mollusca

EES 227: Paleobiology

Spring 2004

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Lecture 18: April 13

Reading:

Chapter 15 textbook, pages 272-279
Johnson, C. J., 2002. The rise and fall of rudist reefs. American Scientist, Vol. 90, p. 148-153 (abstract and some figures available online)
Vermeij, G. J., 1999. Inequality and the Directionality of history. The American Naturalist, vol. 153 (3), p. 243-253

Web resources for this lecture:

Great page on Cephalopoda
Search for the giant squid!
For beautiful computer-generated shells look at this web site, a sub page of web site on computational paleontology.
Shells of molluscs can be derived mathematically as variants on the equiangular spiral. d'Arcy Wentworth Thompson described this spiral and its various versions in his book 'On Growth and Form'; computer derivations were first described by Raup (1962, Computers as aid in describing form in gastropod shells, Science 138, p. 150-152). Raup's mode of description can be used at this web site (see also lecture 11)

Lecture Notes: Mollusca

Mollusca are a group that strongly diversfied in the Mesozoic, and have been mentioned as one of the main groups participating in the Mesozoic Marine Revolution.

Mesozoic Revolutions:

On Land: evolution of Angiosperms (lecture 15)
- External causes: high CO₂ levels (??)
- Internal causes: co-evolution with insects and/or herbivorous dinosaurs
In the Oceans: rapid evolution of more active life styles, higher metabolic rates (e.g, snails, starfish. sharks and rays, modern fish, crabs, marine reptiles)
- External causes: nutrients in sea water (large igneous provinces; ????)
- Internal causes: arms races

The Mollusca are the second most diverse phylum of animals (after Arthropoda), with some 55,000 living and 35,000 fossil species described. The total diversity is estimated to be much larger, maybe up to 130,000 species. The most common group are the Gastropoda (snails, ~40,000 species), followed by Bivalvia (about 13,000 species). Maybe it is because of their great diversity that their cladistics are not yet clear (see below).

The name Mollusca indicates one of their distinctive characteristics: a soft body. Molluscs are an incredibly diverse group, containing forms that do not at all resemble each other superficially: from intelligent, large-brained, fast-moving predators with large eyes such as a squid, to non-moving animals without eyes, a head or a clear brain that live on symbiont algae such as the giant clam. One can derive all these diverse forms from some kind of basic type, proto-mollusc, however. Mollusca possess at least some or all of the following characteristics (see figure 15.3, text book p. 277 1^st edition, 285 2^nd edition):

a muscular fleshy foot, which gastropods (snails) used to crawl, clams to burrow, and which in cephalopods is modified into tentacles
a visceral mass containing the digestive, excretory, and reproductive organs
a mantle, usually two folds that enclose the gills or lungs, and secretes the shell
a radula, a tongue-like organ equipped with rows of microscopic teeth that scrape food off hard surfaces or grind through shells of other molluscs
a respiratory gill (the ctenidium) or lung
a shell made of calcium carbonate.

Note that the shell is not really a skeleton; it encloses the soft body (to greater or lesser extent) and protects it.

I inserted a cladogram for the Mollusca (T. R. Waller, 1998, Origin of the Molluscan Class Bivalvia and a Phylogeny of Major Groups; In: An Eon of Evolution; Paleobiological Studies Honoring Norman D. Newell, edited by P. A. Johnston and J. W. Haggart, Univ. of Calgary Press, p. 1-45). Note the differences with the subdivision in Classes as given in your text book.

For all molluscs, studies on functional morphology have been conducted (see text book p. 108-109; lecture 11). Different shapes of snail shells are better at burrowing or sliding over sediment; the shape of bivalve shells influences how deep and how rapid the animals burrows. The scars of attachment of muscles and of the mantle in clams shows whether the animals had one or two muscles, and whether the animal had a sipho (see below). Similarly, the shape of ammonites (flat, irregular, ornamented) determines whether the animal was a good swimmer or crawler.M

ollusca are subdivided into the following classes. The most common groups are marked in red, lesser groups in blue.

Class Aplacophora (see cladogram), also called Caudofoveata or Chaetodermomorpha or Solenogastres). Worm-like, no shell, spicules of calcite.
Class Monoplacophora: living fossil Neopilina (2 species, known only since the 1950s); fossils long known, early Paleozoic. Limpet-like shells, internally very different structure
Class Polyplacophora (chitons): shell of 8 parts, greatly expanded foot
Class Scaphopoda (tosk shells): tusk-shaped, small shell with openings at both ends
Class Gastropoda (snails). The gastropods are by far the most numerous and diverse: about 40,000 living and 15,000 fossil species. The shell is always one piece, coiled or uncoiled, and is not subdivided into chambers. In the oceans, fresh water, and on land. Examples: snails, limpets, slugs, whelks, sea slugs, abalone. Internally, snails have very unsual, torqued structure,
Class Bivalvia (clams and such). Bivalved molluscs (13,000 recent species) are mostly sedentary filter feeders that depend on currents produced by the gills to bring in food. They have no head and no radula. Deeply-buried forms have a double tube (the sipho) through which they pump water in and out the shell; the tube sticks out above the sediment. Examples: mussels, clams, scallops, oysters, shipworms. Live in salt, brackish and fresh waters.
Class Cephalopoda. The most complex class of molluscs; all living forms are marine, active predators. The foot is modified into tentacles and the mantle edge is grown into a funnel, expelling water from the mantle cavity (jet propulsion). Recent forms have no shells or an internal shell (cuttlebone), with the exception of the chambered Nautilus. Extinct Ammonites also had a spiral, chambered shell. Examples: squids, octopi, nautiluses, cuttlefish, ammonites.

Many snails are fierce predators (there are also many herbivores)

Have grating mechanism mouth: radula; faster than prey.
Snails developed more and more specialized radula to drill holes
Poisonous snails
Snails which use their muscular 'foot' to suffocate prey
Snails which use spines at front of shell to hammer into barnacles and clams
High, narrow snails: fast burrowers, digging up clams.

Especially Geerat Vermeij argues that snails are an important factor in the Mesozoic Marine Revolution, and in the overall Arms Race:

If predator (snails, crabs) invents more ways to attack bivalves and other snails, prey invents way to counter attack (Red Queen hypothesis: see lecture 12)
Bivalves: develop muscles, spines, crenulated edges

Mesozoic Revolution of Marine Invertebrates:

Many new methods of predation developed, specifically as to breaking shells open (bivalves): snails, crabs, lobsters
Fossil record of more and more predation upon shell-bearing organisms, and more and more response of prey species
Modern ways of living in ocean evolved during Mesozoic
No clear linkages between changes in ways of living and environmental changes
Competition/arms races
Increased diversity ('cropping principle')

If this is true:

Paleozoic snails would not have a chance in the present oceans
Environmental influence on evolution may be limited because organisms can not 'call up' a wished-for mutation
But replacement of major groups (e.g., dinosaurs/mammals) may happen only after a major ecosystem disruption (mass extinction), because incumbents are hard to dislodge (see lecture 12).