Tetrapods: Origins & RadiationPART III
VERTEBRATE ZOOLOGY (VZ Lecture11 – Fall 2012 Althoff - reference PJH Chapter 9)
Tetrapod Origins
• Earliest-known tetrapods are from the Late Devonian …~360 MYA
• Genus Acanthostega is now considered the earliest tetrapod…replacing Ichthyostega…found skull and skeletal remains in Greenland area and other subsequent finds in Latvia, Scotland, Australia, Asia, and North America
• Most recent analysis suggest these early tetrapods were more fish-like than originally believed
• Two major lineages that emerge after that were the batrachomorphs and reptilomorphs
By start of Cenozoic….
• ~60 MYA
• Only remaining nonamiotic tetrapods were the lineages still present today:
frogssalamanders
caecilians
• Since late Paleozoic, amniotes have dominated
SIGHTLESS HEAD SMOOTH SKIN
Caecilia nigncans
Batrachomorphs
• Early Carboniferous 354-290 MYA
• Included temnospondyls…the largest and longest lasting group of primitive, extinct _____________ tetrapods (Fig. 9-1, p198)
• Thus, another example of a “successful” group at one time…but not around today
Reptilomorphs
• Early Carboniferous 354-290 MYA
• Both ____________ and _____________
• Small, agile animals. Jaw design that suggested they fed on terrestrial invertebrates
Fig. 9-7 p207 PJH
Acanthostega
Icthyostega
Which one of these tetrapods has more “aquatic” form?1
1a?
2
2a4a
45
5a
3
3a
More Aquatic vs. More Terrestrial Form
Forelimb
Zygapophyses
Ribs
Sacral connection
Tail fin
Acanthostega Icthyostega
Fig. 9-12 p213 PJH
Amniotic egg: progressive development
Characteristic of: turtles, lepidosaurs (lizards & relatives), crocodilians, birds, & monotremes
Placental Mammals: Beyond Amniotic Egg
• Mammals still considered “amniotes”
• Egg-laying mammals (monotremes…duck-billed platypus) have amniotic egg
• Marsupials…transition to true placentals
• For placental mammals, the placenta now is ___________ to certain membranes (chorionic and allantoic) in the amniotic” egg…and yolk is absent because nourishment (and removal of metabolic waste) is accomplished by the placenta). More about this when we examine mammalian reproduction in detail
Fig. 9-15 p217PJH
Anapsid(primitive amniotes
& turtles)
Synapsid(mammals)
Cross-sectional view
Lateral view
Adductormuscle
Adductormuscle
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