Atlasaurus imelakei skeletal

Atlasaurus imelakei

Etymology: "Atlas mountains lizard, the giant"

Time horizon: Middle Jurassic, Bathonian to Callovian epochs (~166 mya)

Length: 15.8m (52 ft.), perhaps more depending on maturity.

Probable mass: 20+ tons


*NOW UPDATED with more accurate femur and distal humerus data*

After several starts and stops, finally an accurate skeletal of Atlasaurus, the bizarre short-necked brachiosaur! This skeletal is piled beyond shoulder-high with data from the description paper as well as from photographs of the museum mount in Rabat, Morocco, which correct a lot of the errors found in GSP's and Michel Fontaine's awful interpretations.

Named for the Atlas mountains in its native Morocco, this odd early member of brachiosauridae is probably the strangest sauropod known from good remains besides Isisaurus - it broke all the rules of a group that was already smashing everyone else's conventions for body size and outrageous neck length. It was one of at least two early brachiosaur genera (the other being an unnamed Portuguese animal paleo-king.deviantart.com/art/… ) which pioneered the "giraffe-platform" model of arms longer than hindlegs that later became the signature feature of the entire family. Earlier macronarians, as well as more primitive sauropod groups such as Mamenchisauridae, already had their shoulders elevated somewhat above the hips, but much of this was due to their long shoulder blades (though few museums mount them at the correct incline). It was these "Atlasaurine" brachiosaurs which first featured arms where the humerus actually did tower over the femur, and the steep slope of the back was not due to big shoulders alone.

However, these early experiments with very long arms were a singular specialization. The very long necks typical of later brachiosaurs still had yet to appear. Atlasaurus was a very short-necked animal as far as brachiosaurs went, and even compared with more "average" sauropod families, the neck still looks painfully undersized. It's one animal that seems to be permanently designed for high-browsing, no matter what you like to imagine the neck doing. The arms are not just long but unusually slender, indicating that the muscles on the arms were relatively light, and that stride length, not torque, was basically the core of this animal's speed. For a sauropod it could have been surprisingly fast, its long arms giving it a clear balance and distance advantage over short-armed diplodocoids. The foot claws, larger than in later brachiosaurs, provided traction as well as flank defense. The tibia and fibula are short as in other brachiosaurs; by comparison the femurs look very oversized. The hands are still rather primitive for a brachiosaur, with the thumb metacarpal being shorter than the others (and, oddly, fused to the index finger's metacarpal), probably supporting a large swiveling thumb claw, a primitive feature which was reduced and fixed in later brachiosaurs. The posterior cervicals and anterior dorsals appear to have forked or at least partially forked spines as in Klamelisaurus, indicating that bifid brachiosaurs did exist but that the trait was later lost -which may coincide the the adoption of a more fully vertical and s-curved neck posture.

It's not clear why Atlasaurus didn't have a longer neck - even relative to more primitive sauropods it seems to have taken a step backwards in that regard. But the rock layers were it was discovered give a clue: the describers notes that the geology of the sandstone indicated "a vast plain close to sea level traversed by powerful rivers", which indicates the possible presence of large boulders getting pushed around by floods. It's possible that Atlasaurus lived on rocky floodplains where big herbivores needed a lot of vertical clearance to get over the huge stones and move quickly over collapse-prone ravines, but where most remaining trees were not very tall and long arms were sufficient for browsing height without needing a very long neck. Apparently the unfortunate type specimen (a teenager, like most of the best brachiosaur specimens) failed to make it over one particular ravine, and was partially scavenged by a theropod (which left behind some teeth) before being buried. It is a remarkably complete skeleton, including pieces of the skull, which appears to have roughly resembled Europasaurus, another basal brachiosaur. The medially curved humeri, as well as the angular front-heavy hips, are distinctly brachiosaurid, and resemble almost nothing else so much as B. altithorax, indicating that its lineage stretched back well into the Middle Jurassic, and that the basic axial and appendicular morphology of brachiosaurs as a unique group existed long before they reached their final iconic proportions.

And of course since this is Atlasaurus we're talking about, the scale figure this time is not Eugen Sandow, but Charles Atlas.


This skeletal has been credited as inspiration by:
cisiopurple.deviantart.com/art…
spinoinwonderland.deviantart.c…


REFERENCES:

M. Monbaron and P. Taquet. 1981. Découverte du squelette complet d'un grand Cétiosaure (Dinosaure Sauropode) dans le bassin jurassique moyen de Tilougguit (Haut-Atlas central, Maroc) [Discovery of a complete skeleton of a large cetiosaur (sauropod dinosaur) in the Middle Jurassic Tilougguit Basin (High Atlas, Morocco)]. Comptes Rendus de l'Académie des Sciences à Paris, Série II 292:243-246

M. Monbaron, D. A. Russell, and P. Taquet. 1999. Atlasaurus imelakei n.g., n.sp., a brachiosaurid-like sauropod from the Middle Jurassic of Morocco. Comptes Rendus de l'Académie des Sciences à Paris, Sciences de la Terre et des Planètes 329:519-526.