Soft-tissue anatomy of the plesiosaur pectoral girdle inferred from basal eosauropterygia taxa and the extant phylogenetic bracket

Abstract

Plesiosaurians are highly derived secondarily-adapted organisms (if fishes are primarily-adapted) with a long evolutionary history, and they are closely related with basal eosauropterygians. Attempts to reconstruct soft-tissue anatomy can be complicated due to the lack of extant closely-related species, thus various lines of evidence must be considered. This study aims to reconstruct the pectoral girdle myology of eosauropterygians. Information derived from the extant phylogenetic bracket method was not sufficient to clarify muscle attachments in the pectoral girdle of plesiosaurians. To correctly infer muscle homologies, the extant phylogenetic bracket information had to be complemented with developmental and osteological information, and osteological transformations had to be traced back to Permian basal neodiapsids. The reconstructed pectoral girdle musculature presented here is, thus, significantly different from previous attempts. As in secondarily-adapted aquatic modern analogues, several muscles atrophied (e.g., pectoralis, episternocleidomastoideus) and others specialized (e.g., coracobrachialis, clavodeltoideus) in order to attain a more influential role to the stringent conditions of subaquatic locomotion. The subcoracoscapularis, scapulodeltoideus, scapulohumeralis and supracoracoideus are inferred to be glenohumeral stabilizers. The clavodeltoideus acted as the main protractor muscle and the coracobrachialis as a major retractor muscle, possibly in conjunction with the latissimus dorsi. Several heads of the triceps possibly atrophied, as in whales, serving mainly as a cubital joint stabilizer. The trapezius, serratus and levator scapulae served as pectoral girdle stabilizers.