COMPARATIVE ANATOMY HOME
COMPARATIVE ANATOMY TABLE OF CONTENTS
  OBL HOME OBL REFERENCES
THE PELVIS

     Although there is only one pelvic bone in adult humans, the os coxa, this bone results from the fusion of three separate bones earlier in development, the ilium, ischium, and pubis.

human hip      Placoderms were the first fish to evolve pelvic fins and pelvic girdles.  The pelvic girdle in placoderms and in all fish was composed of one single element (Kardong, 2002, p. 323; Carroll, p. 47).  The pelvic girdle and pelvic fins of a shark are depicted below.
shark
shark

In amphibians, the hip bones were composed of 3 separate elements, the ilium, ischium, and pubis.  The acetabulum for the head of the femur forms where these 3 bones meet, a pubic symphysis joins the two pubic bones, and (after the most primitive amphibians such as Acanthostega), sacral vertebrae attached to the pelvis to enable the legs to bear the weight of the body (Carroll).

ichthyostega

frog

frog

salamander

salamander

turtle

turtle

turtle

alligator

alligator

alligator

In bird his (such as that of an emu below), the pubis has rotated posteriorly, just as in their closest dinosaur relatives like Deinonychus and Velociraptor.

emu

note that an emu's acetabulum is open, just as in the dinosaurs

emu

     In sphenacodont pelycosaurs, the ilium was expanded (Kemp, 1982, p. 33).  In therapsids, the acetabulum was open more ventrally (Carroll).  In the most primitive therapsids, such as Biarmasuchus, the ilium was not as expanded as in later therapsids (such as Titanophoneus in the following image) (Kemp, 1982).
pelycosaur
In therapsids, a dual gate was possible in that the hip accommodated both an erect and a sprawling posture (Kemp, 1982, p. 119).

     The pelvis of cynodonts included an obturator foramen (Romer, p. 211).  In intermediate cynodonts, the ilium was expanded and the pubis reduced to produce a hip which was practically mammalian in its form (Kemp, 1982, p. 192; Carroll, p. 384).  In advanced cynodonts, the ilium extended forward and turned outward,  (Kemp, 1982, p. 201, 245), the acetabulum became deeper and smaller (Kemp, 1982, p. 244), the anterior ilium was expanded, and the pubis further reduced (Kemp, 1982, p. 244).  In Mesozoic mammals, the obturator foramen became larger and the pubis was reduced (Carroll).                

     Many Mesozoic mammals, modern monotremes, and modern marsupials, have small bones extending from the pubis called epipubic bones.  Epipubic bones existed in many primitive mammals (mutlitberculates, eupantotheres, and even some therapsids) of both sexes.  These bones may have served more for muscle attachment than for a pouch.  Although modern placental mammals lack epipubic bones, at least two early species, Zalambdalestes and Ukhaatherium (and probably Barunlestes as well) had them.  Epipubic bones seem to be homologous to the bones which exist in the penis and, more rarely, the clitoris, of many modern placental mammals (Lillegraven; Hu, 1997).

therapsid

platypus

platypus

cat

cat

howler monkey

howler

rhesus monkey

monkey

monkey
monkey

chimp

chimp

chimp

     At least two lineages of ape have evolved bipedal locomotion.  Oreopithecus is a hominoid from the Late Miocene (7-9 million years ago) from Italy.  It was bipedal and had a number of hip adaptations which supported this locomotion such as cancellous bone architecture, a long ischial spine, a prominent anterior inferior iliac spine, a short ischium, and a short pubic symphysis (Rook, 1999).  Given that both early hominids and Oreopithecus independently evolved bipedality, it is not surprising Oreopithecus hands were modified to improve their grasping ability in ways similar to those observed in early hominids (Moya-Sola, 1999).

     The ancestors of hominids were already capable of some bipedal locomotion.  Chimps use a bipedal posture when feeding on elevated foods, carrying objects, gathering fruits from ground, and when standing on branches (Hunt, 1994; Videan, 2002). Human and chimp pelves are depicted in the following photo.

human and chimp human and chimp
     The pelvis and leg of A. afarensis were very similar to those of humans.  The legs were a little shorter than in humans.  The pelvis allowed bipedal walking but there is no evidence of an enlarged birth canal that would have permitted enlarged fetal crania (Johanson, 1979; Susman, 1984; Hill, 1985;  Leakey, 1972; Lovejoy, 1972; Lovejoy, 1993).

chimp

chimp

A. afarensis

A. afarensis

human

human

human

human

human
human human