The Origin and Evolution of Mammals - Moodle

CHAPTER 3
Evolution of the mammal-like reptiles
Mammals, along with the biologically remarkably
similar birds, are the vertebrates that are most completely
adapted to the physiological rigours of the
terrestrial environment. Whilst all the terrestrial
dwelling tetrapods can operate in the absence of the
buoyancy effect of water, and can use the gaseous
oxygen available, mammals have in addition evolved
a highly sophisticated ability to regulate precisely the
internal temperature and chemical composition of
their bodies in the face of the extremes of fluctuating
temperature and the dehydrating conditions of dry
land. From this perspective, the origin of mammalian
biology may be said to have commenced with the
emergence of primitive tetrapods onto land around
370 Ma, in the Upper Devonian.
The vertebrate conquest of land:
origin of the Amniota
Until the 1990s, the only Devonian tetrapod at all
well known was Ichthyostega from east Greenland
(Fig. 3.1(c)), as described by Jarvik (e.g. Jarvik 1980,
1996). Famous for its combination of primitive fishlike
characters such as the lateral line canals, bony
rays supporting a tail fin, and remnants of the opercular
bones, with fully tetrapod characters such
as the loss of the gills and opercular cover, robust
ribcage, and of course large feet with digits,
Ichthyostega provided more or less all the fossil
information there was relating to the transition from
a hypothetical rhipidistian fish to a tetrapod.
Subsequently, however, an ever-increasing number
of other Upper Devonian tetrapods have been
described, and the emerging picture of the origin of
vertebrate terrestriality has become more complicated
and surprising (Ahlberg and Milner 1994;
Clack 2002). The earliest forms are Upper Frasnian
in age, and include Elginerpeton from the Scottish
locality of Scat Craig (Ahlberg 1995, 1998). So far
known only from a few bones of the limbs and jaws,
Elginerpeton adds little detail to the understanding
of the evolution of tetrapods except to demonstrate
that the process had commenced at least 10 million
years prior to the existence of Ichthyostega.
The next oldest tetrapods are Fammenian in age
and include Ichthyostega, and a second east
Greenland form, Acanthostega (Fig. 3.1(a and b)),
which has been described in great detail (Coates
and Clack 1990, 1991; Coates 1996). This genus has
proved to be very surprising because, despite being
of the same age and apparently living in the same
habitat as Ichthyostega, it was evidently not adapted
for actual terrestrial life. In common with
Ichthyostega, enclosed lateral line canals and a fishlike
tail were present, but furthermore, a full set of
ossified gill bars covered by a bony operculum was
still present. The limbs, while certainly tetrapodal in
lacking fin rays, were relatively short, stubby, and
bore eight digits, and could not possibly have supported
the weight of the animal out of water. Given
its more basal cladistic position compared to
Ichthyostega and all later tetrapods, the anatomy of
both the limbs and the gills of Acanthostega led to the
proposal that the tetrapod limb originally evolved
as a specialised organ for aquatic locomotion.
Indeed, Coates and Clack (1995) suggested that the
whole Devonian tetrapod radiation including not
only Acanthostega, but also Ichthyostega and the possible
basal amniote Tulerpeton (Lebedev and Coates
1995) were entirely aquatic animals. Support for this
idea has recently come from the discovery that
Ichthyostega does in fact also possess gill bars, and
has a unique ear structure designed for hearing
under water rather than in air (Clack et al. 2003).
This view is consistent with the argument, put forward
by Janis and Farmer (1999), that a primarily
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