Download Lecture 3 – Cladistics

1. Geological history
a. Earth 4.6 bya
b. First fossils 3.5 bya (Archean Eon)
c. Proterozoic Eon
i. First oxygen
ii. Land masses formed
iii. First eukaryotes (2 bya)
iv. Multicellular organisms (1 bya)
v. First animals (600 mya)
d. Paleozoic Era
i. Cambrian Period (began 540 mya)
1. First vertebrates at very beginning (550 mya)
2. Continental masses separate, covered by water
ii. Ordovician (began 490 mya)
1. First fishes with jaws
2. Complex plants on land
3. Major radiation of fishes
iii. Silurian (began 443 mya)
1. Still shallow seas, but some dry land
2. Vascular plants and arthropods on land
3. First sharks and bony fish
2. Earliest vertebrates
a. Until a few years ago, believed Ostracoderms to be earliest
i. Evolved in Ordovician Period (480 million years ago)
ii. Fish enclosed in thick, bony armor
b. But very recently found earlier vertebrates from the Early Cambrian (550 mya)
i. Myllokunmingia and Haikouichthys
ii. Very small (3 cm)
iii. But both have a cranium and particular type of muscle (myomeres) that indicate
they are vertebrates
iv. But lack bone or scales like ostracoderms
v. And since both Chinese fossils had developed dorsal fins, they are more advanced
than hagfishes
vi. So vertebrates probably diversified well before this time
vii. 550 mya is probably a few million years later than actually evolved
c. Other fossils (bone fragments) found in the early Ordovician
i. Found at several sites, including Bolivia, Australia, and NA
ii. Suggests that by this time, vertebrates were distributed worldwide
iii. Armored, torpedo-shaped fishes between 12-35 cm
iv. Armor was small, close-fitting plates, in the head and gill region formed a shield
d. Later fossils suggest the Ordovician was period of significant radiation of both jawless
fishes and marine invertebrates (possible prey?)
e. But it was in the Silurian Period that there was a major radiation and jawless fishes (and
some fishes with jaws) began to dominate
f. Fishes were originally thought to evolve in fresh water
i. The presence of the kidney
ii. Necessary to rid water of body of excess water due to osmosis
iii. But kidney was fortuitously adapted to the purpose
iv. Now accepted as evolving in marine environment
1. Earliest fossils found in marine sediments
2. All nonvertebrate chordates and invertebrate phyla are exclusively marine
forms
g. One of most important developments in early vertebrates was bone
i. But aren’t sure of its origin or original purpose
ii. Protection from predators
1. Early jawless fish had large predators, so bony head shield would protect
them
2. But small bits of bone wouldn’t necessarily protect and doesn’t explain
origin
iii. Electric insulation for electroreceptive organs
1. Structure in head shield suggests sensory function
2. But no other electroreceptive vertebrates use bone to insulate
iv. Mineral storage
1. Phosphate is rare and limiting element in marine systems
2. Individuals who could collect and store it would be at advantage
3. Also good for calcium storage
4. Would explain that even small bits would be advantageous
5. Important for evolution to be able to explain all intermediate steps in the
process
v. Possibly began as mineral storage, used in sensory function, then protection (not
mutually exclusive)
3. Ostracoderms
a. A paraphyletic group that is not recognized by cladists
b. We will consider them as one large, diverse group
c. All had dermal bone covering, some with an extensive armored shell (carapace)
d. From 10 cm to 3 m
e. Either bottom feeders or predators (with pumping action and large scoop)
f. Most had dorsal fins, but only more advanced had lateral paired appendages
g. Most extinct by end of Devonian (360 mya)
h. Muscular pump for venting gills for efficiency (respiration)
i. Internal skeleton was simple – cartilage
i. Notochord was main support
ii. Gill supports
iii. No jaws, but some had moveable plates
j. Because such a heavy body (given size), had heterocercal tail for greater lift
4. Modern jawless Fishes – two closely related groups
a. Hagfish = SuperOrder Mixini (60 species)
i. Elongated, scale-less species with world-wide distribution
ii. Marine only (not effective osmoregulation, so isotonic with seawater –
exclusively marine)
iii. Appear to lack vertebrae (some consider lost recently), so classification is
questionable (often not placed in SP Vertebrata)
iv. Have 1-15 gill slits
v. Deep-sea scavengers by smell
vi. Produces huge quantities of slime proteinaceous threads) as defense mechanism
(rapidly produced to fend off predators)
1. When predator leaves, ties itself into a knot to scrape off
2. Also sneezes to clear nasal passages
vii. Mouth has 6 tentacles to help search for food (tactile searching)
viii. Rudimentary eyes
ix. Two horny plates that are teeth-like
x. Feed by using knot-system
1. Capture polychaete worms
2. Scavenge fish carcasses
3. Usually enter through gills or anus
4. Eat organism from inside out
5. Rips meat by grasping and knotting self
xi. Unique circulatory system in vertebrates
1. Venous blood drains into large, open sinuses
2. Essentially an open system
3. Venous blood under extremely low pressure
a. Have accessory hearts around gills and in caudal region
b. Drives venous blood back to heart
4. Hearts are aneural – pumping is intrinsic, not regulated by central nervous
system
xii. Conservation Issues
1. Since 1970's, hagfish have had increasing economic impact on fishing
industry by scavenging fish caught in stationary gear (e.g., gill nets)
2. Captured by humans to use skin (eel-skin products are actually hagfish)
a. Appear to be quite common
b. But overfishing is possibility, particularly in Asia and North
America
c. Since we know nothing of their reproduction, can’t manage them
properly (set necessary fishing limits)
i. We do know that have large eggs and direct development
ii. Size of eggs and offspring correlated with reproductive rate
iii. Large eggs means:
1. Long developmental time before first mating
2. Long interval between reproductive episodes
3. Relatively few offspring (K selected)
iv. Together suggests limits on fishing will be necessary
v. But doubtful will be applied
b. Lamprey = SuperOrder Cephalospidomorphi
i. 50 species in 2 genera
ii. Distributed world-wide except polar and tropical regions
iii. Similar by more advanced than Hagfishes
iv. Have vertebral structures
v. Adults are parasitic
1. Attach by suction
2. Rasp hole with horny plates
3. Oral gland produces anticoagulant so feeding is continuous
4. Has simple digestive system
5. Have 7 gill slits
6. Since mouth is used for suction, can’t vent through mouth to gills
a. Gills vent both directions (take in and push out water)
b. Not very efficient
vi. Anadromous
1. Live in marine, swim upstream to breed
2. Male and female build nest together
3. He latches onto her and she onto a rock
4. External fertilization – fertilizes as eggs are laid
5. Breed once then die
6. Larvae were misidentified as a separate species in a different genus
(Ammocoetes)
7. Spend 3-7 years as sedentary filter feeders in mud or sand
vii. Lack jaws, like hagfish
viii. Eyes are well-developed, unlike hagfish
ix. Osmoregulatory systems
1. Associated with gills
2. Kidneys allow precise regulation of body fluids (permits anadromous life
history)
x. Conservation issues
1. Petromyzon marinus were found in Lake Ontario along St. Lawrence
seaway
2. But couldn’t get past Niagra Falls
3. Creation of Welland Canal connected Ontario and Lake Erie in 1829
4. Invaded around 1920s
5. Contributed to major decline of commercial and sports fishery industry in
Great Lakes
5. The transition to jawed vertebrates (or the origin of the Gnathostomata)
a. Some biologists consider the evolution of jaws to be the most important advances in
vertebrate history
i. Allow variety of new feeding techniques and behaviors
ii. Ability to grasp, manipulate, and carry objects
1. Can dig holes
2. Can carry pebbles and vegetation to build nests
3. Can grasp mates during courtship
4. Can grasp juveniles during parental care
iii. With teeth, enable organisms to:
1. Cut food into pieces small enough to swallow
2. Grind hard food
3. So new foods became available
4. Allow greater flexibility
iv. Jawed vertebrates can become larger than jawless verts
b. How did jaws evolve?
i. Embryologically, we know that jaws are derived (developmentally and
evolutionarily) from the same material that makes up the branchial (gill) arches
(cartilaginous rods that supports gill filaments)
ii. Until 5-10 years ago, assumed jaws evolved for feeding
1. More efficient
2. Greater feeding options available
iii. But for evolution to occur, must have a selective advantage at all points along
evolutionary path
1. Will not evolve something that doesn’t have advantage
2. Goes against natural selection
3. Intermediate steps along the way were not necessarily advantageous for
feeding
iv. Suggested that different function could have driven evolution and that the feeding
advantage was simply a bonus
1. Mouth and jaw also used for respiration/ventilation
2. Mallatt (1996) proposed that original function and current function are not
the same; suggests four basic stages, driven by selection for increasing
activity (selection for more efficient gas exchange and feeding efficiency)
v. Basic scenario
1. Ancestral, unjointed branchial arches used in conjunction with relatively
weak ventilation; low overall activity levels
2. Selection favored increase in ventilatory efficiency associated with
increase in activity levels
3. Consequently, selection favored evolution of new ventilatory muscles and
stronger, jointed branchial arches to support them
4. Jointed branchial arches provide enhanced range of motion for powerful
muscles compared to unjointed arches
a. Could expand and contract by straightening and collapsing
b. Allowed greater intake of water and more rapid and forceful
ejection
5. Most powerful muscles were those associated with opening and closing
mouth attached to first branchial arch
6. Advantages are:
a. Prevent leakage of water during expiration by strong mouth
closing
b. Increase suction by being able to open mouth wide quickly
c. Improves ventilation
d. Allows powerful suction feeding (larger, more active prey sucked
in with water)
e. Selection favors biting jaws to help hold larger, more active prey in
place before swallowing
7. Four basic stages in evolution of jaws from branchial arches
a. Ancestral vertebrate: unjointed branchial arches
b. Early pre-gnathostome: jointed internal arches support stronger
ventilation muscles; gives stronger inspiration and expiration
c. Late pre-gnathostome: mouth-closing, ‘ventilatory’ jaw permits
more forceful expiration of water across gills
d. Early gnathostome: feeding jaws