Pelvic girdle and fin ofTiktaalik roseae

Abstract

A major challenge in understanding the origin of terrestrial vertebrates has been knowledge of the pelvis and hind appendage of their closest fish relatives. The pelvic girdle and appendage of tetrapods is dramatically larger and more robust than that of fish and contains a number of structures that provide greater musculoskeletal support for posture and locomotion. The discovery of pelvic material of the finned elpistostegalian, Tiktaalik roseae, bridges some of these differences. Multiple isolated pelves have been recovered, each of which has been prepared in three dimensions. Likewise, a complete pelvis and partial pelvic fin have been recovered in association with the type specimen. The pelves of Tiktaalik are paired and have broad iliac processes, flat and elongate pubes, and acetabulae that form a deep socket rimmed by a robust lip of bone. The pelvis is greatly enlarged relative to other finned tetrapodomorphs. Despite the enlargement and robusticity of the pelvis of Tiktaalik, it retains primitive features such as the lack of both an attachment for the sacral rib and an ischium. The pelvic fin of Tiktaalik (NUFV 108) is represented by fin rays and three endochondral elements: other elements are not preserved. The mosaic of primitive and derived features in Tiktaalik reveals that the enhancement of the pelvic appendage of tetrapods and, indeed, a trend toward hind limb-based propulsion have antecedents in the fins of their closest relatives.A t first glance, the origin of tetrapods (limbed vertebrates) from finned precursors seems an almost insurmountable transition between life in water and life on land. If the basis of comparison were living taxa alone, then the anatomical and behavioral differences among finned and limbed vertebrates could appear vast: for example, fin structure and function differ dramatically from those of limbs. Fossil evidence, in particular vertebrates from the middle and late part of the Devonian period (393-359 Mya), offers intermediate conditions that bridge this gap (1). The fossils that provide the most informative anatomical intermediates are from the tetrapodomorph lineage (also known as stem tetrapods) and have been recovered from a variety of nonmarine and marginal marine deposits from around the globe (2-4). The creatures closest to the node containing the most basal limbed vertebrates-elpistostegalids, such as Panderichthys, Tiktaalik, and Elpistostege-are most enlightening in understanding the primitive conditions from which tetrapods arose. Although most work has focused on revealing homologies and function of the pectoral appendage of these forms (4-7), relatively little is known of the pelvic appendage beyond limited material of Panderichthys (8). Consequently, analyses of the pelvic fin have been given only sporadic attention over the past decades (4, 8-11) largely because they are often poorly preserved or not preserved at all. In most cases, it is thought that this poor preservation of the pelvic appendage is due to its putative small size and fragility (10).Pelv...

Massive increase in visual range preceded the origin of terrestrial vertebrates

“…Our discovery that the evolution of larger eye sockets occurred in animals that were primarily aquatic is in line with other critical conclusions of the past several decades of early tetrapod paleontology, which has found that robust limbs evolved in primarily aquatic animals (9,10) and that fingers and toes evolved in primarily aquatic animals (11). Notably, the increase in eye size starting in the elpistostegalians coincided with a distinct change in the placement of eyes in this group.…”

A critical appraisal of appendage disparity and homology in fishes

“…Panderichthys, Tiktaalik, Elpistostege), there seems to be an evolutionary trend towards the loss of median fins other than the caudal fin Shubin, Daeschler, & Jenkins, 2014). (Furlong & Holland, 2002;Janvier, 2008;Ota, Fujimoto, Oisi, & Kuratani, 2013;Shimeld & Donoghue, 2012).…”

Carpus and tarsus of Temnospondyli

“…Proximal elements of the wrist (ulnare and intermedium) and ankle (fibulare and intermedium) are present in the more fish-like tetrapodomorphs (Andrews and Westoll 1970;Boisvert et al 2008; Davis et al 2004; Shubin et al 2006) predating the appearance of tetrapod digits. A single element in the tarsus of Tiktaalik is a possible intermedium (Shubin et al 2014). Only a single ossified bone identified as the intermedium is known for the carpus of the stem tetrapod Acanthostega (Coates 1996), but a second stem tetrapod Tulerpeton has an ossified ulnare and intermedium (Lebedev and Coates 1995).…”