Shells

Transcript Shells

Phylum Mollusca: stuff to know • Important morphologic features (hard parts only) • Classification: – Subphyla; classes; subclasses within Class Cephalopoda • Molluscan phylogeny • Ammonoid suture types • Pelecypod genera: –

Pecten

Inoceramus

Gryphaea

Exogyra

Mollusca—Phylum overview •

Representatives include

: snails, slugs, mussels, oysters, clams, squids, octopuses •

Size

18m

ranges from

microscopic

(giant squids) (snails) up to •

Inhabit

marine, freshwater and terrestrial environments • Aquatic types may be

benthonic

planktonic

nektonic

, even

flying

(squids) 2

Phylum overview (cont.) • Mollusks are extremely

diverse

, so there are

few features common to all representatives

– Free-living

metazoans

– – – – Dorsal calcareous

exoskeleton

– Muscular

foot

for locomotion – –

Visceral mass Mantle cavity

with major organ systems with gills (digestive and reproductive systems open into mantle cavity)

Radula Head

with mouth (maybe also tentacles and eyes)

Mantle

(rasping structure in mouth) (tissue layer) that surrounds soft parts and secretes shell 3

“typical” mollusk radula 4

Phylum overview (cont.) • • Phylum originated in

Early Cambrian

(earlier?) from a flatworm ancestor – All major classes and subclasses originated by

Middle Ordovician

– Only

one

major class has become

extinct

(Rostroconchia)

Shells

: – mostly

univalved

bivalved

aragonitic

multilayered

, with

growth lines

and

muscle scars

Mollusk shell and musculature 6

Subphylum Amphineura (includes Chitons) Cambrian-Holocene Classification Phylum Mollusca Subphylum Cyrtosoma Cambrian-Holocene Subphylum Diasoma Cambrian-Holocene Class Monoplacophora Cambrian-Holocene Class Gastropoda Cambrian-Holocene Class Cephalopoda Cambrian-Holocene Subclass Nautiloidea (Late Cambrian-Holocene) Subclass Ammonoidea (Devonian-Cretaceous) Subclass Coleoidea (Devonian-Holocene; mostly Jurassic) Class Rostroconchia Cambrian-Permian Class Scaphopoda Ordovician-Holocene Class Pelecypoda Cambrian-Holocene 7

Phylogeny of molluscan classes 8

Monoplacophorans • Cap-shaped to helical shell; bilateral symmetry; soft parts not twisted; paired muscles; apex of shell points anteriorly and overhangs head • Important because

ancestral to most other mollusks

• Only group of organisms to be

described hypothetically

before being discovered,

AND

to be

known as fossils before live specimens were found

Monoplacophorans

Bellerophon

Monoplacophorans • Most important group is

bellerophontids

– Cambrian-Early Triassic – Resemble gastropods – Very common in Late Permian of Tethyan region (e.g., “

Bellerophon

Limestone”) 11

Gastropods • • Characterized by

torsion

of soft anatomy

Head and foot regions

associated combined or closely • External shell usually coiled in a

corkscrew helix

Gastropod anatomy 13

Gastropod shell terminology • • • • • •

Apex

(earliest part)

Aperture

(opening for head-foot)

Operculum

(cap)

Whorl

(coil of 360°)

Suture

(contact between adjoining whorls)

Siphonal canal

(opening for inhalent siphon) 14

Gastropod shells 15

Cephalopods • • Class includes

Nautilus

, squids, octopuses, extinct ammonoids • Highly evolved

nervous system

eyes) (cephalization;

Carnivorous

and capable of swimming (

nektonic

) (up to 70 km/hour) •

Foot

and

head

closely associated (

indistinguishable

in some)—hence the name:

kephalus

poda

• Possess

hyponome arms

tentacles

(funnel for jet propulsion) and 16

Cephalopods • • Shelled forms possess

gas-filled chambers Buoyancy

is controlled by (1)

poise adaptation

of the shell (shell form) and (2)

adding or subtracting fluid

from chambers by the

siphuncle

• Most living forms possess an

ink sac

• Exclusively

marine

Nautilus

soft anatomy (shell not shown) 18

Cephalopod shell morphology • • • Chambered shell is divided into

living chamber

and

phragmocone

• Chambers separated by

septa Suture

wall is junction of septum with the outer shell

Siphuncle

mantle that extends from animal back through phragmocone (usually

ventral

) – – = tube with blood vessels, nerves and

Septal foramen Septal neck

siphuncle = hole through which siphuncle passes = projection of septum surrounding 19

Cephalopod shell morphology 20

Siphuncle terminology 21

Cephalopod sutures • If suture is fluted,

saddles

point toward aperture and

lobes

– point toward apex

Orthoceratitic

unfluted

or with

broadly undulating

lobes and saddles (Cambrian-Holocene) – –

Goniatitic

undivided

= distinct lobes and saddles that are (

mostly

Devonian-Triassic)

Ceratitic

smooth

lobes (

mostly

saddles; Triassic)

serrated

(“saw-tooth”) –

Ammonitic

serrated

Jurassic-Cretaceous) saddles and lobes (

mostly

lobes Cephalopod sutures saddles 23

Cephalopod suture patterns orthoceratitic gonitaitic ceratitic ammonitic 24

Cephalopod classification (must know) Subclass Nautiloidea straight or coiled shell orthoceratitic sutures (Late Cambrian-Holocene) Class Cephalopoda Subclass Ammonoidea straight or coiled shell goniatitic, ceratitic, or ammonitic sutures (Devonian-Cretaceous) Subclass Coleoidea internal shell includes belemnites (Devonian-Holocene; mostly Jurassic) 25

Subclass Coleoidea: belemnites Squid-like organism; typically, the only preserved part is the

guard

(= “

fossil cigars

”) 26

Rostroconchs and Scaphopods • • • Relatively

uncommon Rostroconchs

= strange,

bivalved mollusks

(superficially resemble pelecypods) (Cambrian-Permian)

Scaphopods

= “

tooth shells

” (Ordovician Holocene) water sediment 27

Pelecypods • Clams, scallops, mussels, oysters, rudists • Soft anatomy

lacks a head region

;

no significant sensory organs or radula

• Mostly

infaunal

attached epifaunal

suspension feeders

; some infaunal deposit feeders • Typical shell is

bilaterally symmetrical

, with

right and left valves

closed by

adductor muscles

• Shells held together along

hinge

; line of junction of two valves is

commissure

Pelecypods • • Exclusively

aquatic

; both marine and non marine

Marine forms

abyssal depths

range from

intertidal zone to

• Mostly

aragonitic

; but

oysters are calcitic

• Bizarre variants

lack bilateral symmetry

(oysters, rudists) 29

Pelecypod shell morphology beak commissure hinge 30

Pelecypod internal shell hinge with articulating teeth morphology adductor muscle scar 31

Bizarre pelecypods

Oyster

(yum-yum; pearls, too!)

Rudists

(extinct; up to 2m) 32

Shells

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