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Limb scaling in alligators and archosaur locomotor evolution




Anat Rec (Hoboken). 2009 Jun;292(6):787-97.
Differential limb scaling in the american alligator (Alligator 
mississippiensis) and its implications for archosaur locomotor evolution.
 
Livingston VJ, Bonnan MF, Elsey RM, Sandrik JL, Wilhite DR.
Department of Biological Sciences, Functional Morphology and Evolutionary 
Anatomy (FMEA) Working Group, Western Illinois University, Macomb, Illinois 
61455, USA.
 
Bipedalism evolved multiple times within archosaurs, and relatively shorter 
forelimbs characterize both crocodyliforms and nonavian dinosaurs. Analysis of 
a comprehensive ontogenetic sequence of specimens (embryo to adult) of the 
sauropodomorph Massospondylus has shown that bipedal limb proportions result 
from negative forelimb allometry. We ask, is negative forelimb allometry a 
pattern basal to archosaurs, amplified in certain taxa to produce bipedalism? 
Given the phylogenetic position of extant crocodylians and their relatively 
shorter forelimb, we tested the hypothesis that prevalent negative forelimb 
allometry is present in Alligator mississippiensis from a sample of wild 
specimens from embryonic to adult sizes. Long bone lengths (humerus, radius, 
ulna, femur, tibia, fibula, third metapodials) were measured with their 
epiphyseal cartilage intact at all sizes. Our results show an overall isometric 
pattern for most elements regressed on femur length, humerus length, or total 
limb length. However, negative allometry was prevalent for the ulna, and the 
third metapodials scale with positive allometry embryonically. These data 
suggest that the general forelimb proportions in relation to the hindlimb do 
not change significantly with increasing size in A. mississippiensis. The 
negative allometry of the ulna and embryonicaly positive allometry of the third 
metapodials appears to be related to maintaining the functional integrity of 
the limbs. We show that this pattern is different from that of the 
sauropodomorph Massospondylus, and we suggest that if bipedalism in archosaurs 
is tied, in part, to negative forearm allometry, it was either secondarily lost 
through isometric scaling, or never developed in the ancestor of A. 
mississippiensis.