Origin of paired limbs
For over a century the origin of vertebrate paired fins and limbs has been fiercely debated. One of the first theories put forward proposed that fins evolved from the gill arches of the early limbless vertebrates (Gegenbaur, 1878). Recent theory proposes that the fins of vertebrates evolved from continuous stripes of competency for appendage formation located ventrally and laterally along the embryonic flank (Yonei-Tamura et al. 2008) (Fig. 3A). A continuation of this theory proposed that the paired appendages of jawed vertebrates evolved with a shift in the zone of competency to the lateral plate mesoderm (LPM) in conjunction with the establishment of the lateral somitic frontier, which allowed for the formation of limb/fin buds with internal supporting skeletons (Freitas et al. 2006; Durland et al. 2008; reviewed Johanson, 2010) (Fig. 3A). The conservation of genetic mechanisms (Hox and Tbx expression patterns) between median fins and paired fins of shark and lamprey embryos supports this theory (Freitas et al. 2006).
Figure 3. Diagram of the evolution of paired fins. (A) The evolution of paired fins from the ‘zone of competence’ (Freitas et al. 2006; Yonei-Tamura et al. 2008; Johanson, 2010). A dorsal zone of competence for unpaired fins is the first to arise in jawless vertebrates (Johanson, 2010). The dorsal zone of competence was then duplicated and co-opted to a ventrolateral position along the flank (Freitas et al. 2006). The ventrolateral zone of competence was then shifted to the LPM, which coincided with the evolution of the abaxial region and the lateral somitic frontier (Johanson, 2010). (B) The ‘lateral fin fold’ theory suggests that two paired fins evolved from a single continuous lateral fin (Thacher, 1877; Mivart, 1879; Balfour, 1881). (C) Based on the collinear expression pattern of Hox gene expression, the ‘pelvic before pectoral’ theory suggests that the pelvic fins evolved before the pectoral fins (Tabin & Laufer, 1993). (D) Based on fossil evidence and the anterior–posterior pattern of development, the ‘pectoral before pelvic’ theory suggests that pectoral fins evolved first and were then duplicated to form pelvic fins (Coates, 1993; Thorogood & Ferretti, 1993).
Download figure to PowerPoint
To explain the emergence of ‘two sets’ of paired fins, several theories have been put forward. The Thacher–Mivart–Balfour fin fold theory of the origin of the paired fins suggests that tetrapod forelimbs and hindlimbs evolved by the splitting of a single lateral fin (Thacher, 1877; Mivart, 1879; Balfour, 1881) (Fig. 3B). This theory has been contested due to the inconsistency with the fossil record and a lack of embryonic evidence. In contrast, Tabin & Laufer (1993) suggested that pelvic fins were acquired before pectoral fins in the ‘pelvic before pectoral’ fin model due to the collinear expression pattern of the Hox genes along the embryonic flank and in developing limb buds (Fig. 3C). It was thought that the pattern of Hox gene expression along the flank of the embryo was co-opted into the pelvic fins and then passed to the pectoral fins, which is why only the posterior Hox genes are expressed during limb/fin development (Tabin & Laufer, 1993). However, to date, no fossils have been described which possess only pelvic fins (Coates, 1993; Thorogood & Ferretti, 1993) and these authors suggested that based on fossil and developmental evidence that pectoral fins were acquired before pelvic fins (Fig. 3D). Ruvinsky & Gibson-Brown (2000) proposed that an ancestral Tbx4/5 cluster was initially co-expressed in the first pair of fins to evolve. In modern jawed vertebrates, Tbx4 is expressed in the pelvic appendages and Tbx5 is expressed in the pectoral appendages (Gibson-Brown et al. 1996; Tamura et al. 1999; Ruvinsky et al. 2000). To explain this expression pattern, it was suggested that the ancestral Tbx4/5 cluster underwent a duplication event, either before or Tbx4 became localised at the pelvic level or after the cluster became localised at the pelvic level (Ruvinsky & Gibson-Brown, 2000). In this model, Tbx4 acted in conjunction with Pitx1 to modify the morphology of the developing limb to a pelvic fin/hindlimb identity (Ruvinsky & Gibson-Brown, 2000).
Pelvic fins and the tetrapod transition
The fish-to-tetrapod transition involved a gradual shift towards more coastal and terrestrial environments (Clack, 2000) and with it came a change in pelvic fin function. This transition involved the shift from paired pectoral and pelvic fins to the development of weight-bearing fore and hindlimbs for locomotion on land. It is thought that the fin to limb transition first began in the pectoral fins and that the evolution of pelvic fins into hindlimbs occurred in a relatively brief period of time between Panderichthys and Acanthostega (Coates, 1996; Boisvert, 2005). Unfortunately, there is a real paucity of fossils in this interval with intact pelvic fins. However, the insights gained from recent fossil finds, re-examination of older fossils and evidence obtained from developmental biology challenge the old ideas and suggest that the pelvic fin to hindlimb transition was evolving even before early tetrapods moved out of the water and colonised land. Two key developmental breakthroughs during this time were the elaboration of the distal skeleton and the development of a robust weight-bearing pelvis (Boisvert, 2005; Johanson et al. 2007).
Evidence from the fossil record and developmental studies of living sarcopterygians suggest that during the evolution of the distal pelvic fin skeleton the digits appeared before the full complement of ankle elements (Wagner & Chiu, 2001; Coates, 2003; Clack & Ahlberg, 2004; Johanson et al. 2007). It is currently thought that digits were not an evolutionary novelty of tetrapods, as previously believed, but evolved from the pre-existing distal radials of sarcopterygians (Johanson et al. 2007; Boisvert et al. 2008). During this stage of evolution, polydactyly was plesiomorphic amongst Tetrapodomorpha. Fossil evidence from early Devonian tetrapods indicates that Ichthyostega had seven toes, Acanthostega had eight toes, and Tulerpeton had at least six toes (Coates & Clack, 1990; Lebedev & Coates, 1995; Coates, 1996). It is thought that pentadactyly of later tetrapods did not evolve until the Carboniferous period (Coates, 1994, 1996).
The evolution of the full complement of the central bones of the ankle (the mesopodium) came after the evolution of digits (Wagner & Chiu, 2001; Coates, 2003; Clack & Ahlberg, 2004; Johanson et al. 2007). The pelvic fins of ancestral sarcopterygians possessed the long bones equivalent to a femur, tibia and fibula, and distal radials from which digits would evolve, but did not possess the full complement of bones of the mesopodium (Andrews & Westoll, 1970; Wagner & Chiu, 2001; Coates et al. 2002; Johanson et al. 2007; Boisvert et al. 2008). Recent re-examination of Panderichthys has revealed that the pelvic fin of this tetrapodomorph fish has a proximal mesopodium element, the fibulare, but lacks the central bones of the mesopodium (Boisvert, 2005). Two of the earliest tetrapods with well preserved hindlimbs, Ichthyostega and Acanthostega, had hindlimbs that had more derived characteristics, but still had very few central bones of the mesopodium (Jarvik, 1980, 1996; Coates, 1996; Johanson et al. 2007). The full complement of the central bones of the ankle seems to appear in Tulerpeton, which has 12 preserved tarsal bones, including three central elements (Lebedev & Coates, 1995). Most Carboniferous tetrapods have three to four central elements in the mesopodium, which allows for the ankle flexibility necessary for walking on land (Coates, 1996).
The development of a robust weight-bearing pelvis was a key step in the evolution of the hindlimb during the tetrapod transition onto land. To walk on land, the relatively gracile unattached pelvic girdle of fish gradually transformed into a large tripartite weight-bearing structure connected to the vertebral column (Fig. 5). The pelvic girdle of lobe-finned fish is composed of a crescentric pubis often connected through cartilage at the midline, but lacks an ilium and is not connected to the vertebral column (Fig. 5A) (Ahlberg, 1989). In contrast, the pelvis of tetrapods has an ilium that is fused to the vertebral column and an ischium that is posterior to the pubis. In addition, the ischium and the pubis from both halves of the pelvis are fused along their midline, which creates a weight-bearing pelvis (Fig. 5B) (Clack, 2000). There is much evidence from both sides of this transition, but little information about how this evolution occurred due to the paucity of fossils from this period with intact pelvic girdles. On one side of the transition, the pelvic girdle of the tetrapodomorph Panderichthys is small, flat, club-shaped and distinctly fish-like (Boisvert, 2005). Unfortunately, the pelvic girdle and fin of the more crownward tetrapodomorph Tiktaalik has not been preserved, but the early tetrapods, Ichthyostega and Acanthostega, had already evolved a distinctively tetrapod-like pelvis with an ilium and ischium (Jarvik, 1980, 1996; Coates, 1996).
Figure 5. (A) In sarcopterygian fish the pelvic girdle is supported by the hypaxial musculature and consists of a pubis (pb) with a caudally oriented acetabulum (ac) (articulation to the fin) (redrawn from Andrews & Westoll, 1970). (B) In early tetrapods the pelvic girdle consists of a pubis, an ischium (ish), and an ilium (il), which connects to the vertebral column through the sacral rib (sr). The acetabulum is placed laterally (redrawn from Coates, 1996). Figure modified from Cole et al. (2011).
Download figure to PowerPoint
With the evolution of the distal pelvic appendage skeleton and the pelvis, came a shift in locomotory dominance from ‘front wheel drive’ to ‘rear wheel drive’ during the tetrapod transition (Boisvert, 2005). Non-sarcopterygian fish predominately use body muscle undulations and pectoral fins for locomotion, whereas tetrapods use their hindlimbs for this function (Coates et al. 2002). Recent evidence from African lungfish (Protopterus annectens) has shown that this sarcopterygian fish can use a range of pelvic fin-driven gaits such as walking and bounding and use their pelvic fins to lift their body clear of the substrate in an aquatic environment (King et al. 2011). Descriptions of the paired pectoral and pelvic fins of fossils such as Panderichthys and Ichthyostega also offer insights into the evolution of tetrapod locomotion. Panderichthys probably employed an intermediate ‘front-wheel drive’ mode of locomotion, using its pelvic fins as minor anchors while body-flexion propulsion pushed the fish forward (Boisvert, 2005). A recent study of limb joint mobility of Ichthyostega has shown that this early tetrapod had terrestrially ineffectual hindlimbs, as it lacked the necessary rotary motions in its hindlimbs to lift its body off the ground and therefore could not employ lateral sequence walking (Pierce et al. 2012). This new study indicates that early tetrapods went through a stage of hip-joint restriction before they evolved the locomotory behaviours of modern tetrapods (Pierce et al. 2012). Recently, Swartz (2012) described a well preserved fossil specimen of the extinct genus of sarcopterygian fish from the Middle Devonian, Tinirau clackae. Tinirau shares many advanced features with later tetrapodomorphs in the pelvic elements. Tinirau is the earliest known stem tetrapod to have a significantly reduced postaxial process, and a fibula more like those of later tetrapods. Caudally, the pelvis articulates with a femur that is preserved in association with the acetabulum. The postaxial fibular process is highly reduced and displays a similar ‘lip’ overhanging the postaxial edge of the fibulare. The lack of a prominent postaxial process in the fibula of Tinirau is more similar to the condition observed in crownward taxa. This pattern underscores previous phylogenetic reconstructions of the appendicular skeleton in which conventional crown group limb characteristics first originate in the pelvic fins.
Historically, the evolution of the neural control in the pelvic fins and hindlimbs associated with this transition has not received much attention. However, a recent review has compared the organization of the motor neurons in the spinal cord of various vertebrates which aids in the understanding of the evolution of fin/limb motor circuitry necessary for hindlimb dominated locomotion in vertebrates (Murakami & Tanaka, 2011).
In addition to insights gained from recent fossil finds and the re-examination of older fossils, discoveries of preserved pelvic fins and girdles of more crownward transitional Devonian tetrapodomorph fish are eagerly awaited to shed light on the evolution of the vertebrate hindlimb from the pelvic fins of ancestral fish.