Zoogeography
I.
Principles, definitions, processes
A.
B.
C.
Study of bio/zoogeography
1.
There are two goals in zoogeography:
a.
Delineation and characterization of faunal areas
b.
Determine the evolutionary history of these faunas, in geographical
context
2.
Two general processes shape the geographic distribution of taxa:
a.
Historical factors: evolutionary origin and spread of taxon
b.
Ecological factors: distribution of abiotic and biotic factors that
influence distribution
c.
We will focus for today on historical zoogeography
Types of distributions
1.
Endemic: found only in a particular region. Hawaii and Bermuda have
many endemic fishes. A taxon can be endemic to an island, an ocean, or a
continent.
2.
Circumglobal, circumtropical, circumpolar: widely distributed across
globe, throughout tropics, around temperate or arctic latitudes. Such
widespread distributions are often found in epipelagic fishes, e.g. many
oceanic tuna, billfishes
3.
Antitropical: absent from tropics but present at higher latitudes, in both N
and S hemisphere. Such distributions can occur in an individual species,
e.g. Scomber japonicus, or in a higher taxon e.g. a genus, family
Historical processes influencing distributions
1.
Dispersal
a.
The distribution of a taxon and its subtaxa are determined by
movement to new areas
b.
Freshwater fish may disperse:
1.
freshwater or overland dispersal:
a.
rivers change course
b.
stream capture, as headwater erosion cuts through
watershed boundary
c.
primary freshwater fishes are groups strictly
confined to freshwater, must disperse this way
2.
dispersal via marine waters
a.
secondary freshwater fishes are usually limited to
freshwater but occasional individuals or species
may be estuarine or marine;
b.
peripheral fishes have evolved freshwater lifestyle
from marine group, or migrate between (diadromy)
c.
freshwater fishes on isolated oceanic islands are
secondary or peripheral; gobies on Hawaii,
Dominica in Caribbean
d.
D.
II.
Example: Plotosidae (catfish) is a marine group that
has secondarily invaded freshwater in Australia,
New Guinea. Was this secondary or peripheral?
c.
Marine fish can certainly disperse
d.
Dispersal involves random events, acting on individual taxa
2.
Vicariance
a.
distribution of a taxon and its subtaxa are determined by the
formation of geographic barriers causing speciation.
b.
continental breakup and drift (freshwater fishes)
c.
for marine fishes,
1.
spreading of sea basin; isolation by distance
2.
formation of isthmus of Panama isolated West Atlantic
from East Pacific, cutting species in two.
d.
These events are likely to occur to many taxa
3.
Competing ideas
a.
These are two basic models for how distributions arise
b.
Both likely to be required for full explanation of any taxon; but
lively debate on relative importance
c.
Classically, explanations were dispersalist
d.
Relatively modern evidence for continental drift gave life to
vicariant hypotheses
Modern methods used in analyzing processes:
1.
seek common patterns of distribution by comparing phylogenies and
distributions for diverse taxa
2.
Determine distribution of taxon and subtaxa
3.
Determine phylogeny for group
4.
Repeat for other taxa in same areas
5.
If possible, also develop an 'area cladogram' based on known geological
history: e.g., Australia, Africa and South America
6.
Fossils can be very useful
a.
Paleodistribution
b.
establishing a minimum age
Test of a vicariance hypothesis: the fishes of Africa and S. America
A.
B.
C.
D.
These continents share a large number of taxa (table 8.1 Lundberg 1993)
Given a hypothetical clade, C, made up of C1 and C2, on different continents.
There are four possible models for this distribution (schematic by ES)
1.
Simple drift/vicariance: continental drift followed by allopatric speciation
2.
Pre-drift intercontinental speciation: species divergence prior to
continental drift
3.
Post drift dispersal: species divergence following cross-ocean dispersal
4.
Indirect dispersal: the common ancestor for the pair of taxa isn't present on
either continent
Geological evidence for split between continents: it occurred sometime between
106 and 84 Ma, in the mid-Cretaceous
Lungfishes: support drift/vicariance model
1.
There is a well supported phylogeny (fig 8.2, Lundberg)
2.
E.
F.
III.
Model 3 is rejected: there is a fossil neoceratodontid from early
Cretaceous; so the ancestral lepidosirenid, sister taxon to the
neoceratodontid, had to have originated before drift.
3.
Model 2 is rejected: subsequent fossils of the two genera are limited to
current continental ranges and earliest is late Cretaceous
4.
Model 4 is rejected: marine dispersal for lungfishes is out of the question
Arapaimidae supports pre-drift speciation model (fig. 8.3c, Lundberg; fig. 16.10
Helfman et al. illustrates these spp)
1.
Model 1 is rejected: two fossils: Aptian (110 Ma) age of Laellichthys
indicates that split between Arapaima and Heterotis occurred before drift
2.
Note that the split between Laellichthys and Heterotis+Paradercetis might
have been model 1 vicariance
Bottom line (again, review Table 8.1); diversification of freshwater fauna on these
two continents cannot be fully explained by one geological event.
North American freshwater fish zoogeography
A.
B.
Relict fishes of North America; several ancient fish families are present here
1.
Pangaea (Triassic, ca. 200 Ma; fig. 1.1, Hocutt and Wiley 1986) was
dominated by early Actinopterygii and Sarcopterygii.
a.
However, there are no extant sarcopterygians in N. America.
2.
Polyodontidae
a.
fossils limited to N. America, earliest are Cretaceous (Laurasia was
still a continuous landmass even into the Cenozoic; fig. 1.6, Hocutt
and Wiley)
b.
there are extant spp in China and Mississippi
3.
Gars and bowfins
a.
earliest fossils of gar are Cretaceous, present in S. and N. America,
Eurasia; now gar are present only in N. and central America
b.
earliest amiiforms are Jurassic, N. and S. America, Eurasia and
Africa; now there is only Amia calva, present in N. America
4.
Hiodontidae:
a.
Osteoglossiformes is present worldwide (fig. 16.11 Helfman et al).
Earliest fossils of the order are Jurassic
b.
Hiodontid fossils are known from the Cretaceous in China, but the
family is extant only in North America.
Ostariophysan zoogeography
1.
Gondwanaland, in Jurassic, dominated by ostariophysans. Their origin
was in the western part, Africa/South America.
2.
Diversification of catfishes and characins in Jurassic and Cretaceous
3.
Ictaluridae (endemic family): catfishes invaded Eurasia and N America in
Cretaceous. Ictalurid fossils are known from beginning of Cenozoic. This
family is closely related to Asian African Bagridae, so invasion from Asia.
4.
Catastomidae: (fig. 30-4 Bond) were a late arrival. N American fossils
mid-Eocene.
5.
Cyprinidae (fig. 30-5 Bond). Earliest fossils in Asia, Eocene. Earliest
fossils in Europe and N America are Oligocene. Persistent land bridge in
C.
Beringia may have permitted invasion, 32 ma. Even later invasion of
Africa
Zoogeography of the northeast
1.
Diversity in the area is pretty low; certainly relative to the Mississippi
drainage basin
2.
Why? Pleistocene glaciation (fig 30-7 Bond) completely cleared the area
of fish (see also fig 5.2 in Hocutt and Wiley 1986)
3.
Fish must have recolonized from glacial refugia. There were several
places that continued to have fish throughout. Obviously, down south; also
some unlikely places, such as a spot off the Georges Banks.
BIOLOGY 2210 LECTURE NOTES
SECTION 1. THE GNATHOSTOMES (THE JAWED VERTEBRATES)
GNATHOSTOMES
TWO SIGNIFICANT EVENTS IN EARLY VERTEBRATE EVOLUTION:
DEVELOPMENT OF JAWS.
DEVELOPMENT OF TWO PAIRS OF FINS (PECTORAL & PELVIC).
THESE ARE DERIVED TRAITS OF THE GNATHOSTOMES.
GNATHOSTOME LINE INCLUDES ALL LIVING VERTEBRATES EXCEPT
LAMPREYS.
FIG.
OLDEST FOSSIL GNATHOSTOMES FROM LATE ORDOVICIAN (440 MYBP),
BUT GNATHOSTOMATA PROBABLY EVOLVED FROM AGNATHAN ANCESTORS IN
LATE CAMBRIAN (>500 MYBP). EARLY GNATHOSTOMES HAD PROMINENT
NOTOCHORD AND LITTLE OSSIFICATION OF SKELETON. DID NOT
READILY FOSSILIZE.
OLDEST FOSSIL GNATHOSTOMES ARE ACANTHODIANS. APPEARED IN
FOSSIL RECORD ABOUT 100 MY AFTER FIRST OSTRACODERMS (RANGE:
440 MYBP-280 MYBP).
ACANTHODIANS WERE GENERALLY SMALL (~20 CM) MARINE FISHES WITH
ROWS OF DORSAL, VENTRAL, OR LATERAL SPINES.
FIG.
ACANTHODIANS HAD PROMINENT NOTOCHORD BUT ALSO HAD OSSIFIED
VERTEBRAE WITH NEURAL ARCHES AND NUMEROUS SMALL SCALES & SOME
DERMAL ARMOR.
FIG.
THE PLACODERMS WERE A FOSSIL GROUP THAT FIRST APPEARED IN THE
DEVONIAN SEVERAL M.Y. AFTER THE ACANTHODIANS (400 MYBP) AND
PERSISTED INTO THE CARBONIFEROUS (350 MYBP).
PLACODERMS HAD HEAVILY OSSIFIED DERMAL ARMOR THAT WAS
COMPLETELY FUSED TO FORM A HEAD SHIELD.
FIG.
PLACODERMS DOMINATED THE FISH FAUNA OF THE MIDDLE DEVONIAN
BUT WERE REPLACED BY THE CHONDRICHTHYES AND OSTEICHTHYES
LEAVING NO DIRECT DESCENDANTS.
LIVING GNATHOSTOMES
THE MOST PLEISOMORPHIC OF LIVING GNATHOSTOMES (MOST LIKE THE
COMMON ANCESTRAL GNATHOSTOME) ARE THE CHONDRICHTHYES (SHARKS,
RAYS, CHIMERAS; 800 SPECIES). APPEARED IN FOSSIL RECORD AT
SAME TIME AS FIRST PLACODERMS. LIVING CHONDRICHTHYANS ARE
GENERALLY LARGE ACTIVE PREDATORS OR BENTHIC FEEDERS.
CHONDRICHTHYES HAVE NO DERMAL EXOSKELETON, NO OSSIFICATION OF
THE ENDOSKELETON, NO SWIM BLADDER, GILLS WITH NO OPERCULUM
(EXCEPT CHIMERAS), AND CONTINUOUS TOOTH REPLACEMENT.
FIG.
THE CAUDAL FIN IN THE CHONDRICHTHYES IS TYPICALLY
HETEROCERCAL.
FIG.
THE SYSTEM OF PAIRED AND MEDIAL FINS IN FISHES SERVED
ORIGINALLY TO CONTROL MOVEMENT RATHER THAN PROVIDE PROPULSIVE
FORCE.
FIG.
MEDIAL FINS (DORSAL) CONTROL ROLL AND YAW; THE PAIRED FINS
(PECTORAL AND PELVIC) SERVE TO CONTROL PITCH, ROLL AND YAW
AND PROVIDE LIFT WHILE MOVING FORWARD (HYDROFOIL). A
HETEROCERCAL CAUDAL FIN PROVIDES FORWARD PROPULSION AND
PITCHES UP WHILE THE HEAD PITCHES DOWN. THE DOWNWARD PITCH OF
THE HEAD CAN BE COUNTERACTED BY THE ANGLE OF THE PECTORAL
FINS.
NEUTRAL BUOYANCY IN THE CHONDRICHTHYES IS ACHIEVED BY A LACK
OF OSSIFICATION AND A LARGE OILY LIVER THAT REDUCES OVERALL
DENSITY CLOSE TO THAT OF WATER.
THE CHONDRICHTHYAN JAW IS MADE UP OF THE MANDIBULAR ARCH
(PALATOQUADRATE AND MECKEL'S CARTILAGE) AND PART OF THE
HYOID ARCH (HYOMANDIBULAR).
FIG.
FERTILIZATION IS INTERNAL, MALES USE "CLASPERS" TO TRANSFER
SPERM TO THE CLOACA OF FEMALE. INTERNAL FERTILIZATION HAS LED
TO THE EVOLUTION OF OVOVIVIPARITY AND VIVIPARITY (LIVE
BEARING). VIVIPAROUS FORMS HAVE A FULLY FUNCTIONAL PLACENTA
ANALOGOUS TO THAT OF THE EUTHERIAN MAMMALS.
RAYS ARE ADAPTED TO A BENTHIC EXISTENCE AND HAVE A GREATLY
EXPANDED PECTORAL FIN.
CHIMAERAS ARE A SMALL GROUP (30 SPECIES) OF DEEP WATER
CARTILAGINOUS FISHES ABOUT WHICH LITTLE IS KNOWN (OPERCULUM
IS ANALGOUS TO THAT OF OSTEICHTHYES)
FIG.
OSTEICHTHYES (BONY FISHES+TETRAPODS)
FIRST APPEARED IN FOSSIL RECORD IN EARLY DEVONIAN, AT ABOUT
SAME TIME AS THE CHONDRICHTHYES (~400 MYBP).
THE OSTEICHTHYES HAVE AN OSSIFIED SKELETON WITH A WELL
DEVELOPED DERMATOCRANIUM (CF CHONDRICHTHYES).
FIG.
ASSOCIATED WITH SKELETAL OSSIFICATION IS A SWIM OR GAS
BLADDER WHICH COMPENSATES FOR THE HIGH DENSITY OF THEIR
OSSIFIED SKELETON ALLOWING THEM TO REMAIN NEUTRALLY BUOYANT.
FIG.
THE SWIM BLADDER ARISES AS AN EVAGINATION OF THE EMBRYONIC
GUT. AIR CAN BE ADDED TO OR REMOVED FROM THE SWIM BLADDER TO
MAINTAIN CONSTANT VOLUME (AND DENSITY) REGARDLESS OF DEPTH
(PRESSURE). AIR IS EXCHANGED WITH THE BLOOD (ADDED ON DESCENT
AND REMOVED ON ASCENT) THROUGH CAPILLARY NETWORKS.
DENSITY = MASS/VOLUME
DOWNWARD FORCE DUE TO GRAVITY (PER UNIT VOL.) IS PROPORTIONAL
TO DENSITY
IN WATER
BUOYANCY = UPWARD FORCE EQUAL TO WEIGHT OF DISPLACED WATER.
POSITIVE BUOYANCY = OBJECT IS LESS DENSE THAN WATER AND
FLOATS.
NEGATIVE BUOYANCY = OBJECT IS MORE DENSE THAN WATER AND
SINKS.
NEUTRAL BUOYANCY = OBJECT IS OF EQUAL DENSITY TO WATER AND IS
STATIONARY.
FIG.
NEUTRAL BUOYANCY IS ACHIEVED BY ALTERING BODY VOLUME (NOT
MASS).
NEUTRAL BUOYANCY IS UNSTABLE BECAUSE PRESSURE INCREASES (AND
VOL. OF BLADDER DECREASES) WITH DEPTH.
FIG.
THE VOLUME OF THE AIR BLADDER IS KEPT CONSTANT WITH CHANGES
IN DEPTH BY RELEASING AIR ON ASCENT AND ADDING AIR ON DESCENT
TO COMPENSATE FOR PRESSURE CHANGES WITH DEPTH (RATE OF CHANGE
IN DEPTH IS LIMITED BY GAS DIFFUSION RATE).
FIG.
EVOLUTION OF THE CRANIUM
CRANIUM IS COMPLEX IN STRUCTURE, EVOLUTIONARY ORIGINS, AND
MODIFICATIONS.
ORIGIN RELATED TO INCREASE IN CHORDATE BRAIN SIZE.
PROTECTS AND SUPPORTS BRAIN AND SENSE ORGANS OF HEAD.
CRANIUM OF OSTEICHTHYES CONTAINS 3 GROUPS OF BONES WITH
SEPARATE ORIGINS.
1. CHONDROCRANIUM (PLESIOMORPHIC CRANIUM)
THIS WAS THE FIRST COMPONENT OF THE CRANIUM TO EVOLVE AND
FORMS THE ENTIRE CRANIUM IN THE HAGFISH AND LAMPREY.
IN THE CHONDRICHTHYES THE CHONDROCRANIUM FORMS THE ENTIRE
BRAIN CASE.
THERE IS NO OSSIFICATION OF THE CHONDROCRANIUM IN THE
CHONDRICHTHYES.
IN THE OSTEICHTHYES THE CHONDROCRANIUM IS MORE OR LESS
OSSIFIED. IT IS ALSO ENCASED BY AND FUSED TO THE
DERMATOCRANIUM THAT FORMS MOST OF THE BRAIN CASE IN
THE OSTEICHTHYES.
2. SPLANCHNOCRANIUM
ORIGINATED AS THE PHARYNGEAL SKELETAL SUPPORTS FOR THE
PHARYNGEAL (GILL) ARCHES.
THE PLESIOMORPHIC NUMBER OF GILL ARCHES IS BELIEVED TO BE 7.
IN THE GNATHOSTOMES THE MOST ANTERIOR OF THESE ARCHES (1) IS
BELIEVED TO HAVE BEEN MODIFIED (SLOWLY THROUGH EVOLUTION) TO
PRODUCE THE ORIGINAL GNATHOSTOME JAW BONES (PALATOQUADRATE &
MANDIBULAR CARTILAGE). THE ADJOINING ARCH WAS ALSO
INCORPORATED INTO THE JAW AS A SUPPORTING STRUCTURE
(HYOMANDIBULAR).
THE REMAINING 5 ARCHES FORM THE SUPPORTS FOR THE GILLS.
IN THE CHONDRICHTHYES THE JAW RETAINS THIS PLESIOMORPHIC
ARRANGEMENT AND IS NOT OSSIFIED.
IN THE OSTEICHTHYES THE BONES OF THE SPLANCHNOCRANIUM ARE
REDUCED AND REPLACED AS THE PRIMARY JAW ELEMENTS BY BONES OF
THE DERMATOCRANIUM.
3. DERMATOCRANIUM
THE DERMATOCRANIUM CONSISTS OF OSSIFIED TISSUE PRODUCED BY
THE DERMAL LAYER OF THE SKIN. ORIGINATED AS SCALES WHICH
EXPANDED TO PRODUCE BONY HEAD ARMOR (E.G. OSTRACODERMS).
THE CHONDRICHTHYES HAVE NO DERMATOCRANIUM.
IN THE OSTEICHTHYES THE DERMAL BONES OF THE HEAD OVERLIE THE
BONES OF THE CHONDROCRANIUM AND SPLANCHNOCRANIUM, FUSE WITH
THESE BONES AND OFTEN REPLACE THEM.
PATTERN OF BONES IN DERMATOCRANIUM WAS VARIABLE AMONG TAXA
EARLY IN VERTEBRATE.
PATTERN BECAME STANDARDIZED EARLY IN ANCESTORS OF TETRAPODS.
FIG.
WHY DID SELECTION PRODUCE A DERMATOCRANIUM TO REPLACE
PREEXISTING CRANIAL STRUCTURES?
1. GROWTH & OSSIFICATION
INTERLOCKING PLATES OF DERMATOCRANIUM ALLOW CONTINUOUS GROWTH
OF OSSIFIED STRUCTURE. OSSIFIED CARTILAGE SPHERES MAY NOT
REQUIRE FUNDAMENTAL CHANGES IN PHYSIOLOGICAL PROCESSES.
2. TEETH
TEETH ARE CALCIFIED STRUCTURES THAT ARE DERIVED FROM SCALES
(DERMAL). THE SPLANCHNOCRANIUM CANNOT PRODUCE SCALES OF
TEETH.
IN CHONDRICHTHYES THE TEETH ARE PRODUCED BY A LAYER OF SKIN
THAT OVERLIES THE JAW BONES.
THIS MAY LIMIT THE POTENTIAL AMOUNT OF ANCHORING OF THE TEETH
IN THE JAW (SHARK TEETH FALL OUT CONTINUALLY)
DERMAL BONES OVERLYING THE SPLACHNOCRANIAL JAWS CAN PRODUCE
TEETH AND THESE CAN BE MORE FIRMLY ANCHORED TO THE DERMAL
BONES THAT PRODUCE THEM.
THIS MAY EXPLAIN WHY THE BONES OF THE MANDIBULAR ARCH
BEEN REPLACED BY TOOTH BEARING DERMAL BONES IN THE
OSTEICHTHYES.
HAVE
IN ADDITION TO DERMAL JAW BONES (MAXILLA AHD DENTARY) THE
OSTEICHTHYES HAVE AN OSSIFIED, HINGED DERMAL OPERCULUM
COVERING THE GILL ARCHES.
THE OSTEICHTHYES HAVE AN OSSIFIED DERMAL OPERCULUM COVERING
THE GILLS AND A DERMATOCRANIUM (DERMAL HEAD BONES DERIVED
FROM THE SKIN) THAT COMPLETELY ENCASES THE CRANIUM AND
REPLACES SOME BONES OF THE CHONDROCRANIUM (ORIGINAL CARTILAGE
CRANIUM AS IN CHONDRICHTHYES) AND SPLANCHNOCRANIUM
(PHARYNGEAL ARCHES MODIFIED TO PRODUCE LOWER AND UPPER JAWS
AS IN CHONDRICHTYHES).
THERE ARE TWO LINEAGES OF OSTEICHTHYES: ACTINOPTERYGIANS AND
SARCOPTERYGIANS. BOTH APPEARED IN THE FOSSIL RECORD AT ABOUT
THE SAME TIME.
FIG.
ACTINOPTERYGIANS (RAY FINNED FISHES)
THERE ARE TWO GROUPS OF ACTINOPTERYGIANS: THE PLESIOMORPHIC
CHONDROSTEANS, AND THE NEOPTERYGIANS WHICH DISPLAY A NUMBER
OF DERIVED TRAITS.
THE LIVING REPRESENTATIVES OF THE CHONDROSTEANS ARE THE
BICHER, THE PADDLE FISH, AND THE STURGEON.
FIG.
THERE ARE TWO LIVING GENERA OF PLESIOMORPHIC NEOPTERYGIANS;
THE GARS AND THE BOWFIN.
THE ADVANCED NEOPTERYGIANS OR TELEOSTEI ARE DIVIDED INTO FOUR
GROUPS:
1. OSTEOGLOSSOMORPHA
SMALL NUMBER OF TROPICAL FRESHWATER FISH INCLUDES THE LARGEST
OF THE STRICTLY FRESHWATER FISHES (ARAPAIMA >4.5M).
FIG.
2. ELOPOMORPHA
CHARACTERIZED
BY LEPTOCEPHALUS LARVAE. INCLUDE THE TARPONS
AND EELS. SOME EELS HAVE CATADRAMOUS LIFE CYCLES:
DIADRAMOUS - PART OF LIFE CYCLE IN FRESH AND PART IN
SALT WATER.
ANADRAMOUS - ADULT FORM DEVELOPS IN SALTWATER BUT EGGS
ARE LAID IN FRESHWATER.
CATADRAMOUS - ADULT FORM DEVELOPS IN FRESHWATER, EGGS
ARE LAID IN SALT WATER.
ANADRAMOUS LIFE CYCLES ARE MORE COMMON THAN CATADRAMOUS ONES.
FIG.
3. CLUPEOMORPHA
MOSTLY MARINE GROUP SPECIALIZED FOR FEEDING ON PLANKTON. E.G.
HERRING, ANCHOVIES. MANY ANADRAMOUS SPECIES.
4. EUTELEOSTEI
I. OSTARIOPHYSANS (>6500 SPECIES)
PREDOMINANT FISHES OF THE WORLD'S FRESHWATERS. NAME REFERS TO
SMALL BONES THAT CONNECT THE SWIM BLADDER TO THE INNER EAR TO
ENHANCE SOUND RECEPTION. E.G. CARP (LARGEST GROUP), MINNOWS,
CATFISH.
FIG.
II.SALMONIFORMS(=PROTOCANTHOPTERYGIANS)(500 SPECIES)
E.G. SALMON, TROUT, PIKES, SMELTS (CAPELIN), LANTERNFISH.
FIG.
III. PARACANTHOPTERYGIANS (1200 SPECIES)
E.G. CODS, MANY COMMERCIALY IMPORTANT MARINE FISHES
FIG.
IV. ACANTHOPTERYGIANS (>14,000 SPECIES)
(SPINY-FINNED FISHES)
E.G. MOST MARINE FISHES (IN SPECIES), PERCH, BASS, SUNFISH,
TUNA, MACKEREL, ALMOST ALL CORAL REEF FISHES.
CLADOGRAM
SARCOPTERYGIANS
THE OTHER MAJOR CLADE OF OSTEICHTHYES, THE SARCOPTERYGIANS,
APPEARED IN THE FOSSIL RECORD AT THE SAME TIME AS THE
ACTINOPTERYGIANS. THEY WERE ABUNDANT THROUGHOUT THE DEVONIAN
(40-350 MYBP), BUT HAVE DECLINED STEADILY SINCE (EXCEPT FOR
THE TETRAPOD LINE). TODAY THERE ARE ONLY FOUR GENERA - THREE
FRESHWATER LUNGFISHES AND THE MARINE LATIMERIA (7
SPECIES TOTAL).
FIG.
DERIVED TRAITS OF THE SARCOPTERYGIANS INCLUDE THE PRESENCE OF
INTERNAL OPENINGS TO THE MOUTH FROM THE NASAL SAC (CHOANA),
LUNGS, DOUBLE CIRCULATION, AND LOBED FINS.
THE LUNGFISHES DEPEND ON THEIR LUNGS (TO A GREATER OR LESSER
EXTENT) FOR RESPIRATION AND FEED PRIMARILY ON MOLLUSKS AND
CRUSTACEANS. THE AFRICAN LUNGFISH CAN SURVIVE PERIODS OF
DROUGHT BY BURROWING INTO THE SUBSTRATE, FORMING A WATERPROOF
CASING AROUND THEMSELVES AND GREATLY REDUCING THEIR METABOLIC
RATE. SOME INDIVIDUALS HAVE SURVIVED 4 YEARS IN THIS STATE.
THE SARCOPTERYGIANS GAVE RISE TO THE TETRAPODS IN THE LATE
DEVONIAN (~370 MYBP