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Laboratory VII Mollusks and

Objective: This is the third of three labs that will introduce you to the main groups of − forming marine . 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 and link them to discussions of basic . You should also learn to identify the major groups. And (3) you will be exploring environmental interpretations of burrowing .

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 fossils, particularly in and 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 that covers much of the body and secretes the 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 , 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 .

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Gastropods − Snails and slugs. This is the most rich group of mollusks, owing in large part to their varied ecologies. Gastropods graze and vascular 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 for processing food as it enters the mouth. The is a fold of the mantle that allows water circulation both for aeration of the 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 . These features have been important in both the classification of gastropods and studies of their evolution.

Cephalopods − , , , , 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 is characterized by a well−developed head with a complex eye and . 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 ( − Recent) − straight or coiled shells with the penetrating the center of each between chambers. Endoceratoids () − 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 − ) − like a straight 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 () − 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 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 that made them were apparently the favorite

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food of extinct ichthyosaurs.

Pelecypods (Cambrian − Recent)− Clams, , . 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 . Pterioids − oysters, scallops, and giant extinct inoceramids. Veneroids − most common clams including razor clams, cockles, and deep−burrowing chemosymbiotic lucinids. Myoids − and with asymmetrical valves. Hippuritoida − extinct rudistids that built reefs during the Cretaceous. Unionoids − most common freshwater

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 and . 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, 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 . 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 . Studies of this type have given rise to a whole new field: Conservation .

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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 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 like us. That means that their blastopore becomes their and they have stem cells capable of differentiating into any type of cell well into development. These features unite them with , making echinoderms our closest relatives among the .

Echinoderms are strictly marine, with no representatives in fresh and few in brackish water. However, they occupy a variety of ecological roles including herbivores, predators, scavengers and filter feeders.

Echinoderms are a morphologically diverse groups, but are distinguished by several key features. First, most have five−fold (or sometimes radial) . Although this is a great general rule and will work well for living echinoderms, some forms have a variety of other or no symmetry at all! Those crazy echinoderms! While we’ve seen radial symmetry before, five−fold symmetry is unique to echinoderms. Echinoderms have an internal or composed of individual plates, held together by −like proteins. Individual plates are composed of single of high−Mg calcite, another unique features of the echinoderms. The morphological variety of echinoderms comes from the wildly variable

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The fossil record of groups is also highly variable. Some, like , have huge fossil records. Others, like stars, fossilize mostly as tiny isolated plates.

Crinoids (Cambrian − Recent) − Sea lilies or feather stars. These are abundant and diverse in the Paleozoic. They are composed of a stalk attached to a cup or calyx. The mouth, surrounded by arms, is at the top of the cup. Living stalked crinoids are mostly restricted to deep water. Non−stalked species live in cryptic reef habitats. However, Paleozoic crinoids dominated shallow where the columnals that composed their stalks compose most of the sediment.

Blastoids (Middle Ordovician − ) − Morphologically they are a cross between an urchin and a . Plates in the calyx are arranged around a narrow mouth with reduced arms. Most are stalked. They are very abundant and diverse in the Pennsylvanian and Permian, but do not survive the end Permian extinction.

Ophiuroids and Asteroids (Ordovician − Recent) − Brittle stars and sea stars. Although each gets its own class in conventional classification, it is unclear whether the brittle stars and sea stars represent independent monophyletic groups. Most are active predators and have been blamed for the failure of and crinoids to recover previous diversity or ecological dominance after the Permian extinction. Note that in these groups, individual plates are embedded in the "skin" of the animal.

Holothuroids (Middle Cambrian − Recent) − Sea cucumbers. This group differs from the conventional echinoderm design. Its plates are reduced and embedded throughout the tissue of its body. It has a head, tail and cylindrical body. There is only a trace of five−fold symmetry but sea cucumbers do have abundant and a . Their fossil record is poor because they are primarily soft−bodied. However, their distinctive plates have been found, testifying to their presence in the Cambrian.

Echinoids (Late Ordovician − Recent) − Sea urchins and sand dollars. Echinoids are the most numerous and best represented Mesozoic echinoderms. The group includes surface−dwelling (spiny) forms and burrowing (irregular shape and lacking spines) forms. Echinoids suffered substantial losses of diversity at the Eocene−Oligocene extinction and have never recovered either their species richness or their ecological importance, although urchins are central players in the ecology of kelp forests in cool seas.

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Questions for Further Thought

1. Just about all of the major groups of marine invertebrates took a major hit on diversity at the end of the Permian. However, some groups recovered and contributed significantly to Mesozoic and Cenozoic diversity (e.g., gastropods and ), while others just didn’t recover (e.g. crinoids and brachiopods). It’s hard to argue that those that didn’t recover were somehow "less fit" than the rest of the fauna because, after all, brachiopods and crinoids were both diverse and dominant throughout most of the Paleozoic. Not the mark of an "inferior" clade. Some have argued that there was a whole new world of in the Mesozoic that included asteroids and . Lineages without anti−predator defenses just didn’t have a chance. Remembering our discussions of historical and experimental science, how might you test this hypothesis? What data (that you could actually gather) would falsify it? What data (that you could gather) would support it?

2. Discuss some of the factors that have contributed to the tremendous species richness of snails.

3. Above, we discussed the widespread convergence of shell form in pelecypods. We note that this convergence is related to the environment and lifestyle preferred by individual species. This is all correct, but why do you think we see so much more convergence in this lineage than in snails or cephalopods?

4. Because they are stalked and lift themselves above the sea floor, crinoids introduced tiering into Paleozoic bottom communities (see Prothero pp. 321−322). How might this innovation have contributed to diversity in these communities, not just in crinoids, but in other organisms as well. Now consider when crinoids introduced this tiering. Who was building reefs at this time? How might the relative lack of reefs have intensified this effect?

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