Vertebrate diversity and phylogeny (cladogram on handout)
Note origin in Cambrian
Origin of jaws (Gnathostomata)
Tetrapoda (4 limbs – terrestrial vertebrates)
Amniota – amniote or cleidoic egg. Hard or leathery egg that allows passage of gases (respiration) but not water. A membrane (the amnion) encloses the embryo, another collects waste, a yolk provides food. Freedom from aquatic development.
Skull types further subdivide the amniotes.
n note mammals branching from synapsids
n note birds branching from dinosaurs
Terrestrial (continental) diversity (Figure on handout)
Terrestrial diversity
Steady, one phase, exponential increase (contrast to marine). Effects of extinction not so evident.
Terrestrial evolutionary themes:
Early: Independence from water (exploiting a variety of habitats)
Later: Increased diversity. Diversification as an exponential process
Problems of terrestrialization
· Physical support and locomotion - greater weight without buoyant effect of water. Support of skeleton in animals; hydrostatic or woody support in plants
· Drying-out. Conserving water; preventing dessication; impervious skin in animals, waxy cuticle in plants
· Nutrient supply - unlike water, where many animals can extract food suspended in the water (bivalves, brachiopods, corals), air is much less dense and does not support a significant "plankton”. No filter-feeders; gotta go and get the food.
Similar problem in plants – can’t absorb nutrients directly from surrounding water; must come from roots in moist soil. Root system, vascular system for conducting water and nutrients.
Most scenarios for the transition to a terrestrial way of life involve a transition not from the marine environment to land, but from ephemeral lakes and ponds to land.
Not only does the direct evidence suggest this, but if you think about it, you should see that only in environments that periodically dried up - like many lakes, would adaptations for life on dry land be useful.
In other words, many of the adaptations that we recognize as adaptations for life on land were originally adaptations for surviving intervals of time when the lakes dried up, or to enable movement from pond to pond
There are two important messages about evolution here:
1. Evolution is opportunistic. There is no internal 'drive’ toward a goal of terrestrial life, only a taking advantage of new habitats.
2. Structures that evolved for one function (resistance to dessication- lungs,
impervious skin) may be used later in evolution for another function (terrestrial life)
The transition to land was made independently by many groups - and at different times.
Three important transitions:
1. The fossil record of the first terrestrial plants
Now, note that things like cyanobacteria (blue-green algae) are common on wet surfaces and can survive many episodes of drying out. These aren't usually counted as "true" terrestrial plants, though I'm sure they have been present since sometime in the Precambrian.
The earliest true land plants date from the Late Silurian.
They are termed true land plants because they clearly have what is called a vascular system: elongated dead cells that serve to conduct water up through narrow tubes.
2. Arthropods. Earliest record is from Late Silurian; though the best documented record is Early Devonian. Aquatic arthropods well-equipped for life on land: a hardened and waterproof exoskeleton limits drying out and walking legs provide mobility.
Spiders, mites, centipedes, insects.
Food chain probably based on decaying plant debris along shores of lakes and deltas.
3. Terrestrial vertebrates - Oldest amphibian fossil is Late Devonian in age.
derived from a group of fish called lobe-finned fish in that their fins are attached to their vertebral column with bones and muscles.
Ichthyostega and Acanthostega the no-so missing links: limbs and pelvic and pectoral girdle (hips and shoulders)
Note seven and eight digits.
n Amniotes (reptiles, mammals, birds) first appear in the Carboniferous. Stronger shoulder and pelvic girdles. Stronger limbs.
n Dinosaurs first appear in the Triassic, diversify rapidly, then extinct at the end of the Cretaceous.
n Class Aves (Jurassic - Recent) poor fossils; lightly constructed, often live in habitats not conducive to fossilization of their remains
n Class Mammalia (Triassic - Recent) Note that they originated early in the Mesozoic but did not diversify greatly until the Tertiary - after the demise of the dinosaurs.