Lec 8/9 Flashcards
Evolution of Jaws: Origination
First appear in Placoderms and acanthodian fishes
Early Jawed Vertebrates
Placoderms “plate skin”
- -Appeared >400mya but NO descendents
- -Some got large (10m)
Acanthodians
- -Contemporary with placoderms but persisted longer
- -Clade possibly gave rise to extant cartilaginous or bony fishes (maybe paraphyletic)
Major Clades of Gnathostomes
Placodermi - Placoderms (extinct)
Chondrichthyes - sharks, rays, chimaeras
Acanthodii (extinct)
–UNCERTAIN placement on tree
Osteichthyes - bony fishes (paraphyletic)
–Historically did not include tetrapods
Tetrapoda
–Humans closer to goldfish than those goldfish are to sharks
Approximately HALF of described extant chordate species are fishes
Class Chondrichthyes
2 major groups:
1) Elasmobranchii: Sharks and rays
2) Holocephali: Chimaeras
Cartilaginous skeleton (NOT a synapomorphy)
~1200 species
External gill slits (operculum in holocephalians)
LACK lungs/gas bladder
SYNAPOMPORHY:
- -Internal fertilization via claspers in males
- –Claspers: type of intromittent organ
- —Placoderm fossils show vivipary; most basal EXTINCT lineage with internal fertilization
- -Placoid scales
Oviparous or viviparous
Lateral line system and electroreception (various other lineages too)
Teeth:
- -Homologous to placoid scales
- –Developmentally derived from skin denticles (epidermal and dermal components)
Spiral valve in intestine:
- -Increases time food is in intestine to prolong digestion (also some other lineages - Petromyzontiformes, basal bony fishes, lungfish)
- -UNCLEAR origination; NOT a synap
Chondrichthyes: Elasmobranchii
Sharks (45%) and rays (55%)
Appear ~420mya (good fossils by 370mya)
–Teeth fossilize very well (enamel is hardest substance in chordates)
~1150 species
Body fusiform in SHARKS
Body depressed (dorsoventrally flattened) in RAYS
Heterocercal caudal fin with upturned vertebral column for support
–Lateral ungulations with tail: Upturned tail prevents sinking
Ventral mouth
–Most fish have terminal mouths: sharks and rays have rostrum at end
Spiracle (ancestral: gnathostome synap)
- -Constricted opening homologous to first pharyngeal slit
- -Present in rays and SOME sharks
- -Unobstructed water intake for gill ventilation in rays
- -Removes substrate for rays (bottom feeders)
- -Purpose UNCLEAR in sharks
Sharks:
–Range in size from 25cm (green dogfish) to >12m and >20 tons (whale shark)
Rays:
- -Includes skates (one group of rays)
- -Benthic in shalow waters
- -Dorsoventrally flattened (depressed)
- -Wings are enlarged pectoral fins and locomotion via waves through fins
- -Teeth flattened for crushing hard invertebrates (manta ray is filter feeder)
- –Molariform teeth: Flat, used to crush
- -Tail defensive lash in some (e.g. stingrays)
- –Hard cartilaginous structure contains VENOM
Chondrichthyes: Holocephali
Chimaeras or ratfishes
Sister to sharks/rays
–Split ~420mya
~40 extant species
Retain notochord (REVERSAL)
Venomous dorsal spine in some
Lateral Line System
Long recessed grooves with neuromasts
Also in other fishes and aquatic amphibians (larval and some adult)
Arises from ectodermal placodes (vert synap)
Functions in mechanoreception
- -Info on direction of movement by detecting water movement along body
- -Prey and/or predator detection
- -Navigation (blind cave fish)
Can be thought of as a PAIRED sensory system
Neuromasts provide mechanoreception
–These neuromasts have hair cells for detection and acts as mechanoreception (bending of hairs)
Neuromasts with hair cells
Gelatinous cap over hair cells
Movement of hair cells changes receptor membrane potential thus providing neural signal
Signal sent to sensory nerve and brain
Lineages with lateral line system
Cyclostomata
Ostracaderms and placoderms
Chondrichthyes
Osteichthyes “bony fishes”
Stem tetrapods (extinct ones)
Extant emphibians:
–Only aquatic stages and some specialized adult aquatic forms
Lost in lineage leading to Amniota
- -CROWN is EXTANT only and is a clade
- -NO amniote has a lateral line
Electroreception
The ability to detect electric potentials
Animals give off small electric potentials during neuromuscular activity
- -VERY small
- -Primary role: Prey detection
- -Secondary: Navigation and communication
- -Likely role in navigation (sensing Earth’s magnetic field) in some taxa
Present in:
- -Petromyzontiformes (but NOT myxini)
- -Chondrichthyes
- -Some osteichthyes
- –Retained in basal lineages of Actinopterygii, then lost, then evolved at least 2X in teleosts
- –Extant non-tetrapod Sarcopterygii
- -Some extant amphibians (aquatic)
- -Lost in lineage leading to Amniota
- –Evolved independenlty (with different underlying mechanisms) in platypus (on bill) and dolphins (some)
- —Evolutionary REVERSAL and ANALOGY: Analogous to electroreceptive cells
Electroreception: Ampullae of Lorenzini
Specialized sensory cells in pits along snout and head
Pits filled with gelatin (good conductor)
Detect electric currents
Impulses carried to brain
Electric Organs: Electrical Discharge
A lineage of rays and some lineages of actinopterygians have evolved ability to discharge high-amperage currents
Electrocytes: Electricity producing cells
- -Modified muscle cells (homologous to muscle cells) concentrated into masses called electric organs
- —Retain electric potentials only
- -Do not contract like typical muscles
- -Cells arranged in stacks that discharge (depolarize) simultaneously: thousands discharge at once, one won’t harm prey
Electric rays (Torpedo) generate high electrical output (nearly one kW)
Primary Functions of Electric Organs
Stun prey
Protection from predators
Secondary: Navigation and communication
Animals with Electric Organs
Chondrichthyes
-Electric ray (Torpedo)
Actinopterygii:
- Electric catfish (Africa)
- Electric eel (South America)
- Stargazer (eastern seaboard)
ALL INDEPENDENTLY EVOLVED
NOT due to common ancestry
Combined Electrical Discharge and Electroreception
Some species generate relatively weak pulsating electric field to NAVIGATE (electrolocalization)
Some species use weak electrical signals to COMMUNICATE (i.e. females and males may discharge different signals)
Reproduction in Chondrichthyes
Internal fertilization:
- Males with specialized pelvic claspers extending from cloaca (synap)
- Clasper inserted into female’s cloaca for fertilization
