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