The Origin and Evolution of Mammals - Moodle

indisputably to occur prior to the end of the
Cretaceous. Even within the Early Palaeocene,
between 65 and 60 Ma, there are representatives only
of Carnivora, and possibly stem Primates.
Otherwise, all of the modern orders made their
actual appearance in the fossil record in a narrow
window of time, either side of the Palaeocene–
Eocene boundary (Archibald and Deutschman
2001). Thus a great stir was caused when Kumar and
Hedges (1998) published a comprehensive timescale
for the divergences of placental orders based on a
molecular clock. From an analysis of 658 nuclear
genes, and a calibration of the clock based on the fossil
date of divergence of birds from mammals, they
estimated that at least five lineages of modern placentals
had diverged more than 100 Ma, and most of
the orders had differentiated before the end of the
Cretaceous at 65 Ma.
All subsequent molecular-based studies, undertaken
by several groups of workers using different
methods and molecular databases, have supported
early divergence times. Waddell et al. (1999), on the
basis of mitochondrial DNA data, estimated that
divergence times of some orders could have been as
early as 150 Ma. In another study, also based on
mitochondrial DNA, Cao et al. (2000) used several
presumed reliable fossil calibration points as the
basis of a clock. They calculated a divergence of
Afrotheria from Xenarthra 102 Ma, and the differentiation
of the Laurasiatherian orders from one
another about 77 Ma. Scally et al. (2001) found the
mean estimate for divergence of Afrotheria from
the northern groups to be 105 Ma, and that the basal
splits within both took place in excess of 85 Ma.
Eizirik et al. (2001) estimated possibly somewhat
younger dates, finding that Afrotheria diverged
from the rest of the placentals in the period 76–102
Ma, and xenarthrans from the northern groups
72–104 Ma. Divergence times of orders within the
northern supra-ordinal groups were all in the range
of 65–95 Ma. Springer et al. (2003) have conducted
the most comprehensive analysis, using a large,
diverse data set of nuclear and mitochondrial genes
for 42 placentals representing all the living orders
(Fig. 7.25). Their results generated divergence dates
for the Afrotheria of about 107 Ma, and for
Xenarthra of about 102 Ma. The two northern
super-orders, Laurasiatheria and Euarchontoglires,
LIVING AND FOSSIL PLACENTALS 275
separated about 94 Ma. All the individual orders
within these super-orders were differentiated from
one another during the Late Cretaceous, between
82 and 77 Ma, apart from the Afrotheria. In the
latter case, the Tenrecida separated from the
chrysochlorida about 65 Ma, right on the K–T
boundary, and the three Paenungulata orders (sirenians,
hyraxes, and elephants) diverged from one
another about 60 Ma.
This marked and consistent lack of agreement
between the pictures based respectively on the fossils
and the molecular evidence has led to considerable
controversy over attempts at a resolution
(Foote et al. 1999; Archibald and Deutschman 2001).
There are four possible explanations.
Incompleteness of the fossil record. In order to test
this, Foote et al. (1999) created a model based on reasonable
expectations of preservation rates of
Cretaceous mammals, coupled with the assumption
that if they were present, members of the living
orders would be morphologically recognisable. The
outcome of the study was that if the ordinal-level
divergences really had occurred in the Late
Cretaceous, then in order to explain the absence of
fossils of members of these modern groups, the
mean preservation rate would have had to be a
whole order of magnitude less than had been
assumed likely. This is taken by the authors to be an
unrealistically low rate, and therefore they conclude
that simple incompleteness of the fossil record cannot
be the explanation for the discrepancy between
fossils and molecules. Hedges and Kumar (1999)
questioned whether the preservation rate assumed
in the model to be reasonable should have had such
a high value. The early representatives of the living
orders might have been very rare animals.
However, other studies based on similar kinds of
models of the preservation process by Alroy (1999b),
and Archibald and Deutschman (2001) have
reached the same conclusion, that the surge in origination
of ordinal lineages of placentals in the early
Cenozoic is a real phenomenon.
Incorrectness of the molecular clock. By supporting the
fossil record, these latter studies explicitly conclude
that the problem lies with the inappropriateness of
assuming a clock-like rate of molecular evolution,