Biology 2108
Lecture
Biodiversity: Animals
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Defining characteristics of animals
(metazoans)
- Multicellular,
heterotrophic eukaryotes feeding mainly by ingestion
- Lack
cell walls; cells held together by structural proteins (mainly collagen)
Theories origin of
animals
Syncytial
theory - Metazoa evolved from multinucleated
ciliate
('syncytial' refers to protoplasm that
contains numerous nuclei not separated from each other by plasma
membranes).
Evidence
to support this idea:
- Many protozoans are
multinucleated,
including
ciliates
- Bilaterality in ciliates as in
most animals
- Such an ancestor might be similar
to
ciliated
'planula' larva found in some radiata
and parazoa groups
Colonial
Theory -Metazoa evolved from hollow, spherical
colonial
flagellates
Flagellate protozoans formed a
colonial
ring of organisms, termed a blastaea then increasing division of labour
including invagination to form a jellyfish-like gut.
Support
for Colonial Theory
- Similar to the embryology of
many animals
- Flagellated sperm widespread in
animals
- Colonial tendency in flagellates
- Similarity between
Choanoflagellates
and Choanocytes
of sponges
Recent
molecular data supports a monophyletic origin of metazoans,
and that choanoflagellate protists are sister to the monophyletic
Metazoa. Does
this suggest that two or more of these theories might be correct
concerning
the origin of metazoans?
Early
history
of multicellular animals
Cambrian
Explosion
- the rapid (~10 million years) appearance
of major animal taxa in the early Cambrian. Several
important
fossils discoveries
have extended the length of time over which this diversification
occurred.
 |
Cambrian
macrofauna explosion-First
fully
skeletonized organisms, many that are clearly related to modern
groups, as
well as
other
taxa
that can
not be confidently placed into modern phyla |
The
Shelly Fauna
- The first
evidence
of
animals
with hard parts are fossils that are small (generally 1 to 5 mm).
|
The
earliest
‘trace’ fossils
- Burrows
and intense
disturbance
of sediment layers (clear
indications of trace
fossils as early as 570 million years ago) indicate the presence of
soft-bodied animal with relatively sophisticated muscular and nervous
systems.
|
 Ediacara
“fauna”
- Quilt-like body
plans
forming
feathery fronds, pouches, disk and worm-shapes, without heads or
obvious
circulatory, nervous or digestive systems. There is controversy
over
whether these organisms are ancestors of modern animals,
a failed experiments’
in early
animal evolution that left no modern day representatives, or
not even
multicellular
animals at all. |
The bottom
line is
that
the evolution of multicellular animal life may be older than previously
believed
but was still relatively rapid in geological
terms (40 million
years), and when compared with
life today
produced
most of the major animal body plans. The last 500 million years
has
been mostly tinkering.
Modern major groups
of animals
There
are ~35 modern phyla of animals. However, the evolutionary
relationships among these groups have been difficult to determine, in
part due to their relatively rapid apparence during the cambrian
explosion.
Traditionally the
major grouping of animals and their relationship to one another (i.e.
their phylogeny) has been based on layout of the body
(or coelomic) cavity (aceolomate,
pseudoceolomate,
or ceolomate) and on early
developmental characteristics of the embryo (protostome or deuterostome)
Below is a
older ('traditional) version
of
the animal phylogenetic tree (don't memorize this one):
There has always been some controversy
concerning this tree of the major groups of animals.
Characteristics used in
traditional animal groupings have been questioned (e.g. some of these
characteristics may be the result of neotony, or comparisons are made
from shared primitive characteristics).
More recently, researchers using molecular
analysis (DNA/RNA sequencing) have proposed major changes in the
animal
phylogenetic tree (a simplified version below), forcing a
re-examination of
morphological data:
Some
traditional groupings of animal phyla
have been supported
by recent molecular evidence:
Many phlya
have been
re-grouped based on recent molecule analyses and
morphological similarities have be recongized within many of these new
groupings
(these
groups together all share bilateral symmetry
and 3 tissue layers):
Platyzoa (e.g.
flatworms, rotifers) - phyla in this group are related to one another
(and to the lophotrochozoa)
based mainly on
molecular similarities
Lophotrochozoa
(e.g.
segmented worms, squid...) - phlya in
this group either share a similar larval type (trochophore)
or a
similar feeding structure (lophophore- tentacle-bearing
ridge or arm )
Ecdysozoa
(e.g. crabs
and insects, round worms) - phlya in this
group
all possess a cuticle
(non-cellular out covering) that molts
Molecular sequence data has also supported the traditional
grouping of deuterstome phyla
Deuterostomes (e.g.
starfish, chordates including mammals) - phlya in this group share development that is characterized by a number of developmental characteristics
including the
mouth not
derived from
the blastopore
