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Do not misundreshtmate the king was Re: Evolution of tyrannosauroid bite power



"Strong" is a relative term. Carpenter and Smith (2001) estimated the maximum force capable of being generated by the M. biceps in T. rex to be about 1955 N, or about 199 kg (440 pounds) per arm.

As you mention, this is just the biceps alone.

Furthermore, the shapes and the cortex thicknesses of the arm bones are stupefying, and the scapulocoracoid is large, even though the arm is not.

Here are a few lines from Table 9.1 of Carpenter & Smith (2001); I recommend to compare the measurements (all in cm) to your own:

                       MOR 555 (gracile)            FMNH PR 2081 (robust)
humerus length                37.7                            37.3
humerus distal width        8.4                                8.9
ulna length                        > 19.5                        21.9
ulna prox. anteropost. length 6.7                        7.1
ulna width prox.                6.4                            4.4
radius length                    > 15.1                        17.3
radius prox. anteropost. length 4.2                    5.2
radius prox. width                2.9                        3.7
metacarpal II length            9.4                        10.9
metacarpal II prox. anteropost. length -            4.1
metacarpal II prox. width        -                        4.9
metacarpal II distal width        -                        3.9

"Finally, the least amount of forearm motion is that of *T. rex* (fig. 9.13), which has short arms and the very low MA [mechanical advantage] of an FBS [force-based system]. The limited ROM [range of motion] and short lever arm of the forelimb provided a very stable platform for the very powerful M. biceps. This indicates to us that the forelimbs were used to hold a [sic] struggling prey. In support of this interpretation, we note the pathology along the medial side of the humerus in FMNH PR 2081. The [huge] site of damage corresponds to the medial head of the M. triceps humeralis, which serves to adduct and extend the lower arm. As noted above, the pathology is characteristic of partial avulsion caused by abnormally high stress loads. Such loads might occur while clutching a large, struggling animal, such as an adult hadrosaur (see Carpenter in press)[.] Indeed, the straight shaft of the humerus, as compared with that of *Allosaurus* (see Gilmore 1920), is precisely what is expected for maximum strength per unit mass (Bertram and Biewener, 1988). Such conditions occur where the bone must resist axial compression, as it would do in this case with the powerful M. biceps (see fig. 9.12). Furthermore, the very low K [ratio of marrow cavity radius to bone radius] and R/t [bone radius to cortical thickness] values for the humerus, ulna, and radius indicate bones selected for ultimate strength or impact loading. Finally, to ensure that the struggling prey not escape while the mouth is attempting to kill it, the two ungual claws point somewhat inward (fig. 9.13C) so that they do not slip out of the prey easily." (Carpenter & Smith 2001:112sq.)

Clearly, there was _strong_ selection _for_ power, even though the arms (except the hands) are short (not small -- just short). Carpenter & Smith even suggest that the arms are short in order to bring the distal ends of the bones closer to the muscle attachment sites on the same bones, decreasing speed and increasing force. And the force was with *T. rex*, even though not enough of it to prevent the abovementioned partial tendon avulsion.