Sedimentology and Palaeontology Lecture 5: The Coiled Molluscs

Question 1

Define Gastropod

Answer 1

Cambrian to Recent:
increasingly abundant and
important throughout geological
time

Most diverse group of modern
molluscs: nearly 100,000
modern species

Question 2

Describe Gastropod anatomy and shell anatomy

Answer 2

-Typically aragonite (calcium carbonate)

• Tight-coiling in the vertical plane creates an internal columella
• Siphon canal holds the inhalant siphon,but the exhalent siphon is marked by the
  slit

There is Considerable morphological diversity in the shells of Gastropod species.

The gastropod itself is Attached to the shell only by retractor muscles

• Aperture can be sealed with the operculum

The Gastropod radula - Diverse ‘teeth’ reflect grazing, predatory and parasitic feeding habits

Question 3

Describe cephalopods

Answer 3

Cambrian to Recent

Entirely marine

Most sophisticated of
molluscs:
- High metabolic rates
- High levels of mobility
- Excellent eyesight
- Advanced brain
- Developed nervous
system
- Strong encephalisation
- Highly modified foot

Question 4

Describe Cephalopod anatomy and shell anatomy

Answer 4

Muscular contractions
of the mantle cavity
used to jet-propel
through water

Funnel or hyponome
used to
control direction

Cephalopod shells are mostly aragonite. There are external and internal shell species. Nautiloidea and Ammonoidea = external shell species, Coleoidea = internal shell species.

Question 5

Describe Nautiloidea

Answer 5

Cambrian – Recent

• Very successful in the Palaeozoic,

but hit hard by the P/T extinction
- Includes the orthocones

Question 6

Descrine Nautiloidea anatomy and shell anatomy

Answer 6

Shell shapes:
Straight – orthocones
Biserially enrolled – planspiral

Some orthocones may have attained lengths of 6 m or more

Nautiloids have Smooth shell divided into camerae (chambers) by septae

Position of septa (sing. ‘septum’) is marked externally by a
suture

Siphuncle perforates the middle of the septa: comprised of
non-porous septal necks and porous siphonal tubes
- Integral to the nautiloid buoyancy mechanism

Nobbly, leathery hoods used
for protection
- Poor ‘pinhole camera’ eyesight
- 80 – 90 differentiated
tentacles used to find and
manipulate food

Question 7

Describe Ammonoids

Answer 7

Nine main groups

• Devonian – Cretaceous:
  true ammonites (Ammonitidae)
  appear in the Triassic
• Very important zone fossils
  (i.e. for biostratigraphy)

Question 8

Describe Ammonite/Ammonoid anatomy and shell anatomy

Answer 8

Sexual dimorphism is known in a number of ammonoids

Ammonite radulae (mouthparts) are
poorly known, but they appear
relatively slender and weak
- Indicative of planktivory

Evidence suggests ammonoids consumed isopods.

Question 9

Describe Coleoidea

Answer 9

Cuttlefish, squids, octopuses
and belemnites

• Carboniferous to Recent
• Dominate modern
  cephalopod diversity
• Only belemnites have
  substantial fossil record

Question 10

Describe Coeloidea: belemnites

Answer 10

Triassic - Cretaceous

• Locally abundant in
  Jurassic and Cretaceous
  rocks
• Simple internal skeleton,
  principally comprised of
  the calcitic guard

Gladius: thin aragonite shell remnants extending along the
mantle