Chronological order of intermediates
Fossilized intermediates should appear in the correct general chronological order based on the
standard tree. Any phylogenetic tree predicts a relative chronological order of the evolution of
hypothetical common ancestors and intermediates between these ancestors. For instance, in our
current example, the reptile-mammal common ancestor (B) and intermediates should be older
than the reptile-bird common ancestor (A) and intermediates.
Note, however, that there is some "play" within the temporal constraints demanded by any
phylogeny, for two primary reasons: (1) the statistical confidence (or conversely, the error)
associated with a phylogeny and its specific internal branches, and (2) the inherent resolution of
the fossil record (ultimately stemming from the vagaries of the fossilization process). As
mentioned earlier, most phylogenetic trees have some branches with high confidence, because
they are well-supported by the data, and other branches in which we have less confidence,
because they are statistically less significant and poorly supported by the data. See also the
associated with phylogenetic analysis.
When evaluating the geological order of fossils, remember that once a transitional species
appears there is no reason why it
become extinct and be replaced. For instance, some
organisms have undergone little change in as much as 100 to 200 million years in rare cases.
Some familiar examples are the "living fossils", such as the coelacanth, which has persisted for
approximately 80 million years; the bat, which has not changed much in the past 50 million
years; and even the modern tree squirrel, which has not changed in 35 million years. In fact,
paleontological studies indicate the average longevity of 21 living families of vertebrates is
approximately 70 million years (Carroll 1997
, p. 167).
Furthermore, the fossil record is demonstrably incomplete; species appear in the fossil record,
then disappear, then reappear later. An exceptional instance is the coelacanth, which last
appeared in the fossil record 80 million years ago, yet it is alive today. During the Cretaceous (a
critical time in bird evolution), there is a 50 million-year gap in the diplodocoidean record,
greater than a 40 million-year gap in the pachycephalosaurian record, greater than a 20 million-
year gap in the trodontidiae, and about a 15 million-year gap in the oviraptosaurian fossil record