![]() ![]() In the osteolepid ancestors of tetrapods, this bone was fused to the inner surface of the cleithrum but was the only element contributing to the glenoid in which the pectoral fin articulated ( Jarvik, 1980 Clack, 2000). In the most primitive tetrapods, the endochondral part of the shoulder girdle was a single element termed the scapulocoracoid. B, C, K in dorsal view F, H–J in ventral view G in right lateral view L in left lateral view. Note the ossification progress in E–G and extension of the epicoracoid from the procoracoid in E. Dermal ossifications in dark grey, cartilages in light grey, glenoid shaded. (A) Pectoral girdle of adult in ventral view, with scapula, suprascapula and cleithrum deflected. The clavicles and cleithra were still relatively broad, thin plates in extinct temnospondyls, whereas in anurans the clavicles are slender and elongated, and the cleithra are represented by vestigial dermal ossification adjoining the suprascapular cartilage.ĭiscoglossus pictus. However, this dermal element was lost in anurans and the pectoralis muscles became attached to new, median elements of endochondral origin. Variation in the size and proportions of the temnospondyl interclavicle occurred even within species and was obviously ontogenetic ( Warren & Snell, 1991). This was preserved in extinct temnospondyls, sometimes with a posterior extension that presumably developed in connection with the pectoralis (i.e. The interclavicle was a large median element connecting both halves of the girdle in early tetrapods and their piscine ancestors ( Jarvik, 1980, 1996 Clack, 2000). However, as with the general trend in all tetrapods, the dermal part is strongly reduced. 1A) retains nearly all the skeletal elements of early tetrapods. This explains why the origin of anurans is meaningfully explained as a result of abbreviated somatogenesis. permanently water-dwelling Dissorophoidea or Capitosauroidea, which did not undergo metamorphosis into a terrestrial adult. Comparisons of the anuran skeletal structure with that of Permo-Triassic temnospondyls (roughly dated to 300–250 Ma) revealed that it most closely recalls some neotenic, i.e. Like caudates, they are contemporary survivors of the late Permian and Triassic members of the amphibian group Temnospondyli (although the modern amphibian orders, in a strict sense, belong among Temnospondyli, the term ‘temnospondyls’ is often employed in its traditional, informal sense as shorthand for extinct ‘non-lissamphibian’ temnospondyls). anurans, caudates and apodans, often grouped under the name ‘Lissamphibia’), which are known by their peculiar, saltatory locomotion. The sternal elements seem to be neomorphs for the most part, which help to cushion the shock of landing in jumping anurans but which also evolved as functional substitutes (insertion area for the pectoralis muscles) of the temnospondyl interclavicle.Īnurans are one of three clades of modern amphibians (i.e. The almost simultaneous appearance of dermal and endochondral ossifications may be explained by the accumulation of developmental events during a short, distinct metamorphosis (which did not occur in neotenic temnospondyls living permanently in water). The overall reduction in ossification of the anuran pectoral girdle supports the hypothesis of a paedomorphic origin for Anura. In parallel, the dermal ossifications of the girdle were reduced to a small clavicle and cleithrum. In the endochondral girdle, the separate scapula and coracoid of Discoglossus and other anurans (completed by suprascapular and procoracoid cartilages) evolved from the compact scapulocoracoid of temnospondyls by paedomorphosis. Basic developmental features were compared with those of extinct Temnospondyli, considered to be the most probable anuran ancestors, and with Triadobatrachus, an early Triassic proanuran. Using cleared-and-stained whole mounts and computer-aided three-dimensional reconstructions made from serial histological sections, we studied the development of the pectoral girdle in Discoglossus pictus, an extant member of an ancient frog lineage, represented for example by Eodiscoglossus from the Middle Jurassic to Early Cretaceous periods in Europe. ![]()
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