The Importance of Patterns
A new door has opened in the world of evolutionary biology with the unearthing of a fossil that dates back 350 million years ago. The reassessment of an early tetrapod-like organism, Rhizodus, has sparked new interest in the evolution of limb patterning as well as in developmental embryology. Rhizodus hibberti, a member of the tetrapod stem group, displays a unique pelvic limb patterning (Jeffery 2018). Instead of the usual one-to-two pattern (seen in virtually all tetrapods) of a femur, tibia and fibula, scientists have observed a one-to-three pattern. This trichotomous articulation carries implications that warrant further research in the evolution towards this limb patterning and the developmental process that created it. In order to understand the gravity of this discovery, we must first highlight the findings, revisit the early tetrapod forms and discuss the science behind limb patterning.
In sarcopterygian fishes, limbs arose as a modification of the pectoral and pelvic fins. Fossil evidence of pelvic material from stem-tetrapods is rare, however, one well conserved fossil is that of Rhizodus hibberti which was found in an oil shale from the Asian Wardie Shales. This particular sarcopterygian fish is one of the largest in its group, growing up to approximately 3.5 m long. This well preserved fossil allows a peek into early tetrapodal form from the Carboniferous period. After analyzing the pelvic fin, researchers noticed that instead of a femur articulating into two distinct radials (the tibia and the fibula), it branched into three, morphologically unique, radials (Figure 1). The three radials are similar to the originating segment in that they are also cylindrical “long” bones (Jeffery 2018). It should be noted that a trichotomous articulation has been seen in other sarcopterygian fin skeletons (including Tiktaalik), however they only occur distally to the two radials. In contrast to the pelvic fin, the pectoral fin of Rhizodus more closely resembles that of later period tetrapods. This suggests that the evolution of the bifurcating radials occurred gradually, originating first in the pectoral fins and then in the pelvic fins. This bit of evidence can be used to help explain other early tetrapod and sarcopterygian forms with differing pectoral and pelvic fin patterns.
For the majority of the 20th century there was very little research on early tetrapod forms. The only representative fossils were that of Ichthyostega and Acanthostega which, although helpful to scientific research, were incomplete making it difficult to draw significant conclusions. However, in 1971 a complete fossil of Acanthostega was found in East Greenland which led to an explosion of research and discoveries pertaining to tetrapod evolution. One particular area of focus was the origin of limbs and digits, because these Devonian tetrapods displayed more than the normal, highly conserved pattern of five digits (Clack 2005). A set of Hox genes in these species that control digit formation did not display the normal proximal to distal expression, instead their action was the opposite and proceeded from the posterior of the limb bud to the anterior. These Hox genes were not only controlling digit formation, but also the number and identity of the digits (Clack 2005). The pattern of digit evolution in early tetrapods is one that reflects Williston’s Law. In Devonian tetrapods there is high variability in digit characteristics followed by specialization and reduced complexity. This high variability does not imply randomness, instead it shows that function in these early forms were diverse. As species evolved and began to take on new forms one characteristic remained constant, the humerus always connected to the radius and the ulna which then radiated into phalanges (Clack 2005). Although there is variety in digit number and shape in differing vertebrates, the pattern in anterior-posterior region of the limb remains the same. With the discovery of the Rhizodus a new question arises; what is the molecular difference that led to such an unconventional limb development? To answer this we must review the mechanisms that control limb patterning.
The basic layout that is ubiquitous to all vertebrates contains three elements; the stylopod, zeugopod, and the autopod. The stylopod will develop into the humerus in the forelimb and the femur in the hind limb. The zeugopod will develop into the ulna and radius in the forelimb and the tibia and fibula in the hindlimb, while the auto pod is associated with more distal skeletal elements like phalanges (Jeffery 2018). Some of the most important molecules involved in limb patterning and development are the hedgehog genes, individually named Sonic hedgehog (Shh), Desert hedgehog, and Indian hedgehog. The molecule that controls the number and identity of digits is the Sonic hedgehog gene (Tabin 2008). Researchers in 1993 discovered that the action of the zone of polarizing activity (ZPA) was completely dependent on Shh expression (Carroll 2006). Shh works by creating a gradient across the emerging embryonic limb and triggering the development of unique digit types dependent on the concentration of Shh. This same gene controls human digit development and when mutations are present, variation in digit number and form appears. An example of this variation is polydactyly. When trying to understand the embryonic process by which the unique pattern of trichotomous articulation arose in Rhizodus, researchers looked to Shh for an answer. Experiments with chicks and axolotls have shown that grafts of ZPA tissue from the posterior section of a donor limb bud placed on the anterior region of the host limb bud cause a mirror image duplication of the distal skeletal elements (autopod) and an expanded zeugopod that contains three radials. Additionally the spacing between the three radials is proportional, meaning that these elements are established through a mechanism that takes into account its surrounding environment. “The wider the primordial field, the more radials will form” (Jeffery 2018). In figure 2 the skeleton under “A” is a chicken wing, underneath it are the two variations of the zeugopod with the ZPA grafted tissue. The skeleton under “D” is an axolotl’s limb followed by another illustration of the zeugopod after experimental alterations. However, although these results are important and exciting, they do not explain the Rhizodus limb pattern. The three radials seen in these experiments with the chicks and the axolotl were mirror duplications, meaning that the bones developed in the order of ulna-radius-ulna or radius-ulna-ulna instead of three distinct, differing radials. This means that on its own, a change in Shh expression did not cause the unique morphology, however it could have played a role. The major implication of this fossil and its oddities are that it may be a link between sarcopterygian fishes and their later tetrapod forms.
This discovery highlights one the major transformations in evolutionary history, the development of the tetrapod form from sarcopterygian fishes. The Rhizodontida, the order of which Rhizodus belongs to, are thought to be elementary members of the tetrapod lineage (Jeffery 2001). This research on Rhizodus hibberti reveals a preliminary stage in the evolution of limb development. In this stage the stylopod and the zeugopod were not restricted in the way that they are in extant tetrapod forms (Jeffery 2018). Although the researchers did not address the question of why this unique form existed, I believe the answer is the same no matter what unique feature is in question of any organism. Natural Selection. At some point in earth’s early history this development of a third radial was beneficial and aided to the organisms survival and reproduction. Further research on this species will hopefully someday identify just how it was possible that these tree radials erupted. Until then, Rhizodus hibberti remains a unique and incompletely understood fossil that carries enormous scientific potential.
Works Cited
Carroll, Sean B. Endless Forms Most Beautiful: the New Science of Evo Devo and the Making of the Animal Kingdom. Quercus, 2012.
Clack, Jennifer A. “The Emergence of Early Tetrapods.” Palaeogeography, Palaeoclimatology, Palaeoecology, vol. 232, no. 2–4, 29 July 2006, pp. 167–189., doi:10.1016/j.palaeo.2005.07.019.
Jeffery, Jonathan E. “Pectoral Fins of Rhizodontids and the Evolution of Pectoral Appendages in the Tetrapod Stem-Group.” Biological Journal of the Linnean Society, vol. 74, no. 2, 1 Oct. 2001, pp. 217–236., doi:10.1111/j.1095–8312.2001.tb01388.x.
Jeffery, Jonathan E., et al. “Unique Pelvic Fin in a Tetrapod-like Fossil Fish, and the Evolution of Limb Patterning.” Proceedings of the National Academy of Sciences, vol. 115, no. 47, 2018, pp. 12005–12010., doi:10.1073/pnas.1810845115.
Tabin, C. J., and A. P. Mcmahon. “DEVELOPMENTAL BIOLOGY: Grasping Limb Patterning.” Science, vol. 321, no. 5887, 2008, pp. 350–352., doi:10.1126/science.1162474.