Spring 2004
Lecture 21: April 22
Within the Phylum Chordates, we saw (lecture 20) evolution of the Craniates (Chordates with skulls), then of the Vertebrates (animals with backbones), than the Gnathostomes (jawed vertebrates), and finally of the Tetrapods (animals with 4 feet).
Within Tetrapods there are 2 large groups:
Ancestral forms: stem reptiles (Cotylosaurs).
- Anapsida: no holes in skull, eye sockets only
- Euryapsids: one hole in skull (each side), high up towards back of skull; probably derived from diapsids
- Diapsida: two holes (each side) in skull, one like euryapsids, one higher
- Synapsida: single hole (one each side), position as upper one in diapsids
Effect of development of 'holes': jaw muscles more sophisticated control, more force. Development led to explosive radiation in animals with different ways of eating, especially in the Synapsida.
Development from 'belly to ground' (legs to side of body) to belly lifted from ground, legs to side of body, to legs under body, changing posture from sprawling to upright. Separates muscle functions:
Separation: animal can walk and breathe at same time and move faster, longer.
3. Feeding efficiency/ hearing in air:
In animals with head close to ground jaw bones transferred vibrations from ground through jaw -> hearing. Yet another adaptation to life on land: hearing in air.
In addition to these endodermal bones, the bone called dentary (or mandible) became more and more important in the lower jaw, while upper jaw became fused to the skull; quadrate bone in lower jaw articulated on the articular bone in skull in most reptiles.
In mammals: jaw joint no longer between quadrate and articular, but between dentary (only bone left in lower jaw) and squamosum. The ancestral articulate bone evolved into the ear bone called hammer (malleus), the ancestral quadrate into ear bone called anvil (incus). Both send information to inner ear (otic capsule) via the stapes (derived from hyomandibular bone, see above). Mammals thus hear with their jaw bones, or: "With malleus aforethought mammals got an earful of their ancestors' jaw" (J. Burns, Biograffitti, 1975); see also this week's recommended reading by Gould. Click here for look at human ear bones.
but these are shared with other organisms (birds also endotherm), do not usually fossilize.
Typical for mammals, not shared by other organisms, and recognizable in fossils:
Note that Pelycoaurs through Cynodonts show large range in size, common 1-2 m long, large animals. Prototheria and first true mammals were very small, mousey-looking, probably insect-eating animals.
During evolution from Pelycosaurs to mammals: dentary jaw bone larger, other jaw bones loose jaw function, become part of ear. Oldest 'true mammal' very small (paperclip-sized): about 195 Ma (Early Jurassic), Hadrocodium wui.
These trends are usually documented using mammalian evolution. Birds are really very little modified dinosaurs, and cladisiticaly we just distinguish between 'avian dinosaurs' and 'non-avian dinosaurs':
Singing their songs without words
They're small ones, it's true
Warm and feathery too,
But they're there, and we call them birds.
(Cowen, History of Life, 1999).
Typical bird characters include the following. Characters followed by ? may have been shared by small dinosaurs.
Cladistic analyses have strongly confirmed that there are very close family relations between birds and what has been called dinosaurs; in fact, the two belong together as sister-groups, jointly named 'dinosaurs'. Then we distinguish between what we used to call birds and what we used to call dinosaurs by calling them 'avian dinosaurs' (=birds) and non-avian dinosaurs (=dinosaurs). The discovery of what might have been a fossilized heart in a dinosaur, suggesting that it has chambers such as a bird (or mammal) heart, should thus not have been a surprise .
The evolution of birds is much less documented by fossils than the evolution of mammals, because birds have thin, hollow bones which are not easily fossilized, and no teeth. Small changes occurred between birds and small dinosaurs, specifically the group called coelurosaurs and dromaeosaurs (similar to the small compys, Compsognathus, in Jurassic Park. Living birds are warm blooded, but small dinosaurs may also have been warm blooded. Birds have feathers (but scales on their feet), but some small dinosaurs are now known to also have had feathers. Many of these feathered dinosaurs were discovered (excellently preserved) in northeastern China; see the November 1999 volume of National Geographic. A particularly complete specimen was figured in the NY Times on April 26, 2001. Figure on the left, NY Times, April 26, 2001, p. A10: 130-million-year-old dromaeosaur fossil on display at the American Museum of Natural History. Figure on right: Reconstruction of living animal; Mick Ellison, American Museum of Natural History, as shown on front page of NY Times, 26 April 2001. Look at the web site of the Museum itself for more pictures.
 |
 |
Real birds have their two collar bones (clavicles) fused into a wishbone (furcula). They have a breast bone with a huge keel, for attaching the muscles for flying. They lack teeth, and have a beak of some horn-like material. Birds have only a very short set of fused vertebrae in their tail (called the pygostyle), whereas reptiles have long boney tails. The hand of birds has a very characteristic fusing of bones. Their small reptilian ancestors had three-fingered claws (thumb, index and middle finger). In birds the bones of these three fingers are fused and grown together, and in almost all modern birds the fingers carry no claws. Birds have very thin, hollow bones for weight reduction linked to flying.
There has been much discussion on the origin of flight in birds, as well as on the role of feathers. The oldest fossil recognizable as a bird-reptile intermediate is the famous Archaeopteryx from the German Jurassic Solnhofen lithographic limestone formation (about 150 million years old). This fossil had feathers; it would almost certainly have been called a small dinosaur if these feathers had not been preserved. It had teeth, a long reptilian tail, the hand bones had claws and were not yet fused; the breast bone had no keel, the bones were typical dinosaur bones, and not light and hollow as those of birds. But the collarbones were fused into a wishbone. A few years ago there was discussion thaty Archaeopteryx was a fraud, which is (to say the least) extremely unlikely.
There are some rather doubftul older (Triassic) fossils of what might have been a bird ancestor, but these (originally called Proavis) are no longer generally accepted as parts of one organism. This supposed ancestor of birds is now seen as a mixture of bones from various small dinosaurs. The cladistics of bird evolution are very poorly known (because of the poor fossil record). We do know that feathered dinosaurs overlapped in time (during the Jurassic and Early Cretaceous) with fairly modern-looking birds.
During the Cretaceous there was a strong increase in diversity of birds, with the development of the Enantiornithines (birds with a foot-construction different from modern birds), the Ornithurines (more old-fashioned types of birds but with a foot construction as in modern birds), and the Neornithes (the modern birds). The family relations between these groups are not at all well known because of the poor fossil record of birds. The two first groups became extinct at the Cretaceous/Tertiary boundary.
There is a discrepancy between genetic and fossil evidence, with no modern birds known as fossils from before the K/T boundary. Fossil evidence thus suggest a rapid radiation of birds coeval with the radiation of mammals. Genetic evidence, however, suggests that many modern families of birds (the ratites, including ostriches; the parrots, the wrens, the penguins, the shearwaters and loons, and the chickens) were probably in existence long before this extinction.
How did flight originate? There is no agreement . Some facts that need to be included in any theory are that fully-evolved feathers occur in specimens that do not have full adaptations for flying (such as Archaeopteryx and several of the feathered dinosaurs). Archaeopteryx had beautiful feathers, but must have been a poor flyer at best: the skeleton system could not have supported strong muscles for flying (no keel on the breast bone), the wings lacked the large primary feathers on the tips. A possible hypothesis includes feathers being used for insulation and keeping the small dinosaurs warm. We call such an adaptation, a feature that evolved for one purpose (e.g., keeping warm) but then turns out to be very useful for something else (flying) an exaptation. (It used to be called 'pre-adaptation' but that word was thought to sound too much as if it was thought of beforehand by the organism). This hypothesis suffers under the problem that feathers (even in the absence of flying structures in the skeleton existed) were not just downy fluff, but had the structure of modern flight-feathers (with small barbs on the feather-elements keeping the feather from splitting).
Origin of flight:
1. The arboreal hypothesis (down from the trees): a ground-running biped small dinosaurs became adapted to life in the trees, then leaping from branch to branch then parachute-type flight. Problems include the fact that small, running dinosaurs are not particularly well-adapted for life in the trees, and do no look like a modern 'glider' in overall structure. Claws on Archaeopteryx do not look like tree-climbing claws.
2. The cursorial hypothesis (up from the ground). Feathers develop first on the front edge of the wing, maybe to catch insects? Enlarge the surface area of the arms to intercept possible prey easier? Then the wings becomes functional as a device for generating lift-off.
3. The display and fight hypothesis (up from the ground). Feathers developed first for display, attracting birds of the opposite sex, and arms (proto-wings) were used in fighting for access to females. Arms were flapped in display and flight.
4. The pouncing hypothesis (down from somewhere). Proto-birds were catching small prey by pouncing down on it from rocks or trees. The feathers helped in producing more drag, and improved aerial control during the jump.
The origin of flight is thus not clear at all; paleontologists hope that the information gained from studying recently discovered feathered dinosaurs (China) will help, because of the possibility of studying the arm-structure in non-flying, feathered dinosaurs.