Evolution of Mollusks and Brachiopods: Ancient Shelled Creatures

1. Inarticulate brachiopods survive today with shells very similar to those of their early Ordovician relatives. Most use their long pedicle to anchor themselves 10s of cm deep in the sediment. Lingulepsis, an inarticulate brachiopod.

2. Brachiopods are different from mollusks. They anchor their shell to a firm ground using a muscular stem (pedicle). This shell shows an opening for its pedicle. Billingsella, a Cambrian orthid brachiopod

3. The most primitive mollusks are single-valved: monoplacophorans have a conical cap-like valve. They are probably the ancestors of all shelly mollusks. ventral (organs) dorsal

4. Bivalve mollusks: two valves are mirror image of each other. Internal organs are different from those of brachiopods. Most mollusks move around, using their muscular foot.

5. Mollusks: a phylum which, today, spans seawater, freshwater and land ecosystems. • - aplacophorans (cap-like shell) • - pelecypods (bivalved shell) • - gastropods (coiled shell: snails; none: slugs) • cephalopods • (coiled shell: nautilus; • shell shrunk to a bone: octopi, scuttlefish) • ... must have all evolved from some worm-like ancestor that started armoring its body.

6. Nautiloids: shelly mollusks related to modern Nautilus.

7. Nautiloids: shelly mollusks related to modern Nautilus. Chambered shells used to control buoyancy by filling with gas/water

8. Nautiloids: shelly mollusks related to modern Nautilus. Ordovician forms were generally straight or gently curved, with very thick shells.

9. What’s a cephalopod, when you have no soft part? scaphopod Salterella, once thought to be a primitive cephalopod, is now in its own phylum.

10. Hyoliths, conical shells with a “lid”, are interpreted to be primitive mollusks which appeared in early Cambrian times.

11. Echinoderms are a very diverse phylum. Today, they include starfish, sand dollars, sea urchins, sea cucumbers and crinoids (sea lilies). Most show 5 arms or food grooves. Some are grazers, some are predators, and others are filter feeders (i.e. strain tiny bits of food from water).

12. Cambrian and Ordovician echinoderms rested on the sea floor. Their plates of porous magnesian calcite (CaCO3 with a few % of Mg) were similar to those of their descendants, but arranged less symmetrically. Helicoplacoids: lived in shallow burrows? A food groove spiraled down their armor-plated football-shaped body.

13. Gogia, a filter-feeding eocrinoid with long arms, used as baffles. Eocrinoids gave rise to blastoids in Ordovician.

14. Crinoids (sea lilies) will become a highly diverse group during most of Paleozoic. Most were anchored to the sea floor.

15. Echinoderms are the closest relatives of chordates (vertebrate animals). These two phyla are thought to share a common ancestor, which must already have been around during the late Proterozoic or the Cambrian. Hemichordates are animals that have a notochord (precursor of spinal chord) but lack other features found in most chordates.

16. conodonts: mm-size tooth-like fossils of calcium phosphate... can be dissolved out of limestone and separated for analysis.

17. graptolites: colonies of microscopic filter feeding animals (hemichordates)… The Cambrian forms were attached to the sea-floor. But they diversified into planktonic, free-floating colonies during the Ordovician.

18. Graptolites are among the most useful index fossils in the early Paleozoic. They are best preserved in sedimentary rocks originally deposited in deep water (quiet, oxygen-poor sediments). Because of fast turnover of species, conodonts and graptolites make correlation possible within 2-3 million years. Monograptus