EOSC326 Class notes

1. List and describe the various lifestyles and environments exploited by marine invertebrates
- Marine animals exploit both pelagic and benthic lifestyles.
o A pelagic lifestyle refers to a lifestyle whereby the organisms live within the water
column and has high dispersal. There are two types of pelagic modes of life.
 Nektonic: Where the animal within the water column is a free swimmer and can
range in size from small to large (mm to m)
 Planktonic: Where the animal within the water column is unable to swim and is
moved by currents. Typically microscopic.
o A benthic lifestyle refers to a lifestyle whereby the organism lives within the benthos/
ocean floor. The organisms live within or on the ocean floor. The have limited disprsal
There are several different lifestyles that benthic organism employ.
 Infaunal: Organisms that live in the ocean floor/substrate. There are two modes
of life for infaunal organisms
 Boring: Organisms that live and bore into hard substrates (i.e rocks)
 Burrowing: Organisms that live and burrow into soft substrate (i.e sand,
mud)
 Epifaunal: Organisms that live on the ocean floor (surface). There are two
modes of life for epifaunal organisms
 Cementing: Organisms that are sessile and tend to have radial symmetry
or be asymmetric; the cement to their neighbours of live-in cavities or
holes.
 Vagrant: Organisms that move around on the substrate; tend to have
bilateral symmetry
2. Describe the basic biology of trilobites
- Trilobites are typically vagrant benthic marine organisms (few were infaunal or pelagic) that
existed from the early Cambrian- end Permian (mass extinction). They:
o Were bilateral symmetrical down the midline
o Were between 1mm to >75cm long
o Had hard (calcium carbonate) jointed exoskeletons and paired appendages  as
customary to other Arthopoda
 Their exoskeletons were on their dorsal sides
 Their ventral sides were soft tissue (unmineralized membranes)
 Their ventral sides housed their limbs, gills, etc.
 Their exoskeletons consisted of three parts: cephalon, pygidium, and thorax.
o Some had spines
o Many had eyes few with different modifications
 Eyes were of two types holochroal and schizochoral
3. Describe the trilobite exoskeleton and how growth was achieved
- The trilobite exoskeleton consisted of three portions
o The pygidium (=tail region)
 Made up of 1-30 fused segments.
o The cephalon (=head region)
 Contained the sensory systems (i.e eyes), a glabella, and glabellar furrows
 Also had facial lines of different kinds  consistent with age and species of
trilobites
 Proparian: Most primitive; found in the larval stage, drops from the top
of the cephalon before branching to each side
 Gontoparian: Like proparian except intersects each corner of the base of
the cephalon

,  Opisthoparian: More complex; intersects the middle of the base of the
cephalon on each side.
o The thorax (=abdominal region)
 Made of 2-61 articulating segments.
- Trilobite growth occurs from the head region in a process called ecdysis (or molting)
o Ecdysis progresses through three steps
1. Sloughing off old skeleton
 During this stage, the trilobite secretes enzymes from its head which are
responsible for disintegrating the facial sutures.
 Once disintegrated the trilobite exits through the head portion
2. Trilobite swelling up
3. New exoskeleton grows.
 As the trilobite grows/secretes a new exoskeleton (calcium carbonate), it must
hide from predators as it is a very vulnerable state.

4. Compare and contrast the different trilobite eye systems
- There are two eye systems that the trilobites utilize: Holochroal and Schizochoral

Holochoral Schizochoral
- Primitive eye type, widespread amongst - Eyes differ from holochroal based on
trilobites their refractive index and their material
- The entire eye is covered by a single makeup.
membrane. - Larger and separated lenses.
- Eye system occurs in the Cambrian- - It is possible that trilobites with this eye
Permian age. type were able to see 3D images as the
images from each lens were slightly
Lenses different which allow them to get a sense
- Lenses are closely packed together (~100 of depth when image was recapitulated
uM) in diameter - Eye system occurs in the Ordovician to
- Lenses are hexagonal shaped Devonian age.
- 1 -15,000 lenses/eye - Eye system potentially evolved from
- Each lens has a c-axis that is juvenile holochroal eye that was retained
perpendicular to the surface which allows into adulthood. As juvenile holochroal
each lens to produce single white and eyes appear to be miniature schizochoral
black dots. When these dots combine, eyes.
they form a picture. (=like how a dot in a
newspaper form an image) Lenses
- The direction the light enters the eye are
DISADVANTAGE: This eyes system suffers is slightly altered as hits the CaCo3
from double refraction and images can appear surface on the lense, it is then altered
fuzzy. Thus, trilobites with this eye system can again when it hits the interface between
only see light movement but are unable to see the CaCo3 and organic material.  Thus,
clear images. correcting the aberration caused by
double refraction

ADVANTAGE: Image is perfectly focused on
the back of the eye.

5. Discuss some of the unusual eye adaptations found in trilobites
- Some of unusual eye adaptations found in trilobites are having: