Dimetrodon, Cope, 1878

Romer, A. S., 1923, The pelvic musculature of saurischian dinosaur, Bulletin of the American Museum of Natural History 48, pp. 605-617 : 605-611

The ilium of the Saurischians presents few major difficulties; I have previously outlined its history (1923). The type of ilium present in the primitive forms from which the " Archosauria" have been derived is represented morphologically by such a reptile as Dimetrodon ( Fig. 2 ). It had a broad outer surface extending little, if at all, anterior to the acetabulum. The greater part of this surface was utilized as an area of origin by the ilo-femoralis, while the ilio-tibialis arose tendinously from its dorsal edge. Near the posterior angle was the origin of the iliofibularis. The saurischian ilium is comparatively little modified. The external surface is carried forward a short distance without changing the muscular arrangements on its outer surface. This anterior prolongation arches over the pubo-ischio-femoralis internus, a small portion of which arises from the anterior edge of the ilium. Posteriorly, in the Saurischia , portions of the long flexors have gained attachment to the bone, as discussed below. Ventro-posteriorly, the coccygeo-femoralis brevis had an area of origin.

In primitive reptiles ( Romer, 1922, p. 578) such as Dimetrodon , the pubo-ischium was a solid plate, broken only by the foramen for the obturator nerve, with a continuous ventral symphysis extending in an almost straight line antero-posteriorly. Three principal muscle groups arose from the outer surface of the plate. Externally were the long flexors to the lower leg (pubo-ischio-tibialis, flexor tibialis internus, flexor tibialis externus). Deep to these lay the adductor, running to the distal part of the femur. Still deeper lay the pubo-ischio-femoralis externus, attaching to the proximal part of the femur. That portion of the pubis close to the acetabulum gave origin to the ambiens (a member of the quadriceps group) and to the pubo-tibialis (an anterior member of the long flexor series). Internally the pubis was covered by the. puboischio-femoralis internus, which ran up and back to emerge on to the thigh above the ambiens. The ischio-trochantericus took origin from the inner surface of the ischium.

The three chief muscle groups arising from the outer surface of the plate have been greatly modified in the "Archosauria," as known in the Crocodilia and as deduced from the dinosaur pelvis. These changes appear to be correlated with changes in the position of the femur.

In the primitive reptiles, the femur was extended very nearly straight out from the acetabulum in a horizontal direction. The three muscle masses of the plate acted upon it in a plane passing nearly vertically through its long axis, as diagrammatically represented in figure 3.

In the "Archosauria," with a higher type of locomotion, the femur (1) has been turned inward anteriorly so as to bring its shaft to a position nearly parallel to the long axis of the body and (2) has, in consequence of the bipedal character of primitive saurischians, been carried downwards and backwards so that, if the primitive plate-like pubo-ischium had been retained, the femur would lie very close to the plate.

The results of such a position on the musculature of the puboischium are obvious from the diagram on the right in figure 3. The long flexors and adductors would be excessively shortened and rendered of little value, especially if situated near the middle of the plate. To correct this, a movement of the areas of origin upwards might be deduced. Further, the main pull of these muscles is now a backward one; a backward migration or a disappearance of anterior portions of these muscles might be expected.

With regard to pubo-ischio-femoralis externus, no movement to a more dorsal position is possible. It is clear, however, that the imiddle portion of the muscle is at a disadvantage as compared with its anterior and posterior extremities, and the development of anterior and posterior heads of the muscle and the disappearance of the middle portion might perhaps be suggested.

The development of this series of modifications is shown, somewhat diagrammatically, in figure 2. Dimetrodon shows the primitive condition; the alligator and sauropod and theropod dinosaurs show the end forms. In the dinosaurs, the muscle areas can be located with almost the same degree of accuracy as in Alligator , except that I am unable to locate exactly the origins of pubo-ischio-tibialis and adductor 1 in the theropods. Euparkeria and Erythrosuchus from the figures of Broom (1906, 1913) are introduced to show the steps by which the changes have taken place, although I am unable to locate exact muscle areas on these forms.

It will be noted that the main change in the form of the pelvis has been a "buckling" of the pubo-ischium, by which the ventral border has been changed from a straight line to an irregular crescent, convex above. Thus the middle part of the pubo-ischium, functionally of the least value, has disappeared.

The apparent "fenestra" caused by this is not at all homologous with the obturator fenestra of other modern reptiles or the obturator foramen of mammals. These openings are essentially a fenestration within the primitive plate, centering about the pubo-ischio-femoralis externus (obturator externus); the line of the symphysis is ventral to the fenestra. In the "Archosauria," on the other hand, the symphysis turns dorsally as the "buckling" takes place and is continuous in such a form as Erythrosuchus ; the apparent "fenestra" is ventral to the symphysis. Later, apparently in connection with the strengthening of the sacrum, the primitive symphysis is interrupted and confined to separate pubic and ischiadic surfaces. Hence the ventral edge of the pubis and ischium of the Crocodilia and Saurischia may be considered as the equivalent of the primitive symphysis. Parallels to the obturator fenestra may be sought in excavations in the surface of the ischium in the part of that bone serving as an area of origin for pubo-ischio-femoralis 3 (as in Ornitholestes ).

If we take a line from the distal end of the ischium to the distal end of the pubis as marking the ventral line of the body, it will be seen that in the saurischian dinosaurs,, the muscles of the ischium as a whole are located farther dorsally than in primitive forms or even in the Crocodilia . If the lower half of the saurischian ischium were removed, the remaining triangular area would be almost exactly comparable to the whole of the ischium in the Crocodilia . Such a dorsal movement, as has been noted previously, might be expected in bipedal forms, or those whose ancestors were bipedal.

The pubis of primitive reptiles seems homologous with that of such primitive archosaurians as Euparkeria and Erythrosuchus . These in turn are similar to the sauropods as regards the pubis; while the theropods have merely carried the reduction farther and eliminated the obturator foramen. In the Crocodilia , further reduction has caused the exclusion of the bone from the acetabulum. But this reduced crocodilian pubis can apparently be carried back through a series showing slight morphological changes to that of the primitive reptiles.

Von Huene (1908) states that the mnuch smaller number of muscles attached to the crocodilian pubis, as compared with that of other living reptiles, tends to show that the crocodilian bone is not the true pubis. But the reasons for a reduction in muscle on the pubis are apparent. In typical reptiles six appendicular muscles may take origin from the pubis: pubo-ischio-femoralis internus, pubo-ischio-femoralis externus, ambiens, pubo-tibialis, adductor and pubo-ischio-tibialis. Of these, the pubo-ischio-femoralis internus has shifted dorsally, as I have explained elsewhere (1923, 1923a), the pubo-tibialis has been lost, and the adductor and pubo-ischio-tibialis have shifted (as expected) posteriorly. This leaves the ambiens and pubo-ischio-femoralis externus as the only possible muscles which might be expected on the pubis in the Crocodilia ; and it is these two muscles which are found on the bone commonly called by that name.

The rearrangement of the musculature may now be considered in detail.

The long flexors, as I have termed them, consist essentially of two sets in a primitive reptile. (1) An outer group, made of pubo-ischiotibialis, from the ventral edge of the pubo-ischium, and a portion or portions of the flexor tibialis internus originating from the region near the posterior ventral angle of the ischium or the lower end of the ilio-ischiadic ligament. (2) An inner group, consisting of pubo-tibialis, from the pubis near the acetabulum, of a portion of flexor tibialis internus from the region mentioned above, and of the flexor tibialis externus, from the ilioischiadic ligament dorsal to the last mentioned.

These muscles have, for the most part, moved posteriorly or dorsally as expected. Pubo-ischio-tibialis, one of the most anterior members of this group, has been reduced, confined posteriorly to the ischium and, further, has moved dorsally to a position just below the acetabulum on the anterior edge of the ischium. In doing this it has not only entered between the two main heads of pubo-ischio-femoralis externus but, as its course shows, has broken in on to the ischium so as to separate the two heads of the adductor. One portion of flexor tibialis internus (1) is in its primitive position in the alligator; but in the Saurischia its origin is only about halfway down the posterior edge of the ischium. A second portion of flexor tibialis internus (2) has moved dorsally and gained an origin from the ilium.

Of the deeper set, the most anterior member, the pubo-tibialis, has disappeared entirely. The remaining members of the group are posterior and dorsal in position, flexor tibialis internus 3 having migrated up along the posterior margin of the ischium, part 4 having migrated up along the ilio-ischiadic ligament (in the Crocodilia at least; its existence in the Saurischia cannot, of course, be proved) and flexor tibialis externus having moved dorsally to the ilium.

The adductor musculature has also moved somewhat posteriorly and dorsally. It is divided into two portions, both arising from the ischium and not at all from the pubis. In the dinosaurs, the small size of the part of the ischium devoted to appendicular muscles has brought the adductors halfway from the original ventral line to the acetabulum. This is not true of the Crocodilia ; but even here, by the division into two parts, both (especially the posterior) have pushed upwards on either side of pubo-ischio femoralis externus 3 so that they extend some distance dorsally along either edge of the ischium.

Pubo-ischio-femoralis externus, as has been mentioned above, cannot move dorsally. Its differentiation has consisted in a division into two main portions, part 2 from the outer surface of the pubis, and part 3 from the outer surface of the ischium'.

In addition, the Crocodilia possess an additional head (part 1) from the inner surface of the pubis, emerging (in contradistinction to puboischio-femoralis internus) below, rather than above, the ambiens, and joining part 2, of which it is a derivative. This muscle is not found in other living reptiles; and it was apparently also absent in primitive forms. The region through which it passes from the internal to the external surface of the pubis is usually occupied by the insertion of the rectus in modern reptiles. In the Crocodilia , the rectus inserts mainly into the strong last abdominal rib and this leaves the edge of the pubis free for the passage of the muscle. Its existence depends then upon the presence of a strong abdominal rib paralleling the pubis. As far as I know, this condition is not found in the Saurischians. For example, a specimen of Struthiomimus in this museum has a well-preserved "abdominal basket," extending to the pelvis; but there is no noticeable strengthening posteriorly. In default of such evidence it seems probable that pubo-ischio-femoralis 1 of the Crocodilia was not present in the Saurischia .

As I have shown previously, the pubo-ischio-femoralis internus has retreated from the interior of the pubic region in the "Archosauria."

The ambiens remained in position near its primitive place of origin from the pubis near the acetabulum. The ischio-trochantericus also appears to have been unchanged. The coccygeo-femoral muscles appear to have been well developed. The "fourth trochanter" for the insertion of coccygeo-femoralis longus on to the femur is well marked. There is an excavation beneath the posterior end of the ilium for the origin of part of coccygeo-femoralis brevis.

Figures

Fig. 2. The musculature of the pelvis in primitive reptiles, the primitiv6 "Archosauria," Alligator and saurischian dinosaurs, to show especially the steps in the differentiation of the pubo-ischium. Abbreviations: ad., adductor; amb., ambiens; c.fb., coccygeo-femoralis brevis; cfl., coccygeofemoralial ongus; f.obt., obturator foramen; ft., femoro-tibialis- f.t e flexor tibialis externus; f.t.i. flexor tibialis internus; il. fem., ilio-femoralis; il. fib., ilio-fibularis; il.tib., ilio-tibialis; is.tr., ischiotrochantericus; p.i.fe., pubo-ischio-femoralis externus; p.i.f.i., pubo-ischio-femoralis internus; p.t. pubo-tibiali8.