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OK, some naive, pure speculation about those “useless forearms” on large theropods (T-rex, abelisaurs, etc…) Could it be that, in theropods, arms muscles actually compete with head and neck muscles for attachment space on the shoulder girdle? I can’t answer this because I have no idea how the muscles are laid out for theropods. Is there even such a thing as neck muscles that attach to the shoulder girdle, or are these completely unrelated muscle systems? Would reducing arm musculature provide any sort of advantage to the head and neck of theropods? My thought is that, having the combination of having powerful jaws capable of securing a grip on massive victims, then having the necessary neck and head strength needed to either wrench around or yank big chunks off the dangerously powerful prey of the time might be such a key advantage that it would be worth giving up your arms for. This would not mean, then, that their forearms are necessarily “useless”, which might explain why some of these tiny arms remain as well-muscled as they can be (without sacrificing head or neck power). In addition, I heard somewhere that weight and volume grow by the CUBE of size while muscle strength only grows by the SQUARE of size (that is, by the area of a muscle cross-section). I don’t know if it’s really that simple, but it would mean that, as these theropod heads and necks get bigger and bigger, they would need proportionally a LOT more neck musculature to catch up with the faster growth in head weight and inertia. Of course, this would not explain why those tiny, wacky alvarezsaurids have small forearms… that would have to be a totally different selection pressure. Does this make any sense at all or did I just make a complete fool of myself? Be gentle… I’m obviously new at this, lol!

Albertonykus borealis was theropod described by Philip Currie and Nicholas Longrich in 2008 from the lower Maastrichtian of the Horseshoe Canyon Formation in Alberta. Its range also extends into Montana, SD and Wyoming in the Hell Creek and Lance Formation. It is noted for having a single hand claw and is a very small theropod, see photo. Very few folks pay attention to small dinosaurs but this one is very cool and material does come out all of the time so I thought it might be interesting to post. If you collect North American dinosaurs the claws are a must have. Teeth have been found on its Asian cousin Mononykus olecranus but no cranial elements to my knowledge have been found in North America. Maybe one of our forum members can find one or has more information to add. Material found in the Hell Creek and Lance formation should be labeled as Trierarchuncus prairiensis I've seen hand and foot claws as well as phalanges sold but few sellers know what they have and label them theropod. I've personally collected some specimens and have purchased others. Published literature and the New York Museum of Natural History Mononykus display help pull the picture together for identification of specimens. Trierarchuncus prairiensis Was described has the Alvarezsaurid from the Hell Creek Formation July 10, 2020 https://www.sciencedirect.com/science/article/pii/S0195667120302469 Hand Claw : They are typically recurved but can be straight. A shallow vascular groove on either side, ventral foramens are present, see second photo (arrow) Carpal : Called the Alular digit and just has one. Photos L to R Medial, Ventral and Dorsal Views Associated Alular Digit : Foot digits in the following post. My collection: