THE FOSSILS
Many groups of invertebrates are represented in the fossils of D.A.R. State Park. [Plate 1] will help you to identify these fossils. The name, phylum (major group) and age of each fossil are provided in the explanation of the plate. The following paragraphs describe each phylum represented in the Park rocks.
Plate 1. Typical fossils found in the Glens Falls Limestone.
Arthropods. D.A.R. State Park rocks contain trilobites with the following imposing names: Cryptolithus tesselatus (crip-toe-LITH-us tessell-AH-tus[5]), Isotelus gigas (ice-so-TELL-us GIG-us) and Flexicalymene senaria (flex-eye-cal-ah-Mean-ee sen-AREA). These fossils are figured on [Plate 1], 1-A, B, C, D; 2; 3. Within the Park Cryptolithus tesselatus is very common wherever fossils occur. Generally only the cephalon or head portion of this trilobite is preserved. The cephalon is easily recognized by three concentric rows of pits arranged around the brim. Cryptolithus is an excellent index fossil[6] for the Park rocks. The arthropod phylum is characterized by animals with jointed legs, segmented bodies and a jointed outer armour of chitin.[7] For examples, the crabs, lobsters, spiders, scorpions and insects are arthropods. Trilobites appear early in the fossil record but they did not survive beyond the Paleozoic Era.
Explanation for [Plate 1]
(all drawings are X1 unless otherwise indicated)
1-A. Cryptolithus tesselatus, Arthropod (Trilobite), Middle Ordovician (Trenton Stage). Front view of the Cephalon or head. (X2)
1-B. Cryptolithus tesselatus, Arthropod (Trilobite), Middle Ordovician (Trenton Stage). Oblique front-lateral view of the Cephalon. (X2)
1-C. Cryptolithus tesselatus, Arthropod (Trilobite), Middle Ordovician (Trenton Stage). Top view of the Cephalon. (X2)
1-D. Cryptolithus tesselatus, Arthropod (Trilobite), Middle Ordovician (Trenton Stage). Side view of the Cephalon. (X2)
2. Isotelus gigas, Arthropod (Trilobite), Middle Ordovician. Top view of specimen.
3. Flexicalymene, Arthropod (Trilobite), Ordovician to Silurian. Top view of an enrolled specimen.
4-A. Dinorthis pectinella, Brachiopod, Middle Ordovician (Trenton Stage). Exterior view of the brachial valve.
4-B. Dinorthis pectinella, Brachiopod, Middle Ordovician (Trenton Stage). Exterior view of the pedicle valve.
5. Reuschella edsoni, Brachiopod, Middle Ordovician. Exterior view of the pedicle valve.
6. Lingula, Brachiopod, Ordovician to Recent.
7-A. Prasopora, Bryozoan, Ordovician. Top view. (X0.5)
7-B. Prasopora, Bryozoan, Ordovician. Side view. (X0.5)
7-C. Prasopora, Bryozoan, Ordovician. Vertical thin section showing the nature and growth of part of a bryozoan colony. (X18)
8. Bryozoan, “twig-like” type, Ordovician to Devonian.
9. Sowerbyella, Brachiopod, Middle and Upper Ordovician. Exterior view of brachial valve. (X2)
10. Rafinesquina, Brachiopod, Middle and Upper Ordovician. Exterior view of the pedicle valve.
11. Platystrophia trentonensis, Brachiopod, Middle Ordovician (Trenton Stage). Anterior or front view.
12. Hesperorthis tricenaria, Brachiopod, Middle Ordovician (Black River and Trenton Stage). Interior view of the pedicle valve.
Fig. 5. The surface of a layer or bed of Glens Falls Limestone. Pen points to a colonial Bryozoan Prasopora “head.” These “heads” are very common just south or to your left if walking down the ramp.
Brachiopods. Brachiopods are abundant in the Park rocks (see [Plate 1], 4A, B; 5; 6; 9; 10; 11; 12). These invertebrates are small marine animals which generally live in waters no deeper than 600 feet. The two valves of their shell are joined at the back (posterior) end of the body along a hingeline of interlocking teeth and sockets. The shell of the brachiopod is opened or shut by muscles attached to the inside of each valve. Brachiopods are found in the oldest rocks containing definite and abundant fossils. Brachiopods are still living today.
Bryozoans. Bryozoans or “moss animals” are very small marine animals which live in colonies. The bryozoans construct their mutual home or colony of lime which is commonly preserved for the fossil record. Large colonies of the fossil Prasopora (prah-sop-OR-ah) are commonly seen on the weathered surfaces of many of the rock layers in the Park (see [Plate 1]; 7A, B, C; 8 and [Figure 5]). Individuals of one genus common here, can be recognized by their chocolate drop shapes. Bryozoans first appear in lower Paleozoic rocks and are still living today in clear well-circulated shallow to deep marine water. Considering all of the fossils found in the Park rocks, the past environment is thought to have been a relatively shallow and warm sea.
Fig. 6. This photograph shows the typical thickness of the Glens Falls Limestone beds in the Park area. Note the massive nature of the limestone bed. The 5-inch pen in the center of the picture is for scale.
Fig. 7. Sections illustrating the geologic history of D.A.R. State Park.