Exercise 7 Fossils Part 2: Trilobites, archaeocyathids, nautiloids, graptolites

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1 Exercise 7 Fossils Part 2: Trilobites, archaeocyathids, nautiloids, graptolites TRILOBITES: Morphology/Terminology The phylum Arthropoda includes an enormous variety of animals, all of which are characterized by segmented appendages. Examples of arthropods include insects, spiders, crustaceans (crabs, crayfish, shrimp, lobsters), ostracodes, and trilobites. In terms of shear numbers of individuals, the arthropods outnumber all other animal groups combined! Trilobites were arthropods whose shells are divided longitudinally into three lobes: the central axial lobe and two lateral or pleural lobes. The trilobite shell also can be divided into a head region (cephalon), the segmented thorax, and a tail region (pygidium) (Figure 1). Figure 1. Trilobit anatomy and terminology. Dorsal (top) view. The shell of a trilobite is made of the organic substance chitin that has been thickened and reinforced by CaCO 3. Growth of an individual was accomplished in a series of increments. When growth became limited by the confines of the hard shell, the shell was discarded and a new, larger one was secreted in its place. The process of discarding an old shell is known as 7 1

2 molting (or ecdysis). A consequence of this mode of growth is that a single individual could produce several potential fossils. Trilobites as a group exhibit many different kinds of eyes. Some were multifaceted as in certain insects, whereas other were a simple, single-lens type. Many species of trilobites apparently were blind and lacked eyes altogether. Stratigraphic Range Trilibites originated in early Cambrian time and they were abundant in both the Cambrian and Ordovician periods. They decreased in abundance after the Ordovician Period and eventually became extinct in late Permian time coincident with the Mother of Mass Extinctions. Paleoenvironmental Range Trilobites were exclusively marine animals. Most lived in relatively shallow shelf environments, although some apparently preferred slightly deeper waters. Trilobite examples 1. Cryptolithus (Ordovician) (cast). This genus has a large cephalon relative to the rest of the body. Note genal spines and ornamentation (pits) along margin of cephalon Does this trilobite possess eyes? 2. Elrathia (Cambrian). The cephalon of this specimen is partly broken, but otherwise this is exquisite preservation of an actual specimen. Make sure you can distinguish the cephalon, thorax, and pygidium Are segments of the pygidium fused or discrete? 3. Ditymopyge (Pennsylvanian) (cast of an assemblage of individuals). Note that one specimen is partly enrolled. Some trilobites did this for protection against predators, just as do modern rolly polly bugs (pill bugs). Note the well developed eyes Can you tell where the thorax ends and the pygidium begins? 7 2

3 4. Examples of phacopid trilobites (casts and actuals specimens). The order Phacopida includes some of the most abundant and well studied trilobites. You may be asked to identify phacopids on the exam. Note that the thorax consists of a large number of segments Note that segments of the pygidium are not well fused (it s difficult to tell where the thorax ends and the pygidium begins) Note the well developed compound eyes (similar to the multi-faceted eyes in certain insects) 5. Assortment of trilobites preserved in gray shale (Cambrian) (impressions, molds, and actual shell material). Make sure you know what part of the trilobite you re looking at when examining incomplete specimens 6. Peronopsis (Cambrian). These are small, blind trilobites. Use the microscope to see as much detail as possible. How many thoracic segments are present in this genus? Are you able to tell which end is the head and which is the tail? Are the two ends identical? 7. Isotelus (Ordovician). This is a very distinctive trilobite from eastern Iowa. You may be asked to identify this genus on the exam. Note the well developed eyes Note the pygidium, triangular in shape and in which the segments are completely fused 7 3

4 ARCHAEOCYATHIDS: Morphology/terminology Archaeocyathids are enigmatic fossils that superficially resemble certain sponges and certain corals. They differ sufficiently from both sponges and corals so that they have been assigned their own distinct phylum, Archaeocyatha. Archaeocyathids were very primitive, multicellular solitary or colonial animals that secreted a calcareous skeleton. An individual skeleton (cup) consists of a double-walled inverted cone built around a central cavity (Figure 2). The walls are typically perforated by small pores. The region between the wall is known as the intervallum. This region is partitioned by numerous longitudinal septa into elongate chambers (loculi; singular = loculum). The cup itself may have been anchored to the seafloor or some other substrate by a root-like holdfast structure (Figure 2), or it may have been part of a larger colony of cups (Figure 3). Figure 2. Cut-away diagram of a single archaeocyathid cup, showing pores and doublewalled construction with septa dividing the intervallum into loculi. 7 4

5 Figure 3. Line drawings of solitary (left) and colonial (right) archaeocyathid growth habits. Stratigraphic Range Archaeocyathids are known almost exclusively from lower Cambrian rocks. Only a few examples are known from the middle Cambrian, just prior to the ultimate extinction of the phylum. Paleoenvironmental Range Archaeocyathids lived in carbonate shelf environments that probably were similar to those of the modern tropics. They formed massive reef structures in the early Cambrian seas, and as such, they are Earth s earliest reef-forming organisms! Archaeocyathid Examples (all early Cambrian) 1. This is a cross section of a solitary individual. Note the well defined inner and outer walls of the cup, and the intervallum Note the septal partitions and the loculi This specimen has a relatively large central cavity by comparison with the thickness of the walls and intervallum 2. Polished slab containing several individuals in a colonial group. Be sure to examine both sides of this slab. Note that these archaeocyathids possess a relatively small central cavity by comparison with the walls and intervallum Note that the intervallum and septa exhibit a complex, spongy network 3. This sample contains several randomly oriented solitary individuals. Note that the walls, septa, intervallum, and loculi are very well defined 7 5

6 4. This is a thin section of an archaeocyathid from the same rock sample as no. 3, above. Use the microscope to observe the crystalline details of the walls and septa. What is the material filling the central cavity and loculi? What is the mineralogic composition of the skeletal material itself (walls, septa)? 7 6

7 NAUTILOIDS: Morphology/terminology Nautiloids are a special group of cephalopod mollusks, which include among their extant members the octopus, squid, cuttlefish, and Nautilus (Figure 4). Extinct cephalopods include the ammonoids and belemnoids (more on those groups in another lab). Figure 4. Sectioned conch of the modern nautiloid, Nautilus. Living chamber is the large, final chamber at the bottom of the conch. Previously occupied chambers are separated by simple, slightly curved septa. Siphuncle is not preserved, but its trace is evidenced by perforations in the center of each septum. Nautiloids are characterized by an external, multi-chambered shell (or conch) of the mineral aragonite (CaCO 3, but different from calcite also CaCO 3 ). The conch may be coiled or straight. The chambers of the conch are separated from one another by septa. A thin, delicate, calcareous tube, the siphuncle, extends through the septa along the entire length of the shell. The siphuncle is permeable and allows the exchange of gas between the living animal (which resides in the final, living chamber) and previously occupied chambers. The addition and removal of gas to chambers enables the animal to control the bouyancy of its conch, much as the bouyancy of a submarine is governed. Nautiloids are further characterized by simple, unfluted septa. The intersection of the margin of a septum with the outer surface of the conch 7 7

8 is known as a suture. Nautiloid sutures are always smooth and never complex (Figure 5). (Complex sutures are characteristic of ammonoid cephalopods.) Ammonoids (variably complex sutures) Nautiloid (simple sutures) Figure 5. Comparison of suture morphology between a coiled nautiloid and the three kinds of ammonoids. Stratigraphic Range Nautiloids appeared in Cambrian time and they are still extant, although represented today by just a single genus, Nautilus. They are fairly abundant in rocks of early and middle Paleozoic age (Cambrian-Devonian), less abundant in rocks of late Paleozoic age (Mississippian-Permian), and rare in rocks of Mesozoic and Cenozoic age. The Ordovician rocks of northeastern Iowa contains large numbers of well preserved specimens. Paleoenvironmental Range Nautiloids are exclusively marine, nektonic (swimming) organisms. In the modern oceans Nautilus has been observed in neritic, bathyal and even abyssal depths. Empty shells, however, commonly wash ashore on islands. Ancient nautiloids are inferred to have occupied environmental niches similar to that of Nautilus. The occurrence of fossil nautiloids in a sedimentary rock, therefore, conveys little paleoenvironmental information other than that the rock was deposited in a marine setting. 7 8

9 Nautiloid Examples 1. Modern Nautilus and a Cretaceous fossil nautiloid. These are examples of coiled nautiloids. In the modern Nautilus, make sure you note the final living chamber and earlier (unoccupied) chambers, the septa, and perforations in the septa where the siphuncle once extended. Unfortunately, the siphuncle is not preserved in this specimen because it was very thin and delicate. Note that you cannot observe the sutures in the modern Nautilus because the original outer shell material is still intact. The color bands and fine textures you see are simply surface ornamentations. Note that the mineralogy of the conch is aragonite, a relatively unstable form of CaCO 3. When aragonite exhibits an irridescent, pearly luster it is called mother of pearl. What might be the adaptive value of the color bands? In the Cretaceous fossil the outer shell material has been removed. It was probably dissolved during or after burial. Note that because the outer shell is no longer present, you can observe the sutures in this specimen Is the living chamber still preserved? 2. This is a polished section of a straight nautiloid. Note that the siphuncle is still intact as a thin, calcareous tube in the center of the conch Note the simple, gently curved septa 3. Another example of a straight nautiloid exhibiting simple sutures. 4. Dawsonoceras (a 3-dimensional cast and a 2-dimensional impression). This straight nautiloid has slightly inflated chambers, imparting a ribbed appearance to the conch. Note that the cast allows you to determine that the sutures were simple 7 9

10 Note that the impression preserves the fine surface ornamentation of the conch. Approximately 4 or 5 thin, wiggly lines cover each chamber. 5. Straight nautiloids from Ordovician rocks at Graf, Iowa (impressions, sectioned specimen, and many 3-D specimens). Have a close look at these specimens and note that the siphuncle pores are observable in some. Note also the septa and simple sutures in every specimen Some of the specimens still possess the outer shell material, albeit it has been altered from the original aragonite to more stable calcite (no mother of pearl still present) At Graf, many of the nautiloids are preserved in a bizarre cone-incone fashion, just like so many ice cream cones. Nobody has been able to come up with a satisfactory explanation for this type of preservation, but suggestions range from group sex to hydrodynamic sorting to some kind of pressure-related phenomenon. Whaddya think? 6. More straight nautiloids exhibiting simple septa and sutures (internal molds). 7. An assortment of large, straight nautiloids (all internal molds). These are big specimens, but they are by no means the largest known. Some Paleozoic nautiloids reached lengths of several feet, reminiscent of characters in giant squid horror films. Note that one specimen possesses a large hole where the siphuncle once was 7 10

11 GRAPTOLITES: Morphology/terminology Graptolites are a group of extinct, marine organisms that constructed fairly simple to highly complex colonies. Both planktonic and sessile forms are known. Although their taxonomic affinity has been much debated, recent specialists assign them to the rather obscure phylum Hemichordata (not closely related to true chordates). The graptolite skeleton is composed of the proteinaceous substance chitin. A colony, or rhabdosome, consists of a pointed terminal structure (nema) and one or more stipes along which large numbers of thecae (tiny living chambers) are arranged (Figures 6 and 7). Figure 6. Sketches of simple graptoloid graptolite rhabdosomes showing arrangement of thecae along stipes. 7 11

12 Figure 7. Enlarged view of a graptolite stipe showing individual thecae and thecal apertures. A tiny animal resided in each theca. Two main categories of graptolites are recognized. Dendroid graptolites constructed highly elaborate, branching, fan-shaped rhabdosomes that may have been anchored to the seafloor (like certain seaweeds) or suspended from a floating bulb (again, like certain seaweeds) (Figure 8). Figure 8. Fossil rhabdosome of a dendroid graptolite. Note overall fan-shaped morphology. Nema (not shown) is located just out of view at bottom, left of colony. Graptolites are usually preserved as two dimensional, carbonized impressions in fine grained matrix. 7 12

13 Graptoloid graptolites were exclusively planktonic and their rhabdosomes are generally simpler, mostly consisting of four or fewer stipes (Figure 6). Both graptoloid and dendroid graptolites are normally preserved as carbonized, two-dimensional impressions in fine grained rocks. In some instances, though, exquisitely preserved three-dimensional specimens have been recovered intact by gently etching them from enclosing limestone matrix. Stratigraphic Range Graptolites as a group appeared in Cambrian time and persisted into Pennsylvanian time. Graptoloid graptolites, which are extremely useful biostratigraphically, are found only in Ordovician, Silurian, and early Devonian rocks, with specimens being most abundant in Ordovician rocks. As with nautiloids, certains Ordovician rocks of northeastern Iowa contain very good graptolite faunas. Paleoenvironmental Range As they were mostly planktonic marine organisms, graptolites are not especially useful as paleoenvironmental indicators. Their presence in a rock indicates only that the rock was deposited in a marine setting. Because of their small size, delicate construction, and non-mineralized composition, they are best preserved in very fine grained rocks. Such rocks formed under quiet water conditions, probably below fair weather wave base in neritic and deeper bathymetric zones. Graptolite Examples 1. This is a latex model, greatly enlarged, showing several thecae along a stipe and a detailed view of the ultrastructure of an individual theca. 2. Assorted graptoloid graptolites. Because graptoloids evolved very rapidly they are considered index (or guide) fossils. Use the microscope to observe details of the thecae. Note that rhabdosomes consist of 1, 2, or 4 stipes. The number of stipes and the arrangement of thecae are key taxonomic characters that differentiate genera and species Note that graptolites are almost always preserved as 2-dimensional, carbonized impressions 7 13

14 3. Assortment of dendroid graptolites. Curiously, although dendroid graptolites are more complex than graptoloids, they apparently evolved at slower rates and are not as useful biostratigraphically. Note the complex, fan-shaped rhabdosomes, each of which possesses considerably more than 4 stipes 4. Graptoloid graptolites from Ordovician rocks of eastern Iowa. Note how abundant and delicate these specimens are. They come from the same rock unit as the straight nautiloids (no. 5 on the nautiloid table). Under what kind of conditions might these graptolites have been preserved? 7 14