GEOL 104 Dinosaurs: A Natural History
Fall Semester 2000
Life on Land before the Dinosaurs
Dinosaurs were not the first major group of animals to dominate the surface of the Earth.
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.
Problems with living on land:
- Desiccation (air is dry)
- Gravity (air is nowhere near as bouyant)
- Reproduction (sex cells need to be kept wet)
- Breathing (getting oxygen as a gas is different than getting dissolved oxygen from water)
- Senses (sight, smell, and touch similar on land & water, but pressure sense and
electric sense don't work in air, and hearing is very different)
Different groups of organisms did not solve all problems at once.
Vertebrate solutions to problems:
Lungs were already widely distributed among fish:
Primitive bony fish of today still have simple lungs
May have evolved in fresh-water environments; often periods of time when oxygen levels get
low in lakes, so having extra method of breathing is useful
NOTE: "lungfish" (Dipnoi) are just one sort of fish with lungs; some other fish still have
working lungs, and the swim bladders of typical ray-finned fish are modied lungs!
Stegocephalia (vertebrates with digits):
Already had limbs with bone cores (as do the closest relatives of stegocephalians: coelocanths,
lungfish, and some extinct groups)
The earliest (mid-Paleozoic) stegocephalians still retained internal gills
Primitive stegocephalians were still mostly aquatics, but could crawl around on land:
- Useful when lakes might dry up: able to haul themselves to a wet pond.
- Lungfish use a different strategy ("hibernate" in mud at bottom of pond until wet season)
More advanced stegocephalians developed stronger limb girdles, better (fewer)
stronger digits: could spend long times on land, but still had to go into water to
reproduce. These were the first Tetrapoda. Modern amphibians retain the necessity
to go to water to reproduce, but spend a lot of time on land.
During the later Paleozoic Era: huge coal swamps, home to abundant plant and arthropods: potential food
for various tetrapods.
Some tetrapods developed more specializations for life on land:
- Even stronger limbs and girdles
- Claws
- Amniotic eggs: eggs with a shell and an internal “pond” or amnion, so that eggs
could be laid on land
These were the first members of Amniota.
The amniote egg allowed for an adaptive radiation:
Three main divisions within Amniota:
Synapsida, with infratemporal fenestra only
Anapsida, with no temporal fenestrae
Diapsida, with both infra- and supratemporal fenestra.
Cladistic studies place Anapsida and Diapsida as sister taxa within Reptilia.
Synapsids form the first major radiation of amnitoes.
1rst Amniote Radiation – Basal Synapsids:
During the Early Permian Epoch of the Paleozoic Era
Basal synapsids once placed in a paraphyletic group “Pelycosauria”, the “fin backs”
Included carnivores, insectivores, herbivores, and omnivores
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:
Began in the Late Permian Epoch
Once called “mammal-like reptiles”, but in fact are not part of Reptilia
Some evidence suggests Paleozoic and Triassic therapsids may have had hair
Therapsids had many specializations beyond the basal condition:
- 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 therapsids in the 2nd Radiation included many specialized carnivores, herbivores,
omnivores, insectivores, etc. All were quadrupedal; biggest were about the size of oxen.
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 lost
- Probably due in part to huge volcanic eruptions (largest in last half billion
years), with other factors adding to it
- No evidence of asteroid impact associated with it
However, Early Triassic Epoch is still effectively part of 2nd Amniote Radiation, since
therapsids dominate.
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
One of the two branches of Archosauria, Pseudosuchia, becomes more important
throughout Early Triassic.
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