Chapter 34:
Vertebrate Evolution and Diversity
I. Phylum Chordata
All chordates are: deuterostomes (enterocoelomates)
This group includes
all of the vertebrates (Subphylum Vertebrata) PLUS two invertebrate groups:
a) subphylum Cephalochordata
b) subphylum Urochordata
Features common to all chordates:
(present at least in the embryo, if not in adult forms) (Fig. 34.2, CD 34A)
1. notochord--longitudinal flexible rod of cartilage between gut and dorsal nerve cord (precursor of skeleton in vertebrates)
2. dorsal, hollow nerve cord
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forms the central nervous system
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develops from a plate of ectoderm which rolls into a tube dorsal to the
notochord
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unique to chordates, all others are ventral, hollow
3. pharyngeal slits
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lumen of digestive tract opens to outside through slits in sides of pharynx
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function in filter feeding in invertebrates
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modified for gas exchange in vertebrates (precursors to gills, lungs)
4. muscular postanal tail
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contains muscular and skeletal elements
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important in propulsion for many aquatic, terrestrial spp.
A. Subphylum Urochordata (Fig. 34.3)
tunicates (Sea squirts)
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sessile marine animals, may be planktonic, colonial
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incurrent and excurrent siphons, atrium chamber; pharyngeal slits for feeding
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animal cloaked in a tunic of tunicin (cellulose-like)
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adult has no notochord, nerve cord, or tail BUT are all present in larval stage
(metamorphose)
B. Subphylum Cephalochordata (Fig. 34.4)
example in lab: lancelet Branchiostoma (formerly Amphioxus)
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good example of all chordate features persisting into adult stage
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buries posterior in sand, feeds with mucous net across pharyngeal slits,
filtering particles from water entering mouth, exiting through atriopore
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muscular segmentation leads to coordinated swimming movement
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segmentation analogous to annelids,
arthropods; homologous to other chordates
First chordate fossils 550 m.y.o., from the Burgess Shale of
British Columbia & Alberta
first vertebrates arrived 50 million years later, probably from cephalochordates undergoing paedogenesis (precocious sexual maturity) before losing their larval chordate characteristics
C. Subphylum Vertebrata (You belong to this subphylum)
Vertebrates share all chordate characteristics, plus:
1.
highly cephalized
2.
well-developed sensory organs, nervous system, and
brain
3.
skeletal units (vertebrae) enclosing the nerve cord
(non-living matrix with live cells, capable of growth)
a) axial
skeleton--consists of vertebrae, skull,
ribs (“backbone”)
b) appendicular
skeleton--2 pairs of appendages
4. closed
circulatory system (blood always enclosed in blood vessels, recirculated to
heart as in annelids, cephalopods)
5. nitrogenous
wastes removed as blood circulates through kidneys
6. separate
sexes in most; reproduction sexual or via parthenogenesis; fertilization may be external or internal
The phylogeny of vertebrates is under review. Figure 34.7 lists 12 proposed
classes of the extant (“living”) groups:
1-2. Agnathans - Proposed Classes Myxini & Cephalaspidomorphi (jawless fish)
60 spp. Extant (Figs. 34.8 & 34.9)
hagfish and lampreys (these are NOT eels)
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as early as Cambrian (app. 500 m.y. a.); more by Ordovician-Silurian (500-400
mya)
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mouth circular, slitlike--no jaws
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no paired appendages
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cartilaginous skeleton
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started as mudsuckers, suspension feeders
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larval lampreys are free-living, resemble cephalochordates; adults parasitic
bloodsuckers
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hagfish are scavengers
Extinct Class Placodermi--armored fish (up to 10 m long)
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Silurian to Devonian (app. 450-350 m.y.a.)--”age of fishes”
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died out by Carboniferous (350 m.y.a.)
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origin of jaws and paired fins, efficient predators
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pharyngeal slits no longer functioned in feeding
3. Proposed Class Chondrichthyes--sharks, rays--750 extant spp. (Fig. 34.1)
from early Paleozoic
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skeleton of cartilage vs. bone
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jaws and paired fins well-developed
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oil stored in huge liver aids buoyancy; still must keep moving to avoid sinking
(denser than water)
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water flows over gills by swimming or active pumping by jaws and pharynx while at rest--gas exchange
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largest are suspension feeders (plankton) eg. whale shark--1 million L/hour
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most are carnivorous predators
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digestive tract relatively short, spiral valve internally prolongs passage of
food through intestine
sensory organs clustered in head:
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vision--no colors
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nostrils (smell only)
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paired regions for electrical field detection (muscular
contractions)-"ampullae of Lorenzini"
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auditory organs “hear” percussions--no ear drum, sound waves transmitted by
whole body to inner ear
lateral line system
on each flank--row of microscopic organs, sense changes in water pressure,
minor vibrations
all have internal fertilization (copulation)--specialized male
clasper fins on ventral side
may bear young by three methods:
a. oviparous--lay
eggs which hatch outside body
b. ovoviviparous--eggs
hatch within oviduct of female, nourished by egg yolk
c. viviparous--embryo
develops in utero, nourished by a placenta (relatively few)
4-6. Osteichthyes - Proposed Classes Actinopterygii (ray-finned), Actinistia (lobe-finned), and Dipnoi (lungfishes) These
are the bony fishes (Figs.
34.12, 34.13, & 34.14)
Most numerous and diverse of vertebrates--30K spp.
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skeleton reinforced by Ca2PO4
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skin glands secrete mucous to reduce drag through water
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have lateral line system like sharks
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more maneuverable swimmers due to flexible bony fins
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swim bladder to control buoyancy:
transfer of gases between swim bladder and blood changes fish’s density; allows
it to conserve energy, remain motionless without sinking
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respiration via 4-5 pairs of gills protected by bony outer operculum; water
actively pumped through by coordination of operculum and muscular contractions
in gill chamber
probably evolved first in fresh water--swim bladder
originated from primitive lungs
most oviparous, with external fertilization of eggs
three sub-classes by end of “Devonian radiation”: (Fig.
34.13)
- Proposed Class Actinopterygii --ray-finned fished--most familiar to us eg. salmon,
bass, trout
- Proposed Class Actinistia, (formerly Sarcopterigii)--fleshy-finned fishes, remained in fresh water, continued using lungs to augment breathing; developed muscular pectoral and pelvic fins with skeletal support, “walked” under water and finally onto land
· coelacanths (one extant species, Latimeria) and extinct rhipidistians collectively known as lobe-finned fishes (Fig. 34.14)
- Proposed Class Dipnoi
three orders of fleshy-finned fishes: a) lungfishes, b) coelacanths, c) rhipidistians
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lungfish have three extant genera in South America (see these at Moody
Gardens!); can burrow in mud and aestivate
if swamps dry out intermittently
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a rhipidistian (Eusthenopteron) believed
to be direct ancestor of amphibians during the Devonian period (365 m.y.a.)
7. Proposed Class Amphibia
first terrestrial tetrapods, dominant during late
Devonian-early Carboniferous periods (“age of amphibians”)--some up to 4 m
long!
conditions on land during this time favored their class:
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climate mostly warm, moist (continental land masses small, separate, clustered
around Equator); plenty of coastline with ample moisture, rainfall
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amphibian skin highly vascularized, aids lungs in gas exchange
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three-chambered heart for more efficient oxygenation of blood
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external fertilization
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shelless eggs may be laid externally, but dry out quickly without moisture; may
be ovoviviparous or viviparous; incubate eggs in mouth, on back, even in
stomach
name Amphibian means “two lives”:
many frogs undergo metamorphosis from larval tadpole (fishlike, with lateral line,
gills, and tail; herbivorous algae diet); these features resorbed as adult
develops lungs, legs, external eardrums, digestive tract specialized for carnivorous
diet
3 extant orders of Amphibians, (Fig. 34.17):
a. Order Urodela--”tailed
ones”--salamanders--400 species
b. Order Anura--”tailless
ones”--frogs and toads--3500 spp.--better adapted to land walking
c. Order Apoda--”legless
ones”--caecilians, wormlike, burrowing--150 spp.
8-10. Reptiles - Proposed Classes:
7000 extant species
of reptiles, many more extinct
oldest fossils 300 m.y.a.--Carboniferous
predominant land vertebrates for app 200 million years
during the Mesozoic Era = “Age of Reptiles”
as continents converged into one supercontinent, Pangaea, the global climate changed, marking the end of the Paleozoic Era with
the Permian extinction (dying
off of many terrestrial and aquatic species)--(read more on pp. 490-491)
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shorelines were reduced
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ocean basins increased in depth, lowering sea level and draining coastal areas
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continental areas became hotter, drier, more erratic temperatures and rainfall
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most extreme period of volcanism in 500 m.y. , led to :
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immediate darkening, temperature cooling
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eventual temperature increases due to CO2 increases in atmosphere
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reduced CO2 availability in oceans due to reduced currents
adaptations that better equipped reptiles for these
conditions:
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three-chambered heart (more well-divided than in amphibians)
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scales containing the protein Keratin--waterproof, prevents drying (analogous
to cuticle of insects)
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lungs must now provide all respiratory needs
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fertilization now exclusively internal (sex as we know it becomes a necessity)
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most oviparous, some viviparous
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are ectotherms (absorb external heat
via behavioral adaptations)
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evolution of shelled amniotic egg to
withstand incubation away from water (common to Class Aves and Class Mammalia
as well)
Incredibly important adaptation for terrestrial living = amniotic
egg, (Fig. 34.19).
Four extraembryonic membranes surround different parts of the developing embryo:
a) amnion surrounds
embryo, encloses amniotic fluid for hydrostatic cushioning
b) yolk sac, highly
vascularized covering over yolk mass provides nourishment
c) allantois serves
as disposal for embryo’s wastes
d) chorion exchanges
gases between embryo and exterior
Evolutionary radiation of amniotes during early Mesozoic
gave rise to three main groups, (Fig. 34.20):
a) synapsids, a branch leading to therapsids, and ultimately, mammals
b) anapsids
(all branches extinct)
c) diapsids, leading to all other modern amniotes
Triassic radiation
200 m.y.a. gave rise to:
a) dinosaurs (check out those T. rex tracks in Hondo Creek) (Fig. 25.1f)
b) pterosaurs (eg Quetzelcoatylus,
found in west Texas, 35 meter wing span)
Cretaceous Extinction
at end of Mesozoic Era (read
more on p. 491) marked by:
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cooling of the climate over a period of 5-10 million years; shallow seas
receded
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currently believed to be the result of meteoric impact and/or massive volcanic
eruptions, causing large amount of suspended dust to block sunlight, lower temperatures, severe acid rain, and disruption of
food chains (see T. rex
and the Crater of Doom by Walter Alvarez, Princeton University
Press, 1997))
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measurements of iridium, a rare metal
on earth but more concentrated in meteorites, shows 1000X concentration in
layers of sediment dated to 65 m.y.a.
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recent discovery of a 125 mile wide crater Chicxulu on Yucutan coast in Mexico
gives further support to impact theory (theorized to be caused by a 10 km
diameter asteroid)
Extant groups of reptiles:
8. Proposed Class Testudine – turtles (formerly Order Chelonia)
shell for protection; arose directly from ancestral amniote, very ancient
(also Proposed Class Sphenodontia) – tuataras…
Only living member of ancient group. Not a lizard. Read more at the site www.kcc.org.nz/animals/tuatara.htm (it’s a children’s site, but I still like it)
9. Proposed Class Squamata – snakes and lizards
most diverse group; predatory; vestigial limb bones in primitive snakes like boas are
evidence that snakes arose from
lizards
Adaptations
for predatory lifestyle in snakes:
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acute chemical sensors
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sensitive to ground vibrations (lack eardrums)
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pit vipers have organ for heat detection between eyes and nostrils
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toxins injected through hollow or grooved teeth
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tongue transmits odors to olfactory organ on roof of mouth
· loosely articulated jaws, cartilage at center of lower mandible
10. Proposed Class Crocodilia - crocodiles, alligators,
caimans, and gharials
little changed from Mesozoic Era; Crocodilians range in size
from Cuvier's dwarf caiman, which only grows to about 1.5 meters (five feet)
long, to the Indopacific crocodile, which grows to 7 meters (23 feet) long,
(animaldiversity.ummz.umich.edu/chordata/reptilia/crocodilia.html)
11. Proposed Class Aves--8600 spp., 28 orders (Fig. 34.29)
should be grouped as reptiles based on phylogeneny; only modern animal believed to be descended
from dinosaurs; have in common:
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beaks, leg scales, feathers of Keratin
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amniote egg
Divided into two groups:
ratites--non-flying,
breastbone lacks keel eg. emus, penquins
carinates--flying
birds, sternal keel supports enlarged pectoral flight muscles
Adaptations for flight:
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lightweight, honeycombed skeleton (4 oz. for frigatebird with 2 m wingspan)
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only one ovary in females, no penis in males
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no teeth (food ground in gizzard instead--also present in dinosaurs,
crocodilians)
Unique features from earlier animals:
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endothermic--use own metabolic energy
to maintain constant body
temperature
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insulated by feathers, fat
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efficient circulatory system w/ 4-chambered heart to support higher metabolic
needs
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highly developed vision, coordination
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proportionately larger brains :
(see The Minds of Birds
by Alexander F.
Skutch, Texas A & M Press)
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internal fertilization, shelled eggs which must be brooded to hatch (some
evidence that some later dinosaur species had developed endothermy, brooding
of eggs)
Advantages of flight:
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escape predators
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enhance hunting and scavenging
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capture flying insects, large food source
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migration for seasonal survival
First fossils 150 m.y.o. (Jurassic)
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Archaeopteryx (Fig. 34.27) Cool! clawed forelimbs, teeth,
tail with vertebrae not considered a
direct ancestor to modern birds, but rather a sidebranch (dated at 145
m.y.a., would have coexisted with more modern branch of birds)
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Protoavis (Texas, 225 m.y.o.) had
a combination of reptilian and birdlike features
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recently found fossils in China (Sinosauropteryx) even more convincing evidence of feathered dinosaurs evolving
12. Proposed Class Mammalia--4500 spp.
Common features of all mammals:
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hair made of keratin for insulation
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endothermic, like birds
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diaphragm, a sheet of muscle that
assists in ventilating lungs
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4-chambered heart (again, like birds)
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mammary glands--modified sweat glands
which produce milk rich in sugars,
fats, proteins
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internal fertilization, mostly
viviparous
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large brains
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long parental care
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differentiation of teeth specialized for diet
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3 middle ear bones (develop from gill arches in embryo) (Fig. 34.30)
Diverged from reptilians (therapsids) before birds:
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present by app. 220 m.y.a. (boundary of Permian-Triassic periods at beginning
of Mesozoic--mostly small, insectivorous, nocturnal (eye sockets reveal
this)
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coexisted with dominant dinosaurs throughout Mesozoic
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adaptive radiation of mammals with the extinction of dinosaurs during the
Cretaceous extinction
Led to three distinct groups by Cenozoic Era (app. 65 m.y.a.):
a) Monotremes--egg-laying
mammals (Fig. 34.31)
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oviparous: yolk nourishes embryo (most
reptile-like)
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lower body temperature than other mammals (30C vs. 37C)
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no nipples, young suck milk from fur around glands
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complex pectoral girdle; humerus and femur held lateral to body
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reptile-like cloaca; bird-like skull w/
elongated beak-like rostrum
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eg. duck-billed platypus, echidna
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only in Australia, New Guinea
b) Marsupials--pouch-bearing
mammals (Figs. 34.31 &
34.32)
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semi-ovoviviparous
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egg contains very little yolk
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complete embryonic development in maternal pouch or marsupium
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include opossums, kangaroos, koalas (opossums the only marsupials outside of
Australia
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many species in Australia--radiated to fill niches not taken by placental
mammals
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probably originated in N. America, migrated to South America, Australia while
still joined
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South America and Australia became islands during break-up of Pangea, and
developed their marsupials in isolation from placentals
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placental mammals migrated to S. America after rejoined at Panamanian
isthmus (12 and 3 m.y.a.), but Australia has remained isolated since Cenozoic
65 m.y.a.--much more extensive marsupial speciation
c) Placental (Eutherian) mammals) (Fig.
34.32 and Table 34.1)
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furthest evolved from reptilian therapsid ancestor
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are viviparous--complete embryonic
development in uterus, joined to mother
by the placenta
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all modern orders radiated 70-45 m.y.a. (late Cretaceous to early Tertiary
periods)
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may have diverged from marsupials 80-100 m.y.a.
4 major evolutionary lines: (Fig. 34.33)
Molecular systematists
have grouped the placental mammals into four clades
Human traits--evolved
from early primate ancestors (arboreal lifestyle)
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limber shoulder joints for brachiation (limb-swinging)
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dexterous hands, sensitive fingertips
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opposable thumbs (and toes in some)
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claws gave way to nails
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eyes in front of face--overlapping vision, greater depth perception
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excellent eye-hand coordination (increase in cerebrum, reliance on sight)
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reduction of olfactory region of brain
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extensive maternal care
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diurnal
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social (band-forming)
Earliest primate fossil Purgatorius unio from Cretaceous/Tertiary (Montana)
2 subgroups of primates had diverged by 40 m.y.a.
a) Prosimians (most
like ancestral insectivore)--lemurs, lorises, potto, tarsiers (Fig. 34.34)
b) Anthropoids--monkeys,
apes, humans
Monkeys further divided into
Old World vs. New World monkeys (became separated as continents drifted apart):
New World = all arboreal; prehensile tails;
nostrils open to sides (spider monkeys, squirrel monkeys, capuchins)
Old World =
ground and tree-dwelling; lack prehensile tails; nostrils open downward;
tough seat pad for sitting (mandrills, baboons, rhesus, macaques)
Four genera of apes (Fig. 34.37) Tropical
regions of Asia/Africa
1. Gibbons (Genus Hylobates)
2. Orangutans (Genus Pongo)
3. Gorillas (Genus Gorilla)
4. Chimpanzees and Bonobos (Genus Pan)
Currently believed that humans (Family Hominidae) diverged from common ancestor with apes app. 6-8 m.y.a. (comparisons between human, chimp DNA)