The Origin and Evolution of Mammals - Moodle

the therocephalian pelvis and hindlimb could be
accounted for by a dual-gait hypothesis. This is the
idea that the hindlimb could operate facultatively in
either a sprawling mode (Fig. 4.7(d)) or a parasagittal,
mammal-like mode with the knee turned forwards
and the foot beneath the body (Fig. 4.7(c)), as
is seen today in crocodiles and large varanid lizards.
Most, although not all of the therocephalian features
of the pelvis and hindlimb occur in the more primitive
therapsids, and Kemp (1982) extended the
hypothesis to gorgonopsians. The basal therapsid
hip joint is completely unlike the shoulder, for it is a
conventional ball-and-socket arrangement whereby
the circular, regularly concave acetabulum accepts
the head of the femur congruently, both having the
same radius of curvature. In fact, the femur can fit
comfortably into the acetabulum in two different
orientations. It can extend laterally and swing backwards
in a horizontal plane. Alternatively, because
of the inturned head and sigmoid curvature that it
possesses, the femur can extend antero-ventrally and
only slightly laterally and retract in a more nearly
vertical plane (Fig. 4.7(c)).
The structure of the knee and ankle joints of therocephalians,
including the evolution of a new
intratarsal joint, is one of the strongest pieces of
evidence for the dual-gait hypothesis and, although
not studied with this functional possibility in mind,
the ankle of the gorgonopsian Lycaenops described by
Colbert (1948) appears to match the therocephalian
in the essential features. The two different gaits
involve two different respective movements at both
the knee and the ankle joints (Fig. 4.7(c) and (e)).
During the sprawling mode, there was relative
rotation between the end of the femur and the foot,
about a vertical axis. This movement was accommodated
by rotation of the tibia on the astragalus of
the ankle and rotation of the fibula on the underside
of the femur. During the parasagittal gait, the
movement at the joints were hinging rotations
about approximately transverse axes. The wide
upper end of the tibia forms a strong hinge joint
with the underside of the distal end of the femur,
though the lower end does not hinge on the foot.
Instead, a new intratarsal joint had evolved between
the astragalus and the calcaneum that allowed the
calcaneum-plus-pes as a unit to flex and extend relative
to the tibia-plus-astragalus as a unit. By this
means, the joint between the tibia and the astragalus
EVOLUTION OF MAMMALIAN BIOLOGY 109
does not have to undergo both rotation and hinging,
but only the former. The latter is taken care of
by the new intratarsal joint. Separating the functions
increased the strength of the ankle and its
ability to withstand the two different patterns of
stress associated with the two different gaits. The
nature of the astragalus and calcaneum, along with
the shortening of the foot indicate that the therapsid
hindfoot as well as the forefoot was plantigrade.
The inferred musculature of the hip and hindlimb
also indicates significant changes in the mammalian
direction (Fig. 4.7(c) and (d)). In mammals, the iliofemoralis
muscle has completely taken over from the
caudi femoralis the role of the principal retractor,
becoming the gluteal muscle complex. In primitive
therapsids, the combination of enlarged and somewhat
anteriorly extended ilium, development of a
distinct trochanter major on the hind edge of the
femur, and reduction of the tail, point to an early
stage in this transition. Nevertheless, the retention of
an internal trochanter on the underside of the femur
indicates that a caudi femoralis muscle from the tail
vertebrae, and the posterior section of the puboischio-femoralis
externus from the ischium were still
important in retraction. The second major change in
the arrangement of the hip musculature in the mammals
is an antero-dorsal extension of the pubo-ischiofemoralis
internus muscle, which has become the
principal retractor of the femur, and is named as the
psoas–iliacus muscle complex. This evolutionary
transition had also commenced in the primitive therapsids.
The lower part of the ilium in front of the
acetabulum is occupied by a broad, shallow depression
interpreted as the site of origin of what was to
become the iliacus muscle. The significance of the
tendency to shift the main hip muscles dorsalwards
relates to the facultative adoption of the parasagittal
gait at this stage, for it keeps them well away from
the femur as it passes to and fro much closer to the
ventral part of the pelvic girdle.
Blob (2001) has tested the dual-gait hypothesis by
estimating the nature and magnitude of the stresses
that would have arisen in the therapsid limb bones
and comparing these with the situation in living
crocodiles and iguanas. He concluded that the
hypothesis is indeed a plausible explanation for the
anatomy of the limb bones of animals transitional
between sprawling pelycosaurs and those with
fully mammalian locomotion.