Zoogeography Principles, definitions, processes



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Zoogeography

  1. Principles, definitions, processes

    1. Study of bio/zoogeography

      1. There are two goals in zoogeography:

        1. Delineation and characterization of faunal areas

        2. Determine the evolutionary history of these faunas, in geographical context

      2. Two general processes shape the geographic distribution of taxa:

        1. Historical factors: evolutionary origin and spread of taxon

        2. Ecological factors: distribution of abiotic and biotic factors that influence distribution

        3. We will focus for today on historical zoogeography

    2. 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

    3. Historical processes influencing distributions

      1. Dispersal

        1. The distribution of a taxon and its subtaxa are determined by movement to new areas

        2. Freshwater fish may disperse:

          1. freshwater or overland dispersal:

            1. rivers change course

            2. stream capture, as headwater erosion cuts through watershed boundary

            3. primary freshwater fishes are groups strictly confined to freshwater, must disperse this way

          2. dispersal via marine waters

            1. secondary freshwater fishes are usually limited to freshwater but occasional individuals or species may be estuarine or marine;

            2. peripheral fishes have evolved freshwater lifestyle from marine group, or migrate between (diadromy)

            3. freshwater fishes on isolated oceanic islands are secondary or peripheral; gobies on Hawaii, Dominica in Caribbean

            4. Example: Plotosidae (catfish) is a marine group that has secondarily invaded freshwater in Australia, New Guinea. Was this secondary or peripheral?

        3. Marine fish can certainly disperse

        4. Dispersal involves random events, acting on individual taxa

      2. Vicariance

        1. distribution of a taxon and its subtaxa are determined by the formation of geographic barriers causing speciation.

        2. continental breakup and drift (freshwater fishes)

        3. 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.

        4. These events are likely to occur to many taxa

      3. Competing ideas

        1. These are two basic models for how distributions arise

        2. Both likely to be required for full explanation of any taxon; but lively debate on relative importance

        3. Classically, explanations were dispersalist

        4. Relatively modern evidence for continental drift gave life to vicariant hypotheses

    4. 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

        1. Paleodistribution

        2. establishing a minimum age

  2. Test of a vicariance hypothesis: the fishes of Africa and S. America

    1. These continents share a large number of taxa (table 8.1 Lundberg 1993)

    2. 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

    3. Geological evidence for split between continents: it occurred sometime between 106 and 84 Ma, in the mid-Cretaceous

    4. Lungfishes: support drift/vicariance model

      1. There is a well supported phylogeny (fig 8.2, Lundberg)

      2. 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

    5. 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

    6. Bottom line (again, review Table 8.1); diversification of freshwater fauna on these two continents cannot be fully explained by one geological event.

  3. North American freshwater fish zoogeography

    1. 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.

        1. However, there are no extant sarcopterygians in N. America.

      2. Polyodontidae

        1. fossils limited to N. America, earliest are Cretaceous (Laurasia was still a continuous landmass even into the Cenozoic; fig. 1.6, Hocutt and Wiley)

        2. there are extant spp in China and Mississippi

      3. Gars and bowfins

        1. earliest fossils of gar are Cretaceous, present in S. and N. America, Eurasia; now gar are present only in N. and central America

        2. earliest amiiforms are Jurassic, N. and S. America, Eurasia and Africa; now there is only Amia calva, present in N. America

      4. Hiodontidae:

        1. Osteoglossiformes is present worldwide (fig. 16.11 Helfman et al). Earliest fossils of the order are Jurassic

        2. Hiodontid fossils are known from the Cretaceous in China, but the family is extant only in North America.

    2. 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 Beringia may have permitted invasion, 32 ma. Even later invasion of Africa

    3. 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 HAVE

BEEN REPLACED BY TOOTH BEARING DERMAL BONES IN THE

OSTEICHTHYES.
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


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