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Differential limb scaling in the american alligator (Alligator mississippiensis) and its implications for archosaur locomotor evolution. | LitMetric

Differential limb scaling in the american alligator (Alligator mississippiensis) and its implications for archosaur locomotor evolution.

Anat Rec (Hoboken)

Department of Biological Sciences, Functional Morphology and Evolutionary Anatomy (FMEA) Working Group, Western Illinois University, Macomb, Illinois 61455, USA.

Published: June 2009

AI Article Synopsis

  • Bipedalism evolved multiple times in archosaurs, and shorter forelimbs are common in both crocodiles and non-avian dinosaurs.
  • A study on the limb proportions of Massospondylus revealed that bipedal traits arise from negative forelimb allometry, prompting researchers to investigate this pattern in Alligator mississippiensis.
  • Their findings indicated that while most limb elements displayed isometric growth, the ulna showed negative allometry and the third metapodials experienced positive allometry during embryonic stages, suggesting differences in limb development compared to other archosaurs.

Article Abstract

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.

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Source
http://dx.doi.org/10.1002/ar.20912DOI Listing

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