Eryops megacephalus

ERYOPS MEGACEPHALUS

Eryops megacephalus Cope 1877 is an early tetrapod from the Permian (298.9-251.9 Ma), apparently showing strong similarities to salamanders (2022HerbstEC_HutchinsonJR; 2023CohenKM_CarN). Adults reached an advanced degree of terrestrial habitation and a size of about three meters from head to tail (2002SchochRR).

Eryops megacephalus (Lower Permian); top – reconstruction of the skeleton; bottom – appearance of the animal, sculptor Ya.M. Eglon; exposition of the Orlov Paleontological Museum (Moscow); photo by the author.

Anatomical evidences of the ligamentum capitis femoris (LCF) in Eryops megacephalus is suggested by certain features in fossilized pelvic and femoral bones. Notable LCF indicators on the femur include the fovea capitis, the groove on the femoral head, and the femoral head’s tuberosity. In the pelvic bone, signs of LCF presence include acetabular notches, an acetabular fossa, perforations, and irregularities on the acetabular surface. We used these features to identify LCF in the first amphibians and land species.

According to E.D. Cope (1880) the pelvic bones of Eryops megacephalus resemble «… in their compression below, the corresponding parts in the Anura. The ilia are, however, shorter and worn as in the Urodela. They are flat, and stand at right angles to the line of the ischiopubic symphysis. There is an open concavity of their inferior posterior free border, and a facet-bearing elevation on the inferior border, or that entering into the formation of the acetabulum. The latter is large and half as long again as deep. The anterior and posterior borders of the pelvis descend regularly to the inferior edge, forming with it a triangle. The ischiadic or posterior border is but little thickened; the anterior, or pubic is flat in front and presents a reverted edge outwards. This expands prominently where it is joined by a ridge which bounds the acetabulum below; it there contracts to an inferior apex. Beneath the anterior point of the acetabulum it is pierced by the usual foramen, which issues on the inner edge of the anterior face, just above the symphysis.» (1880CopeED).

Pelvis of Eryops megacephalus; lower center – lateral view, cranial end on the left (1880CopeED, no changed).

A.S. Romer (1922) described the pelvis of early tetrapods as follows: «A constant feature is the presence of a prominent wedge above the acetabulum, merging gradually into the general surface of the bone anteriorly, but posteriorly dropping back sharply as a distinct supra-acetabular notch. This is found practically the same in almost every form examined, from Eryops to the highest therapsids. As suggested by Broili, who first noticed it in dicynodonts (1908, p. 3), it indicates the position of a ligament similar to that found in the cotyloid notch of the ischium of mammals; the changed position of the mammalian femur would account for a shifting ventrally of the notch.»

Right pelvic bone of Eryops megacephalus, lateral view, cranial end on the right; legend: SAN – supra-acetabular notch, TRL – transverse line, F. OBT – obturator foramen, attachment zones of the ilio-ischiadic ligament – lig. il-isch, ilio-puboiliac ligament – lig. il-pub (1922RomerAS, no changed).

Common morphological features between Eryops megacephalus and modern salamanders include a shallow acetabulum (2022HerbstEC_HutchinsonJR). A large notch is clearly visible in the posterosuperior region of the acetabulum.

Pelvis of Eryops megacephalus, modified from Pawley & Warren (Pawley and Warren 2006); illustration from 2018AhlbergPE, no changed).

The acetabular notch is designed to accommodate and attach the LCF. Although the precise positioning of the LCF remains unknown, it is hypothesized to have functioned as a flexible support element activated during terrestrial movement. It is most plausible that the proximal end of the LCF was attached within the supra-acetabular notch, resembling the attachment observed in one of the paired femoral head ligaments in salamanders (1934FrancisETB). The distal end of the LCF in Eryops megacephalus might have attached to the upper edge of the femoral head. In this configuration, LCF tension would restrict medial pelvic tilt during walking, reducing the load on abductor muscles. From our perspective, the LCF in Eryops megacephalus likely did not facilitate pelvic rotation within the horizontal plane.

View of the bones forming the hip joint of Eryops megacephalus, we have marked the supposed localization of LCF; exposition of the Orlov Paleontological Museum (Moscow); photo by the author.

References

Herbst EC, Manafzadeh AR, Hutchinson JR. Multi-joint analysis of pose viability supports the possibility of salamander-like hindlimb configurations in the Permian tetrapod Eryops megacephalus. Integrative and Comparative Biology. 2022;62(2)139-151. [academic.oup.com]

Cohen KM, Harper DAT, Gibbard PL, Car N. The International Commission on Stratigraphy (ICS) International Chronostratigraphic Chart. September 2023. [stratigraphy.org]

Schoch RR. The evolution of metamorphosis in temnospondyls. Lethaia. 2002;35(4)309-27. [idunn.no]

Cope ED. Second Contribution to the History of the Vertebrata of the Permian Formation of Texas. Proceedings of the American Philosophical Society. 1880;19(107)38-58. [jstor.org]

Romer AS. The locomotor apparatus of certain primitive and mammal-like reptiles. Bulletin of the AMNH. 1922;46, art.10:517-647. [digitallibrary.amnh.org]

Broili F. Ein Dicynodonttierreste ausderKarooformation. Neuen Jahrb. Fur Min., Geol., und Paliiont. 1908, I:1-15, P1. I.

Ahlberg PE. Follow the footprints and mind the gaps: a new look at the origin of tetrapods. Earth and Environmental Science Transactions of the Royal Society of Edinburgh. 2018;109(1-2)115-37. [cambridge.org]

Francis ETB. The anatomy of the salamander. Oxford: The Clarendon Press, 1934. [archive.org]

Keywords

ligamentum capitis femoris, ligamentum teres, ligament of head of femur, doctrine, animals, amphibian

The original text in Russian is available at the link: Eryops megacephalus

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500–1 Mya

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