Shoulders and Hips in Megalancosaurus – WP#12

Another interesting area of research that has intrigued me has been the physical and ecological habitus of various megalancosaurids, specifically Megalancosaurus preonensis and Drepanosaurus unguicaudatus. Today, I will mention Megalancosaurus, the more “tame” of the two.

Megalancosaurus preonensis [1] is based on a partial skeleton including the skull, neck, shoulder, and complete forelimb, split into a part and counterpart. Like many fossils from laminated-bedded lagerstätten, when a slab bearing a fossil (referred to as an inclusion in the slab) is split, the fossil is often broken between the slab; normally, one side bears more material, and the other is often an effective negative mould of the material preserved on the better slab (in these cases, it is trendy to refer to the better slab as the main slab or part, and the other as the counter slab or counterpart). Sometimes, however, the slabs split in a very uneven way, preserving most, if not all of the material on one slab, and partially within it, so that the plane of the slab break is irregular to the plane in which the inclusion resides. Yet another form of splitting generally results in a variation of the first (most common) type, but where the specimen is above the plane of splitting, residing on a pedestal-like structure; this is called pedestalling, and the counterpart is often a perfect mould of the pedestalled slab.

The holotype of Megalancosaurus preonensis, MFSN 1769, is preserved in another type of split, where the inclusion is cut clean through in the same plane as the break in the slab, resulting in split bones on both sides. This has resulted in errors in interpretation, and gave Silvio Renesto a tough time trying to figure out the identity of various bones (as none of them are preserved in lateral view, or medial, or really any “view” [2]. We are fortunate, then, that further specimens have been found, and preserved in the first type of splitting, so that superficial structures can be ascertained. Following one of them, MPUM 6008, I produced these illustrations:

Shoulder and pelvic girdles of Megalancosaurus preonensis. A-C, Shoulder girdle, based on MPUM 6008: A, lateral view; B, cranial/anterior view; C, ventral view. D-F, Pelvic girdle, based on MPUM 6008 and MBSN 25: D, lateral view; E, cranial/anterior view; F, dorsal view.

There’s a lot to see here, and much of it described by Wild and Renesto, so I won’t go into too much detail. What is clear is that Megalancosaurus preonensis has very strange shoulder and pelvic anatomy. A few examples:

1. The scapula is mostly a thin rod with a spatualte plate-like base.
2. The scapular glenoid is a shallow socket, as in some mammals, rather than a slot between scapula and coracoid, as in most conventional reptiles.
3. The sternal plates are both extremely large and conjoined to the large coacoids on their posterior half.
4. There were three sacral vertebrae.
5. As in point 2, the iliac glenoid is socket like, but appears to lack a dorsal shelf under which the femur would articulate. This has bearing on something I will show below.
6. The ilium is expanded into a subhorizontal plate, the anterior extent of which has been likened to an antiliac blade (or preacetabular ala) of the ilium.
7. There doesn’t appear to be any ventral opening between the two halves of the pelves, and each half is fused into a innominate-like structure [n1].

Megalancosaurus preonensis shoulder and hip excursion. A, shoulder in cross-section; B, hip in cross-section. 1-3, Extremes of positions of the propodia relative to the girdle sockets: 1, abduction, 2; intermediate; 3, adduction. Abbreviations: CO, coracoid; F, femur; H, humerus; p, pelvis; SC, scapula.

But there’s more: The shoulder and hip sockets (also called the humeral glenoid and acetabulum) are shallow without derived ridges defining a restrictive shelf around most the socket; what is there is very short. While the humerus is known in many views and specimens, permitting a good means of testing the articulation of the shoulder, the hip is much less certain: Only in MBSN 25 does the femur indicate the shape of the caput, and what is there appears to indicate a medially directed caput, but without any neck or semispherical shape; it appears to have firmly fit in a shallow socket, and provided only a restrictive range of motion.

[1] Calzavara, M., Muscio, G. & Wild, R., 1980. Megalancosaurus preonensis, n. g., n. sp., a new reptile from the Norian of Friuli. Gortania 2:49-63.
[2] Renesto, S. 1994. Megalancosaurus, a possibly arboreal archosauromorph (Reptilia) from the Upper Triassic of Northern Italy. Journal of Vertebrate Paleontology 14(1):38-52.

[n1] “Innominate” is the term applied to mammalian pelvises, where the ilium, pubis and ischium are fused into a single bone, even in very young animals.

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