featureless rocks for archival purposes, as there would be no second chance.

Back at the MPM geology lab, we had several choices for processing our rock samples. We could simply put them in storage, which at the time seemed the wisest course, since they appeared to be so barren. Alternatively, we could prepare them by breaking the rock into small pieces to look for fossils on the newly exposed surfaces, slicing the rock with a rock saw to look for cross sections of fossils, or dissolving the rock in hydrochloric acid (HCI). In the later process, the acid dissolves the predominate carbonate portion of the rock, leaving behind any fossils or other particles composed of silica. In the end we decided to try all three procedures, although we obtained relatively little data on fossil content by breaking and sawing the samples.

Our last hope was thus the acid preparation. The samples to be dissolved in HCI were taken to the "acid room" on the fourth-floor roof of the Museum. Here we placed each rock on plastic screening and set it into a wash basin of acid. Every three or four days we added fresh acid to the tubs until all the carbonate rock was dissolved. The insoluble residue left at the bottom of the tubs, consisting of silica, clay, and a little pyrite, was thoroughly washed and sieved through a 230-mesh-per-inch screen. Preliminary inspection of the sieved residues was very heartening, revealing an abundance of small crinoid and brachiopod fossils. The real surprise, however, came when we examined the residues under the microscope. At high magnification, we could see a multitude of tiny siliceous fossils with a wide variety of intricate geometric forms. Sponge spicules! This represented a new and quite unexpected discovery in the Silurian bedrock.

Sponges are bottom-dwelling, sessile animals that filter small food particles from the surrounding water. A typical sponge has a balloon-shaped body with a large central cavity and an opening at the top, called the osculum. The outer surface of the sponge has many small pores lined by special flagellated cells that create water currents. The entire sponge operates like a pump, pulling water in through the pores and expelling it through the osculum. Small bits of organic matter are filtered out of the water as it moves through the sponge.

Spicules are tiny, hard particles that form the skeleton of a sponge, holding the sponge body together and keeping it upright while alive. The spicules may be joined as an interconnected network, or they may be scattered separately through the sponge tissue. Modern sponges have spicules made of silica, calcite, or spongin (a flexible material familiar in the common bath sponge). Spicules may be fossilized as a complete network, revealing the size and shape of the original sponge body, but more commonly, as in the Silurian bedrock beneath Milwaukee, they occur as discrete, isolated particles released by decay of the soft tissue.

Sponges from 420 million years ago were not significantly different from those of today, as indicated by the same classic geometric shapes of fossil and modem spicules. Following discovery of siliceous spicules in the Deep Tunnel rock, our real work began. It was a task called "picking." This involved using a fine brush to transfer individual spicules to microscope slides. Hours were spent each day at the microscope, scanning the residues, looking at a record of life from millions of years ago and picking out spicule after spicule. In the end, we picked over 5,000 tiny specimens that represent 23 different forms of sponge spicules.

The discovery of a diverse assemblage of fossil sponge spicules from Silurian bedrock beneath Milwaukee is scientifically important for several reasons. Biologically, it indicates a greater complexity of ancient life in the Milwaukee area than was previously recognized. Three taxonomic groups of sponges are represented by the recovered spicules, and the high abundance of spicules in the acid residues indicates that sponges were a major component of the Silurian seafloor community.

The spicules are also important for what they tell us about environmental conditions in this ancient sea. A longstanding question among geologists has been the depth of water covering the Milwaukee area. Silurian seas that covered the interior of North America have generally been considered to be quite shallow, ranging from intertidal conditions to depths of a few tens of feet at most. The sponge fauna of Milwaukee's bedrock is an unusual occurrence for this inland sea. Similar occurrences of diverse, siliceous sponge spicules are present in Silurian rocks of the Canadian Arctic and Nevada, areas that represent deep water at the ancient continental margin. This similarity of sponge faunas indicates that Milwaukee was a relatively deep spot in the inland sea, with water depths on the order of 100 to 200 feet.

Discovery of ancient sponges in the bedrock beneath Milwaukee was a fortunate "accident" of an otherwise routine job. The transition from the ordinary to the extraordinary is a common occurrence in scientific research.

Examples of siliceous sponge spicules from the Deep Tunnel rock. Scanning electron micrographs of these specimens were made by Andrei Skylyarov of the Advance Analysis Facility of University of Wisconsin - Milwaukee. Magnifications of the spicules range froom 55 to 220 times life size.