What is a Dinosaur? 1) occured from the mid-late Triassic to the end
of the Cretaceous (220 mil. years ago, mya to 65 mya) 2) they are "reptiles",
(but as we know this is not a natural group), 3) they were terrestrial
(excludes marine plesiosaurs, ichthyosaurs, mososaurs, 4) Upright pillared
legs (an obvious structural "necessity" given their weight. Only
the latter is a good "defining" character, cladistically, since
there are plenty of other organisms that fit descriptions 1-3 that are
not dinosaurs. Moreover, some mammals and birds have pillared legs.
There are two general groups of dinosaurs based on hip morphology
The Saurischia (reptile-hipped) and the Ornithischia (bird-hipped).
In both groups the ilium and the ischium have relatively
similar forms, but in the Ornithiscia, the pubis has a narrow rod-shaped
extension running ventrally and posteriorly along the ventral side
of the ischium. In the Saurischia, the pubis extends ventrally and anteriorly
and only articulates with the ischium (and ilium) to form the hip "socket".
Most of the Ornithischia also have a horn covered beak and bony
rods and vertebral spines.
Within each of these two major groups there are further distinct types.
Within the Saurischia there are two major groups the Therapods (beast-foot)
and the Sauropods. Typical Therapods are Tyrannosaurus rex,
Deinonychus. These are carnivorous, have bird-like feet, bodies
are balanced at the hip with a long powerful tail. Within the Sauropods
are the huge species such as Apatosaurus (~Brontosaurus) and
Brachiosaurus. These walked on all fours, had long whip-like tails
and were herbivorous.
Within the Ornithischia there are five major groups:
Ornithopods e.g. Hadrosaurs, duck-billed dinosaurs
Ceratopians e.g., horned and frilled dinos such as Triceratops
Pachycephalosaurs with large bone-filled heads
Stegosaurs (e.g., Stegosaurus) with large dorsal spines of disputed use in thermoregulation
Ankylosaurs heavily armored and abundant in late Cretaceous
The only living relatives of Dinosaurs are Birds. From the names of
the two groups one might expect that the birds descended from the Ornithischians.
This is not the case. Birds are related to Therapod dinosaurs. The
living sister taxon to birds are Crocodylians; how do they fit in? You
probably think of Pterosaurs as dinosaurs, too, but they are not. Below
is a simplified cladogram of relationships.
Cloning Dinosaurs - Can it be done?
What do we need to do? What are the "parts" needed?
With the right combination of DNA and cell, it could work
BUT: a genome and a cell are remarkably complex "parts"
However, if you wanted to do it, Crichton's (Poinar/Wilson) approach
is a plausible one
Dinosaur DNA from fossil bones and cells of dinosaurs in the bodies of blood sucking insects trapped in amber
DNA extraction - remove tissue from amber with sterile tools, grind tissue in sterile homogenizing buffer, dehydrate and then dissolve in buffer solution.
Vector Cloning: cut DNA into pieces, splice fragments into a cloning vector, introduce vector+DNA into bacterial cells where many copies are made in cell culture
Remove Dino DNA from vector and reassemble DNA fragments by splicing (ligation)
Assemble complete chromosomes by filling gaps with frog DNA
PCR (Polymerase Chain Reaction): Amplify random fragments of DNA, then
Genome: If we do get DNA:
A. what species of DNA is it? = A molecular systematics problem
identify DNA by its affiliation to putative extant relatives
characters, shared derived character states, group membership
Could be different parts of different taxa: DNA extracted could be a mixed bag of different dinosaur species and other vertebrate taxa different. Probably not enough taxonomic resolution to determine the species identity of the DNA sequence. This is important: how do you tell whether a chunk of DNA is Tyrannosaurus or Stegosaurus simply based on phylogeny of these sequences if there are no living members of either linage to provide a basis for discrimination
Here's where bones are key: species (at least genus?) identify available
AND the DNA for the same sample.
B. What part and percent of the genome is it?
Coding DNA and Genome Size, C-value paradox, Numerology:
Cell: Which species of cell?
How do the proteins and other molecules of one species interact with the DNA/genes of another species (will Dino+frog DNA function properly in modified crocodile eggs??)
DNA as a generative program: does the DNA alone cary the info. to tell any old cell how to make a dinosaur? Development involved many important tissue inductive events.
Tissue specific gene expression: Liver cells express liver genes, blood cells express blood genes
De-differentiation: easy in plants; rare in animals. Mitotic arrest:
who says the cells will keep on dividing? These are Big problems; it will
be some time before we clone dinos.
Of all the problems, the Phylogenetic problem of knowing the species identity of any cell from an insect's gut is as serious as the technical problem of getting the DNA in the first place! A reconstructd dinosaur might well wind up as part Ceratopian, part Sauropod and part Therapod (assuming mosquitos were not super host-specific).