Transcript Origin of Craniates & Vertebrates

The Phylum Chordata
• This phylum has 3 subphyla
- Urochordata
- Cephalochordata
- Craniata
• Chordates are deuterostomes
(sensu strictu)
• Possess four unique characteristics
1) Dorsal Hollow Nerve Cord
• Forms from a dorsal ectoderm tube
• Forms the CNS (brain & spinal cord)
• Other animal phyla have ventral solid
nerve cord(s)
2) Notochord
• Longitudinal, flexible cartilagenous rod
• Located between nerve cord and gut
2) Notochord
• Extends most of the length of the organism
• A simple ENDOSKELETON
• For some chordates this is all they have (thus
‘invertebrate’ chordates)
• VERTEBRATE chordates develop a more
complex jointed skeleton
• BUT vertebrates still have vestiges of the
notochord (intervertebral discs !)
3) Pharyngeal Slits
• Early chordates – these connected to the
digestive tract; used for FILTER FEEDING
• Later became modified for gas exchange
• Present (vestigial) in our early embryo stage
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4) Postanal Tail
4) Postanal Tail
• Digestive tract extends most of the body
length in the majority of non-chordates
• Tail extends beyond the anus in chordates
• Contains skeletal elements
• Contains muscles
• Major propulsive force in many aquatic
chordates
• Other uses for the tail ?????
Subphylum Urochordata
• Invertebrate chordates
• Called Tunicates (some called sea squirts)
• Tunic is made of a cellulose-like
carbohydrate called tunicin
• Filter Feeders
• Free swimming as larvae
• Sessile as adults (adhere by ‘head’ region
to objects)
Subphylum Urochordata
• Highly modified as adults
• Scarcely resemble other chordates
• During metamorphosis they lose the tail,
notochord & nerve cord (‘brain’ atrophies)
• Retain pharyngeal slits
• Closed circulatory system
• Possess heart & blood cells (no hemoglobin)
• Most primitive of the chordates (?????)
Subphylum Urochordata (larva)
Subphylum Urochordata (adult)
Subphylum Cephalochordata
• Lancelets
• Laterally compressed
Subphylum Cephalochordata
• Adults somewhat resemble urochordate
larvae
• All 4 chordate traits persist
• Filter feeders; tentacles around mouth
• Marine; burrow tail first
• Feeble swimmers; fish-like movements
(sinusoidal)
• Serially arranged muscle segments
Origin of Craniates & Vertebrates
• Fossils – resembling Cephalochordates – Burgess
Shale of British Columbia ~550mya
• Craniates and Vertebrates first appear during the
Cambrian “explosion” ~ 530mya
• Early Craniates and Vertebrates possessed all four
chordate characteristics and were filter feeders
• Probably derived from a Urochordate-like ancestor
similar to a tunicate larva
• Paedogenesis resulted in a larva achieving sexual
maturity and it did not undergo metamorphosis
Paedogenesis
• Precocious attainment of sexual maturity in
a morphologically juvenile organism
• If successful, natural selection would have
reinforced the absence of metamorphosis
• These larvae were active and natural
selection would favor the most active
• Actively foraging organisms benefit from
good sense organs
• Cephalization is a benefit
Craniate / Vertebrate Characteristics
• All possess the four basic chordate traits (at least
at some stage)
• Cephalization with a highly specialized brain
• The brain is covered by or enclosed by a skull
• MOST Craniates possess a vertebral column that
encloses the nerve cord
• MOST possess jaws
• Skeleton is an endoskeleton
The Endoskeleton
• Composed of cartilage, bone or both
• Grows with the organism
• Cells secrete and rearrange the matrix
• AXIAL SKELETON - skull plus vertebral
column (plus ribs & breastbone, if present)
• APPENDICULAR SKELETON may be
present (supports fins or limbs)
Additional Traits (I)
• Closed circulatory system
• Heart with 2, 3 or 4 chambers
• Arteries, capillaries, veins
• Red blood cells (with hemoglobin) for O2
transport
• Blood oxygenated through skin or more
commonly via gills or lungs
Additional Traits (II)
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Possess kidneys (compact excretory structures)
Reproduction usually sexual
Dioecious (= unisexual)
Gender control variable (XY, WZ,
environmental)
• Some can change gender
• Parthenogenesis found in most classes (but
not common)
Subphylum Craniata
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Nine or ten extant classes
Mixini
Petromyzontidae
Chrondrichthyes
Antinopterygii, Actinistia, Dipnoi
Amphibia
Reptilia
Aves (?)
Mammalia
The Agnathan Classes (I)
• Oldest fossilized craniates were agnathans
• Mud-suckers/filter feeders
• Living forms lack paired appendages and
external armor
• No covers for gill slits
• Two-chambered heart
• Class Mixini (hagfishes)
• Class Petromyzontida (lampreys)
The Agnathan Classes (II)
• Hagfishes (30 species) are eel shaped
• Lack rasping mouthparts
• Some feed on sick or dead fish or on worms
• Marine
• Skull of cartilage
• No vertebrae (are “craniate invertebrates”)
Hagfish
The Agnathan Classes (III)
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Lampreys (about 35 species) are eel shaped
Young are usually suspension (filter) feeders
Young live in freshwater
Adults usually marine but some stay in freshwater
Adults have rasping mouth parts
Attach to live fish and are parasitic
Possess a cartilaginous tube around the notochord
with pairs of projections (Vertebrae); thus a
“craniate vertebrate”)
Lamprey (I)
Lamprey (II)
Lamprey (III)
The First Gnathans
• Classes Placodermi and Acanthodii (extinct)
• Possessed hinged jaws and paired fins
• Jaws developed from skeletal rods supporting
some of the more anterior pharyngeal slits
• Jaws allow for more varied feeding habits
• Vertebrate jaws work UP and DOWN
(arthropod jaws work Side to Side)
• Remaining slits functioned as major gas
exchange sites
Basic Craniate (from text)
Development of Jaws
(from arch supports 3 & 4)
Jaws and Supports
Acanthodian
Placoderm
Class Chondrichthyes (I)
• The cartilaginous fishes (about 750 species)
• Lack of bone in skeleton is considered a
derived condition
• Sharks, rays, skates, and more
• No swim bladder
• Must swim to stay up in the water column
• Some have added buoyancy due to large
amounts of oil stored in the liver
Class Chondrichthyes (II)
• Marine animals – evolved in the sea
• Paired fins, well-developed jaws; most are
carnivores (a few filter feeders)
• No opercula (external gill covers)
• Scaled; Teeth evolved from modified scales
• Reproduce sexually; fertilization internal;
dioecious
• Oviparous, Ovoviviparous or Viviparous
• Claspers on male’s pectoral fins transfer
sperm into the female’s reproductive tract
Class Chondrichthyes (III)
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Cloaca present
Heart two-chambered
Short digestive tract; spiral valve present
Good vision but no color vision
Sharks have a fusiform body
Rays and skates are dorsoventrally
flattened
Black Tipped Reef Shark
Ray
The “Bony” Fishes
• Formerly in one class (Osteichthyes) but now
divided into three classes
• Cloaca absent in all three classes; dioecious; heart
is 2-chambered
• Class Actinistia – coelacanths (believed to have
been extinct for 65 million years) – Two (?) extant
species; a lobe-fin group that evolved in freshwater
and then moved to the ocean
• Class Dipnoi – lungfishes – another lobe-fin group;
3 genera and 7 species; Freshwater; use lungs for
respiration; Gave rise to tetrapods
Coelacanth
Dipnoi (Australian Lungfish)
Class Antinopterygii
• Largest group of vertebrates (~30,000 spp)
• Most of our familiar fish species
• Body covered by flattened bony scales
• Evolved in freshwater; many moved to the
ocean; some have returned to freshwater
• A few species spend a portion of their lives
in freshwater and in saltwater
• Possess a swim bladder for buoyancy – it
developed from lungs
Rainbow Trout
Evolution of lungs & Swim Bladder
Gas Exchange in Fishes
• Chondrichthyes – swimming important –
forces water into mouth and out over gills
• Bony Fish – water drawn into mouth and
forced out over gills by movement of the
OPERCULUM and contraction of muscles
within the gill chambers