The Origin and Evolution of Mammals - Moodle

16 THE ORIGIN AND EVOLUTION OF MAMMALS
demonstrable in the few cases where larval forms
are known, presumably they mostly retained the
anamniotic egg laid in water and hatching into
a gilled, fully aquatic larval form.
Evolutionary speculations concerning the origin
of tetrapods, as for other major new kinds of organisms,
have been dominated by the idea of preadaptation.
This is defined as the process whereby
a structure of an organism that is adapted for a particular
function in a particular habitat, becomes
co-opted for a different function in a different habit.
In fact, this is not a very helpful concept, because
characters such as limbs and lungs in pre-tetrapods
evidently did not change their function on attaining
land. Rather, they were adaptations for aspects of the
environment that were common to both the shallow
water and to the muddy bank alongside. It is more
plausible to view the origin of tetrapods as a transition
along an ecological gradient in which there are
no abrupt barriers. To be adapted to shallow, lowoxygen
water a vertebrate requires the capacity for
substrate locomotion and air breathing, a feeding
mechanism suitable for small terrestrial organisms
falling in, and aerial sense organs. These same adaptations
must exist in a vertebrate adapted to life on
the muddy bank adjacent to the water. Even such
apparently radical differences between the two habitats
as loss of buoyancy, increased dryness, and
exposure to heat are in effect gradual across the
boundary insofar as an organism can ameliorate
them by temporarily returning to the protection of
the water. From this perspective, the radiation of
early tetrapods consists of lineages adapted to various
points along the water to land gradient.
Within the context of the Carboniferous radiation
of tetrapods, the lineage that evolved the most
extreme terrestrial adaptations, to the extent that its
members were eventually completely independent
of free-standing water were the Amniota, represented
today by the reptiles, birds, and mammals.
The modern reptiles, lizards, snakes, crocodiles,
and turtles, possess internal fertilisation and the
cleidoic egg, greatly reduced skin permeability,
a water-reabsorbing cloaca, and use of solid uric
acid for excretion. Such physiological characters as
these cannot normally be demonstrated in fossils,
but fortunately the modern forms also possess a
number of osteological characters that do permit
a diagnosis of the group Amniota, so that exclusively
fossil groups can be added. Laurin and Reisz
(1995) list nine skeletal synapomorphies, including
for instance the entry of the frontal bone of the skull
into the orbital margin, a row of large teeth on the
transverse flange of the pterygoid, absence of
enamel infolding of the teeth, two coracoid bones in
the shoulder girdle, and the presence of a single
astragalus bone formed from fusion of three of the
ankle bones. The earliest known possible amniote is
Casineria (Paton et al. 1999), which comes from the
Middle Carboniferous of Scotland. Unfortunately,
only a partial postcranial skeleton is preserved, so
its correct taxonomic attribution remains dubious.
The earliest certain amniote fossils occur in the
Upper Carboniferous (Middle Pennsylvanian) of
South Joggins, Nova Scotia. Hylonomus and
Paleothyris are found preserved in a remarkable
way, entombed within approximately 315 Ma
petrified lycopod tree stumps (Carroll 1969). They
are relatively small forms with a body length
of about 10 cm, and are members of a group
Protorothyrididae (Fig. 3.2(b) and (e)), which is
related to the diapsid and testudine reptiles (Carroll
1972; Laurin and Reisz 1995; Berman et al. 1997).
Surprisingly perhaps, a second distinct group of
amniotes appears at about the same time, characterised
most significantly by the presence of a
single opening low down in the cheek region of the
skull. This is the synapsid version of temporal
fenestration and is the most prominent, unambiguous
hallmark of the Synapsida, the mammal-like
reptiles and mammals (Fig. 3.2(c)). Carroll (1964)
attributed some fragmentary remains from the
same locality as Hylonomus to the synapsids,
naming them Protoclepsydrops, although in his
monograph on the pelycosaurs, Reisz (1986) was
unable positively to confirm this conclusion. Be that
as it may, there are indisputable pelycosaur synapsids
(Fig. 3.2(f)) only very slightly younger than
Hylonomus, namely Archaeothyris (Reisz 1972).
There is no serious questioning of the monophyly
of the amniotes, including both protorothyridids
and synapsids, but the relationships of the group to
other Carboniferous tetrapod taxa is presently a
matter of intense debate and disagreement. Amongst
proposed stem-group amniotes, the earliest and
most basal is the poorly known Late Devonian