lab/lecture 3: first reefs

Question 1

What are stromatolites?

Answer 1

Laminated organo-sedimentary structures formed by trapping, binding, or precipitation of minerals by microorganisms.

Question 2

Where do stromatolites grow today?

Answer 2

Intertidal to supratidal zones (e.g., Hamelin Pool, Shark Bay, Australia).

Question 3

Describe stromatolite fossils.

Answer 3

Finely laminated micrite with replaced organic seams, porous septa or sparite cement; often form cabbage-shaped heads.

Question 4

Describe the body plan of a sponge.

Answer 4

Porous skeleton with ostia (inhalant pores), spongocoel (inner cavity), and osculum (exhalant opening), reinforced by spicules.

Question 5

How do sponges feed?

Answer 5

They are suspension feeders; water is pumped through ostia by choanocytes with flagella, filtering food.

Question 6

What are the main cell types in sponges?

Answer 6

Pinacocytes (outer surface)

Choanocytes (flagellated, line canals, gather food)

Archaeocytes (amoeboid, digest, transport, turn into sex cells)

Sclerocytes/spongocytes (secrete skeleton)

Question 7

Describe sponge fossils.

Answer 7

Typically preserved as spicules or fused skeletons; some like Astylospongia have massive globular bodies, others like Cliona bore holes in shells (trace fossils called Entobia).

Question 8

Describe (class) Hexactinellida fossils.

Answer 8

Siliceous skeleton with six-rayed spicules forming cubic symmetry; reef-building forms like Hydnoceras.

Question 9

Describe (class) Calcarea fossils.

Answer 9

Calcareous spicules; examples include star-shaped spicules in Astraeospongia and coiled forms like Girtyocoelia in Permian reefs.

Question 10

Describe (class) Demospongiae fossils.

Answer 10

Spongin skeleton or siliceous spicules (never 90°); massive globular forms like Astylospongia, boring traces (Cliona → Entobia).

Question 11

Describe (class) Stromatoporoidea fossils.

Answer 11

Laminated calcareous skeleton, horizontal laminae, vertical pillars, surface bumps called mammelons with astrorhizae; important Silurian–Devonian reef builders.

Question 12

Describe (class) Archaeocyatha fossils.

Answer 12

Cone-shaped, double-walled skeleton with septa and intervallum; sometimes tabulae or dissepiments; dominant Early–Middle Cambrian reefs.

Question 13

Where are modern sponges found?

Answer 13

Shallow marine (80%) and deep-sea habitats (>500 m).

Question 14

Why are sponges important in paleobiology?

Answer 14

Major reef-builders; fossil record includes demosponge reefs (Ordovician), stromatoporoids (Silurian–Devonian), and hexactinellids (post-Devonian).