INAUGURAL ARTICLE by a Recently Elected Academy Member:Pelvic girdle and fin of Tiktaalik roseae

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.At 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).Pelves, and in some cases pelvic appendages, of taxa that span the fin-to-limb transition are known from Gooloogongia (Rhizodontida) (12), Eusthenopteron (Osteolepida) (10, 13), Panderichthys (Elpistostegalia) (7, 8), and Acanthostega and Ichthyostega (Tetrapoda) (14–17). Comparisons of these forms reveal large differences between the pelvic appendages of finned tetrapodomorphs and tetrapods (Fig. 1). Most noticeable is that, in finned taxa, the entire pelvic appendage is significantly smaller than the pectoral. In particular, the pelvic girdle of finned tetrapodomorphs is diminutive relative to the pectoral: the pelvis represents a small fraction of the length of the body (the maximum length of pelvis-to-body length is 1:20 in Eusthenopteron per ref. 10). In addition, there are major differences in the morphology of the pelvic girdles of finned and limbed taxa. The girdles of Eusthenopteron and Gooloogongia have posteriorly facing acetabulae and lack sacral ribs and ischial bones, among other features (10, 12). Unfortunately, the pelvic girdle of Panderichthys is not preserved in sufficient detail to understand the distribution of these morphological features in elpistostegalids (8). However, the best comparisons available from these data strongly supported the hypothesis that the closest finned relatives of tetrapods were “front wheel drive animals,” possessing enlarged pectoral fins, robust pectoral girdles, and relatively small pelvic appendages that were incapable of providing extensive degrees of body support and propulsion.Open in a separate windowFig. 1.Right pelves of Gooloogongia (A), Eusthenopteron (B), and Acanthostega (C) in lateral view. Gooloogongia is preserved as a natural cast in one orientation. Figures were modified from refs. 10, 12, and 14. Cranial is to the right.Previously undescribed material of the stem tetrapod, Tiktaalik roseae, can inform these issues. The type specimen (NUFV108), recovered in 2004 and described in 2006 (5, 6), has since been revealed to contain a partial pelvic appendage, including the right side of the pelvic girdle and an incomplete pelvic fin consisting of endochondral bones and lepidotrichia (Figs. 2 and ​and3).3). This specimen allows direct comparison of the relative size of the pelvic girdle and appendage with the rest of the body because the type consists of a relatively articulated skeleton from head to pelvis. In addition, work at the same site in Nunavut Territory during 2006, 2008, and 2013 has revealed additional isolated pelves of four other individuals. Together, these specimens offer the possibility to test the front wheel drive hypothesis and provide insights into the sequence in the acquisition of tetrapod pelvic appendage structure and locomotor function.Open in a separate windowFig. 2.Type specimen (NUFV108): ventral surface of cranial block (figured in ref. 6) aligned in preserved position with ventral view of the block containing the pelvic fin. (Inset) Line diagram of lepidotrichia and preserved portions of endochondral bones of pelvic fin. f, fin; i, intermedium?; l, lepidotrichia; r, radials.Open in a separate windowFig. 3.Tiktaalik roseae, stereopairs of the right pelvis from NUFV108 in (A) ventral (cranial is to the Top), (B) dorsal (cranial is to the Bottom), (C) caudal (lateral is to the Right), and (D) cranial views (lateral is to the Left). A, acetabulum; i, ilium; p, pubis; r, ossified ridge; u, unfinished bone.