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Mollusks and Echinoderms

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Mollusks and Echinoderms Laboratory VII Mollusks and Echinoderms Objective: This is the third of three labs that will introduce you to the main groups of fossil− forming marine invertebrates. You have three goals in this lab. (1) To understand the basic morphology of the groups, and (2) to absorb the fundamentals of its classification. In lab, you will have a variety of specimens from most groups to examine. For each, you should observe the morphological features present in the fossils and link them to discussions of basic biology. You should also learn to identify the major groups. And (3) you will be exploring environmental interpretations of burrowing clams. READ: Chapters 15 and 16 in Prothero to accompany this handout. Bring your book to lab. The pictures will help! Mollusks Mollusks are among the most common macroscopic invertebrate fossils, particularly in Mesozoic and Cenozoic rocks. The major groups of mollusks are so different from one another that only a few generalizations are possible. All mollusks are characterized by a thick, fleshy mantle that covers much of the body and secretes the calcite shell (if one is present). Most mollusks also have either a muscular foot that can be used for moving around (creeping or burrowing) or tentacles derived from the same muscular antecedent. Surprisingly, not a lot of work has been done on the relationships within the mollusks. This may be because the wildly divergent morphologies of the major lineage offer few points of homology for morphological phylogenies, and molecular work is only just getting off the ground. Our best guess right now is something like this: This hypothesis leaves out several living groups including the Caudofovaeta and Aplacophora, shell−less worm−like forms, monoplacophorans , and scaphopods (tusk shells), and the some extinct diversity (e.g., rostroconchs, which are probably related to pelecypods). Because they dominate the fossil record, we will look closely at the gastropods, pelecypods and cephalopods. 1 Arens −GEO 390 Gastropods − Snails and slugs. This is the most species rich group of mollusks, owing in large part to their varied ecologies. Gastropods graze algae and vascular plants using their radulae, scavenge, feed on detritus, some filter feed, others are active predators. Moreover, they live in marine, freshwater and terrestrial habitats. Although all are dependent on water for reproduction, there are desert forms that have shortened their larval stages to allow for fast reproduction in ephemeral pools. Gastropods have a distinctly developed head with eyes that can sense light variations and form some images. They have a well−developed "foot" that can be used for creeping and a radula for processing food as it enters the mouth. The siphon is a fold of the mantle that allows water circulation both for aeration of the gills and for removal of waste. This bodies can be withdrawn completely into the shell for protection from predators or to avoid dehydration. In examining specimens, note variation in the height and orientation of the spiral relative to the aperture. These features have been important in both the classification of gastropods and studies of their evolution. Cephalopods − Squid, octopus, cuttlefish, nautiloids, ammonoids. This group varies tremendously in its fossil record depending on whether individual lineages make a hard shell or not. Those that do (nautiloids and ammonoids) have extensive fossil records, those that don’t have very limited and non−diagnostic fossils. The animal is characterized by a well−developed head with a complex eye and nervous system. The "foot" is replaced by a mop of tentacles that surround the mouth. Tentacles have suction disks used to capture and hold prey. The mantle is folded in to a siphon that can be used for jet propulsion. Nautiloids (Cambrian − Recent) − straight or coiled shells with the siphuncle penetrating the center of each septum between chambers. Endoceratoids (Ordovician − Silurian) − straight shells with siphuncle penetrating the lower (ventral) portion of each septum. Actinoceratoids (Ordovician) − similar to straight nautiloids except there is a curved collar around the opening through which the siphuncle passes. Bactritoids (Ordovician − Triassic) − like a straight nautiloid except it has a little round knob at the tip of the straight shell. This knob is the remains of the very first shell formed by the animal after its larval phase. Ammonoids (Devonian − Cretaceous) − Straight, curved or coiled shells with the siphuncle running along the bottom edge of the shell. The most important features of ammonoids are the sutures between the chambers. In their early history and through the Triassic, sutures were straight or curved. In the Jurassic and Cretaceous, sutures became complex. You can roughly seriate ammonite in time by the complexity of their sutures. Be sure to note this as you sketch specimens. Coleoids (Mississippian − Recent) − octopus, squid, cuttlefish. These have a poor fossil record and so their first appearance in the fossil record should be taken with a grain of salt. The only group that left an abundant fossil record was an apparently squid−like form that produced cigar−shaped calcite rods. Theses fossils are abundant in the Jurassic and Cretaceous. The animals that made them were apparently the favorite 2 Arens −GEO 390 food of extinct ichthyosaurs. Pelecypods (Cambrian − Recent)− Clams, oysters, scallops. While gastropods are speciose, pelecypods are numerous. They are the most common type of mollusk fossil and can be the most common fossil in many Mesozoic and Cenozoic localities. We will skim by a lot of diversity in this group, but you should be aware of a few major groups: Nuculoids − very tiny "nut clams" characteristic of the surf zone. Solemyoids − chemosymbiotic "awning clams" with elongated shells. Mytiloids − true mussels. Pterioids − oysters, scallops, and giant extinct inoceramids. Veneroids − most common clams including razor clams, cockles, and deep−burrowing chemosymbiotic lucinids. Myoids − geoducks and shipworms with asymmetrical valves. Hippuritoida − extinct rudistids that built reefs during the Cretaceous. Unionoids − most common freshwater clam Pelecypods protect their soft parts between two (commonly) mirror−image valves that can be held tightly shut with adductor muscles. Pelecypods have lost their head. They have limited sense organs, but do have a rudimentary heart and circulatory system. Their foot is well developed and can be used for burrowing. Mantle siphons can be large and elaborate on pelecypods. Siphons allow circulation of fresh water to the animal while it is buried in sediment. Pelecypods are identified (and classified) based on a variety of features of the shell. Because overall shell form can be highly convergent in many groups, features like muscle scars, tooth, socket, pallial line and sinus, and the shape of the cardinal area are most commonly used for identification. In addition to clues to ecology from shell form (see below) pelecypods have been useful indicators of annual variation in water temperature and salinity. This works because the clam adds growth lines at intervals throughout its life. If a paleontologist samples the chemistry of shell material in successive growth lines, he or she will get a record of the conditions present throughout the life of the clam. A particularly clever application of this approach has been used to show decreasing freshwater input (increasing salinity) of the Colorado River into the Gulf of California. As a number of endangered pelecypod species inhabit this estuary, documenting the relationship between the decrease of fresh water and the decline of these species (also documented by their fossil record) has been an essential component in conservation plans for this ecosystem. Studies of this type have given rise to a whole new field: Conservation paleontology. 3 Arens −GEO 390 Ecology of Pelecypods The systematics of pelecypods has been both problematic and unstable because of disagreement over which characters are best used to classify them. Part of the problem stems from the fact that clam shells are highly convergent. This means that the same form has evolved independently in many different lineages. Rampant convergence suggests that shell form is highly vulnerable to natural in specific environments and scientists have long noticed similarity in shell form between lineages living in the same environment. Some generalizations have emerged. − Burrow in soft sediment = symmetrical valves and muscles − Burrow in hard substrates = asymmetrical valves and muscles − Shallow burrowing or surface−dwelling = thick, sculptured shells − Deep burrowing = thin, smooth shells − Fast burrowing = small bladed or cylindrical, smooth shells − Slow burrowing = large, rounded, textured shells Furthermore, these behavior patterns are associated with specific environments. For example, fast burrowing is characteristic of clams living in coarse sediment and in the surf zone where wave action is likely to excavate and expose them to predators. Conversely, slow burrowing is characteristic of quiet water. Develop an ecological interpretation for the clams presented in lab. Be as detailed as possible. Include a sketch of the clam with your ecological interpretation. Echinoderms Echinoderms are deuterostomes like us. That means that their blastopore becomes their anus and they have stem cells capable of differentiating into any type of cell well into development. These features unite them with chordates, making echinoderms our closest
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