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
1
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
or
bivalved
,
aragonitic
,
multilayered
, with
growth lines
and
muscle scars
5
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
9
Monoplacophorans
Bellerophon
10
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
12
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
or
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
17
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
22
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
or
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
28
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