By: Priya Amin
What is the distal limb pattern of tetrapod forelimbs? The tetrapod distal limb pattern consists of three segments: the stylopod (the first segment of the limb including the humerus), the zeugopod (the second segment of the limb including the radius and ulna), and the autopod (the hand) (1).
What is the basic phylogenetic breakdown of the fin to limb evolution? The vertebrate group Osteichthyes diverges into Sarcopterygii (lobe-finned fish) and Actinopterygii (ray-finned fish) according to several physical characteristics. Most importantly, Sarcopterygii develop lobe fins, which include a central axis and radial fin rays. Within Sarcopterygii, a group named tetrapodomorpha develop a partial distal limb pattern which lacks digits but includes a humerus, radius, and ulna. Early tetrapods like Acanthostega and other amphibians develop digits, and the full distal limb pattern seen in humans is born! Diverging groups of derived tetrapods specialize and adapt this ancestral pattern of a stylopod, zeugopod, and autopod, which define the distal limb pattern.
COELACANTH (Middle Devonian—Present)
This sarcopterygian (“lobe-finned” fish) showcases the ancestral fin model for tetrapods. Accordingly, the lobed fin of coelacanths has a central axis with fin radials. Because radials deviate on both sides of the axis, the fin is considered to be biserial (2).
In addition, because the species Latimeria is considered a ‘living fossil,’ we can study the movement and morphology of the musculature of this lobe-finned fish (3). Accordingly, a study by Miyake et al. revealed that the musculature in the ‘shoulder’ and ‘elbow’ joints of living coelacanths shows an ancestral condition of movement with the human stylopod (humerus) (4). In particular, the stylopods of Latimeria and Homo sapiens both have paired musculature that is necessary for maintaining stable posture and achieving weight-bearing positions. The presence of this musculature in Latimeria suggests that the basic muscle arrangement needed for terrestrial life may have existed at the earlier stages of tetrapod evolution.
EUSTHENOPTERON (Late Devonian)
Eusthenopteron, a tetrapodamorph, features some of the earliest evidence of a distinct humerus, ulna and radius, which evolved from the ancestral sarcopterygian lobe-fin. Research has shown that its humerus has growth patterns similar to those of tetrapods (5). However, its tetrapod-like humeral characteristics are not terrestrial adaptations. A study by Meunier and Laurin revealed that the long bones of Eusthenopteron are very similar to tetrapod long bones (6). Furthermore, the long bones of Eusthenopteron seem to have been capable of supporting more mechanical stress than the fins of extant acintinistians like Latimeria, which has implications for enhanced movement.
PANDERICHTHYS (Late Middle Devonian)
The flat, L-shaped humerus of Panderichthys represents a key early adaptation for tetrapod evolution: the limb would now be able to prop a large head and support limited front-to-rear movement needed for walking (7,8). The blade-like radius and ridge-and-groove surface of the ulna of both Panderichthys and Tiktaalik represent another ancestral condition of the tetrapod forelimb. In addition, in a study by Boisvert et al., a CT scan revealed previously undiscovered distal radials (9). Before this discovery, the prevailing notion was that tetrapod digits were newly evolved structures. Consequently, the distal radials of Panderichthys (as well as the distal radials of Tiktaalik) have been identified as the possible ancestral condition for digits and human fingers.
TIKTAALIK (Late Devonian)
The pectoral fin of Tiktaalik represents the key functional and morphological transition state between fins and limbs. Like other tetrapodomorphs, Tiktaalik has retained dermal fin rods. Like early tetrapods, its partial distal limb morphology includes a humerus, radius, and ulna; however, it lacks digits. The fin is relatively narrower and stouter than the fins of other tetrapodamorphs. In addition, the fin of Tiktaalik differs significantly from other tetrapodomorphs due to its expanded endoskeleton and reduced dermal exoskeleton. This limb morphology suggests a specialized adaptation for locomotion in shallow floodplains, including the capability for flexion and extension (10).
ACANTHOSTEGA (Late Devonian)
Possessing eight digits, Acanthostega was a primarily aquatic, early tetrapod with limited limb movement (1). Its relatively flat articular surfaces suggest limited flexibility at the wrist and elbow; as a result, the limb most likely acted more as a swim paddle than a more derived tetrapod limb. With the presence of digits, the digital arch of Acanthostega is noticeably more curved (2). Furthermore, in comparison to primitive tetrapod fins, the limb of Acanthostega is distinctly broad and flattened (7). Acanthostega demonstrates the full distal limb pattern ancestral to all derived tetrapods, including a stylopod, zeugopod, and autopod.
HOMO SAPIENS (Present)
Much further derived than Acanthostega, we have evolved a very specialized form of the distal limb pattern. The stylopod includes the humerus, the zeugopod includes an elongated radius and ulna, and the autopod consists of carpals, metacarpals, and phalanges (11). Capable of full flexion and extension, the human limb has fully adapted for movement on land.
From lobe-finned fish to humans, the evolution of the distal limb pattern can be traced along the phylogenetic tree. This pattern defines the three segments of the tetrapod forelimb. By studying this transition, we can see how our arms and legs have evolved for movement on land. And, we can appreciate our fish ancestors!
Priya Amin ’19 is a junior in Pforzheimer House concentrating in Integrative Biology.
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