Evolution__3rd_Edition

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Time
(a) Observed incompleteness
within lineages
(b) Adjusted times of origins
for (a)
CHAPTER 23 / Extinction and Radiation 673
(c) Incompleteness due to
missing lineages
(d) Adjusted times of origin
for (c)
1 2 1 2 1 2 1 2
Observed image
Unobserved image
Figure 23.14
Use of observed incompleteness in the fossil record to adjust
estimated times of origin for a taxon. (a) Observed fossil
records for two lineages that both appear to originate at the
same time. (The x-axis has no meaning.) (b) Taxon 1 has a
more incomplete record, suggesting that it had an earlier time
of origin than taxon 2. (c) Taxon 1 and 2 are observed to
The fossil dates are being
“corrected” back in time
originate at the same time in the fossil record, though
taxon 1 has fewer missing lineages than taxon 2 (dashed
lines indicate lineages unrepresented in the fossil
record). (d) Taxon 2 probably had an earlier
origin than taxon 1. The illustrated adjustments to
time of origin in (b) and (d) are not quantitatively
exact.
look at the time from its first to its last fossil appearance, During that time, there will
often be gaps, for times when the lineage is not represented. We can use the extent of
those gaps to estimate how much earlier the lineage really originated than its first
appearance in the fossil record (Figure 23.14a and b). Foote et al. (1999) used this
method, and concluded it was unlikely that the mammal orders had originated much
before 55 million years ago.
Foote et al.’s (1999) method only used incompleteness within observed lineages. The
fossil record is also incomplete in missing out whole lineages (Figure 23.14c). Tavaré et
al. (2002) recalculated the chance that the primates originated in the mid-Cretaceous,
taking account of both kinds of incompleteness a incompleteness within lineages, and
missing whole lineages. Their adjusted fossil estimate for the origin of the Primata was
81 million years ago. This is well before the observed date of about 55 million years ago
and not far off the molecular date (around 90 million years ago). Tavaré et al.’s (2002)
adjusted date implies that primates existed for an extensive ancestral period that
happens to be unrepresented in the fossil record.
The earlier date for the origin of the modern mammal orders has been supported by
discoveries of earlier mammal fossils. Ji et al. (2002) reported a 125 million-year-old
fossil eutherian from the Yixian formation in China. The fossil pushes back the
eutherian record by about 50 million years. While the oldest eutherian was from about
80 million years ago, it was difficult to see how the modern mammal orders could
have originated much before the Tertiary. Now, with the oldest eutherian at 125 million
years ago, ample time exists for those orders to have originated by 80–90 million
years ago.
The molecular evidence has led biologists to accept an earlier time for the origin of
the modern mammal orders. However, many biologists suspect that mammals “lay