From _Science_, Vol. 262, Dec. 24, 1993 A Closer Look at the Dinosaur-Bird Link Arrangemen

           From _Science_, Vol. 262, Dec. 24, 1993
                              
           A Closer Look at the Dinosaur-Bird Link

     Arrangements for family reunions, even for partly
extinct families, rarely go completely smoothly.  Over the
past several years, dinosaur paleontologists have built a
case that birds--not reptiles--are dinosaurs' closest living
relatives by comparing the shapes of arms, shoulders, and
claws.  While this evidence is strong for one of the two
main dinosaur groups, researchers have had difficulty
fitting the second group into this family picture.  Now
comes evidence bringing that group into the avian fold, but
the link is microscopic: the shapes of ancient cells.
     Those cells belonged to a juvenile Maiasaurus, a duck-
billed bipedal dinosaur--and a member of the disenfranchised
group, known as the Ornithischia--that lived in Montana some
72 million years ago.  On p. 2020 of this issue,
paleontologist Claudia Barreto of the University of
Wisconsin's School of Veterninary Medicine in Madison and
her colleagues report that cells within the dinosaur's
growth plates--discs of cartilage near the ends of bones
that allow bones to grow--bear a striking resemblance to the
cells of chicken growth plates.  And they look very
different from those of the growth plates of contemporary
reptiles.  "This work is very careful, very cautious, and
very convincing," says paleontologist Kevin Padian of the
University of California, Berkeley.  "It means people can no
longer say that dinosaurs are like reptiles because here
they're doing things that we know only birds do."
     Growth plates are made up of cells called chondrocytes.
At the actual bone, chondrocytes die off, leaving behind
their calcified extracellular matrix to serve as scaffolding
for the osteoblasts (bone-forming cells) and blood vessels
as the push into new territory.  Using a light microscope,
Barreto's team compared the Maiasaurus growth plates to
those from a dog, a monitor lizard, and a chicken.  They
found that the plate-bone boundary of the dinosaur was very
irregular, undulating up and down just as it does in
contemporary birds.  In contrast, the boundary zone in
mammals and reptiles forms a straight line.
     The team next found that the remnants of dinosaur
chondrocytes themselves resemble those of birds.  In mammals
and reptiles, the cells are tall and have four distinct
sides.  In birds, the cells are shorter and ovoid in shape.
That's the shape Barreto's group saw in the dinosaur plates.
The researchers then used a scanning electron microscope
(SEM) to peer into the extracellular matrix.  In the
Maiasaurus "the SEM showed calcified walls all around,"
Barreto says, as well as calcified lumps known as
calcospherites.  Again, this is identical to the pattern in
birds and very different from what's seen in mammals and
reptiles, who only have calcification and calcospherites on
the longitudinal walls.  "These growth plates point to a
common ancestor for birds and dinosaurs.  It's too complex
to have evolved twice," Barreto concludes.
     Paleontologists who are more partial to reptilian
relativity for dinosaurs, such as Larry Martin of the
University of Kansas, argue that such a statement is too
broad, and all Barreto has shown is a link between birds and
the ornithischian branch of the dinosaurs.  To include the
other branch, the Saurischia, she will have to find this
avian pattern in them as well.  Barreto hasn't examined a
juvenile saurischian yet ("That's the next step," she says),
but she argues that many other features tie birds to
saurischians.  This, together with the growth plate evidence
in ornithischians, suggests to her that all the dinosaurs
are related to birds.
     And that general pattern not only links the ancient
animals to modern avians, Barreto says, it also indicates
the the dinosaur branches had one common ancestor.  It had
been argued that the two branches emerged separately from a
diverse group of primitive reptiles called thecodonts.  But
here even paleontologists who favor the common ancestor
theory think Barreto hasn't done the right comparisons to
support that claim.  "You need to look at crocodiles," says
Jacques Gauthier of the California Academy of Sciences.
     Crocs, next to birds, are presumed to be dinosaurs'
nearest living relatives, deriving from the same general
pool of ancient reptiles.  If the birdlike growth plates are
missing in crocs, Gauthier says, it implies the pattern
arose in a dinosaur ancestor after the croc lineage went its
own way.  But if crocs do have these plates, the feature
must have been older and more generalized, and says nothing
about a common ancestor for the two dinosaur groups.  So
Barreto is off on another big game hunt, this time for
crocodiles, but once again she's looking for something very
small.

                                   -- Joshua Fischman