GEOL 104 Dinosaurs: A Natural History
Fall Semester 2006
Life on Land before the Dinosaurs
Surface of the Earth (and even position of continents) changed radically over Earth history:
- Plate tectonics: continental (and smaller) plates move
- Sea level changes: due to tectonics, ice ages, etc.
First organisms on land would have been various forms of algae back in the Precambrian. Plants
colonized land in early part of Paleozoic; arthropods (arachnids, millipedes, insects)
followed.
Why should vertebrates colonize land?
- Untapped food sources
- Hide from predators
- Move from pond to pond in times of drought
- Etc.
Some of the problems with living on land:
- Breathing
- Desiccation
- Gravity
- Reproduction
Vertebrate solutions to some problems:
- Lungs (synapomorphy for Osteichthyes)
- Various forms of skin protection (scales, mucous, aglandular skin)
- Strong limbs (synapomorphy for Stegocephalia), strong limb girdles,
complex digits (synapomorphies for advanced stegocephalians), claws (synapomorphy
of amniotes)
- Amniotic eggs (synapomorphy for amniotes)
1rst Amniote Radiation – Basal Synapsids:
Early Permian Epoch of the Paleozoic Era
Basal synapsids once placed in a paraphyletic group "Pelycosauria", the "fin backs"
All quadrupeds, all had sprawling gait
Most advanced basal synapsids had teeth differentiated between front of mouth and back of mouth
Anapsids and diapsids were present, but minor component of ecosystem.
Basal synapsids evolved themselves to extinction: were replaced by their own descendants,
the Therapsida.
2nd Amniote Radiation – Therapsids:
Late Permian Epoch of the Paleozoic Era into Early Triassic Epoch of the
Mesozoic Era
Once called “mammal-like reptiles”, but NOT part of Reptilia
Therapsids had many specializations:
- Elaborated the differentiated teeth into nippers, biters, and crushers
- Infratemporal fenestra became quite large
- Post-dentary mandible bones reduced in size
- Forelimbs become better developed than hindlimbs: "front wheel drive"
The end of the Permian Period (and thus the Paleozoic Era) and the beginning of the
Triassic Period (and thus the Mesozoic Era) (boundary at 251 Ma):
- Biggest mass extinction in last half billion years
- Tremendous extinction of marine invertebates
- Many groups of therapsids and other tetrapods lost
- Probably due in part to huge volcanic eruptions (largest in last half billion
years), and possibly comet impact, with other factors adding to it
- Primary killing factor on land seems to be crash in oxygen levels; in sea, too high carbon dioxide levels
Anapsids and diapsids still present, but relatively minor components. However, one type
of diapsid becoming more important in Early Triassic:
Archosaurs:
- Characterized by mandibular and antorbital fenestrae
- Like other diapsids, hindlimbs are longer and better developed than forelimbs:
"rear wheel drive"
- Could probably rear up on hindlegs when running quickly
- May have had specialized "belly breathing"
One of the two branches of Archosauria, Pseudosuchia, becomes more important
throughout Early Triassic.
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Last modified: 14 July 2006
